For a successful granulation process in activated sludge systems, the stimulation of slow growing organisms such as glycogen accumulating microorganisms (GAOs) is a key factor. Here we show that the introduction of an anaerobic feast followed by an aerobic famine phase successfully transforms bulking sludge, caused by the abundance of genus Kouleothrix, to a hybrid floccular-granular sludge. Two sequencing batch reactors (SBRs) were operated for 228 days treating the same industrial wastewater derived from the cleaning of trucks transporting liquid food (the cargo consists of approximately. 70% chocolate and 30% beer). By respectively applying a fast and slow feeding in two parallel SBRs, different degrees of substrate build-up were achieved in the two reactors during the feast phase. The F/M ratio over the feeding time was 1.41 ± 0.48 and 0.57 ± 0.16 kg COD·(kg VSS*d)−1 for the fast-fed and the slow-fed SBR SBR_fastF and SBR_slowF respectively. Our results demonstrate that substrate build-up during the anaerobic selection step is not necessary to obtain well settling granular-like sludge.
Increasing the substrate gradient in the bulk liquid by applying a faster anaerobic feeding regime did not lead to better granulation.
By only applying a feast (anaerobic)/ famine (aerobic) regime as selection pressure for granulation, a hybrid of floccular and granular sludge was achieved in the timeframe of 228 days while treating industrial wastewater derived from a tank truck cleaning site.