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Biological treatment can be aerobic, anoxic or anaerobic depending on whether oxygen is present. Most biological wastewater treatment schemes are aerobic and based on the conventional/classical activated sludge (CAS) process, from which MBRs are adapted.
MBR processes can be configured as immersed or sidestream, and the membranes as flat sheet, hollow fiber or multi-tube. Membranes can also be employed as a 'polishing' step downstream of the biological process, rather than integrated with it as an MBR.
Media processes in wastewater treatment can be based on either agitated or fixed media and be aerated actively or passively, with active aeration applied to submerged media (or 'flooded filters'). Example processes include MBBRs, SAF, BAF and IFAS.
A membrane aerated biofilm reactor (MABR) is an aerobic biological treatment process where the biofilm is fixed to the membrane surface. The membrane additionally permits highly efficient oxygen mass transfer into the biofilm for enhanced biotreatment.
An MBR is a wastewater treatment process where membrane separation is integrated with a biological activated sludge process to completely retain the biological solids (or biomass). The perm-selective membrane replaces the conventional settlement stage.
MBR membranes can take one of three module configurations: flat sheet (FS), hollow fibre (HF) and multitube or multichannel (MT/MC). MBR process take one of two configurations: immersed and sidestream. Biotreatment can be aerobic, anoxic and/or anaerobic.
The design of an MBR system relates to the configuration of the membrane (hollow fibre, flat sheet or tubular), the membrane separation process (immersed or sidestream), and the biological process (aerobic, anaerobic and/or anoxic - often in combination).
MBR OPEX components comprise the energy consumption, staffing requirements, membrane replacement, chemicals consumption, and waste management and disposal. The most significant of these is the energy consumption incurred by membrane and process aeration.
The vast majority of the wastewater treatment capacity provided by MBRs worldwide is for municipal wastewater treatment. Target treated water quality parameters can include nutrients and pathogens, alongside suspended solids, organic carbon and ammonia.
Compared with municipal effluents, industrial effluents tend to have higher and more widely fluctuating concentrations of organic matter than in municipal effluents. The organics are also generally less biodegradable, relected in the lower BOD/COD ratios.
MBR O&M parameters on the membrane side include flux, permeability, membrane aeration rate and physical and chemical cleaning cycle protocols. On the biological side, O&M parameters include the hydraulic and solids retention time and sludge recycle rate.
Membrane bioreactors offer increased removal of most pollutants compared with the conventional activated sludge process. This is because of both intensified biological treatment and filtration of the mixed liquor through a <0.1 µm pore size membrane.
The key challenges imposed by MBR technologies relate largely to the complexities of operating the membrane separation component. These become more evident when the installed pretreatment is insufficiently rigorous, leading to various clogging phenomena.
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For conventional wastewater applications, the two main sludge streams are from the primary sedimentation and secondary biological treatment stages of wastewater treatment scheme, respectively called primary sludge and waste activated sludge (WAS).
Sludge stabilisation processes reduce its odour and putrescence and its level of pathogenic organisms. Conditioning refers to the pre-treatment required in sludge thickening and dewatering processes, to assist the separation of the water from the solids.
Anaerobic digestion is an extensively-used process which provides biological stabilisation of the sludge in the absence of air. The bioreactor generates a biogas containing methane, which is converted to both thermal and electrical energy for use on site.
Thickening processes remove water from the sludge by mechanical means to reduce its volume while leaving a pumpable product. Processes used are relatively low in energy and include sedimentation, dissolved air flotation, centrifugation and draining.
Sludge dewatering employs various processes, including centrifugation and pressing, to reduce the sludge moisture content by mechanical means. The energy reqirement is higher than that of thickening processes and the end product lower in moisture.
Drying of sludge is employed to reduce its moisture content. It demands a heat source, usually from either an upstream anaerobic digester or a downstream thermochemical process. Drying is often needed to sustain operation of downstream incineration.
Thermochemical methods are used to either significantly reduce the sludge solids content, whilst also capturing the sludge latent energy from its organic carbon content, or upstream of anaerobic digestion to increase the organic carbon biodegradability.
