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Large grey plastic drums bob up and down in rows in the waters of Hake Fjord, a bay in Sweden. They are the only visible sign that an offshore area east of Tjörn is a biogas cultivation site, and that it is harvest time. The water is mirror-calm. As the boat approaches shore, sea squirts are visible growing on ribbons below the surface.
“They are basically mussels without shells – sessile filter feeders that eat phytoplankton. After about a year, they die and sink to the bottom, where they become food for other animals,” says Fredrik Norén, head of research and development at Marin Biogas.
The company aims to extract biogas and eco-fertilizer from sea squirts, which are about to be harvested on a large scale for the first time. “It’s actually the world’s first sea squirt harvest,” says Norén, a marine biologist who also works on projects for the IVL Swedish Environmental Research Institute.
Biogas consists mainly of methane, and is a product of anaerobic digestion – bacteria that don't need oxygen. Yes, it's sea-squirt farts.
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Weeds of the sea
The idea of growing sea squirts has been around for years. In 2009, Norén applied to patent a method for extracting biogas from them. The sea squirt is an invertebrate that grows rapidly, up to two centimeters a month, making it a good potential source of biogas. “You can produce first-rate biomass in a short space of time,” says Norén.
Until now, their rapid growth rate made sea squirts a nuisance for mussel farmers. The invertebrates are like weeds of the sea: they like the same habitats as mussels and can invade mussel farms.
“The cultivation we have here was originally an unsuccessful mussel farm that sea squirts took over. If our project is successful, it could enable mussel farmers to get something out of even unsuccessful cultivations,” Norén suggests.
The sea squirts are growing on ribbons attached to wires between the grey drums. When the boat’s crane lifts one of the wires, the ribbons form a curtain. The pale brown bunches of 10-cm-long sea squirts are ripe for harvesting.
Deckhand Alexander Olofsson raises the wire, undoes a clip securing the ribbon and runs the ribbon between two rollers, brushing off the sea squirts. This method is usually used for harvesting mussels, as is the boat.
“Sea squirts are easier to harvest because they are not attached as firmly as mussels,” says Alexander Arvidsson, the boat’s skipper.
The harvest is transported on a conveyor belt and tipped into a combined pump and grinder, killing the sea squirts and removing some of the water. “We take great care to kill them immediately after harvesting; it is the most humane way,” says Norén.
From the pump grinder, a pipe runs down to large plastic crates . The harvest is a success, and a brown-beige slurry runs down at a good rate into the crates, which fill up gradually. The smell of the sea is heavy in the air.
There are eight crates on the boat. Each one holds one cubic meter (around a tonne) of sea squirts, and has small holes allowing water to drain out. “About 95 percent of what is in the crates at the moment is water. This means that each crate only contains around 50 kilograms of material,” says Norén.
Looks like a limp cigar
As the large-scale cultivation of sea squirts is a new venture, one of the challenges the project faces is finding harvesting methods that are both efficient and profitable.
“We need to find methods for getting rid of as much water as possible at an early stage. Transporting water is not particularly efficient,” says Rebecca Gmoser, the company’s development engineer.
She monitors the hatch where the sea squirts are being tipped down into the pump grinder, removing mussels and other hard substances to prevent them from ending up in the grinder. She will take the mussels home for dinner.
Each tonne of harvested sea squirts produces biogas equivalent to 20 liters of petrol, along with fertilizer. Today’s harvest should produce the equivalent of around 160 liters of petrol, which doesn’t sound like much.
Norén points out that they are still at a trial stage. In order to make the process profitable, they would have to be paid for the environmental benefits that the farming and harvesting process offer.
When sea squirts eat phytoplankton, they also absorb substances that cause eutrophication, or excessive enrichment of the water from chemical nutrients. “When we harvest the sea squirts, we also remove nitrogen and phosphorous from the sea,” says Norén. “It’s the only way of removing pollutants that are known as ‘diffuse discharges,’ which come from many sources such as agriculture. Water treatment takes care of other discharges.” He would like to see some form of compensation for initiatives that clean the seas, similar to those made for catch crops that reduce nutrient leaching on land.
Norén holds up a sea squirt in his hand; it looks like a limp cigar. He points to two small tubes underneath the body –water enters one and is expelled through the other. A soft sheath surrounds the inner body, a bit like a sleeping bag.
“It’s called a tunic. It is rich in cellulose, while the inner body is rich in proteins and fats, which have many different potential uses, not just making biogas and fertilizer,” he says.
The unassuming sea squirt contains many attractive substances. “Proteins can be used to make food products, such as animal feed, and the cellulose is also appealing,” says Norén. “We have many ideas in the pipeline.”