Meiden drives in control at water plant

At the recently commissioned Southern Water plant at Ashlett Creek, near Southampton, variable speed drives are employed to control various key operations of the waste water treatment process.

At the recently commissioned Southern Water plant at Ashlett Creek, near Southampton, variable speed drives are employed to control various key operations of the waste water treatment process.

At the heart of the process is simultaneous aeration of the liquor as it is fed into the CASS (Cyclic Activated Sludge System) tanks after solids have been removed.

Then, after aerobic bacterial treatment, the skimmed liquor is decanted at a controlled rate to a smaller balance tank from where it is pumped out to sea.

All operations are co-ordinated by a PLC and the systems were engineered by Southern and Redfern Ltd, Bradford.

The success of the treatment process, developed by Earthtech of Barnsley, demands effective control of the motors involved at each stage.

This is provided by Meiden VT210SA drives, ranging in size from 0.75kW to 45kW, in response to signals from the central PLC controller.

The Meiden drives control two 45kW positive displacement blowers, two skimmer arms, and two submersible pumps.

For the blowers, the drives provide controlled operation for the aeration process, according to levels of dissolved oxygen (DO) in the liquor.

This is critical to ensure that the correct levels of DO are maintained to effect the natural process of biological digestion of impurities by bacteria.

The drives are governed by a complex PID loop that modifies blower operation according to variable predefined ramp signals matched to DO levels.

After aeration of liquor, settlement is allowed to take place when both remaining solids and bacteria sink.

The top third of the liquor is then decanted in to the balancing tank via a drain mechanism with skimmer boom that tracks the height of the liquid.

Here the drives provide constant torque during the emptying process in response to the depth monitoring system's feedback.

This enables the motor/reduction unit to lower the drain mechanism under load at the correct speed to the required depth.

For the submersible pumps in the 300 m?lance tank, the drives control pump speed to discharge the gravity fed treated liquor coming from the 500mÁSS process tanks.

Discharge rates are variable, being based upon the flow from the CASS tank and monitored depth in the balance tank.

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