The thousand and one uses of oyster shells, a rediscovered bioresource – Mer

From feeding chickens to paving, from improving agricultural soils to hunting down limestone in flushing toilets: the quest for eco-designed products leads to discovering, or rather rediscovering, a thousand and one uses for oyster shells, a resource in abundance.

In Périgny, near La Rochelle, real oyster shell “slag heaps”, about ten meters high, supply an imposing machine for sorting, drying, crushing the empty shells, up to the desired size: flakes or breaks, micro-breaks, powder. A machine itself covered with a thick layer of shell dust.

The Ovive company (turnover of around 1.5 M €) has been crushing shellfish – mainly oysters – for more than 30 years: 1,000 tonnes of finished product crushed per year in the 2000s, 2,000 tonnes ten years later, and 4,000 expected this year.

Because the gaze changes on this vestige of table available in profusion, thanks to the 130,000 tonnes of oysters produced per year in France, the largest European market. A renewed interest which amuses Jean-Luc Saunier, founder of Ovive: “the Romans already made lime by burning them at 900 degrees”, he reminds AFP.

“For decades, he continues, the agricultural world has reduced them to powder, to enrich the soil in the fields, and to feed the chickens”, because calcium facilitates the digestion of the seeds and improves the quality of the eggshell.

How about toothpaste?

“Oyster farmers also use it to reinforce the paths that lead them to the foreshore”, part of the coastline that emerges at low tide where the oyster beds are located. “And our grandmothers used them to fight the lime scale in the house, the shell attracting and fixing the lime deposit. It’s still happening! “, Assures Jean-Luc Saunier, nevertheless sometimes surprised by the craze:” This morning I was asked to make toothpaste! “.

Recent uses: the shells filter the air in sewage treatment plants, instead of peat beds, to trap bad odors. In Charente-Maritime and Vendée, they are used as a substrate for green roofs. At the end of 2020, the Gironde department was experimenting with incorporating 30% crushed shells, instead of sand, into the mortar to fill underground quarries.

In Brittany, the achievements are more elaborate. The shell powder incorporates cosmetic exfoliating creams, spectacle frames or pavement paint. And regional shellfish farming committees (CRC) are experimenting with artificial reefs mixing concrete and shells, to restore natural beds of flat oysters, decimated by parasites in the 1970s.

In the bay of Quiberon and the bay of Brest, the “Forever” project (for “Flat Oyster REcoVERy”), launched in 2018, uses the tendency of the flat oyster to seek to attach itself to a support, “or the shell of its congeners suits him perfectly, ”explains Philippe Le Gall, president of CRC Bretagne Sud.

The oyster softens the seas

Researchers from the School of Construction Engineering (ESITC) of Caen are participating in this bio-colonization of the marine environment, but also in the development of an “eco-paving” incorporating 20 to 40% of recycled shells – especially Saint-Jacques, more porous.

These “draining and breathable” paving stones allow more rainwater to flow, and in hot weather, the soil perspires, “creating islands of freshness” in the city, explains Mohammed Boutouil, research director at ESITC. Even if due to less resistance to the load, the use must be adapted: ground, sidewalk, parking …

If oyster shells can allow water to pass through, they can also rebalance the pH, the limestone in the shell “fixing” the carbon. A not insignificant track, against a backdrop of ocean acidification, proven by scientists since the beginning of the industrial era in the 18th century.

“Shell powder alkalizes sea water”, summarizes Fabrice Pernet, researcher at Ifremer. An experiment will soon begin over three years, to see if it is powdered, or crushed, that the shells would be more effective. “It will not solve the problem of ocean acidification, recognizes the researcher, but could restore coastal areas where oysters live. We need more and more ”.

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