Abstract:
This study investigated the nitrogen removal efficiency of a combined nitrification - denitrification reactor from recirculating aquaculture systems. The fibrous polymer BiocordTM and pumice stones (1 - 3 mm in size) were used as the biofilter media. The study consisted of 3 experiments. The first experiment was an evaluation of nitrification and denitrification rates of the biofilter media. The results showed that BiocordTM and pumice stone had the maximum ammonia removal rate of 42.4 ± 0.8 and 640 ± 387 g - N/m3(packing volume)/day, respectively, and the maximum nitrate removal rate of pumice stone was 169.1 ± 8.8 g - N/m3(packing volume)/day. Thereafter, the second experiment was performed using the integrated packing of BiocordTM and pumice stone in a single reactor for nitrogen removal. The combined nitrification - denitrification reactor was a 20 x 20 x 35 cm glass tank. The pumice stone was packed as a layer of 5 cm height at a bottom of the reactor together with 1 meter length of BiocordTM. The ammonia and nitrate removal efficiencies were evaluated after an addition of ammonium chloride with or without methanol as the carbon source at the COD : Nitrate - N of 5:1. It was found that nitrification process could occurred although the reactor was covered with plastic sheet. The small circulating pump was sufficient to maintain dissolved oxygen concentration within the desire level for nitrification. Addition of methanol with ammonium chloride had no inhibition effect to the nitrification process. On the other hand, denitrification was found only with limited dissolved oxygen after turned off an internal circulating pump with an addition of methanol. BiocordTM, however, was not necessary for nitrification treatment since the performance of nitrification process in pumice stone layer was sufficient for ammonia treatment. Finally, in the last experiment, the nitrification - denitrification reactor was set up and used for the real nitrogen waste treatment from the recirculating fish culture system. The 100 L combined nitrification - denitrification reactor with the optimal operating condition from previous experiments was attached to the 100 L Tilapia fish tank for nitrogen waste treatment. Recirculating fish culture system was operated for 4 months. It was found that nitrogen treatment tank containing pumice stone could maintain ammonia and nitrite concentration at below 1 mg - N/L throughout the experiment period. The treatment tank with combined nitrification - denitrification reactor prevented nitrate accumulation and kept nitrate concentration below 50 mg - N/L while nitrate in control tank was as high as 352.47 ± 9.67 mg - N/L.