Optimization On A Gas/water Alternate Membrane Bioreactor Treating Domestic Sewage

The traditional membrane separation bioreactor (MSBR) has many disadvantages such as low oxygen utilization efficiency, bad denitrification effect, and severe membrane fouling. To remove these drawbacks, a new type of membrane bioreactor, the gas/water alternate running membrane bioreactor (AMBR), was developed by us, and some research work including selection of membrane materials and primary study of AMBR's operation effect had been investigated in the former study. In order to make AMBR have a better application in practice, an optimization study of AMBR's operation parameters was carried out in this paper, and its long-time operation characteristic was also investigated.Before the optimization study of AMBR, the influence of DO on pollutants removal by a membrane aeration bioreactor (MABR) treating synthetic domestic wastewater was investigated to provide an instruction for AMBR's experiments. The results showed that when DO in the mixed liquor was 0.5mg/L around, MABR realized favorable simultaneous removal of carbon and nitrogen. The removal rate of COD, NH4+-N and TN was 80%, 86.4% and 66.5%, respectively. However, the effluent SS was a bit too high, which reached to 14mg/L. When DO was above 2mg/L, COD and NH4+-N removal rate increased to more than 88.8% and 94.5%, respectively, and the effluent SS also dropped to 5mg/L around; whereas, TN removal was terrible, and its removal rate was below 24%.In order to optimize the operation parameters of AMBR, the influence of DO, C/N ratio, alternate cycle, and loading rate on AMBR's operation effect were investigated through a series of bench scale experiments. The results showed that when DO in the mixed liquor was 0.5mg/L around, AMBR realized favorable removal of carbon and nitrogen simultaneously. The COD, NH4+-N and TN removal rate was 94.5%, 93.4% and 78.4%, respectively. Nevertheless, neither too high nor too low DO value was helpful to simultaneous carbon and nitrogen removal. When the influent C/N ratio of AMBR was in the range of 3~10, increase of C/N ratio had no influence on COD and NH4+-N removal. However, increase of C/N ratio was beneficial to TN removal by AMBR. Alternate cycle showed great influence on membrane fouling relieving in AMBR. When alternate cycle was 0.75~3h, AMBR appeared favorable membrane fouling relieving effect, while neither too long nor too short cycle was beneficial to relieve membrane fouling. Besides, prolonging alternate cycle helped to reduce oxygen losing rate. Thus, it was determined that the optimization alternate cycle of AMBR was 3h. Under the condition that the intra-membrane oxygen pressure was controlled below 0.2MPa, the maximum removal loading rate of COD, NH4+-N and TN per unit membrane aera by AMBR was 84.9g/m2·d, 6.6g/m2·d and 6.5g/m2·d, respectively.Through the investigation of AMBR's long-time operation characteristic, it was noticed that AMBR had a stable and favorable operation performance. During the 120 days'continuous operation, the average removal rate of COD, NH4+-N and TN reached to 97%, 92% and 75.4%, respectively, and the effluent SS was below 1mg/L. Moreover, AMBR appeared favorable membrane fouling relieving effect. During the 120 days'operation, TMP of the two membrane modules in AMBR maintained -0.017MPa around. From the view of the stable long-term operation performance, AMBR has a great application value in practice.