UV delivered to four Wessex sites
Wessex Water's plans for AMP5 included refurbishment of four groundwater sourced drinking water treatment plants. Natasha Wiseman reports
Improvements to four of Wessex Water’s drinking water treatment works (WTWs) were proposed in the utility’s final drinking water quality submission to the Drinking Water Inspectorate (DWI) in November 2008. The use of UV for cryptosporidium inactivation was proposed in all the cases.
In its final assessment letters of support in January 2009 the DWI supported the schemes, which were included in the final business plan submitted to Ofwat by Wessex Water in April 2009. The key objective was to improve the protection against cryptosporidium.
All four sites are fully operational WTW, critical to the network supply within Wessex Water. Large-scale shutdowns of these supplies to enable refurbishment and upgrade was simply not acceptable, meaning that all works had to be carefully programmed, managed and communicated to ensure security of the water supplies was maintained throughout.
However, the opportunity was also taken to maximise any additional benefits of the UV treatment with regard to disinfection in accordance with the Wessex Water disinfection policy. The projects were achieved through multi-disciplinary teamwork, using real and virtual technology.
Case study 1: Dunkerton WTW
Dunkerton WTW is a spring sourced plant with an output of up to 5Ml/d. It is located in Maiden Bradley on the outskirts of Bath.
The treatment works occupies a small clearing at the edge of a wooded area, which is bounded by a stream on the eastern side. The spring collection system extends throughout the wooded area adjacent to the water bearing outcrop.
The AMP5 programme for Wessex Water included this scheme for UV treatment for protection against cryptosporidium for the site. The existing works consisted of raw water collection, filtration and disinfection and dechlorination.
The raw water system consisted of three sets of spring collection chambers, each set having its own common final chamber with manual diversion to waste. The three final chambers then fed into a single main collecting chamber.
After collection, raw water gravitated through a micro-strainer screen located in the second compartment of the main collecting chamber. Screened water was then disinfected by super-chlorination before passing through three parallel, large-diameter disinfection contact mains to a second collecting tank.
The disinfection process used a chlorine gas system where chlorinated carrier water injection took place downstream of the micro-strainer. Dosing was adjusted based on chlorine residual measured downstream of the contact mains.
The chlorine dosing was started and stopped when the control valve downstream of the contact mains was opened and closed respectively. From the second collecting tank, the flow passed through the control valve and was partially de-chlorinated using sulphur dioxide gas (flow based on chlorine residual trim) in carrier water injected en route to the pumping wet well suction tank.
There were three submersible borehole pumps located inside the pumping chamber and two re-lift pumps, with associated surge vessels, to deliver to the distribution reservoirs.
The treated water was analysed for turbidity and nitrate using on-line instruments. The final treated water sample tap and cryptosporidium sample line were connected upstream of the pumping tank after the addition of sulphur dioxide.
Following a detailed review of the options for refurbishment of Dunkerton, it was decided to auto-divert high turbidity water from three feeder spring sets. Improvements were also made to the drainage of surface water from the spring collection area.
A new raw water monitoring and control kiosk was installed along with a new chlorine dosing system and mixing loop and modifications to the existing micro-strainer unit. The flow control valve was replaced and the stream compensation flow automated.
Modifications were made to the pumping station that pumps flow to the Mapperton service reservoir.
James Henderson, programme manager for Trant Construction, said, “Surge analysis was carried out on the new pump regime and it was deemed necessary for a new surge vessel on the two Grundfos multi-stage inline centrifugal pumps.”
A new building for the UV reactors was constructed. Two Trojan Swift SC reactors, which use low-pressure, high-output UV lamp technology, were selected. They have automatic lamp cleaning incorporated. The design criteria was based on a maximum flow of 5Ml/d and a UV transmission of 90%.
Case study 2: Tatworth WTW
Tatworth is a shallow groundwater source plant operated by Wessex Water, with an output of up to 1.4Ml/d. It is located near Chard in Somerset. Prior to the start of the project, the existing treatment process comprised raw water collection and filtration and disinfection.
Raw water was collected in a shallow adit (underground storage tunnel) and transferred to a well which acted as a sump. Submersible pumps installed within the sump transferred the raw water to the treatment plant. The incoming pumped raw water was then treated by a pressurised granular-activated carbon (GAC) adsorber before disinfection via chlorination. Sodium hypochlorite solution was injected into the common pumping main before the treated water was pumped to two service reservoirs serving the Tatworth and South Chard area.
It was necessary to replace the existing raw water submersible pumps with two new borehole type submersibles from Grundfos at Tatworth. The redundant granular activated carbon (GAC) backwash header tank, that was situated in the existing GAC treatment building, was demolished.
A single-storey building was constructed to house the two Trojan Swift SC UV disinfection units. The design criteria was based on a maximum flow of 1.4MLD (16l/s) and a UV transmission of 95%.
A new surge vessel was supplied and installed and it was necessary to construct a new contact main consisting of 124m of 800mm-diameter pipework. The existing life-expired motor control centre unit was replaced and a new combined flow meter, chlorination, mixer and monitoring chamber constructed. On-site surface drainage was improved.
Case study 3: Dewlish WTW
Dewlish WTW is a borehole sourced plant with an output of up to 8Ml/d, which supplies Sturminster Newton in Dorset and surrounding areas. The raw water at Dewlish WTW was considered to be at high risk from cryptosporidium contamination, however, prior to the upgrade works, the site was operated using a set of control rules which reduced the risk of contaminated water passing into supply.
The use of UV light for the inactivation of cryptosporidium oocysts was accepted by the DWI for this site. The Wessex Water design standard for UV systems requires a validated UV dose of 40mJ/cm2 (often referred to RED - required equivalent dose) to ensure a 4 log inactivation.
The selected validated UV disinfection units were two Trojan Swift SC reactors using low-pressure, high-output UV lamp technology, with automatic lamp cleaning incorporated. The design criteria was based on a maximum flow of 8.6Ml/d and a UV transmission of 93%.
Case study 4: Upton Scudamore WTW
Upton Scudamore WTW is supplied by on-site springs and boreholes along with an external source known as Diver’s Bridge. Divers Bridge is a spring source with an output of around 4Ml/d, which is pumped to the works to be blended with water from the on-site sources.
The total output from Upton Scudamore typically varies between 8 and 13Ml/d and supplies Trowbridge, Westbury, Warminster and the surrounding areas.
The new works consisted of a new UV disinfection plant with all associated ancillaries. The selected validated UV disinfection units were two Trojan Swift SC reactors using low-pressure, high-output UV lamp technology, with automatic lamp cleaning incorporated. The design criteria was based on a maximum flow of 9.0Ml/d and a UV transmission of 90%.
The schemes were delivered by the Treatment 2 Workstream for Wessex Water, which comprised:
- WECS (Wessex Engineering Construction Services) - civil
contractor, responsible for civil engineering construction services
- Trant – mechanical & electrical (M&E) contractor, responsible for
M&E procurement, installation and testing
- Grontmij - design consultant, responsible for outline
and detailed design
- WECS - technical contractor, responsible for automation, process
commissioning and environmental services
The project teams comprised Trant, WECS, Wessex Water’s commissioning & operations team, Trojan Technologies, DMS Engineering, MBH Industrial, Statiflo, Siemens and Quantum Engineering Development (QED).
James Henderson, programme manager for Trant Construction, said, “We had to ensure the projects were turned around quite quickly as the sites were operational. The installation of new equipment and its transfer was done in close relationship with WECS and Wessex Water operations, to ensure this site was switched off for the minimal amount of time.
“Trant Construction is delighted to have been involved in these interesting and challenging projects. We’d like to take this opportunity to thank staff at Wessex Water and WECS for their considerable expertise and commitment to these projects.”
Kirstie Hearn, project manager for Wessex Water, said, “An excellent relationship has developed between the project delivery team, which assisted greatly in successfully managing these four critical cryptosporidium projects through to completion.”
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