Investigations

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

Report on drinking water quality incident at:
Bradan Water Treatment Works in April 2007

Scottish Water Executive Summary

On Saturday 21st April at approx 09:00a.m the air saturated water plant at Bradan WTW failed to operate. The bearings had failed on the electrical motor of the duty compressor. The standby compressor was not available as this had failed two and a half weeks prior and had been subsequently assessed as beyond economical repair. Arrangements were in hand to procure an equivalent replacement.

The resultant impact of the failure was the inability to float the floc material within the clarifiers. The floc material then passed over onto the primary filters increasing head loss and eventually causing turbidity and aluminium breakthrough.

Although there is two stage filtration and intermediate lime addition for manganese removal purposes the quantities of residual aluminium from the primary filter stage eventually also over loaded the secondary filters with the same effect as the primary's. The effect of this caused the aluminium levels within the CWT and distribution system to breach prescribed concentration values (PCV).

The operator in an attempt to maintain water quality under the circumstances washed both primary and secondary filters during the day, concentrating on the latter as wash space became available within the wash water tanks.

The repair and maintenance team managed to build one compressor from the two faulty ones, as different elements of each unit had failed. This was completed and operational by 16:30. Scottish Water (SW) had instigated plans to hire a compressor by mid day. This arrived later and was installed and commissioned as the standby to prevent further failure.

New compressors have been sourced and are scheduled for delivery and installation before the end of June 2007. The new compressors have been deemed more suitable for purpose by both SW and the compressor company.

DWQR Assessment on the incident

This incident occurred due to a failure of one of the two air compressors providing air to drive the Dissolved Air Flotation clarifiers (DAFs) due to a lack of lubrication. The other compressor had failed just over two weeks previously, also due to lubrication problems, and had been deemed uneconomical to repair. The compressors were 15 years old. The failure of the second compressor resulted in the filters being overloaded and elevated concentrations of aluminium in the water entering supply. Aluminium concentrations in the distribution system breached the prescribed concentration or value (PCV) at several locations in the hours following the incident.

DWQR considers that Scottish Water on-site staff responded to the situation promptly and made strenuous efforts to maintain final water quality by undertaking frequent filter washes. A temporary compressor was constructed using serviceable parts from the two failed compressors, and DAF operation was restored within seven hours.

Maintenance of the compressors had been omitted when maintenance tasks were transferred to a new task scheduling system in 2004. Consequently, there is no record of routine maintenance of these compressors, including lubrication, being undertaken at the required frequency, if indeed it was carried out at all. The error also meant that the compressors had not been considered for replacement due to their age.

Scottish Water has installed two new compressors on the site and a lubrication schedule has been introduced to prompt and record routine lubrication tasks. A company wide review of risk-based maintenance is underway to ensure that all such tasks are picked up across the asset base and while DWQR welcomes this, an appropriate maintenance schedule should have been in place already. In common with other recent incidents, DWQR had concerns over the completeness of distribution water sampling in response to the process failure. In particular no additional microbiological samples were taken to verify that disinfection had not been compromised. Measures to address this deficiency are already underway and again, DWQR welcomes this.

In response to the incident, DWQR visited the site to discuss the incident with Scottish Water staff and undertake an audit of the relevant treatment processes. This audit highlighted a number of recommendations. The most serious of these was that some of the primary filters were losing a significant quantity of media during backwashes. Studying backwash flow patterns also highlighted significant concerns over the effective operation of the filters as a barrier against Cryptosporidium. These issues were highlighted in a previous audit by DWQR and it is extremely disappointing that Scottish Water has not acted on these recommendations. Scottish Water plans to progressively modify each filter and replenish lost media commencing September 2007. DWQR has requested to be kept informed of progress with this work through three-monthly updates.