Effects of oxygen on the nitrogen transformations in a slow‐flowing open channel†

Abstract

This study investigated the effects of dissolved oxygen ( DO) concentration and flow conditions on the conversion rates and pathways of nitrogenous compounds in an pilot open channel. Ammonium sulfate and glucose were added externally as the substrates in the investigation. The experimental results showed that under conditions of constant flow velocity and DO concentration above 1 mg/1, the removal rates of total nitrogen (including nitrite, nitrate and ammonium nitrogens) varied linearly with the removal rates of ammonium nitrogen. Comparison of the removal rate for each individual nitrogenous component reveals the following order of removal rates: ammonium nitrogen>total nitrogen>nitrate nitrogen> nitrite nitrogen. Moreover, the nitrite concentration was found to increase in all tests and the nitrate concentration was increased only when the DO concentration was within the range of 3 to 5 mg/l. In contrast, when the DO concentration was less than 1 mg/l, all nitrogenous compounds were removed and the following order of removal rates of nitrogenous compounds was observed: total nitrogen>nitrate nitrogen>nitrite nitrogen>ammo‐nium nitrogen. The results indicated that the conversion pathways of nitrogenous compounds can be divided into four different types depending on the DO concentration level. The test results also revealed that an increase in the flow velocity significantly influenced the conversion rates of nitrogenous compounds. While a V‐shaped relation of the removal rates of total nitrogen vs the Reynolds numbers was observed for low DO below 1.5 mg/1, an inverted V‐shaped relation occurred at high DO above 5 mg/1.