Coupling physical selection with biological selection for the startup of a pilot-scale, continuous flow, aerobic granular sludge reactor without treatment interruption

Effects of Nitrate Recycle on the Sludge Densification in Plug-Flow Bioreactors Fed with Real Domestic Wastewater

“…But the hindrance might be compensated to some extent by the NRCY location change. Testing these two hypotheses will not only provide engineering solutions, but also shed light on the continuous-flow aerobic granulation mechanism by crosschecking with the previous understandings of the feast and famine selection roles [2,11,12].…”

“…To achieve aerobic granulation, hydraulic selection pressure has always been utilized as a driving force to selectively return better settling sludge to the feast zone or the phase of either plug-flow reactors (PFRs) or sequencing batch reactors (SBRs). This mode of operation selectively favors the coaggregation of microbial sludge into aerobic granules by washing out their diffuse flocculent sludge counterpart [1,2]. As a result, the transition from flocculent sludge to densified sludge often came at the cost of substantial sludge washout over the course of the reactor startup.…”

“…As a result, the transition from flocculent sludge to densified sludge often came at the cost of substantial sludge washout over the course of the reactor startup. This in turn directly resulted in the loss of treatment capacity until the granular sludge inventory within the reactor was slowly re-established [2,3]. However, in full-scale applications, WRRFs cannot withstand such a treatment interruption, even if it is temporary, and have to comply with discharge permit limits all the time.…”

“…For this reason, an alternative strategy should be developed to provide sufficient selection strength to promote successful continuous-flow sludge densification, while maintaining the startup treatment performance required for large full-scale applications. To this end, [2] demonstrated a new strategy by continuously returning the sludge in the underflow and overflow of a continuous upflow gravity selector to the feast and famine zones of a plug-flow reactor, respectively, which can effectively drive a smoother transition from flocculent-activated sludge to granular sludge, without dramatic sludge concentration and effluent quality decline during the reactor startup. However, this strategy was only proven successful in conventional-activated sludge pilots without biological nitrogen removal (BNR) [2,11].…”

“…To this end, [2] demonstrated a new strategy by continuously returning the sludge in the underflow and overflow of a continuous upflow gravity selector to the feast and famine zones of a plug-flow reactor, respectively, which can effectively drive a smoother transition from flocculent-activated sludge to granular sludge, without dramatic sludge concentration and effluent quality decline during the reactor startup. However, this strategy was only proven successful in conventional-activated sludge pilots without biological nitrogen removal (BNR) [2,11]. Its application in a modified Ludzack-Ettinger (MLE) configuration with Nitrate Recycle (NRCY) has yet to be explored.…”