Vacuum sewer systems
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Δίκτυα Αναρρόφησης: Μία Νέα Τεχνολογία Αποχέτευσης Λυμάτων Με Οικονομοτεχνικά, Περιβαλλοντικά και Κατασκευαστικά Πλεονεκτήματα ΒΑΣΙΛΕΙΟΣ Λ. ΜΑΛΙΩΚΑΣ ΜΑΡΙΑΝΘΗ Β. ΜΑΛΙΩΚΑ Δρ. Πολιτικός Μηχανικός M.Sc. Πολιτικός Μηχανικός Περίληψη Σε περιοχές με επίπεδο τοπογραφικό ανάγλυφο, υψηλό υδροφόρο ορίζοντα, ασταθές ή βραχώδες υπέδαφος, καθώς και σε περιβαλλοντικά ευαίσθητες περιοχές αναφορικά με την προστασία των υπόγειων υδροφοριών, την ύπαρξη αρχαιοτήτων κλπ., η κατασκευή συμβατικών δικτύων αποχέτευσης (βαρυτικά δίκτυα) σχετίζεται με την ύπαρξη σημαντικών προβλημάτων (κατασκευαστικών, λειτουργικών , οικονομικών , κλπ.). Στις περιπτώσεις αυτές τα δίκτυα αναρρόφησης δίνουν λύσεις με οικονομοτεχνικά και περιβαλλοντικά πλεονεκτήματα συγκριτικά με τα συμβατικά δίκτυα. 1. ΕΙΣΑΓΩΓΗ Ο ρόλος των συστημάτων αποχέτευσης λυμάτων είναι καθοριστικός για την προστασία της δημόσιας υγείας. Σκοπός των δικτύων αποχέτευσης λυμάτων είναι η συλλογή των λυμάτων και η μεταφορά αυτών στις εγκαταστάσεις επεξεργασίας και διάθεσης . Σύμφωνα με την κείμενη νομοθεσία δίκτυα αποχέτευσης αστικών λυμάτων θα έπρεπε να διαθέτουν μέχρι την 31.12.2000, οι οικισμοί με ΙΠ πάνω από 15.000 κατοίκους και μέχρι την 31.12.2005, οι οικισμοί με ΙΠ από 2.000 μέχρι 15.000 κατοίκους [1], [2]. Τα συστήματα αποχέτευσης λυμάτων διακρίνονται στα συμβατικά και στα εναλλακτικά συστήματα. Τα συμβατικά συστήματα αφορούν στα βαρυτικά συστήματα ή συνδυασμό βαρυτικού και συστήματος υπό πίεση. Τα εναλλακτικά αποχετευτικά συστήματα αφορούν στα συστήματα αναρρόφησης , τα αμιγώς υπό πίεση συστήματα και τα συστήματα μικρής διαμέτρου με βαρύτητα [3], [4]. Τα αποχετευτικά συστήματα αναρρόφησης λειτουργούν με υποπίεση η οποία δημιουργείται με αντλίες κενού. Τα αμιγώς υπό πίεση συστήματα προϋποθέτουν τη χρήση αντλιών για τη μεταφορά των λυμάτων . Τα συστήματα μικρής διαμέτρου με βαρύτητα πλησιάζουν τη λειτουργία των συμβατικών βαρυτικών συστημάτων. Χαρακτηριστικό των συμβατικών συστημάτων είναι το υψηλό κόστος κατασκευής , ειδικά σε αραιοκατοικημένους οικισμούς που χαρα κτηρίζονται από την επιπεδότητα του εδάφους, το οποίο προκύπτει από τις σχετικά μεγάλες διαμέτρους, τις βαθιές εκσκαφές και τον μεγάλο αριθμό φρεατίων επίσκεψης . Μείωση του κόστους κατασκευής μπορεί να επιτευχθεί με τα εναλλακτικά συστήματα αποχέτευσης , τα οποία χαρακτηρίζονται από τη χρήση αγωγών μικρής διαμέτρου, την τοποθέτηση αγωγών σε μικρά βάθη και την αποφυγή φρεατίων επίσκεψης. Σημειώνεται ότι τα εναλλακτικά συστήματα αποχέτευσης είναι η μόνη λύση σε περιπτώσεις με μη ευνοϊκό τοπογραφικό ανάγλυφο , υψηλό υδροφόρο
description
Vacuum sewer systems (in Greeks)
Transcript of Vacuum sewer systems
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1. 5673/400/97 (192//14.03.97)., 19661/1982/99 (1811//29.09.99) 48392/939/28.03.02 ( 405//03.04.02) 91/271/EEC 211991.
2. 91/271/EEC,M1991..
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3. Water Environment Federation (WEF), (1986). Alternative Sewer Systems.Manual ofPracticeFD12.
4. United States Environmental Protection Agency (USEPA), October 1991. ManualAlternativeWastewaterCollectionSystems.EPA/625/191/024.
5. Metcalf & Eddy and George Tchobanoglous, May 1981. Wastewater Engineering:CollectionandPumpingofWastewater.cGrowHillCollege.
6. ., 2005. .,31,.:2830.
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VacuumSewerSystems:ANewSewerTechnologyWithTechnoeconomic,EnvironmentalandConstructionAdvantages
VASSILIOSL.MALIOKAS MARIANTHIV.MALIOKADr.CivilEngineer M.Sc.CivilEngineer
Abstract
Inareaswithflattopography,highwatertable,structurallyunstablesoilorrockyconditionsas well as environmentally sensitive areas (e.g. protected groundwater aquifers,archaeological sites), the use of conventional gravity sewers is relatedwith considerableproblems in construction, operation, cost, etc. Under these particular conditions, thevacuum sewer systems offer solutions with technoeconomic, environmental andconstructionadvantageswhencomparedtoconventionalsewers.
1. INTRODUCTION
Theroleofthesewersystems isdecisive inprotectingpublichealth.TheaimofthesewersystemsisthecollectionofwastewaterandtheconveyancetotheWWTPfacilities.
Accordingtothecurrentlegislation,sewersystemsarerequireduntil31.12.2000fortownswith PE>15,000 inhabitants and until 31.12.2005 for settlementswith PE from 2,000 to15,000inhabitants[1],[2].
The sewer systems are distinguished in conventional and alternative systems. Theconventionalsystemsincludethegravitysystemsorthecombinationofgravityandpressuresystems.Thealternative systems include the vacuum systems, thepurepressure systemsandthesmalldiametergravitysystems[3],[4].
Theconventionalsewersystemshavehighconstructioncost,especiallyforsparsepopulatedcommunitieswith flat topography, resulting from relatively largepipelinediameters,deepexcavationsand largenumberofmanholes.Thealternativesewersystemscanreduce theconstructioncostwiththeuseofsmalldiameterpipelines,theplacementofthepipelinesinshallowdepthsandtherequirementofasmallnumberofmanholes(collectionchambers).
Itshouldbenotedthatthealternativesewersystemsaretheonlysolutionunderconditionsofunfavorable topography,highwater table,unfavorable soil (instability, sand, rock) andenvironmentalconstraints[5].
Thepresentworkpresentsthetechnology,advantagesandapplicationofthevacuumsewersystems. Finally,a case study ispresented forMDofCapetanMitrousi,Prefecture Serreswhichhasthetypicalcharacteristicsfortheapplicationofthevacuumtechnology.
Abbreviations
MD MunicipalDistrict
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WWTP WastewaterTreatmentPlantHSA HellenicStatisticalAuthorityPE PopulationEquivalent
2. DESCRIPTIONOFTHEVACUUMSEWERTECHNOLOGY
Theuseofvacuum(negativepressure)forthecollectionofwastewaterandothereffluentswasapplied initially in1860byCharlesT. Liernuranddeveloped further in1950by JoelLiljendhalfromSweden.Atypicalvacuumsewersystemincludesthefollowingthreecoreelements:
1. Thecollectionchambers,whichcollecttheeffluentwastewaterfromtheconnectedhouseholdsandthroughthemwastewaterentersthevacuumpipelinenetworkviapneumaticvalves.
2. Thevacuumpipelines,whichformthepipelinenetworkthroughwhichthevacuumpressure is transferred to the pneumatic valves in the collection chambers fromwherewastewaterisconveyedtothevacuumpumpstation.
3. Thevacuumpumpstation,whichisthecentralpartofthewholesystem,wherethenegativepressure(vacuum)iscreatedfortheentiresewersystemforthecollectionofwastewaterandthen,throughdischargepumpsandapressurepipelinenetwork,itsconveyancetotheWWTPfacility.
Additionally, a vacuum sewer system includes amonitoring system for the statusof thepneumaticvalvesandalertsystemwhen thewastewater level in thecollectionchambersexceedscertainlimits[6].3. ADVANTAGESOFVACUUMSEWERSYSTEMS
Thevacuumsewersystemsofferreducedconstructioncostsandavoidtheconstructionandoperationproblemsencounteredbytheconventionalsystems(gravitysewers).
The wastewater conveyance within the pipelines, being under pressure, allows theplacementofthepipesinupwardslopesandthebypassofseveralobstacles.
Theconstructioncostappears tobe reducedup to40%due to lowerexcavationvolumes(maximum excavation depth for pipelines and collection chambers is 1.50m and 2.00mrespectively) and requirement for only one pump station in contrast to the conventionalsystemwhichrequireseveralpumpstations.Problems associatedwith the vacuum sewer systems are the complexityof thehydraulicoperation of them and the need for constant monitoring. In order to overcome theseproblems,goodknowledgeofthehydraulicconditionswithinthepipesandinstallationofamonitoringsystemareneeded.
4. APPLICATIONFIELDOFTHEVACUUMSEWERSYSTEMS
Vacuum sewer systemsare ideal forareaswith flat topography (plainsand coastalareas)where the conventional gravity sewers would require deep excavation depths. Theplacementofthevacuumpipelinesinshallowdepthsisthemosteconomicsolutioninareas
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withrocky,sandyorunstablesoils.Thewatertightnessof thevacuumpipelinessolves theproblem in areaswith highwater tables. Thewastewater leakage is preventedwhich isespecially important in environmentally sensitive areas (e.g. threatened groundwateraquifers).Simultaneously,thewaterinfiltrationintothepipesispreventedandthecollectedwastewatervolumedoesnot increaseduring itsconveyancetotheWWTPfacility.Theuseof smalldiameterpipelinesand theirplacement in shallowdepths, solves theproblem inareaswithlimitedspace.
Other applications include the main sewer system for rural communities, campsites,archaeologicalsites,oldcitieswithnarrowstreets,hospitals,airportsandrailwaystations,marinesandports,leachatefromsanitarylandfills,etc[6].
Theconstructionofvacuumsewersystemshasbeenwidelyacceptedworldwide.InGreecethelastdecadeparticularinteresthasbeenshowninthesesystems.VacuumsewersystemsareinoperationinPotideaandinsettlementsofKassandra,Halkidiki.InseveralothercasesinGreece,vacuumsewersystemsareunderconstruction.
5. TECASESTUDYOFMUNICIPALDISTRICTCAPITANMITROUSI,PREFECTURESERRES
The MD Capitan Mitrousi has a population of 2.224 inhabitants (HAS, 2001) and incompliancewithDirective 91/271/EC it should had already construct sewer systems andWWTP facility [1], [2]. Currently the sewer infrastructure does not exist and there is aplanning for thewastewater treatment to takeplace at theCityof SerresWWTP facility.Thereisanexistingdesignstudyforagravitysewersystemsince2003[9].
The case study includes a comparison between the conventional and vacuum sewersystems.TheMDCapitanMitrousiwas selectedbecause theparticular conditions governtheareaare ideal for theapplicationof thevacuumsewersystems.Morespecifically, theentire area is flat, the groundwater aquifer is high (1.01.5m below grade), the soil isstructurallyunstable(clayandsand)andthesewerpipelineshavetobypassalargedrainagetrenchthatpassesthroughthemiddleoftheMDarea.
According to the design of the conventional system, the wastewater collection systemrequired9pumpstations,theaverageexcavationdepthwas2.10mandforthe80%ofthetotal pipeline length (approximately 30,000 m) required trench shoring systems (kringstype).Therequiredpipelinediametersrangedfrom200mmto315mmand765manholeswererequiredfortheoperationandmaintenanceofthepipelinenetwork[9].
Accordingto thedesignofthevacuumsystem,thewastewatercollectionsystemrequiredone vacuum liftpump station. It shouldbenoted that thebypassof the largedrainagetrenchwithvacuumpipelinedoesnotrequiretheuseofanextrapumpstation.Theaverageexcavationdepthrangedfrom1.10mto1.40mandduetoshallowdepths,trenchshoringsystemswerenotneeded.Therequiredpipelinediametersrangedfrom90mmto200mmand165manholes (150collectionmanholesand15 isolationmanholes)wererequired fortheoperationandmaintenanceofthepipelinenetwork.
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Theapplicationofthevacuumsewersysteminduceda40%reductioninconstructioncosts.This reduction originates mainly from the reduced costs for earthworks, due to lowerexcavationvolumes,andlowerexpensesfortheconstructionofthemanholes.
Theoperationalcost isdirectlyrelatedwiththeenergyconsumption.Fortheconventionalsystem, the energy consumption for operation of 1 hour is 8.85 KWh,whereas for thevacuum system, the energy consumption for operation of 1 hour is 15.36 KWh. Theaforementioned energy consumptions were extrapolated on an annual basis for thedeterminationoftheannualenergycostpercapitaassumingthecurrentelectricitypriceof0.10 /KWh. The resulted annual energy cost for the conventional system was 1.85/KWh/capita,whereasforthevacuumsystemwas3.21/KWh/capita.
Theoperationalcostisalsorelatedtothemaintenancecostofthepipelinenetworks.Boththeconventionalandvacuumsystemsrequiremaintenanceof thepumps. Inaddition, thevacuum systems require check and maintenance of the manholes (estimated time: 4hours/week)anddetailedcheckandmaintenanceof theequipmentof thevacuumpumpstation(estimatedtime:8hours/week)bytrainedpersonnel.
The maintenance cost of the networks considered, in both cases, the same order ofmagnitude,so for thecomparisonof the totalcosts forconstructionandoperationof thetwo systems, the construction costand theenergy consumption costwere summed. It isnotedthatthepresentvalueoftheenergyconsumptioncostforthelifetimeoftheproject(40years)wascalculated.
It was found that the total cost of construction and operation of sewerage networksdecreasedby40%ifinsteadoftheconventionalmethod,vacuumsewersareappliedinM.D.CapitanMitrousi.
6. CONCLUSIONS
The present work presented the vacuum sewer systems as a current alternative withsignificanttechnoeconomicandenvironmentaladvantages.
Inareaswith flat topography,highwater tablesand structurallyunstable soils, theuseofvacuum sewer systems offers solutions with low costs and facile construction whencomparedtoconventionalsewers.
The construction cost is significantly lowerwhereasoperational cost ishigher. It isnotedthattheincreaseoftheoperationalcostfoundnegligible,comparedtothelargedecreaseoftheconstructioncost.
Inaddition to the technoeconomicadvantages, the vacuum sewer systemsare themostsuitable in environmentally sensitive areas. They are watertight (they do not allowwastewater leakage)ensuring theprotectionof thegroundwateraquifers.Further,due tothewatertightnessofthesystem,groundwatercannotenterthepipelinesandthecollectedwastewatervolumedoesnotincreaseduringitsconveyancetotheWWTPfacility.
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7. REFERENCES
1. Joint Ministerial Decision [JMD] 5673/400/97 (Greek Government Gazette [GGG]192//14.03.97). Guidelines for urban wastewater treatment, as amended by JMD19661/1982/99(GGG1811//29.09.99)and48392/939/28.03.02(GGG405//03.04.02)with the addition of the list of sensitive areas for urban wastewater disposal CompliancewiththeEuropeanCouncilDirective91/271/EECof21May1991concerningurbanwastewatertreatment.
2. EuropeanCouncilDirective91/271/EEC,May1991.Urbanwastewatertreatment.
3. Water Environment Federation (WEF), (1986). Alternative Sewer Systems.Manual ofPracticeFD12.
4. United States Environmental Protection Agency (USEPA), October 1991. ManualAlternativeWastewaterCollectionSystems.EPA/625/191/024.
5. Metcalf & Eddy and George Tchobanoglous, May 1981. Wastewater Engineering:CollectionandPumpingofWastewater.cGrowHillCollege.
6. Maliokas V., April 2005. Vacuum Sewer Systems.GreekMagazine Hydrooikonomia,Issue31,pp2830.
7. Vassilios Maliokas & Associates LTD, October 2008. Liquid Waste Master Plan forPrefecture of Western Greece. Prefecture of Western Greece Authority ofManagementProgramforWesternGreece.
8. VassiliosMaliokas&AssociatesLTD,May2006.UpdateofLiquidWasteMasterPlanforPrefecture of Central Macedonia. Prefecture of Central Macedonia Authority ofManagementProgramforCentralMacedonia.
9. VassiliosMaliokas& Associates LTD, October 2003. Design ofWastewater CollectionSystemforMunicipalityofCapitanMitrousi.MunicipalityofCapitanMitrousi.
VassiliosL.Maliokas,Dr.CivilEngineer,G.Papandreou18,Thessaloniki,54645MarianthiV.Malioka,M.Sc.CivilEngineer,G.Papandreou18,Thessaloniki,54645
