Andy Dinsdale began walking the southern English coast in search of a “sea heart,” a tough Mahogany seed carried by ocean currents from Central and South American rainforests.

In his quest to find one, he inadvertently became a plastic pollution expert.

For the last 20 years, Dinsdale has witnessed the transformation of Camber Sands, a 2-mile stretch of golden sand and dunes, from one of Britain’s most cherished stretches of coast into a front line of an escalating environmental crisis.

But he wasn’t prepared for what he saw in early November. As Dinsdale and citizen scientist group Strandliners scoured the beach for a pollution survey, they discovered something peculiar: An astonishing number of black plastic pellets were littering the sand.

Millions of bio-beads — peppercorn-sized plastic pellets used in some wastewater plants to grow bacteria that help break down pollutants during the final step of the cleaning process — had washed into the English Channel after a mechanical failure days earlier at a water treatment plant more than 35 miles up the coast.

An estimated 10 tons — or up to 650 million beads — had escaped into the sea, mingling into sand, slipping into creeks, and infiltrating the salt marshes of the adjacent Rye Harbour Nature Reserve, one of Britain’s most ecologically significant coastal wetlands. The spill represents one of the UK’s worst environmental disasters in years.

After Strandliners and local lawmakers sounded the alarm, Southern Water, the private utility company that owns the plant, accepted responsibility for the October 29 incident, saying on November 10 it was “very sorry.”

News of the spill sparked a mass volunteer effort, with up to a hundred people a day out on the beach, working with kitchen sieves, colanders and buckets in the initial days of the cleanup.

Southern Water has since deployed clean-up crews to the site, working with local authorities and independent contractors. The company has promised to cover all costs associated with the clean-up, although some consumers are concerned the cost will ultimately be passed on to them through their utility bills.

Southern Water said it believed 80% of the beads had been recovered from the beach as of November 11, but acknowledged that more beads would likely wash ashore with future high tides.

“These bio-beads will be here forever,” Dinsdale said, walking up the shoreline on a bitterly cold but sun-drenched day.

“The horrific thing for us is that they’re so small, so when people walk by them, they just think they’re bits of seaweed… or bits of stone or gravel.”

Barbara Plum, who lives a few miles away, was sifting through a windswept patch of sand on Tuesday morning.

“Our beaches are just used as dumping grounds,” Plum said, explaining that she felt compelled to volunteer. She told CNN she hopes increased scrutiny will put pressure on corporations.

“Companies probably need to be forced to make changes. You know that they probably choose the cheapest option, which may involve plastic use.”

Environmentalists have advocated for replacing plastic bio-beads in wastewater treatment plants with plastic-free, natural alternatives such as sand and pumice. These options are porous, effective, and pose fewer long-term environmental threats if lost, unlike their plastic counterparts.

Dinsdale said that an ideal world would be plastic-free but acknowledged that’s not realistic.

“We use (plastic) in so many lifesaving situations, but used in the wrong place, it can be catastrophic to the environment. And in this example, with the wastewater treatment works, the environment was not on the list of the possibilities of what could happen with them.”

Southern Water maintains that the beads are inert and non-toxic, citing their manufacturer. But experts and conservationists have raised doubts, pointing out the beads come from plants that were constructed in the 1990s, when post-consumer recycled plastic frequently contained heavy metals including lead, antimony and bromine, with persistent chemical residues. Southern Water told CNN it was looking into the age of the beads involved in the spill as part of its own independent investigation.

Researchers fear that bio-beads can leach toxins absorbed during wastewater processing, or pick up pollutants as they drift through the sea, such as PFAS — often called “forever chemicals” because they fail to break down fully in the environment. Once ingested by fish, birds, or seals, these chemicals can bio-accumulate and travel up the food chain, including to humans.

Amy Youngman, a legal and policy specialist at the Environmental Investigation Agency, a UK-based NGO, called the incident “essentially an oil spill in solid form, but with added chemical toxicity.”

“Wherever you end up with a pellet washed on your shore, you likely have some chemical contaminants along with them, or they enter the food web when animals eat them, and we eventually eat them as well,” she said.

Chris Saunders came to Camber Sands from a town 20 miles away to join the clean-up Tuesday. It was his only day off work this week, he said, but he was driven by the birth of his new grandchild.

“What’s my grandchild going to inherit? Nothing. Year upon year, it just gets worse. Environmental protection regulations are diminishing,” he said.

Southern Water, which provides water and wastewater services to millions of customers across a swath of southern England, uses bio-beads at five of its plants. The company told CNN that replacing bio-beads “is under consideration as part of the independent review we have commissioned into the incident,” and that upgrading aging systems requires investment and regulation. “These are complex decisions,” a spokesperson said.

Approached for comment, regulator OFWAT referred CNN to the Environment Agency (EA), a government body.

The EA told CNN that if bio-beads are lost and the environment is polluted, it will “investigate and take necessary enforcement action.” The EA added that it is working alongside the water sector on a project looking at “the impact of microplastics generated by wastewater treatment works.”

That impact appears to be spreading beyond the UK.

In late October, volunteers from the Surfrider Foundation Europe reported finding black bio-beads on the northern French coast, between Cap Gris- Nez and Wissant. Bio-beads have since begun appearing on shorelines in Belgium, too, according to the Belgian environment group Proper Strand Loper, affecting Oostende and De Haan.

In England, the bio-bead spill is not an isolated incident.

A steel mesh at a South West Water plant on the Truro River in Cornwall split in 2010, causing around 5.4 billion bio-beads to escape, leaving beaches and estuaries littered with pellets for years, according to a report from the environmental group Cornish Plastic Pollution Coalition. In 2017, deposits of bio-beads were discovered along Dorset and Devon beaches, with surveys showing millions of tiny plastic pellets embedded in the sand, creeks, and estuaries. Those bio-beads also landed in Camber Sands.

In Sussex, concern remains high over the threat the tiny beads pose to the Rye Harbour Nature Reserve, a vital saltmarsh and wildlife site, known for rare birds and home to more than 4,350 species of plants and animals, including 300 that are rare or endangered.

Henri Brocklebank, director of conservation at the Sussex Wildlife Trust, a conservation charity, told CNN that the wetlands have been carefully managed to act as a “dinner plate” for the birds.

“These (bio-beads) look just like little seeds. So it’s not really a huge leap of the imagination to think that these are going to be getting ingested by these rare birds that have migrated literally thousands and thousands of miles to be here,” she said, adding: “To have plastic here is completely contradictory to everything we’ve done to protect them.”

Plastic contributes to the deaths of up to 1 million seabirds each year, along with hundreds of thousands of marine mammals and turtles. A recent study by Ocean Conservancy, a US nonprofit, found that ingesting just three sugar-cube-sized pieces of plastic can be 90% fatal for seabirds like Atlantic puffins, with even tiny amounts threatening turtles and marine mammals.

The latest bio-bead spill underscores a wider plastic problem along Europe’s coast — and the global crisis of plastic pollution.

Overall, up to 23 million tons of plastic enters our aquatic ecosystems annually, according to the United Nations Environment Programme, which says it’s the equivalent of 2,000 garbage trucks full of plastic being dumped into the world’s oceans, rivers, and lakes every day.

At Camber Sands, the global crisis feels painfully local. Volunteers armed with sieves and combs continue their painstaking efforts to counter an industrial spill that could have far-reaching consequences.

“We don’t know how many are still out there. We don’t know what beaches they’re going to hit next,” Dinsdale said.