DENITRIFICATION IN POROUS-MEDIA DURING RAPID, CONTINUOUS LEACHING OF SYNTHETIC WASTE-WATER AT SATURATED WATER-FLOW

Increasing interest is given to land treatment and especially rapid infiltration (RI) systems as a method to remove N in wastewater by denitrification. The purpose of this article is to investigate denitrification in porous media at conditions corresponding to RI land treatment systems. Denitrification in six porous media columns (98% decomposed granite or sand and 2% field soil) during saturated leaching at constant flow rates of synthetic wastewater was investigated. Concentrations of NO3-N and methanol between 20 and 60 mg L-1 were applied and vertical concentration profiles of NO3-N, NO2-N, and NH4-N were measured for 45 d at 30 °C (three columns) and 10 °C (three columns). A time lag in denitrification of 10 d was found at 10 °C. Complete N removal was obtained after 3 d at 30 °C but was not obtained at high N-loading at 10 °C. A NO2-N leaching was observed between Day 1 and 3 at 30 °C. Assuming first-order denitrification at steady state, the corresponding first-order reaction rate coefficients (k1) for NO3 reduction in the columns were estimated to be between 1.4 and 5.0 h-1 at 30 °C and between 0.017 and 0.14 h-1 at 10 °C. The NO3-N concentration in the influent found to be the most optimal for denitrification at steady state was 20 mg N L-1 at 10 °C and 40 mg N L-1 at 30 °C. No significant differences in k1 values in columns containing decomposed granite and sand, respectively, were found. Steady-state NO3 profiles were obtained between 1.5 to 4.5 wk after the experiments were started.