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MBRs were first implemented in the early 1990s, with their subsequent rapid market penetration in both the municipal and industrial sectors promoted by increasingly stringent environmental legislation.
The technology incurs higher operating costs while achieving a higher treated water quality at a lower plant footprint than the conventional treatment process. A number of studies have concluded the overall cost of an MBR to be lower than that of the conventional process when a high treated water quality is required.
The vast majority of the wastewater treatment capacity provided by MBRs worldwide is for municipal wastewater treatment, since the total flow of sewage is far greater than that of industrial effluent. Municipal MBR installations are generally larger than the industrial plants.
Similarly, most of the research into MBRs, focused largely on membrane fouling, relates to municipal wastewater treatment.
Compared with municipal effluents, industrial effluents tend to contain higher pollutant concentrations, which may vary across daily and seasonal cycles. Industrial effluents also tend to contain less biodegradable organic matter.
Significant variations in water quality and treatability can be found across the different industrial sectors, e.g. between food & beverage and landfill leachate.
There are two main components of a membrane bioreactor whose operation must be sustained: (a) the biochemical processes in the biological tank responsible for degrading the organic material and nitrogenous species, and (b) the permeation of the treated water through the membrane.
Maintaining membrane permeation demands regular physical and chemical cleaning of the membrane.
Membrane bioreactors allow for increased removal of most pollutants compared with the conventional activated sludge process.
This is because, among other reasons, the mixed liquor is filtered through a membrane with an effective pore size of <0.1 µm.
The key challenges associated with MBRs relate largely to the membrane separation component. This makes the MBRs more complex to operate compared to the conventional activated sludge process (CAS).
Sustaining membrane permeability is essential to ensure that the flow capacity of the process is maintained. This becomes problematic if the installed pretreatment is insufficient or ineffective.