Science - January 11, 2018

Fish feed from waste streams in new reactor   

Albert Sikkema

At Wetsus, Bob Laarhoven has designed a reactor for breeding freshwater worms using waste streams from the food industry. The worms can be used as fish feed. More time and research is needed to scale up the process and apply for authorization.

Blackworms like to settle in this reactor. ©Bob Laarhoven

Laarhoven investigated the blackworm (Lumbriculus variegatus). This freshwater worm is found in streams and wastewater treatment plants. The worm eats and digests silt, which significantly reduces the amount of waste so that the water treatment companies have less waste material to dispose of. The disadvantage is that while the worms reproduce fast in the summer, the population collapses in the autumn.

That was why Tim Hendrickx, Laarhoven’s predecessor at water technology institute Wetsus, was looking for a reactor in which the worms could function well all year round. He developed a reactor with a kind of tea strainer in which the worms could settle. That turned out to be a stable system: the worms digested the silt all year round, substantially reducing waste streams. But the focus in that project was on waste water treatment rather than the value of the worms.

In his PhD research, Laarhoven studied the worms’ growth and reproduction in waste streams with the aim of cultivating as many worms as possible. Blackworms are very suitable as fish feed. He modified the reactor’s design by replacing the tea strainer with a vertical gravel column. The worms felt more at home there and the growing conditions were better. The reactor consisted of a 30-centimetre tube with a diameter of seven centimetres. He filled it with 100 grams of worms, which ate and digested the silt in the tube within one or two days. This reactor design was patented several years ago by Wetsus.

The plan was to scale up this prototype and use it for the mass production of fish feed. However companies are hesitant, in part because of the tricky legislative situation. The worms have not yet been officially recognized as production animals or as fish feed for fish farming.

Waste streams
Laarhoven cultivated the worms using a clean waste stream from the food industry as a way of adding value to the food supply chain. He used waste streams from a potato processing factory. ‘The aquaculture sector wants alternatives for the use of fishmeal but they are mainly betting on vegetable protein.’ As in the cultivation of insects for animal feed, companies have cold feet when it comes to using worms for feed in fish farming, notes the PhD candidate.

He sees more potential in worms as feed for ornamental fish in aquariums. Laarhoven has started up a company, Dutch Blackworms, to serve this market. AS

Bob Laarhoven promoted on december 15th by Cees Buisman, professor Biological Recovery and Reuse Technology.