| Arbitrary discharge of untreated sewage in rural areas is one of the main reasons forwater pollution and eutrophication of water bodies. Rural domestic sewage has manycharacteristicsgenerally such as discharging dispersion and widely varied water quality andquantity. Restricted by conditions of the rural economy, the adopted technology of themunicipal sewage plant process and operation management mode is not applicable to the ruralsewage treatment. Characteristics of wastewater discharging and quality and actuality ofregional water pollutionwere investigatedin this study. The traditional step-feed Anoxic/Oxic(A/O) biological nitrogen removalprocess had improved to be suitable for low C/N ratioruralsewage treatment. Nitrogenremoval efficiency of the modifiedstep-feed Anoxic/Oxic (A/O)biological nitrogen removal processwas studied combine with the theoretical model and theartificial water distribution test research. The following conclusions were drawn:(1)Gutian town, a typicaltown in Fujian Province, was selected as the study area,wherethe characteristics of wastewater discharging and quality were investigated.Resultsrevealed that the domestic sewage had characteristics of high nitrogen and phosphorusconcentrations (TN=113.3mg/L, TP=9.58mg/L), and low carbon/nitrogen ratio(C/N=3) due tosmall amounts of organic matter.The discharging rule of sewage diurnal variation was closelyrelated to daily timetable of rural residents. Chemical oxygen demand dischargeis6.351~19.053Kg/p·a, and total nitrogen discharge is0.621~1.643Kg/p·a.Untreated sewagedirectly discharged into the stream which surround village.With the inflow of wastewater,receiving waterbody was deterioratedwhich showed higher nitrogen and phosphorus contentcompared with uncontaminatedupstream.The mountain spring got into wastewaterditchattributed to this regionwas lack of perfect wastewater collection and treatmentfacilities.Sewage treatment difficulty increases with the flow in wastewater ditchwasincreased and the pollutant concentration was diluted.(2)Theoretical model of the modifiedstep-feed Anoxic/Oxic (A/O) biological nitrogenremoval process was set up base on the principle of organic matter in raw water wascompletely as denitrification carbon source. Expressions of influent flow distribution ratio andthe TN removal rate was deduced. Influent flow distribution ratio contacted with C/N ratio ofinfluent and mount of denitrification carbon source. The more number of segments, the moretheoretical TN removal rate greater. But no more than4segments in the engineeringapplication.(3)Influent flow distribution ratio with the C/N ratio of influent respectively were3.75 and7was calculated respectively based on the theoretical model. The volume of each aerobicand anoxic tank were calculated based on two influent flow distribution ratio. Two reactorscontinuouslyoperated under the condition of artificial water distribution. In the reactordesigned with low C/N ratio of influent, the CODCrand NH3-N averageremoval efficiencywere84.3%and96%respectively, the TN maximal removal efficiency was45%.In thereactor designed with high C/N ratio of influent, the CODCrand NH3-N averageremovalefficiency were92.8%and98.5%respectively, the TN maximalremoval efficiency was82.2%.In the reactor which was reference didn’t design with the theoretical model, the TNmaximal removal efficiency was50%. The cubage burden of pollutants in each tanks was notequilibriumthat caused the sludge bulking, leaded to the TN removal efficiency was notstabilized.(4)The reactor designed with low C/N ratio of influent operated under the condition ofC/N ratio of influent to1,2,3,5,6,7respectively due to research that TN removal efficiencyof the modified process which meet with water quality fluctuation.The TN averageremovalefficiency were20.1%,30.2%,31.6%,38.29%,46.19%,55.31%with the HRT and influentflow distribution ratio of reactor was settled. The study found that the TN removal efficiencycould be higher when the C/N ratio of influentincrease, but the cubage burden of pollutants ineach arobic tank would be greater. The TN removal efficiency could be lower when the C/Nratio of influent reduce, and additionalcarbon source was needed to make sure thatdenitrification reaction on the rails.Based on the above results, the modifiedstep-feed Anoxic/Oxic (A/O) biological nitrogenremoval process can be applied to ruraldomastic sewage treatment with its advantages as highefficiency and energy saving and simply maintenance.The influent flow distribution ratiomustbe set up base on the theoretical model, and the volume of each aerobic and anoxic tank mustbe designed base on the principle ofequalcubage burden. In this case, the modified processcan reach the maximalutilization of carbon source as well as stable operation. |
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