Continuous-flow aerobic granulation in plug-flow bioreactors fed with real domestic wastewater

This pilot study was designed to explore the feasibility of achieving successful aerobic granulation in continuous flow infrastructure like that existing in modern wastewater treatment plants (WWTPs). Results demonstrated that aerobic granulation of activated sludge can be achieved in plug-flow reactors (PFRs) fed with primary effluent from a domestic WWTP with seasonal temperature variation between 10 and 22.5 °C. It took about 90 days during the reactor startup to reach a state of sustained aerobic granulation. The characteristics of aerobic granules formed were comparable to those measured in sequential batch reactors (SBRs). The feast-to-famine concentration profiles measured in the plug-flow pilot reactors were found to be in line with those present in the full-scale treatment trains, lending support to the feasibility of converting existing infrastructure to continuous flow aerobic granulation systems. A selection pressure based on settling velocity (Vs) was applied in a Vs selector to retain bioparticles with Vs greater than ~9–9.75 m h−1. It was theorized that an external Vs selection pressure would be necessary but would not be the sole condition sufficient to drive aerobic granulation. The alternating feast-to-famine internal selection provided by the PFRs is also believed to be a required condition to transform biomass from flocs toward dense and compact aerobic granules. While the pilot-scale Plug-flow Aerobic Granulation (PAG) reactor achieved similar COD and NH3 removal efficiencies as the full-scale WWTP treatment train, its effluent from Vs selector contained an average of 138 mg L−1 total suspended solids (TSS) as a result of the biomass ‘wash-out” by the Vs selection pressure. Pilot results suggest a second clarifier for polishing, in addition to the Vs selector, may be needed in a full-scale application of the technique unless other downstream processes (flocculation, sedimentation, filtration) are provided to reach final water quality goals.