Monitoring the denitrification of wastewater containing high concentrations of nitrate with methanol in a sulfur-packed reactor

Abstract.

Biological denitrification of high nitrate-containing wastewater was examined in a sulfur-packed column using a smaller amount of methanol than required stoichiometrically for heterotrophic denitrification. In the absence of methanol, the observed nitrate removal efficiency was only about 40%, and remained at 400 mg NO₃ ^–-N/l, which was due to an alkalinity deficiency of the pH buffer and of CO₂ as a carbon source. Complete denitrification was achieved by adding approximately 1.4 g methanol/g nitrate-nitrogen (NO₃ ^–-N) to a sulfur-packed reactor. As the methanol concentration increased, the overall nitrate removal efficiency increased. As influent methanol concentrations increased from 285 to 570, 855, and 1,140 mg/l, the value of Δ mg alkalinity as CaCO₃ consumed/Δ mg NO₃ ^–-N removed increased from –1.94 to –0.84, 0.24, and 0.96, and Δ mg SO₄ ^2– produced/Δ mg NO₃ ^–-N removed decreased from 4.42 to 3.57, 2.58, and 1.26, respectively. These results imply the co-occurrence of simultaneous autotrophic and heterotrophic denitrification. Sulfur-utilizing autotrophic denitrification in the presence of a small amount of methanol is very effective at decreasing both sulfate production and alkalinity consumption. Most of methanol added was removed completely in the effluent. A small amount of nitrite accumulated in the mixotrophic column, which was less than 20 mg NO₂ ^– -N/l , while under heterotrophic denitrification conditions, nitrite accumulated steadily and increased to 60 mg NO₂ ^– -N/l with increasing column height.