Gravity-Driven Membrane System Treating Iron And Manganese Containing Groundwater

Underground water was the main sources of potable water in rural areas,the main type of water supply is the decentralized drinking water supply.The state of groundwater pollution is serious,iron,manganese,ammonia nitrogen and pathogenic microorganisms threaten the safety of water supply.Conventional water treatment technology is difficult to be popularized in rural areas due to its problems such as complicated operation,unstable treatment efficiency and large operation and maintenance.Therefore,it is very necessary to develop the new treatment technology suitable for the decentralized water supply project in rural areas.Combined with the process characteristics of ultrafiltration and filter tank,Gravity-driven membrane(GDM)system has been developed and applied to groundwater treatment to remove iron,manganese,ammonia nitrogen.In view of the absence of flushing and the lower TMP,the energy consumption of GDM is considerably lower than for conventional UF.Since the GDM does not need supervision,no skilled personnel is required.The operation & maintenance(labor)costs are therefore are expected to be lower than in conventional UF.This study comprehensively explores the efficiency of iron,manganese,and ammonia nitrogen removal in GDM system,analyzes the flux stability mechanism of GDM system,and forms the rapid start-up strategy of GDM system.At the beginning of the GDM system,it can remove iron efficiently,and the iron content in the effluent is less than 0.1 mg/L.The removal of iron ions is mainly through chemical catalytic oxidation,and the product of oxidation(ferric hydroxide flocculant)is intercepted and separated by ultrafiltration membrane.In addition,the retention of iron oxide on the membrane surface also contributes to the formation of filter cake layer on the membrane surface.With the prolongation of the operation time,a bio-cake layer with efficient manganese catalytic oxidation developed on the membrane surface,and the manganese content in the effluent significantly decreased(<0.1 mg/L).The material of ultrafiltration membrane does not affect the startup time of GDM system,and the addition of manganese oxide can shorten the startup time of GDM system,after 35 days of operation,the manganese concentration in the water was less than 0.1mg/L(the concentration specified in the Sanitary Standard of Drinking Water(GB5749-2006)).GDM system can effectively remove particulate matter,suspended matter and colloids in the water,and the turbidity of the effluent is less than 0.1 NTU.It also has a certain effect on the removal of natural organic matter,the removal rate is about 15%.Under the condition that the water temperature is about 6.8 ? and the dissolved oxygen is about 8.5 mg/L,after the stable operation of the GDM system,the limit removal concentration of manganese and ammonia was 3.2 mg/L and 0.85 mg/L respectively.Under the condition of iron and manganese pollution,the flux stability of GDM system in long-term operation is the key to the successful application of this technology in groundwater iron and manganese removal.The results show that the flux of GDM system can reach a stable state in long-term operation,but it takes a relatively long time for the outflow flux to reach a stable state,and the stable flux level also decreases slightly.The amount of microorganisms and inorganic particles in the groundwater is small,the water temperature is low,and the formation time of biofilter cake layer is relatively long and the structure is relatively compact.The addition of manganese oxide in GDM system,increases the roughness,porosity and thickness of the cake layer and the stable flux value.The material of ultrafiltration membrane also has an influence on the stable flux of GDM system.The stable flux of GDM system made in PVDF material is higher than that of PES material.In the GDM system,EPS content in the membrane surface filter cake layer is at a low level,the higher the ATP concentration in the membrane surface cake layer of GDM system,the higher the stable flux of GDM system.In addition,the composition analysis of the cake layer in GDM system shows that the cake layer contains not only a large amount of iron and manganese oxides,but also iron bacteria,manganese oxidizing bacteria and nitrifying bacteria.The mechanism of iron and manganese removal is mainly chemical oxidation,while biological oxidation is auxiliary;the mechanism of ammonia nitrogen removal is mainly biological oxidation,and chemical catalytic oxidation may exist.In this study,the GDM system can be successfully applied to groundwater synchronous removal of iron,manganese,ammonia nitrogen,turbidity,and other pollutants,forming a rapid start-up strategy and revealing the flux stability mechanism and membrane fouling characteristics of the GDM system.The study provides theoretical basis and technical support for GDM system applied to decentralized drinking water supply in rural areas.