Energy Savings:        1,442,845kWhr a year

Cost:                           £570,000

Payback:                    4.7 years


This is a medium sized works with a population equivalent of 33,169.  The quoted DWF is 8,630m3/day, with a FFT of 25,920m3/day.  In addition, the works treats the sludge from a nearby site, which is pumped in via a pipeline.

Treatment comprises an inlet works followed by primary settlement in two circular primary settlement tanks.  The settled sewage then gravitates to secondary treatment in Activated Sludge Plant, which comprises a four-cell aeration tank, with the cells or pockets arranged in series.

The aeration was originally provided by four 30kW vertical cone aerators, which were replaced in 2003 with 12 off 21kW submersible jet aerators due to noise complaints.

Following secondary treatment, the mixed liquors are settled in the two settlement tanks from where the clarified effluent is routed to the lagoon for further settlement before being discharged into the local river.

The power consumption for the submersible aerators was over 220kW which is very high for a carbonaceous ASP.


Air Technology Limited had identified this works as a high-power use works three years previously, but due to concerns regarding noise from the blowers, proposals to replace the submersible aerators with diffused aeration were originally rejected.  Air Technology continued to monitor the works power consumption and process performance, which showed that the power was progressively increasing and the FE quality was deteriorating.  The amount of solid in the lagoon was getting to a point that it would require emptying at a cost of over £1 million.

 A full aeration survey was carried out in 2011, which involved DO surveys, pollutant abatement mapping and power measurements.  The survey identified that treatment was very poor, with low DO levels throughout the aeration tank despite the power consumption being over 240kW on occasions.  The poor levels of treatment and low DO levels resulted in high levels of foaming on the tank surface as can be seen in the picture.  The low DO levels resulted in high SSVI and consequently poor settlement in the FST’s.   The amount of solids from the FST’s were steadily filling up the lagoon and there were concerns that it would have to be emptied.

The survey also established that the reliability of the submersible aerators was poor and typically four units a year needed to be changed at considerable expense as a crane was required to remove and install a unit as well as the repair costs.

 Air Technology recommended replacing the submersible aerators with fine bubble diffused aeration with the blowers housed in acoustic enclosures in the old press building.  This would save over £120,000 on energy alone, with the maintenance savings being estimated at over £30,000.


The installation was managed by a Framework Partner, with the design and specification provided by Air Technology Limited who also designed and carried out the control system upgrade and commissioned the system.

The site work was started in November 2012 and completed in December 2012.  The improvements were immediate, with much reduced levels of moussing and a vast improvement in FE quality, especially solids carryover from the FST’s.  The lagoon has not had to be cleaned.


The energy savings were almost 4,000kWhr a day as can be seen below:

 Validation Chart

Another benefit was a reduction in reactive power charges as the blower motors have a far better power factor than the submersible aerator motors. Maintenance costs have been much reduced as the blowers and DO meters have been reliable and require far less maintenance.

The quality of the final effluent has improved and at the solids carryover from the clarifiers has reduced dramatically the lagoon has not had to be cleaned out, which has saved over £1million in cleaning costs.




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