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 4.5.3 DESIGN METHODOLOGY
The design of an aeration or oxygenation system should begin with a clear definition of the
reservoir thermal and dissolved oxygen stratification characteristics. If the hypolimnetic DO
concentration drops below an acceptable level relative to the project objectives, then the
aeration/oxygenation system would be activated to maintain in-reservoir or release oxygen
concentrations above prescribed minimums. The general methodology described below is adapted
from Lorenzen and Fast (1977). Modifications have been made to include developments since 1977.
a. Determine the hypolimnetic volume. Using historical thermal profiles during the summer
months, the highest elevation of the hypolimnion must be determined. Then, using a depth-volume
curve, the volume of water residing in the hypolimnion can be determined.
b. Estimate the hypolimnetic oxygen degradation (HOD) rate. Once thermal stratification has
begun, the hypolimnetic DO content is measured periodically until the bottom DO concentration
reaches 1.0 mg/L. The DO for each sample period is plotted against time with the slope of the resulting
curve representing the HOD. The empirical model PROFILE (Walker 1987) can be used to compute
the HOD and provide an analysis of various thermocline elevations on the HOD. The HOD rate is the
minimum rate at which oxygen can be supplied to maintain aerobic conditions in the hypolimnion. The
determination of the rate of oxygenation that the aeration system should be designed for should be
made based on the desired DO concentration to be maintained in the hypolimnion.
c. Determine the type of system to be designed. Since the depth of the aeration system, as
well as the gas flow rate, will impact the system efficiency, specific design criteria will be dependent on
the type of system selected. One system may inject oxygen for release from the reservoir, while a
different system might diffuse oxygen for in-reservoir improvement. Some manufacturers of aeration
devices supply the air/oxygen delivery rate for their products and should be contacted for gas delivery
rates and oxygen absorption efficiency. But the latest design technology uses materials not usually
associated with air/oxygen injection.
4.5.4 IMPLEMENTATION
Fast and Lorenzen (1976) compiled an overview of hypolimnetic aeration system designs and
experiences. They describe the three basic designs: mechanical agitation, oxygen injection, and air
injection. Fast, Lorenzen, and Glenn (1976) also provided comparative costs for various hypolimnetic
aeration devices and concluded that each approach must be evaluated relative to the aeration site to
determine construction and operation costs. Pastorok, Lorenzen, and Ginn (1982) provide a review of
hypolimnetic aeration and oxygenation experiences from 15 case studies along with theoretical aspects
and impacts on various components in a lake. The Tennessee Valley Authority conducted tests of an
oxygen injection system at Fort Patrick Henry Dam to determine an optimum configuration to improve
hydropower releases (Fain 1978). Various types of diffusers and configurations were tested, achieving
4.5-4
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