Aerobic granulation in an oxidation ditch using the residual sludge after extracting slime-extracellular polymer substances

Abstract

Sludge granulation in continuous flow reactors (CFRs) is still of great challenge with the hardly obtained hydraulic selection pressure. A novel method was investigated for cultivating granular sludge in an oxidation ditch by recycling the residual sludge after extracting slime-extracellular polymer substances (slime-EPS). Compared to the raw sludge, bioactivities of the residual sludge slightly decreased based on endogenous, autotrophic, and heterotrophic respiration rates (SOURe, SOURA, and SOURH). When the slime-EPS extracted sludge was returned to the main reactor, the SOURe reduced and maintained stable later. For the first and second recycling, SOURA increased from 0.80 to 1.69 mgO2/gSS/h, and 0.16 to 1.21 mgO2/gSS/h, respectively, while SOURH exhibited initial decrease, subsequent increase and later stable trends. Moreover, total nitrogen in effluent decreased from 8.09 to 7.22 mg/L, and the protein-to-polysaccharide concentration ratio increased from 0.49 to 1.55. In addition, the mixed sludge of raw and residual sludge exhibited higher coaggregation capacity with decreased supernatant turbidity, triggering the granulation process. After 94-day operations, granules larger than 200 μm accounted for 32 % in the oxidation ditch, where neither hydraulic selection pressures nor feast-famine phases were provided. Therefore, returning the slime-EPS extracted sludge to the CFRs is a promising approach for aerobic granulation.