Analysis of nitrate removal rate in a continuous column denitrification reactor using hydrogen generated by electrolysis

An autotrophic continuous denitrification process, using hydrogen generated by electrolysis with activated carbon anodes, were experimentally demonstrated to be an effective nitrate removal process. Several fixed bed columns with polypropylene packing and honey-comb shaped activated carbon anodes and stainless rod cathodes were set in thermostat chamber of 30 ℃, and then potassium nitrate enriched tap water, nitrogen concentration of 30 mg-N [',was supplied at various flow rates and electric currents (1.5~6 mA). Although the anode is in the same column where microbes grow, sufficient nitrate removal was observed. For example, 94 % of nitrate was removed at the HRT of as short as 1.8 h for an electrical current of 6 mA. A model assuming successive nitrate and nitrite reductions and plug-flow process, nitrate reduction rate = kı [NO3 ][H2], and nitrite reduction rate = k2 [NO2 ][H2] was constructed. Calculated results with k1=1.3 / mmol ! h1 and k2=2.6 / mmol"! h1 agreed well with the all the experimental results.
The effect of increasing electric current on the denitrification rate was investigated in the same columns run at 0.55 h and 1.1 h HRT. At the electric current of 20 mA, 76% of the nitrogen was removed for HRT of 0.55 hr and 91% for 1.1 hr. By fitting the results with model previously constructed: I'NO3 = k1 [NO3 ][H2], ['NO2= k2 [NO2 ][H2], we got values of k1=2.0, k2=2.4 for 0.55 hr HRT, and k1=0.9 and k2=1.5 for 1.1 hr HRT. Although the values of parameters k1 and k2 calculated from the previous and this studies are different, the data of the HRT needed for 80% removal of nitrogen fall on a straight line. The hydraulic retention time can be shortened by increasing the electric current, but hydrogen utilization efficiency drops.
Nitrate removal by continuous column reactors using hydrogen
generated by electrolysis is proved to be an effective method. Results show that the necessary HRT and the hydrogen efficiency is a trade-off. For commercial use, we have to consider the trade-off between the HRT and hydrogen efficiency, and also, a low cost, easily exchangeable carbon anode are required to be developed.