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AERATION AND AIR STRIPPING b.13

75 to 150 gpm (4.73 to 9.46 L/s) at about 10 psi (69 kPa). Nozzle spacing in most in-
stallations is between every 2 and 12 ft (0.6 and 3.7 m). The area allocated to spray aer-
ation varies from 50 to 150 ft2/mgd [ 106 to 318 m2/(m3/s)] capacity, although much larger
areas have been used at some treatment facilities.
Because interior and exterior corrosion can be serious problems in aerator piping, cor-
rosion-resistant materials should be used wherever possible.
Spray aerators providing a high area-to-volume ratio are spectacular to see. They are
rarely housed, so ventilation presents no problem. Gas transfer between water drops and
air proceeds rapidly, and spray-type aerators usually have a relatively high efficiency. In
general, spray aerators remove more than 70% of dissolved carbon dioxide, and removals
as high as 90% have been documented. Disadvantages of spray aerators are principally
the relatively large space requirements, freezing problems in colder climates, short expo-
sure time between water and air, and high head requirements.

Design of Multiple-Tray Aerators
Multiple-tray aerators are generally constructed with three to nine trays and a spacing of
12 to 30 in. (30 to 76 cm) between trays. Space required for an aeration unit ranges from
about 25 to 75 ft2/mgd (2 to 6 m2/ML per day) capacity, with 50 ft2/mgd (4 m2/ML per
day) being about average. Water application rates range from roughly 20 to 30 gpm/ft 2
[17 to 20 (L/s)/m2], These aerators have excellent oxygen adsorption and carbon dioxide
removal capacities.

Ventilation Requirements. Tray aerators are, in many respects, analogous to cooling
towers, and the design is similar. Ventilation and water distribution must be carefully con-
sidered in connection with location and design.
Multiple-tray aerators are usually housed, particularly in colder climates. A good ex-
ample of an enclosed but well-ventilated installation is the Allen substation aerator at
Memphis, Tennessee. Aluminum scroll panels are used to promote good cross-ventila-
tion, and the roof is open except directly over the distributing trays. Carbon dioxide con-
centration in the source water exceeds 90 mg/L, and this aerator has consistently produced
a 90% or greater reduction.
If a tray aerator must be enclosed and there is not sufficient natural ventilation, artifi-
cial ventilation must be provided. This is usually accomplished by supplying air with a
blower at the bottom of the aerator so that it travels counter to water flow.
Important design considerations in designing tray aerators are the use of corrosion-
resistant materials and methods of dealing with slime and algal growths. Aeration units
are generally constructed using concrete, stainless steel, aluminum, and rot-resistant wood.
Slime and algal growths may be controlled by treating the source water with chlorine or
copper sulfate.

Carbon Dioxide Removal. Carbon dioxide removal by multiple-tray aerators can be ap-
proximated by the following empirical equation, developed by Scott (1955):
Cn = CclO -kn
where Cn = concentration of carbon dioxide after passing through n trays, mg/L
Cc = concentration determined originally in distribution tray
n = number of trays including distribution tray
k = coefficient dependent on ventilation, temperature, turbulence, and other
characteristics of installation; generally ranges from 0.12 to 0.16