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View of the Rapid Gravity Filters

View of the Coagulation Area

Introduction

This M&E contract for Scottish Water involved the design, installation and commissioning of a number of treatment processes, including chemical dosing, coagulation, multi media rapid gravity filtration, sludge thickening, water and chemical storage facilities, instrumentation, associated pipe-work and control systems. The plant is capable of treating up to 5Ml/day.

This treatment facility is part of Scottish Waters initiative to replace or upgrade water treatment works with more advanced modular treatment systems that are reliable, flexible and capable of ensuring the most cost-effective use of process chemicals. The new works will ensure a continuous flow of water from the treatment works to the existing clear water tanks supplying the areas of Beauly, Drumnadrochit and Inverness (West).

Plant Description

Main process

A new water main carries raw water from Loch Bruicheach to the works inlet, where it is monitored for pH, turbidity and colour. The flow is automatically adjusted by a control valve linked to the clear water tank level control signal.

The raw water is dosed with coagulant (Aluminium Sulphate) and pH conditioned with sodium Carbonate before entering the 3 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 passes to a stainless steel flow distribution tank feeding four rapid gravity multi-media sand filters, up to three filters are on duty at any time depending on the works throughput.

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

As the level in the Clear water tanks begins to fall, the works flow rate increases in discreet steps and filters are brought progressively on line as required. The rate of increase in flow through a filter entering service is limited to <1.5% per minute of the design maximum flow.

At least one filter is always off-line (either back-washing or in standby mode). The 3.41m diameter fully automatic filters incorporate a gravel base beneath a layer of sand and anthracite - forming a filtration bed of 1,250mm. Constructed from glass fused to steel panels, each filter vessel has a stainless steel nozzle plate floor with Cadar nozzles.

Water from the distribution tank enters each on-line filter through a motorised inlet valve and percolates down through the media and nozzles. A ultrasonic level transmitter provides data for the backwashing control, providing ‘out of normal limit' alarms in filtering mode and a ‘loss of head' alarm.

The Filtered water passes out via electromagnetic flow meters, turbidity meters and flow-control valves before passing to a common filtered water line, where it is dosed with monosodium phosphate and sodium hypochlorite before passing to a twin compartment chlorine contact tank.

After the chlorine contact tank 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 sometimes seen with traditional simple chlorination of organic surface waters. Further sodium carbonate dosing takes place to optimise the conditions for chloramination of the final treated water.

The final treated water passes to the clear water tanks, which feed by gravity to the Glenconvinth service reservoir (supplying Beauly) and via newly installed pumping equipment to the existing clear water tanks supplying the areas of Drumnadrochit and Inverness (West).

Wash Water Recovery

As water passes through the filters, 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 discharging to the backwash holding tank.

During the backwash 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 containing solids flows from the filters to a dirty water balance tank before passing to one of two picket fence thickeners. The thickeners operate as duty/assist and incorporate slow stirring devices to ensure optimum efficiency. From the thickeners, supernatant overflows to the recovered water tank and thickened solids are transferred into a storage tank by two desludging pumps.

The recovered water is 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 is removed by tanker for off-site disposal.