Development of denitrifying and methanogenic activities in USB reactors for the treatment of wastewater: Effect of COD/N ratio

Nitrification–denitrification is the traditional biological process for nitrogen removal from wastewaters. During its second step, denitrification, nitrate formed during nitrification is reduced to gaseous nitrogen under anoxic conditions. Under the presence of organic matter and nitrogen, also methanogenesis and dissimilatory nitrate reduction to ammonia (DRNA) may also take place. COD/N has been referred to be a key factor in the expression of these metabolic pathways. During this research, five upflow sludge bled (USB) reactors were operated at different COD/N ratios, in order to study the evolution of the methanogenic and denitrifying activities in the sludge. The use of nitrogen and organic matter through denitrification, DNRA and methanization was also studied through mass balances, as well as its granule structure. COD/N ratio showed a strong influence on biomass activity, and therefore on the metabolic pathways of nitrate and organic matter utilization. Low values generated high denitrifying activities, and high value, elevated methanogenic activities. Even though it was possible to perform methanization and denitrification in one single reactor, feasible loading rates will be limited by the available activities, so in many cases separated reactors will be more suitable. Granular structure could not be maintained in denitrifying reactors at low COD/N ratio (COD/N 5 and lower): granules disappeared and were replaced by flocculent sludge, with low settling velocities.