Ole-Kristian Hess-Erga from the Norwegian Institute for Water Research (NIVA)
In some parts of the world large quantities of waste matter are discharged into the sea from hatcheries located along the coast. However, as some operators have discovered the waste can also be of great benefit in the agricultural sector.
“If the waste is collected and used as fertilizer in greenhouses, in agriculture or is processed into biogas, it constitutes a resource and not a problem,” says Ole-Kristian Hess-Erga from the Norwegian Institute for Water Research (NIVA). “This use of waste matter will improve the resource utilization, reduce the environmental impact and increase profitability and sustainability for both the fish farmers and the collaborating industries,” he says.
Norwegian juvenile production is moving to larger and fewer units where the water is recycled and cleansed of toxins. Both wastewater and waste matter that are removed from the water contains a variety of nutrients and organic matter. These are more concentrated than before, since a larger proportion of fry and fingerlings are produced in recirculation systems.
In a collaborative project with a hatchery operator and a garden nursery, Norwegian researchers are using wastewater and sludge from a hatchery, both as a fertilizer for plants and as an energy carrier.
Wastewater and sludge
In order to assess whether it would be appropriate to use wastewater and sludge as plant fertilizers, the researchers had to identify the quality of the waste. Through monitoring the water they found marked variations both in the water quality and the amount of waste matter. The nutrient content varied in relation to the intensity of the operation, the fish size and the cleansing measures. The waste water content of plant nutrients such as nitrate, phosphate, boron and calcium showed the greatest variation, because they are directly related to the amount of feed, additives and the water quality.
Thriving in wastewater
The researchers tested whether the wastewater could replace regular fertilizer for production of lilies and salad greens. A reservoir with wastewater from the smolt production unit was connected to the irrigation table with lilies. The trials with lilies showed that the quality of lilies grown with wastewater and lilies cultivated with commercial nutrient solutions could not be distinguished.
“Co-composting hatchery sludge and remains of lilies is possible, and can provide a good growth substrate for greenhouse production of lilies,” says Hess-Erga.
However, the lettuce trial revealed a greatly reduced growth of lettuce plants grown in wastewater, compared to lettuce grown with normal plant nutrition. The reason was that the water had a somewhat high salt level and pH value, which impaired the growth. “Other plant species with higher salt tolerance are likely to thrive better with such wastewater,” says Hess-Erga.
The researchers also evaluated whether plants can be used as a purification method to reduce the level of metal and organic material in the wastewater. This is beneficial if the water is to be recycled back into the fish tanks or discharged into the environment.
“The results of the lettuce trial showed that plants absorb metals such as copper and aluminum, which is toxic to fish. The contents of these metals were measured in the salad greens, and did not exceed the limits for human food,” says Hess-Erga.
The application of hatchery sludge
Sludge from hatchery production certainly has a commercial potential, but needs to be adjusted according to the application, the researchers concluded. They believe that the chosen sludge treatment method should be based on emission allowances, costs and opportunities for local collaborations and application.
Co-composting hatchery sludge and other organic materials is possible, but requires further optimization to form a good growth substrate or as biomass in energy production.
“For larger hatcheries, a collaboration on energy production with other biomass suppliers such as agriculture, slaughterhouses and municipal disposal service may be an option,” says Hess-Erga.
The participants in the project were Harding Smolt, Berge Gard og Gartneri, the Norwegian Institute for Water Research, Akvaplan-niva, NTNU Social Research and the Norwegian agricultural counseling greenhouses. The project was funded by a marine grant scheme from Hordaland county.