Aerobic Granular Sludge in Continuous-Flow Reactors

Abstract

The development of the aerobic granular sludge (AGS) technology for the retrofit of existing continuous-flow reactors (CFRs) of wastewater treatment plants (WWTPs) using conventional activated sludge (CAS) has garnered increasing interest over the past decade, following the worldwide adoption of AGS technology in sequencing batch reactors (SBRs). The better settleability of AGS compared to flocculent AS would allow for process intensification of existing WWTPs, without the difÏcult conversion of often relatively shallow CFRs to deeper AGS-SBRs. The desire to extend the lifespan of the relatively new assets at the Harnaschpolder WWTP (Den Hoorn, The Netherlands) led to the initiation of the HARKOS research project. The primary goals were to determine the conditions for AGS formation in continuous-flow activated sludge (CFAS) systems, and to optimize process configurations for increased biological and hydraulic treatment capacity using AGS. Spontaneous aerobic granulation in CAS systems designed with an anaerobic stage for enhanced biological phosphorus removal (EBPR) has been reported for several WWTPs. Efforts have been made to identify the common denominators between these installations, mostly regarding the anaerobic selector stage. The principles behind the design of anaerobic selectors to select for well-settling flocculent sludge are shared to some extent with the process conditions that select for AGS in SBRs. Significant correlations between the design of the anaerobic selector stage, the wastewater composition and the observed degree of spontaneous granulation have not yet been found. As outlined in Chapter 1, this thesis project aimed to further clarify the role of the anaerobic phase in SBRs in the formation of AGS. In turn, this would facilitate the translation of the required process conditions to CFAS systems for process intensification.....