Investigations

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

Report on drinking water quality incident at:
Mannofield Water Treatment Works in February 2008

Scottish Water Executive Summary

There has been deterioration in the performance of Mannofield Water Treatment Works through the latter months of 2007 and continuing into the early months of 2008 in respect of the iron and manganese standards.

In a coagulation process, iron should be removed by the clarification and filtration process and should not be a problem.

Manganese removal is particularly dependant upon the pH within the rapid gravity filters being controlled to achieve a pH above 8.0.

Investigation of the failures has shown that in the days prior to each of these, the individual filter pH's have been at a much lower pH than is required for effective removal of manganese.

The deterioration in performance appears to coincide with a request from the Process Scientist to reduce the target pH at Mannofield Water Treatment Works from 9.0 to 8.5 following on from a similar reduction achieved at Forehill WTW at which manganese removal remained successful.

The target residual for the rapid gravity filters at Mannofield Water Treatment Works has been reinstated to pH 9.0 since 13/03/08 as a result of the early investigations for the production of this report.

The reason for the increase in iron failures has yet to be established and requires further investigation.

DWQR Assessment on the incident

DWQR requested an incident report from Scottish Water following a deterioration in the quality of water supplied to parts of Aberdeenshire by the Mannofield water treatment works. Specifically, failures of the standards for manganese and iron were reported to DWQR on 10 January and 11 February 2008 in water leaving the works.

The incident report request triggered an investigation by Scottish Water, whereupon it was found that manganese removal at Mannofield had not been as effective since a reduction in the pH of the water was initiated in September 2007. Manganese removal at the plant had been installed on a trial basis in 2004, however its rudimentary nature meant that it was difficult for site operators to ensure pH was optimised for manganese removal across each of the filters. It was evident that the reduction in pH in September 2007, although successful at other treatment works, had caused filtered water pH at Mannofield to drift outside the range at which effective manganese removal can be achieved. Although iron removal is easier to achieve than manganese, it seems that the efficiency of iron removal has also been affected, something Scottish Water is still investigating.

Scottish Water has now made the necessary readjustments to pH control to improve manganese and iron removal at Mannofield. A permanent and robust means of controlling the coagulation and filtration processes at Mannofield is currently proposed for completion in March 2009. Scottish Water is investigating the scope for bringing this date forward. DWQR is satisfied with the findings of Scottish Water's investigations and that a treatment process capable of reliably removing manganese will soon be in place. It is disappointing, however, that Scottish Water does not appear to have been closely monitoring the performance of this works and that it apparently required the involvement of DWQR to ensure that the appropriate investigation and optimisation were undertaken.