What is sludge made of?

Sludge from closed fish farming systems originates from feed. Ideally most of the feed should be eaten.

Scientists at Nofima have analysed sludge from three commercial hatcheries through an entire production year, resulting in precise knowledge about the content of the sludge, presented recently in two reports.

Authorities, suppliers and fish farmers can use this knowledge as a basis for better management and use.

Energy, nitrogen and minerals

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 fish farming on land increases, the amount of sludge needs to be kept to a minimum.

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 the water content in sludge.

With more sludge being produced, a solid effort must be made to find uses for it, in addition to minimising production. The best solution is to get more of the nutrients from the feed into the fish.

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