Effects of Organic Carbon Addition in Controlling Inorganic Nitrogen Concentrations in a Biofloc System

The daily addition of tilapia feed and tapioca starch at the C : N weight ratio of 16:1 was conducted to examine the effectiveness of biofloc-mediated assimilation and nitrification in the zero-water exchange tilapia cultivation tanks. Inorganic nitrogen concentrations in treatment tanks receiving feed and tapioca starch indicated profiles, which resembled the start-up of biofilters. Assimilation was essential for the control of inorganic nitrogen concentrations prior to the occurrence of complete nitrification as confirmed by an increase in suspended solids concentration from 52 to 1180 mg SS/L, a slower rate of total ammonia nitrogen (TAN) and nitrite accumulation, and lower concentrations of TAN and nitrite relative to those in control tanks receiving only feed addition. Effective control of inorganic nitrogen concentrations (i.e., TAN and NO2-N < 1.0 mg N/L) was observed in both systems when complete nitrification was established after approximately 6-7 wk regardless of starch supplementation. Results from the nitrogen mass balance suggested that nitrification and, to a lesser extent, assimilation were responsible for inorganic nitrogen control in treatment tanks.