Portsmouth Water’s Westergate WTW treats up to 22Ml/d of water from three boreholes and supplies potable water to Littleheath service reservoir. Originally, disinfection of the water was by super-chlorination and de-chlorination using chlorine and sulphur dioxide respectively. In order to provide protection against the possibility of Cryptosporidium, Portsmouth Water decided to install ultraviolet lamps as primary disinfection with residual chlorination to protect the water in supply. Accurate UV dosing is ensured by controlling the UV lamp output by the works flow and the UV transmittance together with the UV intensity measured in the reactor. Final chlorination is controlled by dosing in proportion to the works flow to achieve a C.t set point with feedback trim from the final chlorine residual.
As part of the same upgrade, new variable speed borehole pumps would be installed, linked to the level in the service reservoir. Although superficially a simple plant, the Control Philosophy prepared by the process engineers included monitoring of borehole level, level in the service reservoir, borehole water UV transmittance, treated water turbidity and residual chlorine.
Functional Design Specification
From the Control Philosophy, the Automation, Control & Technology (ACT) team developed a detailed Functional Design Specification (FDS) to provide a step-by-step description of the software systems to be installed. Following completion of the FDS, the ACT team developed and tested the software programme for the control of the new works. In addition, they designed and built a new Motor Control Centre (MCC) which, as well as the variable speed drives for the borehole pumps, includes duplicate mains generator ACB with automatic change over, a 100A active harmonic filter and eight feeders of various ratings for the control circuits and hardwired control logic.
Automated UV Dosing and Chlorination
The MCC also has an Instrumentation, Control and Automation section containing a UPS, dual transformers, a Schneider Modicon M340 PLC controller with a Magelis 12.1” HMI communicating with the UV plant using a direct Modbus (serial) connection as well as numerous hardwired interlocks. The ICA section also controls the chlorination system.
Due to the differing fieldbus communications networks being utilised on the site, integrating the new controls with the existing plant SCADA system was a challenge for the ACT team but they completed the project on time and within budget. This was, in no small part, due to all the design, software and panel fabrication being carried out in-house, with the panels being subjected to a full Factory Acceptance Test in the workshop prior to delivery.