Environmental Engineering Reference
In-Depth Information
which became practical because of availability of sewer cleaning equipment
which can readily clean curved sewers. Previously, standard practice was to
allow only straight sewer alignments, requiring a much larger number of costly
manholes
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.
Sewage Characteristics and Needs for Treatment
and Point Source Control
Unfortunately, the international literature on sewage treatment places such
emphasis on removal of BOD that many design engineers tend to forget which
are the parameters that are most important for design of sewage treatment
systems:
Floatable materials, which, if not removed, form surface mats on receiving
waters
•
Settleable solids, which, if not removal, form sludge banks in receiving
waters
•
BOD (together with suspended solids), which if not removed, will be harm-
ful to stream ecology (as well as making it more costly to use the river
waters as raw water supply)
•
Nutrients, which, if not removed, can induce eutrophication
•
Toxics and other hazardous substances, which, if not removed, can disturb
biological treatment processes and harm aquatic ecology (and impair raw
water quality for community water supply)
•
Primary treatment will remove floatable materials and settleable solids, and
complete treatment (primary plus secondary) is required to removal of BOD.
Advanced treatment is required for removal of nutrients, and point source control
is needed for control of toxics and other hazardous substances. For many DC
communities, primary treatment will suffice for the foreseeable future, but for
others, complete treatment should be the target. Rarely in DCs will there be need
for removal of nutrients. Thus far, many DCs have not made sufficient progress
on point source control due to weakness in the enforcement mechanics of most
DC governments.
Sewage Flow Measurement
Measurement of sewage flows used to be a difficult problem, but the development
of the Palmer-Bowlus (“PB”) flume
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greatly simplifies the problem by use of
the PB flume placed in the sewer, which does not restrict the sewage flow.
Beginning in the 1960s, such flumes became commercially available as shelf
items. They can be readily fitted into a sewer of specified size.
Unfortunately, many DC practitioners are not familiar with the use of these
“PB” meters.
