Thames Water chose PID control solution for sewage

In the sewage aeration process at Hogsmill, Thames Water is using high accuracy Eurotherm PID controllers as analogue interfaces between dissolved oxygen probes and the PLC based SCADA control system

As part of the sewage aeration process at its large scale treatment plant at Hogsmill, Thames Water is using high accuracy Eurotherm PID controllers to act as an analogue interface between dissolved oxygen probes and the PLC based SCADA control system.

The controllers, Eurotherm 2404s, feature a high accuracy control algorithm which allows reliable and timely control of the air blowers to maintain the aeration process within tight limits.

Part of the biological treatment of sewage at Hogsmill is a complex three stage process which puts the settled sewage through anaerobic, anoxic and aerobic conditions.

These conditions provide nutrient removal as well as organic load reduction.

To achieve this, mixed liquor (combined settled sewage and activated sludge) is distributed into six treatment lanes.

Dissolved Oxygen (DO) levels in the lanes, essential to maintain the bacterial action which breaks down the sewage, are maintained by a bank of four PLC-controlled air blowers.

Each lane has a DO probe to provide feedback and dedicated air feed line with flow meter and control valve.

Each DO meter is validated locally by comparison to the other meters.

On failure of a DO meter the air control will be operated by average readings from the other lanes.

The air flow to each lane is modulated to achieve the nominal setpoint, within a deadband typically of +/-0.3mg/l.

The blowers increase or decrease air supply in response to the average DO across all lanes.

It is also possible to select a lane 'out of service' mode to allow manual control of the air control valve.

The four air blowers operate on a duty/assist/assist/standby basis and are fitted with guide vanes to control the flow of air to the lanes.

All guide vanes modulate independently such that typically the duty blower will run at maximum load whilst the assist blower may run at a low load.

Assist blowers are stopped and started according to the overall demand on the blowers.

This approach is important in maintaining efficient energy usage.

The Eurotherm 2404 controller was used to replace an existing step control algorithm and has been integrated into the existing software loops to ensure that operator interfaces remain unchanged and also a minimum of on-site wiring modifications.

This solution, designed and engineered by water process specialists Z-Tech Limited, ensures that small changes in the analogue signals output by the probes are amplified proportionally and fed into the PLC input in digital form.

The key to successful operation of the system is the highly accurate PID algorithm which resides in the 2404 controller.

This has continuously evolved over at least 20 years and allows the controller to respond to analogue signal changes rapidly and with exceptional accuracy.

It is able to respond to small analogue signal changes - such as those being produced by DO probes - faster than digital devices such as PLCs.

The 2404 is a low cost, highly versatile 1/4 DIN panel sized controller with an extensive range of options.

Either PID, on/off or motorised valve control outputs can be configured, satisfying a wide range of control siuations, and the advanced adaptive tuning algorithms optimise control performance to suit the application.

The instruments accept a range of plug-in modules for heating, cooling, analogue retransmission, second process value input and remote setpoint.

High speed Modbus, PROFIBUS and ASCII communications provide easy connection to PLCs, supervisory control and data logging systems.

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