Conversion of Polymeric Substrates by Aerobic Granular Sludge

Abstract

Domestic wastewater is treated prior to its return to natural water bodies, to minimize its polluting effect. Biological wastewater treatment removes organic matter and nutrients from the wastewater, by employing the activity of microorganisms, which consume polluting compounds present in wastewater to grow. One of such technologies is aerobic granular sludge (AGS), which consists of self-immobilized microorganisms growing in spherical biofilms. The granular structure facilitates the separation between treated water and the biomass due to its excellent settling properties. This way, energy and space are saved in comparison to flocculent sludge-based treatment.
Despite its many advantages, the granular structure can pose some challenges too, particularly regarding the degradation of polymeric substrates. The higher mass-transfer resistance in granules compared to flocs challenges the degradation of these substrates, which have a size spanning from a few kDa to several micrometres. Polymeric substrates, furthermore, need to undergo hydrolysis before microorganisms can take them up, which is generally a slow process. Most AGS applications rely on microbial selection driven by the application of a sequencing batch reactor (SBR) cycle. The cycle consists of an anaerobic substrate feeding and a subsequent aerobic starvation period, which selects for intracellular polymer-storing organisms, such as polyphosphate accumulating organisms (PAO) and glycogen accumulating organisms (GAO). Substrates that experience high mass-transfer limitation and low degradation rates may interfere with the microbial selection strategy applied to AGS, especially when they are not (fully) taken up in the anaerobic feeding period and continue degrading aerobically in the next cycle phase. Some lab-scale studies have reported detrimental effects of polymeric substrates in AGS structure and activity, while others have managed to maintain a stable granule bed and suggest that the microbial utilization of polymeric substrates can contribute to good nutrient removal. The degradation of polymeric substrates by full-scale aerobic granules is still poorly understood.