Utilization of sludge from recirculation aquaculture systems

Sammendrag

Norway is an ideal location for salmon farming due to its protected coastline and favorable water temperatures. Historically, salmon smolt production has been under flow-through (FT) systems utilizing the adequate freshwater bodies. Because of this, very few recirculation aquaculture systems (RAS) for Atlantic salmon culture have been implemented in Norway. However, increasing interest in RAS-based production has been shown recently due to increased smolt demand and more limited water resources. We have projected that the installed capacity of RAS in Norway will reach 40 million smolts by the end of 2011. A challenge with RAS production systems is that it will produce rather high levels of sludge. Moreover, if the trends continue with the increased growth rate showed during the last 7 years, around 85 million smolts could be produced in RAS in 2015, with an estimated production of sludge of 1.600 ton/year. Importantly, the composition of the sludge makes it suitable for treatment through anaerobic digestion and composting. Considering this, we have evaluated the potential of using fish farming sludge produced at recirculation aquaculture systems facilities as an input factor in a commercial context. It was concluded that exploitation of the energy in the sludge by means of anaerobic treatment (Biogas) may be profitable when assuming it can be operated continuously without process failure. It was also concluded that a large number of fish hatcheries would be required to ensure the minimum supply of sludge needed to make an industrial biogas plant economically viable. Still, sludge production from RAS can contribute to supply rough material to any biogas plant and thus generate a value added to the solid waste generated by fish farming. Further, treated sludge may be disposed by land application on agricultural fields. In conclusion, there are two major options for the use of sludge generated at RAS facilities that are technically viable and must be explored further in order to achieve economic efficiencies: sludge as a source for biogas and sludge as a source of fertilizer. To follow-up this study, the major recommendations are to conduct R+D to analyse in detail the economic potential of a plant for anaerobic digestion at an industrial level, considering the concentration of total sludge production in Norway or in Chile to be processed in one single biogas plant. To involve a significant number of alevin/smolt producers in order to accumulate and then process a bulk of rough material large enough to generate sufficient value-added product (e.g.: fertilizer, biogas or energy). Finally to conduct R+D focused on the design of smaller biogas reactor systems that can be operated on a viable commercial basis and that can be operated in-situ by small amounts of sludge generated at RAS hatcheries.