TM 5-814-3/AFM 88-11, Volume III
13-3. Closed-loop reactor design criteria.
a. General. Table 13-1 presents the design criteria to be used for the design of a closed-loop reactor
b. Aeration tank design. All oxidation ditch plants use looped channels or ditches. A looped channel
with a partition in the middle may be shaped like an oval or a concentric ring. The design engineer may adopt
a specific channel configuration and flow scheme recommended by the equipment manufacturer or supplier.
(1) Channel depth. The number of loops and the channel depth are dependent upon the size of the plant.
A shallow channel, less than 14 feet deep, is used for smaller plants with unlimited land area available. A deep
channel, greater than 20 feet, should be designed for larger plants or to conserve heat.
(2) Number of channels. Multiple-channel or multiple-loop is the preferable design so that part of the
plant can be shut down for repair and maintenance.
(3) Drainage. A drain should be provided for each channel. This provision allows mixed liquor or
accumulated grit material to be drained from the channel without expensive pumping. Many oxidation ditch
plants do not have drains in their channels and are having maintenance problems.
(4) Channel lining. Deep channels are to be built exclusively with reinforced concrete. A concrete liner
can be placed against the earth backing in shallow channels by pouring concrete or gunite (shotcrete) to a
thickness of 3 to 4 inches. The concrete or gunite should provide a minimum compressive strength of 3,000
pounds per square inch in 28 days.
c. Aeration. Depending on the width and depth of the channel, various types of aerators can meet the
oxygenation and mixing requirements.
(1) Rotor aerator. A rotor aerator is a horizontal shaft with protruding blades which rotates, thereby
transferring oxygen into the wastewater and propelling it around the ditch. Figure 13-6 illustrates a typical
horizontal-shaft aerator. The minimum length of shaft is 3 feet; the maximum length of shaft is 30 feet. This
type of aerator is suitable for shallow channels.