the SLudge Appraisal teaM – Developing a sUstainable value chaiN from tanK to product

The Norwegian government has set the goal to become a leading seafood nation by increasing the current Norwegian aquaculture production 5-fold by 2050, targeting 5 million tons of yearly sustainable aquaculture production. This will also generate an expected sludge volume of 11 million tons per year, posing a significant challenge to achieving the country's sustainability goals for aquaculture. Sludge consists of fish feces and feed leftovers and is characterized by a high content of organic matter and nutrients, and high salinities, making its disposal environmentally challenging. To make aquaculture more sustainable and ready for stricter emission requirements, sludge handling solutions need to be increasingly adopted by the industry. Furthermore, fish sludge also represents a valuable source of energy, making it an interesting raw material for biogas production and for the recovery of valuable nutrients. The objective of the SLAM-DUNK project was to design, optimize, and integrate a combination of novel technologies, 1) anaerobic digestion, 2) microwave-assisted pyrolysis and gasification, and 3) microalgae cultivation, to convert fish sludge into valuable products such as energy, fertilizer, and feedstock for innovative materials. The project not only focused on the technology development, but also studied the sustainability of the developed value chain, looking at the environmental impacts, the energy usage, costs, and the logistics and organization of such a value chain. Anaerobic digestion is a collection of microbial processes by which organic matter is converted under anoxic conditions, producing energy-rich biogas (mostly composed of CO2 and methane) and nutrient-rich liquid digestate, and is commonly used to manage animal manure, wastewater or food waste. Through the project we could show that anaerobic digestion of solely fish sludge produced nutrient rich digestate and biogas containing hydrogen, which is an alternative energy source that can contribute to reducing net carbon emissions. The hydrogen production was highly impacted by the salinity, liquid content, and bacterial community composition of the sludge. Microwave-assisted pyrolysis and gasification (MAP-G) is the combination of three technologies, pyrolysis, gasification, and microwave heating, where organic materials can be converted into bio-syngas containing hydrogen. The combined application of these three techniques is novel and brings several advantages compared to conventional pyrolysis and is straightforward in terms of circular economy. This project focused on developing an improved reactor design and tested the design for converting fish sludge into bio-syngas. Microalgal production was assessed on effluent water from salmon production and on the liquid digestate from the anaerobic digestion experiments. Furthermore, different reactor designs and cultivation techniques (biofilm reactors and suspension cultivation) were evaluated for best performance. The microalgae were able to grow on both residual streams, and removed the nutrients contained therein, and photobioreactor design and cultivation processes were improved so to achieve high microalgal biomass yields. The produced microalgal biomass was used for exploring different microalgae-based composite materials. A value chain on anaerobic digestion and micro-algae production to treat fish sludge and effluent water from fish aquaculture was designed. The produced nutrient-rich digestate, heat, CO2- rich flue gas and electricity from anaerobic digestion, can be used for algae production. Additionally, this value chain produces cleaned water, surplus of electricity and nutrient-rich digestate, as well as algae paste. The environmental impact of the value chain was compared to the current situation where fish sludge is treated by anaerobic digestion in Denmark.

The SLAM-DUNK project demonstrated the potential of novel technologies to valorize fish sludge into valuable products, which contributes to a sustainable marine economy and minimizing waste production. Project results showed the feasibility of producing clean energy (H2) from fish sludge through anaerobic digestion and Microwave-assisted pyrolysis and gasification. Using hydrogen as an alternative energy source can contribute in reducing net carbon emissions and integrate with developing infrastructure for faster adoption of renewable energy in fish farming and sustainable aquaculture processes. Additionally, the liquid waste generated through anaerobic digestion can be utilized by microalgae for recycling nitrogen and phosphorus. The outcomes regarding Sludge conversion through MAP-G also provided valuable know-how for designing and building MAP-G reactors. The obtained insight into microalgal production on residual streams and the examined cultivation techniques, are highly valuable for the algae production sector. In addition, the project has shown the feasibility and potential of a closed-loop value chain for sludge and effluent water treatment, locally. This is an important first step to establish local clusters in Norway, that are not only treating waste streams from aquaculture but are also value chains leading to valuable products. Besides these results that are directly applicable to industry, the project has also increased research expertise on sea and coast in Norway by recruiting a PhD student, training and educating master and bachelor students, and disseminating the knowledge in scientific publications and conferences as well as through popular science articles and stakeholder events. Therewith the project has also increased the visibility of Norwegian researchers, research institutions and R&D infrastructure in Europe and contributed to further sustainable growth and employment of both aquaculture and low trophic species production.

Year after year, the Norwegian seafood industry establishes new records in exporting seafood. The Norwegian government's goal for the future is ambitious: becoming a leading seafood nation with 5 million tonnes of sustainable aquaculture production by 2050. The salmon and rainbow trout biomass production of 2017 was 1.3 million ton with a corresponding sludge volume of 2.1 mill ton. For comparison, 0.5 mill. tonnes of sludge annually discharged from open cages is equivalent to the waste generated by 20 million people. By 2050 the sludge volume is predicted to reach 11 mill ton. This huge amount of generated sludge can be considered a showstopper regarding the 2050 sustainability ambition. The aquaculture sector urgently needs to invest in future sludge handling solutions as new and stricter requirements are expected to be enforced by the authorities in the short term. Aquaculture sludge consists mainly of fish secretions and rests of uneaten fish feed with a very high percentage of organic components (50 – 90%) and a low total solid content. Also, fish sludges differ from other sludge types by having a high salinity and a high nitrogen content, especially ammonia. Discharge or reuse of these sludges is environmentally concerning or require high treatment costs. SLAM-DUNK aims to develop a sustainable value and logistics chain for the conversion of fish sludge into valuable products, based on designing, optimizing, and integrating a combination of novel technologies (anaerobic digestion, microwave assisted pyrolysis, and microalgae). This highly transdisciplinary project will not only focus on technology development, but also develop an optimal value chain for the use of sludge from fish production to potential products for several sectors such as architecture, consumer goods, and textile. Moreover, we will determine the sustainability of the proposed SLAM-DUNK value chains with respect to life cycle impact, energetic viability, and quantitative logistic performance.