INVESTIGATION OF SOLID-PHASE BUFFERS FOR SULFUR-OXIDIZING AUTOTROPHIC DENITRIFICATION

We present evidence of high-rate denitrification using autotrophic microorganisms that use elemental sulfur as an electron donor. In this process, for each gram of NO3−-N removed, approximately 0.64 g cells and 7.5 g of SO42− are generated and 4.5 g of alkalinity (as CaCO3) are consumed. Since denitrification is severely inhibited below pH 5.5 and alkalinity present in the influent wastewaters is less than the alkalinity consumed, an external buffer is needed to arrest any drop in pH from alkalinity consumption. A packed-bed bioreactor configuration is ideally suited to handle variations in flow and nitrate load and requires minimal maintenance; therefore, a solid-phase buffer packed with the elemental sulfur in the bioreactor is most suitable. In this research, marble chips, limestone and crushed oyster shells were tested as solid-phase buffers. Bench-scale experimental data and field-scale studies indicate that crushed oyster shell was the most suitable buffer based on (i) rate of dissolution of buffer and rate of buffering agent (carbonate, bicarbonate or hydroxide and) released, (ii) ability of the buffer surface to act as host for microbial attachment, (iii) turbidity of the solution upon release of the buffering agent, and (iv) economics.