The Study On Hybrid Submerged Membrane Biological Reactor For Domestic Sewage Treatment

A lab-scale Hybrid Submerged Membrane Bioreactor reactor (HSMBR) for domestic sewage treatment was investigated in this study. HSMBR was consisted by a conventional activated sludge process that was partially filled with carrier to form biofilm and a membrane module for solid-liquid separation. This innovative system combined the conventional activated sludge process and the biofilm to create a condition for organic degradation by both suspended sludge flocs and attached biomass. Meanwhile, a PVDF microfiltration membrane module with 0.2μm pore size is immersed into the reactor for a good solid-liquid separation.From the experimental results, the HSMBR have high and stable organic matters removal efficiency. The CODcr in effluent is about 8.8mg/L-22.2mg/L with an average value of 15.4 mg/L and the removal rate of 95%. The BOD₅in effluent is about 7.3-10.1 mg/L with an average value of 8.7mg/L and the mean removal rate of 96%. As the main two important factors for system performance, the effects of DO and HRT were studied. It is found that DO have significant effect on the organic degradation performance in HSMBR system. When the DO is maintained between 1.5mg/L and 4mg/L, the removal rate is quite stable; however, when the DO is changed to 0.5mg/L, the removal rate for COD decreased dramatically. As well, HRT have influence on the system performance. When HRT is 1h, the removal rate of CODcr reaches the minimum during operational phases; with the increase of HRT, the removal rate can be recovered; when HRT was larger than 3h, the removal rate of CODcr will be slightly influenced by HRT fluctuation. According to the experimental analysis, 5h HRT was quite good for system operation.The HSMBR has good efficiency for nitrogen and phosphate removal. During the experimental operation for nutrient removal, the removal rates for NH₃-N and TN are stable, which was 97% and 78% respectively. The reason for the good nitrogen removal rate was that the interior of biofilm have an anaerobic (anoxic) area, which is essential for the growth and utilization of denitrifier. In the meantime, it is found that DO have important effect on the removal performance of NH₃-N and TN, when DO is kept more than 0.5mg/L, the removal rate decreased rapidly; when DO is kept between 1mg/L and 2mg/L, the removal rates for both NH₃-N and TN are high, reaching 96% and 80% respectively, implying that both nitrification and denitrification processes are taking place. When Do is above 2mg/L, it has little influence on the removal rate therefore the disposal rate is high. However the removal rate of TN drops slightly with the further increase of DO: when DO is 4mg/L, the removal rate of nitrogen is about 66%. In conclusion the experiment shows that when DO is between 1mg/L and 2mg/L, it is the best for sewage treatment. By studying the influence of HRT on the nitrogen removal, it is found that the best HRT is 5-8h, which offers a 96% removal rate of ammonia and 77% of total nitrogen. However, if the HRT is too short, the removal rate of NH3-N is not good, that is caused by the wash out of denitrifier under short HRT. When HRT is too high, some of bacteria will die for lack of nutrition and the sludge concentration will decrease, leading to a low removal rate of oxidized nitrogen. Because of the carrier filled in the system, the filtration of the membrane module and the existence of anaerobic microorganism, the system has capacity to consume phosphate. The phosphate concentration in the effluent is ranged from 0.9mg/L to 2.1mg/L, and the removal rate is between 58% and 82%..Membrane fouling can be found during HSMBR operation, leading to a flux decline and even a membrane failure. This thesis explains the reasons and the mechanisms of the membrane fouling and discusses seven effective approaches to slow down the development of membrane fouling. The experiment finds that with a stable flux, the trans-membrane pressure will evolution after a period of time. Physical and chemical cleaning can effective recover the membrane permeability.