Characteristics Of Nitrogen Removal From Domestic Sewage With Low Carbon To Nitrogen Ratio Using Anoxic/Oxic Biofilm Process

Recently, the level of nitrogenous pollutants in rural sewage has tended toincrease with the lifestyle changes of residents in China’s rural area. For purpose ofalleviating eutrophication problem, biological nitrogen removal （BNR） is of greaturgency. With regard to some novel BNR technologies such as shortcutnitrification–denitrification and anaerobic ammonium oxidation with both of highmanaging demand and poor stability, conventional nitrification–denitrificationtechnology still is the most appropriate way for nitrogen removal from rural domesticsewage. Considering the development of China’s rural areas and the trend of lowcarbon to nitrogen ratio of rural sewage, the modification and optimization fortraditional BNR to realize nitrogen removal with high efficiency, low costs and easymaintenance and to reuse biomass are significant.This work combines biofilm technology and anoxic/oxic （A/O） process, andaccording to the principle of decentralized treatmet for rural sewage and therequirements of clean and sanitation, a plug-flow integrated reactor incorporatinganoxic fixed bed biofilm reactor （FBBR）, oxic moving bed biofilm reactor （MBBR）and small settler sequentially is proposed. The main research contents and resultsconcluded during a603-day period are as follows:（1） The influence of different carrier packing rates to the fast start-up of reactorwas investigated. The start-up period under polyurethane foam packing rate of45%inFBBR and polypropylene hollow ring packing rate of20%in MBBR is27days; andno sludge return was available after optimization with a22-day period after start-up.（2） Nitrogen removal performace of the reactor treating domestic sewage withlow C/N ratio was optimized. When the influent C/N ratio is of2.5-4.0, controllingthe system N load of0.11kgN/（m³Â·d） and SRT in MBBR of42±4days wereconductive to the formation of nitrifying biofilm and sludge stabilization in oxic area.Relatively long anoxic hydraulic retention time （HRT） of7.2h promoted endogenousdenitrification in FBBR to reduce anoxic observed sludge yield. Intermittent wastedsludge discharge simplified the operation and maintenance, which achieved simplenitrogen removal. Under above circumstance, TN was removed averagely by69.3%atleast under internal recycling ratio （R） of200%, and R of300%could realize effluentTN concentration below15mg/L, which met class A discharge standard of municipal wastewater treatment plant （GB18918–2002） and achieved nitrogen removalefficiently. The economical potentials were tapped due to low internal recycle andcancellations of sludge return and anoxic stir.（3） The operating strategy for enhanced nitrogen removal from rural sewage bysimultaneous nitrification-denitrification （SND） was proposed to the A/O biofilmprocess. TN with proportions of37.7–42.2%in influent was removed by SND underR of200-350%, and the formation of anoxic micro-environment by biofilm andefficient utilization of the carbon source in raw water were the key factors. SND wasless affacted by temperature, the system had capacity to realize simultaneous nitrogenremoval at0.353mgN/（L·h） under temperature of16±0.5℃, DO concentration of3.5±0.5mg/L and C/N ratio of2.5-3.3.（4） The change of nitrogen removal pathway in oxic MBBR under long-termoperation was proved. Anaerobic ammonium oxidation enhanced nitrogen removalfrom real domestic sewage with low C/N ratio alternatively besides SND.（5） Anoxic wasted sludge of the reactor was utilized as microbial carrier inheterotrophic partial denitrification （NO₃^--Nâ†'NO₂^-N） system to investigate itsfeasibility of resource. During denitrification in case of using acetate as carbon source,nitrite accumulation ratio of60%was realized stably by step feed with pH indicationunder batch-flow mode with rapid biodegradable COD to nitrate ratio （RBCOD/NO₃^-）of2.7, which laid the foundation of partial denitrification system in practicalapplication. Alkaline environment with pH of9.0-9.6repressed N2O emission duringendogenous denitrification.（6） Biofilm of the oxic MBBR was utilized as microbial carrier in pre-aerationcoupling partial nitritation-anammox system to investigate the feasibility of reusingoxic biofilm in the reactor. Under condition of treating micro-polluted water, lowdemand of MLVSS concentration and biofilm process with long SRT were able toincrease α factor （the ratio of oxygen transfer coefficient （kLa） between wastewaterand clean water）, which saved the energy consumption of aeration. When the meanNH₄⁺-N and TN concentrations in influent were of2.97and6.52mg/L, respectively,mean TN removal efficiency of35.3%could be reached by controlling MLVSSconcentration of800mg/L and reacting HRT of0.67h.