Membrane Bioreactors or better known as MBR is a technology based on the use of membrane systems combined with biological processes to get superior clear water discharge and it is commonly used in industrial and household wastewater treatment. In the past, membrane applications have generally been put aside as secondary option due to its high service maintenance cost but recent development on producing better and cheaper membranes coupled with more stringent regulatory controls to have higher effluent quality have aroused wide interest to make it as an important treatment option. It is also looked upon as a potential solution to the conventional wastewater processes involving use of clarifier and sedimentation and overall, unlike the old systems, MBR offers better discharge quality, smaller footprint and yet capable of handling high COD loading rate.
Most membrane bioreactor systems selected for industrial applications are basically the submerged type with those that has flat sheet membranes installed with pore size ranging anywhere from 0.2 to 1 micron that will trap even bacteria and preventing other solids from passing through. As opposed to the pump around configuration, whereby the membrane is located externally out of the reactor and require high energy consumption needed to drive the liquid through the membrane, internal submerged MBR will require less power demand and also the pumping cost can be significantly reduced. Hence the submerged type is more of an automatic choice while the side stream external system has become obsolete.The materials for the membrane construction can vary with the most common one using PVDF but more advanced technology has shifted focus towards using PTFE as it offers twice the flux rate and generally with higher porosity, it will help to prevent premature fouling of the system. Grading of membranes produced by different suppliers is basically quoted referring to the panel area and also the total membrane area per modules with both parameters often being used as a guideline towards the selection process. These days, small-scale membrane bioreactors are even built to handle household wastewater generation located underground.
The working principle of a membrane bioreactor is practically quite straightforward. Depending on design by different manufacturers, basically the goal of an efficient system is to allow fast and uninterrupted flux rate through the membrane and at the same time control fouling by creating turbulent flow velocity. Normally systems are built with having two different compartments. The first section is the screening stage where the wastewater enters the unit and at this area; heavy solids are first separated before it moves on and travel to another compartment which houses the membranes. This is important so that the larger molecules such as scum and grit materials will not trap on the surface of the membrane and led to fouling.
In the second compartment where biological process takes place, it has vigorous agitation coming from air bubbles generated from a blower system. This will act to scour and clean the surface of the membrane to prevent buildup of material on the surface and also to provide sufficient oxygen concentration for biological action that supports growth of bacteria. Depending on how the system is designed to ensure efficient air to water oxygen transfer, the household MBR is capable to support up to 4000ppm of MLSS level while large-scale industrial wastewater treatment plant bioreactor can handle up to 20000ppm. A complete unit usually comes equipped with a backflush system whereby discharged wastewater will now move counter flow from the permeate side back again to the system to dislodge trapped material accumulating on the surface. During this process, air scouring will still continue to run so that it will help towards better removal.
