Membrane bioreactor

Principles of MBR

Over the years, MBR has been the focus of extensive research in the field of environmental engineering owing to its many advantages over the conventional biological treatment methods. MBR effectively separates the suspended solids and soluble organic matter in the water, particularly the pathogenic bacteria and viruses; thus, additional purification and disinfection treatment may not be required (Ensano et al. 2016). This reduces the operational costs and further improves the effluent quality by the prevention of formation of toxic and hazardous by-products. MBR also has higher biomass concentration yielding higher rate of BOD and COD removal and lower excess sludge production (Ensano et al. 2019).

Most studies regarding MBR mainly focus on the conventional type, widely known as the pressure-driven MBR. In this paper, the authors define MBR as a water treatment technology that combines biological degradation process and membrane technology in a single reactor. Aside from pressure-driven MBR, two other configurations were defined, namely, the ion exchange MBR and the gas-transfer MBR. Each configuration has its own advantages and disadvantages, which are discussed in detail in the succeeding sections.

Types of MBR

Pressure-driven MBR

In a pressure-driven MBR, the separation of permeate from contaminated water is facilitated by the pressure difference across a porous membrane. The first pressure-driven MBR was built in 1968 via extemal/side stream configuration, wherein an external UF membrane module was installed after the activated sludge tank as a replacement of the secondary clarifier to overcome sludge settling difficulties (Smith 1968). A new and better configuration was designed by Yamamoto et al. (1989), known as submerged membrane bioreactor, wherein the membrane module was immersed directly inside the aeration tank and a pump was used to recover the permeate (Figure 2A). Internal/ submerged MBR is more advantageous than the extemal/side-stream MBR because it has lower cost, lower energy demand, needs less space and operates at considerably low flux. Moreover, since the sludge retention time is independent from the hydraulic retention time, slow growing microorganisms are maintained inside the bioreactor, which is beneficial to complex micropollutants that require longer degradation periods (Velizarov et al. 2005). Despite these advantages, large scale applications of MBR technology are limited due to the fouling caused by deposition of sludge into the membrane pores. Some of the membrane fouling control techniques successfully implemented by various studies include membrane backwashing, excess aeration, frequent membrane cleaning and the integration with electrochemical processes (Ensano et al. 2019). Pressure-driven anoxic

Three types of membrane bioreactor (A) Pressure-driven MBR, (B) Ion exchange MBR, and (C) Gas-U'ansfer

Figure 2. Three types of membrane bioreactor (A) Pressure-driven MBR, (B) Ion exchange MBR, and (C) Gas-U'ansfer


MBR has been widely used in the treatment of several anions including nitrate (N03~); however, little is known about its application in perchlorate treatment.

Ion exchange MBR

Ion exchange MBR (IEMB) is a tailor made technology that combines physical separation of charged pollutant, using ion exchange membranes, with biodegradation of pollutant by a suitable microbial culture in a bioreactor (Figure 2B). Ion exchange membranes can be either anion or cation exchange membrane depending on the charge of the target contaminants. The orientation of the liquid flow in an IEMB is opposite to that of the conventional MBR. In IEMB, membrane separation precedes biodegradation, such that the ionic micropollutant from the aqueous feedstock (water- compartment) passes through a selective barrier (non-porous ion exchange membrane) towards the opposite compartment (bio-compartment) that is enriched with microorganisms for biodegradation (Figure 2B). The ion transfer across the membrane is governed by Donnan dialysis in which the addition of high concentration of driving counter-ions at the bio-compartment permits the flow of the target ions from the water stream to the bio-compartment, hence maintaining overall electroneutrality between compartments (Crespo et al. 2004). Donnan exclusion effect is also observed, wherein co-ions are rejected by the similar charged membrane (Fox et al. 2014). After the target ion crosses the membrane, it enters the bio-compartment where it is degraded into innocuous products by an appropriate microbial culture. For an effective biodegradation performance, the microorganisms should be fed with carbon and energy source as well as essential nutrients. Moreover, accumulation of the ions can create favorable conditions for biofihn growth at the membrane surface in contact with the bio-compartment. This microbial biofilm increases the ion transmembrane flux and prevents the diffusion of the carbon source from the bio-compartment to the water stream compartment (Fox et al. 2016). However, excess biofihn growth may lead to low transport rate of ionic pollutants.

The first application of IEMB in water treatment was successfitlly conducted by Fonseca et al. (2000) when nitrate was removed from a synthetic groundwater using IEMB at denitrification rate of 7gN/nr d. Portuguese National Patent No. 102385 N and European Patent EP1246778 were awarded to Crespo et al. (1999) and Crespo and Reis (2003), respectively. IEMB was then used extensively for the selective removal of ionic pollutants, such as nitrate, perchlorate and brornate (Matos et al. 2005, 2006a, 2008).

The advantages of IEMB include the following (Fonseca et al. 2000, Crespo et al. 2004): (1) it prevents secondary contamination of the treated water by microbial cells, excess nutrients and/or metabolic by-products since biodegradation occurs in the opposite compartment; (2) the hydraulic residence time in both compartments can be separately adjusted which can aid in the optimization of the pollutant extraction; (3) the selection of appropriate driving counter-ions in the bio-compartment keeps the concentration of the target pollutant in the bio-compartment at low levels; (4) water treatment performance of IEMB mainly depends on the transpoxt rate of the target ions through the membrane and not on the pollutant biodegradation rate; hence, the effluent quality is not affected even if the bio-compartment is operated under extreme conditions (e.g., excessive nutrients and carbon source); and (5) 02 in the feed water does not enter the bio-compartment, which can otherwise interfere with the biodegradation (i.e., anoxic or anaerobic) process.

Gas-transfer MBR

In gas-transfer MBR, gases such as 02, H7, and methane (CH4) are used as electron donor by microorganisms to degrade contaminants in aqueous solution. It employs gas-permeable membranes, which allow efficient transfer of the gaseous substrate for microbial consumption without bubble formation. Bubble-less gas transfer prevents the accumulation of explosive amount of the gas substrate in the headspace above water, as in the case of highly flammable HU (Crespo et al. 2004). Gas-transfer MBR differs significantly from the conventional pressure-driven MBR and ion exchange MBR in terms of the substrate introduction and the microbial orientation. In a gas- transfer MBR, the gaseous substrate is introduced into the membrane module (lumen) and diffuses outside the walls towards the bulk liquid (bio-compartment) (Figure 2C). The microbes can either be suspended in the solution or naturally grown on the membrane surface as biofilm, the latter being designated as the membrane biofilm reactor (MBfR). In situ water remediation of contaminated site by gas-transfer MBR can be done by installing the membrane system across the plume while the groundwater flows through it. As the gaseous substrate diffuses from inside the membrane module towards the plume, the biofilm or suspended microorganisms consume it while simultaneously degrading the pollutants.

The concept of MBfR was pioneered by Schaffer et al. (1960) when 07 was introduced inside the permeable plastic films coated with slimes to oxidize organic pollutants in sewage water. Years passed and the application of 07-based MBfR (also known as membrane-aerated biofilm reactor, MABR) did not develop into a major water treatment technology since there are so many well- established methods to provide aeration for microorganism consumption (Rittmaim 2007). In addition, the major drawback of the MABR technique is the lower oxygen transfer to the biofilm due to the occurrence of gas back diffusion, which affects the efficiency of the wastewater treatment process (Nereuberg 2016).

Most of the researchers focusing on the treatment of wastewater by MBfR utilize H,-based and CH4-based gas-transfer MBfR for the effective reduction of micropollutants. Methane-based MBfR uses CH4 as both carbon and energy sources by heterotrophic bacteria under anoxic condition (Luo et al. 2015). CH4 is an inexpensive gas that can either be extracted from large fossil reserves or can be produced via anaerobic digestion of biomass. Meanwhile, the H,-based MBfR has been gathering good reviews from scientific community because of its capability to bio-reduce a variety of oxidized compounds (e.g., NOj, СЮ4, BrOj, SeO^-, H2As04, Cr042-, etc.) (Rittmaim 2007). H7 is consiuned by autotrophic bacteria, which use inorganic carbon as carbon source, therefore preventing the production of excess biomass in the water distribution system. H7 is also inexpensive in teims of electron-equivalent that is required for contaminant reduction. Lastly, H7 is non-toxic to human health and has reliable and safe methods for transport and storage (Rittmaim 2007).

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