TM 5-814-3/AFM 88-11, Volume III
(2) Aerated ponds. An aerated pond is similar to an oxidation pond except that it is deeper and
mechanical aeration devices are used to transfer oxygen into the wastewater. The aeration devices also mix
the wastewater and bacteria. Figure 13-7 illustrates various aerators which can be used in aerated ponds. The
main advantage of aerated ponds is that they require less area than oxidation ponds. The disadvantage is that
the mechanical aeration devices require maintenance and use energy. Aerated ponds can be further classified
as either complete-mix or partial-mix systems. A complete-mix pond has enough mixing energy (horsepower)
input to keep all of the bacterial solids in the pond in suspension. On the other hand, a partial-mix pond
contains a lesser amount of horsepower which is sufficient only to provide the oxygen required to oxidize
the biochemical oxygen demand entering the pond.
b. Aerobic-anaerobic (facultative) ponds. Three zones exist in an aerobic-anaerobic pond. They are the
following:
(1) A surface zone where aerobic bacteria and algae exist in a symbiotic relationship;
(2) An anaerobic bottom zone in which accumulated solids are actively decomposed by anaerobic
bacteria;
and
(3) An intermediate zone that is partly aerobic and partly anaerobic in which the decomposition of
organic wastes is carried out by facultative bacteria. Because of this, these ponds are often referred to as
facultative ponds. In these ponds, the suspended solids in the wastewater are allowed to settle to the bottom.
As a result, the presence of algae is not required. The maintenance of the aerobic zone serves to minimize
odor problems because many of the liquid and gaseous anaerobic decomposition products, carried to the
surface by mixing currents, are utilized by the aerobic organisms.
c. Controlled discharge ponds. Controlled discharge ponds have long hydraulic detention times and
effluent is discharged when receiving water quality will not be adversely affected by the discharge. Controlled
discharge ponds are designed to hold the wastewater until the effluent and receiving water quality are
compatible.
reduce the liquid volume at a rate equal to or greater than the influent accumulation. Favorable geologic or
climatic conditions are prerequisite.
14-3.
Design considerations.
a. Appurtenances. In general, the only appurtenances required for wastewater treatment ponds are flow
measurement devices, sampling systems, and pumps. Information regarding the selection and design of these
treatment system components may be found in chapter 18 of this manual.
b. Shallow aerobic ponds. Shallow aerobic ponds are limited to a depth of 6 to 18 inches so that light
can penetrate the pond to allow algae to grow throughout the pond. This type of pond produces large
amounts of algae which must be separated from the wastewater so that biochemical oxygen demand and
suspended solids effluent limitations can be met. The separation is typically performed by filtration. The
requirement for shallow construction means that this type of pond necessitates a very large amount of land.
This land requirement and the need to filter algae are such significant disadvantages that shallow aerobic
ponds are not recommended.
c. Aerated ponds.
(1) Complete-mix aerated ponds. Complete-mix aerated ponds are designed and operated as flow-
through ponds with or without solids recycle. Most systems are operated without solids recycle; however,
many systems are built with the option to recycle effluent and solids. Even though the recycle option may not
be exercised, it is desirable to include it in the design to provide for flexibility in the operation of the system.
If the solids are returned to the pond, the process becomes a modified activated sludge process. Solids in the
complete-mix aerated pond are kept suspended at all times. The effluent from the aeration tank will contain
from one-third to one-half the concentration of the influent biochemical oxygen demand in the form of solids.
These solids must be removed by settling before discharging the effluent. Settling is an integral part of the
aerated pond system. Either a settling basin or a quiescent portion of one of the cells separated by baffles is
used for solids removal. Seven factors are considered in the design of an aerated pond:
14-3
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