Wastewater denitrification process—the influence of methanol and kinetic analysis

Abstract

The aim of this work was to achieve rapid nitrate removal from synthetic wastewater (SW) without nitrite accumulation and to improve the economic effectiveness of the process. For that purpose, experiments were carried out to determine the influence of methanol on denitrification rate in batch assays and in the continuous-flow stirred cultures. Nitrate–N (200 mgNO 3 −–N/l) was reduced under anoxic conditions during approximately 4–6 h for the MeOH/NO 3 −–N ratio above 2.5. Nitrite concentration was elevated to the maximum of 1.2 mgNO 2 −–N/l and at the end of the tests nitrite concentrations were 0.06–0.1 mgNO 2 −–N/l. At lower MeOH/NO 3 −–N ratios the denitrification process stopped after exhaustion of methanol. The analysis of experimental results showed that denitrification was a zero-order reaction with respect to nitrate and a first-order reaction with respect to the biomass concentration (the first-order overall reaction). In the continuous denitrification process during 45 days the hydraulic retention time (HRT) was decreased from 62 to 28 h. Dissolved oxygen concentration fell from 5.50 to 0.40 mgO 2/l during the first 4 h, but over 3 days of continuous flow it increased to 2.5 mgO 2/l and remained at that level. Accumulation of nitrite ions in SW was similar to that in batch tests, but at an HRT of 28 h the nitrite concentration increased to 6 mgNO 2 −–N/l. Complete denitrification at 25 °C was achieved at nitrate and methanol loading rates of 4.35 mgNO 3 −–N/l h and 23 mgO 2/l h, respectively.