The Scrutiny Committee of the Isles of Scilly Council met on Tuesday 8 September 2026 to receive a presentation from Southwest Water regarding their wastewater proposals for the islands. The meeting focused on the company's progress and plans for improving wastewater treatment and drinking water services.

Wastewater Proposals from Southwest Water

Alan Burrows, Head of Local Government Affairs for Pennon, presented Southwest Water's (SWW) progress on drainage and wastewater management for the Isles of Scilly. SWW took responsibility for public water supplies on all five islands in April 2020, and for wastewater services on St. Mary's and Tresco at the same time. The company is under regulatory requirements to improve drinking water quality across all islands and wastewater treatment on St. Mary's, with an enforcement position in place with the Environment Agency until September 2027 for wastewater services on St. Mary's.

SWW's Drainage and Wastewater Management Plan (DWMP), published in May 2023, outlines nine key actions, with the most significant being the delivery of a new resilient wastewater treatment facility on St. Mary's to meet Urban Wastewater Treatment Directive (UWWTD) requirements. The DWMP is due for an update, and SWW has begun discussions with the Council of the Isles of Scilly regarding local plans and housing targets.

Regarding wastewater services on St. Mary's, SWW plans to deliver secondary treatment following feedback from residents. The company stated that their tidal models are sophisticated, using data simulations to present worst-case scenarios, and are a combination of local intelligence and complex computer modelling. They have engaged with the Inshore Fisheries Conservation Agency (IFCA) and will continue to work with other stakeholders.

In response to concerns about the company's performance, SWW acknowledged the campaign led by Dowr Glan, which advocates for the water industry to be publicly rather than privately owned. SWW stated their business plan incorporates both capital and operational spending to ensure services are provided to the required standard.

For drinking water, SWW aims to provide improved water quality by March 2030. They plan to undertake ground investigations in October/November 2026 to inform potential intake locations and have identified desalination as the only option to guarantee quality and reliability, citing climate change and prolonged dry periods as factors. Interim improvements for drinking water include a new UV disinfection system for St. Mary's by December 2026, a new pipeline for St. Martin's completed by March 2025, and new UV and enhanced treatment for Tresco by March 2027.

Questions were raised about meeting UWWTD requirements, the use of computer models for tidal data, and the success of development given past votes of no confidence in SWW from other councils. SWW confirmed their plan to deliver secondary treatment for St. Mary's and explained their modelling approach. They also addressed concerns about sewage discharge at Morning Point, stating that improvements are underway and monitored by the Environment Agency under a Local Enforcement Position.

SWW also clarified that their Old Town new site is not within a Site of Special Scientific Interest (SSSI) and only approximately 20% is within a flood zone. They are undertaking trial holes to establish the feasibility of a new rising main from Telegraph to Old Town Sewage Treatment Works, due to commence in September 2026, to address connection requirements for the Duchy of Cornwall's housing development. SWW confirmed they are operating under a Local Enforcement Position from the Environment Agency, allowing treated sewage effluent at Telegraph to continue discharge until September 2027.