Microbial community structure and population dynamics of granules developed in expanded granular sludge bed (EGSB) reactors for the anaerobic treatment of low-strength wastewater at low temperature

Abstract

The anaerobic biological treatment of sucrose-based, low-strength wastewater was investigated in expanded granular sludge bed (EGSB) reactors at low temperatures over a 300-day trial period. During the trial, the operating temperature was lowered in a stepwise manner from 20°C to 5°C. As a result, the reactors exhibited sufficient performances until 10°C operation. The COD removal rate was 3.1–3.8 kgCOD m− 3 day− 1 at 10°C. In particular, the COD removal rate increased gradually through the low-temperature operation; indeed, the later stages of the 10°C operation attained a rate similar to those achieved at 20°C and 15°C. This finding is especially practical for applications of psychrophilic methane fermentation. Additionally, the structure of the microbial community in the granular sludge was analyzed by clone analysis based on 16S rRNA genes and fluorescence in situ hybridization (FISH). As a result, the percentage of the phylum Firmicutes, which were assumed to be Anaerobivrio sp. and Lactococcus sp., greatly increased from 0.7% to 8.0% of the total cells, especially in the surface layer of the granular sludge. These bacteria would contribute to the degradation of the sucrose substrate anaerobically at ambient temperatures. Moreover, the results suggest that a Methanospirillum species, which is a H2-utilizing methanogen, increased from 0.5% to 6.7% during the low-temperature incubation, with a significant increase of methanogenic activity from H2/CO2 at 20°C. Thus, the Methanospirillum species detected in this study may have a key role as hydrogen scavenger during hydrogen-metabolism in low-temperature conditions.