MUNICIPAL WASTEWATER 731
A combined system will, at times, exceed the hydraulic
capacity of the treatment plant and flow must be either bypasses
or held in detention tanks until heavy flow has subsided.
Multiple treatment units are provided and treatment is
not interrupted during periods of maintenance or repairs.
Protection must be provided for pumps against large objects,
such as floating pieces of wood. Coarse racks, with clear
openings of more than 2 inches, may be placed at the entrance
to the plant. Racks to be placed in advance of grit chambers
and settling tanks will have clear openings of 1 to 2 inches. In
smaller plants racks are cleaned by hand while larger plants
have mechanically cleaned racks. Disposal is by burial,
incineration or digestion with sewage sludge. Mechanically
cleaned racks have smaller clear openings because head
losses are lower with continuous cleaning. Racks with clear
openings of 1 to 2 inches can be expected to give from 20
to 100 ft^3 of solids per 1000 people annually. Comminutors
macerate floating material into sizes sufficiently small to be
easily handled by centrifugal pumps. Racks and screens with
very small openings have been almost completely replaced
by comminutors.
Rate of flow into the plant will vary over a wide range
during any 24 hour period with smaller plants exhibiting
greater variation. Flow is measured by Parshall flume, Venturi
meter or Sutro (keyhole) weir. The Parshall flume, sometimes
called the open channel venturi, is most commonly used. The
device operates on the principle of critical flow and mea-
surement of water depth upstream of the flume throat. The
governing equation is of the form Q = cWH 3/2 , where Q is
the discharge per unit time, W is the throat width, H is the
water depth, and c is a constant. While c changes with throat
width, it is closely constant for a constant throat width. Flume
liners of reinforced fiberglass are all but replacing steel and
concrete liners. Ease of fabrication, close tolerance, and
corrosion resistance are advantages cited. The true Venturi
operates on closed pipe (pressure) flow and is usually found
in larger plants. The Sutro, or keyhole, weir is shaped as its
name suggests. Its principal advantage is maintenance of a
constant upstream velocity over a wide flow range, but it
does have a high energy loss and metering is lost when the
opening is submerged.
It is necessary to remove grit in order to protect pumps
against excessive wear and to maintain capacity of sludge
digesters. It has been found that digesters in plants serving
low lying sandy areas can, if grit removal is not efficient,
lose up to a third of capacity in but a few years. Grit cham-
bers must operate in a fairly narrow velocity range of from
0.75 to 1.25 ft/sec. Above this range deposited material is
scoured back into suspension and below the lower value
organic material settles out. The resulting material, called
detritus, is unsuitable for landfill uses due to its highly
putrescible nature. Grit chambers are usually designed to
remove particles with a specific gravity of 2.65 and a mean
diameter of 0.02 cm. Because flow variation with depth fol-
lows a parabolic function, Q = cWH · H 1/2 , the grit chamber
is often given an approximately parabolic shape and better
velocity control is attained. The amount of grit collected per
million gallons flow is found to vary from 1 to 12 cubic feet.Grit is removed manually in small plants and continuously
by mechanical means in larger plants.
Settling tanks are provided for removal of larger, heavier
organic particles, oil, and grease. Oil, grease, and other
materials lighter than water are skimmed continuously from
the surface and led to digestion. Both circular and rectan-
gular surface configurations are used. Rectangular tanks
of the flow through variety have length to width ratios of
4/1 to 6/1. Circular tank size is usually limited by structural
requirements of trusses carrying skimming devices. Tank
depths vary from 7 to 15 feet. Bottoms are sloped about 1%
in rectangular tanks and about 8% in circular tanks to facili-
tate sludge removal.
Design is on the basis of hydraulic loading. A com-
monly used figure is 1000 gal/day × ft^2 surface area. It can
be expected that a BOD removal of 30% will be achieved in
a well operated primary sedimentation unit.
If treatment includes only screening, sedimentation, and
chlorination of effluent, the treatment is classed as primary.
Primary treatment, while inadequate for most areas, is
better than no treatment. The adequacy of secondary treat-
ment is now being seriously questioned. Nonetheless, it
fits the economics of the situation. Only in the 1960’s did
wastewater treatment become of interest to any but a small
number of people. Sanitary engineers were wont to say
“It may be sewage to you but it is bread and butter to me.”
Theirs was not a profession to which much glamour was
attached. Financing bodies were reluctant to invest adequate
sums in waste treatment facilities. Hopefully, this has now
changed.
There are two main processes utilized for biological
(secondary) treatment. These are (1) the trickling filter and
(2) activated sludge.
The trickling filter is not a true filter. It can best be
described as a pile of stones, or other coarse material, over
which sewage flows. This is the most widely used biological
treatment process. Present day biological treatment technol-
ogy is a logical development from sewage farms (irrigation
areas) to intermittent sand filters to contact (fill and draw)
beds to trickling filters and activated sludge units. Numerous
modifications of the basic processes have evolved but the
underlying principles remain unchanged. In biological
treatment a suitable environment is provided so that micro-
organisms may thrive under controlled conditions. The suit-
able environment is one rich in food and maintained in the
aerobic state. The zoogleal mass remains fixed on the filter
media in the trickling filter while the sewage flows past. In
the activated sludge process the sewage and organisms flow
together. In both cases the microorganisms come from the
sewage itself. Traditional secondary treatment plants operate
in the declining growth phase.
Irrigation by sewage provides water return and some
waste stabilization but this means of sewage disposal is in
conflict with sound public health practice and ought not be
used where there is a possibility that sewage can pass with
little change into the groundwater table. Irrigation is best
applied in arid regions. When it is utilized for food growing
areas, care must be taken so that edible plants and fruit areC013_007_r03.indd 731C013_007_r03.indd 731 11/18/2005 10:43:41 AM11/18/2005 10:43:41 AM