Space-age Swansea plant complete with UV
Welsh Water's showpiece Swansea Bay STW has been opened by the minister for Wales, Alun Michael. Designed by Hyder Consulting and OTV Birwelco, the plant has been praised by environmental groups, including Surfers Against Sewage.
Waste was previously pumped out to sea with minimal screening or no treatment at all during storms, creating a threat to the safety of bathing waters off some of the most popular resorts in Wales, including the Mumbles and Gower peninsula. Mr Michael added: "With the opening of this plant our aim of achieving blue flag status for 50 beaches in Wales under our 'Green Seas' initiative is now within reach."
The significance of the project cannot be overstated, according to Surfers Against Sewage (SAS) director Chris Hines, who was present at the opening of the plant.
He said: "We consider a UV system, used here - or some form of microfiltration - as essential for the effective treatment of sewage."
The plant has been designed to cope with a storm lasting up to 2.5hrs, as required by the Environment Agency (EA).
Interceptor tunnels have now been built which prevent sewage overflowing from 13 outfalls to the east and west of the city, except in the most extreme rain conditions.
In dry weather, the flow to the plant is around 58Ml/d. The maximum flow that can be treated to tertiary level is 112Ml/d.
During storms, flow exceeding 112Ml/d is diverted to a storage tunnel 22m below ground at the plant, which can contain up to 10,000m3 of stormwater.
A further 156Ml/d can be passed through the plant when the tunnel is full and flow from the network exceeds 112Ml/d. In this case, the 156Ml/d is only subjected to settlement.
The maximum that can pass through the plant at any time is is therefore a total of 268Ml/d.
According to project director Maurice Clift: "The consent requirements of the EA for BOD and SS were 50/60 mg/l."
"However, due regard has been given to the future requirements of the urban waste water directive and the EEC bathing waters directive."
"The process designers therefore chose to provide a discharge capability of 35/35 mg/l for BOD and SS (95 percentile)."
The treatment process is based on primary screening and settlement, followed by secondary activated sludge treatment and final tertiary UV disinfection.
- Sewage from the interceptor sewers and storage tunnel is pumped to the inlet works.
- The inlet works feature 6mm screens, grit and grease removal systems.
- Four 24m long primary lamellar clarifiers then trap solids by settlement. The remaining wastewater then passes into four huge activated sludge lanes, 58m long by 35m wide by 8m deep, with a capacity of 3000m3.
- Any remaining solids are then settled out in a series of 12 secondary lamellar clarifiers.
- Disinfection is by a Trojan 4000 UV system, which produces a log 2.1 reduction in faecal coliforms, achieving a discharge of 200 FC/100 ml.
The sea outfall pipe has a diameter of 1.3m and is 3.5km long. At the far end there is a manifold incorporating 10 diffuser heads, from which the treated effluent is released.
The manifold is submerged at depth at the lowest tides and its position marked with a buoy.
Two sludge digestion tanks treat the sludge to a standard suitable for reuse on agricultural land.
The temperatures and residence times satisfy the requirements of the EC's 'sludge matrix agreement' legislation which has recently been transposed into UK law.
All air inside the plant is treated in a three-stage 'Detoxair' plant, which passes the air through a series of filters containing treatment chemicals such as sulphuric acid to oxidise any malodorous compounds.
Even lorries entering the plant to collect treated sludge must pass through an air-lock to prevent any risk of odour release. The atmosphere inside the plant is continuously monitored and controlled to ensure the safety of plant staff.
Cardiff STW underway
Construction of a new £180M STW is now also underway in Cardiff. When completed in 2002 the plant will serve a PE of 900,000; Welsh Water's largest ever project.
The plant has been designed by Hyder to treat a maximum flow of 523Ml/d. The secondary treatment stage will include 16 huge sequential batch reactors (SBRs). Although there are no plans at present for tertiary treatment at this site, project director Jeff Martin said: "The results of our modelling show that no pathogens will reach the shore - the outfall is 4.2km long."
Mr Hines was not concerned by the lack of UV: "We wholeheartedly support all of Welsh Water's schemes - in this case we are confident that UV will be added at a later date."
- SAS backs Welsh Water over prices
Ian Byatt has maintained his position that price cuts of 14% across the country will not qualify as an excuse for water companies to delay sewage treatment upgrades. However, Mr Hines claims: "At least 61 schemes in England and Wales will be under threat of delay if the prices go ahead as planned.”