Denitrification Kinetics of High-Strength Nitrate in Granular Sludge Reactors

Biological denitrification of high-strength nitrate is challenging due to building-up of toxic nitrite to high levels. In this study, denitrification kinetics of high-strength nitrate is investigated in a granular sludge reactor operated in sequencing batch mode. The nitrate concentrations are increased from 677, 1355, 2032, 2710, 3387, 4065-4742mgL(-1) NO3N in the reactor. Carbon and an electron source (acetate) are supplied at a fixed acetate-carbon to nitrate-nitrogen ratio of 1.5. At steady state, complete denitrification of 4742mgL(-1) NO3N in the feed is observed within the first 20h of sequencing batch reactor (SBR) cycle. High denitrification rate constants are observed for NO3N concentrations from 677 to 2032mgL(-1). However, a decrease in denitrification rate constants is observed for nitrate-N concentrations of 2710-4742mgL(-1), due to inhibition caused by transient accumulation of high levels of nitrite. Denitrifying biomass formed in the SBR is predominantly consisted of granular sludge, containing cocci, and rod-shaped microorganisms. Cultivation of nitrite-tolerant, denitrifying microbial community in granular sludge will help in the development of high rate reactors needed for process application.