Influence of temperature on aerobic granular sludge formation and stability treating municipal wastewater with high nitrogen loadings

This study investigated the influence of temperature (20 and 30 degrees C) on the formation and stability of aerobic granules in sequential batch reactors (SBR). Therefore, two lab-scale SBRs operated at 20 and 30 degrees C (SBR20 and SBR30) were used. The reactors were fed with municipal wastewater (CODt:TN:TP 100:15:1.7), leading to mean organic loading rates (OLR) of 1.3 +/- 0.4 kgCODt m- 3 day-1. Both reactors had the same height/diameter ratio of 4.2 and were inoculated with activated sludge from a municipal wastewater treatment plant. The operational conditions were also the same for both temperatures and lasted in stable process parameters for over 100 days. By optimizing the aeration and oxygen concentration, a high removal efficiency of NH4-N (-99%) and COD (-90%) was achieved in both reactors, despite the poor C:N:P ratio at the influent. Furthermore, a relatively low oxygen concentration of 2 mg L-1 was defined as the set point for the control strategy. Nevertheless, granulation at 30 degrees C was significantly faster, resulting in more stable sludge volume index (SVI) values (SVI10/SVI30 < 1.1). The granules formed at 30 C were also larger, more compact, and considerably more stable against system disturbances. However, at higher temperatures, larger granules might be required for nitrate removal because of the increased oxygen diffusion rates. Finally, microbiological 16S rRNA gene amplicon analysis for both systems indicated major differences relatively to the inoculum sludge only for nitrogen-degrading organisms.

Automated summary

This study investigated the influence of temperature on the formation and stability of aerobic granules in sequential batch reactors. The results showed that granulation at 30 degrees C was faster and resulted in larger, more compact, and more stable granules. The operational conditions were the same for both temperatures and achieved high removal efficiency of NH4-N and COD.