TM 5-814-3/AFM 88-11, Volume III
Biochemical oxygen demand removal;
Solids separation; and
Hydraulic retention time.
Biochemical oxygen demand removal and the effluent characteristics are generally estimated using a
complete-mix hydraulic model and first order reaction kinetics. The complete-mix hydraulic model and first
order reaction kinetics will be used by the designer of U.S. Army wastewater treatment facilities. Oxygen
requirements will be estimated using equations based on mass balances; however, in a complete-mix system,
the power input necessary to keep the solids suspended is much greater than that required to transfer
adequate oxygen. Temperature effects are incorporated into the biochemical oxygen demand removal
equations. Solids removal will be accomplished by installing a settling pond. If a higher quality effluent is
required, then intermittent sand filtrations, as described in paragraph 14-4, should be used to produce an
acceptable effluent quality.
(2) Partial-mix aerated ponds. In the partial-mix aerated pond system, no attempt is made to keep
all of the solids in the aerated ponds suspended. Aeration serves only to provide oxygen transfer adequate
to oxidize the biochemical oxygen demand entering the pond. Some mixing obviously occurs and keeps
portions of the solids suspended; however, in the partial-mix aerated pond, anaerobic degradation of the
organic matter that settles does occur. The system is frequently referred to as a facultative aerated pond
system. Other than the difference in mixing requirements, the same factors considered in the complete-mix
aerated pond system are applicable to the design of a partial-mix system, i.e., biochemical oxygen damand
removal, effluent characteristics, oxygen requirements, temperature effects and solids separation. Biochemical
oxygen demand removal is normally estimated using the complete-mix hydraulic model and first order
reaction kinetics. The only difference in applying this model to partial-mix systems is the selection of a
reaction rate coefficient applicable to partial-mix systems.
d. Facultative ponds. Facultative pond design is based upon biochemical oxygen demand removal;
however, the majority of the suspended solids will be removed in the primary cell of a pond system. The
solids which settle out in a pond undergo digestion and provide a source of organic compounds to the water,
which is significant and has an effect on the performance. During the spring and fall, overturn of the pond
contents can result in significant quantities of solids being resuspended. The rate of sludge accumulation is
affected by the liquid temperature, and additional pond volume is provided for sludge accumulation in cold
climates. Although suspended solids have a profound influence on the performance of pond systems, most
design equations simplify the incorporation of the influence of suspended solids by using an overall reaction
rate constant. Effluent suspended solids generally consist of suspended organism biomass and do not include
suspended waste organic matter.
e. Controlled discharge ponds. No rational or empirical design model exists specifically for the design
of controlled discharge wastewater ponds. However, rational and empirical design models applied to
facultative pond design may also be applied to the design of controlled discharge ponds, provided allowance
is made for the required larger storage volumes. These larger volumes result from the long storage periods
relative to the very short discharge periods. The unique features of controlled discharge ponds are long-term
retention and periodic control discharge, usually once or twice a year. Ponds of this type have operated
satisfactorily in the north-central U.S. using the following design criteria:
-- Overall organic loading: 20-25 pounds biochemical oxygen demand per acre per day.
-- Liquid depth: not more than 6 feet for the first cell, not more than 8 feet for subsequent cells.
-- Hydraulic detention: at least 6 months of storage above the 2 feet liquid level (including
-- Number of cells: at least 3 for reliability, with piping flexibility for parallel or series operation.