MIL-HDBK-1138
immersion-type heaters to maintain higher oil temperatures.
Sometimes multi-viscosity year-round synthetic oils may be
appropriate. Gear boxes tend to collect moisture and condensate,
which may either degrade lubrication oil or cause corrosion.
This condition should be monitored. Frost may arc across some
electrical circuits; a small warm-air fan at the motor control
center vent will keep these components warm and dry if this is a
local issue.
7.3.3
Process Concerns. Settling characteristics change
when temperatures lower; solids settle at a slower rate in colder
(near freezing) waters. Stoke's Law governs the physics of
settling in primary clarifiers, and it is affected by
temperature. For example, when the temperature drops from 68F
(20C) to 38F (3C), the settling time for a particle increases
64 percent. Under these conditions, additional clarifiers may
facilitate the process. However, proceed with caution because
the increased detention time caused by additional clarifiers will
result in lower water temperatures and a higher risk of freezing.
If the solids loading is very high (above 2,000 milligrams per
liter [mg/L]), Stoke's Law does not apply.
7.4
Biological Systems. Biological reaction rates
depend upon temperature. A drop in temperature of 10C (18F)
decreases the reaction rates by one-half. Mechanical equipment
providing aeration is affected by freezing temperatures, and ice
may affect the components. Table 9 suggests some solutions to
freezing problems in biological systems.
Table 9
Winter Problems with Biological Systems
Problem
Solution
Ice buildup on surface aerator
Remove by hand.
Run on high speed for
15 minutes.
Turn off for 1/2 hour, allow
mixed liquor to warm aerator,
and turn on high speed.
Steam ice off.
Bump aerator on and off
carefully.
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