Thames Water and Anglian Water are running a six-month trial of fat-coated “BioBullet” pellets in their ongoing fight against zebra mussels.

Huge infestations of the mussels from the Black and Caspian Seas are costing UK water companies millions of pounds a year by clogging up their pipes and treatment works. Thames Water spends £1m a year on clearing zebra mussels from its raw water pipes and water treatment facilities, and on applying heavy doses of chlorine to keep the molluscs at bay, while Anglian Water spends £500,000 a year tackling the problem.

Zebra mussels have plagued America ever since decamping to the Great Lakes in 1985, probably from ships' ballast water. Estimates vary but it is thought that the zebra mussel nuisance costs US water and power industries up to $500m a year in clearing tunnels and pipelines.

The microscopic mussel-killing pellets are now being used at Thames Water's Kempton Park water works, home to the company's Innovation team, and at its Walthamstow water works in northeast London.

It is believed zebra mussels hitch rides in the ballast water of ships heading to Britain from Eastern Europe before decamping from the docked vessels and settling in raw water pipes at water treatment plants. 

Mussel numbers "ballooning out of control"

With no known predators in Britain to control them, their numbers are ballooning out of control. Adult zebra mussels, which are no good to eat, can grow up to 4 cm long, and females can release more than 30,000 eggs per year. The free-swimming larvae quickly attach to surfaces where they live for up to five years.

Build-ups of the mussels and the shells of dead ones narrow, and can eventually block, water companies' pipes. Nearly 800 tonnes of zebra mussels were removed recently from Thames Water's main raw water tunnel into Walthamstow water works in northeast London – a £250,000 job. A further 635 tonnes were removed from the company's Ashford Common water works in southwest London.

Picture: Engineers clear thousands of tonnes of zebra mussels from a tunnel at Walthamstow water works in London

The BioBullet, which is about 50 micrometres in diameter, has a fatty outer coating irresistible to zebra mussels, covering an inner centre that is toxic to mussels but harmless to humans and other creatures living in the water. The mussel-killing chemical is fully approved for use in drinking water treatment.

Dr Piers Clark, commercial director for Thames Water, said:

"Zebra mussels were prevalent in the middle of the 20th century but have recently become a problem again with an influx in trade from Eastern Europe since the 1990s. They eat by filtering small particles out of the water, so in the right place they are potentially beneficial to us as they feed on algae in the reservoirs, but in the wrong place, they block up parts of the water treatment process like fat furring arteries. We believe BioBullets will save hundreds of thousands of pounds in operational costs in a way that has no adverse impact on the environment."

Barrie Holden, water innovation manager at Anglian Water, added:

"Zebra mussels are a growing problem for us. They narrow the pipes, forcing us to pump water through at higher pressure and at much higher cost. We estimate that costs us £350,000 in increased electricity bills alone. We are very hopeful BioBullets are the solution we've been looking for having trialed them at our treatment works at Alton in Suffolk and Pitsford in Northamptonshire. In both cases we saw a significant reduction in the numbers of mussels without any impact on the treatment process or the environment."

The trial at Kempton Park water treatment works in southwest London has £550,000 of backing from the Government's Technology Strategy Board.