Understanding permeability decay of pilot-scale microfiltration in secondary effluent reclamation

Performance of a pilot microfiltration (MF) system for water reclamation of the secondary effluent from a municipal wastewater treatment plant was evaluated with and without a preceding sand filter. Preceded with a sand filter, the MF system, which had a total filtration area of 56 m2, maintained a permeate flux of 60.7 L m−2h−1 for 480 h before a chemical clean-in-place (CIP) was necessary. Under the same operating conditions except by-passing the sand filter, the MF maintained the flux for only 250 h. During the filtration within a CIP cycle, the transmembrane pressure increased while the specific permeate flux (SPF) or permeability decreased exponentially with filtration time because of the increases in resistance to permeation. Using municipal secondary effluent as feed, the average resistance of the MF membrane to permeate flux at the end of a filtration cycle was at least 10 times of that using clean water as feed and the resistance was comparable for filtration with and without the sand filter. The decay inSPF as a result of the resistance during the filtration followed a first order kinetics with a half-live time of 198 h with and 74 h without the sand filter in front of the MF. The concept of the SPF decay kinetics should be meaningful in implementation of a robust control in membrane filtration process.