Influence of C:N ratio on the start-up of up-flow anaerobic filter reactors

The long duration of the start-up period is a major drawback of the anaerobic wastewater treatment systems. A better understanding of the factors affecting biomass aggregation and adhesion, the two main mechanisms of biomass retention, could make the start-up more efficient and rapid. The objective of the present work is to determine the effect of the carbon:nitrogen ratio on the efficiency of biomass retention in upflow anaerobic filters. For this purpose, two substrates of two different C:N ratios (nitrogen limited, NL, and nitrogen balanced, NB) were considered. In fed-batch operation it was shown that a nitrogen limitation in the very first phase of reactor start-up seems to enhance appreciably biomass adhesion. A feeding strategy to upflow anaerobic filters, consisting on maintaining a N limitation during the first two weeks resulted in a more efficient biomass retention, with a higher specific methanogenic activity. By using this policy, the total methanogenic capacity attained after 4 weeks exceeded that obtained in the system fed with the balanced substrate from the beginning of the operation by 20%.