Hydraulic Control Of Ponds

In the past, the majority of ponds were designed to receive influent wastewater through a single pipe, usually located toward the center of the first cell in the system. Hydraulic and performance studies have shown that the center discharge point is not the most efficient method of introducing wastewater to a pond (Finney and Middlebrooks, 1980; Mangelson an dWatters, 1972). Multiple inlet arrangements are preferred even in small ponds (<0.5 ha [<1.2 ac]). The inlet points should be as far apart as possible, and the water should preferably be introduced by means of a long diffuser. The inlets and outlets should be placed so flow through the pond is uniform between successive inlets and outlets.

Single inlets can be used successfully if the inlet is located at the greatest distance possible from the outlet structure and is baffled, or the flow is otherwise directed to avoid currents and short-circuiting. Outlet structures should be designed for multiple-depth withdrawal, and all withdrawals should be a minimum of 0.3 m (1 ft) below the water surface to reduce the potential impact of algae and other surface detritus on effluent quality.

Analysis of the performance data from selected aerated and facultative ponds indicates that four cells in series are desirable to give the best BOD and fecal coliform removals for ponds designed as plug flow systems. Good performance can also be obtained with a smaller number of cells if baffles or dikes are used to optimize the hydraulic characteristics of the system.

Better treatment is obtained when the flow is guided more carefully through the pond. In addition to treatment efficiency, economics and esthetics play an important role in deciding whether or not baffling is desirable. In general, the more baffling is used, the better are the flow control and treatment efficiency. The lateral spacing and length of the baffle should be specified so the cross-sectional area of flow is as close to a constant as possible.

Wind generates a circulatory flow in bodies of water. To minimize short-circuiting due to wind, the pond inlet-outlet axis should be aligned perpendicular to the prevailing wind direction if possible. If this is not possible, baffling can be used to control wind-induced circulation to some extent. In a constant-depth pond, the surface current will be in the direction of the wind, and the return flow will be in the upwind direction along the bottom.

Ponds that are stratified because of temperature differences between the inflow and the pond contents tend to behave differently in winter and summer. In summer the inflow is generally colder than the pond, so it sinks to the pond bottom and flows toward the outlet. In the winter, the reverse is generally true, and the inflow rises to the surface and flows toward the outlet. A likely consequence is that the effective treatment volume of the pond is reduced to that of the stratified inflow layer (density current). The result can be a drastic decrease in detention time and an unacceptable level of treatment.

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