Evaluating the main and side effects of high salinity on aerobic granular sludge

Salinity can adversely affect the performance of most biological processes involved in wastewater treatment. The effect of salt on the main conversion processes in an aerobic granular sludge (AGS) process accomplishing simultaneous organic matter, nitrogen, and phosphate removal was evaluated in this work. Hereto, an AGS sequencing batch reactor was subjected to different salt concentrations (0.2 to 20 g Cl- l(-1)). Granular structure was stable throughout the whole experimental period, although granule size decreased and a significant effluent turbidity was observed at the highest salinity tested. A weaker gel structure at higher salt concentrations was hypothesised to be the cause of such turbidity. Ammonium oxidation was not affected at any of the salt concentrations applied. However, nitrite oxidation was severely affected, especially at 20 g Cl- l(-1), in which a complete inhibition was observed. Consequently, high nitrite accumulation occurred. Phosphate removal was also found to be inhibited at the highest salt concentration tested. Complementary experiments have shown that a cascade inhibition effect took place: first, the deterioration of nitrite oxidation resulted in high nitrite concentrations and this in turn resulted in a detrimental effect to polyphosphate-accumulating organisms. By preventing the occurrence of the nitrification process and therefore avoiding the nitrite accumulation, the effect of salt concentrations on the bio-P removal process was shown to be negligible up to 13 g Cl- l(-1). Salt concentrations equal to 20 g Cl- l(-1) or higher in absence of nitrite also significantly reduced phosphate removal efficiency in the system.