TM 5-814-3/AFM 88-11, Volume III
(4) Diffused aeration plus slow mixer. This type of aeration is more suitable for deep channels. Air
bubbles are introduced into the mixed liquor through a pipe grid system with diffusers to provide oxygenation
while a slow propeller mixer provides the flow circulation and mixing.
(5) Aerator sizing. Aerators should be sized to provide adequate mixing and oxygenation. However,
the same size rotor provides different levels of mixing and oxygenation depending on the degree of its
submergence. First, the oxygen requirement must be calculated for a level that will satisfy the carbonaceous
biochemical oxygen demand removal as well as nitrification-denitrification (if needed). Oxidation ditch
equipment manufacturers provide tables or charts for selecting the aerator size for any given speed and
submergence (immersion) based on the calculated oxygen requirement. The aerator size should also be
checked against the mixing requirement set by the manufacturers. Preferably, more than one aerator should
be used per channel; they should be placed at different locations so that if one breaks down, the channel will
still function. The procedure for selecting the jet aerator size is similar except there is no submergence factor.
The sizing of the induction aerator and the diffused air plus slow mixer units is not precise. Design data for
these new aeration systems are not yet available. One reason for this is that the amount of energy required
for mixing relative to the energy required for oxygenation is uncertain since it depends a great deal on the
channel geometry, which varies among plants. More testing data must be collected before a design criterion
can be established.
d. Sludge dewatering and disposal. Sludge from oxidation ditch plants operating in the extended
aeration mode (sludge retention time of 20 to 30 days) can be wasted directly to open drying beds. It can also
be wasted directly to tank trucks which spread the liquid sludge on the plant grounds or on adjacent land.
The degree of sludge stabilization in the oxidation ditch is equivalent to that of a conventional activated
sludge plant operated at a 10-day sludge retention time followed by aerobic digestion of the sludge for 7 to
15 days. In most climates, 1.0 square foot of drying bed surface area per population equivalent (0.17 pound
biochemical oxygen demand per capita per day) should be used. This capacity can accept 2.2 cubic feet of
wasted sludge per 100 capita per day, which is typical for domestic wastewater treatment. Double units of
drying beds should be used so that half of them can be taken out of service for maintenance.
e. Cold climate. In moderately cold areas, ice buildup on clarifier scum collection boxes can cause
problems and eventually jam the skimmer mechanisms. Therefore, final clarifiers should be covered. In cold
areas, the spray from surface aerators will freeze on adjacent structures, bearings, gear reducers, etc., making
maintenance difficult. Drive components and walkways near the aerators should be covered to shield them
from spray, or mounted in isolated compartments. In very cold areas, heated covers for surface aerators
should be provided. Ice fences should be installed across the channel upstream of brush-type aerators to
prevent chunks of ice from breaking the brushes.
13-9
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