November 2005

Water treatment works part of £210 million package for Scottish Water

A new Water Treatment Works recently installed and commissioned at Ballachulish, on the shores of Kinlochleven, is the latest of a series of major projects completed by ACWA Services to meet the ongoing expansion initiatives of Scottish Water. The Ballachulish Water Treatment Works forms part of a £210M package of improvements by Scottish Water to provide the highest quality water supplies for the Highland regions of Scotland.

The Ballachulish contract is the latest of a number of significant contracts designed and built by ACWA over the last three years. These include a WTW for the Fort William Area Water Supply Scheme and Nanofiltration membrane plants at Mallaig, Dalmally, Badachro & Inverasdale – installations designed to treat and remove colour from peaty Highland river water. More recently the company installed a coagulation and rapid gravity filtration plant at Glenconvinth which turns highly coloured water from the nearby Loch Bruicheach into high quality drinking water.

Built to a variety of ACWA process designs, these installations are allowing Scottish Water to maintain tight control of operational cost-effectiveness – using some of the very latest developments in treatment technologies to ensure full compliance with (RDWS), the Regulator’s Drinking Water Standards.

ACWA’s success in gaining many of the Scottish Water design and build contracts stemmed from its proven ability to successfully implement the latest technologies, backed by a tried and tested pro-active approach to process contracting and customer care. The treatment processes were carefully selected to suit not only the nature and condition of raw water sources, which included boreholes, rivers, lochs and reservoirs, but also the stringent operational reliability criteria demanded by remote locations. Processes varied considerably, from conventional filtration systems to highly sophisticated, "state-of the art", membrane technologies.

Ballachulish

The most recently commissioned ACWA treatment process installed at the WTW in Ballachulish is a relatively conventional installation using Sand/Anthracite Filters, Carbon Contactors, a Sodium Hypochlorite Dosing System, a Chlorine Contact tank and a Limestone Contactor.

Under normal circumstances, the feed water to the new ACWA plant is provided by pre-lift pumps, which enable the required works flow to be achieved. In the event of a power failure, the plant is designed to stop treating water at full flow and run albeit at a reduced flow. The quality control instrumentation, dosing pumps and the PLC continue to function for up to 12 hours powered by a UPS (Uninterruptible Power Supply) to maintain production during the power outage.

The process

Raw water from the inlet enters to two fully automatic 1.5m dia x 2.5m Tan/Tan/ skid-mounted sand filters, through distributors at the top of each vessel. Water passes down through the filter beds and out through a nozzle plate installed at a lower tangent. The filter bed media comprises a layer of filtration sand (600mm) and anthracite (300mm) which is backwashed at regular intervals to maintain continuous efficiency.

The two filters are backwashed consecutively (or at least at short intervals in order to replenish the backwater storage tank) at double the backwash interval. This procedure is carried out because - after backwashing and returning a single filter to service - the pressure drop tends to result in a high proportion of the flow passing preferentially to the clean filter. During a backwash, when one of the filters is taken offline, the maximum flow is reduced temporarily by 50%. Filter washes are initiated on a timed basis but, should high turbidity be detected at the filter outlet, unscheduled backwashes are initiated automatically and a warning light alerts the operator at the main control panel.

The 45m3/hour duty/standby backwash pumps are rated at 1 bar and are fitted with suction, discharge isolation and discharge non-return valves. The common discharge manifold contains a low-flow switch and a pressure relief valve and the backwash system incorporates a filter which is provided to collect debris, back-washed or scoured out of the main process vessels before the discharge of wastewater.

From the sand filters, treated water passes via top mounted distributors into two secondary carbon filters containing 23 tonnes of carbon media. Filtered water passing downwards through the carbon media is discharged through nozzle plates at a lower tangent of each filter and after being discharged, passes through a Magflo type flowmeter.

Having passed through the flowmeter, filtered water is dosed with Sodium Hypochlorite at the inlet of a static mixer. Filtrate is dosed at a rate of 0-3.5 litres/hour by two (duty/standby) speed controlled dosing pumps fitted with isolation and pressure relief valves and a pulsation damper. Sodium Hypochlorite is stored in a 500 Litre capacity polypropylene tank, installed in a MDPE bund with a conductivity type level switch.

After dosing, the treated water passes through a chlorine contact tank and enters a limestone contact tank though a nozzle plate installed at a low tangent. Water passes upwards though the limestone chips and out through a collector at the top of the vessel from which it is piped to the inlets of the clear water tanks.

Control of the plant is through local panels and controllers which suit the simplicity of the process. Data from controllers, instrument transducers and other instruments have been marshalled and taken to a telemetry outstation. The plant operates automatically, using the level transmitter in the clean water tank to initiate start-up.

During the early stages of project planning for four new treatment plants, Scottish Water worked closely with the Highland Council to ensure that all new buildings were designed to reflect the environmental characteristics of the surrounding areas, using Ballachulish slate for roofing.

Now operating successfully alongside other new water treatment installations in Glenachulish, Glencoe and Kinlochleven, the new ACWA plant is helping to provide the highest quality drinking water for more than 3000 inhabitants in some of the more remote Highland communities.

Prior to work being started on these four treatment plants, Hugh MacPherson, the Project Manager for Scottish Water Solutions said: "This is one of many projects that shows we’re committed to rural communities. It’s a big investment and will mean superb quality drinking water."

The completed installations, now operating successfully, are more than capable of providing a quality of water to RDWS requirements and, what is just as important, were constructed with minimum disturbance of water supplies to the local communities.

In a recent comment on the ACWA installation, Robert White, Scottish Water’s treatment co-ordinator in the Highlands said: "Ballachulish is running well as a well engineered and reliable plant."