| With the development of social economy, rapid urbanization and increasing environmental awareness, water resource stress becomes more evident and serious in city areas recently. Municipal sewage, called "the second water source", is widely reclaimed and reused to meet the growing city water pressure in all over the world. Membrane bioreactor (MBR), combined with membrane separation technology and biological treatment process, has been much respected among numerous municipal sewage reusing technologies by water treatment researchers. However, membrane fouling has been a main obstacle preventing wide utilization of MBRs because it causes a significant decline of permeate flux, requires more cleanings and replacement of fouled membranes, and consequently, increases the energy consumption and operating cost of the treatment process.In this study a submerged metal membrane bioreactor (MMBR) coupled with stainless steel flat membranes was performed for treating municipal sewage. The feasibility of MMBR in treating municipal sewage was investigated. Membrane fouling behaviors were also investigated to discuss the effect of membrane fouling control and membrane cleaning. This study will provide theoretical foundation and technical support for the wide utilization of MMBR in municipal sewage and industrial waste treatment.Operationc parameters affected membrane fouling greatly. Therefore, an orthogonal optimization experiment of operational parameters was carried out before long period running. Four parameters including the ratio of suction time to non-suction time, aeration rate, backwashing frequency and backwashing flux were chosen as the factors. Average daily change rate of trans-membrane pressure (TMP) was used as the evaluate index. Lastly the optimal operational parameters and its influence intensity sequence were obtained by the L9(34) orthogonal experiment.According to the optimal operational parameters, continued experiment was carried out for 270 days at the permeate flux of 0.4～1m/(m2·d). The results indicated that MMBR had good removal efficiencies of chemical oxygen demand (COD), ammonium and suspended solids (SS) under both oxic and anoxic/oxic (A/O) mode in stable state. COD removal efficiency was improved by the fine filtration of stainless steel membrane. Simultaneous nitrification and denitrification (SND) due to high mixed liquor suspended solids (MLSS) concentration and special inner circulation structure resulted in additional total nitrogen (TN) removal.The study on membrane fouling, composition of filtration resistance and membrane cleaning showed that the predominate filtration resistances under oxic mode and A/O mode were cake layer resistance and internal resistance, respectively. Membrane fouling under oxic mode was alleviated effectively by mechanical cleaning or on-line NaOCl back washing and MMBR ran stably for 115 days at the permeate flux of 0.8～1m3/(m2·d) without off-line cleaning. But membrane fouling could not be controlled only by membrane cleaning under A/O mode. Mixed liquor characteristics must be controlled to reduce the amount of fine particles and slow down the worsening rate of internal fouling.Comparative experiments on effect of dosing powdered activated carbon (PAC) to mixed liquor characteristics were performed. The influence of PAC on contamination removal efficiencies, mixed liquor characteristics, filtration resistance and membrane fouling was investigated. The results showed that the anti-shock loading capability was improved by PAC in MMBR. The mixed liquor characteristics was also modified by PAC as increased mean particle size, enhanced intensity of floc, lower concentrations of fine particles, soluble substance and extracellular polymeric substance (EPS), and lower viscosity. Therefore membrane fouling was well controlled, which resulted in slower increasing rate of TMP and long operation life of MMBR.Because of the environmental damage due to NaOCl used in membrane cleaning, green and safe membrane cleaning reagents were needed. Ozone was used as a new membrane cleaning reagent to control and remove membrane fouling in the study. On-line back washing and off-line washing with ozone solution under different concentration, pH, washing time and washing frequency were conducted. At last an effective membrane cleaning method with ozone solution was established. Finally a comparative study between stainless steel membrane and polyethersulfone (PES) membrane was carried out. The stainless steel membrane showed lower membrane intrinsic resistance and higher critical flux. In the treatment of municipal sewage, stainless steel membrane and PES membrane showed almost the same contaminant removal efficiencies, but the stainless steel membrane indicated higher anti-fouling ability and slower TMP increasing rate. Combined with other advantages such as high intensity, strong corrosion resistance, good antioxidation, convenient preservation at dry state and wide utilization scope, the stainless steel membrane must have a promising future in wastewater treatment. |
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