Research on the Self-Clean Capacity of Shallow Channel

• Main Authors: LEU, HORNG-GUANG, 呂鴻光
• Other Authors: C.F.Ouyang
• Format: Others
• Language: zh-TW
• Published: 1996
• Online Access: http://ndltd.ncl.edu.tw/handle/25548035956180805948

博士 === 國立中央大學 === 土木工程學系 === 84 === This study showed that varied flow conditions affected not only the significant mechanisms of pollutants(including, organic matter, suspended solids, nitrogen compounds) removal and deoxygenation but also the rate of pollutants and DO variations in a shallow channel. Two different flow conditions ( laminar and turbulent flow ) are utilized to describe the transportation of pollutants between the water column and biofilms. A series of well-controlled batch tests were carried out to investigate the transformations of pollutants in an artificial channel at varied flow conditions. Several conclusions relating to this study are noted as follows.The analytical solution for reaeration rate coefficient, K2 is directly proportional to the square root of mean velocity but was inversely proportional to flow depth and the square root of downstream distance. Measured K2 is better described by the analytical solution than by thefive semi-empirical formulae mentioned in the paper.In laminar flows, SS concentration variations with the test time are like a V-shaped curve; yet, SS concentration varied with the test time similar to an exponential curve at a turbulent flow condition. Changes in SS removal efficiency with increasing the Reynolds numbers is in this study like an inverse N-shaped curve. This indicatesthat turbulent flow, at a low Reynolds number regime, is the most effective flowcondition to remove SS from the water column, compared to that which was removedin either laminar flow or turbulent flow at a higher Reynolds number condition. Varied flow conditions influenced organic matter removal which shows that the deoxygenation rate coefficient (K1) of Streeter- Phelps equation maintained a constant value in a steady flow, but changes in K1 with increasing the Reynolds numbers is like a V-shaped curved. Therefore, Streeter-Phelps models should be modified in order to use them to predict BOD and DO concentration effectively in a shallow slow-flowing channel. When flow conditions are within a laminar flowregime (Re between 1050 and 2500), average organic matter removal rate (AORR) decreased with decreasing SS concentration during the first several hours of tests. SS concentration decreased and later abruptly increased. Sloughing of benthic biofilms occurred which increased suspended bacteria concentration and the rate of organic matter removal. Sedimentation and sloughing were the significant mechanisms of organic matter removal. Total deoxygenation rate remained almost constant except the first several hours of the tests. These mechanisms can be divided into 3 stages which are as follows. When Reynolds number is within 3200 to 5150 (turbulent flow), DO concentration increases to a high level (＞4mg/L); SS and BOD concentration is eliminated quickly to a lower level. Initial adsorption and later diffusion dominate the rate of BOD removal. A higher portion of soluble organic matter transported to the inner biofilm layer where metabolism was in either an anoxic or anaerobic state caused the deoxygenation rate to be less than the average organic matter removal rate. Total deoxygenation rate maintained a fairly constant low rate. During these conditions, diffusion controlled the rate of DO consumption.A particular type ofwater 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 increases when flow velocity increases; nitrate and ammonium nitrogen is converted slowly; organic nitrogen and total nitrogen concentration gradually decreases with the test time. When the flow condition transferred from laminar flow to turbulent flow, 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 compounds conversion and transport can be recognized when organic substances concentration and flow conditions are known.