Deterioration mechanisms of sludge settleability in sludge reduction systems with metabolic uncouplers

The deterioration of sludge settleability is a common problem in sludge reduction systems with metabolic uncouplers (MUs). The deterioration mechanisms were studied by conducting laboratory-based tests for 85 days. The sludge volume index (SVI30) increased from 88.0 mL/g SS to 149.5–158.9 mL/g SS with MUs. The addition of MUs decreased the N-acyl-L-homoserine lactone (AHLs) content of the sludge by increasing the AHL-degrading bacteria (Paenibacillus spp., Bacillus spp. and Microbacterium sp.) and decreasing or eliminating the AHL-producing bacteria (Paracoccus sp.). Meanwhile, the addition of MUs increased the extracellular polymeric substances (EPS) of sludge and decreased its protein-polysaccharide ratio (PN/PS). The decrease of AHL content and the changes of EPS resulted in the deterioration of sludge settleability in the MUs systems. The statistical analysis suggested the decrease of AHL content was more important than the change of sludge EPS on deteriorate the sludge settleability.