The clean-up of the River Nidd looks set to take another step forward, if an application to build three kiosks at Knaresborough sewage treatment works is approved.

The ‘kiosks’ – in reality, green-coloured rectangular huts between 9 and 14 metres long – would house equipment to remove harmful phosphorous from the river water, just downstream from Knaresborough Lido.

The kiosks would fulfil different functions. The smallest of them would house a motor control centre, and the other two would house caustic dosing and ferric dosing stations.

These are two processes that combine to remove phosphorous from the water. Phosphorus is a nutrient that can cause excessive algal growth, blocking sunlight and reducing the levels of oxygen that aquatic life depends on.

Ferric dosing involves adding ferric salts such as ferric chloride, which binds with the phosphorus, removing it from the wastewater.

Caustic dosing involves adding sodium hydroxide – also known as caustic soda – to the water, which optimises the conditions for the chemical reaction between the ferric salt and phosphorus.

The lido, just upstream of the sewage treatment works, has been the epicentre of efforts to clean up the Nidd, and Environment Agency scientists take water samples there throughout the summer bathing season to keep track of the river’s health.

So far this year, levels of both intestinal enterococci and E. coli – which are pathogens that can cause severe stomach upsets among swimmers – have been far lower than they were last year.

This is thought to be largely due the the drier weather this summer, which has seen less field run-off enter the river.