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Interaction between the treatment plant and the sewer system in Halmstad: Integrated upgrading based on real time control
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Pergamon PIT: 50273-1223(98)00280-7
Wat Sci. Tech. Vol. 37. No.9. pp. 127-134.1998. «> 1998JAWQ. Publishedby ElsevierScienceLtd Printedin Oreal Britain. 0273-1223/98 S19-00+ 0-00
INTERACTION BETWEEN THE TREATMENT PLANT AND THE SEWER SYSTEM IN HALMSTAD: INTEGRATED UPGRADING BASED ON REAL TIME CONTROL C. Hernebring*, L. Ohlsson**, M, Andreasson*** and L.-G, Gustafsson*** • Urban Hydroinformatics Centre AB. Box 1848, S-581 17 Linkdping, Sweden •• TheMunicipality ofHalmstad; Vdrtra Strandens reningsverk; Smdbdtsg 2, 5-302 38 Halmstad; Sweden ••• Urban Hydroinformatics Centre AB, Box 276, S·351 05 Vlixjo, Sweden
ABSTRACT In Halmstad, Swedengreat effortshavebeen madeduringthe 1990's to improvethe functionality and to reduce the environmental impact of the sewer system and the wastewater treatment plant, The investment and rehabilitation programincludes to a great extent an effectiveuse of existingresources. The wastewater treatmentplant is reconstructed to meet increased nutrientremovaldemands. A five year rehabilitation plan for the sewersystem is undercompletion. wherethe measures mainlyare motivatedby the aim to reducethc combinedsewer overflowvolumes and to minimize the risk of local flooding. It was soon realized that an integrated use of storage volumes at the wastewater treatment plant and within the sewer system could improvethe general conditions for the treatment at the plant, To implement this strategya real time control system was introduced by installing controllable weirs and flow control devices in the main sewer. The article descnbes the background of the sewerage masterplan, how the upgrading work has been carried out by means ofsimulations and measurement, gives examples of some expected potentialbenefits.and outlines plans for the future. ~ 1998lAWQ. Published by ElsevierScienceLtd
KEYWORDS combined seweroverflow, flooding, MOUSE, pollution transport. real timecontrol, sewer, STOAT, TYP, wastewater treatment plant. INTRODUCTION Halmstad municipality on the westcoastof Sweden is situated withinan areaidentified to be the most sensitive to nutrient transport by rivers from the drainage basin. Thushighnitrogen (andphosphorus) removal efficiency at thewastewater treatment plant (WWfP) has beenenforced by the environmental authorities with highpriority. Theplantwasreconstructed in 1992-94 at a costof IS MECU. To reducethe combined seweroverflow volumes (eSO) into local receiving water, i.e, RiverNissan - historically an important salmon habitat - a fiveyearrehabilitation planfor the sewersystem is implemented. The most effective measure in reducing eso outletsis the construction of a covered equalization tank ( 3 500 m', investment cost 3 MECU) located withinthe sewersystem, takenintooperation in 1996/97. The abovementioned improvements of the sewerage andthe wastewater treatment are part of a general environmental program of action carried out since 1990, alsoincluding a vast scheme of wetland restoration andconstruction. It was realized thatan alternative to reduce the required investments wasto improve the operation, the control andthe coordination of the operation of the sewerage network andthe treatment plant (Hemebring and Falk, 1993) 127
C. HERNEBRING It al.
128
Halmstad WWTP V4stra Stranden receives wastewater from about 100000 p.e. with a total catchment area of26 km2 • The sewersystemis partlycombined, withthe connected impervious area(170 ha) mainly concentrated to the centralpartsof the city. Mostof the CSOare concentrated withinthe main interceptor alongthe River Nissan. When the workwith the sewerrehabilitation plan started1991-92, it was found that the functioning and capacity of the sewersystemwerenot acceptable: a CSOdischarge from the sewersystem of2.5 % of the total annual wastewater volume and someoutletsactiveas oftenas 60-80times/year. In someparts of the sewersystem occasional flooding problems occurred. Thecriteriaof maximum allowed CSO frequency for the majoroutletswas formulated to be lessthan5 times/year as a guideline for the sewerrehabilitation planning. For flooding, the risk levelof 10 yearrain events were considered. To reachthesegoals in a cost effective way the MOUSE (Lindberg et al., 1989) software was usedas a planning tool.
MODELLING OF THEHYDRAULIC PROCESSES IN THE SEWER SYSTEM AS A BASISFORUPGRADING Intense hydraulic modelling studies wereperformed to serveas a basisfor the sewerage masterplan,based on continuous hydrological modelling of highlydistributed sub-catchments for surface stormrun-off(fast run-offcomponent) and wastewater loadsandclustered modelling of infiltration of storm- and groundwater C'baseflow"- slow run-offcomponent). The methodology has beendescribed by Gustafsson et al, (1996). The verified hydrodynamic (for transport description) and hydrological (for infiltration and inflow) model was usedto outlinemeasures in the sewerin orderto minimize the risk for localflooding and CSOvolumes. Theoptimum use ofexistingtrunksewers was an important concern in implementing effective sewerage masterplan measures. The effectof rehabilitation and upgrade measures in the sewersystemis continuously followed up by modeladjustments and verifications. Basedon the understanding of the sewersystemloads and performance givenby the verified MOUSE modeldescriptions, different what-ifalternatives couldbe tested.
Measu[ed fIowotVOstrO StrondenWWTP MouseRTC -colculotecHlowotWWT? 1.600 1.400 1.200 1.000 0.800 0.600 0.400 0.200 0.000
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14/11 15/11 16/11 13/11 17/11 1994 FIGURE 1. Example ofMouseRTCflow verification results,takingfullhydrodynamics into account (CSO volumes excluded).
The first estimate how to reachthe functional demands, founded on the knowledge of the localdistribution of inflowload and flow capacity, included a totalrequired storage volume distributed withinthe sewer network of appr 14000m' . This figure couldessentially be reduced (by 75 %) afterhydrodynamical studies, wherecombinations of regulations, optimized flowtransport, increased pumping capacity, removal of bottlenecksand separation of impervious area from the combined sewerweretested. Besidesthe use of existing sewervolumes for storageby flowregulations at critical sites,the needfor extrastoragevolume was reduced
Integrated upgrading based on realtime control
129
to 3 500 rn' , which could be located at one central location within the sewer system . The covered equalization tank Vallgraven was recently taken into operation. The completion of the five year rehabilitation plan for the sewer system (1995-99) will result in more than 90% reduction of CSO volumes from the sewer system according to model estimates.
FIGURE 2. Mean annual CSO frequency and volume at major outlets before and after planned measures
THE WASTEWATER TREATMENT PLANT The design of the WWTP is based on a line with high treatment efficiency, but with limited flow capacity, combined with a open equalization tank of 6 000 rn' for peak flows. The storage volume includes pre-precipitation. Before the outlet to the recipient all sewage will pass through polishing ponds . The des ign was supported by computer simulation studies of the activated sludge process . The plant was reconstructed in 1992-94. Swedish National Franchise Board for Environmental Protection will in the near future present final outlet standards for Vastra Stranden WWTP. At the time being , provisional outlet target values are: BOD 7 - 15 mgll (monthly mean value) total nitrogen - 12 mg/l (yearly mean value) total phosphorus - 0.4 mg/l (quarterly mean value) In the final outlet standard fonnulation enhanced conditions concerning the nitrogen and phosphorus removal efficiency are expected , down to 8-10 mgtl total nitrogen and 0.3 rng/l total phosphorus,
130
C. HERNEBRING et al .
respectively. The requirements shouldbe met includinguntreated or partly treatedstorage overflow volumes occurringfrom the equalization tank at the treatment plant. measured at the outlet of the polishing ponds. This may be a difficult task. mainlydepending on the high P-removal efficiency demanded.
INFLUENT
BIOLOGICAL TREATMENT
PRIMARY CLARIFIER
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SECONDARY SETTLER
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FIGURE3. Halmstad wastewater treatment plant flow scheme
COORDINATED USE OF STORAGE - REALTIME CONTROL For the operationconditions at the WWTP it was found that the treatment efficiencycould be increasedwith a more effectiveuse of vacant resources (volumes) in the sewersystem. not only during heavy storms. but also during the dominantpart of the year with more or less dry weatherprevailing. The formulated operation strategieswere tested by means of MOUSE modelcalculations. A coordinateduse. includingreal time control (RTC) actions. of the coveredequalization tank in the sewer system and the open equalization tank at the WWTP wil1 result in a decreased by-pass volume(partly treated)at the treatmentplant. Such a coordination is prepared by the installation of two movable weirs and flow regulationequipment in the main sewer adjacent to the storagevolume. together with six telemetered flow and water level monitorsat critical sites. The control1able weirsare recently taken into operation. The formulation of control strategiesand the functional specifications of the flow control devices were preparedby MouseRTC-simulations. The main purposeof real time control actions is to bring as much waste water volumes as possible through the ful1 treatment line of the WWTP, without causing unacceptable overflowvolumes neither from the sewer systemnor from the storageat the WWTP. This can, for example, be expressedin a control strategy implyingthat, when the WWTPstorage is beginningto be fil1ed up. the covered storage within the sewer system starts to decreasethe outlet flow to the main collector. This will continue up to a certain degree of storage. when the outlet flow gate again is raised. becausean overflow at the WWTPis preferredcomparedto CSO volumes from the sewer system. The CSO weir at the sewer storage is regulatedin such a way that overflow is avoidedas long as possible.but when an overflow is inevitable. the weir is loweredto permit a rapid discharge of overflow water to minimizethe risk of upstream floodings. In figure4 is shown an exampleof the effect of the described control actionsduring consecutive rain events occurringapproximately 2-3 times/year.
Integrated upgrading basedon real timecontrol WATERLEVEL,TREATMENT PLANTSTORAGE
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WATERLEVEl, SEWER STORAGE VAUGRAVEN
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m ~:fVQ~GE~,~N,~]=R m:rrR,~,S~~,~D,~,E~~j~ 211012:00 0:00 311012:00 211012:00 0:00 311012:00 FIGURE 4. Example from MouseRTC-simulations showing utilization of storage and controlactions. Ifno regulation of the sewerstorage outletis applied, about3.3 % of the total annual wastewater volume (during a yearwith meanprecipitation conditions) willbe discharged as by-pass flow at WWTP and consequently onlypartlytreated. By-pass volumes needto be keptto a minimum to reducesludgedeposit problems in the polishing pondsandthe riskof affecting the outletwaterqualityto such an extentthat the standards of the WWTP final outletcannotbe met (Ohlsson, 1997). The resulting storageoverflow volume at the WWTP is naturally highlydependent on the applied flow restriction through the full treatment train. It shouldbe mentioned that the flow limitation is relatively severe: a constant valueof2000 mvh, only 65 % abovethe yearlymeanflow. This flow is determined by sludgethickening and clarification capacity of the secondary settlers. The capacity of the sedimentation tanks couldvary somewhat because of e.g, seasonal sludgeproperty variations. In this analysis a constant valuehas been applied, nearthe actual operational plantexperience of the settlercapacity. If the described controlstrategy is maintained, the yearly WWTP storage overflow volumewill according to MouseRTC simulations be reduced by 30 % compared to staticconditions. If the monthswith high autumn/winter baseflow conditions (here: Nov-Dec) are excluded whenregulations are less effective, the reduction is about40 %. At the sametime,as a consequence of the RTC actions, a slightincrease ofCSO volumes fromthe sewersystem will occurfrom 0.08 to 0.11 % of the total annual wastewatervolume, however not affecting the CSOfrequency.
The real RTC-implementation is somewhat moresophisticated thanthe one described above,wheremore conditions describing the stateof hydraulic loadandboundary conditions are considered. A numberof controlstrategies have beendefinedas normal situations, risk situations and extreme conditions, respectively, depending on the waterlevels and flows in the sewersystem, sea/river leveland storage capacity at the WWTP. A supervisory controland data acquisition (SCADA) system has beenbuilt up for flowand precipitation measurement data fromdifferent pointsin the sewernetwork, and for process data fromthe WWTP. A data basesystemservesas a basis for specific eventanalyses andprovide data for different modelcalibration applications. It is an essential prerequisite in the realoperation situation to obtainreliable and accurate leveland flow measurement data fromdifferent pointsof the sewersystem, especially whencontrolling the storage
132
C. HERNEBRINO et al.
utilization. Therefore, six flow and levelmeters, based on ADS technology (Petroff 1995) wereinstalled at critical sitesconnected on-line to the SCADA system. INTEGRATED MODELLING Halmstad is participating as a regional dissemination partner in a Technology Validation Project (TVP): "Integrated Wastewater" underthe EDInnovation Programme. It willbe executed during a threeyear period, andstarted late 1996. Theproject hasbeenproposed by a group of leading specialized European R&D and consulting organizations in thewastewater sector as wellas a number of end-users from 6 European member states. Theworkwithin the project includes technology adaptation/development of integrated planning anddesign modelling toolsas wellas pilotstudies aiming to demonstrate and validate the integrated approach andthe corresponding technology. TheTVPwillbe focused on developing an integrated modelling tool including hydrology, hydrodynamical andwaterquality aspects in the sewernetwork andtreatment processes at the WWTP foroperational control andto demonstrate the effect of alternative planning measures. Theaimis to support thepermanent work andplanning process of rehabilitation of thesewersystem andoptimizing treatment processes at the WWTP. Thedeterministic Mouse TRAP (Garsdal et al., 1995) model willbe usedto describe the transport in the sewersystem and the STOAT model (Dudley et al., 1994) willbe used fordescription of the processes at the WWTP. In the firstphaseof the project thesetwomodels willbe coupled together on an off-line basisso the models willrun simultaneously and exchange datadynamically. Hence, during a simulation the WWTP will be theboundary condition to the model and visaversa. I e Mouse andSTOAT willact as one model. Laterin the project thiscoupled model maybe put intorealtimeoperation (on-line), but a final decision concerning this hasnot beentakenyet. Within the Halmstad regional TVPtheplansareto evaluate solutions which are shown to meetexisting standards by longtermmodel simulation, andinvolving control strategies based on real-time flowand quality data. EXAMPLE OF INTEGRATED STUDY ISSUES Within the integrated studies the investigation offlow forecasting possibilities concerning conditions in the maincollector andthe WWTP respectively is envisaged. Theuse of the storage couldforexample include equalizing diurnal flow variations during dry weather conditions, withthe possible conflict of having some storage volumes occupied whenneeded during occurring heavy storm events. In thatcase,a flow forecast e.g.based on rain gauge measurements or weather radarinformation could be usedto change the applied control strategy. Longtermevaluation of possible alternative strategies also includes seasonal changes for example wastewater temperature dueto snowmelt induced run-offconditions, which all canhavecritical effects on the treatment processes at thetreatment plant. Oneissue,to be the subject of further studies, is the establishment of a malting factory withinthe WWTP catchment during1997. Its wastewater discharge represents a substantial portion of the totalBODloadto the treatment plantand canbe considered to be a potential valuable carbon source to be usedwithinthe treatment denitrification processes at the plant. Theposition of the malting factory, andpossibilities to regulate othermainsewerbranches, is suchthat it should be possible, in the nighttime and during dry weather, to transport the discharge essentially undiluted to storage at the treatment plant,where that substance could replace the addition of ratherexpensive external
Integrated upgrading basedon real timecontrol
133
carbon source to support the sewage composition during timeperiods or treatment stepswith lackof easily biodegradable matter. Thisindustrial BODloadis about 10% of the totalloadat the inlet,andis supposed to be about20% after pre-sedimentation (including chemical pre-precipitation). It is of the sameorderof magnitude as the dosage of external carbonsource dosage actually applied to support the denitrification process at the plant. In figure S is shown the results of preliminary STOAT simulations (a careful calibration of the model
parameters still remains), where different patterns to applythis industrial loadhavebeentested. The Nremoval without the actual BOD-load is compared witha constant additional industrial discharge, andthe loadconcentrated to 6 hours at nightor twodifferent occasions daytime. A constant application of the additional carbonsource can,naturally, be seento be generally beneficial compared to no addition. The application at nightmeans a moreequalized BOD-concentration during the 24 hours of the day, but the mean nitrogen removal efficiency willbe slightly lowered compared to the constant application. This is also the case if the loadis applied lateduring theday,whilea startof the application at noonin factimproves the Nremoval efficiency a little. The referred results aredepending on a variety of factors not presented here,for example on the actual analyzed inflow concentration profiles. In the figure, concentration variations of total nitrogen in the outletare depicted. To evaluate these, flowvariations (not presented) also haveto be taken intoaccount. Total 11 _-----~-----_,_-----_r_-----....., Nitrogen, mg/l
10
-I------------.------L-j-L.~~~~_,_____
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FIGURE S. Example from preliminary STOAT-simulations showing the dailyprofileof total nitrogen in the WWTP outlet, dependig on thedischarge pattern of a substancial industial carbon source in the sewernetwork.
CONCLUSION Processes andconditions in sewersystem andwastewater treatment plantsare highlydynamic and varying on different time scales. Dynamic modelling of the inflow and infiltration into the sewersystem has shown to be extremely useful in finding costeffective remedies to functional problems. Thesamereasoning is applicable whenwastewater treatment processes areconcerned. Theuse of integrated modelling in the future, as a planning tool and at the operational phases, will hopefully shown to be evenmorevaluable to develop an effective composite useof existing resources in the sewersystem and at the wastewater treatment plant.
C. HERNEBRING et al.
134
REFERENCES Dudley. J.• Bryan. D. and Cbamben. B. (1994). STOAT - Development and application of a fully dynamic sewage treatment works model. International User Group Meeting: Computer Aided Analysis and Operation in Sewage Transport and TreatmentTechnology. June 13-15. Gl>teborg, Sweden. GandaJ. H.• Mark, 0 .• Derge, J. and Jepsen. S.-E. (1995). MOUSE TRAP: Modelling of water quality processes and the interaction of sedimentsand pollutants in sewers, Wat Sci. Tech.•31(7), 33-41. Gustafsson, L.-Q•• Tomicie,B. and Groselj.S. (1996). Ljubljana City sewerage masterplan project- Scandinavian contribution to 'best practice'in master planningfor urbandrainagesystems. 6th Stoc/cholm WaterSymposillm, August 4-9. Stockholm. Sweden. Hernebring, C. and Falk, J. (1993). Coordination between the operation of the sewerage network and the wastewater treatment plant:demonstration of differentstrategies in Halmstad, Sweden. Wat. Sci. Tech., 17(12),177-181. Undberg, S.• Nielsen.J. B. and Carr. R. (1989). An integrated PC-modelling systemfor hydraulic analysisof drainagesystems. The First AllStralian Conference on Technical Compllting in the Water IndllStry: WATERCOMP '89. Melbourne. Australia. Mark,0 .• Hernebring, C. and Magnusson. P. (1998).Optimisation and controlof the inflowto a wastewater treatment plant using integrated modelling tools. Wat Sci. Tech•• 37(1).347·354. Ohlsson. L. (1997). Halmstad Wastewater Treatment Plant - additional treatment in polishing ponds (in Swedish). Nordic Conference on Nitrogen and BiologicalPhosphorllS Removal. 28-30January. Stockholm. Sweden. Petroff. A. M. (1995). Digital Doppler Velocity technology improves performance and accuracy of sewer flow measurements. WEFSpecialityConference Series. Automating to impieve waterquality;achieving benefitswith computertechnology. 25-28June. Minneapolis. Minnesota.
Pergamon PIT: 50273-1223(98)00280-7
Wat Sci. Tech. Vol. 37. No.9. pp. 127-134.1998. «> 1998JAWQ. Publishedby ElsevierScienceLtd Printedin Oreal Britain. 0273-1223/98 S19-00+ 0-00
INTERACTION BETWEEN THE TREATMENT PLANT AND THE SEWER SYSTEM IN HALMSTAD: INTEGRATED UPGRADING BASED ON REAL TIME CONTROL C. Hernebring*, L. Ohlsson**, M, Andreasson*** and L.-G, Gustafsson*** • Urban Hydroinformatics Centre AB. Box 1848, S-581 17 Linkdping, Sweden •• TheMunicipality ofHalmstad; Vdrtra Strandens reningsverk; Smdbdtsg 2, 5-302 38 Halmstad; Sweden ••• Urban Hydroinformatics Centre AB, Box 276, S·351 05 Vlixjo, Sweden
ABSTRACT In Halmstad, Swedengreat effortshavebeen madeduringthe 1990's to improvethe functionality and to reduce the environmental impact of the sewer system and the wastewater treatment plant, The investment and rehabilitation programincludes to a great extent an effectiveuse of existingresources. The wastewater treatmentplant is reconstructed to meet increased nutrientremovaldemands. A five year rehabilitation plan for the sewersystem is undercompletion. wherethe measures mainlyare motivatedby the aim to reducethc combinedsewer overflowvolumes and to minimize the risk of local flooding. It was soon realized that an integrated use of storage volumes at the wastewater treatment plant and within the sewer system could improvethe general conditions for the treatment at the plant, To implement this strategya real time control system was introduced by installing controllable weirs and flow control devices in the main sewer. The article descnbes the background of the sewerage masterplan, how the upgrading work has been carried out by means ofsimulations and measurement, gives examples of some expected potentialbenefits.and outlines plans for the future. ~ 1998lAWQ. Published by ElsevierScienceLtd
KEYWORDS combined seweroverflow, flooding, MOUSE, pollution transport. real timecontrol, sewer, STOAT, TYP, wastewater treatment plant. INTRODUCTION Halmstad municipality on the westcoastof Sweden is situated withinan areaidentified to be the most sensitive to nutrient transport by rivers from the drainage basin. Thushighnitrogen (andphosphorus) removal efficiency at thewastewater treatment plant (WWfP) has beenenforced by the environmental authorities with highpriority. Theplantwasreconstructed in 1992-94 at a costof IS MECU. To reducethe combined seweroverflow volumes (eSO) into local receiving water, i.e, RiverNissan - historically an important salmon habitat - a fiveyearrehabilitation planfor the sewersystem is implemented. The most effective measure in reducing eso outletsis the construction of a covered equalization tank ( 3 500 m', investment cost 3 MECU) located withinthe sewersystem, takenintooperation in 1996/97. The abovementioned improvements of the sewerage andthe wastewater treatment are part of a general environmental program of action carried out since 1990, alsoincluding a vast scheme of wetland restoration andconstruction. It was realized thatan alternative to reduce the required investments wasto improve the operation, the control andthe coordination of the operation of the sewerage network andthe treatment plant (Hemebring and Falk, 1993) 127
C. HERNEBRING It al.
128
Halmstad WWTP V4stra Stranden receives wastewater from about 100000 p.e. with a total catchment area of26 km2 • The sewersystemis partlycombined, withthe connected impervious area(170 ha) mainly concentrated to the centralpartsof the city. Mostof the CSOare concentrated withinthe main interceptor alongthe River Nissan. When the workwith the sewerrehabilitation plan started1991-92, it was found that the functioning and capacity of the sewersystemwerenot acceptable: a CSOdischarge from the sewersystem of2.5 % of the total annual wastewater volume and someoutletsactiveas oftenas 60-80times/year. In someparts of the sewersystem occasional flooding problems occurred. Thecriteriaof maximum allowed CSO frequency for the majoroutletswas formulated to be lessthan5 times/year as a guideline for the sewerrehabilitation planning. For flooding, the risk levelof 10 yearrain events were considered. To reachthesegoals in a cost effective way the MOUSE (Lindberg et al., 1989) software was usedas a planning tool.
MODELLING OF THEHYDRAULIC PROCESSES IN THE SEWER SYSTEM AS A BASISFORUPGRADING Intense hydraulic modelling studies wereperformed to serveas a basisfor the sewerage masterplan,based on continuous hydrological modelling of highlydistributed sub-catchments for surface stormrun-off(fast run-offcomponent) and wastewater loadsandclustered modelling of infiltration of storm- and groundwater C'baseflow"- slow run-offcomponent). The methodology has beendescribed by Gustafsson et al, (1996). The verified hydrodynamic (for transport description) and hydrological (for infiltration and inflow) model was usedto outlinemeasures in the sewerin orderto minimize the risk for localflooding and CSOvolumes. Theoptimum use ofexistingtrunksewers was an important concern in implementing effective sewerage masterplan measures. The effectof rehabilitation and upgrade measures in the sewersystemis continuously followed up by modeladjustments and verifications. Basedon the understanding of the sewersystemloads and performance givenby the verified MOUSE modeldescriptions, different what-ifalternatives couldbe tested.
Measu[ed fIowotVOstrO StrondenWWTP MouseRTC -colculotecHlowotWWT? 1.600 1.400 1.200 1.000 0.800 0.600 0.400 0.200 0.000
I,
,
:
,'\ -r
I
·.
'
t\
,
..
\
,
\,0 _
_
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,-- ,- ...
14/11 15/11 16/11 13/11 17/11 1994 FIGURE 1. Example ofMouseRTCflow verification results,takingfullhydrodynamics into account (CSO volumes excluded).
The first estimate how to reachthe functional demands, founded on the knowledge of the localdistribution of inflowload and flow capacity, included a totalrequired storage volume distributed withinthe sewer network of appr 14000m' . This figure couldessentially be reduced (by 75 %) afterhydrodynamical studies, wherecombinations of regulations, optimized flowtransport, increased pumping capacity, removal of bottlenecksand separation of impervious area from the combined sewerweretested. Besidesthe use of existing sewervolumes for storageby flowregulations at critical sites,the needfor extrastoragevolume was reduced
Integrated upgrading based on realtime control
129
to 3 500 rn' , which could be located at one central location within the sewer system . The covered equalization tank Vallgraven was recently taken into operation. The completion of the five year rehabilitation plan for the sewer system (1995-99) will result in more than 90% reduction of CSO volumes from the sewer system according to model estimates.
FIGURE 2. Mean annual CSO frequency and volume at major outlets before and after planned measures
THE WASTEWATER TREATMENT PLANT The design of the WWTP is based on a line with high treatment efficiency, but with limited flow capacity, combined with a open equalization tank of 6 000 rn' for peak flows. The storage volume includes pre-precipitation. Before the outlet to the recipient all sewage will pass through polishing ponds . The des ign was supported by computer simulation studies of the activated sludge process . The plant was reconstructed in 1992-94. Swedish National Franchise Board for Environmental Protection will in the near future present final outlet standards for Vastra Stranden WWTP. At the time being , provisional outlet target values are: BOD 7 - 15 mgll (monthly mean value) total nitrogen - 12 mg/l (yearly mean value) total phosphorus - 0.4 mg/l (quarterly mean value) In the final outlet standard fonnulation enhanced conditions concerning the nitrogen and phosphorus removal efficiency are expected , down to 8-10 mgtl total nitrogen and 0.3 rng/l total phosphorus,
130
C. HERNEBRING et al .
respectively. The requirements shouldbe met includinguntreated or partly treatedstorage overflow volumes occurringfrom the equalization tank at the treatment plant. measured at the outlet of the polishing ponds. This may be a difficult task. mainlydepending on the high P-removal efficiency demanded.
INFLUENT
BIOLOGICAL TREATMENT
PRIMARY CLARIFIER
y
-,
l~~i! I
1
AHOXJC
AEROBIC
SECONDARY SETTLER
r..
I r
IlORAO« OVERflOW
RNER NISSAN . . . ._
.....1
FIGURE3. Halmstad wastewater treatment plant flow scheme
COORDINATED USE OF STORAGE - REALTIME CONTROL For the operationconditions at the WWTP it was found that the treatment efficiencycould be increasedwith a more effectiveuse of vacant resources (volumes) in the sewersystem. not only during heavy storms. but also during the dominantpart of the year with more or less dry weatherprevailing. The formulated operation strategieswere tested by means of MOUSE modelcalculations. A coordinateduse. includingreal time control (RTC) actions. of the coveredequalization tank in the sewer system and the open equalization tank at the WWTP wil1 result in a decreased by-pass volume(partly treated)at the treatmentplant. Such a coordination is prepared by the installation of two movable weirs and flow regulationequipment in the main sewer adjacent to the storagevolume. together with six telemetered flow and water level monitorsat critical sites. The control1able weirsare recently taken into operation. The formulation of control strategiesand the functional specifications of the flow control devices were preparedby MouseRTC-simulations. The main purposeof real time control actions is to bring as much waste water volumes as possible through the ful1 treatment line of the WWTP, without causing unacceptable overflowvolumes neither from the sewer systemnor from the storageat the WWTP. This can, for example, be expressedin a control strategy implyingthat, when the WWTPstorage is beginningto be fil1ed up. the covered storage within the sewer system starts to decreasethe outlet flow to the main collector. This will continue up to a certain degree of storage. when the outlet flow gate again is raised. becausean overflow at the WWTPis preferredcomparedto CSO volumes from the sewer system. The CSO weir at the sewer storage is regulatedin such a way that overflow is avoidedas long as possible.but when an overflow is inevitable. the weir is loweredto permit a rapid discharge of overflow water to minimizethe risk of upstream floodings. In figure4 is shown an exampleof the effect of the described control actionsduring consecutive rain events occurringapproximately 2-3 times/year.
Integrated upgrading basedon real timecontrol WATERLEVEL,TREATMENT PLANTSTORAGE
m.:: 1C,::;:£$d~ 12:00 ~ 2110
3110
131
WATERLEVEl, SEWER STORAGE VAUGRAVEN
m
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m ~:fVQ~GE~,~N,~]=R m:rrR,~,S~~,~D,~,E~~j~ 211012:00 0:00 311012:00 211012:00 0:00 311012:00 FIGURE 4. Example from MouseRTC-simulations showing utilization of storage and controlactions. Ifno regulation of the sewerstorage outletis applied, about3.3 % of the total annual wastewater volume (during a yearwith meanprecipitation conditions) willbe discharged as by-pass flow at WWTP and consequently onlypartlytreated. By-pass volumes needto be keptto a minimum to reducesludgedeposit problems in the polishing pondsandthe riskof affecting the outletwaterqualityto such an extentthat the standards of the WWTP final outletcannotbe met (Ohlsson, 1997). The resulting storageoverflow volume at the WWTP is naturally highlydependent on the applied flow restriction through the full treatment train. It shouldbe mentioned that the flow limitation is relatively severe: a constant valueof2000 mvh, only 65 % abovethe yearlymeanflow. This flow is determined by sludgethickening and clarification capacity of the secondary settlers. The capacity of the sedimentation tanks couldvary somewhat because of e.g, seasonal sludgeproperty variations. In this analysis a constant valuehas been applied, nearthe actual operational plantexperience of the settlercapacity. If the described controlstrategy is maintained, the yearly WWTP storage overflow volumewill according to MouseRTC simulations be reduced by 30 % compared to staticconditions. If the monthswith high autumn/winter baseflow conditions (here: Nov-Dec) are excluded whenregulations are less effective, the reduction is about40 %. At the sametime,as a consequence of the RTC actions, a slightincrease ofCSO volumes fromthe sewersystem will occurfrom 0.08 to 0.11 % of the total annual wastewatervolume, however not affecting the CSOfrequency.
The real RTC-implementation is somewhat moresophisticated thanthe one described above,wheremore conditions describing the stateof hydraulic loadandboundary conditions are considered. A numberof controlstrategies have beendefinedas normal situations, risk situations and extreme conditions, respectively, depending on the waterlevels and flows in the sewersystem, sea/river leveland storage capacity at the WWTP. A supervisory controland data acquisition (SCADA) system has beenbuilt up for flowand precipitation measurement data fromdifferent pointsin the sewernetwork, and for process data fromthe WWTP. A data basesystemservesas a basis for specific eventanalyses andprovide data for different modelcalibration applications. It is an essential prerequisite in the realoperation situation to obtainreliable and accurate leveland flow measurement data fromdifferent pointsof the sewersystem, especially whencontrolling the storage
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utilization. Therefore, six flow and levelmeters, based on ADS technology (Petroff 1995) wereinstalled at critical sitesconnected on-line to the SCADA system. INTEGRATED MODELLING Halmstad is participating as a regional dissemination partner in a Technology Validation Project (TVP): "Integrated Wastewater" underthe EDInnovation Programme. It willbe executed during a threeyear period, andstarted late 1996. Theproject hasbeenproposed by a group of leading specialized European R&D and consulting organizations in thewastewater sector as wellas a number of end-users from 6 European member states. Theworkwithin the project includes technology adaptation/development of integrated planning anddesign modelling toolsas wellas pilotstudies aiming to demonstrate and validate the integrated approach andthe corresponding technology. TheTVPwillbe focused on developing an integrated modelling tool including hydrology, hydrodynamical andwaterquality aspects in the sewernetwork andtreatment processes at the WWTP foroperational control andto demonstrate the effect of alternative planning measures. Theaimis to support thepermanent work andplanning process of rehabilitation of thesewersystem andoptimizing treatment processes at the WWTP. Thedeterministic Mouse TRAP (Garsdal et al., 1995) model willbe usedto describe the transport in the sewersystem and the STOAT model (Dudley et al., 1994) willbe used fordescription of the processes at the WWTP. In the firstphaseof the project thesetwomodels willbe coupled together on an off-line basisso the models willrun simultaneously and exchange datadynamically. Hence, during a simulation the WWTP will be theboundary condition to the model and visaversa. I e Mouse andSTOAT willact as one model. Laterin the project thiscoupled model maybe put intorealtimeoperation (on-line), but a final decision concerning this hasnot beentakenyet. Within the Halmstad regional TVPtheplansareto evaluate solutions which are shown to meetexisting standards by longtermmodel simulation, andinvolving control strategies based on real-time flowand quality data. EXAMPLE OF INTEGRATED STUDY ISSUES Within the integrated studies the investigation offlow forecasting possibilities concerning conditions in the maincollector andthe WWTP respectively is envisaged. Theuse of the storage couldforexample include equalizing diurnal flow variations during dry weather conditions, withthe possible conflict of having some storage volumes occupied whenneeded during occurring heavy storm events. In thatcase,a flow forecast e.g.based on rain gauge measurements or weather radarinformation could be usedto change the applied control strategy. Longtermevaluation of possible alternative strategies also includes seasonal changes for example wastewater temperature dueto snowmelt induced run-offconditions, which all canhavecritical effects on the treatment processes at thetreatment plant. Oneissue,to be the subject of further studies, is the establishment of a malting factory withinthe WWTP catchment during1997. Its wastewater discharge represents a substantial portion of the totalBODloadto the treatment plantand canbe considered to be a potential valuable carbon source to be usedwithinthe treatment denitrification processes at the plant. Theposition of the malting factory, andpossibilities to regulate othermainsewerbranches, is suchthat it should be possible, in the nighttime and during dry weather, to transport the discharge essentially undiluted to storage at the treatment plant,where that substance could replace the addition of ratherexpensive external
Integrated upgrading basedon real timecontrol
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carbon source to support the sewage composition during timeperiods or treatment stepswith lackof easily biodegradable matter. Thisindustrial BODloadis about 10% of the totalloadat the inlet,andis supposed to be about20% after pre-sedimentation (including chemical pre-precipitation). It is of the sameorderof magnitude as the dosage of external carbonsource dosage actually applied to support the denitrification process at the plant. In figure S is shown the results of preliminary STOAT simulations (a careful calibration of the model
parameters still remains), where different patterns to applythis industrial loadhavebeentested. The Nremoval without the actual BOD-load is compared witha constant additional industrial discharge, andthe loadconcentrated to 6 hours at nightor twodifferent occasions daytime. A constant application of the additional carbonsource can,naturally, be seento be generally beneficial compared to no addition. The application at nightmeans a moreequalized BOD-concentration during the 24 hours of the day, but the mean nitrogen removal efficiency willbe slightly lowered compared to the constant application. This is also the case if the loadis applied lateduring theday,whilea startof the application at noonin factimproves the Nremoval efficiency a little. The referred results aredepending on a variety of factors not presented here,for example on the actual analyzed inflow concentration profiles. In the figure, concentration variations of total nitrogen in the outletare depicted. To evaluate these, flowvariations (not presented) also haveto be taken intoaccount. Total 11 _-----~-----_,_-----_r_-----....., Nitrogen, mg/l
10
-I------------.------L-j-L.~~~~_,_____
~
6-1-----------"c--::7''"---
sL00:00
r-
--6-- Inddisch6 h at night00- , -e--.Ind disch 6 h daytirrll12. _Ind disch 6 h daytirrll18-
--=========--J
.:.06:00
tii6lllCOIldltbna Constant Inddischarge
12:00
18:00
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FIGURE S. Example from preliminary STOAT-simulations showing the dailyprofileof total nitrogen in the WWTP outlet, dependig on thedischarge pattern of a substancial industial carbon source in the sewernetwork.
CONCLUSION Processes andconditions in sewersystem andwastewater treatment plantsare highlydynamic and varying on different time scales. Dynamic modelling of the inflow and infiltration into the sewersystem has shown to be extremely useful in finding costeffective remedies to functional problems. Thesamereasoning is applicable whenwastewater treatment processes areconcerned. Theuse of integrated modelling in the future, as a planning tool and at the operational phases, will hopefully shown to be evenmorevaluable to develop an effective composite useof existing resources in the sewersystem and at the wastewater treatment plant.
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REFERENCES Dudley. J.• Bryan. D. and Cbamben. B. (1994). STOAT - Development and application of a fully dynamic sewage treatment works model. International User Group Meeting: Computer Aided Analysis and Operation in Sewage Transport and TreatmentTechnology. June 13-15. Gl>teborg, Sweden. GandaJ. H.• Mark, 0 .• Derge, J. and Jepsen. S.-E. (1995). MOUSE TRAP: Modelling of water quality processes and the interaction of sedimentsand pollutants in sewers, Wat Sci. Tech.•31(7), 33-41. Gustafsson, L.-Q•• Tomicie,B. and Groselj.S. (1996). Ljubljana City sewerage masterplan project- Scandinavian contribution to 'best practice'in master planningfor urbandrainagesystems. 6th Stoc/cholm WaterSymposillm, August 4-9. Stockholm. Sweden. Hernebring, C. and Falk, J. (1993). Coordination between the operation of the sewerage network and the wastewater treatment plant:demonstration of differentstrategies in Halmstad, Sweden. Wat. Sci. Tech., 17(12),177-181. Undberg, S.• Nielsen.J. B. and Carr. R. (1989). An integrated PC-modelling systemfor hydraulic analysisof drainagesystems. The First AllStralian Conference on Technical Compllting in the Water IndllStry: WATERCOMP '89. Melbourne. Australia. Mark,0 .• Hernebring, C. and Magnusson. P. (1998).Optimisation and controlof the inflowto a wastewater treatment plant using integrated modelling tools. Wat Sci. Tech•• 37(1).347·354. Ohlsson. L. (1997). Halmstad Wastewater Treatment Plant - additional treatment in polishing ponds (in Swedish). Nordic Conference on Nitrogen and BiologicalPhosphorllS Removal. 28-30January. Stockholm. Sweden. Petroff. A. M. (1995). Digital Doppler Velocity technology improves performance and accuracy of sewer flow measurements. WEFSpecialityConference Series. Automating to impieve waterquality;achieving benefitswith computertechnology. 25-28June. Minneapolis. Minnesota.