Investigations

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

Report on drinking water quality incident at:
Dalchreichart Water Treatment Works in October 2006

Dalchreichart Water Treatment Works supplies an average of 0.018 ML/Day to a population of approximately 75. The treatment process consists of raw water well which feeds two up- flow sand and carbon filters and then onto a storage tank. The disinfection is by means of sodium hypochlorite dosing as the water exits the storage tank and enters into supply. The pump is powered by mains power which is converted to 24v by the mains power charging two 12 volt batteries which are linked in series to produce the required 24 volts for the chlorine dosing and other ancillary equipment.

On Monday 23rd October 2006 at 14:45 hours, on arrival at the works the treatment operator noticed that the chlorine residual at the statutory sample tap was 0.01 mg/l; the online chlorine monitor was showing 0.44 mg/l. After further investigation it was noticed that the sodium hypochlorite dosing pump had air locked, the airlock was immediately removed and the dosing recommenced with the pump put to 100% for 20 minutes until a free chlorine residual of 1.20mg/l was achieved leaving the clear water tank and going into supply, the pump was then returned to the normal setting. The works was then monitored for 11/2 hours to ensure that the chlorine pump and residual were satisfactory. The operator then took one final water sample and two distribution samples.

On Tuesday 24th October 2006, the works was checked at 10:00 and the Operator reported that verything was okay. The samples that were taken the previous day were reported as having passed.

On Wednesday 25th October 2006, the works was checked at 11:00 and the Operator reported that everything was okay.

The root cause for the disinfection failure at Dalchreichart WTW was due to the sodium hypochlorite dosing system air-locking. A low chlorine alarm should have been received at this point via the telemetry; however the duration of the disinfection failure was prolonged due to a fault with the on-line residual chlorine monitor which resulted in the low level alarm of 0.2mg/l not being reached.

A work request was immediately raised for the Scottish Water ICAT team to attend site to repair and service the monitor. This work was undertaken and completed on the 25/10/06.

The problem with the online chlorine monitor was found to be a restriction in the sample flow which would have prevented a zero calibration being carried out. A zero calibration is vital, as it re-sensitises the chlorine probe allowing a span calibration to be performed.

An assessment of the sodium hypochlorite dosing system will be carried out to establish whether the installation of degassing heads will be sufficient to prevent air locking of the system in the future and whether further modifications could minimise gassing of sodium hypochlorite.

DWQR Assessment on the incident

The main cause of the disinfection failure was due to an air lock in the dosing system. A low level chlorine alarm would normally have been activated and the problem dealt with quickly. However, there was an additional fault on the chlorine monitor which meant that it was reading higher than it should have, so was above the actual low level and an alarm was not therefore sent. Once found, 4 days after the most previous visit when the works was operating effectively, the problem was dealt with fairly quickly. The actions being taken by Scottish Water as a result of this incident are appropriate, including taking steps to ensure as much as possible is done to prevent air locks in the dosing system and that operational staff have refresher training in instrument calibration and procedures for reporting critical equipment faults.