WASTEWATER TREATMENT PLANT OPERATION
Section I. PRIMARY WASTEWATER TREATMENT
Primary wastewater treatment is essentially a physical process even though a
limited amount of biochemical activity characteristic of secondary treatment sometimes
occurs. Primary treatment operation at the plants include screening and grinding to
remove larger floating solids; velocity reduction to remove grit, sand, and cinders; a
means to measure the amount of incoming wastewater; primary
clarification/sedimentation to remove settleable residue; and removal of sludge from the
settling tanks. The units will need to be maintained for maximum production.
2-2. SCREENING AND GRINDING
Wastewater treatment plants are equipped with screens, grinders, or shredders.
Their function is to prevent the entrance into the plant of large objects (pieces of wood,
rags, dead animals, etc.) which may clog pipes or damage pumps and other equipment.
a. Shredders (External). These units are located outside the wastewater flow
path and receive large objects that need to be shredded or grounded prior to being
returned to the wastewater flow for further treatment. Except at large plants, shredders
are operated only when enough screenings have been collected to justify operation.
Shredders are carefully washed, cutting edges inspected and adjusted, and parts
lubricated according to manufacturer's instructions after each run.
b. Screens. There are several types of screens such as bar, fixed-bar, and fine
screens. The most commonly used type of screen is the fixed-bar screen. This type of
screen usually consists of a grid of steel bars spaced on centers varying from 3/4 to
2 1/2 inches. It is installed at an angle in the direction of the flow of the wastewater
influent (see Figure 2-1). Fine screens are not used during this process.
(1) Need for frequent cleaning. In small installations, screens are hand-
cleaned. At larger plants, they are usually cleaned mechanically by rakes that traverse
the screens either continuously or intermittently. The bar screen must be kept clear by
raking at frequent intervals. Neglect of cleaning may cause wastewater to back up in
the influent pipe and overflow the screen chamber. Clogged bar screens cause material
to settle in the influent line where it becomes septic and odorous. This septic material is
suddenly washed into following elements of the treatment plant when the screen is
cleared. This is objectionable because best plant performance is secured when the
wastewater is received fresh and in uniform strength.