Use of ozone for Vibrio parahaemolyticus inactivation alongside nitrification biofilter treatment in shrimp-rearing recirculating aquaculture system

Abstract

Disease suppression is an essential aspect of shrimp farming. To control water quality, a recirculating aquaculture system (RAS) is employed in worldwide. In RAS, water quality is achieved by nitrification biofilter treatment, whereas disease prevention is achieved by the application of an oxidizing agent such as ozone (O3). This study investigates the proper residual ozone concentration (ROC) dosage for controlling pathogenic bacteria Vibrio parahaemolyticus (VP) and minimizing mal-effects on nitrifying bacteria activity in the biofilter media inside an RAS. Juvenile Pacific white shrimp, Litopenaeus vammanei, were cultured for 4 months in a lab-scale brackish RAS integrated with ozonation. Results showed that although the system biofilter's nitrification activity was influenced by ROC, the 0.3 mg L−1 ROC dosage provided an optimum condition which effectively eliminated VP in the biofilter but did not inhibit ammonia removal. At a higher ROC (approximately 0.4 mg L−1) ozonation dosage, the biofilter's ammonia oxidation rate significantly reduced. Over a 40-day shrimp-rearing operational period, the viable VP count in an aerated (non-ozonated RAS) gradually increased, but the viable VP counts in both biofilm and water in a 0.3 mg L−1 ROC treated RAS significantly reduced down to undetectable levels. As such, the conclusion was drawn that a 0.3 mg L−1 ROC ozonation treatment in the biofilter tank of a RAS can effectively control VP pathogens without influencing the activity of the nitrification biofilter, thus providing a balanced control between water quality and safety to the shrimp.