The Purification Efficiency And Sludge Characteristics Of The Moving-bed Biofilm Multi-stage A/O Process

At present,the urban sewage treatment mainly use biochemical methods,includingactivated sludge and biofilm.Biofilm purify the pollutants by the microorganismgrowing attached on it.Compared with the activated sludge,it has someadvantages,including diversity of the microbial phases,long length of the food chain,tosurvive longer generation microorganisms;large number of microorganisms,thepurification function significantly improved,good sludge settling performance;easy tosolid-liquid separation;less excess sludge production can reduce the sludge treatmentand disposal costs; resistance to shock loading, strong adaptability to changes in waterquality and quantity,able to deal with low concentrations sewage;easy operation andmanagement,reduce the bulking problem.Traditional biofilm wastewater treatmentprocess include biological filter, rotating biological, biological fluidized bed andbiological contact oxidation. Moving bed biofilm reactor is a new technology developedon the basis of the traditional biofilm.Its advantdges are no clogging, no backwash,continuous operation,improving the sludge concentration with no increasing of reactor’svolume,easily and simply appling to the transformation of existing wastewater treatmentplant. Step feed A/O process will be a combination of A/O process and pointsinfluent.It is without nitrification liquid reflux and can make full use of the carbonsource.The process will combined moving bed reactor and step feed A/O process.Takinginto account the generation time difference of the autotrophic nitrifying bacteria andheterotrophic denitrifying bacteria,suspended filler is dosed in the aerobic zone,which isconducive to the autotrophic nitrifying bacteria growth.The select of suspended filler isvery important.It should meet the requirements of good water mechanical property,highmechanical property,stable biological, chemical and thermodynamic property,conduciveto biofilm attachment,moderate proportion,low cost.The test research the start-up of thereactor,analyse the removal effect of COD,ammonia,TN and TP in differentcinditions,and analyse the biomass,oxygen uptake rate,sludge apparent form,sludgereduction and microbial community structure.The test complete start-up in29days by using closed-loop method.After16days’continuous culture,biofilm mass is about920mg/L, MLSS is about1935mg/L andMLVSS/MLSS is about0.85.Analysing the pollutants removal effect is by four cinditions. Influent distribution ratio is6:3:1, the anoxic zone and aerobic zone volumeratio is1:2,design SRT is13d,design HRT is8h,filler dosing ratio is40%and sludgerecycle ratio is100%in case1. Influent distribution ratio is5:3:2, the anoxic zone andaerobic zone volume ratio is1:2in case2. Influent distribution ratio is4:3:3, the anoxiczone and aerobic zone volume ratio is2:3in case3. Influent distribution ratio is5:3:2,the anoxic zone and aerobic zone volume ratio is2:3in case4.COD and ammonianitrogen removal effect is better and the average effluent concentration can achieve an Astandard in four conditions.Case4is best in TN removal and the average effluentconcentration can achieve an A standard;TP removal effect is limited in fourconditions.By estimate the carbon source used in assimilation, oxidation, nitrification,denitrification and phosphorus removal in different conditions,the highest CODproportion used in denitrification is35.91%in case4which is also best in carbonsource utilization.The highest COD proportion used in oxidation is48.09%which wastethe carbon source.COD used in phosphorus removal is low in different conditions whichis only3.05%to4.00%.The carbon source used in phosphorus removal doesn’t fullyutilized and it is also one of the reasons that phosphorus removal is ineffective.The concentration of activated sludge and biofilm was progressively decreasingalong the reactor.There are a large number of filamentous bacteria which form a densemesh, spherical bacteria which is coated by viscous extracellular polymer and porestructure which extend into the biofilm on the biofilm surface.A large number of biofilmsurface to form a dense mesh of filamentous bacteria,viscous extracellular polymercoated spherical bacteria within the biofilm extension of the pore structure.Activatedsludge oxygen uptake rate (OUR) show the trend that increase in the anoxic zone andreduce in the aerobic.The contribution of biofilm to OUR is only25.7%to38.76%,lower than activated sludge.It describes that activated sludge oxidation toorganic matter is greater than biofilm oxidation to organic matter,which is very differentto the traditional biofilm reactor.The average sludge yield is0.15KgMLSS/kgCOD incase4which is only about1/5of the conventional activated sludge process.Sludgereduction effect is obvious.There may be three reasons,the first is that the biofilm’salternating aerobic-hypoxia-anaerobic environment from outside to inside leads to theoccurrence of uncoupling metabolism,the second is that long sludge age of the biofilmis favorable to organisms lysis,the third is that biofilm is favorable to the formation ofmore types of micro-organisms and long food chain and improve the role of biologicalpredator.The process has large number of microbial species and obivious dominant species. Shannon-Weaver index is about3.00,and the biofilm diversity index slightlyhigher than the activated sludge.It shows a high degree of microbial diversity and moretypes of microorganisms in the biofilm.Similarity coefficient of different reacting zonesis more than70%,indicating that microorganisms in activated sludge and biofilm ismostly similar.Similarity coefficient of biofilm is more than90%.Biofilm’s similarcoefficient is significantly higher than activated sludge,which is related to the higherdegree of microbial flora diversity in the biofilm.