Forming densified activated sludge in a modified simultaneous nitrification–denitrification and phosphorus removal (SNDPR) system by introducing shift work mode

Abstract

Forming granular-like sludge in the Simultaneous Nitrification-Denitrification and Phosphorus Removal (SNDPR) process would promote nutrient removal and improve the sludge settleability. This study introduced the “shift work mode” into an SNDPR system, which featured that 50 % of sludge was temporarily transferred into and “rested” in a side-stream reactor a cyclic time rather than let it keep “working” in the mainstream SNDPR reactor. Results showed that 77.6 % of inorganic nitrogen and 80.6 % of orthophosphate were removed well even after the combination, which attributed to the co-work of Candidatus_Competibacter (10.1 %–29.0 %), nitrifiers, and phosphate accumulating organisms. The 30-min settled sludge volume and sludge volume index were 8 % and 45.8 mL/g, respectively, the sludge density was 1.045 g/mL, and cumulative 90 % of the particle diameter was less than 80 µm; these indicated the densified activated sludge was formed. After analyses, the increase in the food-to-microorganisms ratio and temporary enrichment of filamentous bacteria Thiothrix might be the causes. Such shift work mode promisingly serves as a simple and practical way to improve settleability and promote granulation.