Civil Engineering Reference
In-Depth Information
CHAPTER 9
Aeration and Air Stripping
INTRODUCTION
Water treatment facilities have come to employ a wide variety of gas transfer processes,
although the basic theory of these processes has remained relatively unchanged since
the 1920s. Historically, aeration has been used for taste and odor control and to remove
excess carbon dioxide from water. Since the 1970s, it has also been used for the
removal of radon and volatile organic compounds (VOCs). In December 1997, the
United States Environmental Protection Agency (USEPA) released a drinking water
advisory for methyl tertiary butyl ether (MTBE), a volatile gasoline oxygenate, after
this compound was detected in numerous water supplies throughout the United States.
In October 1999, the USEPA released the proposed drinking water standard for radon,
which cited high-performance aeration as the proposed best available technology for
radon removal. These recent developments will likely spur additional interest in the
use of aeration for drinking water treatment.
This chapter is organized into three parts: The first section discusses the funda-
mentals that govern all aeration processes; the second part covers the different types
of unit processes used for aeration, as well as the associated design considerations;
and the third part includes specific applications of aeration in water treatment.
GAS TRANSFER FUNDAMENTALS
Aeration in water treatment depends upon two basic principles: equilibrium and mass
transfer kinetics. In the case of removing VOCs, the water is initially supersaturated
with the contaminant gas of concern, and the gas will tend to diffuse from the water.
In other cases, such as the addition of oxygen to groundwater, the water is initially
undersaturated with a desired gas and the goal is to increase the concentration of that
gas in the finished water. In both cases, the limit to how much gas can be transferred
is defined by the equilibrium between the gas in solution and in the vapor phase. The
rate at which this transfer occurs is governed by kinetics.
Equilibrium
Equilibrium conditions define the limits of the gas transfer process. Aeration drives a
system toward equilibrium regardless of whether water is undersaturated or oversatur-
ated with a gas. Equilibrium is affected by the properties of the gas, the temperature,
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