Trojan Horses for water courses 

Zebra mussels inhale their food. They taste the tiny morsels and, if they like what they find, down it goes. If they don’t (and they’re fussy eaters), they bind it in mucus and vomit it as pseudo faeces.

It might sound an unpleasant response but it’s at the heart of protecting our water supplies from non-native species of mussels. 

Invasive species are in the top five threats to the natural environment, costing the UK economy £1.8 billion per year. Among the worst offenders is the zebra mussel, a species native to Eastern Europe but now spread around the world. 

As well as driving native species to extinction, zebra mussels are a major headache to industries that depend on the movement of water. Unlike native freshwater mussels, zebra mussels can attach to hard surfaces, leading to build-up on the inside of the pipes. They also breed at alarming rates, with females producing as many as one million eggs in a single season.  

Every year, the UK water industry spends £8 million on controlling invasive species such as zebra mussels by flushing clogged raw water and water treatment pipes with chlorine. But this causes the mussels to clam shut – and remain shut for up to three weeks – meaning companies have to administer a continual dosing of this dangerous chemical. An alternative solution was urgently required. 

Zoologist Professor David Aldridge and chemical engineer Professor Geoff Moggridge came up with the idea of sending them a Trojan Horse: tiny fat-coated particles called BioBullets that might look like food to a zebra mussel but which carry a combination of salts that are lethal to the invaders but not native species. The otherwise harmless formulation then dissolves away, without affecting water quality. 

“Zebra mussels have only recently become a problem again with an influx in trade from Eastern Europe since the 1990s. We believe BioBullets will tackle this problem, saving hundreds of thousands of pounds in operational costs, and all in a way that has no adverse impact on the environment.”

Dr Piers Clark, former Commercial Director for Thames Water

First, though, the researchers had to understand more about how zebra mussels feed and to do this they used a mussel-sized endoscope.

Aldridge fed the endoscope through the inhalant siphon of a live feeding mussel and watched as particles were transported and sorted, selected or rejected.

More recently, he’s taken this a step further and used a micro CT scanner to observe the particles’ journey through the mollusc’s gut.  

He found that when mussels filter feed, they end up concentrating the particles without detecting that they have consumed a toxin, and that any uneaten product dissolves to harmless concentrations within hours. 

Then came the clever chemistry needed to disguise the biocide. “Zebra mussels are fairly fussy eaters,” he says. “They will decide what to eat based on the size, shape and surface texture of the particles. We also had to get the buoyancy right to make sure that, once the capsules were released into water, they would be carried to the invaders.” 

In 2000, Aldridge and Moggridge founded the company BioBullets Ltd. Within three years they had a European umbrella patent for the encapsulation of any material for delivery to filter feeders like mussels, and further patents followed. 

There was no point trying to develop the technology further in the lab because to be commercial it needed to be scalable, safe to handle and have a sufficiently long shelf life.

“And so we collaborated with established manufacturer TasteTech, which produces encapsulated flavourings for the food industry,” explains Aldridge.  

Collaboration on the BioBullets project has helped the Bristol-based company develop and grow, including the creation of a new Project Manager job within the company.

Meanwhile, the researchers secured funding from Innovate UK to make sure their products were fit for purpose by working directly with Thames Water, the UK’s largest water and wastewater services provider, and Anglian Water, which supplies water and water recycling services to almost seven million people in the East of England and Hartlepool. 

“We knew we needed to engage with water companies from the outset to make sure we understood the sort of situations the product would be used in,” says Aldridge. “You don't want to be seen as a Cambridge boffin trying to tell the end users what they want.”  

To date, BioBullets have been trialled by seven UK water companies in England, Northern Ireland and Wales that collectively supply 34.7 million people with drinking water, changing how the industry approaches the control of mussel infestations. One company has estimated that the technology is 69% cheaper to use than the alternative chlorine-based control methods that are more hazardous.  

Aldridge and his team have continued to investigate the biology, spread and management of invasive mussels – and they’ve discovered that the zebra mussel is not alone in its biofouling activities. 

Sea squirts, golden mussels, freshwater sponges... as the list of invasive species grows, so do the research and development capabilities of Aldridge’s team. They can now tailor products to offer specialist control solutions for a range of freshwater and marine pests. 

In 2014, the UK Government Department for Environment, Food and Rural Affairs asked Aldridge to carry out a risk assessment to identify the highest-risk future invaders, leading to the first discovery of quagga mussels in the UK. In 2018, the team identified the gulf wedge clam as posing a significant threat to biodiversity and water provision in Lincolnshire.  

New formulations of BioBullets have been created with higher toxicity, smaller particle size and controllable breakdown rates. These innovations reduce the amount of product needed, enable a much more targeted and efficient delivery to mussels and are better for use in longer pipelines.

The new formulations (Silver Bullets 1000 and Silver Bullets 2000) have been approved by the UK Drinking Water Inspectorate – the only mussel control solution that has been approved for use under Regulation 31 of the Water Supply (Water Quality) Regulations. 

“With climate change facilitating the continued spread of invasive species, mussel biofouling is probably going to get worse where we already have it, and start to appear in new locations too,” says Aldridge. “So it’s even more crucial to have these different tools in the chests to deal with the problem.” 

Looking back, he remembers a major milestone when one water company set them a target to reduce energy needed to pump water through pipes narrowed by mussels. 

“We didn’t just achieve the target, we went a step above. That’s when we knew we’d gone from quirky academic blue skies research to something where we actually could say ‘wow this has achieved something’.”

Professor David Aldridge, Department of Zoology and co-founder of BioBullets Ltd

Published 5 May 2022

With thanks to
_{David Aldridge
Anna Davies}

Infographics
_{Alison Fair}

Images
<sub>Banner and section header images of zebra mussels (credit: sdphotography iStock/Getty Images Plus)

The text in this work is licensed under a Creative Commons Attribution 4.0 International License</sub>