Investigating the influences of quorum quenching and nutrient conditions on activated sludge flocs at a short-time scale

Quorum sensing signals regulate various functions within activated sludge processes such as formationof microbial aggregates. Disturbance of this signaling system, known as quorum quenching (QQ), providesopportunities for eliminating some problems related to biological wastewater treatment (e.g.,biofouling and excess sludge production). However, it is poorly understood how and to what extent QQsystems can affect the microbial aggregation processes and the following floc formation. In particular, anin-depth structural characterization at the scale of microbial aggregate while considering nutrientconditions in the reactor is still largely disregarded. Here, we evaluated the QQ effects at the short-termtime scale (i.e., after 4 h for the exogenous period and 19 h for exogenous/endogenous period), bycombining advanced techniques for microbial characterization (flow cytometry, CARD-FISH, and confocallaser scanning microscopy) and conventional physical-chemical assessments. The results indicated thatby implementing QQ agents (immobilized Acylase I enzyme in porous alginate beads) the abundance ofsingle cells and suspended microbial aggregates in the supernatant did not show significant changesduring the exogenous period. Conversely, at the end of the exogenous/endogenous period a significantincrease of single prokaryotic cells, small and large microbial aggregates favored the growth of grazers,including free-living nanoflagellates and ciliates. Flocs became looser and thinner than those in thecontrol reactor, thus affecting the sludge settling behavior. Inability of microbial community in degradationof soluble protein during the endogenous period confirmed that the QQ agents are likely to inhibitthe secretion of protease enzyme within microbial communities of activated sludge.