Identification of surfactants emerged in aerobic granulation

In this study, aerobic granules were cultivated in sequencing batch reactors with activated sludge as the seed. The reactors were operated for 12 h per cycle with the organic loading rate (OLR) increasing in double stepwise from 0.5 to 4.0 g COD L⁻¹d⁻¹. Within the 40 d running, black granules with regular and smooth morphology were cultivated, which had high wet density and high settling velocity. During the granulation process, foams emerged and disappeared in the reactor, coinciding with the proliferation of filamentous microorganisms in the granules, implying that surfactants might exist and play an important role in the granulation. Using ultra performance liquid chromatography-tandem mass spectrometry, the surfactants were identified as homologous compounds of polyethylene glycol (PEG) with molecular weights ranging mainly from 100 to 500 Da. Their general formulas were proposed as HO-[CH₂-CH₂-O](n)-H. The source of PEG still needs further investigation.