Articles & Case Studies

Cryptospiridium treatment within Wessex Water

Posted: Wednesday 26th June 2013

Improvements were proposed in the final drinking water quality submission to the Drinking Water Inspectorate (DWI) in November 2008 for four key sites in the Wessex Water region.

The improvements were based on using UV for cryptosporidium inactivation. 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.

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 utilising real and virtual technology with Trant Construction, the water engineering specialist, playing a key role.

The schemes were delivered by the Treatment 2 Workstream for Wessex Water. Partners for this were;

Responsible for civil engineering construction services

  • Trant - M&E contractor

Responsible for M&E procurement, installation and testing

  • Grontmij - design consultant

Responsible for outline and detailed design

  • WECS - technical

Responsible for automation, process commissioning and environmental services

James Henderson, Programme Manager for Trant Construction, said: “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.”

Specific points related to the schemes

All sites are fully operational water treatment works, critical to the network supply within Wessex Water. Large scale shutdowns of these supplies to enable refurbishment and upgrade are 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. This was successfully completed through close integration between the project teams consisting of Trant, WECS, Wessex Water’s commissioning and operations team, Trojan Technologies, DMS Engineering Ltd, MBH Industrial Ltd, Statiflo, Siemens and QED (Quantum Engineering Development).

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.”

Here’s how the schemes were delivered:

Dunkerton Water Treatment Works

Dunkerton WTW is a spring source with an output of up to 5Ml/d, 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.

Existing works

Prior to project commencement the existing works consisted of the following treatment process:

· Raw water collection

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.

· Filtration and disinfection

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 utilised a chlorine gas system where chlorinated carrier water injection took place downstream of the micro-strainer (dosing was trimmed 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.

· Dechlorination

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.

New works

The final solution after a detailed options review process consisted of the following:

· Auto divert of high turbidity water from three individual feeder spring sets

· Improvement of drainage to remove surface water from spring collection area

· New raw water monitoring and control kiosk

· New chlorine dosing and mixing loop and modifications to existing micro-strainer unit

· Replacement of flow control valve

· Automation of stream compensation flow

· Pumping station modifications

· Surge vessels and surge analysis

· New UV building housing UV reactors

The selected UV disinfection validated units were Trojan Swift SC reactors utilising low pressure high output UV lamp technology, with automatic lamp cleaning incorporated. The design criteria was based on a maximum flow of 5MLD (57l/s) and a UV transmission of 90%.

Tatworth Water Treatment Works

Tatworth WTW is a shallow groundwater source operated by Wessex Water with an output of up to 1.4Ml/d, located near Chard in Somerset. The AMP5 programme for Wessex Water included this scheme for the provision of UV treatment for protection against cryptosporidium.

Existing works

Prior to project commencement the existing works consisted of the following treatment process.

· Raw water collection

Raw water was collected in a shallow adit and transferred to a well which acted as a sump. Submersible pumps installed within the sump transferred the raw water to the treatment plant.

· Filtration and disinfection

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.

New works

The final solution, after a detailed options review process, comprised:

· Replace the existing raw water pumps.

· Demolish the redundant GAC backwash header tank situated in the existing GAC treatment building

· Construct a new single storey building to house the proposed UV plant and associated equipment.

· Supply and install a surge vessel.

· Construct a new contact main consisting of 124m of 800 mmØ pipework.

· Replace existing life expired motor control centre unit.

· Construct a new combined flow meter, chlorination, mixer and monitoring chamber

· Improve the on-site surface drainage

The selected validated UV disinfection units were Trojan Swift SC reactors utilising low pressure high output UV lamp technology, with automatic lamp cleaning incorporated. The design criteria was based on a maximum flow of 1.4MLD (16l/s) and a UV transmission of 95%.

Dewlish Water Treatment Works

Dewlish WTW is a borehole source with an output of up to 8Ml/d, which supplies Sturminster Newton 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 in to supply.

The use of ultraviolet 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 Trojan Swift SC reactors utilising low pressure high output UV lamp technology, with automatic lamp cleaning incorporated. The design criteria was based on a maximum flow of 8.6MLD (99l/s) and a UV transmission of 93%.

Upton Scudamore Water Treatment Works

Upton Scudamore water treatment works (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 Upton Scudamore WTW 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 Trojan Swift SC reactors utilising low pressure high output UV lamp technology, with automatic lamp cleaning incorporated. The design criteria was based on a maximum flow of 9.0MLD (104l/s) and a UV transmission of 90%.


UV reactor chambers – outlet stage and manifold.

UV reactor chambers – inlet stage and manifold.




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