Prof. Christof Holliger, Dr. Sirous Ebrahimi, Sébastien Gabus, David Weissbrodt
Internal project, no specific funding
October 2005 – June 2007
Mr. A. Baumann, Ara Thunersee, Uetendorf
Aerobic granular sludge-based reactors represent an attractive alternative to conventional activated sludge systems due to their small footprint and excess sludge production. Earlier research has shown the importance of slow growing microorganisms such as polyphosphate- and/or glycogen- accumulating organisms (PAO/GAO) on the density and stability of aerobic granules. These organisms have normally been selected by application of a feast-famine regime in a SBR system. In other wastewater treatment systems it has been observed that at temperatures above 20°C, activities of the biological phosphorus removal process deteriorates and GAO become dominant microorganisms. As temperature increases to 35°C, both PAO and GAO disappeared and ordinary heterotrophic organisms (OHO) become predominant. The main purpose of the present study was therefore to investigate aerobic granulation at mesophilic temperatures (30-35°C) using either activated sludge as inoculum or granular sludge developed at 20°C.
The results obtained 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 showed that the microbial community structure present in the activated sludge used as inoculum was completely different from the microbial community present in the first granules. The granules were first dominated by a Zoogloea-like population. The size of this population decreased and disappeared at 20°C but became predominant again in the reactor operated at 35°C. Sphingomonas-like populations were present under all temperature conditions and accounted for 10-40% of the total community. A population that affiliated with a clone isolated in a study on GAOs in laboratory- and full-scale plants appeared after 6 weeks of reactor operation and accounted for approximately 30% of the total community at 20°C and for approximately 50% at 30-35°C. A Rhodocyclus-like population also appeared after 6 weeks of reactor operation at 20 as well as 30°C. It accounted for 20-30% of the total community at 20°C but disappeared at 35°C. A Nitrosomonas-like population appeared at 30°C after 6 weeks of reactor operation and was absent at 20°C which in agreement with the nitrification observed at 30°C and its absence at 20°C.