TM 5-814-3/AFM 88-11, Volume III
APPENDIX D
SEWAGE TREATMENT IN HOT CLIMATES
D-1. General considerations.
Although this manual is intended to serve U. S-based activities, the U.S. Army and Air Force often must
establish bases in areas which may be hotter; drier or wetter than conditions faced in the U.S. Certain factors
generally found in hot climates bear special consideration.
D-2. Establishment of flow rates.
Calculation of wastewater flow may be based, as in temperate climates, upon fresh water use. However;
sewage flow may be either much larger or much smaller than water use. In areas of high rainfall and water
abundance, personnel will bathe more often, use water for cooling and will tend to be less water conscious.
Wastewater flow may be greater than expected for a given population. Although brackish water may be used
for washing, cooling or cleaning, if it is allowed to enter the wastestream, the increased salinity will lower
biological process efficiencies. High dissolved solids concentrations have an impact on the treatment process
efficiency. This condition should be considered in designing these systems. On the other hand, greywater may
be separated from the wastestream and recycled for purification, or used directly onsite for lawns, plants,
washing or cooling. Under these circumstances, wastewater flow will be low and much more concentrated.
Loss of water by evapora-tion and from pipelines into the ground may further decrease flow to the treatment
plant.
a. Typical low flow regimes where desert posts utilize a standpipe, practice water conservation and
recycle water may require only 5-20 gallons per day per person of stored fresh water and only 2-10 gallons
per day per person may arrive at the treatment plant.
b. Typical high flow regimes in the humid tropics are subject to considerable infiltration, use of warm
rooftop or cistern stored water and high shower utilization require 100-125 gallons per day per person of
fresh water and 130-150 gallons per day per person may appear at the treatment facility.
c. The engineer must, therefore, assess the water use, storage and transport patterns of the site in order
to assign reasonable sewage flows.
D-3. High temperature parameters.
A major design parameter will be water temperature. Use of rooftop rain storage, cistern water; brackish
water; and the ambient conditions will result in very warm sewage. The engineer shall expect high salt
content, including sulfate, chloride, phosphate, borate and nitrate anions, and both alkali and alkaline earth
cations. Oxygen levels will be very low and chalcogenides as well as dissolved hydrogen sulfide should be
anticipated. The most dramatic effect of high temperature will be upon biochemical reaction rates. The
general formula relating temperature to rate constants is as follows:
K20 1.047(T-20) where,
Kt =
Kt =
the rate constant for the sewage treatment reactions involved;
K20 = the rate constant for that same reaction at 20 degrees Celsius.
Using this equation, the rate constants can be calculated at 10 degrees Celsius and 35 degrees Celsius. The
result indicates a 3-fold increase in the rate coefficient. It is this high rate of biological reaction which results
in unexpectedly high sulfide levels, very low dissolved oxygen concentrations and large biomass
accumulation. All chemical moieties found in the sewage will arrive in the reduced state in a much more
microbiologically rich liquor.
D-1
Previous Page
