In a bid to extract the value from aquaculture waste products, scientists at Nofima have analysed sludge from three commercial hatcheries through an entire production year, and have recently presented their findings in two reports.
Sludge is a growing problem in aquaculture and the projects may help to turn it into an asset – or at least allow it to be handled better – by finding out what it’s made of. Authorities, suppliers and fish farmers can use this knowledge as a basis for better management and use.
The analyses show that sludge has a high content of energy, nitrogen and minerals, such as phosphorus. Knowledge about the composition of sludge is essential if it is to be used as fertiliser and to produce biogas.
The content of organic pollutants is low and does not constitute an environmental risk for use in fertiliser. By contrast, the zinc and cadmium levels in sludge are a challenge.
Sludge has a relatively high content of long-chain fatty acids, posing a challenge for biogas production, which can be resolved by co-processing with manure or other waste.
The researchers also created a model for calculating the amount of waste feed in sludge based on energy content. This model is particularly useful when trying to convert the sludge to biogas or fertiliser.
Minimising sludge production
As land-based fish farming increases, the amount of sludge needs to be kept to a minimum. Nofima’s research indicates two main areas where steps can be taken: the first is to prevent feed waste without compromising the growth and health of the fish; the second is to recover waste feed and fertiliser promptly in the pipe system without the particles disintegrating and before they absorb much water. This will make sludge recovery more efficient and reduce its water content. The best solution to the increase is sludge production is, according to Nofima, to get more of the nutrients from the feed into the fish.