Performance of integrated household constructed wetland for domestic wastewater treatment in rural

Ecological Engineering 37 (2011) 948–954
Contents lists available at ScienceDirect
Ecological
Engineering
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e c o l e n
g
Performance of integrated household constructed wetland for domestic wastewater treatment in rural areas
Shubiao Wu a ,David Austin b ,Lin Liu a ,Renjie Dong c ,∗
a
Key Laboratory of Agricultural Engineering in Structure and Environment of Ministry of Agricultural,College of Water Conservancy &Civil Engineering,China Agricultural University,100083Beijing,PR China b
CH2M HILL,Mendota Heights,MN,USA c
College of Engineering,China Agricultural University,100083Beijing,PR China
a r t i c l e i n f o Article history:
Received 1May 2010
Received in revised form 24January 2011Accepted 15February 2011Available online 2 April 2011Keywords:
Constructed wetland Rural areas
Household wastewater treatment
a b s t r a c t
As environmental legislation has become stricter in recent years,the issue of wastewater treatment in rural areas has become an increasing concern.Choice of the most suitable on-site purification systems is based on the key issues of affordability and appropriateness in Chinese rural areas.This paper describes an integrated household constructed wetland (IHCW)system planted with willow (Salix babylonica )to treat household domestic wastewater in rural villages in northern China.The precast frame structure of IHCW is strong and waterproof.It can be mass-produced and installed per a standard set of specifications.The IHCW has achieved high overall removal efficiencies for BOD 5,TSS,NH 4-N,and TP:96.0%,97.0%,88.4%and 87.8%,respectively.A 0.4m biomass layer cover on the system provided significant system thermal insulation,maintaining high treatment performance in freezing winter conditions.The system is cost effective and does not need any operational energy inputs,demonstrating its feasibility for single-family use in developing countries.
© 2011 Elsevier B.V. All rights reserved.
1.Introduction
Domestic wastewater treatment in rural areas is essential to prevent pollution of aquatic environments,which has been of increasing concern for both researchers and government officials (Ichinari et al.,2008).These concerns are a prelude to toughing environmental legislation.
Estimates from the World Health Organization (WHO)and the Water Supply and Sanitation Collaborative Council indicate that <18%of rural populations have access to sanitation services in developing countries (Massoud et al.,2009).In fact,the amount of domestic wastewater treated in China is just 11%for county towns and <1%for rural villages (Pan et al.,2007).To address this situ-ation,in 2005,the Chinese government put forward the strategic plan “New Socialist Countryside Building”.Methods of improving the living environment and dealing with domestic wastewater in rural areas have been an urgent concern of the China State Council and State Environment Protection Administration.
Households in rural areas that do not have public sewers must depend on on-site treatment systems to manage their wastewater.Many on-site wastewater treatment technologies,such as septic
∗Corresponding author.
E-mail address:wsb4660017@ (R.Dong).
tanks,drain-field systems,lagoons,aerobic biological treatment units,membrane bioreactors (MBRs)and constructed wetlands are available (Nakajima et al.,1999;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 effluent unsuitable discharge to water bodies (Carroll et al.,2006).Traditional leach-field 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 goons tend to be unpleasant from an aes-thetic perspective and because of odor production (Burkhard et al.,2000;García 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 efficiency,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 Engineering37 (2011) 948–954
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Fig.1.Schematic diagram of the integrated household constructed wetland system (the doted red line shows the waterflow path).(For interpretation of the references to color in thisfigure legend,the reader is referred to the web version of the article.)
tural factors.That is the underlying reason some currently available 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 into an alternative disposal system based on local requirements and 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-flow,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-flow model to avoid damage from low temperatures in winter (Fig.1).The system consisted of a two-stage sedimentation tank and a verticalflow constructed wetland bed section.The frame structure was precast with magnesia cement andfiber glass fabric which is strong and waterproof.In plan view the structure is ellipti-cal with the bottom smaller than the top to facilitate transportation. 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 of0.5m3.The empty volume of the wet-land bed section is1.2m3(area×depth:1.2m2×1.0m).A steel sieve was installed in the inlet basin to prevent large solids(such as vegetable leaves andfish scales from the kitchen)fromflow-ing into the tank.Wastewaterflows into thefirst segment from the inlet basin and then into the second segment via afloating valve installed in thefirst segment to allow intermittent system feeding.In order to maintain normal operation during the winter period,0.4m of sawdust insults the bed.Wastewaterflows from the sedimentation tank downwards through a60mm diameter perfo-rated plastic pipe with5mm holes located at the top of the sand layer and then trickles through the wetland bed.The effluentflows into the bottom gravel layer and then through the dewatered alum sludge placed in the outlet,andfinallyflows 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 modified according to the stan-dard design criteria(Brix and Arias,2005):a15cm layer of washed gravel with particle size of10–30mm,15cm of washed pea gravel with particle size of5–12mm,and90cm of washed sand.The effective size of the washed sand is0.45–1mm with a uniformity coefficient of3.8.Seventyfive kilograms of washed dewatered alum sludge derived from drinking water treatment plant with particle size of0.5–1mm was put in the outlet of the system.The medium surrounding the distribution pipe network was10cm of gravel with particle size of10–30mm.
A willow(Salix babylonica)with a trunk diameter of40mm was planted in the verticalflow 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. The experience of some constructed wetland systems with willows 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 November2007. Sampling evens occurred from March2008to February2009.Dur-ing the experiment operation,household wastewater influent was comprised of kitchen and laundry effluents.The hydraulic loading rate was about0.12m d−1.Water samples of approximately200ml were collected from the influent,sedimentation tank(second seg-ment),and effluent(Fig.1)at7–10days intervals to evaluate the treatment performances.Wastewater parameters of biochemical oxygen demand(BOD5,5210B.5-day BOD test),total suspended solids(TSS,2540D.total suspended solids dried at103–105◦C), ammonia-nitrogen(NH4-N,4500G.automated phenate method), and total phosphorus(TP,4500F.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-ronment of China Ministry of Agriculture according to the Standard 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.The temperature of the vertical constructed wetland 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 among158home-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
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Fig.2.Concentrations of influent,sedimentation tank and effluent in integrated household constructed wetland system ((a)biochemical oxygen demand (BOD),(b)total suspended solids (TSS),(c)ammonia-nitrogen (NH 3-N),and (d)total phosphorus (TP)).
Table 1
Construction cost of one integrated household constructed wetland system.Items
Cost (US dollars)Precast frame structure 147Gravel and sand 36Installation
30Pipes and joints 22Excavation 22Willow 2Sum
259
of BOD 5,TSS,NH 4-N and TP in the influent were 302.4mg/l,128.6mg/l,30.7mg/l and 5.0mg/l,respectively,and the aver-age concentrations in effluent were all continuously reduced to 11.8mg/l,3.8mg/l,3.5mg/l and 0.6mg/l,respectively (Fig.2).
Average removal efficiencies of BOD 5,TSS,NH 4-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 efficiency for BOD 5,TSS,and NH 4-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 259US dollars,including precast frame structure,gravel and sand,and installation (Table 1).Of the householders polled on cost issues,23%respondents were reluctant to pay any money for wastewater treatment facilities construction,
32%took a “wait and see”position,and 45%were willing to pay (Fig.5a).The percentage of householder’s willingness to pay ranges from 3to 6dollars per year was 54.1%,but just 6.4%were willing to pay more than 12dollars per year (Fig.5b).These attitudes may be closely related to local economic conditions,especially the farmer’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 BOD 5and 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 BOD 5and 46%of TSS (Fig.6).This treatment performance was similar to that observed for primary settling (Metcalf and Eddy,2003).
Anaerobic degradation in the sedimentation tank may have also played an important role in treatment.Domestic wastewater from a single rural family traditionally comes from kitchen and laun-dry effluent.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 NH 4-N and TP was just 15%and 17%.As expected,nitrification 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 fulfils primary treatment buffering of BOD 5and TSS to the treatment system by moderating high influent concentrations (Fig.7).The clear bene-fit of the sedimentation tank section lies not only in concentration reduction of BOD 5and TSS,but also to avoid or relieve clogging
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Fig.3.Pollutants removal efficiencies in integrated household constructed wetland system ((a)biochemical oxygen demand (BOD),(b)total suspended solids (TSS),(c)ammonia-nitrogen (NH 3-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;Álvarez et al.,2008;Barros et al.,2008;Chen et al.,2008).
Phosphorus removal in this system exhibited 87.6%overall removal efficiency 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;Yates and Prasher,2009).Phosphorus removal in constructed wetlands is an integrated process including settlement,plants uptake,and bacteria adsorption and substrate affinity (Greenway and Woolley,1999;Vymazal,2007;Kadlec and Wallace,2008),but compared to adsorption to substrate other mechanisms of phos-phorus removal are not significant 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 landfill.At present,there is
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Fig.5.Willingness to pay (a)and payment level (b)for the householders to treat their domestic wastewater in rural areas.(158respondents 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 (NH 3-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 influent 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 effluent 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 10years,which was calculated in batch tests as 31.9mg P/g by alum sludge (Babatunde et al.,2009).
Seasonal influence 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 nitrifica-tion.Although aerobic biofilm systems can maintain nitrification below 6◦C (Choi et al.,2008),nitrification 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 nitrification.The insulation layer allows effluent 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,Mæhlum 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
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Table 2
Summary of hypothetical EPA rural community wastewater treatment technologies costs (1995US dollars,adapted from USEPA,1997)and the integrated household constructed wetland system costs.Technology
Total capital cost Annual operation and maintenance cost Total annual cost Centralized system
2,321,840–3,750,53029,740–40,260216,850–342,500Alternative small-diameter gravity sewers 598,100729055,500Collection and small on-site systems
510,00013,40054,500Integrated household constructed wetland system
34,965
700
3518
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 450people in 135
homes.
Fig.7.Sedimentation tank and effluent concentrations change with different influent loading concentrations ((a)biochemical oxygen demand (BOD),(b)total suspended solids (TSS),(c)ammonia-nitrogen (NH 3-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 Mæhlum et al.(1995)using a filter medium with high phosphorus adsorp-tion.
Cost estimates on a national and/or international basis for wastewater treatment systems are difficult 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 household to con-struct a integrated household constructed wetland system between the average annual payment levels of $5.66(calculated from Fig.5)and $26.1total annual cost (calculated from Table 2).The local gov-ernment would need to fill the gap.Choosing a “Most Appropriate
Technology”is not an easy task,but is necessary to avoid failure.The two key issues in managing a treatment technology are afford-ability and appropriateness (Grau,1996).Affordability relates to the economic conditions of the ernment support is important in developing countries to realize implementation of sanitary infrastructure,even when decentralized.Appropriate-ness relates to environmental and social conditions,which requires appreciation of local cultures,active participation,and correct maintenance training of local people.
5.Conclusion
1.Without public sewers and limited economic conditions,the integrated household constructed wetland system planted with willow effectively treated household domestic wastewater.The high overall removal efficiencies for BOD 5,TSS,NH 4-N,and TP were achieved for 96.0%,97.0%,88.4%and 87.8%,respectively.
2.The 0.4m insulating biomass layer maintained bed temperature above 6◦C in the face of freezing temperatures common in north-ern China winters.There was no significant loss of treatment during the winter as a result.
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3.Dewatered alum sludge obtained from a drinking water plant
and incorporated into the wetland bed proved to effectively remove phosphorus.If widely used,this method would have the additional benefit of reducing alum loading of landfills.
4.The frame precast structure(magnesia cement andfiber glass
fabric)is strong and waterproof.It could be industrially produced and installed to standard specifications during con-struction.
5.The system is much more cost effective than others reported in
the peer-review literature,but there is still an affordability prob-lem.Support from the government will be needed to implement this infrastructure.
Acknowledgment
The authors are pleased to acknowledge thefinancial support provided by Agricultural Scientific and Technical Achievements Transform Project(Project No.21318008).
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