CRISPY SEAFOOD Credit: Energy Fuels

Before that beautiful salmon filet lands on your plate, a lot of less appetizing stuff gets stripped away: By one estimate, the global seafood industry produces 64 million metric tons of waste each year. A new study suggests a potentially sweeter fate for all those heads and guts: They can be turned into a coal-like substance called hydrochar, which could be used as fuel or added to soil to improve fertility and sequester carbon (Energy Fuels 2015, DOI: 10.1021/acs.energyfuels.5b01671).

A few years ago, McGill University graduate student Shrikalaa Kannan learned that the city of Gaspé, Quebec, which has a large fishing industry, prohibited local shrimp processing plants from disposing of seafood waste in municipal landfills. Although solid seafood waste can also be processed into fish meal for fertilizer and feeding livestock, such factories often generate complaints because of their fishy odor. Meanwhile, liquid seafood waste often ends up in the sewage system or in water bodies, where its high nutrient content can stimulate harmful algal blooms. The industry in Gaspé was looking for environmentally friendlier alternatives. Kannan, who was researching technologies to turn waste into biofuel, wondered if she could find a solution that could be used not only in Gaspé, but all over the world.

FISHING FOR FUEL [+]Enlarge Credit: Shutterstock

Previous studies had applied a process called hydrothermal carbonization to transform products including wood and plant residue, sewage, and food waste into hydrochar. During this process, waste is heated with water under pressure and at temperatures of 150–300 °C, generating a series of reactions that produce a carbon-rich solid. The solid can be burned as fuel or added to soil to improve water and nutrient retention while sequestering carbon. But all these previously studied waste streams contain cellulose, which breaks down easily with acid or base treatment, easing carbonization. Kannan wanted to adapt the method for seafood waste, which contains more complex carbohydrates, proteins, and fats.

So she and her colleagues ground up samples of fish and shrimp waste from a local market, and heated them by microwaving the waste in quartz vessels at high pressure and 150 °C for an hour. But they obtained no hydrochar, not even when they treated the waste with acid first. So Kannan tried again, first treating the waste for 16 hours with three commercially available enzyme products used in a previous study that made hydrochar from food waste (Bioresour. Technol. 2014, DOI: 10.1016/j.biortech.2014.03.022). The enzymes, including lipase and protease, hydrolyze complex macromolecules in the food waste into simpler compounds such as glucose. This time, it worked: The team recovered 29% of the dry weight of the fish waste as char, and 36% of the shrimp waste. The char smelled like coffee, Kannan says; she suspects this may be a sign of the Maillard reaction, the reaction between amino acids and carbohydrates that takes place when food is browned.

Next, Kannan plans to determine the carbon content and calorific value of the char to evaluate its potential as fuel. She also wants to test whether treating the waste at higher temperatures could increase the yield.