| Rural sewage treatment in recent years in large scale has brought about great improvent in environment quality in China.However,long term maintainance of these facilities showed a serious problem,that is,the low phosphorus removal efficiency.An experiment in this thesis was carried out in 4 districts in Zhejiang Province to investigate phosphorus removal efficiency and phosphorus forms in influx and efflux in rural domestic sewage.The result revealed that the phosphorus removal rate in average was only 19.3%,and phosphorus in both influx and eflux was mainly in the form of dissolved state,which indicated that removal of dissolved phosphorus was the key factor enhancing phosphorus removal efficiency.Our experiment also showed that addition of biofilter or constructed wetland to A/O process with a low hydraulic loading rate could increase phosphorus removal efficiency.The current investigation showed that there are two strateges to improve the efficiency of phosphorus removal,which are improving constructed wetland and addition of phosphorus removal agent whith slow release mechanism.The results runn as follows.The sewage treatment systems comprising of constructed wetlands with high hydraulic load and low phosphorus removal efficiency were screened for our study.A stratege combining the merits of both constructed wetland and bio filter was practised,and a new mixed flow constructed wetland-biofilter composite treatment process(MFWB process)was proposed.The improved system included new types of bio-media with a specific arrangement in the reactor which had high efficiency in N,P,and CODcr removal,an aeration system to improve ORP status,and a water circulating and distributing system to increase the efficiency of biomedia.An improved system with a scale of 5t/d running for 10 months showed a great enhancement in CODcr,NH3,TN and TP.Neverthless,analysis of the structure of the reactor indicated that the removal rate of TP and TN could still be improved further.A newly designed MFWB was employed to a system with 30t/d in scale,and much higher removal efficiency of CODcr,NH3,TN and TP were observed,and the parameters of effluent stably met the Class B emission standard.Analysis of the microbial diversity of MFWB unit fillers demonstrated that the dominant microbial species of different packing modules varied greatly.It is speculated that the MFWB unit realized the partitioning of pollutant removal function.In this thesis,a slow release phosphorus removal agent was developed to improve long-term phosphorus removal efficiency and decrease the labor cost in maitainance.Reagent A was used as the coating material,and Reagent B was used as the dephosphorization agent to prepare the slow and sustained release phosphorus removal agent.The study was focused on optimum processing conditions for preparing the slow-release phosphorus removal agent.The result showed that Reagent A and Reagent B were stirred for dispersion at the ratio of 1:1 and the mixture was shaped into a cylinder and then recoated by Reagent A.The product immerged in water showed a release period of Fe in 23 days.The release curve conforms to the zero-order release model and the release rate is constant and stable.The slow-release phosphorus removal agent has a strong and stable phosphorus removal effect.When the slow-release phosphorus removal agent is added at a molar ratio of Fe:P=3:1,the effluent TP concentration can meet the first-class A emission standard.The stable release period could reach up to 15 days or more without any increase in CODcr was observed.Our experiments gave a conclusion that process improvement and application of slow release phosphorus removal agent is highly applicable to improve phosphorus removal effect of rural domestic sewage with low labor cost. |
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