Comparison of the filtration characteristics between biological powdered activated carbon sludge and activated sludge in submerged membrane bioreactors

The filtration performances of submerged membrane bioreactors (SMBR) with and without the addition of powdered activated carbon (PAC) were investigated respectively under the same feed and operation conditions. A series of experiments were conducted to analyze near-critical flux, effect of air-scouring rate and time of stable filtration operation of both systems. The experimental results demonstrated that pronounced flux enhancement was achieved by adding 1.2 g/L PAC. The near-critical flux for the biological powdered activated carbon (BPAC) system was about 32% higher than that for the activated sludge (AS) system. Increasing the air-scouring rate led to a more significant flux improvement for the BPAC system compared to the AS system. Long-term operation indicated that, at constant flux, the TMP increasing rate of the BPAC system could be lagged and thus cause the extension of operating intervals about 1.8 times compared to the AS system. Quantitative calculations showed the total hydraulic resistance of the BPAC system was about 44% lower than that of the AS system, and this decrease was mainly caused by the reduction in cake resistance. Analyses were then made from various aspects such as floc size distribution and apparent viscosity of the mixed liquor to elucidate the major factors giving rise to different filtration characteristics.

In this study the filtration characteristics of two types of SMBR systems (BPAC and AS) were investigated in parallel. Factors affecting hydraulic filtration resistance were discussed. The following conclusions can be drawn from the study: 1. Effective control of flux reduction was accomplished by adding PAC. The membrane permeability of the BPAC system was better than that of the AS system. As a result of adding PAC, the near-critical flux for the BPAC system could be raised by about 32% compared to the AS system. The near-critical flux increased almost linearly with an air-scouring rate within a certain range. At a higher air-scouring rate, the flux improvement with the addition of PAC was more prominent than that without PAC. Long-term operation showed that the membrane fouling could be reduced effectively by adding PAC, and operating intervals could be extended about 1.8 times compared to the AS system. 2. Resistance calculations showed that the total resistance for the BPAC system was about 44% lower than that of the AS system. Adding PAC not only reduced the fine colloids and EPS in the bulk phase, but also increased the incompressibility of the floc structure. Accordingly, a more dynamic, looser cake layer with higher porosity was formed on the membrane surface in the BPAC system, which caused a decline of the cake resistance and thus the total resistance compared to the AS system.