Introduction
With the rapid growth of the Norwegian aquaculture industry, also the demand for suitable aquaculture waste management is rising. Current waste treatment practices include both partial release into the surrounding water as well as capture, dewatering and shipment to other countries such as Denmark or Vietnam where it is used in biogas production or as fertilizer . Here we evaluated a waste-treatment process in Norway, where anaerobic digestion of the fish sludge was used for biogas production in Norway and the leftover nutrients, such as nitrogen (N) and phosphorous (P) from the aquaculture effluent water were used for microalgae cultivation. The final products from this proposed value chain are energy and heat from biogas, liquid digestate (organic fertilizer), as well as microalgae biomass. To create a better understanding of the environmental and economic benefits of the proposed value chain, as well as any drawbacks, a life cycle analysis as well as preliminary economic assessment was performed.
Material and Methods
Four different scenarios were investigated: 1) representing the current situation of sludge treatment from an average fish production site with sludge-capturing mechanisms and shipping 50% of the captured sludge to Denmark for biogas production, 2) assuming 100% of the captured sludge to be shipped to Denmark for biogas production, 3) nutrients of the reject water from sludge dewatering are remediated through microalgae cultivation, and the dewatered fish sludge is used as sole substrate for biogas production in Norway, and 4) the same as in 3, though with the distinction that the dewatered fish sludge is added as an additional feedstock to a municipal anaerobic digestion plant in Norway. All scenarios we re based on the aquaculture waste streams from an assumed average-sized fish production facility located on the west coast of Norway. A life cycle assessment was performed to compare the potential environmental impact of these four different fish sludge treatment scenarios with each other and identify environmental hotspots in the proposed value chains. The assessment followed the ISO14040 and ISO 14044 guidelines. O ne year of treatment of fish sludge and reject water from an average salmon production site using Recirculating Aquaculture System (RAS) and producing 5000 tons fish per year (Bregnballe, 2022), was chosen as the functional unit for the LCA. Moreover, A preliminary economic assessment was performed to quantify the differences in operational costs of the scenarios.
Results and Discussion
Shipping and treatment of 100% of the fish sludge in Denmark, as described in scenario 2, gave lower environmental impacts than the baseline scenario where only 50% of the fish sludge was shipped and trea ted in Denmark. Geographical differences in electricity sources in Norway and Denmark are a major factor that determine the environmental sustainability. System expansion , for example to account for the lack of imported sludge in Denmark and subsequent local energy production (biogas) compensated by energy from the local grid, was a major contributor to the environmental impacts. Even though the amount of energy produced from the biogas production in scenario 2 and 4 was the same, the compensated energy coming from the local grid had a significantly different environmental footprint , with the Danish energy mix based on wind power (58%), biofuels (16%) , 10% solar power, and 16% non-renewable energy, and the Norwegian energy mix based for 89% on hydropower, and another 10% based on other low-carbon energy sources.
In scenarios 1 and 2, the system was expanded with the production of microalgae on commercial fertilizer and electricity from the national Norwegian grid alone . This was a major contributor to the environmental impacts and had a greater impact than the treatment of fish sludge in Denmark. When all scenarios were compared without a system expansion for compensation of local energy production in either Norway or Denmark, scenarios 3 and 4 became the most environmentally sustainable scenarios in all impact categories.
Scenario 4, where fish sludge was co-digested with other feedstock, showed a potentially lower environmental impact than when fish sludge was the sole substrate for anaerobic digestion (scenario 3) , due to variations in the biogas productivity, and thus production of less energy that could be used in the microalgae production. Optimisation of anaerobic digestion with high inclusion levels of fish sludge can play an important role here.
As expected, in the scenarios where microalgae were cultivated on aquaculture wastewater and thus recovered the nitrogen from this water , the impact category marine eutrophication was clearly improved .
The direct comparison of only the treatment of fish sludge for biogas production (when using co-digestion with other feedstocks) showed that local treatment of the sludge in Norway resulted in less environmental impact in most of the impact categories compared to treatment in Denmark. This is mainly caused by a lower energy consumption (as no drying of fish sludge is necessary, only dewatering up to 20 % DW) and shorter transport distances (no shipment, only transport by road). However, infrastructure for anaerobic digestion is far less developed in Norway, and the use of the produced digestate needs also to be taken into account.
References
Bregnballe J. (2022) A guide to Recirculation Aquaculture – an introduction to the new environmentally friendly and highly productive closed fish farming systems. FAO and Eurofish International Organisation, Rome 10.4060/cc2390en.