Aerobic granular sludge formation in a sequencing batch reactor treating agro-industrial digestate

Most of nitrogen emissions can be ascribed to agro-industrial activities. Advanced, technical- and cost-effective technologies must be developed and applied in order to significantly reduce the impact of agro-industrial residues on the environment. In this study, aerobic granules were successfully cultivated in a granular sludge sequencing batch reactor (GSBR) fed with the ammonium-rich (approx. 2,500 mg L-1) liquid effluent of a 3-stage anaerobic digester treating agro-industrial residues. The peculiar characteristics of such wastewater required a 2-step operating strategy aimed at the selection of nitrifying biomass (Step 1) and the formation of aerobic granular sludge (Step 2). During Step 1, biomass selection was achieved by properly regulating the cycle length. During Step 2, the progressive increase in selective pressures (i.e., minimum settling velocity and volumetric organic loading rate) led to the formation of compact and well settling granules. Results were promising, as the GSBR removed up to 87% of organic matter (as COD), 71% of NH4-N and 55% of total nitrogen (as sum of NH4-N, NO2-N and NO3-N), showing stable performance.