Leveraging feast and famine selection pressure during startup of continuous flow aerobic granulation systems to manage treatment performance

Hydraulic selection pressure that selectively washes out flocculent microbial sludge has been a conventional approach to drive flocculent systems towards aerobic granulation. Such substantial sludge washout inevitably compromises treatment performance during reactor startup, which is unacceptable in full-scale applications of aerobic granulation technology. This study explored the possible use of feast and famine conditions to increase selection pressure to drive aerobic granulation while avoiding substantial sludge washout during startup. Experimental results demonstrated that continuously returning the sludge in the underflow and overflow of a gravity selector to the feast and famine zones of a plug flow reactor, respectively, can effectively drive a smoother transition from flocculent activated sludge to granular sludge without dramatic sludge concentration and effluent quality decline during reactor startup. In comparison to a more conventional approach with only hydraulic selection pressure, adding the microbial sludge selection afforded through feast and famine conditions offers a novel startup strategy that can better maintain treated water quality by controlling system solids retention time. It is anticipated that the outcome from this study will provide further engineering guidance to promote full-scale applications of continuous flow aerobic granulation technology.