Changes in process performance and microbial characteristics of retained sludge during low-temperature operation of an EGSB reactor

Abstract

A lab-scale expanded granular sludge bed (EGSB) reactor was seeded with granular sludge and operated to investigate the influence of temperature decrease on both process performance and the microbial community structure of the granular sludge. Synthetic wastewater containing sucrose and volatile fatty acids was used as feed. The EGSB reactor was brought online at a starting temperature of 15°C and was reduced stepwise to a final temperature of 5°C. The reactor exhibited sufficient COD removal efficiency between 10°C and 15°C. However at 5°C serious deterioration of process performance was observed. The methane-producing activity of the retained sludge increased when it was compared to the activity of the seed sludge (day 0) during 10°C to 15°C operation. When hydrogen fed, sludge showed much higher methanogenic activity as compared with seed sludge activity at test temperatures of 15°C and 20°C on day 196 of reactor operation. At this time, proliferation of the genus Methanospirillum in the retained sludge was observed and a decrease in Methanobacterium species was also measured. Throughout the experiment, the genus Methanosaeta was detected in abundance and the community structure of the Domain Bacteria was stably maintained. The sugar-degrading acid-forming bacteria, Lactococcus and Anaerovibrio were detected in the retained sludge throughout the experiment as well and the propionate-degrading acetogen Syntrophobacter fumaroxidans was also detected, although its population size decreased at 5°C.

Keywords:

• EGSB
• low-strength wastewater
• low temperature
• Methane fermentation
• Methanospirillum
• microbial structure

Acknowledgments

A part of this study was supported by New Energy and Industrial Technology Development Organization (NEDO). This study was also supported by NIES special-research program. We are grateful to Mr. Keita Nishiyama, Mr. Masashiro Okawara, Ms. Kaori Ooki and Ms. Yue Qin Fang for their technical assistance and to Dr. Robert A. Kanaly (Yokohama City University, Japan) for proof reading of the manuscript.