P2 Options PIP
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Project information paper In 2001 the independent Thames Tideway Strategic Study was set up to look at the problem and come up with a solution. In 2005 the study concluded that improvements to existing sewage treatment works and two tunnels to intercept the combined sewer overows (CSOs) were the best solution. A lot of further work has been done since this study, which continues to reinforce its conclusions. Three other ways to reduce the CSO discharges were considered as part of the study. Here are the reasons why they were ruled out: Separation of the sewerage system The root of the problem that London faces is the combined sewerage system. This means that rainwater and foul sewage all go into a single pipe. A separate sewerage system is built with one pipe t hat conveys rainwater (normally to the nearest watercourse) and a second pipe that conveys foul water to a sewage treatment works. Modern developments are built with separate systems, but a signicant amount of London was built with the combined system. Separating the combined system would involve building a second pipe and making sure that all connections from buildings and roads are connected into the correct system. Initial estimates showed that the construction cost of a new foul sewerage system would be between £1 3bn and £20bn. The construction work would need to take place over several decades and the disruption to London would be enormous, requiring deep pipes to be constructed in almost every street. Options We looked at a number of di erent ways to reduce the amount of untreated sewage that over ows into the River Thames from London’s largely 19th Century sewerage system. Other options have been assessed as costing more, being more disruptive and not achieving the required environmental standards.
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Transcript of P2 Options PIP
8/3/2019 P2 Options PIP
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Project information paper
In 2001 the independent ThamesTideway Strategic Study was set upto look at the problem and come upwith a solution. In 2005 the studyconcluded that improvements toexisting sewage treatment worksand two tunnels to intercept thecombined sewer overows (CSOs)were the best solution. A lot of further work has been done sincethis study, which continues toreinforce its conclusions.
Three other ways to reduce the CSOdischarges were considered as part of the study. Here are the reasons why
they were ruled out:
Separation of thesewerage systemThe root of the problem that Londonfaces is the combined seweragesystem. This means that rainwaterand foul sewage all go into a singlepipe.
A separate sewerage system is builtwith one pipe that conveys rainwater(normally to the nearest watercourse)and a second pipe that conveys foulwater to a sewage treatment works.
Modern developments are builtwith separate systems, but asignicant amount of London was
built with the combined system.
Separating the combined systemwould involve building a second pipeand making sure that all connectionsfrom buildings and roads areconnected into the correct system.
Initial estimates showed that theconstruction cost of a new foulsewerage system would be between£13bn and £20bn. The constructionwork would need to take place overseveral decades and the disruption toLondon would be enormous,requiring deep pipes to beconstructed in almost every street.
Options
We looked at a number of dierent ways to reduce the amount ofuntreated sewage that overows into the River Thames from London’slargely 19th Century sewerage system. Other options have beenassessed as costing more, being more disruptive and not achieving therequired environmental standards.
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Options
Sustainable urbandrainage systems (SUDS)SUDS involve a variety of measuresthat reduce the amount of rainwaterentering the sewerage system in therst place. They also slow down anyrainwater that does enter the system.SUDS measures include green roofs(ie. grassed ‘living’ roofs), soakaways,rainwater detention ponds, rainwaterharvesting and grassy swales,but unfortunately they need a lotof space.
The implementation of SUDS in newdevelopments is essential to help stopthe situation getting worse. This willplay an important part in ensuringthe future-proong of the ThamesTunnel, by helping to reduce theamount of surface water entering thesystem. They can also provide a full orpartial solution to localised oodingissues. We are therefore playing a fullpart in promoting the use of SUDS
through the London Plan. However,SUDS cannot resolve the massiveproblem of CSO discharges thatalready exists, and certainly not inany realistic timescale.
SUDS can enhance the environment,eectively manage surface waterooding and have a low carbonfootprint, not to mention very lowwhole-life operating costs. However,there are limitations to sustainabledrainage, particularly in the GreaterLondon area, where the drainagesystems are complex, most of theland is already developed and there ishuge potential for ooding.
The scale of SUDS that would benecessary in London is not practical or
cost-eective, taking into account theextent of densely built developmentthat already exists in the capital andthe underlying geology.
Retrotting SUDS in London on thisscale would have a detrimentalimpact on virtually every household,driveway, road and open space inevery borough.
Grassed ‘living’ roofs
Grassy swales
Case studyRetrotting of SUDS in the Putney and Wandsworth areas
Catchment area andnumber of hectaresfor modication
Cost(£m)
Reduction in thevolume of discharge %
Approxnumber of footballpitchesrequired
Would the UrbanWaste WaterTreatment Directive(UWWTD) standardbe met?
Cost of connecting theCSO to theThames Tunnel(£m)
West Putney,36 hectares
27 33 72 6.9
Putney Bridge,52 hectares
45 76 104 44.6
Frogmore (BuckholdRoad), 135 hectares
56 93 270 25.4
Retrotting SUDS would take over
30 years and cost several times asmuch as the Thames Tunnel.The cost is estimated to be at least£13bn, and would not solvethe problem.
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Bubblers and skimmersWe currently use ‘bubbler’ and
‘skimmer’ boats’ to reduce theimpact of untreated sewageoverowing to the River Thames.Biological material in the sewagereduces the amount of oxygen inthe river, which can kill large numbersof sh, so our two bubbler boatsinject oxygen into the river helpingsh survive sewage discharges.
Our two skimmer boats skim o the surface litter that also gets intothe river when the sewers overow.The boats tackle the symptom, notthe cause, and aren’t an eective,long-term solution. A substantialeet would be required to keep upwith the growing frequency of discharges. There are also severelimitations as to where theseboats can go, due to tides and
bridge heights.
Skimmer boat
Storage and transferNone of the above options wereconsidered viable, when comparedwith the benets of building a tunnelwith CSO interceptions.
A storage and transfer tunnel is asimple and robust solution to preventlarge volumes of sewage discharginginto the River Thames. It will:
• tackle discharges from the 34 most
polluting CSOs
• provide storage to hold thedischarges, avoiding the harmfulpollution of the river
• transfer intercepted discharges to asewage treatment works forprocess. We will also recover thesludge for energy generation
• provide a spine of continuous, safe
and integrated storage that isavailable, no matter where therainfall (and therefore the CSOdischarges) is concentrated
• capture the ‘rst ush’ of all CSOdischarges, which is the mostpolluting part
• future-proof the impacts of London’s sewerage system on theriver by providing exibility andmuch needed extra capacity, for atleast a century.
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For further information see our website: www.thamestunnelconsultation.co.uk or call us on 0800 0721 086.
Phase two consultation (Autumn 2011)
Options
Jacobs Babtie Review Ofwat commissioned a report byJacobs Babtie to review the workand reports of the Thames TidewayStrategic Study. This was publishedin February 2006 and proposedadditional options for dealingwith the CSO discharges atpotentially lower cost, but with lowerCSO control.
The primary option comprised a
hybrid solution based on fourelements:
1. Enhanced primarytreatment at Abbey Mills.
2. Construction of a west tunnel – anine kilometre tunnel, 7.2m indiameter from Hammersmith toHeathwall, with an associatedscreening plant at Battersea andanother at Earl Pumping Station.
3. The deployment of
skimmer vessels.
4. A medium to long term strategyof implementing SUDS and othermeasures to reduce sewer ows.
The west tunnel, following a routeunder the River Thames, would storesewage until there was capacity inthe current system to transfer it tothe Beckton and Crossness sewagetreatment works. The screening
facility at Battersea would dischargeinto the river when the west tunnelwas full.
Unscreened discharges wouldcontinue at all other CSOs notconnected to the tunnel, with theexception of Earl Pumping Station.
Although cheaper to construct, this
scheme was not adopted becausethe much shorter tunnel wouldnot have enough volume tocapture a sucient proportion of the discharges.
Also, the current collection systemhas virtually no spare capacity thatcould be utilised to convey storedstorm sewage from the tunnel fortreatment. This would mean thatsewage would need to be stored inthe tunnel for long periods, waitingfor spare capacity. This would leadto septicity and odour problems inthe vicinity of the tunnel.
The enhanced primary treatmentplant at Abbey Mills would onlyremove a small proportion of thepolluting load, meaningenvironmental objectives would notbe met.
The deployment of skimmer craft toremove sewage-derived litter in theriver from the remainingunconnected and uncontrolled CSOswould not reduce the polluting loadin the river. It would also not be
compliant with the UWWTD.
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RelateddocumentsProject information papers
• Regulatory framework Technical reports
• Needs Report
