Scottish Water plant get the all clear
11 Aug 2010
Edinburgh, UK – Water quality engineering firm Panton McLeod has carried out testing on a new facility at Scottish Water’s Roberton Water Treatment Works in Hawick to check that it was ready to be put into service. The recently refurbished Roberton site is currently under extra scrutiny following an incident there last year that led to an investigation by regulatory authorities in Scotland.
Over two days, Panton McLeod engineers treated and de-chlorinated millions of litres of water at the Scottish Borders. The water was sent through the facility, to test its operational capacity and to check for any faults.
Barclay Smart, civil engineer at the engineering firm, explained: “The main storage tanks were filled with millions of litres of water to check that they would work at full capacity, which was then passed through the whole facility to ensure it functioned perfectly. Our team was tasked with de-chlorinating and neutralising all of this clean water as it is passed through the facility
“The water has to contain chlorine to ensure that the facility remains clean during the test, but it has to be de-chlorinated correctly afterwards - otherwise you cannot safely discharge it back into the local water course.”
The project is the latest work that Panton McLeod has completed for Scottish Water. For the past 15 years, the firm has provided expertise in cleaning and repairing water structures across Scotland - which has included contributions to some of the biggest water projects Scottish Water has undertaken, such as its £120-million Loch Katrine scheme.
Earlier this year, the firm agreed a new deal with Scottish Water - arranged as part of an ongoing price and supply agreement - that will see it clean and disinfect more than 400 service reservoirs and water tanks across the Ness, Don, Tayside and Ayrshire regions.
The Roberton WaterTreatment Works was the focus of a £14.7-million investment by Scottish Water to combine three plants into one. The refurbished site is now be capable of producing 16 million litres of drinking water every day, replacing capacity at ageing works at Dodburn and Acreknowe, which are being made redundant.
Meanwhile, an incident at the Roberton works in late 2009 led to a serious breach of water quality regulations, which prompted an investigation by the Drinking Water Quality Regulator for Scotland (DWQR)
On 29 Oct, the plant was being shut down to allow a changeover of live power connections on the new works which had just been completed. At 11.01hrs, the plant was re-started but neither of the polyaluminium chloride coagulant dosing pumps (duty and standby) would start under an auto setting.
The standby pump was set to manual and it started dosing correctly. The other pump was left live but, as it was still set on auto, it was not running and therefore not dosing chemical. At this point the works was running normally.
The operator left site at 14.00hrs. An orthophosphate dosing alarm was received at 15.25hrs and a standby operator attended the works at 1600hrs to deal with the problem. When on-site he also cleaned the turbidity meter which was found to be dirty. He left the site at 1700hrs when the plant was operating normally.
At 20.03hrs a low dosed pH alarm was passed to the Standby Operator who took no action because he believed that it was a false alarm linked to the earlier plant shutdown.
When he was passed a high clarified water turbidity alarm at 21.17hrs the Standby Operator discussed the situation with the Standby Team Leader who checked the other water quality trends on the telemetry system. These were found to be normal at the last dial out at 21.15hrs.
At 22.05hrs and 22.23hrs the standby team leader dialled into the site to check water quality trends and noted that the clarified turbidity was on a rising trend and so instructed the standby operator to attend the works. He arrived at 23.00hrs and found the coagulant pump that was set on auto but not dosing earlier had started dosing, along with the pump set on manual, leading to overdosing of the polyaluminium chloride coagulant.
The pump set on manual was switched off and the plant left running with the coagulant pump on the auto setting. The plant was left to recover clarified water quality before filter washing started at 01.30hrs on 30 Oct and continued overnight.
During the incident the turbidity of the water from all six filters reached values in excess of 2NTU (the maximum allowed under the Regulations being 1NTU),while the mixed filtered water residual aluminium was over the maximum allowed concentration for approximately 12hrs.
The final water leaving the works residual aluminium exceeded the maximum allowed for around 16hrs but the turbidity remained within the standard. Good chlorine residuals were maintained throughout the incident.
DWQR found that a time lapse of three hours occurred between the receipt of the low dosed water pH alarm and the standby operator arriving on site to deal with the problem. This, it said, is an unacceptable delay in reacting to a potential problem in the critical coagulation treatment process.
However DWQR also noted that steps have been taken to prevent a repeat of this incident namely, issuing site instructions on the steps to be taken should the coagulation pumps fail to start after a shut down and, linking the coagulant flow meter to the telemetry system along with alarm settings.
