A New Soil Inflltration Technology for Decentralized Sewage Treatment- Two-Stage Anaerobic Tank

Pedosphere18(3):401–408,2008ISSN1002-0160/CN32-1315/Pc 2008Soil Science Society of ChinaPublished by Elsevier Limited and Science PressA New Soil Infiltration Technology for Decentralized Sewage Treatment:Two-Stage Anaerobic Tank and Soil Trench System∗1YE Chun1,3,HU Zhan-Bo1,2,KONG Hai-Nan1,∗2,WANG Xin-Ze1and HE Sheng-Bing11School of Environmental Science and Engineering,Shanghai Jiaotong University,Shanghai200240(China).E-mail: yechbj@2School of Light Industry and Food Engineering,Guangxi University,Nanning530004(China)3Research Center of Lake Ecological Environment,Chinese Research Academy of Environmental Sciences,Beijing100012 (China)(Received August24,2007;revised January23,2008)ABSTRACTThe low removal efficiency of total nitrogen(TN)is one of the main disadvantages of traditional single stage subsurface infiltration system,which combines an anaerobic tank and a soilfilterfield.In this study,a full-scale,two-stage anaerobic tank and soil trench system was designed and operated to evaluate the feasibility and performances in treating sewage from a school campus for over a one-year monitoring period.The raw sewage was prepared and fed into thefirst anaerobic tank and second tank by60%and40%,respectively.This novel process could decrease chemical oxygen demand with the dichromate method by89%–96%,suspended solids by91%–97%,and total phosphorus by91%–97%.The denitrification was satisfactory in the second stage soil trench,so the removals of TN as well as ammonia nitrogen(NH+4-N)reached 68%–75%and96%–99%,respectively.It appeared that the removal efficiency of TN in this two-stage anaerobic tank and soil trench system was more effective than that in the single stage soil infiltration system.The effluent met the discharge standard for the sewage treatment plant(GB18918-2002)of China.Key Words:decentralized treatment,nitrogen removal,soil trench system,subsurface infiltrationCitation:Ye,C.,Hu,Z.B.,Kong,H.N.,Wang,X.Z.and He,S.B.2008.A new soil infiltration technology for decentralized sewage treatment:Two-stage anaerobic tank and soil trench system.Pedosphere.18(3):401–408.INTRODUCTIONAs one of the soil treatment technologies,subsurface infiltration system has been used as an on-site treatment alternative to conventional intensive sewage treatment technology in many countries(US EPA,2002;Zhang et al.,2004).Conventional subsurface infiltration system consists of a septic tank and a soil absorptionfield where wastewater is treated prior to its discharge to the ground water.There are over22million typical subsurface infiltration systems in operation in the US now(Van Cuyk et al., 2001).However,it is still one of the contributors of pathogens and nutrients to ground water,which can lead to health problems if the ground water is directly used as a drinking source.Recently,an improved subsurface infiltration system,referred to as soil trench system,has been developed.It has been proven to be an effective method for treating dispersed sewage,especially in developing countries(van Buuren et al.,1999;Kong et al.,2002;Inamori et al.,2003).Its main treatment frame is the soil absorption unit,which is enclosed by a waterproof material.The depth of the infiltration layer usually varies from0.6to1.2m.The treated effluent is collected at the bottom of the gravel layer.With this set-up,construction and operating costs are reduced by about one-third to half that of the active sludge technology.In addition,the effluent can be reused for non-drinking ∗1Project supported by the National High Technology Research and Development Program(863Program)of China (No.2002AA601012-01).∗2Corresponding author.E-mail:hnkong@.402 C.YE et al. purposes(Sun et al.,1998;Kong et al.,2003).During the last four years,over100full-scale,on-site soil trench systems have been set up and put into successful operation in different climatic zones in China. These systems operate at aflow rate between5and500m3day−1(Kong et al.,2002;Hu et al.,2004).Soil trenchfilter combined with an anaerobic tank in the single-stage soil trench process has been found to achieve chemical oxygen demand(COD),total phosphorus(TP),suspended solids(SS),and NH+4-N removals of80%–92%,80%–96%,80%–86%,and70%–81%,respectively.However,one limita-tion of single-stage soil trench system lies in its low total nitrogen(TN)removal,which is only around 23%–55%(Kong et al.,2002,2004;Sun et al.,1998).During the percolation process of the traditional single stage soil infiltration system,both nitrification and denitrification occurred within the same soil infiltration unit.As the carbon content decreased when the organic substances were removed along the soil depth from the top to the bottom,it was found that the carbon source was not enough for the denitrification that followed the nitrification stage.This led to a low level of TN removal.Therefore, further research to increase the efficiency of denitrification is important for biological nitrogen removal in soil trench systems.The use of soil mixed with peat and plastic foam asfilter materials arranged in a multilayer fashion, is one approach to improve the nitrogen removal efficiency.By this system,a mean removal of75.5%for TN has been attained at certain period(from the thirtieth tofiftieth day)during a lab-scale operation (Zhang et al.,2004).A previous study also proposed the use of a sandfilter plus a post-positioned denitrification tank,where a carbon source such as grey water or ethanol was fed into the denitrification tank for increasing nitrogen removal efficiency(van Buuren et al.,1999).However,up to now,very little attention has been paid to the combination pattern of the multistage anaerobic tank and soil trenchfilter for improving the TN removal efficiency,especially in practice,for treatment of decentralized domestic wastewater.Therefore,such a system was evaluated in this research for improving TN removal.The raw sewage was distributed into two stage anaerobic tanks separately.The aim of the process configuration was to utilize the raw sewage fed into the second stage as a carbon source for denitrification of the nitrate and nitrite in effluent that came from thefirst stage soil trench.In this article,a full-scale,two-stage anaerobic tank and soil trench system for the treatment of sewage was designed and operated to evaluate the feasibility and performances in treating sewage from a school campus for over a period of one year.MATERIALS AND METHODSSystem descriptionThe full-scale system was located in a new middle school campus,which is around8km from Wenzhou City,China.It was set under the ecological lawn around the dormitory building.The raw sewage was collected from the dormitory and refectory with a population of1000–1200people,produced by toilets,bathing,laundry,cooking,and washing dishes.The system had a treatment capacity of60 m3day−1in winter and about twice that in summer.The hydraulic loading rate was100L m−2day−1. The raw sewage stayed in the anaerobic tank with a retention time of24h.The treatment facility consisted of two anaerobic tanks and two soil trenchfilters(Fig.1).In this process,60%of the raw sewage was fed into thefirst stage anaerobic tank and the other40%was fed into the second stage anaerobic tank.The effluent from thefirst stage anaerobic tank was transported into thefirst soil trenchfilter,where the liquid was dispersed evenly by a branched tube distributor.The sewageflowed uniformly within the constructed soilfiltration system by capillary action.The effluent of thefirst-stage soil trenchfilter was then allowed toflow into the second anaerobic tank,where it was mixed with40%raw sewage.Finally,the effluent from the second anaerobic tank was fed into the second-stage soil trenchfilter for further treatment.A NEW SOIL INFILTRATION SYSTEM403Fig.1Schematic diagram of the two-stage anaerobic tank and soil trench system.Sampling and analytical methodsThe behavior and performance of the full-scale system were investigated from Sep.2002to Dec.2003. The samples were taken from the influent and effluent of the system and then chemical oxygen demand with the dichromate method(COD cr),TN,TP,NH+4-N,and SS were determined in accordance with the standard methods(Chinese EPA,1997).At the beginning of the operation,the sampling frequency was once a week in thefirst9weeks and then once two weeks in the following8weeks.The thirteenth samples were taken in Jan.2003,and from then on,the samples were taken at the interval of one month until Dec.2003.RESULTS AND DISCUSSIONOrganic pollutants and suspended solidsInitial operation of the system was conducted at an average influent COD cr concentration of512mg L−1and SS concentration of184mg L−1.Results showed an excellent removal of both COD cr and SS at89%–96%and91%–97%throughout the normal operation period,respectively(Figs.2and3).The residual COD cr and SS were19–50and6–17mg L−1,respectively.This was in agreement with the report of previous researches.The removal of biochemical oxygen demand(BOD)was greater than90%for the typicalfilter bed subsurface wastewater infiltration system(SWIS)(Darby et al.,1996;Emerick et al.,1997;MoEJ,2002).The organic substance was removed in an anaerobic tank and soilfilter.On-site wastewater treatment system containing a septic tank and SWIS was capable of removing nearly all the suspended solids and biodegradable organic compounds,if properly designed,installed,and operated (US EPA,2002).The COD cr and SS removals were not significantly influenced by climactic changes, so the uniform performance of the system was maintained even during the cold winter season.In the subsurface infiltration system,suspended solids and some dissolved organic matter were removed by theFig.2Variations in the concentration and removal of chemical oxygen demand with the dichromate method(COD cr)in the influent and effluent during the startup(15weeks)and normal operation(Jan.2003–Dec.2003).There was a two-week interval between the startup and normal operation.404 C.YE et al.Fig.3Variations in the concentration and removal of suspended solids(SS)in the influent and effluent during the startup (15weeks)and normal operation(Jan.2003–Dec.2003).There was a two-week interval between the startup and normal operation.combination of physical(i.e.,sedimentation,absorption,filtration,and trapping)and biological degra-dation processes.In general,the surface layer,which extended to a depth of0.3–0.4m,of the soil filter was considered biologically active because of the presence of a large quantity of bacteria,protozoa, and metazoan.Hence,biodegradation of organic substances occurred mainly in this oxygenated section around the distributed pipe.This mechanism was similar to that of intermittent sandfiltration,where similar biological decomposition took place within thefirst20cm of thefilter surface(Sabbah et al., 2003).PhosphorusPhosphorus(P)removal ranged from91%–97%throughout the entire study period(Fig.4).After four weeks,the total phosphorus(TP)removal was around94%.This corresponded to a mean effluent concentration of0.23mg L−1.The highest removal(over94%)was recorded during the summer of 2003,from June to September.Variations of the P removal could be because of thefluctuations in the capacity of the soil at different periods(Rodgers et al.,2005;Karczmarczyk,2004).Fig.4Variations in the concentration and removal of total phosphorus(TP)in the influent and effluent during the startup(15weeks)and normal operation(Jan.2003–Dec.2003).There was a two-week interval between the startup and normal operation.The main mechanism of P removal in soil trench systems is generally through physicochemical absorption by the soil as well as by nutrient uptake by natural growths in the lawn.With regard to the former,minerals such as Ca,Al,and Fe oxides serve as important binding sites and their presenceA NEW SOIL INFILTRATION SYSTEM405 in considerable amounts would enhance the P sorption capacity,resorption rate,and the permeability of the soilfilter.Therefore,a significant basis for the selection of soil media is the presence of high levels of Ca,Al,and Fe oxides.Now,with the relatively high P removal of above90%,even after more than a year of operation,it indicates that the soilfilter applied exhibits high capacity for P.However, for any sorption processes,the equilibrium uptake is generally affected by certain factors,particularly pH and redox potential,so thatfluctuations on these parameters during climatic changes will result in variations in P binding.Similarly,changes in the density and nature of plant growth with the season will also influence the extent of P removal.NitrogenIn addition to the satisfactory COD cr and SS removals,high removal efficiency for TN was also at-tained.Nitrogen is generally removed from sewage by several processes,including ammonification,mi-crobial nitrification-denitrification,soil and plant uptake,and ammonia volatilization(Martinez,1997). In this study,nitrification resulted in96%–99%removal of NH+4-N(residual:0.5–1.2mg L−1)(Fig.5). Previous researches have also achieved more than98%NH+4-N removal(residual:0.3–0.5mg L−1)in a full-scale infiltration system(Mottier et al.,2000;Lance,1972).These high nitrification efficiencies are mainly attributed to the high oxidation activity of nitrifier present,particularly in the upper soil layer. It has been determined that approximately35%–50%of ammonium is oxidized at the top layer of30 cm in the infiltration system.In addition,NH+4-N can also be immobilized in the soil particles through ion exchange and can be further assimilated by plants through their root systems(Mitsch et al.,2001).Fig.5Variations in the concentration and removal of NH+4-N in the influent and effluent during the startup(15weeks) and normal operation(Jan.2003–Dec.2003).There was a two-week interval between the startup and normal operation.In previous soil trench systems,both biological conversion processes of nitrification and denitrifi-cation for ammonia and nitrate removal,respectively,occur within a single stage(Sun et al.,1998). Nitrification takes place in the upper soil layer,which is in oxic condition,whereas,denitrification proceeds in the lower sections,where oxygen level approaches zero(Kong et al.,2002).During the percolation process in soil trench systems,the carbon content decreases as the organic substances are removed by absorption and biological degradation in the soil.As excellent biological denitrification requires a carbon source,its performance in usual single stage soil trench system will be drastically reduced.Specifically,to accomplish denitrification satisfactorily,a BOD:NO−3-N ratio of at least2.86 is required(van Buuren et al.,1999;Martinez,1997).The C:NO−3-N ratio becomes a limiting factor for denitrification in single-stage soil trench system because of the lack of an available carbon source in the deep layer.This is the main reason for the low removal of TN in the conventional single-stage soil trench system.In the full-scale two-stage system,both nitrification and denitrification were well established with406 C.YE et al. TN reduction in the range of68%–75%and a mean TN value of12.07mg L−1in effluent(Fig.6).This result may be attributed to the treatment configuration applied,which is a combination of the two-stage anaerobic tanks and the soil trench system.Specifically,the effluent fromfirst stage soil trenchfilter was mixed with40%of raw sewage and then fed into the second anaerobic tank.The raw sewage supplies the necessary carbon source to denitrify the remaining nitrite and nitrate in the effluent offirst stage soilfilter.Meanwhile,40%of raw sewage was not treated by thefirst soil trench by nitrification.It may restrain higher TN removal efficiency.Since the theoretical maximum ratio of BOD:NO−3-N in the second stage anaerobic tank is higher than3:1,this means that NO−3-N can be converted to gaseous nitrous oxide(N2O)and molecular nitrogen(N2)completely(MoEJ,2002).Any remaining organic substance and nitrogen are thenfinally treated by the second soil trenchfilter,which leads to a stable and high denitrification efficiency.Fig.6Variations in the concentration and removal of total nitrogen(TN)in the influent and effluent during the startup (15weeks)and normal operation(Jan.2003–Dec.2003).There was a two-week interval between the startup and normal operation.Application of a similar full-scale,two-stage anaerobic tank and soil trench system for the treatment of sewage from a hotel located in a tourism area in China has also been reported(Kong et al.,2003). High TN removal of80%–85%has also been achieved.This is considerably higher than the TN removals achieved in conventional secondary biological treatment and in single stage soil trench system,which ranged from15%–20%(Erol Nalbur et al.,2003)and23%–55%,respectively.As for COD cr,SS,and TP,no significant difference was observed between the single-stage and two-stage anaerobic tank and soil trench system.Variation of COD cr and nitrogen removals in influent and effluentDuring the startup period,nitrification did not commence until the eighth week.The removal of NH+4-N was initially around24.0%–44.3%during thefirst few weeks and then it increased to77.2%at the seventh week.It gradually increased further until it reached98.8%during the thirteenth week(Fig.5). At that period of time,the removals of COD cr and TN were45%–85%and19%–50%,respectively. Gradual improvement and stabilization were observed for the removal of COD cr,TN,and NH+4-N from the thirteenth to thefifteenth week of operation.Effluent NH+4-N concentration was below1.7mg L−1 after one year of operation.Therefore,in this study a period of about11–15weeks was found necessary for the full exertion of bacteria activity in soilfilter under the prevailing experimental conditions.In other words,the two-stage anaerobic tank and soil trench system was able to attain steady state after11–15weeks of continuous operation.Thefirst11to15weeks can be regarded as the startup period.Then a consistent effluent containing low level of COD cr,NH+4-N,and TN could be maintained thereafter.A NEW SOIL INFILTRATION SYSTEM407CONCLUSIONSUnder the conditions studied,the full-scale,two-stage anaerobic tank and soil trench system demon-strated a stable and advanced treatment performance.The effluent met the discharge standards of pollutant for municipal wastewater treatment plant(GB18918-2002)of China.The treated water pre-sented the potential for lawn watering and road washing by the reuse criteria(GB/T18920)of China. 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