Wessex Water STW upgrade nears completion
8 Feb 2010
Swanage, UK – Wessex Water’s sewage treatment works at Swanage is nearing the completion of a major plant upgrade. The project is intended to introduce automation and energy-saving measures, whilst increasing the size of the advanced membrane treatment plant, to provide spare capacity for future demand at the site.
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.
Swanage STW treats sewage for a population of around 10,000 which can more than double when the area is busy with holidaymakers. Following preliminary screening and grit removal, sewage enters Kubota membrane plants installed in six large tanks for biological treatment and disinfection. 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 36, 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.
Central to the upgrade, Profibus network-enabled Rotork IQT intelligent electric actuators have been installed to operate butterfly and plug valves on new pipework throughout the plant. The actuators were supplied under a framework agreement that Rotork holds with Wessex Water.
On each of the membrane plants, the 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. A total of 51 actuators are installed on each tank to provide automatic sequencing of the sewage treatment, membrane cleaning and backwashing processes.
The unit employs 14 Profibus-DP two-wire networks to 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.
With the high level of process automation enabled by the new actuators, come the added benefits of process integrity, audit ability and highly reliable and accurate process control, saoid Rotork. Running costs, it added, are expected to reduce due to more effective control of the plant, saving electrical power, cleaning agents and water. The Profibus technology has allowed significant installation savings in cabling, terminations, PLC hardware and labour.
