Assessment Of Modified Septic Tanks Treating Domestic Sewage

Traditional septic tanks have been used to remove precipitated and suspendedsolids and digest anaerobic sludge for fertilizer150years before. Up to now, the septictanks have been widely used all over the world. In recent years, government invested alarge number of funds in the construction of urban sewage treatment facilities, so therate of sewage collection and treatment has been greatly improved. In order to save theland and funds, improve the quality of living environment and avoid accidents, some ofbig cities have perfect sewage treatment systems. However, rural areas have imperfectsewage treatment systems, so septic tanks have to been used. It is necessary to improvethe quality of wastewater by modifying the design of septic tank.Whether septic tank should be built in the all building is controversial，but waterquality survey of septic tank is lack. A septic tank, located in Harbin, was investigatedfor water quality. The survey analyzed the change of water quality of septic tank withinone day and at different temperature by sampling three times one day.Recent decades witnessed development of anaerobic wastewater treatmenttechnology, so as traditional anaerobic treatment technology of horizontal flow, septictank should be modified, based on the new technology. Three on-site septic tanks,namely ABR-septic tank, YDT-carrier-septic tank and traditional septic tank, wereoperated in parallel to evaluate the modified septic tanks. The test can be separated intotwo phase, namely start-up and operation. Aim of first phase is to evaluate the threeseptic at different temperature for treating sewage. In the second phase, operatingconditions of septic tank were analyzed in different HRT and temperature for treatingsewage and input of primary sludge.In the start-up phase, the achieved COD_tot removal efficiencies in R1and R2increased by30.5%and9.2%, respectively, compared with R3at the stage of of24oC,while the value reduced to10.9%and9%in the phase of16oC. The achieved removalefficiencies of TN and NH4+-N arrived at5%³0%, and the value of TP wasnegligible,with approximately5%.In the operation phase, the functions of traditional septic tanks are acidification andblocking of organic matter. The achieved COD_tot removal efficiencies in ABR-septictank and YDT-carrier-septic tank increased by10-20%, compared with traditional septictank. The achieved removal efficiencies of TN and TP were negligible. The removalefficiencies of NH4+-N and PO43--P were negative, due to ammonification andmineralization, respectively. ABR-septic tank achieved a better specific methaneproduction rate. The two modified septic tanks improve biochemical characters and attain betterremoval efficiency for domestic sewage. The results provide a technical support and atheoretical basis for the modification of septic tank.