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Drinking water standards

Drinking water standards

Compliance with drinking water standards - The DWI annually calculate and report mean zonal compliance (MZC) - it measures performance against 39 parameters representing the quality of potable water supplied to customers. This is measured at customer taps; so failures can be caused by customers plumbing issues.

In our business plan we proposed a target of 99.98% throughout representing our highest ever historic performance. The DWI consider anything other than targeting 100% is inappropriate, so Ofwat have set this as the target from 2017-18 onward.

This measure has the potential for an underperformance payment of £770k if in any year performance falls below 99.95%. 

In 2018-19 we had 9 sample failures out of the approximately 25,000 tests contributing to mean zonal compliance, giving  MZC of 99.96%. Of these sample failures, 9 were attributable to domestic plumbing and service pipe issues and none were found to be associated with Wessex Water assets. 

We are continuing to work with Water Regulations Advisory Scheme (WRAS), the water fittings agency, to influence domestic plumbing to reduce domestic failures. 

We continue to review and modify our company lead pipe replacement strategy. This is a long term issue that is not expected to change headline compliance in the short term. Therefore, we will not achieve 100% compliance in this AMP (2015-20) but we can expect to always be above the Ofwat deadband target of 99.95%. 

A new method of measuring compliance with drinking water standards called 'Compliance Risk Index' (CRI) is being introduced by the DWI. This will eventually replace MZC. CRI includes sample failures from reservoirs and treatment works as well as customer taps and the scoring takes into account the type of breach as well as the scale of the potential impact.