Characteristic identification of extracellular polymeric substances and sludge flocs affected by microalgae in microalgal-bacteria aggregates treating wastewater

The microalgal-bacterial aggregates (MABAs) treating wastewater have gained more attention. However, the introduce effect of microalgae on extracellular polymeric substances (EPS) and sludge flocs is not well clarified, which presents great impact on the operation stability of MABAs. This study selected the algae-based sequencing batch suspended biofilm reactor (A-SBSBR, Ra) as model to achieve both suspended (i.e., sludge phase) and fixed (i.e., biofilm phase) MABAs. Results showed that compared to control SBSBR, although the content of LB-EPS in suspended MABAs of Ra was reduced by 15.23%, the content of total EPS was increased by 4.54%, besides, both the types and contents of functional groups were enriched. Microalgal-bacteria biofilms (MABBs, i.e., fixed MABAs) showed similar change trends. In addition, sludge flocs in Ra presented better settleability and smaller particles, with the decrease rate of sludge volume index (SVI) and particle size of 14.76% and 9.17%. Morphology analysis proved that microalgae cells were mainly absorbed on LB-EPS layer. And this adhesion process made LB-EPS easier to fall off and form more soluble microbial products, with the increase rate of 16.67%. Moreover, oxygen produced by microalgae photosynthesis was favorable for enhancing the activity of bacteria, and then the production of EPSs was improved. It is expected that the characteristic identification of EPSs and variation evaluation of sludge flocs can provide some new insights for MABAs treating wastewater.