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Wessex Water cleans up with Rotork

09 March 2010

The Wessex Water sewage treatment works at Swanage, UK, claims its effluent not only exceeds EU standards but is cleaner than the sea into which it is discharged. The water company has implemented membrane bioreactor technology as part of a plant-wide upgrade.

Rotork IQT actuators installed on new pipework in the chambers between the membrane plants, operating the sewage inlet and outlet, air, sodium hypochlorite, wash water and permeate valves
Rotork IQT actuators installed on new pipework in the chambers between the membrane plants, operating the sewage inlet and outlet, air, sodium hypochlorite, wash water and permeate valves

The upgrade includes automation and energy saving measures and will increase the size of the membrane treatment plant to provide spare capacity for future demand. Central to the upgrade are Profibus network-enabled Rotork IQT intelligent electric actuators that have been installed to operate butterfly and plug valves on new pipework throughout the plant.

Swanage STW treats sewage for a population of around 10,000, which can more than double when the area is busy during holiday seasons. Following preliminary screening and grit removal, sewage enters Kubota membrane plants installed in six large tanks for biological treatment and disinfecting. The pores in the membranes are so small that they can prevent microscopic bacteria and viruses from passing through. The upgrade involves increasing the number of membrane packs in each tank to thirty-six, thus considerably increasing the treatment capacity without exceeding the confines of the existing works’ footprint. Automation of the treatment process also enables it to run with increased efficiency and reduced energy consumption.

Rotork IQT actuators controlling butterfly valves on the pipework carrying air to the membrane plants
Rotork IQT actuators controlling butterfly valves on the pipework carrying air to the membrane plants

On each of the membrane plants, the Rotork IQT actuators operate inlet isolation and outlet modulating valves for the sewage, together with air, sodium hypochlorite and wash water injection valves. Additional IQT actuators control the permeate isolation valves used for automated sequential flushing to maintain membrane efficiency. In total, fifty-one actuators are installed on each tank to provide automatic sequencing of the sewage treatment, membrane cleaning and backwashing processes.

Fourteen Profibus-DP two-wire networks link the actuators to PLCs in the site’s central MCC. These are controlled by a PLC and SCADA system written by Wessex Water’s in-house Automation Team. The control PLC is a twin CPU configuration, with one dedicated to the control and monitoring of more than 300 actuators, whilst the other runs the complex automation process, resulting in the collection of control, status and full diagnostic data from each actuator, without any PLC speed or memory limitations. Diagnostic information is available, locally at the valve or nearby HMI, on the site SCADA system and at the regional operation centre at Bath. Remote diagnosis is a cost-effective method of maintenance and service because the right operational staff can be sent to site when required.

One of the Rotork actuator diagnostic touch screen HMI displays on the site’s SCADA system
One of the Rotork actuator diagnostic touch screen HMI displays on the site’s SCADA system

Running costs are expected to reduce due to more effective control of the plant, saving electrical power, cleaning agents and water. The effective use of Profibus technology has allowed significant installation savings in cabling, terminations, PLC hardware and labour.

The Damar Group is the principal contractor for mechanical and electrical installation at the Swanage upgrade project, including the on-site fabrication and installation of pipework and actuated valves. The project manager is Wessex Engineering and Construction Services and the design consultant is Mott MacDonald. The Rotork actuators have been supplied in accordance with the framework agreement that the company holds with Wessex Water.


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