ACWa Services is near to completing a WTW for Scottish Water’s (SW) new Fort William area water supply scheme – a project in which ACWa is responsible for the process design, supply, installation, testing and commissioning of all process systems at the wellfield, WTW and service reservoirs. When considering the projected population growth for the Fort William area, SW recognised the existing water supply structure had limitations and may soon become inadequate.
It was established that a new works would be required and investigations were carried out to determine the best source of water supply. It was decided abstraction of raw water from a proposed wellfield site, adjacent to the River Lochy, would be the most preferable option and a series of tests were carried out at various locations within the site to determine the optimum positions of boreholes. The new WTW will be capable of continuously handling and achieving specified quality criteria for a range of raw water flows between
50 l/s (equivalent to 4.3Ml/d) to 139 l/s (equivalent to 12Ml/d). The works will initially comprise seven operational boreholes with associated wellhead pumping stations, flow control, cascade aeration, orthophosphate dosing, disinfection (chloramination), pH correction and pumping stations to the reservoirs at Camisky and Spean Bridge.
Due to seasonal flood conditions, the seven electro-submersible pumps, associated head-works and control systems have been designed by ACWa to operate continuously without interruption, damage or operator intervention in partially submerged conditions.
Head-works are being firmly secured on reinforced concrete plinths and totally enclosed in removable GRP kiosks. Each of the seven boreholes incorporate a pressure transducer to monitor water levels and inhibit/enable the pump. Water will be conveyed from the boreholes to the inlet of the new WTW by seven dedicated rising mains. At the new WTW, all seven rising-mains enter a flow control chamber where dedicated flow meters measure individual flows. Flow rates from the boreholes will be adjusted individually and each rising main can be isolated without interrupting the operation of the WTW.
Readings from the flowmeters will be transmitted to the plant’s PLC for control purposes and displayed on the operator’s HMI control panel. A sample of water taken from each rising main will be transferred along dedicated sample lines to the laboratory area of the treatment building and monitored for turbidity.
After the flow control chamber, individual flows will combine in a single pipeline to feed the cascade aerator. A sample line taken from the combined line will also be monitored for turbidity and the ‘turbidity high-high’ signal used as a regulatory alarm to comply with the Scottish Executive Cryptosporidium Directive. The feed pipeline to the cascade aerator divides into two streams, each feeding one of the two inlet chambers.
Water will be aerated naturally – entraining oxygen from the atmosphere as it flows over a three-step system to achieve 75% oxygen saturation. The design of the aerator is such that one side alone will be capable of handling the duty flow of 10.5Ml/d – allowing either side to be isolated for maintenance work without disrupting throughput.
Aerated water leaving both sides of the cascade aerator will combine in a single pipeline and be disinfected with sodium hypochlorite before passing through a static mixer. The chlorinated water will then be dosed with sodium orthophosphate (to reduce plumbo solvency) and a sample transferred to the laboratory for residual chlorine analysis.
The chlorinated water will then enter the base of the contact tank, which is divided into two equal sides fitted with internal baffles to direct the flow and eliminate short-circuiting of water across the tank. Chlorinated water from each side of the tank will exit the system and combine in the first chamber of a rapid mixer tank. A second sample of chlorinated water will be continuously drawn from this chamber and transferred to the laboratory for residual chlorine analysis. The chlorinated water will overflow a weir into the second chamber of the mixing tank, where it will be dosed with a lime solution to correct the pH and with ammonium sulphate to convert the chlorine to chloramines (chloramines remain in the water longer than chlorine and will continue to disinfect during prolonged periods of storage). An impeller mixer installed in the second chamber will ensure complete mixing of the chemicals. Treated water from the above processes will then overflow a final weir in the mixing tank and enter one of two pumping station wet wells, where three centrifugal pumps, operating as duty/duty assist/standby, transfer it to the service reservoir at Camisky.
The operation of the pumps will be governed by level controls installed in the pumping station wet wells. Sodium hypochlorite, sodium orthophosphate and ammonium sulphate held in dedicated storage tanks, which will be periodically topped-up from IBCs via filling points on the outside of the building. The tanks will be installed with ultrasonic level sensors to measure their contents and high-level probes to provide hard wired ‘high-high’alarms. Each storage system will utilise two diaphragm pumps to transfer chemicals from the bulk tanks to dedicated day tanks. To maintain environmental integrity, chemical storage facilities will be surrounded by a dedicated bund, complete with sloping floors to enable the flow to the sump and facilitate the removal of any chemical spillages.
Hypochlorite, sodium orthophosphate and ammonium sulphate will be dosed from dedicated day storage tanks by two dosing pumps. Dosing is flow proportioned, based on combined flow-rates from all the boreholes – with residual control on hypochlorite based on a residual chlorine monitor installed downstream of the dosing point. The operator will pre-set the required concentration of chemicals via the HMI.
For pH correction, the lime solution will be prepared in two make-up tanks, each with a dedicated dosing pump, ultrasonic level transmitter and conductivity level probe. The tanks and associated dosing pumps will be displayed on the HMI, so it will be possible for the operator to adjust their settings. The lime solution dosing will be flow proportional, based on combined flow rates from all the boreholes – with residual control based on a pH monitor installed downstream of the dosing point. The operator will be able to pre-set the required pH via the HMI.
Powdered lime delivered to the treatment site will be stored in a 22t duty silo located in the process basement. The silo will be fitted with a vibrator and aeration pads to ensure the smooth movement of lime into the system. The lime powder will be discharged to each make-up tank by dedicated screw conveyors and accurately measured by load cells in each hopper. The quantity of lime to be transferred during each make-up of lime solution is set via the HMI.
Pumps will transfer make-up water from the wet well in the pumping station to the make-up tanks, where impeller mixers will agitate the contents to ensure optimum mixing of the solution. Treated water for the local consumer mains supply system will be stored in two service reservoirs constructed at nearby Camisky and Spean Bridge. Both reservoirs will be divided into two halves – each incorporating a level switch and ultrasonic pressure transducer to monitor the level of water in each section of the tank. The reservoir at Camisky will control the operation of the borehole pumps via a control algorithm in the PLC – a feature designed to stop and start the flow from each well-head according to operational requirements. Final water from the Camisky reservoir will gravitate back to the WTW and join the consumer main feeding Fort William.
Two centrifugal pumps, operating duty/standby, will transfer a proportion of final water from the Fort William gravity pipework to the second service reservoir at Spean Bridge. A sample of final
water will then be transferred to the laboratory for pH and turbidity monitoring.