Northumbrian Water Bran Sands

Following the installation of a Variable Speed Drive for energy saving the treatment works Anaerobic Digester, Northumbria Water have seen the following results.


There are 6 off 90KW pump motors supplying 3 anaerobic digesters originally running at fixed speed with a star delta start at 1500 RPM. Each Digester runs 24/7 (8736 hours per annum) and the pumps run duty standby. (4368 hours per pump per annum.)

Following the installation and testing of two ACS800’s the speed was reduced down to 900 RPM at which point the digester could run without causing excess foaming.

Quantum Controls then came back to site to measure the difference in kW’s between two pumps running simultaneously, with the first pump being Star Delta and the second being VSD controlled.

We can assume all other variables remain equal as both digester tanks are the same size and were full and both pumps are the same age.

In order to work out the costs for running each pump we have used the area average of 8p per kWH

Cost of Upgrade

The cost per pump is £4875 this is based on one engineer being assisted by an NWL engineer and taking two days to upgrade each Star Delta panel to a VSD panel, this also allows for the cost of the VSD and all ancillary equipment needed when upgrading the panel. This cost is also fully inclusive of any commissioning.

Overall summary

Overall saving based on these results would be £106,928.64 this equates to approximately 75% of the current costs.

The estimated cost of project is £29,250.00 (including delivery and Labour) divided by annum savings of £106,928.64 means a payback period of 0.27 years or to put it another way approximately 100days. These figures are based on reducing the speed of the motor by 40% from 1500rpm to 900rpm. The savings for this have also been proven with the first two installations.

Further savings are possible as reducing you kWH you’re reducing your Co2 footprint by 721.80 tonnes and with the current carbon tax credits this reduction equates to an additional saving of £8661.66.

This brings to total savings up to £115,590.24 with a return on investment of 92 days.

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