Sewage works saves energy on filter bed application

20 September 2021

Variable speed drives, supplied by ABB, have improved energy efficiency, with payback of less than three years, at a sewage treatment works.

Severn Trent Water is saving 15,000 kWh per year after installing two ABB variable speed drives (VSDs) at its Edwinstowe filter bed sewage treatment works in Nottinghamshire in the UK. 
The sewage treatment works includes a stone filter bed that requires a minimum circulation flow. If the inflow to the stone filter bed drops below its 75l/sec minimum threshold, the bed risks drying out, killing the bacteria needed to break down the sewage. When this threshold is reached, two 9kW fixed-speed pumps, operating in duty/ standby, recirculate water to maintain the wetting of the filter bed. However, as the pumps are constant speed, they may transfer more than the required circulation, leading to excessive energy use and high operational costs. 

Inverter Drive Systems – a member of the ABB Value Provider network in the UK – suggested retrofitting an 11kW VSDs to each pump. By picking up a 4-20mA signal from the inlet flowmeter, the VSD detects when flow falls below the 75 l/s threshold and regulates the pump speed according to the sewage works’ inflow. Once flow exceeds the minimum threshold the VSD stops the pump. In addition, if the recirculation well falls below an operable level a low-level float switch sends a stop signal to prevent the pump running dry. This has resulted in energy saving of 15,000kWh per year and a return on the investment in just under three years.

"The skill to identifying the energy saving is to find the fine balance between pump speed and flow rate," said Blaise Ford, managing director at IDS. "This we achieve during the commissioning process with the drives activating the pumps when the inflow drops below 75l/sec, initially controlling them at minimum speed. If inflow to the works continues to fall, the drives control the pumps in a linear speed ramp up."

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