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Contact Tanks

Treatment Building

Introduction

This contract for Scottish water was an M&E project in which ACWA were responsible for the process design, supply, installation, testing and commissioning of all process systems, at the wellfield water treatment works and service reservoirs.

Scottish Water recognised that the existing water supply infrastructure and treatment works at Fort William had limitations, considering the projected population growth for the area. A new treatment works was required and investigations were carried out to determine the best source of water supply. It was decided that abstraction of raw water from the wellfield site adjacent to the river Lochy was the most preferable option and a series of tests were carried out at various locations within the site to determine the optimum borehole positions.

The WTW is capable of continuously handling raw water flows between 50 l/s (equivalent to 4.3 Ml/day) to 139 l/s (equivalent to 12Ml/day). The works comprises of 7 operational boreholes with wellhead pumping stations, flow control, cascade aeration, orthophosphate dosing, disinfection (chloramination), pH correction and pumping stations to the reservoirs at Camisky and Spean Bridge.

Because of seasonal flood conditions, the seven electro-submersible pumping stations were designed by ACWA to operate continuously without interruption, damage or operator intervention in partially submerged conditions and transfer raw water to the new WTW via seven dedicated rising mains.

Plant Description

The seven rising mains with dedicated flow control and monitoring discharge into a flow control chamber. Individual samples are taken from the each and transferred to the on site laboratory to monitor for Turbidity.

After the flow control chamber, the combined flow feeds the cascade aerator where the flow is divided into two streams, each feeding one of the two inlet chambers. The water is aerated naturally absorbing oxygen from the atmosphere as it flows over the three step aerators to achieve 75% oxygen saturation.

The design of the aerator is such that one side alone will be capable of handling the average duty flow of 10.5Ml/day—allowing either side to be isolated for maintenance work without disrupting throughput.

The aerated water leaving both sides of the cascade aerator combines in a single pipeline and is disinfected with sodium hypochlorite before passing through a static mixer. The chlorinated water is also dosed with sodium orthophosphate (to reduce plumbo solvency) and a sample is transferred to the laboratory for residual chlorine analysis.

close up of Cascade Aerators

Cascade Aerator

The chlorinated water enters the base of the contact tank, which divides into two equal compartments fitted with internal baffles to direct the flow and eliminate short-circuiting of water across the tank. The water from each compartment of the tank recombining in the first chamber of a rapid mixer tank. A second sample of chlorinated water is continuously drawn from this chamber and transferred to the laboratory for residual chlorine analysis.

The water then overflows a weir into the second chamber of the mixing tank where it is dosed with a lime solution to correct the pH and 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).

Treated water overflows the final weir into one of two pumping station wet wells, where three centrifugal pumps operating as duty/duty assist/standby transfer the treated water to the service reservoir at Camisky.

Sodium Hypochlorite, Sodium Orthophosphate and Ammonium Sulphate are held in dedicated storage tanks which are periodically filled from IBCs via filling points on the outside of the building; and are dosed from dedicated day storage tanks by two dedicated dosing pumps. The dosing is flow proportioned, based on the combined flow-rates from all the boreholes with residual control on Sodium Hypochlorite based on a residual chlorine monitor installed downstream of the dosing point.

Lime solution is prepared in two make-up tanks, The dosing is flow proportioned based on the combined flow-rates from all the boreholes with residual control based on a pH monitor installed downstream of the dosing point.