外文原文-构建组合人工湿地系统对农村地区污水处理的作用.pdf

Ecological Engineering 37 2011 948 954 Contents lists available at ScienceDirect Ecological Engineering journal homepage Performance of integrated household constructed wetland for domestic wastewater treatment in rural areas Shubiao Wua David Austinb Lin Liua Renjie Dongc aKeyLaboratoryofAgriculturalEngineeringinStructureandEnvironmentofMinistryofAgricultural CollegeofWaterConservancy Abegglen et al 2008 A Most Appropriate Technology is the one that is economically afford able environmentally sustainable and socially acceptable On site treatment systems often do not meet these requirements High total suspended solids TSS biochemical oxygen demand BOD total fecal coliforms total nitrogen TN and total phosphorus TP make septic tank effl uent unsuitable discharge to water bodies Carroll et al 2006 Traditional leach fi eld systems are prone to failure in areas with impermeable heavy clay soils and also pro vide inadequate treatment in areas with highly permeable soils and high water tables Lagoons tend to be unpleasant from an aes thetic perspective and because of odor production Burkhard et al 2000 Garc a et al 2001 Aerobic biological treatment unit and membrane bioreactors MBRs effectively remove pollutants but have high capital operations and maintenance costs that are not affordable in developing countries Nakajima et al 1999 Daude and Stephenson 2004 Ichinari et al 2008 Ren et al 2010 Con structed wetlands have high pollutant removal effi ciency as well as low cost and simple operation Brix and Arias 2005 Siracusa and La Rosa 2006 but can be limited by seasonal changes in treatment capacity and large area requirements Brix 1994 It is apparent that a successful and sustainable system entails a wide range of criteria including environmental technical and social cul 0925 8574 see front matter 2011 Elsevier B V All rights reserved doi 10 1016 j ecoleng 2011 02 002 S Wu et al Ecological Engineering 37 2011 948 954949 Fig 1 Schematic diagram of the integrated household constructed wetland system the doted red line shows the water fl ow path For interpretation of the references to color in this fi gure legend the reader is referred to the web version of the article turalfactors Thatistheunderlyingreasonsomecurrentlyavailable practices adopted from other countries can be incompatible with local requirements limitations and conditions Massoud et al 2009 Ren et al 2010 It is therefore essential to conduct research intoanalternativedisposalsystembasedonlocalrequirementsand conditions for the treatment of wastewater from a typical single family in rural China This paper describes a new on site wastewater treatment sys tem Integrated Household Constructed Wetland IHCW for rural household wastewater treatment The system consists of a two stage sedimentation tank and a vertical fl ow constructed wetland bed The precast structure is strong and waterproof Modular con struction allows for installation with unskilled labor It is expected that the system may overcome the local limitations of soil con ditions and unskilled construction Additionally the insulating biomass layer at the wetland bed surface allows the system to run normally in freezing temperatures This concept appears to offer advantages for household wastewater treatment in develop ing countries where really low cost convenient construction and operational simplicity are essential 2 Materials and methods The experiment took place in the backyard of a rural family in Chang Ping Beijing China It was an insulated at grade vertical fl ow model to avoid damage from low temperatures in winter Fig 1 The system consisted of a two stage sedimentation tank and a vertical fl ow constructed wetland bed section The frame structure was precast with magnesia cement and fi ber glass fabric whichisstrongandwaterproof Inplanviewthestructureisellipti calwiththebottomsmallerthanthetoptofacilitatetransportation It can be directly installed after excavation The two stage sedi mentation tank consists of two segments with equal empty bed volume for each segment of 0 5m3 The empty volume of the wet land bed section is 1 2m3 area depth 1 2m2 1 0m A steel sieve was installed in the inlet basin to prevent large solids such as vegetable leaves and fi sh scales from the kitchen from fl ow ing into the tank Wastewater fl ows into the fi rst segment from the inlet basin and then into the second segment via a fl oating valve installed in the fi rst segment to allow intermittent system feeding In order to maintain normal operation during the winter period 0 4mofsawdustinsultsthebed Wastewaterfl owsfromthe sedimentation tank downwards through a 60mm diameter perfo rated plastic pipe with 5mm holes located at the top of the sand layer and then trickles through the wetland bed The effl uent fl ows into the bottom gravel layer and then through the dewatered alum sludge placed in the outlet and fi nally fl ows into the ground The dewatered alum sludge is a byproduct from drinking water treat ment plants and has been reported to enhance P removal due to its high content of amorphous aluminum Babatunde and Zhao 2007 2009 Razali et al 2007 The bed media from the bottom to the top are washed gravel pea gravel and sand which was modifi ed according to the stan dard design criteria Brix and Arias 2005 a 15cm layer of washed gravel with particle size of 10 30mm 15cm of washed pea gravel with particle size of 5 12mm and 90cm of washed sand The effective size of the washed sand is 0 45 1mm with a uniformity coeffi cientof3 8 Seventyfi vekilogramsofwasheddewateredalum sludge derived from drinking water treatment plant with particle size of 0 5 1mm was put in the outlet of the system The medium surroundingthedistributionpipenetworkwas10cmofgravelwith particle size of 10 30mm A willow Salix babylonica with a trunk diameter of 40mm was planted in the vertical fl ow wetland bed section The willow was selected as the wetland plant for several reasons In China people would prefer a tree in their backyard rather than wetland plants Theexperienceofsomeconstructedwetlandsystemswithwillows in Demark has successfully provided a showcase of good perfor mance in nutrients and heavy metals removal Hasselgren 1998 Sander and Ericsson 1998 probably due to the well developed root system willows produced Willows are also cold hardy in the harsh climatic conditions of northern China The system was planted and then seeded in November 2007 Sampling evens occurred from March 2008 to February 2009 Dur ing the experiment operation household wastewater infl uent was comprised of kitchen and laundry effl uents The hydraulic loading ratewasabout0 12md 1 Watersamplesofapproximately200ml were collected from the infl uent sedimentation tank second seg ment and effl uent Fig 1 at 7 10 days intervals to evaluate the treatment performances Wastewater parameters of biochemical oxygen demand BOD5 5210 B 5 day BOD test total suspended solids TSS 2540 D total suspended solids dried at 103 105 C ammonia nitrogen NH4 N 4500 G automated phenate method and total phosphorus TP 4500 F automated ascorbic acid reduc tion method were measured on the same day of collection in the Key Laboratory of Agricultural Engineering in Structure and Envi ronmentofChinaMinistryofAgricultureaccordingtotheStandard Methods AWWA 1999 The pH and DO were measured in situ for each sample using a portable meter Orion 5 Star 510M 62 For each of the parameters samples were collected and analyzed in triplicate Mean and standard deviation values were reported The daily air temperature of meteorological data in terms of maximum and minimum was provided by the Beijing Meteorological Bureau Beijing China Thetemperatureoftheverticalconstructedwetland bed was determined by a temperature sensor Pt1000 Yonghua China installed in the middle depth of the bed Fig 1 Face to face questionnaires were conducted among 158 home respondents to evaluate farmers willingness to pay for treatment and the potential application of the integrated household con structed wetland in rural villages 3 Results The wastewater used for experiment was generated in a single household excluding toilet wastewater Average concentrations 950S Wu et al Ecological Engineering 37 2011 948 954 Fig 2 Concentrations of infl uent sedimentation tank and effl uent in integrated household constructed wetland system a biochemical oxygen demand BOD b total suspended solids TSS c ammonia nitrogen NH3 N and d total phosphorus TP Table 1 Construction cost of one integrated household constructed wetland system ItemsCost US dollars Precast frame structure147 Gravel and sand36 Installation30 Pipes and joints22 Excavation22 Willow2 Sum259 of BOD5 TSS NH4 N and TP in the infl uent were 302 4mg l 128 6mg l 30 7mg l and 5 0mg l respectively and the aver age concentrations in effl uent were all continuously reduced to 11 8mg l 3 8mg l 3 5mg l and 0 6mg l respectively Fig 2 Average removal effi ciencies of BOD5 TSS NH4 N and TP out side of winter were 96 3 97 3 90 0 and 87 6 respectively and 95 0 96 2 84 6 88 2 in winter period Fig 3 There was negligible decrease of average removal effi ciency for BOD5 TSS and NH4 N during winter 1 3 1 1 and 5 4 respectively while an increase of 0 6 was achieved for TP removal in winter period The insulting sawdust layer is most probably responsible for the minimal change between winter treatment performance and the rest of the year Wallace et al 2001 The sawdust insulating layer kept the wetland bed temperature constantly above 6 C even as the minimum air temperature decreased to 8 C during winter Fig 4 The total construction cost of the integrated household con structed wetland system was 259 US dollars including precast frame structure gravel and sand and installation Table 1 Of the householderspolledoncostissues 23 respondentswerereluctant to pay any money for wastewater treatment facilities construction 32 took a wait and see position and 45 were willing to pay Fig 5a Thepercentageofhouseholder swillingnesstopayranges from 3 to 6 dollars per year was 54 1 but just 6 4 were willing to pay more than 12 dollars per year Fig 5b These attitudes may be closelyrelatedtolocaleconomicconditions especiallythefarmer s income level Clearly the cost of wastewater treatment technology must be extremely low before rural villages will adopt it 4 Discussion High removal rates of BOD5and TSS in the study system were similar to those observed with constructed wetlands Haberl et al 1995 Lakatos et al 1997 Pretreatment in the sedimentation tank also plays an important role by removing 32 of BOD5and 46 of TSS Fig 6 This treatment performance was similar to that observed for primary settling Metcalf and Eddy 2003 Anaerobicdegradationinthesedimentationtankmayhavealso played an important role in treatment Domestic wastewater from a single rural family traditionally comes from kitchen and laun dry effl uent It is highly biodegradable Elmitwalli and Otterpohl 2007 The sedimentation tank had dissolved oxygen DO concen trations 0 8mg l which was essentially an anaerobic condition with little temperature variation Sedimentation tank removal of NH4 N and TP was just 15 and 17 As expected nitrifi cation was limited by low dissolved oxygen concentration in the sedimenta tion tank Beccari et al 1992 Paredes et al 2007 The rate of TP removal was consistent with a sedimentation process Metcalf and Eddy 2003 The sedimentation tank clearly fulfi ls primary treatment buffering of BOD5and TSS to the treatment system by moderating high infl uent concentrations Fig 7 The clear bene fi t of the sedimentation tank section lies not only in concentration reduction of BOD5and TSS but also to avoid or relieve clogging S Wu et al Ecological Engineering 37 2011 948 954951 Fig 3 Pollutants removal effi ciencies in integrated household constructed wetland system a biochemical oxygen demand BOD b total suspended solids TSS c ammonia nitrogen NH3 N and d total phosphorus TP Fig 4 Meteorological air temperature and wetland bed temperature change with experiment operation of the constructed wetland bed by excessive particulate or organic loading Austin et al 2007 lvarez et al 2008 Barros et al 2008 Chen et al 2008 Phosphorus removal in this system exhibited 87 6 overall removal effi ciency outside of the winter period and 88 2 in the winter period Fig 3 This removal rate is far higher than what can be achieved in conventional constructed wetlands Vymazal 2007 YatesandPrasher 2009 Phosphorusremovalinconstructed wetlands is an integrated process including settlement plants uptake and bacteria adsorption and substrate affi nity Greenway and Woolley 1999 Vymazal 2007 Kadlec and Wallace 2008 but compared to adsorption to substrate other mechanisms of phos phorus removal are not signifi cant in wetland systems Brix 1997 Vymazal 2007 Therefore this study did not detect phospho rus uptake by willows Since traditional wetland systems employ gravel and or sand as the substrate P removal is often poor due to their limited P adsorption capacity Removal of P in all types of constructed wetlands is low unless special substrates with high sorption capacity are used Vymazal 2007 Some studies have demonstrated that dewatered alum sludge has the potential to enhance P removal due to its high content of amorphous aluminum Babatunde and Zhao 2007 2009 Razali et al 2007 It is a byproduct from drinking water treatment plants and commonly be disposed as landfi ll At present there is 952S Wu et al Ecological Engineering 37 2011 948 954 Fig 5 Willingness to pay a and payment level b for the householders to treat their domestic wastewater in rural areas 158 respondents were investigated Fig 6 Average contribution to pollutants removal by sedimentation tank and constructed wetland bed a biochemical oxygen demand BOD b total suspended solids TSS c ammonia nitrogen NH3 N and d total phosphorus TP no report of published studies and demonstrated cases for wet lands constructed with alum sludge in China Therefore this study investigated use of dewatered alum sludge as a substrate in the treatment system to remove phosphorus The average concentra tion of TP in infl uent and the volume of domestic wastewater produced from a single rural family per day is assumed to be 5 0mg l and 120l d The TP concentration in effl uent is required to be 1 0mg l according to China regulation This standard requires 480mg phosphorus removal per day The longevity of the 75kg alum sludge in this system could be at least 10 years which was calculated in batch tests as 31 9mgP g by alum sludge Babatunde et al 2009 Seasonal infl uence on constructed wetland performance can be particularly important in cold climates Microbial activity is linked to temperature with bacterial growth and metabolic rates reduced with decreasing temperature Faulwetter et al 2009 Low wastewater temperature is a special concern for nitrifi ca tion Although aerobic biofi lm systems can maintain nitrifi cation below 6 C Choi et al 2008 nitrifi cation has commonly been observed to drop off rapidly below 6 C Werker et al 2002 Xie et al 2003 In northern China the lowest winter temperature often drops to 8 C An insulating layer such as a surface layer of sawdust is important to prevent freezing of the wetland bed and also to ensure cold temperatures do not inhibit nitrifi cation The insulation layer allows effl uent temperature to remain above 6 C in winter The observation that the average removal effi ciency in winter was not much less than the rest of the year may be attributed higher wastewater temperatures maintained by the insulating layer The low temperature performance of this technol ogy is therefore robust For cold climate constructed wetland design M hlum et al 1995 advocated a system consisting of an aerobic pretreatment step followed by constructed wetland units that perform nearly the same during winter and summer seasons Expense and energy S Wu et al Ecological Engineering 37 2011 948 954953 Table 2 Summary of hypothetical EPA rural community wastewater treatment technologies costs 1995 US dollars adapted from USEPA 1997 and the integrated household constructed wetland system costs TechnologyTotal capital costAnnual operation and maintenance costTotal annual cost Centralized system2 321 840 3 750 53029 740 40 260216 850 342 500 Alternative small diameter gravity sewers598 100729055 500 Collection and small on site systems510 00013 40054 500 Integrated household constructed wetland system34 9657003518 Assumptions All technology options presented are assumed to have a 30 year life span All of the options considered are capable of achieving the secondary treatment level The rural community consists of 450 people in 135 homes Fig 7 Sedimentation tank and effl uent concentrations change with different infl uent loading concentrations a biochemical oxygen demand BOD b total suspended solids TSS c ammonia nitrogen NH3 N and d total phosphorus TP requirements of this system however are not appropriate for Chi nese rural householders with limited income Phosphorus removal for the study technology was similar that achieved by M hlum et al 1995 using a fi lter medium with high phosphorus adsorp tion Cost estimates on a national and or international basis for wastewater treatment systems are diffi cult to develop primarily due to varying conditions of each community such as popula tion density land costs and local performance requirements The USEPA developed cost estimates of centralized and decentralized approaches to wastewater management for a hypothetical 135 home rural community USEPA 1997 The study revealed that decentralized systems such as onsite systems are generally more cost effective for managing wastewater in rural areas than conven tional centralized wastewater treatment system The integrated household constructed wetland system is even more cost effective Table 2 Despite the fact that the IHCWs system is more suitable there is still a 20 4 US gap for a Chinese rural house