*3.1.1.4 Reverse and forward osmosis*

Reverse osmosis (RO) is often referred to as a tight membrane and has been widely used in brackish and WWT. Its effectiveness in desalination was found to be more effective than conventional thermal multistage flashing [49]. High external pressures of 15 to 150 bars is a result of the hypertonic feed and is usually greater than the osmotic pressure which is applied to retain dissolved solute, and prevent

**Figure 3.** *Schematic diagram of ED adapted from Obotey 2020 [47].*

and allow for solvent permeation at a MWCO around 100 Da through diffusion mechanism [47]. Some advantages of the RO system that have been reported in previous studies include low energy consumption, simple configuration and operation, low membrane fouling tendencies and high rejection of a wide range of contaminants. With a concentration gradient as the driving force, the separation and concentration in forward osmosis (FO) occurs as the concentrated solution (e.g. salts such as NaCl) draws water from a less concentrated feed solution. The use of FO operates at ambient conditions, hence irreversible fouling is low. However, to attain the desired process flow and optimum configuration, ROs are arranged in stages and passes. The sequence of the stages has the concentrate stream of the first stage as the feed inlet to the second stage. In addition, the permeate streams from both stages are directed into one discharge channel.

#### *3.1.1.5 Electro-dialysis (ED) and electro-dialysis reversal (EDR)*

These processes combine the principles of electricity generation and ion-permeable membranes in the separation of dissolved ions from water [45]. A difference in electric potential leads to a transfer of ions from a dilute solution to a concentrated solution through an ion-permeable membrane. During electro dialysis, two types of ion exchange membrane are used as shown in **Figure 3**. One is permeable to anions and rejects cations, while the other is permeable to cations and rejects anions. There are also two streams which are the concentrate and the diluate (feed). When an electric current is passed through the system, ions from the diluate migrate into the concentrate through oppositely charged membranes (cations migrate to the cathode whiles anions migrate to the anode). The cations are then retained by the positively charged anion-exchange membrane (AEM). Likewise, the anions are retained by the cationexchange membrane (CEM). The outcome of this is a feed stream depleted of ions, while the concentrate stream becomes rich in ions [44].

#### *3.1.2 Applications of membrane technology (MT)*

A wider scope of industrial and environmental applications of MT are based on its advantages such as (1) clean technology, (2) energy saving (in most cases) and (3) ability to replace conventional processes; such as filtration, distillation, ion exchange, and chemical treatment systems [52]. A schematic representation of the applications of MT is depicted in **Figure 4**. Other advantages are (4) its ability

*Emerging Trends in Wastewater Treatment Technologies: The Current Perspective DOI: http://dx.doi.org/10.5772/intechopen.93898*

**Figure 4.** *Application of membrane processes adapted from Obotey 2020 [47].*

to produce high-quality products and (5) its flexibility in system design. Because of its multidisciplinary application, this technique is applied in several industries, including water treatment for domestic and industrial water supply, chemical, pharmaceutical, biotechnological, beverages, food, metallurgy, and various separation processes.

#### **3.2 Microalgal wastewater treatment (MWWT)**

Water-security is a perspective which defines the reliable availability of an acceptable quality and quantity of water for health, livelihoods and production; coupled with an acceptable level of water-related risks [53, 54]. However, population dynamics and the proliferation of industrial set-ups have induced an imbalance in the water-resource equation. Domestic use of water and the demand for water in the production sector of the economy, coupled with commercial services and the agricultural sector, have surpassed the supply capacity of potable water sources [54]. The unethical discharge of wastewater from some of these sources results in serious social, health, and environmental problems. In addition, freshwater-scarce nations have the growing need to encourage strategies for water reuse, because of inadequate precipitation and lack of capacity to harvest rainwater,, which in turn aims to reduce effluent wastewater disposal. Functional wastewater treatment plants (WWTP) for municipalities across the globe have proven to be highly demanding to run in terms of chemical input and energy. Although the basic stages of treatment are primary, secondary, and tertiary, the effluent from these plants contribute to secondary pollution as they are unable to meet the green-drop guidelines [54]. Phytoremediation is a green strategy that sequesters residual pollutants from wastewater and renders it potent for re-injection into the water supply system. The use of microalgae-based WWT systems has received serious scrutiny in the research community; and in synergy with industry, various wastewater technologies and strategies have been developed to address specific needs in the sector [55].

#### *3.2.1 Microalgal intervention*

Standard culture media have been optimized for specific microalgae strains and are subsequently modified to cultivate many other strains. These are then used as

templates to define wastewater characteristics and to select the microalgal strain or microalgae consortium that would best be able to treat a given wastewater source. The microalgae intervention protocol (MAIP) is mainly designed to rid the effluent wastewater from WWTP of the residual pollutants and concurrently produce high value products, thereby meeting the green-drop requirements [2, 3]. MAIP is therefore integrated into regular WWTP and upgrades it to advanced WWTPs (AWWTPs). This in turn confers the ability to sequester nitrates and orthophosphates, which, if unsuccessful will result in eutrophication to be induced and propagated in the receiving waters [3]. The need to regulate nitrogen and phosphorus discharge to the environment is born out of the following: (i) as free ammonia, ammonia-nitrogen is harmful to fish and other aquatic biota, (ii) ammonia consumes dissolved oxygen (DO) and therefore presents the potential of DO depletion, (iii) both phosphorus and nitrogen are plant nutrients and therefore contribute to eutrophication, (iv) is the NO3- ion, nitrate-nitrogen reacts and combines with hemoglobin, which contributes to infant mortality. In addition, nitrate-nitrogen can be reduced to mutagenic nitrosamines in the gastrointestinal tract thereby posing more hazards to infants [56]. Various research teams [57–60] reported the presence of emerging pollutants (EP) in WW and the possible undesirable effects many of them can have on the environment and living organisms. These EP include, among others, pesticides, pharmaceuticals, and personal care products; and some technologies have been proposed for their removal; such as physico-chemical and biological treatment strategies. EP removal using pure microalgae strains has been proven to be effective. However, microalgae-based EP removal technologies have not received appreciable attention in the global research community.

The advocacy for employing microalgae to sequester wastewater nutrients, as a treatment option has attracted global acceptance. However, there are skepticisms in employing wastewaters for microalgal cultivation to produce biomass and bio-products. This is primarily due to the reality that wastewaters are of a wide variety of sources and therefore have a wide range of properties whose stability is in question. Pre-treatment is therefore a necessary stage for microalgal WWT, which imposes on the economy of the process. This brings to bear the necessity to adopt the integrated microalgal WWT protocol [61–64].

### *3.2.2 Microalgal WWT strategies*

Aside from the ability of microalgae to sequester NH3-N, NO3- -N and PO43-, microalgae also removes heavy metals as well as organic carbon from wastewater, while preventing secondary pollution. However, previous research has indicated that microalgae can rarely grow in undiluted wastewater due to high concentrations of ammonium and other compounds frequently present in wastewater. Different microalgae species present different growth indices in each wastewater treatment application. It is therefore paramount to select a suitable microalgal strain to treat a given wastewater source. Ungureanu and co-workers [63, 65] reported that the microalga *C. mexicana* recorded the highest removal of N, P and C from piggery wastewater compared with five other species (*C. vulgaris*, *M. reisseri*, *Nitzschia cf. pusilla*, *O. multisporus* and *S. obliquus*). On cultivation of the microalga C. zofingiensis with piggery wastewater using different dilution ratios, 79.84% of COD, 82.70% of total N and 98.17% of total P were removed [63]. In another study with *V. vulgaris*, 60–70% of COD and 40–90% of NH4+ -N were removed from diluted piggery wastewater [65]. The highest removal percentage was obtained with 20-fold diluted wastewater. Whilst tertiary treatment of municipal wastewater effluent and remediation of animal waste streams are an additional technological and economic pressure on municipalities and farms that threatens their economic sustainability,

#### *Emerging Trends in Wastewater Treatment Technologies: The Current Perspective DOI: http://dx.doi.org/10.5772/intechopen.93898*

but at the same time it also presents an opportunity [63]. However, there are several challenges with current microalgae growth systems. For example, algae grown in an open pond or raceway system are suspended in the water in the presence of soluble and suspended waste and can be extremely difficult to harvest because oilagenous microalgae are approximately 5–10 micrometers in diameter. Many of the highly productive microalgae cannot be easily filtered and harvested through centrifugation which is an expensive unit operation. Algae can be harvested by sedimentation; however, this is a slow process and requires considerable floor space. Metal salts can be used as flocculants to facilitate sedimentation; however, this results in water contamination. Algal pond systems are also susceptible to washout, where algae leaves the system and enters surface waters [63, 65]. Integrated microalgal WWT systems are examples of green technology, which incorporates both the conventional WWTP and the microalgal WWT protocol which is primarily considered to address imperative issues such as global warming and climate change. The microalgal biomass generated during wastewater treatment, represents a carbon sink, and thus mitigates the negative effect of CO2 by photosynthetic sequestration of this greenhouse gas [66].

#### *3.2.2.1 Open ponds*

Open ponds are grouped into natural systems, artificial ponds, and containers. Natural systems include the lakes and lagoons; artificial ponds which are either unmixed open ponds, circular open ponds mixed with a center pivot mixer, or raceway ponds; and containers. The commonly used forms include raceway ponds, circular ponds, and tanks, of which raceway ponds have received the most attention [64].

Waste stabilization ponds are used for wastewater treatment by tens of thousands of small communities around the world. These ponds are low cost, simple to operate and provide effective wastewater treatment in terms of organic carbon and pathogen removal. However, phosphorus removal in waste stabilization ponds is often low, generally between 15 and 50% [62, 64]. Because of this, there is increasing pressure from regulators to upgrade pond systems to prevent eutrophication of receiving water bodies. The problem is that current upgrade options often involve the use of chemical dosing which contributes to secondary pollution that makes recovery and reuse of the phosphorus very difficult, and in some cases almost impossible. What is needed is a sustainable low-cost solution to remove phosphorus from the wastewater and ideally allow the phosphorus to be recovered and reused. A potentially emerging environmental process technology has been identified whereby microalgae in waste stabilization pond systems may be triggered to excessively accumulate phosphorus within their cells. While microalgae in lakes can store polyphosphate there is the potential of using this natural phenomenon to optimize for phosphorus removal in algal wastewater treatment ponds [62, 63].

**Figure 5(A)** Is the raceway pond that uses a motorized paddle wheel (PW) to initiate and sustain movement and mixing of the microalgal cell (MCs,) thereby preventing them from settling to the reactor bed. It enhances exposure of the MC to light and nutrients and promotes interphase mass transfer. However, while the mixing energy requirement of a PW is relatively low, efficiency of gas transfer is also low. In some instances, aerators are used to supplement CO2 to improve microalgae growth, and hence promote nutrient sequestration from the broth. The pond operates at the prevailing temperature and light intensity depends on the incoming solar insolation [68]. **Figure 5(B)** is a rectangular open unmixed pond (ROP). The MCs here do not have the privilege of equal exposure to light. The MCs that are near the bottom are shielded from light by those above, thereby creating blind zones to photosynthetic activities resulting in reduction in cell density (CD)

#### **Figure 5.** *Microalgal open pond systems [66–68].*

and productivity. **Figure 5(C)** shows open circular containers (OCC) which are unmixed. **Figure 5(D)** shows circular open pond systems (COPS) equipped with mixers [15, 16].

## *3.2.2.2 Closed bioreactor (CBR) systems*

Closed photobioreactor systems are characterized by (i) efficient photosynthetic activities associated with adequate control of the operational variables, (ii) lower risk of contamination and (iii) minimization of water loss by evaporation, which is a serious concern in open systems. However, closed systems are more expensive, since they must be constructed with transparent materials, and are more complicated to operate and challenging to scale up. Closed photobioreactors vary in configuration, and the main types are bubble columns, airlift reactors, tubular (loop) and stirred tank reactors. Photobioreactors employing microalgae to treat wastewater and produce biomolecules have (i) elevated efficiency in the use of light energy, (ii) an adequate mixing system, (iii) ease of control of the reaction conditions, (iv) reduced hydrodynamic stress on the cells [69–71].

**Figure 6** gives a pictorial view of photobioreactor scenarios for bubble column, airlift, and annular configurations. A bubble column reactor is basically a cylindrical vessel with a gas distributor at the bottom. The gas is sparged in the form of bubbles into either a liquid phase or a liquid–solid suspension without mechanical agitation. During operation, mixing and CO2 mass transfer are carried out through the action of the spargers with an external light supply. The configuration of a gas sparger is important since it determines the properties of bubbles; such as bubble size, which in turn affects gas hold-up and other hydrodynamic parameters associated with bubble columns. Photosynthetic efficiency depends on the gas flow rate, which further depends on the photoperiod as the liquid is circulated regularly from central dark zones to external photic zones. This exposes more MCs to the nutrients in the medium, which in the context of this chapter, is wastewater. Photosynthetic efficiency can be increased by increasing the gas flow rate (≥ 0.05 m/s), which in turn leads to shorter photoperiods [69, 70]. This type of reactor has advantages of higher mass transfer rates; and low operational and maintenance costs due to

*Emerging Trends in Wastewater Treatment Technologies: The Current Perspective DOI: http://dx.doi.org/10.5772/intechopen.93898*

**Figure 6.** *Bubble column reactors P*ł*aczek et al., 2017 [47].*

fewer moving parts. However, back-mixing and coalescence have been identified as major challenges for these reactors. There is an upper limit for increasing the flow rate, beyond which the heterogeneous flow formed will eventually cause the back-mixing of gas components. Scalability and economics of microalgae cultivation using photobioreactors remain the challenges that have to be overcome for large-scale microalgae production.

Hom-Diaz and co-workers [57], in an outdoor pilot 1200 L microalgal photobioreactor (PBR) used toilet wastewater (WW) and evaluated the PBR's ability to remove pharmaceutically active compounds (PhACs). Nutrients (ammonianitrogen, nitrate-nitrogen and total phosphorous) were removed and chemical oxygen demand (COD) was efficiently reduced to the extent of 80%, whilst as much as 48% of the pharmaceutical residues were removed, thereby satisfying the green-drop requirement.

Airlift photobioreactors comprise of two interconnecting zones, called the riser and the down-comer, in an annular setup. Generally, there are two types of airlift photobioreactor: (i) the internal-loop and (ii) the external-loop [19]. For the internal-loop airlift reactor, the two regions are separated by either a draft tube or a split cylinder, whilst for an external-loop airlift reactor, the riser and downcomer are separated physically by two different tubes. Mixing is done by bubbling the gas through a sparger in the riser tube, with no mechanical agitation. A riser is synonymous with bubble column, where sparged gas moves upward randomly and haphazardly, and decreases the density of the riser making the liquid move upward. Gas hold up in the down-comer significantly influences the fluid dynamics of the airlift reactor thus forcing the liquid downwards The external-loop which is a draft tube confers certain advantages to the airlift bioreactor, namely, preventing bubble coalescence by directing them in one direction; distributing shear stresses more evenly throughout the reactor. This exposes more MCs to the nutrients, minerals, volatile organic compounds and a host of other pollutants for sequestration and for cell growth; enhancing the cyclical movement of fluid, thus increasing mass and heat transfer rates [71–73].

Fully closed tubular photobioreactors are potentially attractive for large-scale axenic culture of microalgae and is one of the more suitable types for outdoor mass culture. Tubular photobioreactors consist of an array of straight, coiled, or looped transparent tubes that are usually made of transparent plastic or glass. Algae are circulated through the tubes by a pump, or airlift technology [21].

Many factors contribute to the inability of microalgae to remove nutrients and produce biomass. Some minerals, such as calcium, iron, silica, magnesium, manganese, potassium, copper, sulfur, cobalt, and zinc, also influence microalgae development in wastewater, along with pH, temperature, light, mixing, and dissolved oxygen, which influence development rates and chemical composition of microalgae in wastewater treatment systems [74, 75].
