corrodes concrete, masonry, and steelwork. Infrequent or insufficient cleansing of
wastewater containers permits the accumulation of gas and explosive vapors. Thus,
while the pumping station may be of the automatic type, it still requires daily attention.
1-10. WASTEWATER TREATMENT
There are basically three types of stages or processes that take place to render
wastewater for disposal. These processes are called primary, secondary, and tertiary
treatment. Likewise, there are three types of treatment plants -- primary, secondary,
and tertiary -- that reduce the pollutant load in wastewater and chlorinate it before
a. Primary Treatment. Primary treatment, essentially a physical process,
includes the removal of settleable and floating residues. The process order is
screening, followed by grinding or shredding if the facility or equipment is present (in
this subcourse, we will assume that it is), and then grit removal, primary clarification of
sedimentation, and sludge removal.
(1) Screening and/or grinding. Screening and grinding prevent the
entrance into the treatment plant of large objects such as rags, pieces of wood, dead
animals, and other objects which may clog pipes, pumps, or other mechanical
equipment. Types of equipment most commonly used are fixed-bar screens and
Fixed-bar screens -- a series of evenly spaced bars set in the
(b) Communitors -- machines for cutting or shredding solids and
passing them to the wastewater flow.
(2) Grit removal. Grit removal is the preliminary removal of nonfiltrable,
inorganic material (sand, gravel, cinders, etc.). This material, if not removed, will
damage pumps and other equipment. It will also settle in digesters (see para 1-10b(4)),
thus reducing their capacity and treatment efficiency and necessitating frequent and
costly cleaning. Grit removal is accomplished by passing wastewater through a grit
chamber that retards the flow enough to permit the grit to settle. There are two types of
grit removal units commonly used -- the flow rate controlled grit removal unit and the
aerated grit removal unit. These units are discussed in lesson 2.
(3) Sedimentation. When fresh domestic wastewater stands quiescent or
flows very slowly, a considerable portion of the nonfiltrable residue will settle fairly
rapidly. Under average conditions, most of the settling occurs within 1 hour. The
quantity and rate of sedimentation that occurs after 2 hours is almost negligible.
Sedimentation may be accomplished in any of the following devices.