Membrane Bioreactor Fundamentals

MBRs combine biological and membrane treatment to effectively remove contaminants of concern from wastewaters. As illustrated in Fig. 1, MBRs are similar to conventional activated sludge systems (CASs) with the exception that the biomass (i.e., microorganisms) responsible for removing the contaminants of concern are retained within the bioreactor component of the system using membranes (Fig. 1b and c) rather than secondary clarifiers (Fig. 1a). Early designs of MBRs simply replaced the secondary clarifier of CASs with an external membrane (Fig. 1b). However, most MBRs are now designed with the membrane submerged within the bioreactor component of the system (Fig. 1c). The treatment performances of external and submerged MBRs are similar; however, the capital and operating costs for submerged membrane systems are typically much lower than those for external systems, and comparable to those for CASs [53]. Primary clarifiers are typically used prior to CASs to remove material that can easily settle by gravity, reducing the contaminant load on these systems.

Influent Treated

Wastewater

Effluent

Primary

Bioreactor

Secondary

Clarifier

Clarifier

Waste

Primary

Return Sludge [ Sludge

Sludge

Sludge a) Conventional activated sludge system a) Conventional activated sludge system

b) External Membrane MBR c) Submerged Membrane MBR

Figure 1 Bioreactor process schematics.

b) External Membrane MBR c) Submerged Membrane MBR

Figure 1 Bioreactor process schematics.

However, since MBRs can sustain higher loading rates than CASs, primary clarifiers are typically not required. On the other hand, fine mesh screening (0.5-3 mm) is typically used prior to MBRs to remove large debris and fine material (e.g., hair), which can negatively affect the performance of the membrane component of these systems.

Although MBRs and CASs are relatively similar, their process configurations in wastewater reuse applications differ substantially. This is because CASs are not as effective at removing the contaminants of concern present in wastewaters (see Section 3), and therefore the effluent from these systems typically must be further treated (e.g., using granular media filtration) before reuse applications [3]. Typical MBR and CAS configurations used for wastewater reuse applications are presented in Fig. 2. As illustrated in the figure, MBRs usually have fewer unit processes than CASs, leading to reduced system complexity and improved operability [4]. In addition, the total life cycle cost of MBRs is lower than that of CASs with granular media filtration [5]. It is also interesting to note that the total life cycle cost ofproducing reverse osmosis (RO) quality water from seawater is approximately twice that of producing RO quality water from MBR effluents [6]. The final effluent from MBRs and CASs is typically disinfected to kill and/or inactivate any pathogens that may remain in the treated effluent.

Influent Wastewater

Influent Wastewater

Primary Clarifier

Primary Sludge

Treated

Disinfection

Waste Sludge a) MBR system

Bioreactor

Secondary Clarifier

Return Sludge

Waste Sludge

Secondary Clarifier

Gra hi

nuiar m

edia.:

Treated Effluent b) Conventional system

Figure 2 Typical process configuration for wastewater reuse application.

Bioreactor

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