TM 5-814-3/AFM 88-11, Volume III
CHAPTER 6
SMALL FLOW TREATMENT SYSTEMS
6-1. General considerations.
Treatment systems handling less than 1.0 million gallons per day are generally considered small treatment
systems. For some packaged treatment systems, the principles of design are no different but the choice of
equipment will usually differ from that used in large plants. This is usually due to the effect of economies of
scale, whereby certain operations are economically feasible only on a large scale. In other cases, certain
treatment systems such as septic tanks, Imhoff tanks, waterless toilets, mounding systems and composting
toilets are only applicable to very small flows. Small packaged plants must make larger safety factor
allowances for flow variation and temperature effects relative to total wastewater flows. Smaller package
plants inherently have less operational flexibility; however, they are capable of performing effectively and
efficiently. These small packaged plants may consist of trickling filter plants, rotating biological discs,
physical-chemical plants, extended aeration activated sludge plants, and septic tanks. (Barnes and Wilson,
1976.) Design criteria for septic tanks, Imhoff tanks, waterless toilets, mounding systems, composting toilets,
and filtration/reuse systems are given below. Criteria for other processes have been presented in other
chapters of this manual. (See also: Hutzlet, et al., 1984; Grady and Lim, 1985.)
6-2. Septic tanks.
Septic tanks, with appropriate effluent disposal systems, are acceptable as a treatment system for isolated
buildings or for single-unit residential buildings when permitted by regulatory authority and when alternative
treatment is not practical. When soil and drainage characterictics are well documented for a particular site,
septic tank treatment may be permanently feasible. Septic tanks perform settling and digestion functions and
are effective in treating from 1 to 300 population equivalents of waste, but will be used only for 1 to 25
population equivalents, except when septic tanks are the most economical solution for larger populations
within the above range. Minimum size will be at least 500-gallons capacity. In designing tanks, the length-to-
width ratio should be between 2:1 and 3:1, and the liquid depth should be between 4 and 6 feet (fig 6-1). (See
Military Standard Drawings No.26-20-01 and 26-20-02 for details of construction.) Detention time depends
largely on the method of effluent disposal. When effluent is disposed of in subsurface absorption fields or
leaching pits, 24 hours detention time based on average flows is required. The septic tank must be sized to
provide the required detention (be low the operating liquid level) for the design daily flow plus an additional
25 percent capacity for sludge storage. If secondary treatment such as a subsurface sand filter or an oxidation
pond is provided, this can be reduced to 18 hours. Open sand filter treatment can further reduce detention
time to 10 to 12 hours. Absorption field and leaching well disposal should normally be limited to small
facilities (less than 50 population equivalents). If the total population is over 50, then more than one entirely
separate field or well would be acceptable. For 10 or more population equivalents, discharge of effluent will
be through dosing tanks which periodically discharge effluent quantities near 80 percent of the absorption
system capacity.
6-1
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