Introduction
One of the main problems associated with any food production value chain is valorisation of by-products and side-streams to reduce the environmental impact, while optimizing the use of resources (Newton et al., 2017) . This is especially needed for the aquaculture sector, where circularity principles are key elements to meet sustainability requirements . In the H2020 project "GAIN " , we have used different strategies to address this issue by investigating innovative processes aimed at reusing: fish sludge, generated in Recirculating Aquaculture Systems (RAS) , fish mortalities, by-products generated by fish processing (heads, frames and trimmings, skins, etc..), and shells from bivalves generated by the cannery industry . Main results and developments that will significantly contribute to the eco-intensification of this productive sector are presented.
Material and Methods
As a basis for investigating and developing technolog ies for valorizing aquaculture side-streams we analysed wastewater from land-based Atlantic salmon smolt production farms for its chemical characteristics with respect to its environmental footprint and the underlying valorisation potential. To recover particulate, flocculated matters, the S3 filterdryer system was developed, including a fine meshed filter cloth (mesh size 6 µm) capable to remove particles from aquaculture wastewater. In addition, the S3 filterdryer sanitizes and dries the resulting sludge (DM content 91-94%) without the use of added chemicals (polymers, coagulants, etc.) . Besides, an alternative system, adding polymers and drying the sludge using the Waister superheated steam dryer to sanitise it, was also tested.
For the sanitation of fish mortalities, a new superheated steam dryer was developed, the Waister 15 generation.
In order to estimate the potential for valorisation of by-products , the yield of these by-products in salmon, trout, seabream, seabass, turbot and carp were determined. These by-products were used as raw material for performing enzymatic hydrolysis to obtain fish protein hydrolysates (FPH).
The suitability of bivalve shells for being used as filling material of a newly designed biofilter for nitrification and phosphate removal from RAS waste water was investigated. 10L volume biofilters were used at lab-scale level for compative experiments between plastic material and crushed and whole mussel shells. For phosphate removal experiments, crushed shells biofilters were compared with calcite material, the latter simulating the abovementioned material calcined shells.
Results
The main challenge to purify aquaculture wastewaters lies with the very low levels of dry matter (DM) and the small particle size of solids present in wastewaters. The use of the newly designed S3 filtration system provides a 93 + 2.8 % removal of suspended solids (SS) and a 80 + 6.4 % removal of organic content determined as chemical oxygen demand (COD) in the treated wastewaters, confirming the utility of the S3 filter dryer as a technology meeting the regulatory demand of a 50% SS and a 20 % COD reduction as a standalone unit. The S3 system dries the sludge in parallel, achieving a DM content of 91-94%, without the use of chemicals and extremely low energy consumption: 300W per cubic meter wastewater for the entire process.
Sludge dried using conventional heating, as a superheated steam dryer, resulted in a product with excellent physicochemical properties and DM contents of 90-95 %, outperforming airdried fish sludge as a bioenergy source in cement production. Analysis of dried fish sludge shows that it is an excellent bio-fertiliser product with high content of N and P, while low level of K. The Zn content is currently the most limiting element for allowed spead of dried fish sludge on farmland.
Waister implemented a novel drying technology of mechanical fluidisation and superheated steam for sludge of typically 10-50 % DM from polymer-based dewatering technologies. This technology can be combined with the S3 dewatering technology. Both systems stabilise the sludge and minimise shipping costs.
Regarding the treatment of fish mortalities, a single loop Waister 15 dryer was installed in Helgeland Smolt, finalising a well-functioning technology for mortalites disposal. The main challenges with the treatment of salmon mortalities were the high fat content and the presence of excess water mixed with the fish. The first problem was solved with the incorporation of additives, such as wood chips or dried spent grain, while the second was solved with manual feeding of the fish into the dryer. The dried product obtained can be used for bio-energy production, biofertiliser, or potential animal feed ingredient depending on the additive used prior to the drying.
The potential volume and economic value of by-products generated by the main aquaculture species in Europe was estimated.
Most by-products present a significant protein content : therefore, the use of proteolytic enzymes to breakdown these proteins and use the resulting peptides for different applications, i.e. as ingredient of aquafeeds was investigated, in order to determine optimal operational, e.g. temperature, pH and other parameters.
Mussel shells were tested as a replacement for plastic filling in a RAS biofilter. Results suggest that mussel shells could be effectively used, as t hey provide a suitable substrate for nitrifying bacteria and, due to their carbonate content, contribute to stabilize pH. Also, they showed interesting properties to remove dissolved phosphate from water, improving the quality of the effluent.
Acknowledgements
The research leading to these results has also received funding from the GAIN project, European Union’s HORIZON 2020 Framework Programme under GRANT AGREEMENT NO. 773330.
References
Newton et al. 2014. Perspectives on the utilization of aquaculture coproduct in Europe and Asia: prospects for value addition and improved resources efficiency. Critical reviews in Food Science and Nutrition 54(4): 495-510
Morris, J. P., Backeljau, T., & Chapelle, G. (2019). Shells from aquaculture: a valuable biomaterial, not a nuisance waste product. Reviews in Aquaculture, 11(1), 42-57.
Newton et al. 2014. Perspectives on the utilization of aquaculture coproduct in Europe and Asia: prospects for value addition and improved resources efficiency. Critical reviews in Food Science and Nutrition 54(4): 495-510