The River Towy (Afon Tywi) is one of the most important salmon and sea trout rivers in Wales. In the 1980s it was found to be under threat from acid rain caused by air pollution. The local geology and conifer forests make this area especially vulnerable to the effects of acid rain. From 1984 to 1991, the Government funded research into the link between land use and the impact of acid rain in the upper Tywi catchment. This included liming some of the headwaters upstream of Llyn Brianne.

Surveys showed that 17km of the Tywi downstream of Llyn Brianne was also impacted. To deal with this problem, 500 tonnes of powdered limestone was applied by boat to the reservoir every six months from 1991 to 1996. This improved water quality and fish populations downstream of the reservoir.

In 1996 improvements were made to change the way the powdered limestone was applied. Two lime dosing silos were installed on the Tywi and the Camddwr above Llyn Brianne. This was a more efficient and safe method of liming, which proved effective in maintaining the water quality improvements and benefited the salmon, trout and overall ecology downstream of the reservoir. Dwr Cymru Welsh Water, who own and operate the reservoir, Carmarthenshire Fishermen's Federation and Carmarthenshire Rivers Trust supported this work over the years.

In 2008, EAW started work to upgrade the lime dosers and increase their efficiency. This was a challenge as the dosers are in a very remote area. Financial support was received from the Environment Agency's Carbon Reduction Fund and from the European Union and Welsh Government.

The dosers were initially powered by solar panels and backup petrol generators, later supplemented by wind turbines. A hydro-generator installed in 2009 provided enough energy for dosing when it was most needed, under high flows. Installing vibrator mechanisms reduced caking of limestone powder and improved reliability. Telemetry systems were also installed to reduce site visits. These developments ensured a steady improvement in water quality in the upper Tywi. Maintenance visits became less frequent and the backup petrol generators were replaced with more efficient methanol fuel cells to reduce the carbon footprint of the operation. This project was cited in the Sunday Times as one of the achievements that led to the Environment Agency winning an award in their 2009 Green List.

The hydro-generator was stolen in 2010, but the installation of more efficient solar panels made it possible to rely on solar power with only occasional use of the methanol fuel cells.

Lower industrial emissions of nitrogen and sulphur have led to a reduction in acid rain in recent years, but the recovery of upland streams in mid-Wales has been slow. More sustainable forestry management and further reductions in industrial and vehicle emissions will help the process. Liming was therefore seen as a crucial medium-term solution to prevent the deterioration of ecological potential in the Tywi, as required by the Water Framework Directive, before the expected natural recovery in the longer term.

Liming ceased in April 2019, due to mechanical failure. The need to carry out further liming is currently under review.