residues in wastewater are also classified as fixed residues and volatile residues. The
fixed residues are the materials remaining after the volatile residues have been ignited
(1) Total residues. The total residues (total solids) are determined by
evaporating a known volume of wastewater over a steam bath, drying the residue, and
determining the weight of the residue. This test is used to determine the efficiency of
sludge digestion.
(2) Nonfiltrable residues. The nonfiltrable residues (suspended solids) are
found by filtering a known volume of wastewater, drying the filtered material, and
determining its weight. This test is used to determine the efficiency of both primary and
secondary processes (except for sludge digestion).
(3) Settleable residues. Settleable residues (settleable solids) are
determined in two ways--by volume and by weight. The simple test by volume is made
by allowing a one-liter sample of wastewater to settle for about one hour in a quiescent
condition in a specially designed (lmhoff) cone. The quantity of material that collects in
the bottom of the cone is reported in milliliters per liter. The more accurate method of
determining settleable solids is to allow the sample to stand quiescently in the lmhoff
cone, siphon off the supernatant, evaporate the settled solids to dryness, and weigh the
residue. The number of milligrams of the material weighed is the parts per million of
settleable solids in the sample. This test is also used to determine the efficiency of
primary and secondary processes as in (2) above.
(4) Fixed and volatile residues. If the dried and weighed filtered residues
obtained from the total residues determination are ignited (tat is, heated sufficiently to
burn off the volatile matter), the weight of the remaining substances is the weight of the
fixed residues or approximately the weight of the mineral matter. The difference
between this value and the weight of the dried (unignited) residues is the weight of the
volatile residues, or approximately the weight of the organic matter. This test is used
with (1) above to determine the efficiency of sludge digestion.
c. Biochemical Oxygen Demand (BOD). The BOD of a liquid is defined as the
quantity of oxygen required for the biochemical oxidation of the decomposable (organic)
matter at a specified temperature, within a specified time (usually 5 days at 20C), and
under specified conditions. The BOD indicates the amount of decomposable organic
matter in the wastewater, is an index of the concentration of the wastewater, and aids
the engineer In evaluating both plant efficiency and effluent effect on receiving streams.
(1) Determination of the BOD. The recommended method of determining
the BOD is to dilute a known volume of wastewater with a known volume of specially
prepared dilution water that is saturated with dissolved oxygen. The dissolved oxygen
content of the mixture is determined at the beginning and at the end of the incubation
period on separate bottles of the wastewater dilution water mixture. The difference or
depletion in oxygen is corrected for the loss of oxygen in a similarly incubated sample of
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