Influence of osmotic stress on the formation and performance of aerobic granules under high (conventional SBR) and low (simultaneous fill/draw SBR) selection pressure.

Application of salinity during aerobic granulation has proven to be quite effective, with even more promising results when applied intermittently using pulses. This pulsed doping has been little explored, with studies restricted to conventional batch systems with variable volume, although the most common full-scale operating mode is simultaneous fill/draw (constant volume). Four reactors were monitored, differing in operating mode and salt addition: R1, constant volume without salt; R2, constant volume with salt pulses; R3, variable volume without salt; R4, variable volume with salt pulses. Salinity favored granulation, but at constant volume, the granulation time was shorter. R4 was considered granulated after 21 days, while R2 required more than twice that time (45 days). Nevertheless, the systems operated at constant volume achieved higher solids concentrations than the variable volume systems, with average concentrations of 4 g/L and peaks above 6 g/L in the presence of salt (R2). These systems also excreted more EPS (>600 mg/gVSS), but the salt addition did not make a difference. Under the influence of salt, the granules formed in the constant volume system showed greater resistance to deformation, especially with diameters greater than 600 µm (G' = 49.4 Pa). In the rotational tests, granules with a diameter greater than 200 µm in R2 demonstrated that they maintained their structure longer than granules of the same size in R4. Therefore, NaCl pulses applied in constant volume reactors proved to be a promising strategy, introducing additional selective pressure into a system characterized by lower hydrodynamic shear, thus improving granulation.