Advancements in Osmotically Driven Membrane Systems

#### **Chapter 3**

## Forward Osmosis Membrane Technology in Wastewater Treatment

*Deniz Şahin*

#### **Abstract**

In recent times, membrane technology has proven to be a more favorable option in wastewater treatment processes. Membrane technologies are more advantageous than conventional technologies such as efficiency, space requirements, energy, quality of permeate, and technical skills requirements. The forward osmosis (FO) membrane process has been widely applied as one of the promising technologies in water and wastewater treatment. Forward osmosis uses the osmotic pressure difference induced by the solute concentration difference between the feed and draw solutions. The proces requires a semi-permeable membrane which has comparable rejection range in size of pollutants (1 nm and below). This chapter reviews the application of FO membrane process in wastewater treatment. It considers the advantages and the disadvantages of this process.

**Keywords:** Desalination, Forward osmosis, FO-Based Hybrid System, Integrated FO System, Wastewater, Wastewater treatment

#### **1. Introduction**

Membrane separation processes are widely used in the last decade for industrial, commercial, and domestic activities such as water and wastewater treatment, energy-efficiency. Within the concentration-driven processes, FO has gained increasing prominence due to its advantages such as possibility of low fouling, high salt rejection, and high water recovery. However, FO does have inherently disadvantages such as; reverse solute diffusion (RSD), lower flux, concentration polarization (CP), and membrane fouling. These obstacles oblige the developing new processes, synthesis of different membrane materials or modifications, and finding new draw solution (DS). There is therefore an exigent need to develop new FO membranes by optimization of thickness, porosity, tortuosity of active/support layer of FO membrane.

This chapter is divided into two parts. In this first part of chapter, basic principles of FO phenomenon, advantages and challenges of FO over conventional membrane processes are addressed by the literature review and scholarly articles. The second part of which states applications of FO process for wastewater remediation, and recent developments in FO process.

#### **2. General aspects of forward osmosis**

#### **2.1 Process description**

Forward osmosis is one example of water separation processes and a potential acceptable alternative/complement to reverse osmosis (RO) process for power generation, wastewater treatment and desalination. Forward osmosis is a membrane process in which requires little or no hydraulic pressure. Unlike the RO process, in the FO process, an osmotic pressure gradient through a semi-permeable membrane is the driving force of water transport from the feed solution (FS) to the DS [1]. Thus, the concentrated DS generates an osmotic pressure and drives water from the feed through the membrane while most of the contaminants and salts are rejected by the membrane, then separating the water from the diluted DS [2]. **Figure 1** illustrates the principle of operation of RO and FO processes.

The general equation used to describe theoretical water flux across the RO and FO membrane (*Jw*) is calculated using Darcy's law [1]:

$$J\,w = A\,w \times \left(\sigma\,\Delta\,\pi - \Delta\,P\right) \tag{1}$$

where, *Aw* is the membrane pure water permeability coefficient, *σ* is the reflection coefficient which indicates the rejection capability of a membrane (for a perfect semipermeable membrane *σ* = 1), Δ*π* is the osmotic pressure differential across the membrane, and Δ*P* is the applied external pressure. Therefore, in FO, Δ*P* is zero thus making the water flux to be directly proportional to the difference in osmotic pressure, while for RO, Δ*P* > Δ*π*. The relation between water flux and applied pressure is illustrated in **Figure 2**.

#### **2.2 Draw solution**

Both FO and RO processes involve semi-permeable membranes as key component, which has comparable rejection range in size of pollutants (1 nm and below). One of the major factors in the development of FO membrane is selecting an appropriate DS [4]. The ideal DS should have following characteristics: high osmotic pressure, low molecular weight (MW), non-toxicity, relatively low-cost, high water solubility, and efficiently regeneration [5, 6]. Sodium chloride (NaCl)

**Figure 1.** *Schematic illustration of the (a) RO, and (b) FO processes.*

*Forward Osmosis Membrane Technology in Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.97483*

#### **Figure 2.**

*Schematic representation of FO, RO process: (a) FO process where no external force is applied on the high concentration solution. The natural flow of water is from the low concentration side to the high concentration side; and (b) RO process where applied pressure on the high concentration solution exceeds the osmotic pressure difference across the membrane, so the water flux is opposite to the flux in FO process; and (c) classification of these processes in a water flux vs. applied pressure. Adapted from [3, 4].*

is among the most commonly used draw solute in FO because it has highly water solubility and it is also relatively easy to reconcentrate using classical desalination processes [1]. In the past few decades, vast studies have been performed to determine desirable DSs, the different DSs are presented in **Table 1**, such as (1) inorganic compounds (e.g., NaCl, sodium nitrate (NaNO3), magnesium sulfate (MgSO4)) (2), organic compounds (e.g., glucose, fructose, 2-methylimidazole-based compounds) (3), functionalized nanoparticles (e.g., magnetic nanoparticles (MNPs)), Na+ functionalized carbon quantum dots (Na-CQD).

The different DSs allow the generate of high osmotic pressure and can be easily regenerated or recovered. Nevertheless, their costs have not been successfully determined [39].

#### **2.3 Membrane material**

The identification of an ideal membrane in FO process is a key component which needs to be addressed to further advance this process. A perfect semipermeable membrane should have high water flux and solutes rejection, low propensity to fouling, and high chemical and thermal stability and so forth [2].

The FO membrane can be either synthetic or natural. In the early studies, the variety of natural materials used has included animal bladders and intestines [4]. A few decades ago, investigators have been examined different materials for FO membrane fabrication that include cellulose, rubber, and porcelain [4, 40, 41]. Although synthetic FO membranes have been currently commercially available; but this technology is still in its infancy. As a result, many types of FO membranes have been investigated that are able to perform well under a very wide range of applications [42–51]. **Table 2** provides information about membranes used in wastewater treatment.

As can be seen from **Table 2**, CTA-FO membranes have been used in the most of the experimental working on wastewater treatment due to its relatively higher tolerance to chlorine, insensitive to bio-degradation, and low fouling potential [66–68]. Despite its advantages, there are still some drawbacks such as narrow pH range, relatively low water permeability and high NaCl permeability [69–71]. Compared with CTA membranes, TFC membranes have higher fouling propensity, higher surface selectivity, a wider pH range, and better chemical stability [72–75]. Although CTA membranes have also a chlorine tolerance of up to 1 ppm (part per


#### **Table 1.**

*Overview of the different DSs in FO process.*

million), TFC membranes have limited tolerance to chlorine attack [76]. On the other hand, TFC membranes prone to membrane fouling which negatively impacts their operational and maintenance costs.

*Forward Osmosis Membrane Technology in Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.97483*


#### **Table 2.**

*Some previous and recent researches on FO membranes.*

In addition to fouling of membrane, concentration polarization has an impact on the water flux, particularly at the support layer, which leads to the severity in internal concentration polarization (ICP). A low ICP requires a low S-value (structural parameter) [43, 77].

The membrane structural parameter S is defined as [2]:

$$\mathbf{S} = \mathbf{KxD} = \frac{\mathbf{ts} \times \mathbf{D}}{\mathbf{E}} \tag{2}$$

where *D* is the diffusion coefficient of the draw solute, *ts* is the thickness of the support layer, Ԏ is the tortuosity, and ε is the porosity of the support layer.

Recently, new materials have been investigated for FO membrane fabrication to increase water flux, reduce ICP, and enhance the tolerance to water quality.

#### **3. Application in wastewater treatment**

As an emerging membrane technology, FO has been investigated over the last decade for seawater or brackish water desalination, wastewater treatment, power generation, pharmaceutical applications, and food&dairy processing in both academic research and industries [78, 79].

The most attractive usage of FO is its application for wastewater treatment. Consequently, there are two clusters of applications (i) desalination and (ii) water reuse (**Figure 3**) [80].

Key attributes of this process are:


However, the main challenges in this process are related to:


#### **3.1 Desalination**

Saline water (e.g. seawater or brackish water) and an osmotic reagent (e.g. a non-volatile or a volatile salt) are used as the FS and DS, respectively, in the *direct FO desalination* [81, 82]. In this process, after the FO process, an additional step is needed to recycle the draw solutes as well as to produce purified water [83, 84]. One of the first examples of FO application in water desalination was published in 1975. This study was intended to desalinate Atlantic Ocean seawater to produce an emergency water supply on lifeboats by *direct* osmosis (**Figure 4**) across a CA-FO membrane with a hypertonic glucose solution as the DS [85]. In another study, a flat-sheet CTA-FO membrane was used in seawater desalination, yielding a high water flux and high salt rejection (over 95%) with 6 M ammonium bicarbonate (NH4HCO3) as DS [84]. Also, polymer hydrogels particles have been studied as draw agents in FO desalination. Smaller polymer hydrogel particles led to higher FO water flux in these tests. Similarly, higher salt concentration led to lower FO

*Forward Osmosis Membrane Technology in Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.97483*

#### **Figure 3.**

*Applications of FO in the water treatment industry.*

#### **Figure 4.** *FO process for desalination of seawater or brackish water.*

water flux. Meanwhile, the use of a commercial FO membrane was more suitable than RO membrane [83]. Another study modified magnetic particles covered with thermo sensitive polymer investigated as DS and about 93% of salt recovery was obtained [34]. The world's first commercial forward osmosis desalination plant for direct sea water treatment was established in Al Najdah, Oman. This facility is still in operation and has reduced chemical consumption and provides higher throughput and longer membrane life, significant operational and capital costs and to be more reliable than traditional methods [86]. Membrane fouling and scaling problems at RO stage mitigate due to the use of FO as a pretreatment step for the RO process.

*Indirect FO desalination* uses a high salinity water (e.g. seawater or brackish water) as a natural DS and quality-impaired water source (e.g. wastewater effluent or urban storm water runoff) as the feed solution [87, 88]. The diluted seawater or brackish water can potentially couple with low pressure reverse osmosis (LPRO). The FO-LPRO hybrid process has lower costs for producing water compared to pure reverse osmosis [89]. These experiments have demonstrated the ability of FO membranes to reject nutrients from wastewater, especially chemical oxygen demand (COD) and phosphate, and moderately nitrogen compounds [88, 90]. As an example, a submerged membrane module which makes it possible to adapt the process to a primary clarifier tank has been employed for partial desalination of seawater. The findings indicated that FO membranes have high rejection of heavy metals present in the wastewater (~99%). This study also showed that the use of biopolymers-like substances resulted in the fouling layer on the membrane surface [88]. A similar result has been reported in the use of osmotic membrane bioreactor (OMBR) for municipal wastewater treatment [91].

**Figure 5.** *Scheme of the two FO processes for desalination (a) direct, (b) indirect (adapted from [92]).*

Direct and indirect arrangements of desalination systems using FO membrane are shown in **Figure 5**.

On the other hand, the pretreatment of wastewater has not yet been reported in the study of FO process. The reason, probably, is that the FO system is considered as a pre-treatment step to concentrate wastewater and then concentrated wastewater can be used to recover biogas or other valuable compounds [88, 93, 94].

#### **3.2 Wastewater treatment**

Forward osmosis has been utilized to treat various types of wastewater such as municipal wastewater (sewage) [95–98], oily wastewater [67, 99, 100], tanner effluent [101], automobile effluents [102], dairy streams [102, 103], produced water [104–106] besides synthetic wastewater [107, 108].

Lately, the current systems on FO application on wastewater treatment may be classified into two groups: FO and FO-based hybrid processes, and integrated FO processes. Both in FO and FO-based hybrid systems, the FO membrane is used to recover fresh water and reject of pollutants from the feed solution. In the integrated FO system, the FO membrane gradually replaces conventional membrane in the bioreactor, such as the FO membrane in membrane bioreactor (MBR). The function of the membrane is to concentrate the wastewater and improve the performance of the modified system.

Therefore, FO has been extensively applied in wastewater treatment and reuse, resource recovery, seawater desalination, and food/medicine manufacturing as shown in **Table 3**.

The FO process shows promising results for the treatment of wastewater, and has many advantages in comparison to the conventional wastewater treatment processes. When high process recoveries are obtained, FO processes become viable. Forward osmosis also provides a more sustainable flux and reliable removal of contaminants.

#### *3.2.1 FO and FO-based hybrid system*

Hybrid desalination systems using emerging FO process and combined with traditional process like reverse osmosis, membrane distillation, nanofiltration, electrodyalsis (ED) could potentially reduce the energy consumption of the desalination process, and decrease obstacles in the implementation of process. In these systems, FO is used as a pre-treatment step, while RO, NF, and ED are


*Forward Osmosis Membrane Technology in Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.97483*

#### **Table 3.**

*Application of FO in different industries.*

known as water recovery or draw solution regeneration/reconcentration step [116, 117]. An overview of FO and FO-based hybrid system configurations is depicted in **Table 4**.

#### *3.2.1.1 Hybrid FO-MD system*

The performance of the FO process can be improved by its combination with other system to take advantage of the unique strengths of the individual processes. For this reason, FO process is often combined with an MD process (**Figure 6**). As an example, the FO-MD hybrid system was employed for raw sewage [93] at water recovery up to 80%. This process also achieved high removal efficiency for trace organic contaminants (TrOCs) that rates 91–98%. In another study, this hybrid system was used for oily wastewater treatment. The findings indicated that 90% feed water recovery could be readily attained with trace amounts of oil and NaCl [99]. A vapor pressure driving FO-MD system was studied for treatment high salinity hazardous waste landfill leachate [129]. Total organic carbon (TOC) and total nitrogen (TN) rejection rates were higher than 98% while rejection rate of salt was higher than 96%. NH4 + -N, and heavy metal ions were also completely removed. Similar performance could also be seen in the application of dairy wastewater and grain possessing wastewater treatment [103, 130].


#### **Table 4.**

*An overview of FO and FO-based hybrid systems.*

#### **Figure 6.**

*Schematic diagram of hybrid system consisting of FO and MD processes.*

#### *3.2.1.2 Hybrid FO-RO system*

Due to the current scenario of global water crisis, seawater desalination has become one of the practical solutions to produce water of potable quality. Membrane based desalination processes have been used to desalinate seawater have been widely reported. Among the various desalination processes, RO is the most consistent and reliable process which offers a number of advantages due to its high salt rejection

#### *Forward Osmosis Membrane Technology in Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.97483*

rate, high quality drinking water, high water recovery, and green technology [131]. Despite the aforementioned advantages, several shortcomings, such as high energy consumption and severe fouling propensity remain the obstacles [132]. In recent years, the hybrid system of the FO and RO processes has gained increasing prominence among researchers [8, 116, 117, 119]. As can be seen in **Figure 7**, the hybrid system consists of two stages. The first stage begins with the migration of fresh water from the seawater feed solution to join the draw solution. In the second stage, the product fresh water is separated from the draw solution in the RO unit [89].

In the first study focusing on this FO-RO hybrid system, the authors demonstrated that the approach may provide four major benefits over stand-alone RO desalination: lower energy use, multi-barrier protection of drinking water, beneficial reuse of impaired water, reduction in RO membrane fouling [89]. Similar interest has also been conducted that compaires the hybrid FO-RO system and the standalone RO process for seawater desalination [119]. The study showed that the hybrid FO-RO system can be highly competitive depending on the salinity of seawater and type and concentration of the draw solute. Interestingly, total power consumption in a hybrid FO-RO system was higher than that in RO process, yet the FO process alone was only contributed 2–4% of the total power consumption in the FO-RO hybrid system. Therefore, most of the power consumption in the FO-RO system was realized in the high hydraulic pressure RO regeneration unit [119]. In another study, FO process used as a pre-treatment for a hybrid FO-RO desalination system. The optimal parameters such as water flux, water recovery and final draw solution of this FO pretreatment process were determined by modeling and were experimentally validated by using real brackish water [116]. In a further study, FO-RO hybrid system for coal-fired power plant wastewater treatment, seawater after UF was investigated as DS. Results showed that the total energy consumption of the FO-RO system was 15% less than that of a typical seawater desalination RO [121].

#### *3.2.1.3 Hybrid FO-NF system*

The literature includes theoretical studies on the strengthening economic and environmental potential of the large-scale FO-based systems but very few experimental reports exist on these issues [133–135]. Examples include discussion on pilot-scale FO coupled with NF and other distillation processes for treating wastewater effluents. For example; a pilot-scale FO-NF hybrid closed loop system was developed for the treatment of tannery wastewater at a rate of 52–55 L/m<sup>2</sup> h and rejections of 98.5% COD, 97.2% chlorides and 98.2% sulfate were achieved [136].

**Figure 8.** *Schematic diagram of the hybrid FO-NF system for seawater desalination (adapted from [137]).*

In addition, a hybrid FO–NF system designed for brackish water desalination was investigated and also presented promising results such as lower hydraulic pressure, less flux decline [122]. In another study, a hybrid FO-NF system with two NF passes for the post treatment was used for desalinating seawater [123]. A proposed configuration of a hybrid FO-NF process for seawater desalination is shown in **Figure 8** [137].

#### *3.2.1.4 Hybrid FO-ED System*

Electrodyalsis is a membrane-based separation process in which ions across ion-selective membranes under an electric field. A FO-ED hybrid system was investigated by using diammonium phosphate (DAP), as DS to achieve wastewater reuse and mitigation of salinity buildup on the feed side. Electrodyalsis was able to significantly recover the 96.6 ± 3.0% reverse-fluxed DAP under 3.0 V 1-h daily operation [125]. Forward osmosis process was tested upstream to ED-RO system for an access to DS with higher electrical conductivity in the FO-ED-RO hybrid system [126]. In another study, FO-ED-RO hybrid system proposed to produce high-quality water from secondary-effluent or brackish water is shown in **Figure 9**. Results showed that the water from this system contains a low concentration of total organic carbon (TOC), carbonate and cations derived from the feed water [127].

#### *3.2.2 Integrated FO system*

The integrated FO system includes an osmotic microbial fuel cell (OsMFC) and osmotic membrane bioreactor (OMBR). Recent research has elucidated how

**Figure 9.** *Schematic diagram of a novel photovoltaic powered FO-ED system (adapted from [127]).*

the integration of osmosis in MFC and MBR was used through the application of FO membrane for simultaneous recovery of osmotic water, the concentration of wastewater, and the improvement of effluent quality [138, 139].

#### *3.2.2.1 OsMFC*

The system uses FO integrated into a microbial fuel cell (MFC) to improve the quality of the treated wastewater and the performance of the fuel cell. A FO membrane is placed between the anode chamber with wastewater and the cathode chamber full of DS and water flux through this membrane transports protons from the anode to the cathode [140–145]. An OsMFC (**Figure 8**) achieved water flux of 3.94 ± 0.22 L/m<sup>2</sup> h with a catholyte containing 2 M NaCl, while there was no obvious water flux in a conventional MFC [140]. In a further study, FO membrane is integrated into an air-cathode MFC (AAFO-MFC) for enhancing bio-electricity and water recovery from low-strength wastewater. The AAFO-MFC system produced a high quality effluent, with the removal rates of organic contaminants and total phosphorus (P) of more than 97% [145].

There are also some drawbacks for OsMFC application in wastewater treatment such as the lower water flux of the FO membrane, membrane fouling and salt accumulation (**Figure 10**) [146].

#### *3.2.2.2 OMBR*

Hollow fiber or flat-sheet MF and UF membranes are commonly used membranes in MBR. A major problem associated with the operation of MF-UF-MBRs is membrane fouling. A novel MBR-named OMBR- has been developed and widely used to reduce fouling and promote the reuse of treated wastewater. In OMBR, FO membrane module is displaced in the wastewater. A combination biological treatment and an OMBR uses to remove water from the mixed liquor to the draw side under the osmotic pressure gradient. The pollutants, activated sludge and solids are all rejected by the membrane. The OMBR-based hybrid system, for the first time, was utilized to direct recovery nutrient from municipal wastewater with over 90% of nutrient. In this study, nutrient and mineral salts were rejected via FO membrane and enriched within the bioreactor and then recovered by chemical precipitation [147]. The OMBR has several advantages, including higher rejection rate, lower energy consumption, and higher quality of treated wastewater compared to the traditional MBR. However, OMBR still has some disadvantages, such as salinity accumulation and membrane fouling. Based on the OMBR hybrid system, an integrated UF or MF membrane system in the OMBR system was investigated to remove the soluble inorganic salts in the reactor [148]. This process has a longer sludge

**Figure 10.** *Schematic diagram of an OsMFC (adapted from [140]).*

**Figure 11.** *Schematic diagram of an OMBR (adapted from [150]).*

residence time (SRT) than the traditional OMBR system, so a higher sludge concentration can be obtained. Similarly, MF membrane was added to the system for phosphate recovery from the raw sewage, in which MF and FO membranes function in parallel. The results show that the phosphate can be recycled up to 98%. The MF membrane retained phosphate and mineral salts in the bioreactor, so phosphate was precipitated as calcium phosphate precipitates without the input of Ca2+ ions [149]. In another study, the OMBR system was operated in treating of Chromium (Cr) and Lead (Pb) metals of the high strength wastewater. The findings revealed that industrial wastewater containing more than 5 mg/L of Cr and more than 2 mg/L of Pb is not recommended for the OMBR due to poor sludge characteristics, and high membrane fouling (**Figure 11**) [150].

#### **4. Conclusions**

The FO membrane process is a promising process for drinking water purification and wastewater treatment technology due to its excellent high rejection rate performance and relatively low membrane fouling characteristics. Hence it is likely to gain an very important place in the membrane technology.

The engineering of the FO process application is relatively scarce, due to the FO investigations and applications are still in the laboratory scale and progress in practical applications still requires further proof of the pilot. The research on membrane fouling mechanism is also needed, which still has a large gap in the current research results. Over the past decade, a large number of research papers has been published on membrane development (to increase water flux) and process design (i.e., to increase osmotic pressure, to change sludge retention time) and the number of papers in these issues has also increased year by year. The researchers' focus is to develop next-generation membranes by advanced membrane fabrication methods as well as hybrid systems where the FO process can really add value.

This chapter focuses mainly on forward osmosis either individually or in combination with other processes for wastewater treatment. For example; the FO removes the large molecular weight trace organic compounds while the combination of the MBR and NF/RO process for removing TrOCs from synthetic wastewater is feasible. The key concepts mentioned in the chapter provide better understanding for further promoting the utilization of FO process and its new applications for water resource recovery and wastewater treatment development.

*Forward Osmosis Membrane Technology in Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.97483*

#### **Author details**

Deniz Şahin Faculty of Science, Department of Chemistry, Gazi University, Ankara, Turkey

\*Address all correspondence to: dennoka1k@hotmail.com

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 4**

## Integration of Forward Osmosis in Municipal Wastewater Treatment Applications

*Stavroula Kappa and Simos Malamis*

#### **Abstract**

In recent years, the research community has made constant efforts to develop new technologies for the recovery and valorization of water, nutrient and energy content of municipal wastewater. However, the recovery process is significantly limited due to the low-strength of sewage. Over the last 10 years, the Forward Osmosis (FO) process, has gained interest as a low-cost process with low membrane fouling propensity, which can convert municipal wastewater into a concentrated low-volume effluent, characterized by high organic and nutrient concentration. This chapter presents the main configurations that have been implemented for the concentration of municipal wastewater using FO, including their performance in terms of contaminant removal and water/reverse salt flux (Jw/Js). Furthermore, the draw solutions and respective concentrations that have been used in FO for the treatment of sewage are reported, while at the same time the positive and negative characteristics of each application are evaluated. Finally, in the last section of this chapter, the spontaneous FO followed by anaerobic process is integrated in a municipal wastewater treatment plant (WWTP) and compared with a conventional one. The comparison is done, in terms of the mass balance of the chemical oxygen demand (COD) and in terms of the energy efficiency.

**Keywords:** forward osmosis, municipal wastewater, configurations, draw solution, COD mass balance

#### **1. Introduction**

Water scarcity is one of the most serious threats which our planet faces [1]. Globally, water demand is predicted to increase by 35% more than sustainable supply by 2040/50, if the linear water management model continues to be implemented [2]. The European Union (EU) encourages the implementation of a circular economy model, through its strategy called "Closing the loop—a EU action plan for the Circular Economy" in 2015 and European citizens must seize the opportunity to close the loop of water, resource and energy management [3]. Among various types of water, seawater and wastewater are two alternative sources, which are readily available, especially in coastal, arid areas [4]. Both need to be treated before they can be rendered suitable for use. Membrane processes such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse

osmosis (RO) are particularly effective in the purification of non-conventional water sources and count many applications in both the wastewater and the desalination sector [5]. In particular, the RO, holds a prominent position in water desalination, compared to traditional thermal desalination processes [6]. As high energy consumption is required to overcome the osmotic potential, reverse osmosis is not applied in many water-stressed areas [5].

Forward osmosis is one of the most attractive membrane-based processes that requires two solutions of different osmotic concentrations (high and low), separated by a semi-permeable membrane to be realized [7]. Water molecules are spontaneously diffused from the low osmotic potential solution (feed side) to the high osmotic solution (draw solution or DS), to equalize the concentration difference, while the semi-permeable membrane acts as a barrier that rejects the salts and contaminants [8]. The natural osmotic pressure of FO makes it stand out from conventional RO, by offering high water recovery, reduced membrane fouling potential, greater effectiveness, low cost, and reduced energy demand [8, 9]. All these positive aspects have led to a notably high trend of publications on FO applications in various water sources, such as seawater and wastewater, with more than 97.5% of publications since 2009 [10]. Among them, several researchers investigate the feasibility of integrating the FO process in a novel sewage treatment system based on the circular economy concept, as the main goal is to valorize the chemical energy, water, and nutrients of sewage. This innovative application of FO and its combination with appropriate downstream technologies is really promising. As the results show, the wastewater is converted into a small volume liquid, characterized by a high concentration of organic matter, as it can be concentrated up to 8–10 times, while the recovery of phosphorus can reach up to 90%, replacing the need for chemical fertilizers [11, 12]. However, there are many challenges that need to be overcome for this application, the most important of which is the selection of the most appropriate DS, which despite the significant efforts has not been found to date [13–15].

This chapter presents the main configurations that have been implemented to concentrate municipal wastewater using FO, including their performance in terms of contaminant removal and Jw/Js. The draw solutions and their concentrations that have been used in the FO process for the treatment of sewage are reviewed, while at the same time the positive and negative characteristics of each application are evaluated. Finally, in the last section of this chapter, the spontaneous FO followed by an anaerobic process is integrated into a municipal wastewater treatment plant and compared with a conventional activated sludge process (CAS), in terms of COD and corresponding energy efficiency, emphasizing the key impact of the FO in the latter process.

#### **2. Forward osmosis configurations and performance in municipal wastewater management**

The main benefit of the FO process in municipal wastewater treatment is that it converts sewage from low-strength liquid to a concentrated bulk, which consists of high a concentration of organic matter and nutrients [16, 17]. According to Korenak et al. [18], the FO process is characterized by high membrane fouling reversibility, while it can significantly minimize space requirements in a municipal WWTP. Considering all the above, three basic configurations have emerged for the integration of the FO process in the municipal WWTPs, which are illustrated in **Figure 1**.

*Integration of Forward Osmosis in Municipal Wastewater Treatment Applications DOI: http://dx.doi.org/10.5772/intechopen.95867*

**Figure 1.**

*Configurations for the integration of the FO process in the municipal WWTPs.*

#### **2.1 Osmotic membrane bioreactor (OMBR)**

In 2008, an innovative system was introduced, in which FO membranes were submerged into a typical membrane bioreactor (MBR) module; this system was called OMBR (**Figure 1(A)**) [19]. The replacement of UF or MF membranes in the conventional system by FO membranes resulted in better performance in terms of contaminants' rejection (79.7–100% of COD, **Table 1**). In addition, the absence of hydraulic pressure contributed to lower fouling tendency and probably lower energy requirements. Despite the benefits of OMBR over traditional systems, two major challenges are still under investigation; low Jw rate and salinity accumulation [19, 31]. The findings confirm that the decline in Jw was greatly affected by the salt accumulation, even with the implementation of improved membrane materials, such as thin-film composite (TFC), achieving an average rate equal to 3.9 0.5 L m<sup>2</sup> <sup>h</sup><sup>1</sup> [32]. In addition, the microbial community of the reactor can either be partly or fully inhibited, due to the gradual building-up of salts, which occurs due to the Js [31, 32].

#### **2.2 Anaerobic OMBR (An-OMBR)**

The combination of MBR technology with the anaerobic process has been extensively investigated in the last 10 years, due to the environmental benefits of both [33]. However, the low-strength nature of sewage is a major obstacle to the effective application of the anaerobic process in municipal WWTPs; containing a high amount of water with low organic and nutrients concentration. Due to the methane's solubility in water (22.7 mg L1, at room temperature), a large part of the produced gas escapes with the treated effluent of the anaerobic process (ranges between 20 and 60%) [34]. Due to the aforementioned barriers, it is difficult to implement anaerobic processes for municipal wastewater treatment particularly in areas, where the sewage temperature drops below 15°C, during the winter period. The incorporation of FO, either as a pre-treatment step or submerged into the MBR system, significantly enhances the resource recovery potential in the anaerobic


#### *Osmotically Driven Membrane Processes*

#### **Table 1.**

*Osmotic Membrane Bioreactor applications to treat municipal wastewater.*

#### *Integration of Forward Osmosis in Municipal Wastewater Treatment Applications DOI: http://dx.doi.org/10.5772/intechopen.95867*

process. Compared to OMBR, An-OMBR (**Figure 1(B)**) is characterized by significantly lower energy requirements, due to the replacement of energy-demanding aeration, while biogas production contributes to the coverage of specific energy needs. According to Zhang et al. [35], due to the 2–3 times smaller pore size of the FO membranes over conventional UF or MF membranes, the dissolved methane content in the An-OMBR treated effluent was eliminated, even as a function of different operating parameters. Regarding the yield of methane, Zhang et al. [35] and Gu et al. [36] observed particularly satisfactory production that reached 0.256 L CH4 g<sup>1</sup> COD and 0.25–0.3 L CH4 g<sup>1</sup> COD at mesophilic conditions, respectively. In addition, anaerobic biomass showed high resistance to increasing salt concentrations and was not affected even when the concentration was equivalent to 200 mM sodium chloride (NaCl) [36]. As shown in **Table 2** the FO membranes achieve high rejection of contaminants; specifically the Total Organic Carbon (TOC)/COD and PO4-P removal was 95% and 73%, respectively. However, due to the lack of ammonia removal, its accumulation has been observed in the reactor, but not in concentrations that can lead to the interruption of the anaerobic process [36]. In recent years, an alternative configuration has been proposed, which includes the addition of MF membranes both to OMBR and An-OMBR systems, the so-called Microfiltration- Osmotic Membrane Bioreactor (MF-OMBR). The main goal of this hybrid system is 1) to balance the salts concentration in the reactor so as to prevent an inhibition event and 2) to apply resource recovery methods to its nutrient-rich


**Table 2.**

*Anaerobic Osmotic Membrane Bioreactor applications to treat municipal wastewater.*

treated effluent. Nonetheless, according to Wang et al. [39], the FO membranes achieved much lower ammonia rejection rates (39–50%) compared to an An-OMBR system (62.7–81.2%), while the addition of another membrane significantly raises both the maintenance and the investment cost of the entire system [39, 43].

#### **2.3 Pre-concentration with FO**

Alternatively, the FO unit can be applied as a pre-condensation step in municipal WWTPs (**Figure 1(C)**), achieving a similar goal to the previously analyzed configuration, as it can be combined by suitable downstream processes for resources and energy utilization. As reported by Ansari et al. [34], the submerged FO configuration is significantly disadvantaged compared to the separate one, as the former gets in contact with the dense activated sludge, while the latter with the diluted primary treated effluent. In contrast, a recent study that examined both approaches in parallel, direct osmosis showed a significant decline in Jw performance compared to OMBR system [7]. On the other hand, a prolonged biodegradation study (approximately 7 months) of both cellulose triacetate (CTA) and TFC membranes demonstrated that the long-term exposure to activated sludge significantly affects their performance, in terms of water permeability and Js [44]. Sun et al. [7] found that the direct FO module is characterized by reversible membrane fouling over the submerged OMBR membrane, mainly due to the lower abundance in the microbial load of the feed solution. In terms of performance, as shown in **Table 3**, this FO configuration achieves the retention of organic load by a percentage ranging from 71.9 to 100%. At this point, it should be noted that based on the current literature most studies refer to FO as either a separate or integrated system of an Anaerobic Membrane Bioreactor (An-MBR), while alternative anaerobic treatment systems are not frequently investigated.

#### **3. Draw solutions**

In contrast to other osmotic, membrane-based technologies, the application of high osmotic potential is the driving force in the spontaneous FO process [52]. Therefore, the selection process of the most effective solution acts as a cornerstone of the FO and plays a crucial role on its performance as well as on downstream processes [15]. In an ideal physicochemical context, the parameters listed in **Table 4** must be met to classify a solution as appropriate [52–54].

In recent years, significant efforts have been made by researchers to combine the above parameters and develop an ideal DS, which will be compatible with the application of FO in the municipal wastewater treatment sector [15, 55, 56]. Alternative systems have been developed; different configurations have been applied to integrate the FO in several stages of a municipal WWTP; as pre-treatment, secondary and post-treatment steps for nutrient recovery. Obviously, the treatment level and the quality-target of the recovered product must be considered in the DS selection process [57]. First on the list and most commonly used as DS is NaCl, even in high concentrations up to 4 M, due to its high aqueous solubility, small molecular size, high availability, and relatively low cost [58]. As shown in **Table 5**, the 0.5 M concentration is most frequently applied, as it simulates the osmotic pressure of seawater [53]. The ultimate goal is to adopt a circular solution, by applying an abundant water source without any economic burden or a process' by-product, such as the RO brine as DS (**Table 6**) [58, 65]. High rejection rates of TOC/COD and PO4-P have been reported using NaCl as DS in OMBR systems, equal to 100% and 95.6%, respectively, although the same is not achieved for ammonium nitrogen


#### *Integration of Forward Osmosis in Municipal Wastewater Treatment Applications DOI: http://dx.doi.org/10.5772/intechopen.95867*

**Table 3.** *Pre-concentration*

 *of municipal wastewater*

 *using FO.*


#### **Table 4.**

*Main parameters that characterized the ideal DS.*

(NH4-N), which in most studies ranges between 43 and 90% [25, 59]. Nevertheless, the biggest challenge in OMBR systems using NaCl as DS is the accumulation of salts in the concentrated stream of mixed liquor and the subsequent negative effect on bacterial growth, due to reverse sodium leakage [12]. Relevant mitigation measures of the above obstacles have been proposed, such as the reduction of sludge retention time (SRT), but also the application of hybrid solutions, such as MF and UF membranes downstream for the parallel recovery of phosphorus [32].

Similar results are demonstrated in bench and pilot scale FO systems for the preconcentration of municipal wastewater using NaCl. The bidirectional diffusion of monovalent ammonium ions from the feed to the sodium cations of DS remains a major drawback [17]. In a recent study, Yang et al. [49] demonstrated the effect of the pH parameter on low NH4-N rejection rates and suggested a functional range of less than 8 for optimized performance. More specifically, at elevated pH as the main form of ammonium nitrogen is ammonia, diffusion becomes independent of the reverse sodium leakage [49]. Alternatively, the application of divalent molecular compounds as DS (**Tables 7** and **8**), such as magnesium chloride (MgCl2) and magnesium sulfate (MgSO4), which are characterized by lower reverse salt transport than NaCl, is suggested in many investigations [16]. Another superiority of inorganic solutions containing Mg ions is their combination with MF-OMBR hybrid systems and the utilization of the reverse Mg flux in the mixed liquor to nutrients' recovery, after proper pH adjustment. Although, a comparative study demonstrated that Mg transport leads to the formation of both organic and inorganic fouling in the active and support layer of the TFC membrane, correspondingly, causing a dramatic reduction in membrane flux [56]. As shown in **Table 8**, a highly charged compound, ethylenediamine tetraacetic acid disodium salt (EDTA 2Na) was applied as DS to remove the water from the activated sludge in a hybrid Forward Osmosis – Nanofiltration (FO-NF) system; the NF module was used for the recovery of DS. Water flux dropped rapidly after 8 operating hours (8.45 to 4.22 L m<sup>2</sup> h<sup>1</sup> ), mainly due to the reduction of the osmotic driving force and the formation of a cake layer on the membrane surface. It is worth noting that the reverse salt flux was equal to 0.2 g m<sup>2</sup> h<sup>1</sup> , while suspended solids were concentrated from 8 g L<sup>1</sup> to 32 g L<sup>1</sup> [75].


*Integration of Forward Osmosis in Municipal Wastewater Treatment Applications DOI: http://dx.doi.org/10.5772/intechopen.95867*

**Table 5.**

*Sodium chloride as DS in FO treating municipal wastewater.*


**Table 6.**

*Seawater, Brine, and industrial effluents as DS in FO treating municipal wastewater.*

To enhance the valorization of the resources contained in municipal wastewater, through the application of the anaerobic process several organic and ionic organic draw solutions have been investigated [13, 14, 74]. Bowden et al. [14] compared 10 different ionic organic compounds as DS and slightly altered the selection methodology proposed by Achilli et al. [15], introducing the parameter of biodegradability


*Integration of Forward Osmosis in Municipal Wastewater Treatment Applications DOI: http://dx.doi.org/10.5772/intechopen.95867*

#### **Table 7.**

*Magnesium chloride as DS in FO treating municipal wastewater.*

of the DS in the protocol. A bench-scale FO unit was used, while CTA membranes (Hydration Technology Innovations, HTI, USA) were applied to all experiments; the main purpose of this study was to evaluate the applicability of ionic organic solutions to OMBR systems.

Magnesium acetate (C4H6MgO4) and sodium propionate (C3H5NaO2) recorded the best performance as DS in terms of Js, potential recovery, and biodegradability. Siddique et al. [76] showed similar results with the application of synthetic wastewater, highlighting C4H6MgO4 as suitable DS for OMBR applications, while sodium acetate (C2H3NaO2) led to the development of dense membrane biofilm. Despite the many benefits of ionic organic solutions, it should be noted that their potential application is limited, as the re-concentration cost is high compared to inorganic solutions.

A recent study aimed to integrate all the parameters of **Table 4** with the compatibility of FO as a pre-treatment step preceding the anaerobic process [83].



*Integration of Forward Osmosis in Municipal Wastewater Treatment Applications DOI: http://dx.doi.org/10.5772/intechopen.95867*

#### **Table 8.**

*Fertilizers, organic, inorganic, and ionic organic compounds as DS in FO treating municipal wastewater.*

Among the 5 different zwitterions solutions tested, glycine (C2H5NO2), L-proline (C5H9NO2), and glycine betaine (C5H11NO2) exhibited comparable Jw to NaCl (4.3– 4.9 L m<sup>2</sup> h<sup>1</sup> ), with lower Js. From a physicochemical perspective, the process efficiency depends significantly on the pH value, affecting both the charge and the molecular size. Despite the rapid biodegradation (Adenosine triphosphate (ATP) levels range from 7 to 14 μg L<sup>1</sup> after degradation tests) of all zwitterions compounds, the replacement cost, which is 3–4 times more than the cost of commercially available solutions, is a potential barrier to their implementation in municipal wastewater streams. It is worth noting that the above experiments were performed with deionized water as feed, which favors the overall performance over the application of a more complex ionic matrix, such as sewage [83].

Commercial fertilizers are another largely inorganic solution medium that has been tested in various effluents resulting from a WWTP, such as typical secondary and MBR permeate and raw municipal wastewater. As illustrated in **Table 8**. Li et al. [82] compared the effect of 3 different commercial fertilizers on the downstream anaerobic process when applied as draw agents directly in raw wastewater. The following order of compatibility with the anaerobic treatment revealed Potassium Nitrate (KNO3) > Potassium Chloride (KCl) > Potassium dihydrogen Phosphate (KH2PO4), with their reverse solute flux showing a similar sequence when the concentration of all DS was equal to 1 M. Water flux can be dramatically reduced by applying KNO3 as DS, as extensive biofouling has been observed, while increasing nitrate concentrations can inhibit the subsequent anaerobic process, rendering them as unsuitable [80]. The implementation of different fertilizers in a hybrid FO-RO

system to concentrate MBR permeate proved that the amplification of enhanced NaCl with Diammonium Phosphate (DAP) ((NH4)2HPO4) can reduce reverse solute leakage by 35%, achieving ΝΗ4-Ν rejection rates more than 95% at different flow rates (1.2 and 2 L m<sup>2</sup> h<sup>1</sup> ) [66]. In addition, a long-term study examining the pilot application of a hybrid FO-NF system that treated MBR permeate found that Sodium Polyacrylate ((C3H3NaO2)n) was inappropriate for irrigation practices. On the contrary, the combination of MgCl2 with NF membranes significantly improved the process efficiency and operating costs, as the application of chemical cleaning was not required. However, a notably high loss of the osmotic agent was observed [78]. A particularly interesting investigation was carried out by Adnan et al. [51] in which the possibility of applying 9 different fertilizers to the direct FO for the wastewater valorization and its parallel application in agricultural practices was examined. Water recovery was high by applying KCl (Jw = 21.1 L m<sup>2</sup> h<sup>1</sup> ; Js = 11.2 g m<sup>2</sup> h<sup>1</sup> ; Osmotic Pressure (OP) = 44.6 bar) and Ammonium Chloride (NH4Cl) (Jw = 21.1 L m<sup>2</sup> h<sup>1</sup> ; Js = 7.5 g m<sup>2</sup> h<sup>1</sup> ; OP = 43.5 bar), while other fertilizers recorded particularly low reverse flux, such as Ammonium Sulfate (SOA) ((NH4)2SO4) (Jw = 15.5 L m<sup>2</sup> h<sup>1</sup> ; Js = 1.7 g m<sup>2</sup> h<sup>1</sup> ; OP = 46.7 bar), KH2PO4 (Jw = 13.2 L m<sup>2</sup> h<sup>1</sup> ; Js = 2.3 g m<sup>2</sup> h<sup>1</sup> ; OP = 36.5 bar), and NH4H2PO4 (Monoammonium Phosphate, MAP) (Jw = 13.8 L m<sup>2</sup> h<sup>1</sup> ; Js =1gm<sup>2</sup> h<sup>1</sup> ; OP = 44.4 bar). However, this process becomes inapplicable, as a large amount of water is required to dilute the concentrated fertilizer (at least 1/100), to reach the irrigation limits [51].

The analysis of the existing literature makes it clear that the FO process is still under investigation and the determination of the ideal DS plays a vital role in upgrading the process of this technology. Despite the properties of the DS, the selection of the suitable configuration, the techno-economic factors, and the recondensation method should be combined during the selection process; the optimization of the FO membrane's properties is a major challenge that can solve many issues. The development and fabrication of higher rejection membranes can be the answer to the implementation of both monovalent and divalent ions, which have been widely used as DS and their performance is already known to the research community.

#### **4. Integration of FO followed by anaerobic treatment in a WWTP**

#### **4.1 COD valorization in municipal WWTPs**

For more than a century, the CAS process has been applied as the main urban wastewater treatment system worldwide, making a significant contribution to environmental protection and public health. However, the low energy efficiency of the CAS process ranks WWTPs among the largest energy consumers in a country; on an annual basis, in developed counties, about 1–3% of electricity consumption is spent on their operation [84]. In addition, WWTPs are characterized by a high energy and carbon footprint, as during biological processes, large amounts of greenhouse gases are produced, mainly carbon dioxide generated due to the oxidation of organic matter and indirectly by electricity consumption [85]. Therefore, about 0.3–0.5 kWh m<sup>3</sup> of energy is required for sewage treatment by applying the CAS process, while the contained chemical energy and nutrients are not utilized [86].

According to Wan et al. [87] the traditional CAS process needs an average of 0.45 kWh to treat one m<sup>3</sup> of sewage, which equals to 1620 kJ m<sup>3</sup> . Assuming a concentration of 600 mg L<sup>1</sup> COD, energy consumption becomes 2.7 kJ g<sup>1</sup> COD.

#### *Integration of Forward Osmosis in Municipal Wastewater Treatment Applications DOI: http://dx.doi.org/10.5772/intechopen.95867*

As shown in **Figure 2(A)**, the energy recovery in convectional CAS systems occurs through the anaerobic digestion of the primary and secondary sludge, which corresponds to 32–39% of the organic material in COD terms. The latter percentage is equal to 2.9–3.5 kJ g<sup>1</sup> COD, since 1 g of methane-COD is equal to 13.9 kJ (65% methane percent in produced biogas). Considering that only 35% of the produced methane can be utilized for the production of electricity [86], about 1–1.2 kJ g<sup>1</sup> COD can be recovered from municipal wastewater, by applying anaerobic digestion to the sludge treatment line. Comparing the aforementioned energy requirement, 2.7 kJ g<sup>1</sup> COD, it is estimated that about 40% of it can be recovered using anaerobic digestion (1–1.2 kJ g<sup>1</sup> COD). The anaerobic digestion process also generates approximately 50–55% heat, part of which is used to heat the digesters. The excess heat can only be valorized locally [88].

#### **Figure 2.**

*(A) COD mass flow in a convectional WWTP, (B) COD mass flow, when FO followed by anaerobic treatment is integrated into a WWTP.*

Obviously, COD capture, and subsequently valorization of the chemical energy contained in municipal wastewater can lead WWTPs to sustainable development, transforming WWTPs from energy consumers to producers, while significantly reducing the environmental footprint and operating costs.

The integration of FO in municipal wastewater treatment and the benefits of its application have been investigated in various studies [17, 49]. This chapter presents the combination of FO and anaerobic treatment in a typical WWTP for the utilization of the chemical energy, which is inherently present in sewage. As shown in **Figure 2(B)**, by placing the FO in the main treatment line of a WWTP and taking into account the efficiency of a typical anaerobic system, such as An-MBR, which is equal to 80% in ambient conditions [89], 46–55% of COD is converted to biogas (65% of the aforementioned percent corresponds to methane). Following the same procedure as before, the energy recovery in the main treatment line through the implementation of anaerobic treatment is between 4.2–5 kJ g<sup>1</sup> COD. Another 1.3–1.6 kJ g<sup>1</sup> COD of energy is recovered from the anaerobic digestion of the sewage sludge (13.9 kJ g<sup>1</sup> methane-COD). Since only 35% of the produced methane can be converted into electricity [86], the power production from the wastewater treatment line ranges between 1.3–1.7 kJ g<sup>1</sup> COD, while from the sludge treatment line it is equal to 0.4–0.6 kJ g<sup>1</sup> COD. On aggregate, 1.9–2.3 kJ g<sup>1</sup> COD of electricity can be utilized from this innovative treatment scheme, which can counterbalance 80% of the existing energy consumption of a typical municipal WWTP. The treated effluent of the anaerobic system is rich in nutrients, which can be valorized by applying recovery technologies for the production of slow-release fertilizers, while the reclaimed water content can also be reused.

#### **4.2 Salinity, the greatest impact of FO on anaerobic treatment**

Despite the benefits of the wastewater management system presented in the above section, there are two factors that can be particularly limiting to the subsequent operation of the anaerobic process. The solute flux that characterized the FO system results in the accumulation of salts in the feed stream, potentially resulting in partial or complete inhibition of the downstream anaerobic and aerobic biological treatment processes [14, 17, 32]. Salinity has been identified as an inhibitory agent of the anaerobic process, as the increased osmotic pressure across the cell membrane can cause plasmolysis, leading to cell death and total inhibition of the anaerobic process. More specifically, Lefebvre et al. [90] stressed that the activity of methanogenic bacteria is inhibited at concentrations of NaCl equal to 5 g L<sup>1</sup> , while acidogenic microorganisms are affected at much higher concentrations, i.e. 20 g L<sup>1</sup> . Ansari et al. [91] studied the effects of NaCl on anaerobic treatment of concentrated wastewater effluents in batch mode experiments and observed that by increasing water recovery rates of FO (from 50 to 90%), the anaerobic process achieves higher methane production (approximately 5 times higher), while the presence of salinity has a negligible negative effect.

Based on the existing literature, the limiting parameter of salinity has been investigated and observed only in aerobic/anaerobic systems, where the FO unit is plugged into MBR systems for a relatively short time, while in pre-concentration systems few studies have examined the effect of salinity on the downstream anaerobic process and suggest mitigation measures. Chen et al. [37] and Wang et al. [39] did not observe significant effects of salinity on anaerobic reactors by recording an average methane yield of 0.2 and 0.3 L CH4 g<sup>1</sup> COD, respectively, in studies that cannot be characterized as long-term. As mentioned above, the application of

*Integration of Forward Osmosis in Municipal Wastewater Treatment Applications DOI: http://dx.doi.org/10.5772/intechopen.95867*

minimization strategies such as the corresponding regulation of the hydraulic residence time (HRT) seems to regulate the salinity conditions to which the biomass is exposed. Accordingly, the addition of MF membranes is a particular interesting approach for the minimization of salinity and the parallel application of nutrient recovery methods. Another interesting perspective is the acclimatization of the anaerobic biomass to high salinity conditions. This mitigation technique is not recent as the presence of specific microorganisms, such as halotolerant bacteria has shown particularly high efficiency in the anaerobic treatment of saline industrial wastewater [92]. In a recent study, where no acclimatized biomass was used, Gao et al. [93] separately investigated the effect of high salinity and ammonia nitrogen concentration and the combination of the two inhibitors in the anaerobic treatment of pre-concentrated municipal wastewater. The results showed that the presence of NH4-N and NaCl concentrations separately, up to 200 mg L<sup>1</sup> and between 5 and 8gL<sup>1</sup> , respectively, did not significantly affect the activity of anaerobic microorganisms. The combination of the two parameters in non-acclimatized and acclimatized biomass showed that the latter had significantly better performance and can respond without the risk of inhibition. Further research into anaerobic biomass acclimatization should be conducted in the future, as higher condensation rates could be applied from the upstream FO unit.

All the acquired knowledge of the above studies would be particularly interesting to be used in the long-term investigation of a FO system combined with a downstream anaerobic process, in which all the limiting parameters and the proposed mitigation measures can be examined in-depth, for the rational assessment of its performance.

#### **5. Conclusion**

There is no doubt that FO is a promising technology that has been investigated for a range of applications at various stages of a municipal WWTP. Among them, its combination with the anaerobic process has significant advantages, as much of the chemical energy inherently contained in sewage can be recovered as biogas, while resource recovery technologies can be applied downstream, utilizing the nutrientrich effluent. However, the transition of the FO from laboratory scale to full-scale applications requires further research to address important issues, such as the salinity accumulation in the downstream technologies and the reduced rejection of NH4-N by existing FO membranes. The application of NaCl indicates a possible suitability for the concentration of municipal wastewater. The background knowledge available on the basic criteria of FO has to be utilized for the development of membranes with higher selectivity. Future investigations should carry out extensive long-term monitoring and targeted combination/interaction of different parameters for the concentration of real wastewater, to assess from a technical, environmental and economic perspective the feasibility of applying FO technology to municipal wastewater management.

#### **Acknowledgements**

This research work was supported by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the "First Call for H.F.R.I. Research Projects to support Faculty members and Researchers and the procurement of high-cost research equipment grant" (Project Number: 4008).

### **Nomenclature**


*Integration of Forward Osmosis in Municipal Wastewater Treatment Applications DOI: http://dx.doi.org/10.5772/intechopen.95867*

### **Author details**

Stavroula Kappa\* and Simos Malamis Department of Water Resources and Environmental Engineering, School of Civil Engineering, National Technical University of Athens, Athens, Greece

\*Address all correspondence to: stavroula\_kappa@windowslive.com

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 5**

## Recent Advances in Measuring and Controlling Biofouling of Seawater Reverse Osmosis SWRO: A Review

*Rana H. Idais, Azzam A. Abuhabib and Sofiah Hamzah*

#### **Abstract**

This study presents recent literature that sheds light on the SWRO membrane biofouling, Inventory of causes, consequences, measurement, and possible solutions. In particular, biofouling of SWRO is considered as one of the challenges faced by seawater desalination today. For seawater desalination, mitigating membrane biofouling is essentially required and yet to be overcome. Specific shortcomings and prospective solutions are reviewed towards understanding the biofouling mechanism, pretreatment impacts, level of assimilable nutrients, and real-time monitoring. Accordingly, this review aims to address recent advances in biological fouling measurements and control to better understand biofouling and the best ways of dealing with such a challenging issue.

**Keywords:** biofouling, membrane, pretreatment, reverse osmosis, seawater

#### **1. Introduction**

Beyond doubts, Seawater desalination is commonly considered as a significant method towards producing and supplying potable water across the globe, especially in areas like the Middle East and North Africa (MENA) region characterized by a dry climate, low precipitation, and insufficiency of surface water. Despite the availability of various desalination technologies, membrane technology presented by Reverse Osmosis (RO) witnessed significant growth dominating about 60% of the desalination industry worldwide. The newly developed RO membranes characterized by high rejection and high flow membranes were allowed to operate at high pressures (up to 80–90 bar), thereby making conversions to 55–60% economically feasible. Such advancements towards simplifying RO processes from a two-stage treatment change to a single-stage array which in turn reduced capital and operational costs [1–3].

The high demand and global climate change have contributed to water scarcity in a significant way. As such, 71% of the world's population live under conditions of moderate to extreme water shortage, and about 66% (4.0 billion people) live in severe water deficiency. This well-felt scarcity could be a binding limitation on the socio-economic development of many countries according to Goal #6 [SDG6] of the Sustainable Development Goals cover all aspects of managing water for fair access, sustainability, and environmental protection. Having said so, seawater desalination is reliably seen compared to other sources, especially with the

long-term satisfaction tends to be achieved fully or partially for the demand in areas around the globe where water scarcity is felt, such as Australia, Southern Carolina, the Middle East, and Northern Africa [4].

Seawater desalination technology by RO is proven to be an extreme energyefficient compared to other conventional thermal distillation methods and therefore is economically feasible. Membrane technologies application in the desalination industry has witnessed some rapid development and growth over the past 20 years. However, SWRO membranes are highly sensitive to the feedwater characteristics and to the concentration of certain organic compounds that potentially lead to membrane fouling phenomena [1, 5–7].

#### **2. Desalination pretreatment**

Pretreatment is crucial to SWRO, as it influences membrane efficiency and life expectancy by fouling reduction. Practically, it is essential to enhance the raw water quality before passing through RO vessels to promote high and effective performance. Yet, Membrane fouling and scaling remain challenging even though the perfect design and operating conditions can be significantly helpful. Both source and quality of feed water influence the pretreatment choice towards better fouling control. Technically, the silt density index (SDI) and turbidity are the two main parameters determining pretreatment performance [8–10]. In addition, microbial foulants characterization can be found in [11]. Pretreatment techniques are designed to eliminate the microbial loads on high-pressure membranes but may scavenge nutrients and potentially provide a suitable environment for microbial growth. A comparison of the bacterial community composition can, therefore, answer whether pretreatment compartments serve as inoculum for high-pressure membranes. Physical and chemical water treatment processing feed water in desalination industry is referred to usually as pretreatment, as shown in **Figure 1**, usually proceed by a series of methods: coagulation and flocculation, followed by granular media filtration (e.g., anthracite coal, silica sand, or garnet) and cartridge filtration. Biocides such as chlorine and Peracetic acid, in addition to ozone or ultraviolet (UV), can be used when biofouling is a problem [2, 11].

Furthermore, membrane biofouling cannot be removed by conventional pretreatment methods such as coagulation, flocculation, ultrafiltration, and cartridge filters (CF), as they are unable to remove all passing microorganisms tending to colonize the membrane. Sand filtration combined with chemical disinfection is more efficient in reducing microbial contaminants, including viruses, to acceptable levels meeting drinking water standards. Technically, the pressure-driven process presented by membrane filtration can provide high-quality drinking water. However, it could be faced with vital challenges including system demanding, relatively high cost, clogging, scale formation, and biofouling. Moreover, membranes have a limited lifetime regardless of how good they are and so they may not reject all pollutants of concern for drinking water after a certain time of operation even if microorganisms are successfully removed. One consideration in large-scale applications is that membrane filtration systems produce considerable amounts of more concentrated wastewater per unit of purified water. Having said so, membrane selection must take into consideration the nature of the contaminants associated or extracted [10, 12, 13].

Microbial colonization at the membrane surface is traditionally overcome by overall applying disinfection to the feed water. Ideally, any disinfectant should not be expensive or hazardous. However, it must be highly toxic to microbes with zero

*Recent Advances in Measuring and Controlling Biofouling of Seawater Reverse Osmosis SWRO… DOI: http://dx.doi.org/10.5772/intechopen.95782*

#### **Figure 1.**

*Stages of an RO membrane system.*

effect on the desalination plant productivity. Traditionally, there are many disinfection processes applied in water treatment including but not limited to chlorination, ozonation, and UV radiation. Although ozonation is found to be an effective disinfection technique using oxidative effects in removing microorganisms, it is a bit costly and unstable in addition to producing carcinogenic bromates as by-products in the treated water occasionally. Chlorine on the other hand is the most commonly used disinfectant characterized by easiness use and low cost. During the chlorination process, the biomass on the RO membrane is effectively destroyed. Besides and due to molecular analysis, some bacterial groups appear to tolerate this biocide. Well-known bacterial classes potentially resisting chlorination, such as Bacillus and Clostridia due to their ability to sporulate, are very much identified on fouled membranes [2, 14].

For many reasons, biofouling is challengingly difficult to manage in RO systems. Some membranes like polyamide-based membranes tend to be sensitive to oxidizing agents such as chlorine leading to significant limitations for such use. Generally, Commercial plants are not observably in sterile environments. Therefore, any microorganism that enters the system can rapidly multiply. Surprisingly, it takes only 30 minutes for some bacteria to duplicate their population, showing exponential growth. The free chlorine presented during the chlorination process may potentially lead to membrane damage and salt rejection deterioration. Another downside of applying chlorine as a disinfectant is its capacity of breaking down the organic and humic material to Assimilable organic carbon (AOC), resulting in the rapid growth of biofilm which in-turn leads to accelerated incremental of feed channel pressure drop. In some treatment plants, Mono-chloramine is usually applied to achieve biofouling control. Nevertheless, mono-chloramine can be used to produce N-Nitrosodimethylamine (NDMA), which is a human carcinogenic material that can result in public health issues. Furthermore, contaminated water with mono-chloramine may potentially result in the damage of membrane in the iron and manganese presentation [8, 12, 15].

Surprisingly, various bacterial types and groups were found to be succeeding and thriving when membranes are cleaned intermittently with various cleaning agents. One thing to think of is the inclusion of citric acid leading to several community compositions compared to when chlorine was used alone. *Acinetobacter, Ralstonia, Comamonadaceae,* and *Diaphorobacter, Stenotrophomonas*, and *Enterobacteriaceae* are dominantly shown on cleaned membranes via chlorination. When chlorination combined with citric acid cleaning Silicibacter*, Rhodobacteraceae, Pseudomonas, Pedobacter*, and *Janthinobacterium,* they became abundant. This is confirmed based on physiological features assigned to taxonomically related bacteria and Adenosine Triphosphate (ATP) concentrations. It is, therefore, suggested that spore-formers, Gram-positive bacteria, and Acidophiles are better resisted citric acid treatment. These suggestions should be taken into consideration with caution because simply, there is no evidence provided that bacteria are recalcitrant against citric acid [16].

#### **3. SWRO membrane fouling**

Membrane fouling is practically seen as a chronic drawback hindering the development and operation of SWRO desalination processes. Fouling results in overall membrane performance deterioration with operational pressure drop and more frequent cleaning leading to operational costs increase and eventually full loss of membrane. From hydrodynamics perspectives, fouling development mainly in space-filled channels of the membrane is influenced by water quality, operational conditions, and spacer and membrane design. Technically, membrane fouling issues vary from organic and inorganic fouling to colloidal and biofouling contributing to increase cost of operation as well as affecting the quality of water produced. Amongst, biofouling seems to be way too complicated and hard to be controlled due to the excessively increase of biofilm formation on the surface of membrane surface, consequently leading to deteriorated performance. Additionally, the capability of lived bacteria inside biofilms in terms of high tolerance to antibiotics and other antimicrobials than planktonic cultures is problematic. As such, various techniques including pretreatment, membrane surface alteration or modification, disinfecting of feed water via chlorination, and cleaning are developed to overcome and/or control biofouling simply by treating biofilm formation on membrane surfaces [2, 17, 18].

*Recent Advances in Measuring and Controlling Biofouling of Seawater Reverse Osmosis SWRO… DOI: http://dx.doi.org/10.5772/intechopen.95782*

### **4. Biofouling**

#### **4.1 Definition**

Presently, several foulants considered or categorized as microbial ones including various microorganisms and organic compounds, known also to be aquatic, such as polysaccharides, proteins, and lipids, called extracellular polymeric substances (EPS). Identically, the biofouling process involves in adhesion of organisms that are aquatic along with their metabolic products presented on membrane surface or feed spacers. As shown in **Figure 2**, strong biofilm growth can be observed and found on the feed spacer strands. More than 45% of all membrane fouling is biofouling originated mainly by unicellular or multicellular microorganisms and therefore seen as one of the major issues of concern to reverse osmosis membrane filtration processes. Although membrane biofilm majority is formed by bacteria, other organisms such as fungi, algae, and protozoa may potentially be attracted by the membrane surface and add up to the formation of biofilm in a significant manner. Various studies confirm that *Pseudomonas, Bacillus, Arthrobacter, Corynebacterium, Flavobacterium,* and *Aeromonas* are the most predominant bacteria identified in fouled RO membranes [9, 11, 12, 19–22].

Membrane biofouling takes place gradually in sequential steps, as shown in **Figure 3**. Firstly, the microbial cells get to membrane surface attachment, causing the forming of the biofilm as layer, involving communities of different microorganisms' types (e.g., bacteria, algae, protozoa, and fungi). Initially and acting as mediator for the attachment of microbial substances, electrokinetic and hydrophobic interaction, the growth and multiplication of cell usually follows at the expenses of nutrients being soluble in water feed or membrane surface adsorbing organics. The roughness and charge of the membrane surface are considered as key factors contributing to the enhancement of the microorganisms attached to the membrane surface [9].

Several environmental factors raising bacterial growth such as nutrients amount and types which strongly affect the microbial composition and biofilms density. Also, membrane characteristics such as type, roughness, charge, and hydrophobic/ hydrophilic characters very much influence the biofouling microbial film establishment. Producing RO membranes highly resistant to biofouling as well as other fouling types remains challenging. Typically and from operational point of view, biofouling poses itself as a challenge, especially for saline waters having natural organics at high levels. Seasonally, biofouling tends to be problematic during extreme algal blooms or in time of having accident entrance to the open intake of the plant in rainy season with highly organic water [2, 11].

**Figure 2.** *Biofouling in RO sample (left: top view, right: cross-section) [12].*

**Figure 3.** *Steps of the biofilm [20].*

Commonly, biofouling attributes to the increased probability of bacteria producing polysaccharides and natural adhesives. It occurs at all open-ocean desalination plants such as the Jeddah SWRO desalination station in KSA. Mature biofilms exhibit anti-bactericidal properties and are also resistant to detachment. Biofilm formation results in biofouling when exacerbated in desalination systems by water production efficiency deterioration of membrane degradation, leading to a significant increase in operational costs associated with cleaning regiment and shortened membrane lifespan [23, 24].

#### **4.2 Factors affecting biofouling**

Generally, the saline feed water biofouling potential is influenced by several interrelated factors including microorganisms' concentration; content of readily biodegradable compounds; nutrients concentration and composition in the source water; temperature; the salinity of the feed water as well as operating parameters such as cross-flow velocity [11, 25, 26].

The study of [25] elucidated Algal organic matter (AOM) impact on biofouling affecting various membranes modules (capillary and spiral wound) by algal blooms. They found that measuring Adhesion force illustrate that AOM has the propensities towards adhering to a membrane surface and would need massive force to be removed from the membrane. Also, the seawater capacity supporting bacterial growth illustrated a correlated positive linear with AOM concentration levels in the water. It was linked to the tending of AOM, especially, transparent exopolymer particles (TEP), to nutrients concentration absorption from the feed water feeding attached bacteria. Also, fastened experiments of biofouling made with spiral wound and capillary membranes evidently show that when biodegradable nutrients presented in the feed water unlimitedly, a high level of AOM concentration in water feeds or as membrane attachment may significantly speedup biofouling. Further observation is that lower biofouling rates occurred once membranes are exposed to feed spikes with AOM or nutrients [25].

#### **4.3 Microbial communities in RO systems**

The bacteria can tolerate a wide range of pH (0.5–13) and temperature (−12–110 °C) while being able to colonize on all membrane surfaces in RO plants

#### *Recent Advances in Measuring and Controlling Biofouling of Seawater Reverse Osmosis SWRO… DOI: http://dx.doi.org/10.5772/intechopen.95782*

under different conditions. Various studies were carried out to investigate frequently observed microorganisms on membranes in RO plants. As concluded by [28] some bacterial groups are presented with some potential finger print significantly related to biofouling. Their study mostly opened some future window towards focusing on having already-cleaned membranes treated prior to installation. Also, paying more attention to primarily target troubling colonizers, or developing pre-treatment designs considering biofouling measures through the bacterial load minimization attempting to access membrane unit feed. While, a pilot-scale study of [29] was implemented to compare bacterial populations (membrane biofilm) in seawater, CF, and from Carlsbad plant at California, USA.

Observably, population of biofilm for seawater and membrane tend to have some similarity, but the CF harbored other biofilm community type. It was a relatively firm study because it concluded the findings of different communities of biofilm in five fouled SWRO membranes than those of other found around the globe. Apparently, such unique occurrence was due to differences observed in operational conditions and sampling across the year. Various mutually and dominantly existence of bacterial group could be observed in all samples. As such, strong suggestion was made about certain group being conformed to the membrane surfaces growing under chemolithoheterotrophic conditions oligotrophically [27–29].

Similarly, [30] results found that members of the *Ruegeria, Pseudoruegeria, Parvularcula, Legionella,* and *Shigella* were the only bacterial groups shared between the CF and RO membranes. *Phaeobacter*, *Leisingera, Kangiella,* and *Bacillales* are abundant in the CF, while *Haliangium* and *Limnobacter* are abundant on the RO membrane. The presence of bacteria belonging to taxa harboring facultative and obligate chemolithotrophs, such as *Geobacter, Desulfuromusa*, and *Thioalkalivibrio,* on the CF potentially indicate the effective removal made by the pre-treatment compartments for certain nutritional compounds, such as ferrous iron or sulfurConsequently, published studies do not uniformly present the composition of the bacterial community at the same taxonomic level, hardening the comparison of bacterial diversity. For instance, a review of [14] compared 33 studies investigating bacterial communities on fouled high-pressure membranes. They classified the identified bacteria at the order level. A total of 35 bacterial orders from those fouled highpressure membranes have been recorded. These orders were used as a benchmark to compare the microbial diversity of feed water, pre-treatment compartments, and fouled membranes, and to detect the role of specific selection pressures on the microbial composition [14, 30].

A review of [14] found that the most commonly detected bacteria on fouled membranes are *Burkholderiales, Pseudomonadales, Rhizobiales*, and *Sphingomonadales,* and *Xanthomonadales.* Whereas *Burkholderiales* and *Xanthomonadales* have not been identified in earlier studies, but studies of nextgeneration sequencing (NGS) have frequently identified these orders of bacteria on fouled membranes. Due to its ability to study bacterial community compositions in a culture-independent and high-performance way [31]. In [32] they compared the bacterial diversity of the surface water and the membrane population. They concluded that the biofilm actively produced on the membrane surface, rather than being a concentration effect of bacteria. In general, the composition of the bacterial population on the membrane varies from the feed water because only a fraction of the bacterial feed water diversity accumulates at the membrane surface, indicating that the membrane surface provides bacterial selection pressures. However, [33] found that the bacterial composition of a mature fouling layer was similar to the feed water composition [14, 31–33].

In the experiment of [34], a lab-bench cross-flow RO system was used to explore the impact of chlorine disinfection on reverse osmosis membrane biofouling.

No significant distinctively chlorine-resistant bacteria were detected in the sample without chlorine dosage and with 1 mg-Cl2/L chlorine dosage. However, in the samples with 5 and 15 mg-Cl2/L chlorine, kinds of significantly distinctive chlorine-resistant bacteria were presented included *Methylobacterium, Pseudomonas, Sphingomonas*, and *Acinetobacter.* These results indicated the significant selection effect of chlorine on the chlorine-resistant bacteria. Results of [35] found *Proteobacteria*, *Bacteroidetes*, *Firmicutes,* and *Planctomycetes* are the most abundant phyla with the application of high throughput sequencing. Microbial community succession was revealed during biofilm formation, in which *Proteobacteria*, *Planctomycetes,* and *Bacteroidetes* played significant roles [34, 35].

The research of [28] analyzed the biochemical properties by selecting a good-model bacteria include *Paracoccus, Burkholderia, Pseudomonas, Acinetobacter, Pseudoalteromonas, Cytophaga, Microbacterium, Bacillus, Marinomonas, Rhodococcus, Exiguobacterium,* and *Staphylococcus* which may influence its ability in terms of forming insurgent biofilms cumulatively at membrane surfaces. In this study, bacteria was isolated across stages of all plant. Predominant organisms were detected and seem to be significantly involved in biofouling as well as including almost all isolated cultures by culturing and next-generation sequencing (NGS) through applying 16S rRNA meta-barcoding. Researchers have also found that as biofilm community influenced by bacterial community on seawater reverse osmosis membranes, it is compulsory to have customized/designed controlling measures targeting the invading microbial elements related to the plant's geographical spot [28].

#### **4.4 Biofouling potential indicators and measurements**

A biofilm has a high content of water and organic matter (70–95%), high numbers of colony-forming units and cells, high contents of carbohydrates and proteins, high content of ATP, and low content of inorganic matter. Indicating biofouling potential can be proposed by multiple parameters as ATP, AOC, and biodegradable dissolved organic carbon (BDOC). Generally, the previously mentioned parameters are generally applicable for fresh waters and yet to be extended to be applied for desalination plants [25, 36]. Meanwhile, the study of [37] suggested some testing sets to allow for the determination of the water samples capacity of microbial support. In addition to using fluorescence intensity microplate analysis to determine biofouling potential on RO membranes [35, 37, 38].

Measurement of RO feed water biofouling tendency is not an easy task. To do so, several in-practice parameters are indicatively considered like: Silt density index SDI, turbidity, and total suspended solids (TSS). Having said so, biologically-based data is yet to be obtainable supporting such measurements. The RO feed water microbial support capacity (MSC) is practically determined by factors associated with the algal activity, such as TOC, the ratio of TOC:TN: TP, the increase in RO train DP, Chlorophyll a, TEP, bacterial activity (e.g., ATP), total bacterial count, microscopic observation, and nutrients concentration (Total N, Total P). Biologicalbased factors such as AOC and BDOC are used in waters with no salinity. Also, many consistent monitoring systems like monitoring of biofilm and the MFS had been developed to determine formation rate of biofilm. These monitoring systems cannot predict the feed water potentiality for biofouling but simulate overall plant operation [11, 19].

The concentration of TOC is widely applied to indicate the potentiality of saline water to biofouling whereas the rate of DP increase is indicatively used for the rate of biofouling. From operational point of view, potentiality to biofouling tends to be significantly increased when TOC concentration raises to 2 mg L−1.

*Recent Advances in Measuring and Controlling Biofouling of Seawater Reverse Osmosis SWRO… DOI: http://dx.doi.org/10.5772/intechopen.95782*

Practically, the weekly measurements made for ratio of TOC:TN: TP to indicate biofouling increasing. Consequently, ratios above 20% of 1:1:1 indicates an elevation requires bacterial EPS generation leading to have the bacteria encouraged to cause membrane fouling [11, 39].

The concentration of Chlorophyll *a* for the feed water can be indicatively seen as a sign related to the green pigmentation algae content in the water [11]. The total count of algae is potentially determined via online methods or through lab experiments. Technically, there are three KPIs (key-performance-indicators) determining the algal high content impact in feed water including: efficiency of solid removals deterioration at pre-treatment stage due to filtration overload, fouling acceleration of CF, and finally RO train productivity deterioration [11].

#### **4.5 Membrane biofouling impact**

In SWRO systems, biofouling has many adverse effects, as increases in differential driving pressure and feed channel pressure drop. These are required to maintain the same production rate due to biofilm resistance. In addition to increased energy consumption associated with high pressure to achieve the biofilm resistance and flux decline. Biofouling eventually leads to the biodegradation of cellulose acetate membranes caused by acidic by-products concentrated at the membrane surface. Also, it leads to reducing the active membrane area, and therefore decreased flux of permeate due to the formation of a low-permeability biofilm on the membrane surface. Other main consequences of biofouling decreased membrane permeability, increased the frequency of chemical cleaning, and the possible increase in replacement frequency of membrane [9, 19, 24, 40].

Research conducted by [41] investigated the biofouling effect on the sequentially declining in reverse osmosis membranes in terms of membrane operational parameters like membrane permeability, pressure drop in feed, salt rejection. Also, the consumption of temporal organic carbon (DOC) is being measured. It could be illustrated that all indicators were influenced by biofouling formation. Observed increase in the pressure drop in the feed channel (FCP) affected permeability and decline salt rejection, consequently leading to prove the FCP sensitivity to biofouling. Besides, [35] found that biofouling can accelerate the formation of scaling, and the mixed foulants can block the membrane pores, leading to a significant flux drop [35, 41]. In brief, biofouling has a potential effect on the following: differential driving pressure, feed channel pressure, energy consumption, the flux of permeate, membrane area, membrane permeability, the frequency of chemical cleaning, and salt rejection.

#### **4.6 Biofouling alleviation and control**

The control of biofilm formation is a complicated and controversial process involving the reduction of microorganisms within the RO water, monitoring strategies, and controlling factors such as nutrient concentrations and physicochemical interactions between microorganisms and membrane surface. Gulf Sea at the Saudi Arabia is known to be having biofouling major challenge uneasy to be controlled. It still the main challenge in membrane filtration installations. Curative or preventive measures are not always efficient. Flocculants provide a suitable habitat for microbial growth, whereas conditioning agents are potential sources of microorganisms and nutrients for the biofilm. Another source of microbial contamination is the piping, storage tanks, and treatment systems before RO, such as ion exchangers and active carbon filters. Biofilm can grow in very low-nutrient habitats with TOC levels as low as 5–100 μg/L. In practice, several methods for biofouling control have been

investigated, such as the application of the pretreatment before SWRO to remove bacteria and biodegradable organic matter, dosing of biocides, and limiting essential nutrients such as carbon and phosphate [9, 40, 42–44].

Membrane cleaning as a method of biofouling control typically done when there a significant decrease in differential pressure drop or permeability. Principally, cleaning process involves removing and/or destroying of the biomass accumulating on membrane surface to reserve membrane permeability. Cleaning process can be applied physically or chemically. Physical cleaning was usually performed before chemical cleaning, involving flushing of air and water. It requires applying pressure mechanically, attributing to the removal of all non-adhesive fouling-based. Membrane manufacturers suggest different chemical agents' forms for cleaning purposes (e.g. alkaline, acids, biocides, enzymes, and detergents). Such process is efficiently eliminating or deactivating non-accumulating microorganisms. Therefore, the residual inactive biomass can be consumed as food by survived bacteria leading to bacteria regrowth acceleration. Base/acid cleaning removes organic foulants on membranes and destroys the microbial cell walls. Metal chelating agents and surfactants were used to disintegrate EPS layers by removal of divalent cations and solubilization of macromolecules, respectively. The efficiency of cleaning agents to remove biofouling is limited because the EPS layer is recalcitrant against cleaning agents. Improvement of cleaning efficiency difficult, particularly for aged biofilms. Membrane cleaning frequently removes only part of the fouling layer and cleaned membranes, therefore, provide a suitable environment for swift microbial colonization. Thus, cleaning processes (physically and chemically based) may partially result in biofouling reduction on the short run without sustainably controlling biofouling on the long run [8, 45–47].

Control of bacterial growth by chemical disinfectants depends on many factors, such as chemical concentration, its mode of action, contact time, the density of organisms, and TSS of feed water. These factors make it extremely difficult to attain absolute disinfection. Besides, chemical disinfectants like chlorine and its derivatives may be hazardous to health. Chlorine is known to oxidize and degrade the humic substances in seawater, thus, resulting in smaller molecules, which are AOC. The AOC is a good nutrient source for marine bacteria, and under such status could also lead to rapid biofilm formation in SWRO plants. Chlorination may foster the formation of trihalomethanes and other chlorinated by-products, which are carcinogenic [48].

Many researchers have concluded that biofouling is inevitable and tend to be difficult to prevent with having the focus shifted towards control strategies aiming to achieve: biofilm formation delay, biofilm accumulation impact reduction or delay on performance, and finally removing biofilm via advanced strategies of cleaning. For many reasons, biofouling control tends to be challenging. As such, various methods were developed towards treating biofilm formation on membrane surface and/or mitigating biofouling effect in general. Instantly, some strategies were applied including: membrane flushing or cleaning, application of chemical additives to target bacterial cell or extra-cellular matrices, membrane surface modification, limiting nutrient content, and the quenching of quorum. All previously mentioned methods have limitations and may result in unwanted membrane degradation [14, 18, 21, 49, 50]. As part of chemical treatments with biocides in addition to anti-microbes were applied mutually as part of industry practices. Chemicallybased cleaning are known to be affecting exclusively the topper biofilm layers by colonizers. The effect of nutrient levels and possible manners to control membrane biofouling poses another potential solution for many membrane installations and should be further investigated. Biofouling impact on membrane efficiency is potentially minimized through a combination of strategies involving early identification,

#### *Recent Advances in Measuring and Controlling Biofouling of Seawater Reverse Osmosis SWRO… DOI: http://dx.doi.org/10.5772/intechopen.95782*

preventive cleaning, substrate limitation for delaying biofouling built-up, and cleaning procedures optimization towards effective biofilm removal [14, 41, 50].

Based on the current knowledge, membrane surface modifications tend to be incompatible for control biofilm formation in full-scale membrane operations because of the drag force that transfers bacteria and nutrients to the membrane surface. As various components are moved to the membrane surface by the drag force, they are easily covered, and membrane surface modifications are rendered less efficient. By applying comprehensive pretreatment, therefore, biofouling can be limited but not eliminated. Practically, membrane biofouling prevention tends to be fully or partially achievable by better pretreatment in new desalination systems. Yet, it might be essential to have old, insurgent biofilms and prolonged membrane operating plants dispersed sufficiently. Most existing techniques in efficiently use an enormous spectrum of biocides and chemicals attacking bacteria to maturely disperse biofilms [14, 26, 28].

Practices presented as clean-in-place (CIP) tend to be less efficient and that successful. This is related probably to various reasons including: wrong selection of chemical, improper pH control, temperature, time of contact, unsuitable recirculating flow rates, and partial biomass removing. The repetitive biocides usage potentially lead to bacterial resistance inducing via bacterial cell modification on membrane surface, permeability deterioration of biocide, and biocides degradation by enzymes development, or gaining more resistance for biocide genes [28].

Strategies for Biofilm control applying enzymes towards degrading of EPS matrix including glycosidases, proteases, and deoxyribonucleases. However, these enzymes targeting specific strains, and their sufficiency in complex multi-species biofilms is yet to be established. Also, enzymes are costly and may not be so practical when applied for membrane treatment or flushing. On the other hand, a bad need for more efficient and cost-effective methods to eliminate biofilms and alleviate biofouling in SWRO processes do exist. As such, it is highly recommended to conduct researches investigating novel chemical cleaning agents which may positively contribute to overcome or mitigate biofouling [26, 28].

A study of [18] investigated the *Pseudomonas* quinolone signal (PQS) pathway role in biofouling control in reverse osmosis membranes. They inoculated *Pseudomonas aeruginosa* inside water feed as a sort of biofouling simulation. Conversely, a novel-based method on quorum sensing (QS) biochemically, has triggered considerable interest in controlling biofouling. Several advantages could be concluded, as it is characterized by high efficiency low operational pressure, practically contribute to hindering development capacity of the bacteria. QS Identified as cell-to-cell signals whereby microorganisms applied it for the sake of cell density sensing; reaching to a critical threshold level in terms of signals will trigger responsive sets of genes. Many researchers have found that the interference with such cell density-dependent communication technique formulate a biofilm potential controlling strategy.

The application of bacteriophage in synergetic way combined with some other traditional methods, such as cleaning proven to be mitigating *P. aeruginosa* biofouling-based sufficiently*.* Some alternative options are presented by bacteriophages. *Pseudoalteromonas*, for example, presented in high amount on a marine-based biofilm layer is potentially isolatable and known to be having some lytic footprint, highly efficient. Lastly, it might be of interest to explore the bacteriophage treatment effectiveness in biofilm formation prevention instead of having the structure of biofilm removed. To this regard, bacteriophage activation may be limited by low cell density. Another bacterial hosts might be targeted by taxonomical families performing a more sufficient approach towards maximizing infection impact on the way to achieve biofouling mitigation [51].

In [51] research, the isolation of lytic bacteriophages was used to hinder *P. aeruginosa* growth in planktonic-based mode and varied pH, salinity level, and temperature. Accordingly, bacteriophages were found to be optimally infective with 10-times infection multiplication under salinity mode. It illustrated that the lytic has reasonable abilities over experimental testing temperatures (25, 30, 37, and 45 °C) and pH range of 6–9. When exposed to bacteriophages, Planktonic *P. aeruginosa* found to be significantly exhibiting a longer lag mode and low rates of specific growth, taking into account the application of bacteriophages to P. made in subsequent manner*.* The biofilm presented by *aeruginosa*-enriched was determinant to lowering the relative amount of Pseudomonas-related taxa from 0.17 to 5.58% in controlling to 0.01–0.61% in processed communities of microbes. The findings illustrated the potential application of bacteriophages as a biocidal agent to achieve the mitigation of unwanted *P. aeruginosa* associated with issues in seawater-based applications [51].

In [26] study, biofilm amount and characterization were analyzed concerning membrane performance applying acid/base cleaning. Generally, cell and tissue of the bacteria deactivate chemical agents used in cleaning process to remove mainly the biomass related to biofouling. Chemical-based reactions like dispersing, chelating, solubilization, suspension, peptisation, sequestration, and hydrolyzing are observed during cleaning process. Cleaning by Alkaline-based solutions like Sodium hydroxide was also applied in this study for three types of biofilm to explore biofilm removal efficiency as well as illustrating EPS matrix role. They concluded that with minimum biomass amount at low substrate concentration cleaning was not as efficient as with high substrate concentration, with same observed phenomena for membrane performance restore [26].

While [43] describes the biofouling monitoring technology of the "Megaton Water System" project and verifies the technology in the pilot and real plants in Al Jubail, Saudi Arabia. Biofouling monitoring technology refers to the community of bacteria composition change by chemical usage of the Membrane Biofilm Formation Rate (MBFR) was applied to this project was a positive indication of a reliably system design and operation. Such monitoring technology could be applied to achieve plant operational and reliability improvement throughout the overcome of biofouling issue. It could also assist in environmental impact reduction and lower plant production costs through chemical-free injection [43].

According to [52] study, they developed a simple method where a combination of bubbling and cleaning-based on frequent addition of hydrogen peroxide (H2O2) at lower concentration level at feed water. The same approach was also explored with the use of CuO or PP spacers. The dosage of 0.3% (w/w) H2O2 being applied periodically at 12 h intervals resulting in having no increase in FCP in the tested system, also an indication referring to the tangible biofouling lacking with intermittent H2O2 dosing. For tested fouled membranes fouled over a period of eleven days, a single dose of 0.3% (w/w) H2O2 applied and successfully eliminated almost all spacers and membranes accumulated biofilm in few minutes demonstratively by a FCP of 69% (CuO spacer) and 54% (PP spacer). The biofouling reduction was primarily due to the high shear created by the generated oxygen bubbles in the system, combined with the disinfection effect of H2O2. The reasonably low cost of \$0.009/m3 from intermittent H2O2 dosage was not more than 0.8% of overall assumed cost and 6.5% out of pre-treatment cost, allowing for economical accepted approach to overcome biofouling [52].

It seems that dechlorination water, activated carbon, cartridge filtration, UV irradiation, ozone treatment, hydrogen peroxide, detergents, alkaline, sodium bisulfite, and hot water sanitization are effective techniques and limitations to control biofouling.

*Recent Advances in Measuring and Controlling Biofouling of Seawater Reverse Osmosis SWRO… DOI: http://dx.doi.org/10.5772/intechopen.95782*

#### **5. Conclusion**

Biofouling in SWRO membranes continues to be problematic for operation and maintenance quality. It plays an essential role in the fouling of the membrane parts in full-scale and pilot-scale plants, and it's significant to reduce its occurrence by prediction and prevention. The study demonstrates the RO membrane biofouling mechanisms and the effective fouling control strategies within seawater desalination, where biofouling is a critical drawback. The study aims to evaluate microbial fouling (biofouling) to understand its effect on RO membrane performance. The study highlighted the composition of the microbial community and the functional potential of the RO membrane biofilm. In general, biofouling has affected all performance indicators. The selection of pretreatment seems to be a factor affecting the microbial community composition and functional potential. Analysis of the biofilm bacterial community has shown that seasonal changes in water quality influenced the biofouling bacteria.

The results showed that the accumulation of biofilms on membrane surfaces remains the key obstacle for high-pressure membrane filtration. For future research, it is significant to describe the cleaning agent and cleaning frequency. Also, measuring feedwater temperature, determine the location of the membrane element, and the sampling location at the membrane. These comprehensive researches will use to establish an integrated strategy to control biofouling. Biofouling control should concentrate on improving low fouling feed spacers, and the hydrodynamic conditions reduce the effect of biomass accumulation.

We conclude that to maintain plant productivity and membrane recovery it is necessary to increase the membrane cleaning frequency. In the CF and RO membrane, the microbial regrowth rate is a significant factor that impacts the biofouling rate. We recommend further searches of the strategy of balancing the nutrient levels as a solution for several membrane installations to control membrane biofouling. To measure biofouling, it needs for real tool, sensitive pressure drop data, and systematic methodology. Therefore further studies related to avoiding adverse biofouling processes will be valuable to investigate specific microbial members in more detail using biofilm monitoring and control strategies. Finally, additional SWRO research and development are critical for the efficiency of this growing industry.

#### **Abbreviations**



### **Author details**

Rana H. Idais1 , Azzam A. Abuhabib1 \* and Sofiah Hamzah<sup>2</sup>

1 Water Technology PhD Joint Programme between Islamic University of Gaza (IUG) and Al-Azhar University of Gaza (AUG), Gaza, Palestine

2 Faculty of Ocean Engineering, Technology and Informatics, Universiti Malaysia Terengganu, Terengganu, Malaysia

\*Address all correspondence to: azz200@hotmail.com

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Recent Advances in Measuring and Controlling Biofouling of Seawater Reverse Osmosis SWRO… DOI: http://dx.doi.org/10.5772/intechopen.95782*

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*Edited by Muharrem Ince and Olcay Kaplan Ince*

*Osmotically Driven Membrane Processes* provides an overview of membrane systems and separation processes, recent trends in membranes and membrane processes, and advancements in osmotically driven membrane systems. It focuses on recent advances in monitoring and controlling wastewater using membrane technologies. It explains and clarifies important research studies as well as discusses advancements in the field of organic-inorganic pollution.

Published in London, UK © 2022 IntechOpen © barbol88 / iStock

Osmotically Driven Membrane Processes

Osmotically Driven

Membrane Processes

*Edited by Muharrem Ince* 

*and Olcay Kaplan Ince*