Biomass and porosity profiles in microbial granules used for aerobic wastewater treatment

Aims: To obtain biomass and porosity profiles for aerobically grown granules of different diameters and to determine a suitable range of granule diameters for application in wastewater treatment. Methods and Results: Microbial granules were cultivated in an aerobic granulated sludge reactor with model wastewaters containing acetate, or ethanol plus acetate, or glucose as the main carbon source. Granules were formed by retaining microbial aggregates using a settling time of 2 min. Sampled granules had diameters ranging from 0.45 to 3 mm. Microbial biomass in the granules was detected with the nucleic acid stain SYTO(R) 9 and confocal laser scanning microscopy. The thickness of the microbial biomass layer was proportional to the granule diameter, and had a maximum value of 0.8 mm. The thickness of the microbial biomass layer correlated with the penetration depth of 0.1 mum fluorescent beads into the granule. Conclusions: The microbial biomass and porosity studies suggest that aerobically grown microbial granules should have diameters less than a critical diameter of 0.5 mm, if deployed for wastewater treatment applications. This critical diameter is based on the assumption that whole granules should have a porous biomass-filled matrix. Significance and Impact of the Study: This work could contribute to the development of aerobic granulation technology for effective biological wastewater treatment.