International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 13 Issue: 05 | May 2026 www.irjet.net p-ISSN: 2395-0072
DESIGN AND SIMULATION OF DECENTRALIZED WASTEWATER TREATMENT SYSTEMS FOR PERI-URBAN AREAS USING HYBRID MODULAR UNITS
Abdul Baqui Raheem1, Mr. Ushendra Kumar2
1Master of Technology, Civil Engineering, Lucknow Institute of Technology, Lucknow, India
2Head of Department, Department of Civil Engineering, Lucknow Institute of Technology, Lucknow, India
Abstract - Rapid urbanization in peri-urban regions has intensified the challenge of wastewater management due to inadequate centralized sewerage infrastructure. This study presents the design and simulation of a decentralized wastewater treatment system (DEWATS) using hybrid modular units tailored for peri-urban conditions. The proposed system integrates physical, biological, and natural treatment processes, including screening, sedimentation, anaerobic baffled reactors (ABR), constructed wetlands, filtration, and disinfection. A representative peri-urban case with a projected population of approximately 16,386 and an estimated wastewater flow of about 1770 m³/day was considered. Design calculations were performed based on CPHEEO guidelines, ensuring technical reliability and contextualrelevance.Simulationmodelingusingtoolssuchas BioWin and MATLAB was conducted to predict system performance under varying hydraulic and organic loading conditions. The results indicate that the hybrid modular system achieves significant removal efficiencies for key pollutants such as biochemical oxygen demand (BOD), chemical oxygen demand (COD), and total suspended solids (TSS),meetingCPCBdischargestandards.Sensitivityanalysis further demonstrates the robustness and adaptability of the system. The studyhighlights the potentialofhybridDEWATS as a sustainable, scalable, and cost-effective solution for wastewater treatment inperi-urbanareas
Key Words: Decentralized wastewater treatment; Hybrid modular systems; Peri-urban sanitation; DEWATS; Simulation modeling; Constructed wetlands
1. INTRODUCTION
1.1 Background
1.1.1 Challenges in Peri-Urban Wastewater Management
Peri-urbanareasrepresenttransitionalzonesbetweenrural and urban environments, often characterized by rapid populationgrowth,unplanneddevelopment,andinadequate infrastructure.Theseregionsfacesignificantchallengesin wastewater management due to increasing water consumption, rising wastewater generation, and limited institutional capacity. The variability in land use, ranging from residential to semi-agricultural activities, further
complicates the design and implementation of treatment systems. In such settings, untreated or partially treated wastewaterisfrequentlydischargedintotheenvironment, leadingtogroundwatercontamination,publichealthrisks, andecologicaldegradation.Thesechallengesnecessitatethe development of adaptable and decentralized treatment solutionsthatcanfunctionefficientlyundervariableloading andinfrastructuralconstraints(UN-Habitat,2015;Massoud etal.,2009).
1.1.2 Lack of Centralized Sewer Systems
Amajorlimitationin peri-urban regionsistheabsence or inadequacy of centralized sewer networks. Conventional centralized wastewater treatment systems require substantialcapitalinvestment,extensivepipelinenetworks, andhighoperationalexpertise,whichareoftennotfeasible in rapidly expanding peri-urban zones. As a result, many communities rely on on-site sanitation systems such as septic tanks, which are frequently poorly maintained and inefficientinpollutantremoval.Thiscreatesapressingneed for decentralized alternatives that can operate independentlyoflarge-scaleinfrastructurewhileensuring effectivetreatmentandenvironmentalprotection(Tilleyet al.,2014).
1.2 Literature Review
1.2.1 Existing DEWATS Technologies
Decentralized wastewater treatment systems (DEWATS) have emerged as a viable solution for areas lacking centralized infrastructure. These systems typically incorporate a combination of primary, secondary, and tertiarytreatmentprocesses,includingsedimentationtanks, anaerobic reactors, and natural treatment units such as constructed wetlands. DEWATS are known for their low energyrequirements,minimaloperationalcomplexity,and suitability for community-scale applications. Previous studieshavedemonstratedtheireffectivenessinremoving organic matter and suspended solids, particularly in developing regions where resource constraints are significant (Crites and Tchobanoglous, 1998; Kadlec and Wallace,2009).
International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 13 Issue: 05 | May 2026 www.irjet.net p-ISSN: 2395-0072
1.2.2 Hybrid Systems and Modular Approaches
Recent advancements in wastewater engineering have focusedonhybridsystemsthatintegratemultipletreatment technologies into a modular configuration. These systems combine the strengths of different processes, such as anaerobicdigestionandnaturalpolishing,toachievehigher treatment efficiencies. Modular designs offer flexibility, scalability, and ease of maintenance, making them particularlysuitableforperi-urbancontextswheredemand and conditions may change over time. Hybrid DEWATS configurations,includinganaerobicbaffledreactorsfollowed by constructed wetlands, have shown improved performanceintermsofpollutantremovalandoperational stability(VonSperling,2007)
1.2.3 Gaps in Current Research (Lack of SimulationBased Optimization)
DespitethegrowingadoptionofDEWATS,manystudiesrely primarily on empirical design approaches and field observations,withlimiteduseofadvancedsimulationtools. Theabsenceofsimulation-basedoptimizationrestrictsthe ability to predict system performance under varying conditions,suchasfluctuatingflowratesandpollutantloads. This gap reduces the reliability of design outcomes and limits the potential for system optimization prior to implementation. Incorporating simulation modeling into DEWATS design can enhance predictive accuracy and supportinformeddecision-making(Henzeetal.,2008).
1.3 Research Gap
1.3.1
Limited Integration of Design, Simulation, and Evaluation
Existingresearchoftentreatssystemdesign,performance simulation,andregulatoryevaluationasseparateprocesses rather than an integrated framework. This fragmented approach can lead to inconsistencies between theoretical design and actual system performance. A comprehensive methodologythatintegratesengineeringdesignprinciples with simulation modeling and performance evaluation is essential to ensure reliability, efficiency, and compliance withenvironmentalstandards.
1.3.2
Lack
of Context-Specific Models for Peri-Urban India
Anothercriticalgapliesinthelimitedavailabilityofcontextspecific models tailored to peri-urban conditions in developing countries, particularly India. Wastewater characteristics, climatic conditions, and socio-economic factors in these regions differ significantly from those in developed countries, making direct adoption of existing modelsinappropriate.Thereisaneedforlocalizeddesign frameworks that incorporate regional standards, such as CPHEEO guidelines, and address the unique challenges of peri-urbanenvironments.
1.4 Objectives
1.4.1
Design of Hybrid Modular DEWATS
The primary objective of this study is to develop a hybrid modular decentralized wastewater treatment system that integratesmultipletreatmentprocessesintoacohesiveand efficientconfiguration.Thedesignaimstoensuretechnical feasibility, scalability, and adaptability to peri-urban conditions.
1.4.2
Simulation of System Performance
Anotherkeyobjectiveistosimulatetheperformanceofthe proposedsystemusingadvancedmodelingtools.Simulation enablesthepredictionoftreatmentefficiencyundervarying hydraulicandorganicloadingconditions,therebyreducing uncertaintyandimprovingdesignreliability.
1.4.3
Evaluation against Regulatory Standards
The final objective is to evaluate the performance of the system against established regulatory standards, such as those prescribed by pollution control authorities. This ensures that the treated effluent meets environmental dischargerequirementsandsupportssafereuseordisposal.
2. MATERIALS AND METHODS
2.1
Study Area Description
2.1.1 Peri-Urban Characteristics
The study focuses on a representative peri-urban region characterizedbyrapidurbanexpansion,mixedlanduse,and inadequatesanitationinfrastructure.Theseareastypically exhibitatransitionbetweenruralandurbansettings,where basic services such as wastewater management are often underdeveloped. The variability in land use, including residential, commercial, and semi-agricultural activities, leads to fluctuating wastewater characteristics and poses challenges for designing efficient treatment systems. The selection of such a study area ensures that the proposed decentralized wastewater treatment system (DEWATS) is context-specificandpracticallyrelevant
2.1.2 Population Characteristics (5,000–20,000 Range)
Thestudyconsidersacommunity-scalepopulationranging between5,000and20,000,whichistypicalforperi-urban settlements. For design purposes, a base population of 10,000 is assumed and projected over a design period to accountforfuturegrowth.Thispopulationrangeissuitable for decentralized systems, as it allows for manageable systemsizingandefficientoperationwithouttheneedfor large-scaleinfrastructure
5.1.3 Lack of Sewer Infrastructure
Adefiningfeatureoftheselectedstudyareaistheabsenceof acentralizedsewernetwork.Thislimitationnecessitatesthe
International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 13 Issue: 05 | May 2026 www.irjet.net p-ISSN: 2395-0072
adoption of decentralized treatment solutions capable of functioning independently. Existing sanitation practices, suchasseptictanksoropendischarge,areofteninadequate and contribute to environmental pollution, thereby reinforcing the need for an efficient and sustainable DEWATSapproach
2.2 Research Framework
2.2.1
Engineering Design
The engineering design component is based on standard guidelines to ensure technical reliability and applicability. Designcalculationsareperformedusingestablishedcriteria forhydraulicretentiontime,loadingrates,andsafetyfactors. Thisensuresthateachtreatmentunitisappropriatelysized tohandletheexpectedwastewaterloadwhilemaintaining operationalefficiency.
2.2.2
Simulation Modeling
Simulationmodelingisemployedtoreplicatethedynamic behaviorofwastewatertreatmentprocesses.Advancedtools are used to model biological reactions, flow patterns, and pollutantremovalmechanisms.Thisapproachenablesthe predictionofsystemperformanceundervaryingconditions, reducinguncertaintyandenhancingdesignaccuracy.
2.2.3 Performance Evaluation
The performance of the proposed system is evaluated by comparingsimulatedoutputswithregulatorystandards.Key parameters such as BOD, COD, and TSS are analyzed to determine the treatment efficiency and compliance of the system.This step ensuresthatthedesignedsystemmeets environmentaldischargerequirements
2.3 Design Basis
2.3.1 Population Projection
Population projection is carried out using a geometric growth model to estimate future wastewater generation. Basedonaninitialpopulationof10,000andagrowthrateof approximately 2.5% over a 20-year design period, the projectedpopulationisestimatedtobearound16,386.This projection ensures that the system is designed to accommodatefuturedemandandavoidoverloading.
2.3.2 Wastewater Generation
Wastewatergenerationiscalculatedbasedonstandardper capitawatersupplyvalues.Assumingawatersupplyof135 literspercapitaperday(LPCD)andareturnfactorof80%, thetotalwastewaterflowisestimatedtobeapproximately 1770m³/dayforthedesignpopulation.Thisvalueformsthe basisforsizingalltreatmentunitswithinthesystem
2.3.3 Wastewater Characteristics
The design is based on typical wastewater characteristics observed in Indian conditions. Key parameters include
biochemicaloxygendemand(BOD)rangingfrom200–300 mg/L,chemicaloxygendemand(COD)from400–600mg/L, andtotalsuspendedsolids(TSS)from200–350mg/L.These parametersarecriticalforselectingappropriatetreatment processesandpredictingsystemperformance
2.3.4
Peak Factor Analysis
Toaccountforvariationsinwastewaterflow,apeakfactoris applied.Forthegivenpopulationrange,thepeak factoris estimatedtobebetween2.9and3.0.Thisfactorensuresthat the system is capable of handling peak flow conditions withoutcompromisingperformanceorefficiency
2.4 System Design: Hybrid Modular DEWATS
2.4.1 Treatment Train
Theproposedsystemconsistsofasequentialtreatmenttrain designedtoprogressivelyremovecontaminants.Theprocess beginswithscreeningandgritremoval,followedbyprimary sedimentation.Secondarytreatmentiscarriedoutusingan anaerobicbaffledreactor(ABR),whichfacilitatesbiological degradation. Tertiary treatment includes constructed wetlands for polishing, sand filtration for fine particle removal, and disinfection for pathogen control. This integratedapproachensureshightreatmentefficiencywith lowenergyrequirements.
2.4.2 Design of Units
Each treatment unit is designed using appropriate engineeringprinciples.Thesettlertankisdesignedbasedon detentiontimetoalloweffectivesedimentationofsolids.The anaerobic baffled reactor is designed using hydraulic retentiontime(HRT)toensuresufficientcontactbetween wastewater and biomass for degradation. Constructed wetlands are designed using first-order kinetics to model pollutant removal efficiency. These design approaches ensureoptimalperformanceandreliabilityofthesystem
2.5 Simulation Modeling
2.5.1 Software Tools (BioWin / GPS-X / MATLAB)
Simulation of the treatment system is performed using advancedsoftwaretoolssuchasBioWin,GPS-X,orMATLAB. These tools enable detailed modeling of biological and hydraulicprocesses,providinginsightsintosystembehavior underdifferentoperationalscenarios
2.5.2 Process Modeling
The simulation incorporates key processes, including biological degradation of organic matter, hydraulic flow distribution, and pollutant removal mechanisms. By modelingtheseprocesses,thestudyevaluatestheefficiency ofthesysteminreducingBOD,COD,andTSSundervarying conditions
Volume: 13 Issue: 05 | May 2026 www.irjet.net
2.6 Performance Evaluation
2.6.1 Comparison with CPCB Standards
Theperformanceofthesystemisassessedbycomparingthe simulated effluent quality with regulatory standards prescribedbypollutioncontrolauthorities.Thisensuresthat thetreatedwastewatermeetspermissibledischargelimits andissuitableforsafedisposalorreuse.
2.6.2 Evaluation Parameters (BOD, COD, TSS)
KeyperformanceindicatorsincludereductionsinBOD,COD, andTSS.Theseparametersareusedtoquantifytreatment efficiency and determine the effectiveness of the hybrid modular system in removing organic and suspended pollutants
2.7 Sensitivity Analysis
2.7.1 Variation in Flow Rate and Organic Load
Sensitivityanalysisisconductedtoevaluatetheresponseof thesystemtovariationsininputconditions,suchaschanges in flow rate and organic loading. This analysis helps in understanding the adaptability of the system under realworldfluctuations
2.7.2 System Robustness Evaluation
Theresultsofthesensitivityanalysisareusedtoassessthe robustnessandreliabilityoftheproposedsystem.Arobust systemisonethatmaintainsconsistentperformancedespite variations in operating conditions, which is essential for peri-urbanapplicationswherevariabilityiscommon
3. RESULTS
3.1 Design Outputs
3.1.1 Flow Rates and Unit Sizing
The design outputs of the proposed hybrid modular DEWATSsystemarederivedbasedonprojectedpopulation, wastewater generation, and standard design criteria. The averagewastewaterflowisestimatedatapproximately1770 m³/day,whilepeakflowconditionsareconsideredusingan appropriatepeakfactortoensuresystemreliability.These flow values form the basis for sizing individual treatment unitssuchassettlers,anaerobicbaffledreactors(ABR),and constructed wetlands. The design ensures that each unit operateswithinoptimalhydraulicretentiontime(HRT)and loadingconditionstoachieveefficienttreatment.
Table 1: Design Flow Parameters
3.1.2
System Configuration
Thesystemconfigurationfollowsamodularandsequential arrangement oftreatmentunitsdesigned toprogressively remove contaminants. The treatment train consists of preliminary, primary, secondary, and tertiary stages, ensuring comprehensive treatment. Each unit is hydraulically and functionally integrated to maintain continuousflowandmaximizepollutantremovalefficiency.
Table 2: Hybrid Modular System Configuration
Preliminary Screening
oflarge debris
Preliminary Gritchamber Removalofsand andgrit
Primary Settler
Secondary ABR
Sedimentationof suspendedsolids
Anaerobic biological treatment
Tertiary Constructed wetland Nutrientand organicpolishing
Tertiary Sandfilter Finefiltration
Final Disinfection Pathogenremoval
3.2 Simulation Results
3.2.1 Influent vs Effluent Quality
Simulationmodelingprovidesacomparativeassessmentof influent and effluent wastewater quality. The influent characteristics reflect typical peri-urban wastewater conditions, while the effluent values indicate the
International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 13 Issue: 05 | May 2026 www.irjet.net p-ISSN: 2395-0072
effectiveness of the treatment system. The results demonstrate a substantial reduction in organic and suspendedpollutantsacrossthetreatmentstages.
Table 3: Influent and Effluent Characteristics
3.3 Performance Evaluation
3.3.1 Compliance with Discharge Standards
Thetreatedeffluentqualityisevaluatedagainstregulatory dischargestandardstodeterminesystemcompliance.The resultsindicatethattheproposedhybridmodularDEWATS systemmeetsthepermissiblelimitsforkeyparameterssuch as BOD, COD, and TSS. This confirms the suitability of the system for safe discharge or reuse in peri-urban environments.
Table 4: Compliance with Standards
4. CONCLUSION
Thisstudy presents the designandsimulation ofa hybrid modular decentralized wastewater treatment system (DEWATS)tailoredforperi-urbanareaslackingcentralized sewer infrastructure. The proposed system integrates physical,biological,andnaturaltreatmentprocessesintoa cohesive and scalable framework, ensuring efficient pollutant removal under varying operational conditions. Basedonarepresentativeperi-urbancase,thesystemwas designed using standard engineering guidelines and evaluated through simulation modeling to predict performance.
The results demonstrate that the hybrid system achieves substantial reductions in key pollutants, including biochemical oxygen demand (BOD), chemical oxygen demand (COD), and total suspended solids (TSS), with effluentqualitymeetingregulatorydischargestandards.The incorporation of an anaerobic baffled reactor (ABR) and
constructedwetlandsenhancestreatmentefficiencywhile maintaining low energy and operational requirements. Furthermore,sensitivityanalysisconfirmstherobustnessof thesystemunderfluctuationsinflowandorganicloading, whicharetypicalinperi-urbanenvironments.
Overall, the study validates the effectiveness of hybrid modular DEWATS as a sustainable, cost-effective, and adaptable solution for decentralized wastewater management. The integration of design, simulation, and performanceevaluationprovidesacomprehensiveapproach that can support informed decision-making and practical implementation.Thefindingscontributetotheadvancement of decentralized sanitation strategies, particularly in developing regions facing rapid urbanization and infrastructuralconstraints.
5. FUTURE SCOPE OF RESEARCH
Futureresearchshouldfocusonpilot-scaleimplementation and field validation of the proposed hybrid modular DEWATStoassessreal-timeperformanceandoperational challenges. Long-term monitoring studies are needed to evaluate system stability, maintenance requirements, and seasonal variations in treatment efficiency. Integration of advancedmonitoringtools,suchasIoT-basedsensorsand real-time data analytics, can further enhance system performance and management. Additionally, economic analysis, including life-cycle cost assessment and cost–benefit evaluation, would provide insights into financial feasibility.Researchcanalsoexplorethereusepotentialof treated wastewater for irrigation or non-potable applications.Finally,adaptingthemodeltodifferentclimatic and socio-economic conditions would improve its applicabilityacrossdiverseperi-urbansettings.
REFERENCES
1. Crites, R. and Tchobanoglous, G., 1998. Small and Decentralized Wastewater Management Systems. Boston:McGraw-Hill.
2. Henze, M., van Loosdrecht, M.C.M., Ekama, G.A. and Brdjanovic,D.,2008.BiologicalWastewaterTreatment: Principles, Modelling and Design. London: IWA Publishing.
3. Kadlec,R.H.andWallace,S.,2009.TreatmentWetlands. 2nded.BocaRaton:CRCPress.
4. Massoud, M.A., Tarhini, A. and Nasr, J.A., 2009. Decentralizedapproachestowastewatertreatmentand management: Applicability in developing countries. JournalofEnvironmentalManagement,90(1),pp.652–659.
5. Tilley,E.,Ulrich,L.,Lüthi,C.,Reymond,P.andZurbrügg, C., 2014. Compendium of Sanitation Systems and Technologies. 2nd ed. Dübendorf: Swiss Federal InstituteofAquaticScienceandTechnology(Eawag).
International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 13 Issue: 05 | May 2026 www.irjet.net p-ISSN: 2395-0072
6. UN-Habitat, 2015. Wastewater Management: A UNWaterAnalyticalBrief.Nairobi:UnitedNationsHuman SettlementsProgramme.
7. Von Sperling, M., 2007. Wastewater Characteristics, TreatmentandDisposal.London:IWAPublishing.
8. CentralPollutionControlBoard(CPCB),2013.General StandardsforDischargeofEnvironmentalPollutants. New Delhi: Ministry of Environment, Forest and ClimateChange,GovernmentofIndia.
9. CPHEEO, 2013. Manual on Sewerage and Sewage Treatment Systems. New Delhi: Ministry of Urban Development,GovernmentofIndia.
10. Arumugam, P., Zuma, L., Mercer, S., Govender, L., Pocock, J. and Brouckaert, C., 2023. Decentralised wastewater treatment systems for urban and rural sanitation.WaterSA,49(1),pp.8–18.
11. Domínguez-Solís,D.etal.,2025.Constructedwetlands asdecentralizedtreatmentsystems:Areview.Water, 17(10),p.1451.
12. Ventura, J.R.S. et al., 2024. Advancements and challenges in decentralized wastewater treatment systems.EnvironmentalTechnology&Innovation,35, p.103000.
13. Varma, V.G. et al., 2022. Decentralized wastewater treatment systems in India: A review. Chemosphere, 303,p.135126.
14. Swan, K.H. et al., 2023. Sustainability assessment of decentralized wastewater treatment systems. Sustainability,15(24),p.16756.
15. Kadry,Z.,2025.Centralized,decentralizedandhybrid wastewater treatment systems: A review. Future EngineeringJournal,3(1),pp.1–15.
16. Zhou,X.Q.etal.,2014.ModifiedDEWATSsystemusing constructed wetlands. Advanced Materials Research, 864–867,pp.1275–1280.
17. Dahiya,H.andLaura,J.S.,2025.Constructedwetlandas decentralized wastewater treatment system. EnvironmentalScienceReview,pp.1–12.
18. Massoud, M.A., Tarhini, A. and Nasr, J.A., 2009. Decentralizedwastewatermanagementindeveloping countries. Journal of Environmental Management, 90(1),pp.652–659.
19. Tchobanoglous,G.andLeverenz,H.,2013.Therationale for decentralization of wastewater infrastructure. In: SourceSeparationandDecentralization.London:IWA Publishing.
20. Larsen, T.A., Udert, K.M. and Lienert, J., 2013. Source Separation and Decentralization for Wastewater Management.London:IWAPublishing.
21. Sasse, L., 1998. DEWATS: Decentralised Wastewater TreatmentinDevelopingCountries.Bremen:BORDA.
22. Reynaud, N., 2015. Operation of DEWATS under TropicalConditions.PhDThesis.TUDresden.
23. ESCAP, UN-Habitat and AIT, 2015. Policy Guidance ManualonWastewaterManagement.Bangkok:United Nations.
24. WRC, 2014. DEWATS Process for Decentralised Wastewater Treatment. Pretoria: Water Research Commission.
25. Singh, N.K. and Kazmi, A.A., 2016. Performance evaluationofABRsystemsforwastewatertreatment. EcologicalEngineering,94,pp.545–556.
26. Vymazal,J.,2011.Constructedwetlandsforwastewater treatment:Fivedecadesofexperience.Environmental Science&Technology,45(1),pp.61–69.
27. Kivaisi, A.K., 2001. The potential for constructed wetlands for wastewater treatment. Ecological Engineering,16(4),pp.545–560.
28. Otterpohl, R., Braun, U. and Oldenburg, M., 1999. Innovative technologies for decentralized sanitation. WaterScienceandTechnology,39(5),pp.23–32.
29. Parkinson, J. and Tayler, K., 2003. Decentralized wastewater management in peri-urban areas. EnvironmentandUrbanization,15(1),pp.75–90.
30. Wilderer,P.A.andSchreff,D.,2000.Decentralizedand centralized wastewater management: A challenge. WaterScienceandTechnology,41(1),pp.1–8.
31. Gutterer,B.,Sasse,L.,Panzerbieter,T.andReckerzügel, T.,2009.DecentralisedWastewaterTreatmentSystems (DEWATS) and Sanitation in Developing Countries. Bremen:BORDA.
32. Libhaber,M.andOrozco-Jaramillo,A.,2012.Sustainable Treatment and Reuse of Municipal Wastewater. London:IWAPublishing.
33. Metcalf & Eddy, 2014. Wastewater Engineering: TreatmentandResourceRecovery.5thed.NewYork: McGraw-Hill.
34. Selerio,E.,2024.Multicriteriaanalysisofdecentralized wastewater treatment technologies. Water Systems Research,pp.1–15.