(1) Filterable residuals. Filterable residues comprise about 60 percent of
the total residue. These substances in solution are mainly inorganic compounds such
as mineral salts, many of which are not removed by conventional wastewater treatment
(2) Nonfiltrable residues. Nonfiltrable residues account for about 40
percent of the total residue. Typical residues found in domestic wastewater are feces,
toilet paper, grease, food scraps, hair, and dirt. Considerable quantities of grit will
infiltrate into the wastewater system. Domestic wastewater will also contain refuse
which should have been disposed of in some other way but which has, accidentally or
purposely, been placed in the wastewater system. Examples include broom straws,
rags, towels, bandages, sanitary napkins, sticks, match stems, cigarette and cigar
stubs, tobacco cans and pipes, denture plates, condoms, dead animals, and a variety of
other domestic and industrial materials. Nonfiltrable residues are further subdivided into
floating, settleable, and nonsettleable residues. Settleable residues are those that will
settle to the bottom in about one hour under relatively quiet conditions. The strength of
nonfiltrable residue is determined by filtering a given quantity of wastewater and drying
and weighing the filtrate. Nonfiltrable residues are expressed in milligrams per liter
(mg/l, or ppm).
QUANTITY OF WASTEWATER
The organic strength of sanitary wastewater varies through wide ranges.
Usually, wastewater strength is greatest when the wastewater rate of flow is highest and
lowest when the flow rate is lowest. The strongest wastewater is normally produced
during the morning hours and the weakest at night. Water infiltration into sewers during
rainy weather will dilute the wastewater, thus making it weaker.
a. Per Capita Production. For comparison purposes, wastewater flow is
commonly reported as average daily flow per person. The variation may be from as
much as 200 gallons per capita per day in some large cities to as low as 25 gallons per
capita per day in small communities. Reports from many of the permanent and
semipermanent military camps show a much smaller variation in proportion to
population. An average figure is approximately from 70 to 100 gallons per capita per
day, or about 70 percent of the domestic water consumption under routine conditions.
b. Hourly Flow. The volume of a domestic wastewater varies widely during
each day. The period of peak flow, usually 200 to 300 percent of the average flow, lasts
for a few hours after the period of peak water consumption preceding and following
breakfast. Secondary periods of high flow normally occur in the early afternoon and
again for a few hours in the evening. Minimum flow occurs after 2400 hours to
approximately 0600 hours and may be as low as 10 percent of the average daily flow.
These wide variations in flow are not as pronounced in large cities, but are readily
apparent at Army camps and in small communities that follow a normal activity pattern.