Effect of sodium chloride on the denitrification of ion exchange resin eluates by a sulphur-oxidizing chemoautotrophic bacterium
The treatment and ultimate disposal of saline eluates from ionic exchange resins remain a badly worked out problem. New approaches are needed and biological denitrification seems to be very attractive since it may be implemented in the form of an inexpensively and easily used reactor. The aim of the present work was to better understand the bacterial denitrification process on sulphur by a chemoautotrophic bacterium, Thiobacillus denitrificans, in a saline medium, and to apply this knowledge to the treatment and/or the recycling of these eluates. Claus and Kutzner (1985) reported an inhibitory effect of NaCl starting at a concentration of 20 g/L. A column containing elemental sulphur was continually fed by a solution containing nitrate and nutrient salts. In the first step of the study, a relationship between the volumetric load (Cv) and the denitrification efficiency was established. For Cv < 2 kg NO3-/m3/j, the denitrification efficiency was greater than 85%, whereas it decreased rapidly to 50% for Cv 3,5 kg NO3- /m3/j. Stoichiometric ratios between reduced nitrates and formed/consumed products were determined. For example, the reduction of 1 mole of NO3- induces the formation of 0,98 mole SO42- and requires 0,90 moles HCO3-. The effect of the NaCl concentration was investigated for a volumetric charge of 1 NO3-/m3/j. An uncompetitive inhibition of denitrification by the sodium chloride present in the feed solution was observed. The denitrification efficiency reaches at least 90% for NaCl concentrations up to 30 g/L. The inhibition constant is about 40 g/l. A model linking the denitrification efficiency (r) to the volumetric loading and to the sodium chloride concentration in the regeneration water was developed, based on an examination of the biomass distribution on the fill material and the assumption of plug-flow within the column: r=1 - e[ - 143,5 ] (over) CV (41+I)