Innovative thinking and successful collaboration between Southern Water and ABB ensure a solution is found to accurate coagulant dosing

When Southern Water couldn’t find an exact match to replace its existing coagulant dosing system, different thinking from ABB helped to find a solution.

An innovative coagulant dosing system devised by ABB Measurement and Analytics is helping Southern Water to ensure continued delivery of high-quality water with greater efficiency and reduced environmental impact.

Installed at Southern Water’s Otterbourne Water Supply Works (WSW) in Hampshire, the system has replaced an obsolete automatic coagulant dosing system that was reaching the end of its operational life. With spare parts no longer available, the decision was made to replace the system with an updated alternative, with a deadline for completion set for 31^st January 2020. (Otterbourne WSW pictured below).

In the initial stages of the design process, the team carried out an assessment of the potential replacement systems available and also met with other water utilities providers to compare the available product and gain valuable feedback and share best practice.

Following evaluation of the available systems, Southern Water discovered that there was no solution that was a 100 percent match for its requirements. As such Southern Water set suppliers a challenge to find a solution. ABB were selected for trial from a scoring system set up to evaluate those solutions across several parameters, from design, to operation, maintenance, whole life cost, among others of all the UK available auto-coagulation systems in the market. ABB was the top scorer and therefore engaged to progress to the next step, which was a trial set-up.

The essence of the design brief was to find a solution that replicated the existing system, while also providing additional control and instrumentation improvements that offer an OPEX reduction through chemical cost savings and a reduction in maintenance expenditure. (Legacy coagulant dosing system pictured below

“The challenges presented by the application meant that Southern Water needed something over and above the solutions available in the market,” says Julian Edwards, CWA Product Manager for ABB Measurement and Analytics in the UK and Ireland. “We therefore decided to develop something combining solutions from our existing product portfolio that would provide the degree of accurate dosing control needed by Southern Water.”

ABB’s solution is a continuous measurement and control system combining high and low-range UV organics (DOC) surrogate colour, turbidity and pH analysers with an RVG200 controller / recorder. The purpose of the system is to calculate the optimum amount of coagulant to allow particulates to be effectively removed from the water through sedimentation and filtration. The RVG200 feeds real-time data from the UV organics, turbidity and pH analysers through a fully-configurable algorithm, enabling adjustments to be made to handle potential variations caused by seasonal fluctuations in raw water or physical changes to the process, such as changes in coagulants.

The resulting calculation produces values for the actual required dose (mg/l) and dose rate (ml/min) which are used to control a duty/standby set of coagulant dosing pumps, ensuring that the correct amount of coagulant is always administered.

The use of feedback control modifies the coagulant dose by maintaining colour for the clarified water within a set target band. Referred to as the ‘colour feedback trim’, it fine-tunes the predictive dose to improve both the system response to raw water changes and the efficiency of the coagulation treatment process. This was an improvement on the previous system design that operated solely with a feed-forward control, with no feedback input.

Furthermore, as the new system is fully compatible with the existing communication arrangements between the plant SCADA/PLC/MCC and the coagulant dosing pumps, no changes are required to the existing set-up. Using an ethernet connection in addition to for remote data transmission via a secure VPN link enables remote condition monitoring to be set up.

Through working closely with Southern Water, additional enhancements have been incorporated into the new system including a calculation for ‘supernatant flow return’, sample flow fail protection and alarm for all associated instrumentation, ‘Trim Time’ reset function and coagulant dose expression factor. These enhancements have been easy to carry out using ABB’s RVG200 controller. Combining an intuitive user interface and Windows™ style menus, the RVG200 enables operators to easily adjust the algorithm as and when necessary. Any changes are logged, with details of time, date and the identity of the person who made them.

Another benefit of the system is its reduced cost of ownership. By utilising inputs from the UV organics, turbidity and pH analysers, the system eliminates the need to purchase reagents, filters and peristaltic pump tubes. Frequency of maintenance is also reduced, with the UV organics system only needing to be checked once per quarter.

To assess the performance of this new system, it was tested alongside the existing automatic coagulation system. The accuracy of the system was demonstrated during trials at several SWS sites, with Otterbourne WSW specifically which operated over a 12-month period to capture the widest possible range of raw water conditions. During these trials, the system produced ferric dosing calculations based on raw water quality and flow which were consistently close to Southern Water’s previous auto-coagulation system.