or pumps with cutting edges in the suction inlet are most commonly used for primary
sludge. The density of the sludge varies with the depth as it settles, that on the bottom
being more dense than that which has just settled.
(3) Sludge from Imhoff tanks is permitted to remain for much longer periods
of time since it undergoes complete digestion in the lower portion of the tank.
Withdrawal is at a rate sufficient to remove only that portion of the sludge which is
completely digested. The digested sludge from Imhoff tanks is normally pumped
directly into sludge-drying beds (para 2-15b).
b. Rate. Proper sludge withdrawal is important to the efficiency of primary
settling. If sludge is permitted to remain in a primary settling tank too long, septic
conditions will result leading to the formation of gas and interference with natural
sedimentation. On the other hand, the solids content of the sludge should be as high as
possible for efficient digestion. Therefore, withdrawal must be stopped when the sludge
becomes too thin. Sludge of proper consistency can be recognized from experience by
correlating its appearance with sludge-solids test results. Primary sludge is normally
removed from the settling tank 3 to 4 times each 24 hours.
Section II. SECONDARY WASTEWATER TREATMENT
Secondary wastewater treatment is performed to remove pollution from
wastewater by changing dissolved and colloidal solids to settleable sludge through
biological processes and then removing the sludge. Before the treatment processes are
discussed, two biochemical decomposition terms and processes need clarification.
a. Aerobic. Aerobic decomposition is a relatively rapid process with
stabilization being achieved in a matter of hours. In this process, bacteria decomposes
and decays organic material in an oxygenated ("free" or dissolved oxygen present),
aquatic environment. Aerobic bacteria live and reproduce only in an environment
containing oxygen, which is needed for their respiration (breathing). This process is
different than in anaerobic decomposition. In this process, oxygen that is combined
chemically, such as in water (H2O) or in sulfate (SO4), cannot be used for the bacteria to
breathe. The oxygen must be atmospheric or dissolved in water. As bacteria
decomposes organic matter, enzymes formed as by-products of this biological action
cause a limited chemical action on inorganic and organic compounds, both in
suspension and in solution. The residual material from this biochemical action is
relatively stable and will settle out readily in secondary settling tanks. As long as
sufficient free oxygen is present to support the aerobic bacteria, aerobic decomposition
continues. The reaction results are relatively odorless, with the principal gaseous by-
product being carbon dioxide. If the supply of oxygen becomes insufficient and the
oxygen demand has not been satisfied, anaerobic decomposition begins to occur.