Netheridge is Gloucester’s main sewage treatment works is served by over 30 outstations and treats the effluent of the whole of Gloucester, a population equivalent of 50,000 – 200,000.
Situated next to the River Severn, Netheridge STW is a large urban site which has undergone a significant upgrade program for the anticipated regeneration of Gloucester and future peak demand flows. It operates a traditional process of sewage treatment, initially screening the untreated sewage, then blending it with raw activated sewage before sending it to the distribution chamber, primary settlement tanks and onto the site’s aeration tanks.
Feed to the high level distribution chamber at the start of the treatment process is served by three large Archimedes screw pumps which operate on a duty, assist, stand-by basis. They are designed to handle a dry weather flow through the works of 1200 l/s maximum and up to 8,000m3 maximum flow in storm conditions.
Whilst the pumps had been installed since the 1980’s, a series of pump gearbox and bearing breakdowns began to be experienced. Whilst they were periodic problems, the site is a critical element of the treatment process for Gloucester city centre, and therefore needs to remain operational at all times. At one point, two out of the three screw pumps failed, and because they could not be repaired or replaced quickly, Pump Supplies was needed to bring in over-pumping support because the client could not rely on just one pump working with no back-up alternative.
The criterion was for temporary pumps which could supply 1,000 l/s at 11 metre static head over a 40 metre discharge length within a very short time frame, so the client could be confident it could maintain flows to the works if one or more of the screw pumps continued to fail.
Rob Bessant, Pump Supplies Contract’s Manager, comments ‘We received the classic Friday afternoon call to see if we could supply pumps and equipment to act in the place of the second screw pump immediately. We visited the site that afternoon and following our assessment a solution was agreed. We quickly mobilised our personnel and stocks, so that the selected pumps and associated equipment for the job were prepared on the Saturday, and installed & fabricated and secured with lifting chains on site during Sunday, and by Monday we were fully operational.
Pump Supplies provided 2 x FLYGT 140kW N type submersible pumps, each capable of 500 l/s to cover one screw pump. Variable frequency drives were utilised to control pump speed as required and Ultrasonic level sensors were installed to monitor the depth of the media and control the pump start and stop cycles. All the equipment was installed and maintained by Pump Supplies engineers for the entire duration.