Turning on the waterworks

ACWa Services is currently working on an M&E contract to design, install and commission a new 3.5Ml/d plant for Scottish Water’s Glenconvinth WTW. The plant will treat raw water supplies from nearby Loch Bruicheach to provide high-quality drinking water for the Highland areas of Beauly, Drumnadrochit and Inverness (west).

The contract was awarded to ACWa Services by the Galliford Morgan Joint Venture (GMJV), the in-house delivery partner of Scottish Water Solutions.

The new treatment facility is part of a Scottish Water Solutions initiative to replace the existing Glenconvinth waterworks with more advanced modular treatment systems that are reliable, flexible and capable of ensuring the most cost-effective use of process chemicals. The process design will ensure that there is a continuous flow of water from the treatment works to the existing clear water tanks supplying Beauly, Drumnadrochit and Inverness (west).

The M&E contract involves the installation and commissioning of a number of treatment processes, including coagulation, chemical dosing, mixed-bed rapid gravity filtration and sludge-thickening. Water and chemical storage facilities, instrumentation, associated pipe-work and control systems are also included as part of the total contract.

ACWa Services has assumed project management responsibility for the total sub-contract, including site supervision, the procurement of various items of plant and equipment, cost control, contract scheduling and the supply and control of on-site services.

A new water main will carry raw water from Loch Bruicheach to the waterworks inlet, where it will be monitored for pH level, turbidity and colour. Depending on demand, the flow will be adjusted automatically by a control valve linked to the clear water tank level control signal. In the event of a power failure, the flow control valve will initiate a controlled shut-down, using a battery back-up facility.

At the works inlet, raw water will be combined with recovered backwash and conditioned with sodium carbonate to ensure the effectiveness of the downstream coagulation process. The dose rate will be determined by the flow rate and colour levels of the water.

The coagulation game

At this point the raw water will be dosed with coagulant (aluminium sulphate) before entering the three-stage coagulation system. Each of the three 6.0m3 tanks may be placed on- or off-line to satisfy variable throughput requirements and provide continuity of the treatment process during maintenance.

Following coagulation, the flow will pass to a stainless steel flow distribution tank feeding four rapid gravity multi-media sand filters (three duty with one backwash/standby).

As the water level in the two clear water tanks (CWTs) rises, the works flow rate will be reduced in discreet steps and the duty filters removed from service one by one, maintaining a constant filtration rate. Dosing rates will be reduced under PLC control to match the reduced water demand flow.

As the level in the CWTs begins to fall, the works flow rate will increase in discreet steps and filters brought progressively on-line as required. The rate of increase in flow through the rapid gravity filters will be limited to <1.5%/minute of the design maximum flow.
At least one filter will always be off-line (either back-washing or on standby mode). Each of the four 3.41m-diameter fully automatic filters will incorporate a gravel base beneath a layer of sand and anthracite – forming a filtration bed of 1,250mm depth. Constructed from glass fused to steel panels, each filter vessel will incorporate an under-drain, collection system, air-scour header and stainless steel nozzle plate with Cadar-type nozzles.

Water from the distribution tank will enter each on-line filter through a motorised inlet valve and percolate down through the media and nozzles. In each filter, an ultrasonic level transmitter will provide data for the backwashing control, providing ‘out of normal limit’ alarms in filtering mode and a ‘loss of head’ performance alarm. A sample will be drawn from each filter outlet to supply a local turbidity meter. This will be used for performance monitoring and alarms to the works control system. When required, the sample point will also facilitate bacteriological testing of process water.

Filtered water will be collected in an under-drain and pass out via electromagnetic flow meters, turbidity meters and flow-control valves before passing to a common filtered water line, where it will be dosed with monosodium phosphate and sodium hypochlorite before passing to a twin-compartment chlorine contact tank.

As water passes through the filter, trapped particles will accumulate and cause progressive blinding of the media – producing an increasing pressure drop (loss of head) across the filter. To avoid flow restrictions and solids breakthrough, the filters will be backwashed at regular intervals, staggered to minimise queuing (a back-up timer will initiate back-washing after a maximum time interval if other backwash initiators have not come into play).

On detection of high turbidity at the filter outlet or a high head loss (indicated by high differential pressure across the filter bed) the operator will be given a warning of the need to clean the filters, and the control system will automatically initiate a filter wash.

Keeping it clean

The backwash water holding tank will be filled with un-chlorinated water from the filtered water outlet channel prior to the chlorine contact tank inlet chamber. For the backwash cleaning process, two (duty/standby) air-blowers will provide the filter with ten minutes’ air scour, and wash-water will be supplied by two (duty/standby) pumps. The pumps will incorporate variable speed-drives, suction and discharge isolation valves and discharge non-return valves.

Dirty backwash water, together with any debris that might have accumulated along the way, will pass to a dirty water balance tank before entering the sludge-thickening and water recovery system.

Wastewater, containing solids, will be transferred from the filters to a dirty water balance tank before passing to one of two picket fence thickeners. The picket fence thickeners will be installed to operate as duty/assist and will incorporate slow stirring devices to ensure optimum efficiency of the process. From the thickening process, supernatant will overflow the recovered water tank and thickened solids will be discharged to a storage tank by two desludging pumps.

The supernatant will be re-introduced to the front end of the works for re-processing at a maximum rate of 10% of the incoming flow, and the sludge will be removed by tanker for off-site disposal.

After the chlorine contact tank, further chemical dosing will take place immediately prior to the CWTs.

Ammonium sulphate will be dosed under residual chlorine control to maintain the disinfection level during storage and distribution, whilst minimising the possibility of the trihalomethane (THM) formation sometimes seen with traditional simple chlorination of organic surface waters. A further sodium carbonate dosing will be made to optimise the conditions for chloramination of the final treated water.

The water will pass to two new CWTs, which will feed by gravity to the Glenconvinth service reservoir (supplying Beauly) and via new pumping equipment to the existing clear water tanks supplying Drumnadrochit and Inverness (West).