Geoscience Reference
In-Depth Information
24
Wastewater Calculations
Water is the most critical resource issue of our lifetime and our children's lifetime. The health of our
waters is the principal measure of how we live on the land.
—Luna Leopold
24.1 INTRODUCTION
Standard wastewater treatment consists of a series of steps or unit processes (see Figure 24.1) tied
together with the ultimate purpose of taking the raw sewage influent and turning it into an effluent
that is often several times cleaner than the water in the outfalled water body. Like the water calcula-
tions presented in Chapter 23, this chapter presents math calculations related to wastewater at the
operations level as well as the engineering level. Again, our purpose in using this format is consis-
tent with our intention to provide a single, self-contained, ready reference source.
24.2 PRELIMINARY TREATMENT
The initial stage of treatment in the wastewater treatment process (following collection and influ-
ent pumping) is preliminary treatment. Process selection normally is based on the expected char-
acteristics of the influent flow. Raw influent entering the treatment plant may contain many kinds
of materials (trash), and preliminary treatment protects downstream plant equipment by removing
these materials, which could cause clogs, jams, or excessive wear in plant machinery. In addition,
the removal of various materials at the beginning of the treatment train saves valuable space within
the treatment plant.
Two of the processes used in preliminary treatment include screening and grit removal; how-
ever, preliminary treatment may also include other processes, each designed to remove a specific
type of material that presents a potential problem for downstream unit treatment processes. These
processes include shredding, flow measurement, preaeration, chemical addition, and flow equaliza-
tion. Except in extreme cases, plant design will not include all of these items. Here, we focus on and
describe typical calculations used in two of these processes: screening and grit removal.
24.2.1 s
Creening
r
emoval
C
alCulations
Screening removes large solids, such as rags, cans, rocks, branches, leaves, roots, etc. from the
flow before the flow moves on to downstream processes. Wastewater operators responsible for
screenings disposal are typically required to keep a record of the amount of screenings removed
from the flow. To keep and maintain accurate screening records, the volume of screenings with-
drawn must be determined. Two methods are commonly used to calculate the volume of screen-
ings withdrawn:
3
)
Screenings (ft
3
=
Screenings removed (ft /day)
(2 4.1)
day
667
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