Assessment of a recirculating aquaculture system : with a focus on energy and water/effluent sustainability

Abstract

In response to the urgent challenge of climate change, the United Nations launched the Sustainability Development Goals in 2015 and the European Union has proactively designed and implemented several Directives that are targeted towards decarbonisation and climate change abatement, with focus on the sustainable consumption of energy and the protection of marine and freshwater bodies. Fish production by aquaculture has increased within recent years, with the development of new technologies such as Recirculating Aquaculture Systems (RAS). While these systems reduce the potential output of harmful nitrogenous species into natural waterbodies, it is achieved at the cost of increased energy consumption. The aim of this dissertation was to conduct a sustainability review of such a facility, however a research-scale one. Energy consumption data (kWh) was collected to model the Global Warming Potential (GWP) (kg CO2 eq.) of the RAS when connected to the national grid, while power factor values were used to understand the overall energy efficiency of the system. The power factor of the system ranged between mainly between 0.9 - 1.0 when several variable time-of-use equipment (heat exchangers and drum filter) were on standby mode. However when these started operating, the power factor dropped, indicating the need for improving energy efficiency through the installation of correctors, while also identifying areas for possible improvement. A photovoltaic (PV) alternative was compared with the national grid data, showing that whilst GWP decreased for the PV, its Abiotic Depletion Potential was much higher than the fossil fuelled national grid. Dissolved nitrogen species concentrations (NH3, NO2 & NO3) were also measured from the system, allowing for the Marine Eutrophication Potential (kg N eq.) to be calculated using the characterisation factors for the corresponding species. Due to the low stocking density of the research-scale RAS, the Marine Eutrophication Potential was not high, however larger scale production RAS could have differing values due to economy of scale. Finally, alternatives for improved sustainability were also explored, focusing on water recycling and alternate ‘green’ energy sources.