Effects of flow velocity changes on nitrogen transport and conversion in an open channel flow

This study demonstrates that short-term changes in flow velocity affect nitrogenous compoundsʹ transport and conversion in a shallow, slow-flowing channel. Two different flow conditions controlled by varied flow velocities, including laminar and turbulent flow, are proposed to describe soluble and particulate nitrogenous compounds transported between the water column and biofilms. The incipient turbulent flow was suggested to design a flowing channel which will induce a high rate of nitrification and a low rate of resuspension. A series of well-controlled batch tests were carried out to investigate nitrogenous compound transformations in an artificial channel at varied flow velocities. The results of the batch tests show that a particular type of water motion will control the fate of nitrogen, while organic matter concentration is low in the water column. When laminar flow occurs in the flowing channel, total kjeldahl nitrogen (TKN) removal rate is held approximately constant; oxidized nitrogen generation rate increased when flow velocity increased; nitrate and ammonium nitrogen converted slowly; organic nitrogen and total nitrogen concentration gradually decreased with the test time. In contrast, when the flow condition transferred from laminar flow to turbulent flow, the TKN removal rate decreased when flow velocity increased; the oxidized nitrogen generation rate increased more than it did during the laminar flow, and it maintained a constant rate when flow velocity became higher; nitrate concentration increased and ammonium nitrogen concentration decreased quickly; total nitrogen and organic nitrogen concentration gradually decreased and later increased. The primary pathways of nitrogenous compoundsʹ conversion and transport can be recognized when organic substance concentrations and flow conditions are known.