TM 5-814-3/AFM 88-11, Volume III
f. Complete retention ponds. In areas of the U.S. where the moisture deficit, evaporation minus rainfall,
exceeds 30 inches annually, a complete retention wastewater pond may prove to be the most economical
method of disposal. Complete retention ponds must be sized to provide the necessary surface area to
evaporate the total annual wastewater volume plus the precipitation that would fall on the pond. The system
should be designed for the maximum wet year and minimum evaporation year of record if overflow is not
permissable under any circumstances. Less stringent design standards may be appropriate in situations where
occasional overflow is acceptable or an alternative disposal area is available under emergency circumstances.
Monthly evaporation and precipitation rates must be known to properly size the system. Complete retention
ponds usually require large land areas, and these areas are not productive once they have been committed
to this type of system. Land for this system must be naturally flat or be shaped to provide ponds that are
uniform in depth and have large surface areas.
14-4.
Disinfection.
Wastewater contains bacteria which can produce diseases in humans. Disinfection is the selective destruction
of these disease-causing organisms. Since chlorine, at present, is less expensive and offers more flexibility
than other means of disinfection, chlorination is the most practical method of disinfection. The chlorination
of pond effluents requires consideration of some wastewater characteristics which are unique to pond
effluents. A list of these considerations is presented below; additional information, design criteria and design
examples may be found in EPA Manual 625/1-83-015.
a. Sulfide. Sulfide, produced as a result of anaerobic conditions in the ponds during winter months when
the ponds are frozen over, exerts a significant chlorine demand. For sulfide concentrations of 1.0-1.8
milligrams per liter, a chlorine dose of 6-7 milligrams per liter is required to produce the same residual as a
chlorine dose of about 1 milligram per liter for conditions without sulfide.
b. Chemical oxygen demand (COD). Total chemical oxygen demand concentration in a pond effluent
is virtually unaffected by chlorination. Soluble oxygen demand, however, increases with increasing concentra-
tions of free chlorine. This increase is attributed to the oxidation of suspended solids by free chlorine.
c. Suspended solids. Some reduction in suspended solids, due to the breakdown and oxidation of
suspended particulates and resulting increases in turbidity, are attributed to chlorination. However, this
reduction is less than that resulting from settling. Suspended solids can be reduced by 10 to 50 percent from
settling in chlorine contact tanks.
d. Algae. Filtered pond effluent exerts a lower chlorine demand than unfiltered pond effluent due to the
removal of algae. Chlorine demand is directly related to chlorine dose and total chemical oxygen demand.
e. Temperature. Disinfection efficiency is temperature dependent. At colder temperatures, the reduction
in the rate of disinfection was partially offset by reductions in the exertion of chlorine demand; however, the
net effect was a reduction in the chlorine residual necessary to achieve adequate disinfection with increasing
temperature for a specific contact period.
f. Chlorine residual. Adequate disinfection can be obtained with combined chlorine residuals of between
0.5 and 1.0 milligrams per liter after a contact period of approximately 50 minutes, i.e., disinfection can be
achieved without discharging excessive concentrations of toxic chlorine residuals into receiving waters.
Parameters and pond design are discussed in detail in Baudy et al., 1986; Siegrist and Boyle, 1982;
Winneberger, 1976; and Yonika et al., 1978.
14-5
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