The thermal stratification that occurs in most reservoirs isolates the hypolimnion from circulation
and oxygen absorption at the surface. Because of hypolimnetic oxygen degradation, the hypolimnion
may become anoxic or anaerobic, and allow oxidized elements and compounds to be reduced into
solution. These constituents, such as iron, manganese, and hydrogen sulfide, are toxic to aquatic life
and, along with the low or zero oxygen content, may pose an in-reservoir or downstream water quality
An aeration system can improve the low dissolved oxygen (DO) content of a reservoir or lake
hypolimnion by directly introducing air or molecular oxygen into the water column. These systems
usually consist of an oxygen or air source, supply lines, and a diffuser system to generate bubbles. An
aeration system can be an open, free bubble column in the reservoir or confined within a floating or
submerged system. The selection of the aeration system type depends upon the objectives and
constraints under which the system will operate.
An aeration system may destratify the reservoir by mixing the hypolimnion and epilimnion. This
technique may eliminate the anoxic conditions, but will also warm the hypolimnetic water to a
temperature near that of the epilimnion. If cool water is needed for a cold-water fishery, then the
thermal stratification has to be maintained. Thus, the hypolimnion must be aerated without impacting the
thermal stratification.
4.5.2 THEORY
Hypolimnetic aeration/oxygenation can be accomplished through a variety of approaches,
ranging from pumping the hypolimnetic water to the surface for aeration and returning it to the
hypolimnion, to fine-pore pneumatic diffusers placed in the hypolimnion for the introduction of gaseous
oxygen (Figure 4.5.1). These systems use either air or liquid oxygen as the oxygen source. The
volume of oxygen required for reaeration of the hypolimnion is based on the volume of the hypolimnion
and the hypolimnetic oxygen degradation rate. Pneumatic diffuser systems are designed such that the
rising bubble plume does not mix hypolimnetic water into the epilimnion and thus begin destratification.
Systems that pump hypolimnetic water to the surface for reaeration are designed to minimize the
velocity of the return flow so that mixing of the return flow does not induce destratification.


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