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Located beside the service track to Llyn Llygad Rheidol, this installation appears to be an air vent valve in the raw water pipeline feeding water from Llyn Llygad Rheidol and the abstraction point on Maesnant towards Llyn Craig-y-Pistyll and on to the waterworks at Bont-goch. At this point, the gradient of the pipeline becomes much steeper, and trapped air rising up against the flow could cause an air lock in the bend, hence the air vent at this point.

Nant-y-moch reservoir lies below. It is a hydro-electric reservoir and has nothing to do with the drinking water supply - the water pipeline goes around its base.

North Ceredigion is supplied with drinking water through the water works at Bont-goch SN6886, which are fed with raw water from the Llyn Llygad Rheidol Link and Llyn Craig-y-pistyll Link reservoirs. The supply is supplemented by stream abstractions from the Maesnant Link and the Nant y Moch Link as well as two groundwater bore holes near Lovesgrove SN6281.

Llyn Llygad Rheidol is a natural lake in a glacial hollow on the north face of Pumlumon Fawr which has been dammed to increase capacity. Aberystwyth Town Council, then responsible for water supply to the town, built a 16-mile pipeline to transport untreated water from Llyn Llygad Rheidol directly to Aberystwyth in the 1870s.

The separate Aberystwyth Rural District Council, serving the villages of north Ceredigion, took a water supply from the Afon Leri, regulated by their dam at Llyn Craig-y-pistyll. They also built the treatment works at Bont-goch in 1939.

The two local authorities had begun to make joint plans when the responsibility for the water supply was transferred to the new Cardiganshire Water Board in 1962. In 1967, the water board built a new pipeline connecting the two reservoirs, siphoned around the base of the Nant-y-moch hydroelectric reservoir, which had been built between 1957 and 1961. This new pipeline replaced the old direct one to Aberystwyth to allow processing of all drinking water at Bont-goch. Link (Archive Link )

The water from the two small reservoirs is generally sufficient to supply the needs of north Ceredigion. Dŵr Cymru, the publicly owned water company now in charge of water supply to the area, has a drought plan which establishes that the supply would be topped up by transfers from Nant-y-moch directly into the pipeline linking the two drinking water reservoirs using a temporary installation during a severe drought Link . This has not been necessary to date.

Built from 1957 to 1961, Nant-y-moch reservoir regulates the flow of water in the Rheidol hydro-electric scheme. The dam is 52m tall and the storage capacity is 26 million cubic metres Link . The reservoir isn't usually used to supply drinking water, although temporary use for this purpose during severe droughts has been envisaged Link .