TM 5-814-3/AFM 88-11, Volume III
D-4. Unit operations in the tropics.
Although activated sludge, trickling filter or rotating biological filter processes may be used in hot climates,
strong sunlight and adequate space will often dictate the use of oxidation ponds. Temperature and sunlight
intensity will control algal growth, which will be intense. The most useful type of pond will be the facultative
pond, which is aerobic at the surface and anaerobic at the bottom. Pond retention time may be over 30 days;
depth is usually between 5 and 10 feet.
a. Not only are photosynthetic and microbiological processes accelerated, but gas formation is also
increased as temperature rises. Sludge rising is often a problem since sludge accumulates at a rapid rate and
much gas is evolved in the material. Daily desludging is normally required rather than the usual weekly
desludging. Settlement rate is controlled by viscosity so that the temperature increase does not dramatically
change reten-tion time in primaries, which is usually 1-2 hours in a correctly designed tank.
b. The effect of increased temperature reduces the saturation concentration of oxygen in any process, such
as a trickling filter or packaged activated sludge plant, but, fortunately, the mass transport coefficient is
increased. In any system involving plug flow, initial oxygen demands will be very high. Flow to the plant will
usually be anaerobic. The engineer should, therefore, anticipate 5-15 percent larger blower or bubbler air
demands than required in the U.S. At high altitudes, the oxygen saturation value will again be reduced,
requiring further increased air capacity at about 5 percent per 1,000 feet. Dissolved oxygen electrodes should
be mandatory in hot climate wastewater treatment plant processes because both under and over-aeration will
result in process distrubance. In package treatment plants where gravity return of settled activated sludge is
common, the sludge will usually turn anaerobic, making positive sludge return usually advisable. Aerobic
stabilization of activated sludge is most applicable in hot climates.
c. Trickling filters and
rotating biological disc filters show great promise
in hot climates. Since filter media
volume requirements are proportional to To.15 as temperature increases, the volume decreases.
d. Sludges dry much more rapidly in hot climates; but in the humid tropics, covers will be required. Odor
problems have been common in the sludges produced in hot climates, indicating that aerobic digestion or
aerobic composting are potentially useful. Anaerobic digestion and gas produciton should be investigated
since a hot climate encourages microbiological fermentation reactions.
e. The engineer should examine each unit operation in a proposed system for potential problems caused
by high temperature, torrential tropical rain, and local sewage characteristics variations.