Anglian Water unblocks pumps at Basildon STW

Basildon, UK – Anglian Water has resolved problems that were creating continuous blockages at its Basildon Sewage Treatment Works’ (STW) pumping system.

The Essex Works is fed by an estimated population of 150,000, of which 80% of the flow arriving at the works is handled by the High Level Inlet Pumping Station.

Anglian Water decided to replace the existing inefficient spindle pumps with 350mm diameter discharge dry well submersible type pumps, fitted with channel impellers, and configured Duty/Assist/Standby, with the capability of 400 l/s with one pump running.

Due to dry weather conditions during the first three months following installation the performance was satisfactory. However, at the first sign of significant rainfall, all three pumps began blocking.

Indeed, and in one particular instance, on a Bank Holiday Monday, each pump had to be removed nine times to remove blockages. Even then the site could not be prevented from flooding.

Several months later, after discussions with the manufacturer, run timers were fitted in the panel, with the idea of stopping the pumps after a ten-minute run to prevent rags building up on the leading edge of the impellers, which caused the “soft blocking”. However, this had no discernible improvement on pump efficiency.

One pump was then changed to a screw type impeller with an inlet of 500mm. However, blockages still continued between the screw and liner cone.

Finally, as a last alternative, three smaller 300mm pumps were installed, fitted with semi-open channel type impellers, and the configuration was changed to Duty/Assist/Assist. Unfortunately, this failed to solve the blocking problem.

By this stage, the situation had become serious and very high profile within the local community. Gary Wakley, asset engineer, based at Colchester STW, therefore, contacted ITT Water & Wastewater to carry out a site survey and select a pump which could cope with the heavy rag mass.

Two of the key criteria for selection was that the pump would give a minimum 400 l/s, with one pump running, and would offer Anglian Water spare capacity for when the pumps need servicing.

Nottingham-based ITT selected three Flygt ’N-Pumps’ NT 3301LT with 37 kW motors configured Duty/Assist/Standby for the job which offered 423.6 l/s with one pump running and 827.6 l/s with two pumps running.

Due to the problems encountered with the three types of Impellers tried to-date, the first pump was fitted on a “try before you buy” basis, by ITT for three months to ensure a thorough test. In this period, not one single blockage occurred.

The pump was so reliable that the Anglian Water Operations staff actually stopped unblocking the other two pumps, which meant that the Flygt pump was single-handedly coping with the flow, said ITT.

After three months of running almost continually as the main duty pump, there was no apparent damage or wear, prompting Anglian Water to install two more pumps.

“After three months of operating almost entirely on the trial Flygt pump it was opened and inspected. There was no sign of wear at that point,” Wakley reported.

Anglian Water has also installed variable speed drives on these pumps, which now not only pump the rag without blockage, but are also offering us significant energy savings, the asset engineer added.

“Similarly we have now got pumps that will give a duty of some 1200 l/s if required on only 3 pumps,” said Wakely. “Due to the success of this project, and the addition of VFDs, the energy saving equates to circa £6k - £7k per year.”