PHYSICAL AND CHEMICAL TREATMENT OF WASTEWATERS 977
Sedimentation Sedimentation basins are important compo-
nents in water- and wastewater-treatment systems, and their
performance greatly depends upon proper design. In chemical
treatment of wastewater, the separation of chemically coagu-
lated floc depends on the characteristics of the floc in addition
to the factors normally considered in the design of conven-
tional primary and secondary clarifiers. Field experience indi-
cates that the usual values for surface overflow rates used in
separating chemical floc in water-treatment plants must be
reduced in order to obtain a good efficiency in the removal
of floc from chemically coagulated wastewater (Weber et al.,
1970; Convery, 1968; Rose, 1968; Kalinske and Shell, 1968).
In wastewater-treatment practices, the recommended overflow
rate for removal of alum floc is 30 m/day, while with use of
lime or iron salts, it can be increased up to 40 m/day.FiltrationFiltration of wastewater can be accomplished by the use of
(1) microscreens, (2) diatomaceous earth filters, (3) sand fil-
ters, (4) mixed media filters, or (5) membranes. The filtration
of sludges, on the other hand, is achieved by sand beds or
vacuum filters.
The filtration characteristics of the solids found in a bio-
logical treatment plant effluent are greatly different from
those of the floc formed during chemical coagulation for the
removal of organic matter and phosphates. Tchobanoglous
and Eliassen (1970) have noted that the strength of the bio-
logical floc is much greater than that of the flocs resulting
from chemical coagulation.
Accordingly, biological flocs can be removed with a
coarser filter medium at higher filtration rates than can the
weaker chemical flocs, which may shear and penetrate through
the filter more readily. Lynam et al. (1969) had observed that
the chemical floc strength can be controlled, to some degree,
with the use of polymers as coagulant aids. Their experi-
ments yielded higher SS removal by filtration when 1 mg/l of
anionic polymer A-21 was used along with alum.
The filterability of solids in a conventional biological
plant effluent is dependent upon the degree of flocculation
achieved in the biological process. For example, filtration of
the effluent from a trickling-filter plant normally cannot yield
more than 50% removal of the SS due to the poor degree of
biological flocculation in trickling filters. On the other hand,
the activated sludge process is capable of a much higher
degree of biological flocculation than the trickling-filter pro-
cess. The degree of biological flocculation achieved in an
activated sludge plant was found to be directly proportional
to the aeration time and inversely proportional to the ratio of
the amount of organic material added per day to the amount
of SS present in the aeration chamber (Culp and Hansen,
1967a). It has also been reported that up to 98% of the SS
found in the effluent from a domestic sewage-treatment plant
after a 24-hour aeration time could be removed by filtration
without the use of coagulants (Culp and Hansen, 1967b).Microscreening Microscreens are mechanical filters
in which flow is passed through a special metallic filterfabric placed around a drum. The filter traps the solids and
rotates with the drum to bring the fabric under backwash
water sprays fitted to the top of the machine, in order to
wash the solids to a hopper for gravity removal to disposal.
The rate of flow through the microscreen is determined
by the applied head, normally limited to about 150 mm
or less, and the concentration and nature of the SS in the
effluent.
Extensive tests at the Chicago Sanitary District showed
that microscreens with a 23 μm aperture could reduce the
SS and BOD of a good-quality activated sludge effluent,
20–35 mg/l SS and 15–20 mg/l BOD, to 6–8 mg/l and 3.5–
5 mg/l, respectively (Lynam et al., 1969). It was noted that
the microscreens were more responsive to SS loading than
to hydraulic loading and that the maximum capacity of the
microscreens was reached at the loading of 4.3 kg/m^2 /day
at 0.27 m/min.Diatomaceous Earth Filtration Diatomite filters found their
widest application in the production of potable waters, where
the raw water supply was already of a relatively good quality,
i.e., of low turbidity. Operating characteristics of diatomite
filters can now be predicted under a wide range of operat-
ing conditions by utilizing several mathematical models
(Dillingham et al., 1966, 1967). Several investigators have
studied the filtration of secondary effluent by diatomite filters
whose ability to produce an excellent-quality effluent is well
established (Shatto, 1960; R. Eliassen and Bennett, 1967;
Baumann and Oulman, 1970). However, the extremely high
cost and their inability to tolerate significant variations in SS
concentration limit the usage of diatomite filters in sewage-
treatment practices.Sand Filtration Sand filters have been operated as slow
sand filters or as rapid sand filters made with one or more
media. A slow sand filter consists of a 150- to 400-mm-thick
layer of 0.4-mm sand supported on a layer of a coarser mate-
rial of approximately the same thickness. The underdrainage
system under the coarser material collects the filtrate. The
rate of flow through the filter is controlled at about 3 m/day.
This rate is continued until the head loss through the bed
becomes excessive. Then the filter is thrown out of service
and allowed to partially dry, and 25 to 50 mm of the sand
layer, which includes the surface layer of sludge, is manually
scraped from the top for washing. Disadvantages of slow
sand filtration system are: (1) the filters may become inoper-
ative during the cold winter weather, unless properly housed;
(2) slow sand filters may not be effective due to the rapid
clogging of filters (the normal frequency of cleaning filters
varies from once to twice a month; Truesdale and Birkbeck,
1996); (3) the cost of slow sand filtration is three times the
cost of rapid sand filters and twice the cost of microscreens
(anonymous, 1967); and (4) the large space requirement.
Rapid sand filters consist of about a 400-mm-thick layer
of 0.5- to 0.65-mm sand supported on coarser gravel. The
rate of filtration ranges between 80 and 120 mm/min. At this
high filtration rate, the filter beds need backwashing when
the head loss becomes excessive.C016_005_r03.indd 977 11/22/2005 11:25:17 AM