Date of Award

Summer 8-19-2022

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science in Civil Engineering (MSCE)

Department

Civil Engineering

Advisor

Jean MacRae

Second Committee Member

Onur Apul

Third Committee Member

Deborah Bouchard

Abstract

Recirculating aquaculture systems (RAS), where only approximately 10% of the total system water is exchanged per day, have grown in popularity in recent years due to their potential to provide a high-quality protein source in a contained environment. With increased production comes the need for RAS water treatment to mitigate recirculation and discharge of nutrients produced by fish; mainly phosphorus and nitrogen. When discharged, nutrients can contribute to eutrophication in surrounding water bodies, harming the fish and other aquatic life. Therefore, RAS effluent should be treated before discharge. One method of phosphorus removal is adsorption, a surface phenomenon that is often used to bind dissolved pollutants to a solid-phase medium and remove them from water. Nitrogen is present in RAS as ammonia, which is toxic to fish even at concentrations as low as 0.05 mg L-1. Therefore, the ammonia is transformed to nitrogen’s non-toxic form, nitrate, before recirculation or discharge, by a process called nitrification. Both adsorption and nitrification can be affected by RAS process parameters such as salinity. Many anadromous fish such as Atlantic salmon require a change in salinity over their lifetimes; therefore, both of these processes should be investigated for their response to salinity changes.

An adsorption study was performed on an aluminum oxide-based material, RhizoSorb®, to assess its response to different RAS variables. It was found that both film diffusion and intraparticle diffusion are rate-controlling steps in the adsorption process, and the removal efficiencies in batch tests were affected by time, salinity, and phosphate concentration. The Freundlich isotherm fit the equilibrium data better than the Langmuir isotherm, showing that adsorption is a multi-layer process and that the adsorbent is highly heterogeneous. The Clark model was better suited than the Thomas model for predicting the performance of the RhizoSorb® in a flow-through system. The results of this study showed that RhizoSorb® and other alumina-based sorbents have high potential towards application to the RAS water treatment process to remove and recover phosphorus.

A second study was performed to assess the effects of salinity changes on nitrifying biofilters. Acclimation to a small amount of salinity before transition to a higher salinity may help biofilters recover from the larger shift; therefore, a series of experiments was performed on both freshwater and brackish (3 parts per thousand (ppt)) biofilters to assess their respective levels of recovery after an abrupt change in salinity (3, 20, and 33 ppt). Tests were run for a two-week period in which the nitrification rates were monitored. The freshwater biofilters quickly recovered from a shift to 3 ppt, but did not recover from a shift to 20 or 33 ppt. The brackish filters started to recover at the end of the two-week test, but did not recover from a shift to 33 ppt. DNA sequencing of the variable V4 region of the 16s rRNA gene showed that the heterotrophic communities in the biofilm were lysed at a greater proportion than the nitrifiers, though the nitrifiers were inactivated in higher salinities. A longer series of tests could fully characterize the effects of acclimation to salinity with the effects of ammonia concentration and organic matter, which could help to fully understand the microbial community dynamics.

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