Image description: Blue plastic and steel pipes.

The Drinking Water Inspectorate (DWI) has outlined new standards for water companies to assess, manage and report risks that could lead to a loss of drinking water supplies. This directive expands existing Drinking Water Safety Plans to include “sufficiency” risk assessments of network assets.

Applying a DWSP approach

The guidance provides a methodology for managing the risk of loss of supply using the drinking water safety planning (DWSP) approach, which they say provides a structured way to documenting hazards and hazardous events, scoring risks and applying control measures.

DWI says companies’ overall aim must be to prevent loss of supplies, in accordance with their duty under Section 52(1)(a) of the Water Industry Act 1991, while taking account of consumer vulnerability and assigning appropriate levels of criticality to assets supplying vulnerable consumers.

Companies will also be expected to assess the supply systems they operate for risks that could affect the sufficiency of supply. They should use hazardous-event matrices included in the guidance to identify and assess critical points from source to tap where a loss of supply could occur. The assessment should take account of both asset criticality and asset health.

Companies are expected to use a reasonable worst-case scenario approach when applying the hazardous-event methodology, covering both common and rare risk scenarios.

Companies are expected to reduce the risk of insufficient supplies “as low as reasonably practicable”, with measures prioritised according to the risks identified.

The Inspectorate also says companies should improve the sufficiency of supplies across the parts of the system under their control. Where parts of a supply system are outside their control, companies are expected to forecast, plan and implement mitigation measures to improve resilience to changes outside their control.

Companies should aim to maintain a continuous supply of tap water during loss-of-supply events

A particular expectation is that companies should aim to maintain a continuous supply of tap water during a loss-of-supply incident. DWI says this should be achieved through measures including rezoning and tankering water to storage or distribution mains, rather than using bottled water as a validated contingency for loss of supply.

New formal reporting requirements introduced

Companies are expected to submit sufficiency risk assessment reports for each supply system using the reporting template included in the guidance by 31 October 2027. DWI says an initial iteration of the data may be submitted by 31 March 2027 to allow for ongoing development of the methodology and industry feedback.

The DWI has also introduced an immediate change to event reporting, effective from the date of the Information Letter. Companies are expected to include details of any realised sufficiency risk when notifying DWI of water-quality events where the incident also has implications for continuity of supply. This includes a pressure drop lasting 24 hours or more that affects continuous supply to consumers. This also encompasses situations where a critical asset is taken out of service for planned or unplanned maintenance and this leads to a water-quality event, as well as restrictions on the output flow from treatment works or the storage volume available in treated-water tanks where these are caused by a water-quality event or result in a loss of supply to consumers.