c. Principles of Operation. Undigested sludge drawn or pumped from
sedimentation units enters the digestion tank near the top and is mixed with digesting
sludge already in the tank. As digestion proceeds, four products are formed in the
digester -- stable sludge, supernatant Iiquor, sludge gas, and scum. Within the tank,
these substances are found in layers that are rather well defined.
(1) Stable sludge. This is the principal end product of sludge digestion. It
is a heavy, black solid residue that is not further reduced by anaerobic bacterial action.
It settles on the bottom of the tank and is drawn off for disposal.
(2) Supernatant liquor. The liquid sludge undergoing digestion occupies
space in the digester just above the stable sludge. Nonfiltrable residues are found
mostly in the lower portion of this liquid or liquor. Nitrogenous matter is converted mainly
to ammonia, which remains in solution in the liquor. The upper portion of this liquor
contains only small quantities of suspended matter. This clarified liquid is called
digester supernatant Iiquor. The supernatant liquor commonly is recirculated to primary
treatment units and passed again through primary and secondary treatment (see Figure
1-8). This recirculation is carefully controlled to prevent the high ammonia content and
high BOD of the liquor from upsetting optimum conditions in the primary treatment units.
(3) Scum. Scum consists of hair, match stems, grease, and a foam of
other lightweight, slow-digesting materials. It floats on top of the supernatant liquor.
Normally, scum can be broken up periodically and mixed with digesting liquor by wetting
the scum with raw sludge influent or recirculated supernatant liquor, mechanical stirring,
or raising the temperature inside the digester. Scum that has been allowed to
accumulate too long becomes a thick mat that must be broken up by hand. Scum
mixed with digesting liquor is eventually digested.
(4) Sludge gas. As digestion of organic matter proceeds, large amounts of
methane gas and carbon dioxide are produced. These gases, along with small
quantities of hydrogen sulfide, nitrogen, and hydrogen, collect in the top of the digester
and are drawn off. Since gas produced by sludge digestion (sludge gas) is 60 to 70
percent methane, it has a high heat value and can be used for heating plant buildings
and sludge digesters.
(a) The temperature of the sludge is an important factor in the rate and
effectiveness of sludge digestion. Optimum temperature is on the order of 950F. To
maintain this temperature, most digestion tanks in cold climates have heating coils
either around the inside periphery (see Figure 2-27) or in external heating units.
Sufficient heated water is forced through the coils to maintain the desired temperature.
(b) Sludge gas for heating purposes will need to be stored. Sludge
gas yields on average 15 cubic feet per pound of volatile solid destroyed and has a heat
value of between 500 to 700 British thermal units per cubic foot (Btu/cu. ft.). At some
plants, however, the gas has been used to produce electric power for operating the
plant and for sale to nearby consumers.