Degradation of extracellular polymeric substances (EPS) extracted from activated sludge by low-concentration ozonation

• Degradation of EPS by ozonation was studied to understand activated sludge property.
• Ozone reacts with protein in EPS by attacking amino acid residue on polypeptide chain.
• The polysaccharides/protein molecular weights decreased significantly after ozonation.

Reaction mechanisms between ozone and extracellular polymeric substances (EPS) can be the key of understanding the improvements in microbial aggregates properties by low-concentration ozonation. In this study, EPS are extracted from activated sludge and treated continuously by ozone gas at 270 ± 41 ppm. The reaction between ozone and EPS was investigated by observation of EPS component concentrations, functional groups and molecular weight distributions using UV–Vis spectrometry, excitation-emission matrix fluorescence spectroscopy (EEM), high performance size-exclusion chromatography (HPSEC) and gas chromatography-mass spectrometry (GC–MS). In a 12-hour-ozonation experiment, significant ozone consumption was observed in the first 4 h and protein concentration in EPS solution was reduced by 30 ± 12%. However, the polysaccharides concentration only had a slightly decrease at the end of the ozonation process. UV–Vis spectra and EEM spectra results suggest that ozone removed protein and fluorescent matters (SMP and tryptophan-like aromatic protein) rapidly by attacking specific amino acid residues on polypeptide chain. After ozonation, the molecular weight of polysaccharide and protein dropped by 4 orders of magnitude according to HPSEC results. TOC concentration of EPS solution was reduced by 13 ± 2% after ozonation. The loss in TOC could be explained by the observation of volatile organic compounds such as carboxylic acids, aldehydes and ketones in the off-gas detected by GC–MS. The results in this study can provide a better understanding towards the mechanisms of improvements in activated sludge properties by ozonation.