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

•Pumice was used as media for integrated nitrification-denitrification treatment for aquaculture.•Intermittent ammonia and nitrate treatments were accomplished in a single pumice tank.•Short period of anaerobic denitrification has no effect on nitrification efficiency under intermittent treatment.•Pumice treatment tank was successfully tested with fish recirculating tank.•This technique is suitable for ammonia and nitrate treatment in a low-cost, moderate density, aquaculture 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.