Welsh Water invests in activated carbon

As part of a £1.2B investment, Welsh Water called in Black & Veatch, aided by Laing O'Rourke, to improve drinking water from a Pembrokeshire WTW. Malcolm Bamsey of Black & Veatch reports on the upgrade, which introduced a granular activated carbon stage to the treatment

Dwr Cymru (Welsh Water) has recently completed an £8M scheme to further improve the quality of water supplied to its customers in Pembrokeshire. This investment forms part of £1.2B the company is investing in its water and sewerage network between 2005 and 2010.

The scheme, to improve the quality of drinking water from Bolton Hill WTW, involved the introduction of an additional granular activated carbon (GAC) stage to the treatment process. The scheme was successfully delivered by Black & Veatch with assistance from Laing O' Rourke.

Bolton Hill WTW currently supplies drinking water to more than 80,000 customers in the Pembrokeshire area. The recent improvements will ensure that the quality of water supplied by the works is extremely high, with the capacity for planned growth in the future.

The works is fed from a river source and originally comprised a dissolved air flotation plant followed by rapid gravity filters (RGF) and a chlorine contact tank. The recent upgrade work introduced an additional GAC treatment stage after the RGF filters and prior to the chlorine contact tank. This minimises the concentration of dissolved organic matter in the treated water and also eliminates any pollution risks.

All drinking water contains a small proportion of organic compounds. But minimisation of these organic compounds helps to improve taste and odour in treated drinking water, and reduces disinfection by-product formation.

Trials were carried out by the Water Research Council (WRC) in the early stages of Black & Veatch's process design. GAC media was provided by two suppliers, and testing was carried out using water from Bolton Hill WTW. These tests enabled the company to optimise the period the rapid gravity filtered water needs to be in contact with the GAC media. This time period is referred to as the empty bed contact time (EBCT). A time of 12.5 minutes was determined by the trials as optimum for the water treated at the WTW.

In the trials, samples of RGF-filtered water were spiked with elevated concentrations of the target organic compounds, which simulated typical maximum target compound concentrations for extended periods of time. Using 12.5 minutes EBCT, a GAC bed life of at least ten years was predicted, and was therefore believed to be a satisfactory basis for using 12.5 minutes as the design basis for the plant.

The main components of the scheme are a relift pumping station, a GAC contactor block comprising 12 pressure contactors, a MCC building, and an 800kVA generator, which serves the whole site.

The GAC plant is designed for a maximum throughput of 53Ml/d with one contactor offline for backwashing and one set aside for carbon regeneration. The filtration rate under this mode of operation is 18.4m/h, which is within the recommended limit of 15-20m/h for this type of water. The RGF filtrate at the WTW is also a very good quality, and therefore allows a higher filtration rate.

Both a gravity contactor plant and pressure contactor plant were considered for GAC as well as other treatment process options. The GAC pressure contactor option was ultimately selected as the best process option and reduced the scheme costs and programme to 12 months. The pressure vessel order was split between two suppliers to reduce the overall programme by eight weeks. Interstage pumping was required to facilitate the introduction of the new GAC process due to hydraulic constraints of the original works. These pumps deliver water through the GAC contactors and return the treated water to the chlorine contact tank.

The GAC contactors are 4m diameter, closed pressure vessels containing 3m deep GAC media, giving a minimum EBCT of 12.7 minutes with all contactors in use.
The contactor backwash sequence is initiated on turbidity and time. The design assumptions are that the contactors will be backwashed every 72 hours, although longer contactor run times of up to a week are anticipated without turbidity being compromised.

Prior to backwashing, the vessel is automatically drained to a preset level and on blower start up the air flow is staged to 50%, followed by 100% flow once the air flow is established in the contactor. The backwash rate is automatically selected based on backwash water temperature to achieve a 25% expanded bed for varying temperatures throughout the year.

The pressure contactors have a design pressure of 2.5 bar and incorporate a nozzle plate floor supported by stay bars. All vessels are identical so contactor face pipework is handed to suit the vessel. The vessel incorporates two manways in the base of the contactor, one in the side at platform level and one at the top of the vessel. As well as the main pipework nozzle connections to the contactor, there are six media bed life sample points between the top of the media and the nozzle plate.

The relift pumps are 75kW vertical split casing pumps configured as a duty/assist/assist and standby arrangement drawing from a relift sump with a capacity of 1,000m3 and a very large surface area. The pumps are variable speed and have a single pump duty point of 234l/s at 18m total head. The output of the relift pumps is controlled to maintain a preset level in the relift pumping station sump.

The 900mm feed pipe to the contactors incorporates a flowmeter, which determines the flow set point for each operational contactor. The flow through each operational contactor is via individual outlet flowmeters controlled to the set point by the outlet control valves. This achieves an equal contactor throughput irrespective of contactor head loss.

The timescale for implementation of the upgrade was extremely tight. Construction work started on site in May 2006 and the compliance date was met on March 31, 2007.

The key to this was a short civil construction programme, early orders for the pressure vessels and a first class site team working closely with the two main installation contractors.

Black & Veatch is also delivering a number of other quality schemes across Wales in AMP4. Bolton Hill WTW is the third consecutive quality scheme success of the AMP4 period.

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