Energy Savings:692,040kWhr a year

Cost: £85,000

Payback: 1.3 years


This case study is for a large water recycling centre with a of 164,000.

Up until 2015, treatment was split between three parallel streams, each taking approximately one third of the incoming flow. “A” and “B” Works comprised rectangular trickling filters and the “C” Works built in 1989 comprised an activated sludge plant with fine bubble diffused aeration.

The poor performance of the filter works meant that “C” Works was run to very low ammonia levels to dilute the poorer quality effluent coming off the filter works.

To keep “C” Works discharge ammonia very low, an upgrade was carried out by others in 2007, including the replacement of the diffusers, installation of new anoxic mixer, replacement of the DO meters, upgrading of the blower and system controls and the replacement of the blower inlet filters. 

Since then, the diffusers have been replaced in 2013 due to high back pressures causing the blowers to surge.  It is believed that the increase in diffuser back pressure is due biological fouling, not contaminants such as grit and rag.

In 2015 a new activated sludge plant, known as “D” Works was brought into service taking 80% of the incoming flow, with 20% going to “C” Works.  The filter works were decommissioned once “D” Works was fully operational. 

The reduction in load in “C” Works means that is now treating a load equivalent to a PE of approximately 33,000 compared to around 50,000 previously.  


Several aeration surveys carried out on “C” Works at the higher loads prior to “D” Works coming on line showed that treatment was complete by two thirds of the way along the lane, even during high load periods, with the exit ammonia levels being less than 0.05mg/l, which is less than 1% of the consent limit. 

The aeration surveys also showed very high nitrate levels in the aeration lanes, exceeding 40mg/l on numerous occasions despite there being an anoxic zone in the front of the lane.  Subsequent surveys identified that the anoxic zones were ineffective due to back mixing from the aerobic zone.

When “D” Works was commissioned the works was over treating, with DO levels approaching saturation at the end of the lanes.  This was due to the poor control system and difficult to maintain DO meters due to the mounting arrangement and local of the controllers.

With the reduced load on “C” Works it was also identified that the aerobic volume does not need to be as great.  A trial was carried out which showed that the anoxic zones can be extended, which will improve denitrification and save energy. 


The final scope of work was agreed as:

 Extend anoxic zone by fitting auto valves to the Zone 1 air supply

  • Modify the control system so that the Zone 1 valves will open on Low DO
  • Modify control system to provide burst mixing
  • Replace the DO meters and install on chain mounts
  • Install new controllers adjacent to the DO meters
  • Control system modifications to run on average DO on a selectable number of meters
  • Acid clean the diffusers



The savings have been immediate as can be seen from the blower power consumption trends below:

Validated Aeration Savings


Overall energy savings of 692,040kWhr a year have been found as well as improved process and reduced stress on the diffuser membranes.

The project ended up with a payback of just 1.3 years.

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