Enhancing quality and safety

Guarding public health has seen a Welsh Water treatment plant embark on process control system upgrade. Simon Ellam of Siemens explains

A Dwr Cymru Welsh Water (DCWW) treatment works has tackled potential issues surrounding water quality through a recent £11M process control system upgrade based upon Siemens' PCS 7 process control system and specified by leading system integrator, Oasis Software Solutions. The new control system - which was installed with minimum disruption to the continuous operation of a key treatment site - now ensures that previous concerns about threats to water quality in the local supply network have been eradicated.

The treatment plant has also optimised its daily operational efficiency in areas such as plant downtime and predictive maintenance - and now enjoys a strong foundation to prepare for future expansion of the plant if required.

The Mynydd Llandygai Water Treatment Works (WTW) is located on the outskirts of Bethesda, near Bangor in Gwynedd. There are two raw water sources for the works - the Ffynnon Llugwy and Marchlyn Bach.

In the past both raw water sources have on occasion exceeded the regulatory guideline values for total coli forms and E.coli, and the Ffynnon Llugwy source has also exceeded the guideline value for colour.

World Health Organisation guidelines for drinking water quality requires subjected to coagulation and filtration stages in the overall treatment process. Once treated, the water from the Mynydd works gravitates into an onsite treated water reservoir and then on into general public supply via two pipelines to the Pentir Tank and to Bethesda as part of the networked distribution system. It also supplies the Pant yr Afon local distribution system which serves Mynydd Llandygai village.

In addition to the ongoing issues outlined above, in recent times low concentrations of Cryptosporidium organisms had been detected in the treated water from the Mynydd treatment works.

Historically, the treatment process at the works had not been designed to remove Cryptosporidium so it was expected that the oocysts (the technical term for the cells), could pass through the treatment process into supply.

Presence of the Cryptosporidium organism occurs as a result of faecal contamination in the water normally caused from livestock grazing around the reservoir, or from discharges from sewage works result that no treatment process had been specified at the treatment works to deal with such an issue. However, a Cryptosporidium outbreak in an area served by another water treatment works, which affected approximately 200 people and saw a subsequent Boiling Water Order implemented, alerted management and raised concerns that a similar outbreak could potentially occur at Mynydd Llandygai in the future.

Another problem at the works was that elevated levels of Trihalomethanes (THMs) had over the last few years been detected in the distribution system from the Mynydd treatment works, particularly at the furthest points on the supply system. THMs are formed from the reaction of free chlorine with certain organic precursors present in raw water sources and they are not removed in the treatment process.

The two options to reduce the extent of TMH formation are to remove the organic precursors or minimise the contact with a free chlorine residual found in the water.

Upgrade
Against this background, management at DCWW Welsh Water proactively embarked on an £11M upgrade project to modernise the water treatment works. They wished to address the key operational objectives to improve the colour at the raw water sources, reduce the Trihalomethane (THM) levels, and to proactively prevent Cryptosporidium organisms from the treated water entering the general supply.

The treatment process chosen for the works was a CoCoDaFF plant, which is a proprietary system involving counter current dissolved air flotation filtration. Prior to this major capital investment at the works, the existing process control system in place at Mynydd was obsolete and had proved to be problematical particularly with numerous hardware failures of the system resulting in many call-outs. Such occurrences significantly affected the overall operational efficiency at the treatment works.

Oasis Software Solutions based in Monmouth and part of the Oasis Group of companies, were approached by DCWW to undertake a feasibility study to upgrade the legacy process control system at the Mynydd works. This resulted in Oasis being awarded the contract to upgrade the distributed control system and provide the process control software to ensure the operational objectives could be met.

Oasis software manager, Neil MacDonald explains, "The choice and the design of the control system were crucial as it had to meet the requirements of the system upgrade but also allow for seamless future expansion of the plant as and when required."

With this in mind, the control system was upgraded to a Siemens PCS7 system with a pair of redundant S7-417H AS controllers at the heart of the system providing the performance capability and the high availability that the application required. Two PCS7 single stations were installed in the treatment works control room to provide total and easy visualisation of the entire plant process for operating personnel.

The existing motor control centres and ICA sections were integrated into the centralised redundant AS controllers using distributed ET200M I/O hardware sitting on a redundant Profibus network. The control system solution allowed for integration of the new CoCoDaff plant by ensuring that the new motor control centres and instrumentation were designed to sit entirely on Profibus DP & PA networks - then simply adding additional Profibus networks cards to the PCS7 system.

Commissioning
To ensure minimum disruption to the daily operation at the treatment works during the commissioning phase, the new area of plant was installed in isolation to the main works using a temporary S7-416 AS controller. Once successful dry and wet loop commissioning had taken place the CoCoDaff plant was integrated into the main works simply by disconnecting the CoCoDaff Profibus networks from the temporary AS controller and integrating them into the existing works PCS7 system. Indeed, a whole new area of plant was integrated into the control system within a matter of minutes.

DCWW's head of production, Marc Davies comments: "The project at Mynydd Llandygai WTW was a great success and all the partners involved worked together to deliver the improvements in time to meet the target date agreed with the Drinking Water Inspectorate. The new treatment processes and control system has delivered much more effective treatment and security in terms of the quality of water supplies to our customers. "

A period of process optimisation has followed with Oasis working closely with Black & Veatch and DCWW in order get the very best out of the control system, including the provision of on-going service support and training assisting staff at the works.


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