Ragging is a type of clogging, where clogging relates to agglomeration of solids in the membrane tank. In the case of ragging, the filament solids from textile materials join together to form long rags or braids. These rags, which may contain filaments no more than a few mm in length, are mechanically stable in the mixed liquor and can block the inlet channels of the membrane module and/or wrap themselves around the infrastructure of the membrane tank – including the aerators.
Features - Process biology
Disinfection by MBRs. Unlike regular physical membrane filtration, virus removal by MBRs is not limited to simple size exclusion. Adsorption onto the sludge solids and the membrane cake layer (as well as the membrane itself) represent important removal mechanisms, as well as removal by predation (the feeding on the pathogens by other higher organisms).
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Ever more versatile and sophisticated options for designing and managing plants are being developed. And as the new CAE tools become more accessible and widespread, suppliers continue to develop and refine the tools, so the reliability of their predictions increases – promoting further take-up.
MBR membranes are prone to fouling generally, and biofouling in particular, by organic matter originating from the microbial cells. These biofoulants vary in concentration with the activated sludge characteristics, such as the mixed liquor suspended solids (MLSS) concentration and solids retention time (SRT) (and so the food/microorganism (F/M) ratio), as well as the feedwater chemistry.
MBRs use more energy compared with classical activated sludge (CAS) because the aeration requirements are greater. Aeration is needed both for the biological and membrane tanks for degrading the organics and scouring the membrane respectively. Typically, aeration energy consumption accounts for 70–80% of total energy used for the municipal wastewater treatment process, with 40–60% consumed by the process biology.
Operational costs in MBRs are marginally higher than those of conventional activated sludge (CAS). Firstly, permeating water through a membrane demands energy. In the case of the immersed technologies (iMBRs) this means that the overall specific aeration demand (SAD) is higher, since air is needed both for maintaining the process biology in the aeration tank and scouring the immersed membrane.