Study On Choose And Optimize Of Extra Carbon Source For Denitrification

At present, in most WWTP of China, especially for the southern cities, carboncontent in sewage is observed to be low and become the main factor influencingbiological denitrification efficiency. Herein extra carbon source supplement should beconsidered to provide electron donors needed in biological denitrification process.Methanol is the most desirable extra carbon source used in WWTP, however its highcost, toxicity and transportation problem urged researchers to explore new types ofcarbon source. Therefore, this study concentrated on denitrification characterization ofvarious carbon sources and improved new types of carbon sources. Comparison andchoosen of various carbon sources were studied by lab scale test, and process ofoptimization of new types of carbon sources were investigated. Then selective carbonsources were applied to pilot continues A~2/O reactors on site to explore theirdenitrification effect on real condition.In order to provide theoretical basis for optimal selection of carbon source,denitrification process with several carbon source, i.e. sodium acetate, leachate,hydrolyzed sludge, and starch solution, were studied in SBR batch reactors,denitrification rate and carbon source consumption rate were calculated and kineticsof denitrification were analyzed. It is observed that degradation of nitrate and carbonsource underwent stepwise process. During the initial phaseⅠ(averagely 20mins),concentration of nitrate decreasely sharply with high removal-60%-87%, andconcentration of nitrite accumulated continusouly up to 10-20mg/L. Duing thefollowing phase II (averagely 50mins), degradation rate of nitrate declined largely,and concentration of nitrite became decreasing until nitrate and nitrite were degradeddrastically. By analyzing degradation process of denitrification through piecewisezero-order kinetic model, results showed that the denitrification rate of phase I withsodium acetate as carbon source was the highest, which was 0.835g N/g VSS·d,carbon source consumption rate of initial phase with fresh leachate as carbon sourcewas the highest, which was 5.853g TOC/g VSS·d. For denitrification process witheach carbon source, denitrification rate and carbon consumption rate of phase I werehigher than that of phaseⅡ, and during phaseⅡthe demand of carbon source waslarger. Additionally, variations of nitrate and nitrite during denitrification process withexternal carbon source could be well described by differential equations model based on Monod equations. The obtained kinetics parameters could determine the efficiencyof carbon source, nitrate and nitrite degradation rate of with sodium acetate as carbonsource was the highest, which was1.330g N/g VSS·d and 1.120 g N/g VSS·drespectively.Optimization process of hydrolyzed recycled sludge and straw as carbon sourceswere studied. The results showed concentration of VFAs, whose main component wasacetate, increased with hydrolyzed time (HRT). For product under hydrolyzing 8 dyas,total VFAs accountted for 37.7% of SCOD, and acetate occupied 18.2%, of SCOD. Inthe bath denitrification process with the carbon source of hydrolyzed recycled sludgeunder 8 days hydrolyzing, nitrate degradation rate of phase I reached 0.759g N/gVSS·d, which is second only to that of sodium acetate and higher than that of anyother else. In addition, denitrification process with straw as carbon source werestudied in plug flow biofilm reactor and mixed flow suspended reactor respectively.The denitrification effect of plug flow biofilm reactor appeared better. Under HRT is42.3h in plug flow biofilm reactor, nitrate concentration of inflow was 37.5mg/L andthat of effluence was 5.1mg/L, with nitrate removal 86%. Nitrite concentration wasalways below 1 mg/L during the process. Nitrate concentration of effluence and nitrateremoval increased with HRT prolonged, denitrification rate correlated HRT in relationof negative logarithm. While in mixed flow suspended reactor, nitrate were firstlydegraded through absorbing and endogenous denitrification, carbon source thatdissolve out from straw was little and there was no obvious denitrification effect.Based on research in the lab, continues A~2/O reactor with external carbon sourceof sodium acetate and leachate on site was operated to check the nitrogen removaleffect of the new types of carbon source on real condition, and the effect of new typesof carbon sources were comparied with that of traditional methanol. It is observed thatthe TN removal of system with external carbon source was improved by 15.1%averagly, TN of effluent decreased below 10mg/L which meet the standards firstclass type A of "Discharge standard of pollutants for municipal wastewater treatmentplant"(GB 18918-2002). By comparison nitrate degradation rate between differentdosing quantities of various carbon sources, nitrate degradation rate was the highestwhen dose of sodium acetate is 60mg/L, which is 0.0107mg TN/mg MLVSS·d. Theobtained kinetics parameters could provide reliable basis to technical process design.By economy and technology analysis, leachate was proved to be pragmatic externalcarbon source whose cost was far cheaper than methanol.