A low energy gravity-driven membrane bioreactor system for grey water treatment: Permeability and removal performance of organics

Synthetic grey water was treated with a low-pressure gravity-driven membrane bioreactor (GDMBR) system. The system was operated without any direct shear at the membrane surface and without any cleaning or flushing. In order to reduce energy consumption, one reactor was operated without aeration and the results were compared with an aerated reactor. Although the dissolved oxygen content was low (0.4–0.6 mg/L) in the non-aerated system, a stable permeability was observed at a level of around 20 L/m² h bar (flux of 1 L/m² h). The fouling resistance was dominated by the bio-fouling layer, which could be removed hydraulically. In comparison to the aerated system, the bio-fouling layer grown without aeration exhibited a lower biological activity, was thicker and had a lower surface roughness. Furthermore, the stable permeability of the aerated system was higher (40 L/m² h bar), which could be explained by a lower content and load of extracellular polymeric substances (including extracellular polysaccharides and proteins). In comparison to filtration through a non-fouled membrane, the rejection of biopolymers and humic substances was strongly enhanced, which shows that the bio-fouling layer acts as a secondary membrane. The energy consumption was 0.02–0.04 kWh/m³, which is substantially less than for normal MBRs, and even less than for the traditional activated sludge process. These results show that grey water can be treated in a membrane reactor without any cleaning and flushing and even without any aeration, which makes the process suitable for decentralized wastewater treatment and considerably reduces the energy consumption.