Municipal Secondary Effluent Treatment By A/O Biofiltration Enhanced Physical-Chemical Methods

The shortage of water resources and the pollution of water environment are very serious in China.We are facing the multiple pressures of the increasing water demand,serious shortage of water resources and the deteriorating water quality.Strengthening the advanced treatment of wastewater and utilizing renewable water sources can not only reduce pollutant discharge effectively,but also save water resources,promote the contribution of ecological civilization and guarantee the sustainable development of economy and society.Municipal wastewater has less pollution,better water quality and low treatment difficulty.The potential of resource recovery and utilization is enormous.However,most municipal wastewater treatment processes are still unable to meet the requirements for the reuse of treated effluent.Therefore,it is significance to develop advanced and high efficiency technology for the reuse of secondary effluent of municipal sewage with less energy,more simple process and smaller installation area.Based on the above,this paper intends to systematically analyze the characteristics of typical secondary effluent of municipal wastewater,focusing on the removal effect and the law of pollutants by different biochemical and physicochemical technologies such as A/O biological filtration,advanced oxidation and high-efficiency membrane separation.In order to provide technical reference for the research and development of sewage treatment and its resource utilization,the paper research and develop the key technologies and processes for the secondary effluent of municipal sewage.A/A/O and oxidation ditch are the main processes of sewage plant at present.The quality of these secondary effluent are better than AO process obviously,but the secondary effluent of these three typical processes has a greater common characteristic obviously.The main form of organic matter in secondary effluent of three typical processes is dissolved state,accounting for 75%-85%,and seasonal variation is not obvious.The B/C value of secondary effluent is less than 0.3,which is not conducive to the subsequent biofiltration process.The water quality indexes of the secondary effluent from A/A/O and oxidation ditch process basically meet the first A standard except for a few days,but the total nitrogen and ammonia nitrogen content of the effluent from A/O process are still very high and the water quality is unstable.In order to solve this problem,the secondary effluent of a A/O process was selected as the water for subsequent test.Based on the above analysis of water quality characteristics,the further purification of municipal sewage secondary treatment effluent should focus on the control of organic matter,nutrients and turbidity.Biofiltration technology has comprehensive treatment capabilities such as biological oxidation,filtration and biological flocculation,and can be used as the core unit of municipal secondary effluent advanced treatment and reuse process.Slag composite filter material has good adsorption performance for ammonia nitrogen and phosphorus,and is suitable for treating sewage as A/O biological filter material.In the way of synthetic wastewater and natural film hanging,according to the start-up procedure of single aerobic hanging and anoxic/aerobic acclimation,the start-up of A/O biofiltration system was completed within two months?55d?.The removal rates of COD and ammonia nitrogen in secondary effluent of municipal sewage were stable at 60%and 95%,respectively.Under the typical steady-state conditions of continuous operation,the average removal rates of COD,TOC,ammonia nitrogen,TN,TP and turbidity of A/O biofiltration system were 63.1%,61.9%,96.9%,48.5%,41.03%and 90%,respectively.The average removal rates of COD,TOC,ammonia nitrogen,TN,TP and turbidity were 23.27 mg/L,1.52mg/L,1.49 mg/L,12.67 mg/L,1.97 mg/L and 1.3 NTU,respectively.The effluent indexes except total phosphorus are superior to the first A standard.The effects of temperature and reflux ratio of digestive juice on organic matter treatment in A/O biofilter reactor were investigated.When the influent temperature drops from 25?to5?,the average COD removal rate of the treatment system decreases nearly 30%from63.58%to 33.97%.While the average COD removal rate can be maintained at about 51.27%when the temperature of influent is about 15?.Temperature also has a great influence on the removal efficiency of ammonia nitrogen,when the influent temperature drops from 25?to15?,the average removal rate of ammonia nitrogen decreases from 91%to 83%.However,the removal efficiency of ammonia nitrogen in the A/O biofilter can still reach more than 80%even at low temperature of 5?,the average concentration of ammonia nitrogen in the effluent is 3.37 mg/L.The removal efficiency of COD,ammonia nitrogen and TN in A/O biofiltration system can be significantly improved by increasing the reflux ratio of mixed liquor.The reflux ratio of mixed liquor increased from 100%to 200%,the average removal rates of COD,ammonia nitrogen and TN increased by 30%,11%and 15.2%respectivelyThe advanced treatment of municipal secondary effluent by A/O biofiltration system can achieve good removal effect of organic matter and ammonia nitrogen,but the removal effect of phosphorus is not good.If appropriate enhanced physicochemical methods are added to complete phosphorus removal and further improve the biodegradability of secondary effluent of municipal wastewater,it will not only ensure that organic matter removal meets the standard,but also improve the denitrification carbon source conditions,thereby improving the total nitrogen removal rate.Therefore,the effects of physical and chemical methods such as enhanced coagulation,ozone oxidation,ferrate oxidation and plate ceramic membrane filtration on municipal wastewater secondary effluent treatment were also discussed.Polyaluminium chloride?PAC?,polyaluminium sulfate?PAS?and polyferric sulfate?PFS?coagulants have enhanced organic matter removal effect on the secondary effluent of municipal sewage.Under the weak alkaline condition?pH=8?,the best COD removal effect can be obtained by three coagulants.Among them,PFS is superior to PAC and PAS;the removal rates of COD by PFS,PAC and PAS are 39.84%,35.25%and 35.65%,respectively;the optimal dosage of PAC,PAS and PFS are 120 mg/L,80 mg/L and 120 mg/L,respectively.Ozone oxidation can remove part of the organic matter.In the range of 0-10 mg/L,the dosage of ozone has a linear relationship with the removal rate of COD.With the increase of ozone dosage,the ability of degradation is limited.The removal effect of ozone on UV₂₅₄ in secondary effluent is remarkable.The removal rate of UV₂₅₄ is positively correlated with ozone dosage within the ozone dosage range from 0 to 15 mg/L.Ozone oxidation can destroy most organic compounds with C=C,C=O structure in water,biodegradability is also effectively improved.Within the experimental concentration range,ozonation has limited removal efficiency of ammonia nitrogen and phosphorus in water.Potassium ferrate oxidation can remove organic matter from secondary effluent,remove phosphorus and turbidity through flocculation simultaneously,and improve biodegradability of secondary effluent.But it can not remove ammonia nitrogen and TN.The flux of flat ceramic membranes by clean water can reach 552.9 L/m²·h,and the sub-critical flux is 220L/m²·h.Ceramic membrane has a good effect on turbidity removal of the secondary effluent,and the removal rate is basically stable at 93%-96%.According to the analysis of secondary effluent and the test results of intensified unit,the combined process route and key parameters of?Pre-oxidation-A/O biofiltration-flat ceramic membrane‘were proposed for the advanced treatment of secondary effluent from municipal wastewater.The continuous operation test show that the quality of effluent from combined process is good and get the requirements of municipal miscellaneous water quality standard?GB/T18920-2002?and reclaimed water quality index?GB12941-1991?for landscape environmental water.