Study On The Efficiency And Mechanism Of Advanced Phosphorus Removal By Micro-flocculation Enhanced Biofilm Filter

Nitrogen and phosphorus are the main pollutants causing eutrophication in water bodies.The controlling of phosphorus by wastewater treatment plants has been proved to be one of the most cost-effective methods for preventing eutrophication.Therefore,in april 2015,the state council has promulgated "The action plan for prevention and treatment of water pollution" andrequired the first level A emission standard in Discharge standard of pollutants for municipal wastewater treatment plant(GB 18918-2002)of the effluent from sewage treatment plant.Currently,the process of biological removal of phosphorus is too long and it’ s difficult to reach the discharge standard.While the removal of phosphorus that only depends on chemical methods will consume a high amount of chemicals.In this thesis,an effective method of biofilm process combined chemical augmentation was used to enhance the phosphorus removal efficiency of the biofilm filter by micro-flocculation to achieve advanced phosphorus removal.The target wastewater were the urban sewage effluent with low organic substrate concentration and poor nutrient.First,the optimum selection tests of flocculant and hydraulic conditions were carried out.The microflocculation conditions were optimized to reduce internal carbon sources consumption in the process of phosphorus removal.Then,the effect of phosphorus removal by micro-flocculation on the biofilm filter based on continuous flow experiment were studied and the efficiency of three kinds of phosphorus removal methods were compared.Thereafter,the process parameters of the modified system were optimized and the operation performance was further analyzed.Lastly,the mechanisms and dynamics of the system were revealed.The main results and conclusions are as follows:The influencing factors of micro-flocculationand on phosphorus removal and its effects on biofilm filter were studied.The results showed that surface fraction PAC had the best phosphorus removal effect,followed by PFS;the influences of phosphorus removal by microflocculation was optimized by the response surface methodology(RSM).The results indicated the optimal conditions(the PAC dosage is 20.02 mg/L,the stirring intensity is 149 r/min,and the stiring time is 12 min would have the highest removal rate of TP together with the lowest COD removal rate.It is only need 12 mg/L of PAC and 15 mg/L of PFS to achieve the high efficiency of TP,which reduced by 40%of the orginal agent consumption.In addition,the addition of PAC and PFS had no significant effect on the removal of nitrogen and COD.However,the backwashing interval of the biofilm filter shortened from 7d to 4d.It is recommended that PAC could be used for phosphorus removal when chemicals was added outside the filter column.The synergistic phosphorus removal efficiency was significantly higher than that of chemical process superposed with biological phosphorus removal.Thus it is feasible to apply the biofilm filter enhanced by micro flocculation process for advanced phosphorus removal.The operational parameters of the biofilm filter enhanced by micro-flocculation were selected and the operation efficiency of the optimized process was analyzed.The results showed that all indexes of the effluent could reach the first class A discharge standard during the whole operation period with the average removal rates of COD,TN,NO3--N,TP,and PO43-P were up to 78.9%,63.5%,88.9%,63.6%and 71.0%,respectively;the water yield of the biofilm filter was 1520 L,with the backwashing water consumption of only 6.72 L,accounting for 0.44%of the water yield;compared to chemical phosphorus removal,the consumption of chemicals of the enhanced system reduced by 40%of PAC and 40.2%of PFS,and the price of water per ton was 0.014 yuan cheaper of PAC and 0.014 yuan cheaper of PFS.Based on the study above,the mechanisms and kinetics of advanced phosphorus removal of the biofilm filter enhanced by microflocculation were studied.The results revealed that the charge neutralization and adsorption bridging action both existed in the process of micro-flocculation.The micro flocs contained a large amount of aluminum hydroxyl,which could effectively utilize the filter layer and increase the chance of interrelate collision when they passed through the biofilm filter.Besides,on the filter surface,the micro flocs could also interact with microbes whose surface were possessed by negtive charge,promoting the bio-chemical synergies to improve phosphorus removal;in the process of filtration.The kinetics of phosphorus removal followed a 1/2 kinetic reaction model and of nitrogen removal followed the first-order kinetic reaction model.