Environmental Engineering Reference
In-Depth Information
Primary treatment
Secondary treatment
Influent
Air
Chlorine
Effluent
Collection
system
Screening &
comminution
Grit
chamber
Primary
settling
Chlorine
contact tank
Secondary
settling
Aeration
Activated sludge
Screenings
Grit
Anaerobic
digester
Sludge
dewatering
hickener
Sludge
disposal
figure 5.1 Schematic of standard wastewater treatment process provid-
ing primary and secondary treatment using activated sludge process.
processes sequentially follow and tie into each other. Sections that dis-
cuss unit processes will make frequent reference to Figure 5.1, because
wastewater treatment is a series of individual steps (unit processes)
that treat the wastestream as it makes its way through the entire pro-
cess. A pictorial presentation of the treatment process, along with per-
tinent written information, enhances the learning process. It should
also be pointed out, however, that even though the model shown in
Figure 5.1 does not include all of the unit processes currently used in
wastewater treatment, this handbook does not ignore the other major
processes: trickling filters, rotating biological contactors (RBCs), and
oxidation ponds.
5.2 WasTeWaTer ColleCTion sysTeMs
Wastewater collection systems collect and convey wastewater to
the treatment plant. The complexity of the system depends on the size
of the community and the type of system selected. Methods of collection
and conveyance of wastewater include gravity systems, force main sys-
tems, vacuum systems, and combinations of all three types of systems.
5.2.1 gravity Collection system
In a gravity collection system, the collection lines are sloped to
permit the flow to move through the system with as little pumping
as possible. The slope of the lines must keep the wastewater moving
at a velocity (speed) of 2 to 4 feet per second (fps); otherwise, at lower
velocities, solids will settle out, causing clogged lines, overflows, and
offensive odors. To keep collection systems lines at a reasonable depth,
wastewater must be lifted (pumped) periodically so that it can continue
flowing “downhill” to the treatment plant. Pump stations are installed at
selected points within the system for this purpose.
