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A membrane bioreactor requires two principal operations to be sustained: First, the biochemical processes in the biological tank which are responsible for degrading the organic material and nitrogenous species. Second, the permeation of the treated water through the membrane.
Key challenges which arise in MBRs include ragging (or braiding), foaming and fouling.
The principal MBR design parameter on the membrane side is the permeate flux. The flux required will then determine the various operating parameters and protocols for maintaining the membrane permeability.
On the biological side, O&M parameters mainly comprise the hydraulic and solids retention times, and sludge recycle rate. These parameters, in combination with the feedwater characteristics and load, determine the sludge yield and food to micro-organism ratio.
The key parameter of a membrane system is the flux, the flow rate of permeate per unit membrane area. The membrane permeability is the ratio of the flux to transmembrane pressure (TMP), and is maintained through a combination of air scouring (for an immersed system) or crossflow of retentate (for a sidestream system) with periodic physical and chemical cleaning.
The key biological process parameters for activated sludge-based technologies are the hydraulic and solids retention times (HRT and SRT), along with sludge recycle rates.
In an MBR, the HRT and SRT are uncoupled, allowing greater biological operational flexibility than for a conventional process.
It is necessary to clean the membranes to maintain the permeability at a sustainable level, to achieve the required permeate production rate and reduce the energy consumption.
Membranes can be cleaned by physical or (more usually) chemical means.
The ‘integrity’ of a membrane refers to its ability to sustain the rejection of the target pollutant. An undamaged membrane provides 100% rejection of those microorganisms whose size is substantially larger than that of the membrane pores. If the membrane is damaged, then there is a risk of reduced pathogen rejection.