From several decades of experience of the treatment of domestic sewage on a professional basis I am at a loss to understand how a "pump failure" can lead to the discharge to the Firth of Forth of millions of gallons of untreated sewage.

Standard industry procedures for handling such critically important pumping requirements in sewage treatment plants have long since involved splitting the pumping effort required between a number of separate pumps, plus having a number of standby pumps just in case any of the primary pumps fail.

In this case, I can only assume that the PFI approach to the sewage treatment project at Seafield in Edinburgh has led to the ideal pumping requirements being reduced to the absolute minimum, even beyond, such that the failure of a single pump has led to such a disgraceful discharge of raw, untreated sewage to the Firth of Forth for a period approaching 48 hours.

David W Webster, 1 Middlewood Park, Deans, Livingston.

The Forth sewage crisis raises fundamental questions about how we run and manage our waste-water treatment. The Seafield works is run by a firm - Thames Water - from England where water is privatised under PFI.

This event happened because of the failure of a pump that was more than 30 years old. It is normal in Northern Ireland (where water is publicly controlled) for any pumping station to have two back-up pumps of equal capacity. This appears not to be the case at Seafield. This is indicative of a wider problem with PFI. The first concern of Thames Water is to provide shareholders with a dividend, not the safe and effective treatment of sewage.

The failure of a 30-year-old pump should not be a surprise - but it is astonishing that the chairman of the Water Industry Commission, Sir Ian Byatt, has not ensured there is an effective maintenance and replacement programme. Local people have been warning Scottish Water and the Scottish Executive for years about the problems at the plant. In the last parliament, I supported the local community campaign against the stench from the sewage works.

The Scottish Green Party has campaigned consistently against water privatisation - for the very reasons that we see today. Mark Ballard, Green Speaker on Finance and Public Services.