Mass balance and energy optimisation in recirculating aquaculture systems (RAS), with focus on diurnal variations in water quality

Mass balance and energy optimization in RAS (Recirculating Aquaculture Systems), with a focus on diurnal variations in water quality. Recirculating aquaculture systems (RAS) are becoming increasingly common, and there is rapid technological development in this field. To ensure environmentally friendly, welfare-oriented, and sustainable development, it is important to build and further develop expertise. This project addresses relevant issues related to water quality, optimized operation, and sustainable energy utilization and use. Optimization of Water Quality in RAS Facilities Through studies in both full-scale and pilot-scale facilities, the project has investigated water quality and the cleaning efficiency of CO2 strippers, biofilters, and drum filters. In a descriptive study of a commercial RAS for the production of Atlantic salmon post-smolt, theoretical values for metabolite production and cleaning efficiency were compared with observed values in two production systems with different salinity levels. Two measurement methods for CO2 were also compared. The study showed that in systems with turbulent water flow and a moving bed biofilter, much of the CO2 removal can occur before the CO2 stripper. This depends on bacterial activity, the maturation level of the biofilter, and system design. The CO2 level was affected by the measurement method, and CO2 production was mainly influenced by biomass, feed load, temperature, and pH, rather than salinity. The project also investigated the effect of alkalinity, which measures the water’s ability to resist pH changes, on water quality in a controlled lab experiment using a pilot-scale brackish water RAS with Atlantic salmon post-smolt. The study showed that it is beneficial to maintain alkalinity around 100 mg/L to balance system performance, cost and the risk of high pH. Stable pH is fundamental in RAS as it directly impacts water chemistry, toxicity, fish health, fish welfare, and efficiency of water treatment. Diurnal variations in water quality impose variable loads on the water treatment systems. These variations were studied in both pilot and full-scale facilities, and the effects of continuous versus periodic feeding were compared. Overall, these studies provide valuable knowledge on how to optimize operations to ensure good water quality in commercial RAS facilities. Descriptive studies from commercial facilities are particularly useful for complementing the knowledge base gathered from controlled lab experiments. Energy-Intensive RAS Facilities and Sustainability RAS are known to be energy-intensive, challenging both financial and environmental sustainably. Energy needs also vary with diurnal and seasonal changes. To optimize energy use, a numerical model in Matlab, coupled with the process simulation tool, Aspen HYSYS, was developed and validated against real data from a commercial RAS for the production of Atlantic salmon post-smolt. Using this model, water quality and energy needs for a complete RAS facility could be simulated. Automating the water treatment loop and simulating the fish tanks in Matlab coupled with Apen HYSYS through dynamic modelling has made it possible to accurately dimension energy needs. The study shows that modelling and simulating RAS with these tools work well and can also handle sudden changes in the system. Reduction of Carbon Footprint To reduce the carbon footprint of land-based RAS facilities, the aim is to use locally available renewable energy sources such as wind, sun and waves. To accurately assess the available energy, it is important that the available weather data correctly represents the energy fluctuations at a specific location. In the project, four MCP (measure-correlate-predict) models were tested for wind speed, significant wave height, and maximum wave period. Also, available sun energy at the RAS-facility was estimated over the course of one year. The results showed improved prediction accuracy for all models when appropriate criteria for data sorting and algorithms were used. Production of Biogas from Sludge To investigate the potential for sustainable utilization of sludge from fish farming, the project conducted a series of experiments to determine whether the biochemical methane potential (BMP) during anaerobic degradation of sanitized wastewater sludge is affected when using sludge from aquaculture facilities as a co-substrate. Automatic Methane Potential Test System II (AMTPS II) was used to determine BMP. A heavy metal analysis of the bio-residue was also conducted using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) to assess its suitability for soil improvement. The goal of the experiment was to determine the optimal composition of wastewater sludge and dry and/or wet fish sludge. In the final series of experiments, the extent to which the use of pure fish sludge as a substrate, without mixing with municipal sludge, can affect BMP was also investigated.

New expertise is needed in the field of energy efficient aquaculture systems in general and in recirculating systems especially. The energy intensive nature of recirculation aquaculture systems (RAS) hampers their sustainability and cost effectiveness. By means of dynamic simulations using data collected in existing RAS, this interdisciplinary research project will study and optimise the energy use of RAS if supplied by locally available renewable energy forms like wind, wave and solar energy as well as biogas produced from accumulated sludge from the filter systems while taking diurnal variations in energy demand into account. There is also limited information regarding how both the biofilters and aerators function during diurnal fluctuations of water quality parametres. An understanding of the diurnal variations and better methods of measuring water quality parameters will help the fish farmers to run their fish farms better in relation to fish welfare and fish health. The carbon dioxide excretion from the fish results in acidification of the water in the rearing units and is comparable to increased carbon dioxide levels in the sea in the future. Thus, knowledge generated about this topic can also have important relevance beyond the field of aquaculture. Better utilization of waste and residual products is a central part of the circular economy, and can contribute to increased competitiveness and value creation in Norway. This project will further raise the research competence within utilisation of fish sludge to increase biogas production. Knowledge developed through this interdepartmental cooperation in the field of energy, ocean and aquaculture engineering is relevant to education and research at Western Norway University of Applied Sciences. The related industry will contribute to and profit from this research and the positive environmental and societal impact expected from the new knowledge.