A water treatment works—Welsh Water in Wales, U.K.—is gravity fed from a nearby impounding reservoir with raw water at a pressure of up to 145 psi. The raw water is fed into an open dissolved-oxygen flotation (DAF) tank. The entering pressure is not necessary for the process and would have typically been eliminated by a PRV. The end of the pipe feeding into the DAF plant is elevated to a height of approximately 16 feet. A normal turbine would have to be mounted at this height, which would be challenging and expensive. The flow ranges from 0.22 million gallons per day to 0.68 million gallons per day.
Image 2. Rotary lobe pump installation at Welsh Water
The municipality chose a system that used a generator and rotary lobe pump. The entire system was able to be installed at ground level, and because of its operation, it could generate renewable electricity. The system also maintained a minimum pressure on its outlet, which allowed water to enter the elevated tank. The turbine controlled the raw water flow into the treatment process by controlling its speed to match the desired flow. To provide seamless operation, the turbine was equipped with a load bank that allowed it to continue running even when the electricity supply from the grid failed. This allowed a controlled shutdown, which would prevent any hydraulic issues.
The system has performed well during since commissioned in December 2011. It has been in constant, 24-hour operation. It has exceeded its net water-to-wire efficiencies relative to performance trials.
The municipality is satisfied with the system and has subsequently purchased additional units. Image 2 shows the installation at Welsh Water, in which the generator/pump system operates to provide pressure control in and produce electricity for the plant. P&S