Autotrophic denitrification by Thiobacillus denitrificans in a packed bed reactor

Summary An upflow packed bed reactor with lava stones as support for the microbial growth proved to be very useful for the denitrification of industrial waste water by Thiobacillus denitrificans. The application of the plug flow principle allowed higher concentrations of nitrate to be employed than in a stirred tank reactor because inhibitory concentrations of sulfate from thiosulfate oxidation built up only in the upper part of the column — if at all. In experiments with synthetic media nitrate solutions of different strength (NO ₃ ^– g/l: 1.8; 3.0; 4.3; 6.1) were tested, each at 5 different residence times (5; 3.3; 2.5; 2.0; 1.7 h). The combination of the two parameters which still allowed 95% denitrification was 3 g NO ₃ ^- /l and 2.5 h residence time; this corresponded to a volumetric nitrate loading of about 25 kg/m³·d. Higher nitrate loadings led to incomplete denitrification coupled with the occurence of nitrite in the outflow. Below the critical loading rate nitrite accumulated only in the lower part of the column and was then gradually reduced. Experiments with simulated middle active waste from processing nuclear fuel which contained numerous heavy metals yielded similar results. — Although pure inorganic media were fed into the reactor the microflora developing as a dense layer covering the lava stones consisted not only of T. denitrificans but also of heterotrophic denitrifiers, mainly Pseudomonas aeruginosa.