Enhancement with DAF, RGF and Ultraviolet Disinfection

Cwellyn Water Treatment Works is situated near the town of Caernarfon and supplies drinking water to customers situated in a large area of north west Wales. Dwr Cymru Welsh Water, as part of its early Amp 5 Quality Investment Plan, has enhanced the existing works to meet the requirements of the Drinking Water Inspectorate (DWI) through undertaking a scheme to address the deterioration in raw water quality. The new process was sized to meet existing demand of 20Mld and was designed to address a number of specific requirements such as turbidity, bacteriological and cryptosporidium removal. The combined use of steel and concrete process tanks for Dissolved Air Flotation (DAF) and Rapid Gravity Filtration (RGF) along with a new Ultraviolet (UV) Disinfection system will provide a robust industry standard treatment process well into the future.
Existing works
Prior to the recent investment, the existing treatment process used pressure sand filters along with chemical dosing to clean and disinfect the water before distribution to the supply network. Over recent years the quality of the raw water source has deteriorated, leaving the existing process unable to maintain the required final water quality. At the end of 2005 an outbreak of cryptosporidiosis in the community, linked to water supplied by Cwellyn, focused attention on the treatment process and as a result of this investigation a UV disinfection plant was installed immediately as a short term mitigation. Welsh Water worked to complete the scheme to address long-term water quality issues and this was complete by 31 March 2010.
The preferred option
Black & Veatch, Welsh Water’s Asset Management Alliance (AMA) Water Process Partner for AMP 4 investigated options and decided to implement DAF, RGF and newer, fully validated, low power UV disinfection system. A wash water recovery system, using WRc standard thickeners was also provided along with a chemical dosing plant for coagulation, pH control and sludge thickening. As the site is located within the Snowdonia National Park (SNP), early consultation with the planning department, the local council and community was instigated. Key stakeholders were informed that the two new buildings along with an external process plant would need to be constructed and would use materials and practices that would not be detrimental to the local environment and that would blend into the local area once complete.
The selected option utilised the existing raw water main from Llyn Cwellyn reservoir, two kilometres from the existing works. The new process plant is gravity fed using the existing available head. This provided challenges to the design team when calculating hydraulics for the new works. Following the RGF stage all the available head has been used so an interstage pumping station then pumps from the new to the existing works. The upgrade to the existing DCS control system allowed both old and new parts of the water treatment works to be run as one. All work was required to be completed whilst Welsh Water continued to provide potable water to the network without disruption.

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