Fe(II)-mediated autotrophic denitrification with four different microbial cultures under different pH and EDTA/Fe(II) conditions was investigated in batch bioassays. Initially, the highest nitrate removal (72%) was achieved with an activated sludge inoculum. The use of pure cultures of Pseudogulbenkiania strain 2002 and Thiobacillus denitrificans resulted in a 55 and 52% nitrate removal, respectively. No denitrification was observed for a mixed culture dominated by Thiobacillus thioparus and T. denitrificans. A longer enrichment on Fe(II) and the supplementation of thiosulfate as additional electron donor were needed to stimulate the denitrifying activity of the Thiobacillus-mixed culture. A second subculture on Fe(II) as sole electron donor resulted in higher denitrification efficiencies for all microbial cultures. In particular, nitrate removal reached up to 84% with a specific nitrate removal rate of 1.160 mM·(g VSS·day)-1 in the bioassays seeded with the Thiobacillus-mixed culture. All cultures were favored by decreasing the EDTA/Fe(II) molar ratio from 2.0 to 0.5. The most significant denitrification enhancement was observed for the Pseudogulbenkiania species, indicating a lower tolerance to EDTA. The two pure cultures effectively maintained denitrification at pH 7.0 and were more sensitive to a pH decrease. Conversely, the optimal pH was 6.0 for the Thiobacillus-mixed and activated sludge cultures.
Keywords: Autotrophic denitrification; EDTA; Ferrous iron; Nitrate; Pseudogulbenkiania; Thiobacillus.