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Siltbuster has collaborated with the Environment Agency Wales (EAW) to treat and remove silt-contaminated water from a constructed fish pass at Osbaston Weir on the River Monnow in Monmouth.

Historically, the migration of salmon up the river Monnow had been prevented by a man-made weir at Osbaston, Monmouth.

During an independent construction project to build a small hydroelectric plant, the Environment Agency Wales took the opportunity to build a new fish pass re-opening the Monnow to migratory salmon.

The new fish pass at Osbaston was needed as it opens up some prime spawning grounds and ensures full access to the upper reaches of the Monnow, which in turn helps safeguard the salmon populations of the river Wye.

However, during construction works, heavy rainfall led to flooding in the Osbaston area and deposited tonnes of sediment in the fish pass.

The site contractors tried to remove the silt prior to commissioning the fish pass, but in doing so some of the silt was released to the river causing some discolouration downstream.

‘We were under strict time restrains to get the project completed before the fish began their migration and we needed a quick and effective solution to the siltation problem,’ said James Hepburn, environment officer for the EAW.

‘We contacted Siltbuster because they could provide a temporary treatment plant to counteract the environmental damage caused by flooding and reduce the harmful effects on the aquatic life before the fish migration began.’ Siltbuster provided a temporary treatment plant including a hopper bottom HB50 lamella clarifier and associated chemical dosing equipment with polymer dosing kit and pipe flocculator.

Dr Richard Coulton, Siltbuster’s managing director, said: ‘There was a significant volume of sediment in the pools that form the fish pass, which would be remobilised to the river when the full 1m3/s of flow was released through the fish pass.

‘This could have had a major effect on the local river life so we provided a temporary treatment plant to remove the solids laden water.’ To achieve effective solids/liquids separation Siltbuster treated the water with a coagulant followed by an ionic flocculant, dosed on a flow proportional basis.

The resultant solids were then separated from the water using a Siltbuster HB50 lamella clarifier and the treated water returned to the head race above the fish pass.

The separated solids were then recovered from the hopper bottoms fitted to the HB50 unit and removed offsite for disposal.

‘We had initial concerns about any potential discharge into the Monnow and needed an indication of the loads of silt and the chemicals that would be used to ensure the treatment process didn’t have any adverse effects on the river,’ said Andrew Osbaldiston, environment management team leader (Lower Wye) for the EAW.

‘After discussions with Siltbuster, we were confident the water could be treated in an effective and non-harmful manner.’ The fish pass is now open allowing the migration process to continue and the Salmon to reach their important spawning grounds further up the Monnow.

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