TRACER STUDIES OF A GRAVEL-BED WETLAND

Current rational design methods for subsurface flow wetlands idealise these systems as plug flow reactors. This assumption ignores the exist ence of longitudinal dispersion, short-circuiting, stagnant zones and so on. This paper reports on a series of three tracer experiments perf ormed on a gravel bed wetland with a design hydraulic retention time o f four days in order to study its hydraulic characteristics. The resul ts present the hydraulic retention time distributions. The first trace r run using a single point inlet source had a mean time of 2.71 days. The second and third had the influent distributed across their width u sing an inlet manifold and yielded mean retention times of 3.47 and 3. 41 days respectively. Further interpretation of these results indicate s that the current plug flow assumption used in design can result in o ver estimation of treatment efficiency. Copyright (C) 1996 IAWQ. Publi shed by Elsevier Science Ltd.