MIL-HDBK-1138
7.6.1
Aerobic Digesters. Aerobic digesters will operate
at lower efficiencies. Longer detention times will be required to
obtain the needed levels of stabilization. Digester solids
concentrations should be increased to accommodate these changes.
Thus, decanting the digesters may take longer as a result of
increases in viscosity.
7.6.2
Anaerobic Digesters. It will take more energy to
keep the contents up to the processing temperature because of the
lower feed temperature. Tank insulation (dome and sides) will
need to be inspected in the fall of each year and repaired as
needed.
7.6.3
Dewatering. Dewatering on drying beds can take
longer because of freezing beds. Where solids drying is
necessary, maximize the dewatering in the warmer and dryer summer
months. It is often faster to apply thin layers of solids, allow
the solids to dry, clean the beds, and reapply solids than to
apply one thick layer. If adequate capacity exists, keeping the
solids on the frozen beds throughout the winter is satisfactory.
7.6.4
Equipment Maintenance. All equipment that
contains solids mixtures and can be exposed to freezing
temperatures should be insulated and heat-taped. If equipment is
not required during the winter, drain it completely and store it
for the season. Equipment in which condensate can collect should
be emptied often to prevent freezing. For example, enclosed
cabinets (switch gears, mechanical enclosures, and pneumatic
systems, etc.) that are exposed to liquid streams in which
condensation may be generated should be either heat traced,
drained, or isolated from the condensation source.
Concrete Repair. Concrete exposed to repeated
7.6.5
freeze/thaw cycles is subject to cracking and spalling. In
environmental type structures, the concrete becomes saturated
with water. When temperatures drop below freezing, the water in
the concrete pores will freeze. As the water freezes, it expands
and exerts pressure on the concrete. When the pressure exceeds
the tensile strength of the concrete, it cracks. The cracks get
a little larger each time the concrete freezes. When the water
thaws out, the water shrinks, leaving void areas in the cracks.
Through capillary action the crack fills up with more water.
Repeated freezing and thawing makes the cracks get larger until
the concrete spalls off.
Small spalled areas that do not expose the reinforcing
are not significant since their effect is more cosmetic than
structural. Small spalls should be repaired as part of routine
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