Biofilm in Moving Bed Biofilm Process for Wastewater Treatment

*Shuai Wang, Sudeep Parajuli, Vasan Sivalingam and Rune Bakke*

### **Abstract**

A brief introduction of the long history of biofilm-based wastewater treatment is given together with basics of biofilm behavior and mechanisms in removal and transformation of pollutants. Moving bed biofilm reactor (MBBR) principles and applications of such are presented. Advantages and limitations of such solutions are given together with evaluations of emerging MBBR applications. The basis of biofilm processes and biofilm layer classification based on dissolved oxygen gradient is discussed. Organisms grow at the protected surface of the biocarrier where oxygen gradients create aerobic, anoxic, and anaerobic layers allowing simultaneous nitrification and denitrification in one MBBR (nitrification, nitritation, autotrophic, and heterotrophic denitrification). Combination of MBBR with activated sludge, continuous flow intermittent cleaning (CFIC®), and integration with anaerobic digestion increases the potential usage of MBBR for enhanced efficiency and energy recovery and is partly discussed as case studies (COD, ammonium, and solid removal). Biofilm thickness and scaling control can be crucial for MBBR performance. The type of carriers, filling degree, and operational conditions play a major role for process performance; hence, the effect of those parameters is presented.

**Keywords:** moving bed biofilm, TN removal, scaling on biofilm, biocarriers

### **1. Introduction**

The use of biofilm systems in wastewater treatment is being increased rapidly because of its tempting approach of pollutant removal from wastewater, which has been proved to be effective in terms of both cost and environmental perspectives [1, 2]. Biofilm can have both positive and negative effects in treatment processes depending on the type of treatment concept applied. Processes such as a moving bed biofilm reactor (MBBR) depend on biofilm development, while it can cause problems in membrane bioreactor (MBR) through membrane biofouling. Those processes taking advantage of biofilms have been widely used for the removal of organic and inorganic matters from different wastewaters [1], by mechanisms such as biodegradation, bioaccumulation, biosorption, biomineralization, and bioimmobilization [1, 3, 4].

There are several benefits of using biofilm system in wastewater treatment, as compared to suspended growth system (activated sludge for example), such as flexible procedures, smaller space demand, lower hydraulic retention time, increased resilience to changes in the environment, higher biomass retaining period, high active biomass concentration, as well as low sludge production [3, 5, 6]. The use of biofilm systems also enhances the control of reaction rate and population mechanisms [5, 7].

Microorganisms tend to form clusters/colonies to expedite the organism's growth and facilitate access to food, etc., by forming biofilm [8]. In biofilm or attached growth systems, the growth of the biomass responsible for the conversion of organic material and/or nutrient occurs on the surface of support packing material [9]. Biofilm formation is enhanced by substratum provided to retain and grow microorganisms. The support medium can be rocks, stones, gravels, sand, soil, wood, rubber, plastic, and agglomerates of the biomass itself (granules) or any other synthetic materials [3, 8]. The packing material provides a large surface area per unit volume for biofilm development in high-rate processes; thus, substratum material selection is important to maintain a high quantity of active biomass and to uphold different varieties of microbial populations [10]. The large surface area of the biofilm enables the media to efficiently adsorb a high amount of substrates from the influent wastewater. As the biofilm develops on the media, it provides diverse habitats so that different constituents such as carbon and nitrogen components of the wastewater are transformed and mineralized, thus increasing the removal efficiency of the organic substances from influent wastewater.

There are generally three steps involved in biofilm formation, including biofilm attachment, growth, and detachment (**Figure 1**). Microorganisms attach on to the substratum, such as the surface of carriers in MBBR processes, by adhesion, and the attachment is reversible at the early stage. Tight connections between organisms and the substratum can be gradually established by extracellular polymetric substances (EPS) produced by the organisms. EPS is a mixture of polysaccharides, proteins, and extracellular DNA, and it is recognized to also be important for the communication between biofilm cells, biofilm 3D structure formation, and multicellular living [11]. Biofilm detachment from the surface is a natural mechanism where biomass (individual cells or lumps of cells) is released into bulk liquid. It can be influenced by hydrodynamic shear forces and other environmental conditions such as toxic chemical exposure. Detachment process limits biofilm accumulation and thickness and thus balances the attached biofilm quantity at steady-state conditions [11].

Different species can be found in the same biofilm clusters. They can vary from rapidly growing to inactive organisms, from heterotrophic to autotrophic organisms depending on substrate gradients, mutation, genetic regulatory switches, and signaling pathways [11]. Due to oxygen transfer limitations in an aerated system, the biofilm can contain aerobic, anoxic, or anaerobic organisms at the same time [5, 8]. A well-established biofilm can have any thickness, but around 0.1 mm is considered

**5**

treatment [14].

*Biofilm in Moving Bed Biofilm Process for Wastewater Treatment*

development, and effectiveness of biofilm processes.

**2. Moving bed biofilm reactor**

suitable in an efficient MBBR, where mass transfer in the biofilm structure and between the interphase of biofilm and liquid is critical for efficient mass transfer [12, 13]. Both diffusion and convention can occur [11, 14] in the biofilm mass transfer (**Figure 2**), while substrate diffusion is considered to usually be the rate-limiting process within the biofilm structure. Substrate accessing of biofilm can be enhanced by, for example, enhanced aeration/mechanical mixing to enhance mass transfer from the bulk liquid to the biofilm surface. The internal biofilm growth (**Figure 1**) and external forces such as abrasion are important factors for biofilm morphology,

*Left, graphical illustration of biofilm processes [15]; right, biofilm picture and layer classification based on* 

Biofilm processes applied for wastewater treatment have a long history. Trickling filters (TF) and rotating biological contactors (RBCs) utilizing biofilm growing on the packing medias are biofilm processes being widely applied of low-cost and low maintenance comparing to activated sludge process [1, 3, 16]. Moving bed biofilm process, which was developed in the 1980s [14, 17], has been widely applied for organic and inorganic wastewater treatment of high efficiency, low maintenance, and low operation cost [8, 17, 18]. A membrane-aerated biofilm reactor (MABR) has been developed recently for organic and ammonia removal, based on an aerated membrane where biofilm attaches on the fiber [10]. Biofilm in the form of granular sludge for energy recovery (methane) from wastewater organics, such as by upflow anaerobic sludge blanket (UASB) [8, 19], expended granular sludge bed (EGSB) [20, 21], and internal recycle reactor (IC) [22], and aerobic granular sludge reactors for shortcut ammonia removal, such as anammox process [23], and for simultaneously organic, phosphor, and ammonia removal, Nereda [24] has been developed and is increasingly used in both industrial and municipal wastewater treatments. Biofilm applied in MBBR processes are focused in this book chapter. Commonly applied MBBR and its derivatives processes are introduced and compared. A case study based on MBBR concept for municipal wastewater treatment is also provided.

Moving bed biofilm reactor (MBBR) is an advanced wastewater treatment technology, which employs the benefits of both biofilm and activated sludge processes for highly efficient wastewater treatment [14]. Developed in the 1980s, MBBR has been established in the last two decades worldwide as a simple, robust, flexible, and compact wastewater technology for both municipal and industrial wastewater

*DOI: http://dx.doi.org/10.5772/intechopen.88520*

**Figure 2.**

*dissolved oxygen gradient [12].*

*Biofilm in Moving Bed Biofilm Process for Wastewater Treatment DOI: http://dx.doi.org/10.5772/intechopen.88520*

#### **Figure 2.**

*Bacterial Biofilms*

biomass concentration, as well as low sludge production [3, 5, 6]. The use of biofilm systems also enhances the control of reaction rate and population mechanisms [5, 7]. Microorganisms tend to form clusters/colonies to expedite the organism's growth and facilitate access to food, etc., by forming biofilm [8]. In biofilm or attached growth systems, the growth of the biomass responsible for the conversion of organic material and/or nutrient occurs on the surface of support packing material [9]. Biofilm formation is enhanced by substratum provided to retain and grow microorganisms. The support medium can be rocks, stones, gravels, sand, soil, wood, rubber, plastic, and agglomerates of the biomass itself (granules) or any other synthetic materials [3, 8]. The packing material provides a large surface area per unit volume for biofilm development in high-rate processes; thus, substratum material selection is important to maintain a high quantity of active biomass and to uphold different varieties of microbial populations [10]. The large surface area of the biofilm enables the media to efficiently adsorb a high amount of substrates from the influent wastewater. As the biofilm develops on the media, it provides diverse habitats so that different constituents such as carbon and nitrogen components of the wastewater are transformed and mineralized, thus increasing the removal

There are generally three steps involved in biofilm formation, including biofilm attachment, growth, and detachment (**Figure 1**). Microorganisms attach on to the substratum, such as the surface of carriers in MBBR processes, by adhesion, and the attachment is reversible at the early stage. Tight connections between organisms and the substratum can be gradually established by extracellular polymetric substances (EPS) produced by the organisms. EPS is a mixture of polysaccharides, proteins, and extracellular DNA, and it is recognized to also be important for the communication between biofilm cells, biofilm 3D structure formation, and multicellular living [11]. Biofilm detachment from the surface is a natural mechanism where biomass (individual cells or lumps of cells) is released into bulk liquid. It can be influenced by hydrodynamic shear forces and other environmental conditions such as toxic chemical exposure. Detachment process limits biofilm accumulation and thickness and thus balances the attached biofilm quantity at steady-state conditions [11].

Different species can be found in the same biofilm clusters. They can vary from rapidly growing to inactive organisms, from heterotrophic to autotrophic organisms depending on substrate gradients, mutation, genetic regulatory switches, and signaling pathways [11]. Due to oxygen transfer limitations in an aerated system, the biofilm can contain aerobic, anoxic, or anaerobic organisms at the same time [5, 8]. A well-established biofilm can have any thickness, but around 0.1 mm is considered

*Biofilm life cycle. Adapted from the Center for Biofilm Engineering, Montana State University [11].*

efficiency of the organic substances from influent wastewater.

**4**

**Figure 1.**

*Left, graphical illustration of biofilm processes [15]; right, biofilm picture and layer classification based on dissolved oxygen gradient [12].*

suitable in an efficient MBBR, where mass transfer in the biofilm structure and between the interphase of biofilm and liquid is critical for efficient mass transfer [12, 13]. Both diffusion and convention can occur [11, 14] in the biofilm mass transfer (**Figure 2**), while substrate diffusion is considered to usually be the rate-limiting process within the biofilm structure. Substrate accessing of biofilm can be enhanced by, for example, enhanced aeration/mechanical mixing to enhance mass transfer from the bulk liquid to the biofilm surface. The internal biofilm growth (**Figure 1**) and external forces such as abrasion are important factors for biofilm morphology, development, and effectiveness of biofilm processes.

Biofilm processes applied for wastewater treatment have a long history. Trickling filters (TF) and rotating biological contactors (RBCs) utilizing biofilm growing on the packing medias are biofilm processes being widely applied of low-cost and low maintenance comparing to activated sludge process [1, 3, 16]. Moving bed biofilm process, which was developed in the 1980s [14, 17], has been widely applied for organic and inorganic wastewater treatment of high efficiency, low maintenance, and low operation cost [8, 17, 18]. A membrane-aerated biofilm reactor (MABR) has been developed recently for organic and ammonia removal, based on an aerated membrane where biofilm attaches on the fiber [10]. Biofilm in the form of granular sludge for energy recovery (methane) from wastewater organics, such as by upflow anaerobic sludge blanket (UASB) [8, 19], expended granular sludge bed (EGSB) [20, 21], and internal recycle reactor (IC) [22], and aerobic granular sludge reactors for shortcut ammonia removal, such as anammox process [23], and for simultaneously organic, phosphor, and ammonia removal, Nereda [24] has been developed and is increasingly used in both industrial and municipal wastewater treatments.

Biofilm applied in MBBR processes are focused in this book chapter. Commonly applied MBBR and its derivatives processes are introduced and compared. A case study based on MBBR concept for municipal wastewater treatment is also provided.

### **2. Moving bed biofilm reactor**

Moving bed biofilm reactor (MBBR) is an advanced wastewater treatment technology, which employs the benefits of both biofilm and activated sludge processes for highly efficient wastewater treatment [14]. Developed in the 1980s, MBBR has been established in the last two decades worldwide as a simple, robust, flexible, and compact wastewater technology for both municipal and industrial wastewater treatment [14].
