Scottish Water - Glenconvinth


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Introduction

ACWA undertook an M&E contract to design, install and commission a new 3.5Ml/day Water Treatment Plant for Glenconvinth water works of Scottish Water. The plant treats raw water supplies from nearby Loch Bruicheach to provide high quality drinking water for the Highland areas of Beauly, Drummadrochit and Inverness (West).

Description

The new treatment facility was part of a Scottish Water Solutions initiative to replace the existing Glenconvinth Water works 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 ensures that there is a continuous flow of water from the treatment works to the existing clear water tanks supplying the areas of 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 pipework and control systems are also included as part of the total contract.

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

The process
Raw water flows from Loch Bruicheach to the works inlet, where it is monitored for pH, turbidity and colour. Depending on demand the flow is automatically adjusted by a control valve linked to the clear water tank level control signal. In the event of a power failure, the flow control valve initiates a controlled shut down, using a battery back-up facility.

At the Works inlet, raw water is conditioned by dosing with sodium carbonate to ensure the effectiveness of the downstream coagulation process. The dose rate is determined by the flow rate and colour levels of the water.

Coagulation
The raw water is then dosed with coagulant before entering the 3 stage coagulation system. Each of the three 6.0m3 tanks can be placed on or off line to satisfy variable throughput requirements and provide continuity of the treatment process during maintenance.

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

Multi Media Filtration (Rapid Gravity Filters)
As the water level in the two clear water tanks (CWT’s) rises, the works flow rate is reduced in discreet steps and the duty filters removed from service one by one, maintaining a constant filtration rate.

As the level in the CWT’s begins to fall, the works flow rate increases in discreet steps and filters are brought progressively on line as required.

Each of the four 3.41m diameter fully automatic filters incorporate a gravel base beneath a layer of sand and anthracite - forming a filtration bed of 1,250mm. Each filter vessel incorporates an under-drain, collection system, air-scour header and stainless steel nozzle plate with Cadar type nozzles.

Water from the distribution tank enters each on-line filter through a motorised inlet valve and percolates down through the media and nozzles.

Filtered water will be collected in an under-drain and pass out via 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.

Backwash System
As water passes through the filter, trapped particles 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 are backwashed at regular intervals, staggered to minimise queuing.

The backwash water holding tank is 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 provide the filter with 10 minutes air scour and wash-water is supplied by two (duty/standby) pumps.

Dirty backwash water and any accumulated debris pass to a dirty water balance tank before entering the sludge thickening and water recovery system.

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

Chloramination and pH Control
After the chlorine contact tank, further chemical dosing takes place immediately prior to the CWT’s.

Ammonium Sulphate is dosed under residual chlorine control in order to maintain the disinfection level during storage and distribution, whilst minimising the possibility of Trihalomethanes (THM’s) formation.

Project Details

Client: Scottish Water - Glenconvinth, Scotland

Industry: Rapid Gravity filters, Sludge Thickening and Chemical Dosing

Application: Production of Potable Water from Loch Water

Technologies