WPL Limited has recently supplied the National Policing Improvement Agency (NPIA) with a new sewage treatment plant. Due to an increase in student numbers and tighter consent standards set by the Environment Agency, the existing plant was  too small to cope with forecasted flows and loads.

The existing treatment plant consisted of a 150PE Rotating Biological Contactor (RBC) unit and a 450PE Submerged Aerated Filter (SAF) package plant. The SAF was supplied by WPL Limited in 1996, when pupil numbers were smaller. With an average student population of 1,100 and strict discharge limits of BOD 10mg/l; TSS 10mg/l and NH4-N 2mg/l, the plant needed to be upgraded.

The training college is a Grade 1 listed Jacobean mansion, set within rolling parkland on a beautiful 3,000 acre estate in Hampshire. The design of the sewage treatment plant had to be sympathetic to landscape and therefore was to be below ground with ancillaries not exceeding 1.5m.

WPL ltd undertook the process design utilising data supplied by the consultant Scott Wilson, and supplied a large four stage modular HiPAF plant. Flow is pumped from a balance tank into a 6m conical primary tank, where significant BOD and solids are removed. Settled sludge is removed by airlift to a Robust Aerobic Digestion unit, where sludge volumes are reduced by a minimum of 75%.

After primary settlement, settled liquor flows by gravity through a flow split chamber, where flow is split into two equal streams arranged in parallel. Each stream comprises two  Submerged Aerated Filters (SAF) arranged in series. In these units carbonaceous and nitrifying processes take place and treated liquor flows by gravity into a final settlement Lamella plate clarifier. The final settlement removes biomass which is the by-product of the aeration process, before flowing by gravity to tertiary sand filters for final effluent polishing.

The WPL sewage treatment plant is controlled by a Form 4 Motor Control Centre (MCC) which is housed in an existing building along with two rotary lobe type blowers. The blowers run  duty/stand-by, providing emergency cover should one blower fail, with the added benefit that the blowers have a prolonged life cycle.

Frequency inverters were added to optimise the provision of scour air and reduce power consumption by an estimated 30%.

WPL’s commitment to reducing energy consumption and improving the carbon footprint has led to the development of separate process and scour aeration regimes, that not only provide environmental and financial incentives for implementation, but provide the treatment units with a greater process flexibility that has led to improved discharge quality.