Research On Mechanisms Of Decentralized Treatment Process For Domestic Sewage

The water quality of sewage in Zhejiang province was established by evaluating samples collected from five sites in three typical cities.Based on fluid mechanics and chemical reaction engineering principles,the best Carrier Anaerobic Baffled Reactor (CABR)was then chosen for decentralized sewage treatment.Subsequently,the removal effects on pollutants and the mechanism of anaerobe succession in the CABR were investigated using sewage from the residential district of the Huajiachi Campus of Zhejiang University as influent for four months.The performance and mechanisms of shock resistance to short-term changes in temperatures in the CABR were also evaluated while treating the sewage.Furthermore,to overcome deficient denitrification that often occurs in anaerobic biological reactors,a novel denitrifying reactor without circumfluence was developed and evaluated.The primary results of this study are summarized as follows:(1)The sewage produced in Zhejiang province is characterized by a low organic and phosphorus concentration and a high nitrogen level.This results in the sewage having good biodegradability,but poor microbial denitrification due to the low C/N ratio(approximately 6.10).In addition,the sewage water quality in Zhejiang province was found to vary significantly by season,but not by region.The pollutant concentration was found to be highest in January and lowest in July.Finally,the four typical ratios(B/C,C/N,Alkalinity/TN and NH₄⁺-N/TKN)of the optimized synthesis wastewater were similar to those of actual domestic sewage.(2)The CABR has good structure and effect,with a dead zone of no more than 10%.In addition,the CABR shows a state of plug flow during operation.However,the hydraulic dead zone tended to increase as the number of chambers in the CABR increased in response to the increase in upflow velocity.When the effectiveness and capacity of the reactor was considered,a CABR with six chambers was found to be optimal.(3)The CABR with six chambers started up successfully after approximately three weeks with the influent of actual domestic sewage at a 48 h HRT and 28±1℃. In addition,the TCOD and SS concentrations of the effluent water met the first integrated wastewater discharge standard(NEPA,GB8978-1996).However,this system showed poor removal of nutrients such as N and P.In addition,the CABR was not able to completely eliminate the risk of xenobiotic matter as a result of the formation of humic acid.Although this system was able to remove 99.82%and 98.87%of the fecal coliforms and ascaris eggs,respectively,it must be used in conjunction with appropriate post-disinfection processes to guarantee the safety of the effluent.(4)Evaluation of the microbe development during start-up and running stage revealed that the predominant community in the CABR varied as the process of acclimation.There was high microbe comparability in six chambers without appearance of biofacies separation.In addition,fermentative bacteria and hydrogen-producing acetogenic bacteria reproduce significantly during the initial stage of start-up,while methanogens breed obviously until the fourth month of operation. Furthermore,the concentrations of these three anaerobes differed by approximately 1/10 of an order of magnitude during the stable running stage.Finally,63%and 20% of the substrate in the wastewater was metabolized by these organisms during start-up and stable operation,respectively.(5)The CABR has good performance shock resistance.However,the shock resistance declined as the temperature decreased.Although there was little effect on performance observed at high temperatures(28℃),low temperatures(10℃) significantly reduced the performance of the CABR.In addition,the shock resistance of the CABR was greater when water quality was considered than when water quantity was considered at 10℃-28℃.These findings are based on sensitivity ratios and standard deviations,as well as the variation in indexes used to evaluate microbial activity,the formation of intermediate products,soluble microorganism products and the volume of TCOD removal.(6)The average TCOD of the effluent of the novel denitrifying reactor with no circumfluence was 42.3±12.8mg/L after 90 days of operation.This value is lower than the discharge standard for pollutants released from municipal wastewater treatment plants(GB18918-2002).In addition,when the five different experimental influents were evaluated,this system removed 76%and 89%of the TN and NH₄⁺-N from the wastewater,respectively.The denitrifying reactor without circumfluence also showed a good effect on denitrification with the influent of high C/N ratio.The removal efficiency of TN in this system was dependant on the C/N ratio of the influent, whereas the NH₄⁺-N removal was dependant on the last influent flow distribution ratio. For sewage in Zhejiang Province,it is reasonable to assume that the ratio of the last influent flow distribution was less than 30%.This would indicate that this system removed the highest amount of TN from the wastewater and that the NH₄⁺-N concentration met the discharge standards for municipal wastewater.These findings indicate that the power consumption of the denitrifying reactor without circumfluence was lower than that of treatment facilities of conventional A²/O processes,which would result in at least a 51.1%reduction in energy consumption.Consequently,the denitrifying reactor without circumfluence was found to be the most effective, economical and convenient system for the treatment of decentralized sewage.