Nitrogen removal from a recirculating aquaculture system using a pumice bottom substrate nitrification-denitrification tank

This research investigated the efficiency of a pumice stone biofilter tank for nitrogen removal from a recirculating aquaculture system. The pumice bottom substrate nitrification-denitrification tank was a glass tank packed with a 5 cm depth of pumice stone (approximately 3 mm in diameter) at the bottom. It was found that the pumice stone could perform as a nitrification biofilter under aerobic conditions. When applying methanol as the external carbon source at a COD:N ratio of 5:1 and then covering the tank with a plastic sheet to reduce gas exchange, pumice stone could remove nitrate through denitrification. Thereafter, nitrification and denitrification treatments using the pumice tank were applied to a 100 L moderate density (10 kg m −3 ) recirculating aquaculture system (RAS) under laboratory conditions. The RAS consisted of a 100 L tilapia culture tank connected to a 100 L pumice tank packed with a 5 cm layer of pumice stone. It was found that the nitrification treatment performed by the pumice tank could control ammonia and nitrite concentrations within the required safety range throughout the 121-day culture period. When nitrate accumulated to approximately 50 mg-N L −1 in water, water recirculation was paused, after which batch denitrification treatment was performed by adding methanol at COD:N of 5:1 in the pumice tank. With nitrification-denitrification treatment, ammonia and nitrite concentrations were below 1 mg-N L −1 and nitrate was kept below 50 mg-N L −1 while nitrate in the control tank was as high as 352.47 ± 9.67 mg-N L −1 . Moreover, the pumice bottom substrate tank with methanol supplement had no negative effect on growth and survival of fish in the recirculating system.