A pilot study to assess the potential of denitrifying bioreactors in treatment of aquaculture effluents

Heterotrophic denitrification reactors are typical treatment units used to remove nitrate from aquaculture effluent. However, they face with limitation due to practical and economic reasons as a result of their requirement for external readily available organic carbon sources. In this study, four clear acrylic vertical columns (10cm × 32cm) with upward flow direction were constructed and packed packed with wood chips (from birch tree), and mixture of woodchips with three different carbon sources (potato peels from industrial process; dried sphagnum moss and biochar coals). The purification performance of bioreactors were investigated in treatment of aquaculture effluent under design hydraulic retention time (HRT) of 48-h and temperature of 15.35± 0.31°C (mean± SD) for 55 days. The results of this study clearly showed the instant nitrate (NO₃-N) reduction in all bioreactors and stable removal efficiency after one week. As much as 94% of the nitrate was removed from aquaculture effluent with 35.5 mg NO₃-N L⁻¹ concentration, in bioreactor with woodchips. This removal efficiency corresponded to 21.6 g N m⁻³d⁻¹ removal rate. Reported removal rate and removal efficiency for 35.5 mg NO₃-N L⁻¹ inflow concentration, averaged (21.4 g N m⁻³d⁻¹, 92 %) for woodchips with potato peels, (20.7 g N m⁻³d⁻¹, 91.3 %) for woodchips with biochar and (19.2 g N m⁻³d⁻¹, 78.9 %) for woodchips with dried sphagnum moss (BR2). In addition, bioreactor with potato peels showed higher nitrate removal rate (29.2 g N m⁻³d⁻¹) for the inflow water with high concentration of NO₃-N (35th -55th day). Reported nitrate removal rate in this study were relatively greater than measured in previous studies due to the selected sufficient HRT (48-h) combined with relatively high NO₃-N concentration of influent in the optimal temperature of 15.35± 0.31°C (mean± SD). Start-up leachate of organic carbon, ammonia and phosphorus for start-up period took place in all bioreactors, but with higher rate in bioreactor with potato peels. Rise in alkalinity took palace as an evidence of heterotrophic denitrification in all bioreactors expect bioreactor with potato peels. Phosphorus reductions were observed after two weeks in woodchips bioreactor with biochar, bioreactor with dried sphagnum moss and woodchip bioreactor at the highest removal efficiency of 23%, 38% and 35%, respectively. While, Phosphorus reduction started in bioreactor with potato peels after nearly one month at the rate of 20%. In summary, the master thesis demonstrated that woodchips bioreactors have potential of nitrate removal from aquaculture effluent even with low chemical oxygen demand (COD). Woodchips bioreactors can reach the stable performance within few weeks