Microbiome structure and function in parallel full-scale aerobic granular sludge and activated sludge processes.

Aerobic granular sludge (AGS) and conventional activated sludge (CAS) are two different biological wastewater treatment processes. AGS consists of self-immobilised microorganisms that are transformed into spherical biofilms, whereas CAS has floccular sludge of lower density. In this study, we investigated the treatment performance and microbiome dynamics of two full-scale AGS reactors and a parallel CAS system at a municipal WWTP in Sweden. Both systems produced low effluent concentrations, with some fluctuations in phosphate and nitrate mainly due to variations in organic substrate availability. The microbial diversity was slightly higher in the AGS, with different dynamics in the microbiome over time. Seasonal periodicity was observed in both sludge types, with a larger shift in the CAS microbiome compared to the AGS. Groups important for reactor function, such as ammonia-oxidising bacteria (AOB), nitrite-oxidising bacteria (NOB), polyphosphate-accumulating organisms (PAOs) and glycogen-accumulating organisms (GAOs), followed similar trends in both systems, with higher relative abundances of PAOs and GAOs in the AGS. However, microbial composition and dynamics differed between the two systems at the genus level. For instance, among PAOs, Tetrasphaera was more prevalent in the AGS, while Dechloromonas was more common in the CAS. Among NOB, Ca. Nitrotoga had a higher relative abundance in the AGS, while Nitrospira was the main nitrifier in the CAS. Furthermore, network analysis revealed the clustering of the various genera within the guilds to modules with different temporal patterns, suggesting functional redundancy in both AGS and CAS. KEY POINTS: • Microbial community succession in parallel full-scale aerobic granular sludge (AGS) and conventional activated sludge (CAS) processes. • Higher periodicity in microbial community structure in CAS compared to in AGS. • Similar functional groups between AGS and CAS but different composition and dynamics at genus level.

Case study of aerobic granular sludge and activated sludge-Energy usage, footprint, and nutrient removal.

This study demonstrates a comparison of energy usage, land footprint, and volumetric requirements of municipal wastewater treatment with aerobic granular sludge (AGS) and conventional activated sludge (CAS) at a full-scale wastewater treatment plant characterized by large fluctuations in nutrient loadings and temperature. The concentration of organic matter in the influent to the AGS was increased by means of hydrolysis and bypassing the pre-settler. Both treatment lines produced effluent concentrations below 5 mg BOD7 L-1 , 10 mg TN L-1 , and 1 mg TP L-1 , by enhanced biological nitrogen- and phosphorus removal. In this case study, the averages of volumetric energy usage over 1 year were 0.22 ± 0.08 and 0.26 ± 0.07 kWh m-3 for the AGS and CAS, respectively. A larger difference was observed for the energy usage per reduced population equivalents (P.E.), which was on average 0.19 ± 0.08 kWh P.E.-1 for the AGS and 0.30 ± 0.08 kWh P.E.-1 for the CAS. However, both processes had the potential for decreased energy usage. Over 1 year, both processes showed similar fluctuations in energy usage, related to variations in loading, temperature, and DO. The AGS had a lower specific area, 0.3 m2  m-3 d-1 , compared to 0.6 m2  m-3 d-1 of the CAS, and also a lower specific volume, 1.3 m3  m-3 d-1 compared to 2.0 m3  m-3 d-1 . This study confirms that AGS at full-scale can be compact and still have comparable energy usage as CAS. PRACTITIONER POINTS: Full-scale case study comparison of aerobic granular sludge (AGS) and conventional activated sludge (CAS), operated in parallel. AGS had 50 % lower footprint compared to CAS. Energy usage was lower in the AGS, but both processes had potential to improve the energy usage efficiency. Both processes showed low average effluent concentrations.

A comparison of aerobic granular sludge with conventional and compact biological treatment technologies.

The aerobic granular sludge (AGS) technology is growing towards becoming a mature option for new municipal wastewater treatment plants and capacity extensions. A process based on AGS was compared to conventional activated sludge processes (with and without enhanced biological phosphorus removal), an integrated fixed-film activated sludge (IFAS) process and a membrane bioreactor (MBR) by estimating the land area demand (footprint), electricity demand and chemicals' consumption. The process alternatives compared included pre-settling, sludge digestion and necessary post-treatment to achieve effluent concentrations of 8 mg/L nitrogen and 0.2 mg/L phosphorus at 7°C. The alternative based on AGS was estimated to have a 40-50% smaller footprint and 23% less electricity requirement than conventional activated sludge. In relation to the other compact treatment options IFAS and MBR, the AGS process had an estimated electricity usage that was 35-70% lower. This suggests a favourable potential for processes based on AGS although more available experience of AGS operation and performance at full scale is desired.