| It is becoming more and more serious that decentralized wastewater drainage and water pollution in rural areas of developing countries,which is widely concerned around the world.In China,a typical developing country,wastewater treatment in rural areas has been paid special attention.Rural wastewater is mainly derived from domestic and poultry breeding activities.Due to the lack of pipeline network construction and suitable wastewater treatment technologies,most rural wastewater were discharged randomly without effective treatment.This will bring risks to the local environment,groundwater resources,and human health of rural residents.As a novel ecological technology,multi-soil-layering(MSL)system is suitable and promising for decentalized wastewater treatment in rural China.MSL system is composed of soil mixture blocks(SMBs)and permeable layers(PLs),corresponding to the anaerobic functional zone and aerobic functional zone,respectively.Both SMBs and PLs play the important role in contaminants removal.The unique brick-pattern structure and good permeability of MSL system are conducive to wastewater distribution and prolong the hydraulic residence time(HRT).Various mechanism processes such as physical filtration,adsorption,interception,chemical precipitation,and microbial degradation are involved in MSL system,which has good performance on the removal of organic matters,nitrogen,and phosphorus even under the condition with high hydraulic loading rate(HLR)and high pollution load of wastewater.MSL system also with advantageous features such as small land requirement,easy operation and maintenance,no noise,and no odor.Previously,experimental studies associated with MSL system mainly adopted single-factor comparison scheme.Moreover,there are still many operating factors have not been condidered in lab-scale studies.Furthermore,the interactive effects of operating factors on the treatment performance of MSL system are not clear.Previous studies carried out the field application of MSL system for treating actual rural wastewater.However,besides the contaminants removal performance was mainly focused,the quantitative assessment of economic feasibility and the possible environmental impact of MSL system have not been considered.In addition,emerging organic pollutants such as antibiotics may be present in the wastewater discharged from small-scale poultry breeding activities.It is still not clear that the treatment performance of MSL on antibiotics containing poultry breeding wastewater,as well as the negative effect of antibiotics on microorganisms and the removal performance of MSL system.To address the above challenges,the key objectives and findings of this research are as follows.(1)In the factorial design experiment,8 MSL systems were operated with three factors that submersion of bottommost layer,microbial inoculation,and continuous aeration,to study the performance of MSL system on systhetic rural wastewater treatment.Additionally,the main effect of factor and their interactive effects on contaminants removal as well as the significance of all effects were also revealed through factorial analysis.In this study,a model with functions that similation and inference prediction was developed on the basis of stepwise-cluster analysis(SCA)method,which was used to track the nonlinear relationship of contanimants removal rate data in MSL systems.The results of this study indicated that continuous aeration is advantageous to the degradation of organic pollutants,chemical reaction for phosphorus removal and nitrification,however,unfavorable to the denitrification and the total nitrogen removal.Microbial inoculation was not significant for imoroving the treatment performance of MSL system.Submersion of bottommost layer had negative effect on the structural stability and treatment performance of MSL systems.Without submersion of bottommost layer,there were good wastewater distribution and unobstructed flow in MSL systems,which also with good structural stability and treatment performance.The oxygen demand in MSL systems without continuous aeration could also be ensured through natural reoxygenation,which had less negative effect on denitrification in MSL system.SCA method can effectively track the nonlinear relationship among discrete data of removal rates related to different contaminants treatment processes,which assciocated with the complicated effects of multiple operating factors.The results will provide a favorable reference about operating factor and level combination for the stable operation and effective performance of MSL system.(2)In the factorial design experiment,8 MSL systems were operated with three factors that the additive amount of exogenous activated sludge,the additive amount of polymer solid carbon source,and height of bottom submersion,to study the performance of MSL system on the treatment of systhetic domestic wastewater with low carbon nitrogen ratio.This study was mainly to improve the denitrification and nitrate nitrogen(NO3--N)removal efficiency from the perspective of microbial diversity change.Furthermore,the effects of different operating factors and their interactions on microbial species richness,microbial structure diversity,and relative abundance of denitrifying bacteria in SMBs were investigated on the basis of factorial analysis.The results indicated the long-term operation of MSL systemcould significantly increase the microbial species richness in SMBs compared with the original natural soil.The microbial species richness in samples was directly proportional to the level of factors that the additive amount of exogenous activated sludge,the additive amount of polymer solid carbon source,and height of bottom submersion.The microbial structural diversity among samples was directly proportional to the level of height of bottom submersion,and was inversely proportional to the level of factors that the additive amount of exogenous activated sludge and the additive amount of polymer solid carbon source.Comamonadaceae,Rhodocyclaceae,and Xanthomonadaceae were the dominant denitrifying bacteria selected from the SMBs samples at family level.The higher total relative abundance of denitrifying bacteria in SMBs,the better performance of MSL system on NO3--N removal.The can achieve The best denitrifying perforamcne and the highest NO3--N removal could be achieved in MSL system which with high level of all factors during experimental operation.The results can help understanding the microbial mechanism associated with the enhencement of denitrification in MSL system,and providing the reference of operating conditions for improving NO3--N removal in MSL system.(3)Based on the favorable factors and their levels results through factorial analysis,a gravity-flow integrated ecological bed system(GIEBS)was developed,and MSL system was the core unit for wastewater treatment.In addition,the unit composition,structure design,and technical principle were introduced in detail.GIEBS was applied to treat actual rural domestic wastewater.Under the guidance of the framework of life cycle sssessment(LCA),the environmental impact and economic feasibility of GIEBS were quantitatively evaluated.It was of great significance for improving the biodegradability of wastewater and removing nutrients in MAAT pretreatment unit.In GIEBS,the MSL treatment unit has the best performance on contaminants removal,as the largest contribution to GIEBS.During the construction period of GIEBS,electricity consumption was only 0.1%of the cost.The operation cost of GIEBS was only 700 yuan per year.The average operating cost of GIEBS to treat 1 m3 of domestic wastewater was less than 0.4 yuan.The MAAT pre-treatment unit had the highest contribution(91%)to greenhouse gas(GHG)emissions from GIEBS.However,MSL treatment unit and subsurface flow constructed wetland(SSFCW)post-treatment unit contributed less to GHG emissions.GIEBS had a good performance on both effectively treating decentralized domestic wastewater and reducing GHG emissions in rural areas.Low-cost and high-efficiency GIEBS can effectively solve the challenge of decentralized domestic wastewater treatment in developing countries or remote and economically underdeveloped rural areas.(4)The effects of factors that PLs material,influent sulfamethoxazole(SMX)concentration,and influent pH and their interactions on the performance of MSL system on SMX containing poultry breeding wastewater treatment were studied in factorial design experiment.The most favorable factors with optimized level configuration for SMX removal in MSL system were determined through factorial analysis.Stepwise-cluster analysis(SCA)method was applied to track the nonlinear relationship between the discrete data of SMX removal rate and the potential effect of operating factors on other contaminant removal rates in MSL systems,and a model for similating and predicting SMX removal rate was established.In addition,16s RNA microbial diversity analysis was used to excavate the potential dominant resistance genus in SMBs related to SMX removal.The results indicated that Influent with high SMX concentration had significantly short-term negative effect on the microbial degradation capacity and chemical oxygen demand(COD)removal in MSL system.However,MSL system had a strong self-regulate mechanism and ability to improve and recover its performance on contaminants removal.The influent with low concentration of SMX had no significantlt negative effect on COD removal in MSL system.The influent pH was the key operating factor related to the chemical phosphorus removal process in MSL system,as well the form and stability of phosphorus containing chemical products.After a long-term acid or alkaline corrosion,the surface morphology of medical stone samples was changed obviously.There was a spongy porous structure of medical stone samples and its specific surface area was greatly increased.However,the surface morphology of anthracite samples did not change significantly.MSL system using medical stone as PLs material had better performance on NH4+-N and NO3--N removal than that using anthracite.The functional activity of some denitrifying bacteria was negatively affected in MSL systems using acidic influent for a long time,and such inhibition of denitrification was more severe than that in MSL systems using alkaline influent.The SMX concentration in effluent of among MSL systems increased with the increase of the SMX concentration in influent.As a result,under the conditions of low pH,low SMX concentration in influent,and the use of medical stone as PLs material(MSL4),the MSL system exhibited the best SMX removal performance(91.3%).In MSL system,adsorption was identified as the key mechanism for removing SMX.The difference levels of influent pH had a significant effect on the microbial speciss richness in SMBs.The different levels of SMX concentration in influent had a potential effect on the similarity of microbial structure composition among samples.The results are helpful to better understand the removal mechanism of SMX in MSL system from the perspectives from factorial analysis,numerical prediction and microbial diveristy changes. |
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