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Drinking water standards


Compliance with drinking water standards (MZC) - The DWI annually calculate and report mean zonal compliance (MZC). This 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 set this as the target from 2017-18 to 2019-20.

The target for the whole industry is 100% and we incur an underperformance penalty payment of £770k if performance falls below 99.95% in any year.

In 2019-20, we achieved 99.97%. We had 13 sample failures out of the approximately 29,000 tests contributing to MZC. All of these 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 the Water Regulations Advisory Scheme (WRAS), the water fittings agency, to influence domestic plumbing to reduce domestic failures.

Over the five-year period we have run approximately 149,000 tests for drinking water quality and seen 68 sample failures. Only 11 of these have been on Wessex Water assets, and none in the last two years.

A new method of measuring compliance with drinking water standards called Compliance Risk Index (CRI) has been introduced by the DWI. This will replace MZC for the next five-year period. 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.