Water quality is a critical element to the successful operation and management of recirculating aquaculture systems (RAS) for the land-based farming of fish.
With recent developments in technology, water quality monitoring is now becoming a commonplace and automated process with the integration of a number of different sensors, and how those sensors provide data and feedback for system management.
At Marineholmen RASLab, we are involved in a number of projects and are working with a number of partners to develop and test new sensor technologies as well as integrate our current RAS system sensors with our other data collection, to provide management tools and solutions that can be adapted to the aquaculture industry.
Dr Mark Powell, CEO of Marineholmen RASLab, details the innovations in water quality measurement helping to improve the quality of land-based fish farming
Continuous water quality monitoring
Our RAS systems routinely and continuously monitor dissolved oxygen, temperature, salinity, water flow, and pH, where we use, for example, state-of-the-art optical oxygen sensors. However, there are a number of variables that need to be measured manually, including carbon dioxide using a Franatech CO2 probe, but nitrogenous compounds like ammonium (in both fresh and salt water), nitrite, nitrate, turbidity, alkalinity, and suspended solids are all measured in our lab, using spectophotometric and gravimetric analysis. Other key elements, like hydrogen sulphide and ORP, are also important to us to ensure optimal operation of our RAS systems.
Measurement of hydrogen sulphide at the concentrations that are sub-lethal to fish is challenging. RASLab has a partnership agreement with SeaRAS, a Norwegian company that has developed a unique type of water sensing technology – Aquasense® – allowing measurement of H2S concentrations to 0.1 µg/L. With this technology, we have monitored our systems and see that, even under high stocking densities and organic loads, we reach a maximum of less than 0.75 mg/L H2S in our systems, well below any toxic water thresholds for Atlantic salmon. In fact, under most operational conditions, we register significantly lower values. Further development of ammonium sensing is under development, with RASLab being used as a test lab with SeaRAS.
Nitrogenous compounds in RAS
For the measurement of nitrogenous compounds in RAS, the issues are more challenging. Freshwater allows for the use of ion-selective electrodes for ammonium; however, this is not possible in more saline conditions. In this case, the use of wet-chemistry-based methods must be deployed. Whereas we have been working with partners on advancing these approaches, we are looking forward to finding a better solution for continuous measurement of ammonium.
The development of measurement of compounds like nitrite and nitrate is being challenged with partnerships through the European EEA funding programmes. We are working with Portuguese partners NS2 on a new project, NOxAqua, where we will test an electrochemical test-strip-based sensor technology for measuring nitrite. Approaches for the measurement of ammonium and nitrate are also in development. Although not necessary to measure continuously, nitrite is a critically important compound to measure in RAS, particularly in freshwater systems. It is the end product of ammonium oxidation within the biofilter – an indicator of this first stage of ammonium degradation.
Nitrite ions also compete for chloride ions in entering the fish, and oxidise haemoglobin, inhibiting the transportation of oxygen by red blood cells, resulting in ‘brown blood disease’. While combatted by increasing chloride concentrations in the RAS operating water, monitoring levels are important intermittently (for example, daily) to ensure safe operating levels and monitor the proper functioning of biofilters.
Measurement of water quality by sensor technologies is only of value if the data can be used in management and in steering the safe and productive operation of an RAS system. At RASLab, we are working with the Portuguese company Sensaway on an EEA-funded project, BlueRAS, where we are able to gather our data streams from our RAS systems (including sensors) and integrate them with operational measurements, such as fish growth and welfare scores, with the aim of using Machine Learning algorithms to help steer and guide a more efficient operation of the RAS system to optimise fish production.
Together with our partners, RASLab is exploring new approaches to water quality measurement, building on conventional approaches and challenging them with new innovations for the future of aquaculture.
Please note, this article will also appear in the twelfth edition of our quarterly publication.