Investigations

This section contains summaries of reports on drinking water
quality incidents and investigations.

Report on drinking water quality incident at:
Picketlaw Water Treatment Works in November 2007

Scottish Water Executive Summary

On 11th November 2007 at 18:00 aluminium levels exceeded the Prescribed Concentration

Value (PCV) of 200µg/l at Picketlaw WTW. The main causes of the aluminium failure and the prolonged breach of PCV's were blockages at the lime plant and the inability to maintain a stable coagulated pH. The blockages were temporarily resolved by rebatching lime slurry tank No.1 and flushing the dosing lines, by 00:30 on 12th November 2007 treated aluminium levels were reducing. By 01:30 the levels were reducing back to within the PCV at approx 250µg/l. The system has a duty and a standby lime slurry tank facility. At the time of the incident only tank No. 1 was operational as tank No.2 had been taken out of service and cleaned, but not yet reinstated.

Initially on 11th November at approx. 20:30 lime slurry tank No.1 failed to batch automatically and as a consequence the lime slurry in the tank was emptying. At around 00:30 on 12th November the tank had approximately 25% capacity when the lime system blocked again. This again caused a severe drop in coagulated pH and the loss of treatment of raw water. An alarm was raised to the Balmore Road Operational Management Centre (OMC). Staff within the OMC passed this alarm out to the standby operative.

On arrival at the works the standby operator manually batched lime slurry tank No.1 and put back into service. He also cleared the blockages which appeared to rectify the problem. This re-established the coagulation process and by 02:00 the aluminium levels started to reduce. The standby operator then left site.

On Monday 12th November the site operator arrived on site at 08:00 and noticed the DAF turbidity high at 10 NTU. Site operator informed the Team Leader who in turn informed the Regional Manager and the Public Health Team. Remedial action on site commenced immediately, the site operator checked the alum dose and proceeded to strip out and clean the inline alum filter and the alum dosing line to the inlet suction pump, but this did not improve the high turbidity levels.

At 08:30 Bairdsknowe Service reservoir was isolated and by 09:00 one compartment of High Borland service reservoir was isolated and run to waste.

The final water was being analysed on site for alum levels at regular intervals and samples were collected in distribution and at service reservoirs for analysis by the laboratory. Water quality was back to normal by the 13th November. No customer complaints regarding water quality were received during the course of the incident. Final water cryptosporidium monitoring took place over the period of the incident and all samples were negative.

DWQR Assessment on the incident

It is DWQR's assessment that this incident occurred due to a number of coinciding factors. Had any one of these happened in isolation, it is likely that the exceedence of the aluminium standard in water entering supply would not have been as prolonged nor as severe. In response to this incident, DWQR visited the site to discuss the incident with Scottish Water operational staff and undertake a technical audit of the treatment works. Following the audit, three recommendations and one suggestion were made.

The incident commenced due to problems with the batching of the Lime solution used to control the pH of the treatment process. Although an appropriate alarm was generated, this was not passed out to operational staff in a timely manner. Scottish Water has taken action to investigate and resolve the failure of the alarm monitoring process. Once the alarm was received, the standby operator acted correctly to try to restore the correct pH and regain control of the treatment process. Although apparently successful, further problems with a blocked chemical dosing filter and pH monitoring instrumentation served to prevent the treatment process from stabilising which meant that aluminium concentrations leaving the works remained high. During the incident, Scottish Water progressively increased the concentration of dosed aluminium in an effort to regain control of the coagulation treatment process, taking the dose beyond that considered optimal for the plant. It is likely that this may also have had the effect of prolonging the incident. Following adjustment of the dissolved air flotation process and a re-consideration of the alum dose, water quality was finally restored over 36 hours after the incident commenced.

Scottish Water took measures to reduce the impact of the high aluminium concentrations on consumers, although following the incident it has considered additional measures it could have taken. It is evident from DWQR's investigation of the incident, that the treatment processes at Picketlaw are basically sound, although the lime plant is difficult to work with and prone to failures and blockages. Scottish Water is in the process of replacing the plant which will undoubtedly serve to make the process more robust. DWQR has also identified that there may be a need for site specific written guidance to operators on reacting to failures of the treatment process. This should include clear guidance on appropriate chemical doses given specific situations and raw water quality.