Graduation Year

2023

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

Sarina Ergas, Ph.D.

Committee Member

Kevan Main, Ph.D.

Committee Member

Lior Guttman, Ph.D.

Committee Member

Maya Trotz, Ph.D.

Committee Member

George Philippidis, Ph.D.

Keywords

Sustainable Marine Aquaculture, Resource Recovery, Nitrogen, Algae, Aquaculture Biofilter Combinations

Abstract

Marine aquaculture is a growing industry that has the potential to reduce eutrophication, overfishing, and loss of biodiversity. Recirculating aquaculture systems (RAS) are a type of aquaculture system that decreases discharge, raises biological control, and conserves water; however, RAS are challenged due to high energy cost, low biodiversity, water quality control, and waste nutrients. Periphyton biofilters are a new technology that allows for feed production, water quality control, microbial diversity, and resource recovery in marine aquaculture systems. The overarching question that this dissertation investigates is, how can periphyton biofilters be integrated into RAS to address the challenges of modern sustainable RAS?

The effect of hydrodynamics on periphyton biofilters in RAS was the focus of the first study. The hypothesis was that hydrodynamics effects the periphyton biomass, water quality, mass transport, and nutrient balance. To test this hypothesis, parameters such as total ammonia nitrogen (TAN), NO2-, NO3-, total nitrogen, CO2, periphyton dry weight, and dissolved oxygen (DO) were measured in response to varying flow rates. Effects of system hydrodynamics on periphyton biofilter performance were studied using two pilot scale (2500L) RAS stocked with Ariopsis felis (hardhead catfish) and operated with a modular harvest routine. To operate the biofilter and maintain periphyton in an active growth state, one tank was harvested per week. Periphyton growth rates were found to be maximal at intermediate flow rates. This was because nutrient limitation occurred at low flow rates and sloughing of biomass occurred at high flow rates. Photosynthetic DO production averaged 1.31 ± 0.20 mg DO/(L*m2 *day) and was nearly enough to support microbial and fish respiration without the support of mechanical aeration during the daytime. Periphyton removed 32 ± 4 % of the input nitrogen and 61 ± 3 % of input carbon from fish waste.

The second study investigated the effect of aquaculture biofilters in combination (ABC) on different RAS parameters. The hypothesis was that ABCs effect the water quality, nutrient resource recovery, microbiome, periphyton growth, and water treatment efficiency. The following ABCs were tested: halophytes + periphyton (H+P), moving bed biofilm reactor (MBBR) + periphyton (M+P), and periphyton only (P2). To test the hypothesis, TAN, NO2-, NO3-, total nitrogen, CO2, periphyton dry weight, lipid content, protein content, temperature, and light were measured. The microbiome community was characterized by DNA extraction and sequencing. The halophyte chosen was Sesuvium portulacastrum (sea purslane) due to its high growth rates and use as an edible value-added product. Each trial was run in duplicate on two pilot-scale RAS and repeated in spring and summer to investigate the effect of seasonality on performance. The Sciaenops ocellatus (red drum) stocking density was maintained at 14-18 kg/m3. Lower pH, higher DO concentration, higher TAN concentration, and lower periphyton dry weights were observed in the spring sets, when compared to the summer trial. The M+P combination had the highest nitrification rate (1.75±1.08 TAN-N mg/L/day). The M+P and H+P sets required alkalinity addition (100-200 CaCO3 mg/L) to adjust changes in system pH. Sea purslane growth rates were high (1.0431 ± 0.3361 g/day/plant) compared to literature values. The P2 trials showed a stable balance of alkalinity and pH, while still generating DO. Periphyton biomass was found to contain 4.55 ± 2.24 % of lipids by dry weight. It also contained Ω-3 fatty acids. The periphyton microbiome was found to be composed of filamentous algae, denitrifiers, ammonia oxidizing microbes, nitrite oxidizing microbes, and valuable microalgae, such as Chlorella.

The goal of the third study was to clarify and compare the environmental impacts of periphyton biofilters as compared to conventional MBBRs. The hypothesis was that periphyton biofilters reduce the use of water, energy, nutrients as compared to MBBRs. A life cycle assessment (LCA) with the systems expansion method was utilized on MBBRs and compared to periphyton biofilters. The scope was from raw materials to farm gate to produce 1 kg red drum fish. The life cycle inventory used information from the pilot scale experiments. The LCA analyzed different scenarios for water treatment, feed replacement, and energy savings due to DO generation. The LCA utilized OpenLCA software with agribalyse database running CML IA analysis. The categories of highest impact were marine ecotoxicity, abiotic fossil fuel depletion, global warming potential; although several of the impact categories were related back to the use of non-renewable fuels. Construction costs only make up a small portion (<10%) of the environmental burdens. The initial construction of periphyton biofilters was more expensive, however, the reduced operational expense due to reduced fish feed and electricity costs quickly overcame the initial high construction cost. A 40% replacement of fishmeal based feed with periphyton was significantly lower than the abiotic fuel depletion and marine aquatic ecotoxicity. A 40% replacement of blower electricity with photosynthetically produced DO reduced the energy cost of 1kg of fish by 8 MJ and lowered the global warming potential.

Periphyton biofilters are an emerging solution to address the problems of sustainable feed production, energy savings, wastewater treatment, and microbiome diversity in marine RAS. The harvesting regime and control of hydrodynamics were established as key points in the operations of periphyton biofilters. Integrating periphyton biofilters into ABCs resulted in high resource recovery and energy savings, while maintaining or improving the water quality goals for the fish. Further research may be necessary to optimize the stocking density and understanding seasonal effects more fully. The LCA model showed that there were real energy and ecotoxicity savings associated with the incorporation of periphyton biofilters. This dissertation indicates that the integration of periphyton biofilters into marine RAS improves the sustainability of RAS systems.

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