Influence of aeration intermittency on nitrogen removal in a reactor with aerobic granular sludge treating wastewater

The process of aerobic granular sludge (AGS) has been considered an alternative of great interest for the tertiary wastewater treatment mainly due to the great capacity of granule densification, which makes the system very compact. The simultaneous nitrification and denitrification (SND) occurrence guarantees nitrogen removal at a high level of efficiency and biological phosphorus removal can be optimized in this arrangement with a single treatment stage in sequential batches. In this study, the general behavior of a sequential batch reactor (SBR)/AGS in wastewater treatment and the effect of the air supply regime on nitrogen removal were evaluated. A pilot scale system supplied by relatively high concentration wastewater in a warm climate region was set up over 525 days. The study was divided into three main phases. In Phase I, a cycle of 3 h of total duration and continuous aeration of the sludge was applied with high efficiency in the removal of soluble chemical oxygen demand (sCOD) from the wastewater, but excessive drag of total suspended solids (TSS) with the effluent persistently occurred throughout the cycle. Nitrification went well, but wastewater denitrification was only partial with a notorious and worrisome accumulation of nitrite. In Phase II, the total cycle duration was increased to 4 h, but this increase in the time available for the anoxic and aerobic reactions was not enough to control the problem and the nitrite and nitrate residuals remained high in the effluent. In Phase III, the implementation of an intermittent aeration regime led to the production of an effluent from the treatment system with ammoniacal nitrogen, nitrite and nitrate average concentrations of 4.3 mg N.L-1, 0.4 mg N.L-1 and 5.5 mg N.L-1, respectively. Surveys of concentration profiles were carried out over some batches and it was observed that denitrification occurs mostly simultaneously with nitrification during the aeration period. The general conclusion of this study is that the application of intermittency regime in the air supply of AGS is of great importance for the improvement of nitrogen removal in this process.