Industrial wastewaters have turned out to be a significant problem during the last decades. The best alternative for maize processing wastewaters due to high organic content and slow biodegradability characteristics is anaerobic treatment, which is successfully used in the treatment of medium-high strength industrial wastewaters. This study investigates a full-scale anaerobic expanded granular sludge bed reactor treating a maize industry wastewater in terms of reactor operation, methanogenic activity of reactor sludge, changes in composition and numbers of microbial populations during a 5-month period. Three samples were taken in 2-months intervals when the reactor was re-operated after a shut-down of 2 months. Combination of denaturing gradient gel electrophoresis (DGGE), quantitative real time polymerase chain reactions (Q-PCR) and specific methanogenic activity tests were used to investigate composition, diversity and quantity of microbial community with activity of acetoclastic methanogens. During monitoring period, COD removal efficiencies and organic loading rates varied in ranges of 79–95% and 0.65–3.85 kg COD/m3.d, respectively. Potential methane production rates of the reactor sludges decreased 27% with time that is from 394 to 287 mL CH4/gVSS.d. Archaea, bacteria, and methanogens in the sludge samples were quantified by Q-PCR and the results showed that while amount of archaea was decreasing, quantity of methanogens increased during the monitoring time. The DGGE results of the anaerobic sludge revealed that the microbial diversity dramatically changed, particularly in the last sample. Among these, the differences in archaeal community were more remarkable. Although an average COD removal efficiency of 86%±8.2 which was quite a reasonably stable performance was maintained during the monitoring period, remarkable differences were observed on both acetoclastic methanogenic activity and the methanogenic community. The increase in the numbers of total methanogens despite the decrease of acetoclastic methanogenic activity might be an indicator of dominating hydrogenotrophic methanogenic pathway. Further, construction of clone library was necessary to identify the species and their changes within the all three samples.

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