MIL-HDBK-1005/16
(0.0118 to 0.059 inch for the top 2 feet [0.6 m] of filter) to
allow a dose rate of 5 gpd/sq ft (0.2 cu m/d/sq m).
6.6.1
Recirculation Tank. The recirculation tank receives
some underdrainings from the filter and mixes this with the
septic waste. The recirculation tank should be between
one-quarter and one-half the size of the Imhoff or septic tank.
A simple movable gate directs flow from the drain either to the
recirculation tank or to chlorination or other further treatment
and ultimate discharge. A tee turned upside-down and a rubber
ball suspended in a stainless steel basket under the open end of
the tee will also provide adequate flow control.
6.6.2
Recirculation. Recirculation is kept between 3:1 to
5:1. Pumps are set to dose every 2 to 3 hours and to empty the
recirculation tank. The recirculation pumps are sized so that
4 to 5 times the amount of raw sewage is pumped each day.
Duplicate, alternative pumps are required. Sand and gravel are
placed carefully so as not to crush the plastic or tile pipe
underdrains. Usually two separate sand filters are built so that
filters can be raked each week and allowed to completely aerate.
Prior to winter operation, the top 2 inches (5 cm) of sand on the
filters is replaced. Since these filters are placed on the
surface, they should be surrounded by a fence and landscaped.
Effluent will be of good quality, with biochemical oxygen demand
values ranging between 1 and 4 mg/L. In the winter, ammonia may
range from 40 to 50 mg/L. Pathogens are practically completely
removed.
6.6.3
Design Information. Design concepts are detailed in
Renovation of Secondary Effluent for Re-use as a Water Resource,
Kardos et al., 1974; On-Site Sewage Treatment, American Society
of Agricultural Engineers (ASMAE), 1984; Ecological Aspects of
Used Water Treatment, Curds and Hawkes, 1975; and Alternative
Wastewater Treatment, Eikum and Seabloom, 1982.
6.7
Garbage Grinders and Grinder Pumps. The following
information is provided for consideration by the designers of
small flow and package plant treatment systems.
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