Recently, an increasing level of attention has focused on the emerging technology of anaerobic dynamic membrane bioreactors (AnDMBRs), owing to its merits such as low membrane module cost, easy control of membrane fouling, low energy consumption and sludge production, as well as biogas production.
As such, the combination of an anaerobic treatment process and membrane filtration technology, the so-called anaerobic membrane bioreactor (AnMBR), has been successfully applied at various scales to maintain anaerobic microorganisms. Hydraulic retention time (HRT) and solids retention time (SRT) in AnMBRs can be independently controlled to extend process applicability for treating wastewaters of various strengths from industries and municipalities , as well as solid wastes (such as food wastes and surplus activated sludge). Thus, the HRT could be largely reduced, but SRT could be maintained at a relatively high value (50–700 d) to produce a solids-free effluent with high chemical oxygen demand (COD) removal owing to the retention of slowly degradable organics within the reactor.
It is well-recognized that anaerobic sludge in AnMBRs is characterized by high viscosity and mixed liquor suspended solids (MLSS) concentration. It also contains large amounts of biopolymers and inorganic substances, resulting in higher fouling potential than activated sludge in aerobic MBRs. The attachment and accumulation of solid particles (such as fine sludge particles, biopolymers, and inorganics) on the membrane surface is a common phenomenon in (An)MBRs, causing membrane fouling . Membrane fouling can be divided into two categories, namely cake layer formation on the surface of the membrane and pore clogging . It is commonly accepted that cake layer formation is the main contributor to fouling in aerobic and anaerobic MBRs, as in most cases the cake layer filtration resistance accounts for over 80% of the total filtration resistance. However, the formed cake layer could function as an additional filter (secondary membrane or dynamic membrane) owing to its capability to reject various pollutants and pathogens . As such, the rejection properties are more dependent on the cake layer rather than the underlying membrane, and thus, a cheap support material (such as meshes, woven, or non-woven filter cloth) enabling the formation of a cake layer could be used instead of microfiltration/ultrafiltration (MF/UF) membranes in AnMBRs . The well-formed dynamic membrane (DM) layer (also called a secondary membrane) can be used as a filter prior to the support material, and it can provide effective retention in both aerobic DM bioreactors (DMBR) and anaerobic DM bioreactors (AnDMBRs) combined with several unique merits (such as low membrane cost, high flux, and easy cleaning).