Micro-scale observations of the structure of aerobic microbial granules used for the treatment of nutrient-rich industrial wastewater

The structure and function of aerobic microbial granules from a lab-scale sequencing batch reactor treating nutrient-rich abattoir wastewater were investigated. These wastewater-fed granules were examined using a wide range of micro-scale techniques including light microscopy, scanning and transmission electron microscopy, fluorescent in situ hybridisation (FISH) combined with confocal laser scanning microscopy and oxygen and pH microsensors, in conjunction with a range of measurements in the bulk liquid phase. Interesting structural features were observed in these granules that have not been reported in synthetic-fed granules. The complex nature of abattoir wastewater was suggested to be responsible for accelerating the breaking process of large mature granules due to a rapid clogging of the granules pores and channels and for the very diverse microbial community observed displaying specific spatial distribution throughout the granules. More importantly, the dissolution at lower pH of mineral complexes associated to the granule matrix of extracellular polymeric substances might have caused the structural damages observed on the granules even though some pH buffer capacity was observed inside these granules. Ciliate protozoa were found to be very abundant on the surface of these wastewater-fed granules, which could potentially assist with reducing the high levels of suspended solids usually present in the aerobic granular sludge effluent. All these observations provide support to future studies on aerobic granular sludge treating real wastewater especially with regard to the granule structure and the mechanisms involved in their formation.