Aerobic granular sludge formation and performance at 30 and 35°C in sequencing batch bubble-column reactors
Aerobic granular sludge-based reactors represent an attractive alternative to conventional activated sludge systems due to their small footprint and their low energy consumption and excess sludge production. The granules developed in such systems have high biomass concentration, good settling properties, high COD removal efficiencies and eventually high phosphorus removal capacity. In addition, and depending on bulk oxygen concentrations and granule size, both nitrification and denitrification can occur in the granules. Therefore sequencing batch reactors operated with granular sludge have the potential to achieve organic matter and nutrient removal in a single compact system. So far, aerobic granular sludge development has been studied at 20°C as well as at relatively low temperatures (e.g. 5 and 10°C). Compact wastewater treatment systems are especially interesting for industry. Since they often produce wastewaters with temperatures above 20°C, we investigated whether aerobic granular sludge could be developed at higher temperatures and if so, what the physical and metabolic properties of the sludge were. The formation and performance of aerobic granular sludge was studied in sequencing batch bubble-column reactors. Two start-up strategies were examined. Reactor 1 and 2 were inoculated with activated sludge from a municipal wastewater treatment plant and operated at 20°C and 30°C, respectively. After failure of Reactor 2, it was inoculated with aerobic granular sludge of Reactor 1 and operated with a stepwise temperature increase from 20 to 35°C. Granular sludge with good physical and metabolic properties was obtained in Reactor 1. Virtually all acetate was consumed during the anaerobic period with concomitant high phosphorus release suggesting the predominance of phosphate accumulating organisms (PAO). After the aerobic phase, an average of 63% of phosphate was removed. Nitrogen removal was not observed in this system, not even nitrification. In Reactor 2, a mixture of smooth granules and irregular structures was obtained. Complete COD removal was achieved, however, the sludge had low density (<40 g VSS/LBiomass) and poor settling properties with SVI8 values higher than 153 ml/gTSS. Over time, important biomass washout was observed resulting in a decrease of the reactor sludge bed. Due to the significant decrease in reactor biomass concentration, Reactor 2 was stopped after 67 days of operation. To restart Reactor 2, part of the sludge bed from Reactor 1 was used and the temperature was stepwise increased from 20 to 35°C. At 30°C, the granular sludge maintained good density and settling properties (SVI8, 27 ml/gTSS). However, despite the consumption of 60% of acetate during the anaerobic period, the phosphate removal capacity decreased to 8% suggesting that glycogen accumulating organisms (GAO) became abundant in the system. On the other hand, 85% of ammonium was nitrified. When the temperature was increased to 35°C only 25% of acetate was consumed during the anaerobic phase. There was no net phosphorus removal and nitrification was at 88%. This suggested that “ordinary” heterotrophic organisms (OHO) were the main active guild in the aerobic sludge and that neither PAO nor GAO were active. In conclusion, the results presented here showed that a sequential increase in temperature from 20 to 30 and 35°C permitted to obtain aerobic granular sludge with good density and settling properties at these higher temperatures when starting with granular sludge obtained at 20°C. The stepwise changes from 20 to 35°C resulted in a sequential shift in the dominant guild with PAO at 20°C, GAO at 30°C, and OHO at 35°C. Interestingly nitrification was only observed at 30°C and not at 20°C. A detailed molecular study of the microbial communities is in progress in order to understand the absence and presence of certain metabolic processes at the different temperatures.
Record created on 2011-01-06, modified on 2016-08-09