Development And Operating Performance Of Anaerobic Acidification Integrating With Two-stage Forward Osmosis Process

Currently,activated sludge process is the dominant technology for municipal wastewater treatment,which not only consumes a lot of energy but also emits carbon dioxide.Anaerobic membrane bioreactor（An MBR）,as an upgraded anaerobic digestion process,has several advantages including high resistance to organic loading,low waste activated sludge production,low energy consumption and biogas recovery compared with the traditional activated sludge process.However,An MBR has a low methane recovery rate during municipal wastewater treatment due to the low organic matters concentration of municipal wastewater and the high solubility of methane at room temperature.In the meanwhile,volatile fatty acids（VFAs）,an intermediate product of anaerobic digestion process,can be applied not only as a high-quality carbon source for nitrogen and phosphorus removal but also as the raw material for the synthesis of biofuels and biopolymers.In this case,development of processes for recovering VFAs from municipal wastewater is one of the feasible routes geared toward the synergistic development of pollution reduction and carbon reduction.However,one of the prerequisites for anaerobic acid production to be feasible is the use of highly concentrated waste as substrate.Therefore,how to enrich organic matters in municipal wastewater before VFAs recovery is a key step in the recovery of organic matters from municipal wastewater via anaerobic digestion technology.Forward osmosis（FO）is a novel membrane separation technology that uses the osmotic pressure difference between the solutions on both sides of the membrane as the driving force.Compared with other pressure-driven membrane technologies,FO has some significant advantages including low energy consumption and low tendency of membrane fouling.Based on the characteristics of FO technology,it has been applied in municipal wastewater concentration,and the concentrated municipal wastewater is used for recovering organic matter and nitrogen and phosphorus.Based on this fact,a novel technology combining anaerobic acidification and two-stage FO concentration,i.e.,FO-AAMBR-FO,is proposed in this study for simultaneous recovery of organic matters,nitrogen and phosphorus from municipal wastewater.Since this is a new process proposed in this study,there are no related reports on this process.Thus,the aims of this study is to evaluate the feasibility of the FO-AAMBR-FO process for municipal wastewater treatment,and to optimize the operational performance.The main research contents and conclusions are summarized as follows:（1）Impacts of draw solution concentration and p H value of feed solution on the performance of FO membrane for concentrating municipal wastewater were investigated.The results indicated that the FO membrane achieved a higher membrane flux and concentration efficiency when the draw solution concentration was 4 M Na Cl solution and under controlled salt conditions.The COD loss during municipal wastewater concentration was mainly attributed to pipeline deposition and microbial utilization in the concentrated solution.The optimal conditions for concentrating municipal wastewater by FO membrane were 4 M Na Cl solution as the draw solution with controlled salinity and feed solution p H of 7.5,and the concentrations of COD,NH₄⁺-N and TP of municipal wastewater reached 2866.99±80.29 mg/L,196.95±19.7 mg/L and 33.26±3.6 mg/L at this condition,respectively,with a COD recovery of 81.27%.（2）The feasibility of VFAs recovery from municipal wastewater via AAMBR was evaluated based on the construction of a lab-scale AAMBR.The results showed that the AAMBR achieved stable production of VFAs from municipal wastewater concentrate with the main components of acetic acid and propionic acid during a stable operation of 46 d.During the operation of AAMBR,the MF membrane flux was stable at 2.25 LMH,and the corresponding TMP was always lower than 0.3 k Pa,implying the membrane fouling mainly including organic fouling and biofouling was light.The dominant acid-producing bacteria in the AAMBR reactor were Firmicutes and Actinobacteriota.（3）The optimum conditions for enrichment of VFAs in the MF membrane effluent by FO membrane were investigated,and the nitrogen and phosphorus recovery from MF effluent concentrate was carried out by guano stone precipitation method.The results showed that when the draw solution was 4 M Na Cl solution,the feed solution p H was 5.5 and the feed liquid volume was concentrated by 2 times,the VFAs concentration after the FO membrane enrichment was above 3012 mg COD/L,which met with the requirements for the synthesis of polyhydroxyalkanoates（PHA）.The optimal operating conditions for guano precipitation were Mg:P:N=1.72:1.15:1 and a stable solution p H of about 10.Under these conditions,the recoveries of nitrogen and phosphorus reached 94.53%and 98.59%,respectively.（4）The carbon,nitrogen and phosphorus streams in the whole FO-AAMBR-FO process were analyzed.The results indicated that the recovery of organic matters and nitrogen and phosphorus from municipal wastewater by the FO-AAMBR-FO process reached 44.86%,40.75%and 35.45%,respectively.