In 2021, the installation of ABB VSDs to improve energy efficiency provided a payback period of three years. This was acceptable as the savings would continue through the life of the installation. However, recent electricity price increases mean that for Severn Trent Water (STW) VSD payback periods tumble.
STW is saving 15,000 kWh annually after installing two ABB Drives at a filter bed sewage treatment works in Nottinghamshire. The sewage treatment works include a stone filter bed that requires a minimum circulation flow. If the inflow drops below its 75 litres per second (l/s) minimum threshold, the bed risks drying out. Drying would kill the bacteria needed to break down the sewage and halt the process.
On reaching the threshold, two 9 kW fixed-speed pumps in duty/standby recirculate water to maintain wetting of the filter bed. As the pumps are constant speed, they may exceed the circulation needs causing excessive energy use and higher operational costs.
VSD payback periods tumble
In response, STW fitted an 11 kW inverter drive to each pump. Using a 4-20 mA signal from the inlet flow meter allows them to detect when flow falls below the 75 l/s threshold. It then regulates the pump speed according to the sewage works’ inflow. Once flow exceeds the minimum threshold the VSD stops the pump. Furthermore, if the recirculation well falls too low, a float switch sends a stop signal to prevent the pump running dry.
The changes resulted in energy savings of 15,000 kWh per year to deliver a payback period of under three years at the time of installation. Moreover, with the recent doubling of electricity costs, the VSD payback periods tumble to nearer eighteen months. This puts more projects within acceptable payback times.
According to the installer, “The skill to saving energy is finding the fine balance between pump speed and flow rate. This is only possible during commissioning with the drives activating the pumps when the inflow drops below 75 l/s and controlling them at minimum speed. If inflow to the works continues to fall, the drives control the pumps in a linear speed ramp up.”