CHEMICAL-PHYSICAL PROCESSES
I. J. Kugelman and J. M. Cohen
Physical & Chemical Treatment Research Program
Advanced Waste Treatment Research Laboratory
ADVANCED WASTE TREATMENT AND WATER REUSE SYMPOSIUM
Great Lakes Region, WQO, EPA
Cleveland, Ohio
March 30-31, 1971
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CHEMICAL-PHYSICAL PROCESSES*
I. J. Kugelman and J. M. Cohen
INTRODUCTION
The most recent advance in the art of sewage treatment is the application of
physical-chemical technology to raw sewage treatment. In reality, physical -
chemical treatment of sewage is not a completely new technology. Chemical
treatment of sewage was widely practiced in England and the United States in
the latter portion of the 19th Century. However, this technique gradually
fell into disuse with the advent of activated sludge, because activated
sludge systems achieved higher degrees of treatment (1). In the 1930's a
number of systems employing physical and chemical treatment in combination
were evaluated. These produced treatment superior to primary sedimentation
followed by activated sludge (conventional treatment) but at a cost 1.5 to
2 times as great (2). This additional increment of cost discouraged adoption
of physical-chemical treatment at that time.
During the last decade advances in physical-chemical treatment technology
resulting from Environmental Protection Agency-supported research have signi-
ficantly reduced the cost of physical-chemical treatment. In addition, it is
apparent that higher levels of treatment will be required in the future to
maintain water quality. As a result of these alterations in conditions,
physical-chemical treatment is now an alternative to conventional treatment,
especially for situations where significant phosphorus removal is required.
GOALS OF SEWAGE TREATMENT
The fundamental goal of sewage treatment is sufficient reduction in the level
of pollutants in the wastewater to allow discharge to the environment. The
pollutants in sewage are grouped into classes of similar compounds which have
the same environmental impact rather than being individually dealt with. At
present the five major pollutant groups of interest are: suspended solids,
organic matter as measured by BOD, TOC or COD, phosphorus compounds, nitrogen
compounds and pathogenic microorganisms. It is impossible to specify one set
of effluent standards which is applicable to all or most situations. However,
it is generally agreed that a good quality effluent will have the character-
istics given in Table 1. In addition, complete disinfection will be required.
As yet no general agreement on a nitrogen level is in evidence. It must be
repeated the goals listed above are not design standards, but are guides in
describing a good quality effluent.
*Presented at the Advanced Waste Treatment and Water Reuse Symposium,
Cleveland, Ohio, March 30-31 , 1971.
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THE CLARIFICATION-ADSORPTION PROCESS
The physical-chemical treatment system which at present is most advanced in
development, and which seems the best is the clarification-adsorption system
schematically illustrated in Figure 1. In this system chemical clarification
and filtration are utilized to achieve almost complete suspended and colloidal
solids removal. With a high enough dose of the proper chemicals essentially
complete phosphorus removal can also be obtained. The function of the
activated carbon step is the removal of soluble organics. As in any conven-
tional system preliminary screening, grit removal and final disinfection
are provided. The filtration step is shown as optional but conservative
design dictates its use. The positioning of this step prior to or after carbon
adsorption is dictated by the type of waste-carbon contacting system provided.
The three steps of chemical clarification, filtration and carbon adsorption
will be discussed in some detail below.
This system does not provide ammonia or nitrate-nitrogen removal. Physical-
chemical methods for soluble nitrogen removal are covered in another lecture
of this symposium. Any of these nitrogen removal methods can be integrated
into the clarification-adsorption system.
CHEMICAL CLARIFICATION
Chemical clarification provides the bulk of the pollutant removal achieved
by this type of treatment system. In this unit operation an appropriate
chemical is dosed to the wastewater and the mixture is flocculated and
settled. All of these steps can be conducted in independent units, or
combined into a single unit which is usually referred to as solids-contact
clarifier.
Chemicals which have been successfully used to clarify raw sewage include
organic polymers (3), iron salts (4), aluminum salts (5), and lime (6)(7).
The inorganic coagulants have the advantage of providing for phosphorus
removal. At present there is no rational method for predicting the dose
of chemical required. For planning purposes, jar tests are suggested.
Fortunately field control of the coagulant dose is possible. For all
coagulants except lime the suggested method is monitoring of the clarifier
effluent turbidity. Monitoring of phosphorus also appears promising, as
good clarification is always obtained when sufficient chemical is added to
provide good phosphorus removal. With lime as the coagulant excellent
control is obtained by pH measurement. The pH required to achieve good
clarification and phosphorus removal is dependent on the chemical charac-
teristics of the sewage. In areas where the alkalinity and hardness are
low the high lime process in which the pH is raised to approximately 11.5
is required. Generally, two-stage precipitation with intermediate recarbon-
ation is utilized. In hard water, high alkalinity areas a low lime single-
stage precipitation at pH 9.5 to 10 is sufficient.
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Table 2 illustrates the treatment obtained with chemical clarification in
pilot plants at a variety of locations. In addition to the expected high
levels of suspended solids and phosphorus removal significant organic
removal was obtained. Based on these data and others it can be expected
that chemical clarification of raw sewage will consistently yield organic
removal in the 65-75 percent range. With this degree of organic removal
from chemical coagulation the carbon need provide only a small increment
of additional removal to match the performance of a good secondary treat-
ment system.
Sludge disposal plays a dominant role in the economics of any chemical
clarification system. Only limited data on the characteristics of sludges
resulting from the chemical treatment of raw sewage are available. These
data indicate that with iron or aluminum salts as coagulants:
1. The resulting sludge volume sometimes exceeds and sometimes is
less than primary sludge from the same sewage.
2. The chemical sludge is more difficult to dewater in a vacuum filter
than the corresponding primary sludge.
However, with lime as the coagulation chemical dewatering is extremely rapid
just by gravity thickening. Eimco (8) reported lime-raw sewage sludge solids
concentrations of 15% to 25% after gravity thickening.
In addition to yielding a sludge amenable to rapid and easy dewatering lime
is the only coagulation chemical which can be economically recovered.
Recalcining the lime sludge in a furnace has been successfully conducted in
a tertiary plant at South Lake Tahoe and has been state-of-the-art in water
softening plants for a considerable period of time (9). In addition to
regenerating the lime, the organic solids are incinerated thereby accomplish-
ing ultimate disposal of the sludge. For these reasons it is anticipated
that lime will be the coagulant of choice for most situations.
Design criteria for the coagulation equipment is similar to that used in
water treatment plants. A flash mix of one minute, flocculation for 15 to
30 minutes and sedimentation at upflow rates of 0.5 and 1.0 gpm/ft .
CARBON ADSORPTION
The role of the carbon adsorption step is the removal of soluble organics
from the wastewater. Although the chemical clarification step does the bulk
of the pollution control job, the carbon adsorption step is required to
produce a good quality effluent. The total organic removal achieved by the
combination of clarification and carbon adsorption at several physical-
chemical pilot plants is illustrated in Table 3. It can be seen that not
only is the removal achieved quite high (95%+) but equally important the
residual organics after treatment are quite low. These effluents are
superior to the usual quality of secondary effluent (TOC = 20 mg/1,
COD = 40 to 50 mg/1).
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The results reported in Table 3 were obtained with carbon contacting systems
employing granular carbon. In such systems the waste is passed either upward
or downward through columns containing the carbon. Downflow columns function
as packed beds and accomplish filtration of the wastewater. Flow rates of
2 gpm/ft^ to 8 gpm/ft^ have been employed. In this flow range essentially
equivalent adsorption efficiency is obtained provided the same contact time
is employed. At flow rates below 2 gpm/ft^ adsorption efficiency is reduced,
while at flow rates above 8 gpm/ft^ excessive pressure drop takes place.
Contact times employed are in the range of 30 minutes to 60 minutes on an
empty bed basis. In general increases in contact time up to about 30 minutes
yield proportionate increases in organic removal. Beyond 30 minutes the rate
of increase falls off with increases in contact time and at about 60 minutes
contact becomes negligible. Carbon beds to be operated at the lower end of
the flow rate range are generally designed for gravity flow. Those systems
designed for the higher flow rates employ pressure vessels. A pressure
vessel is more expensive to contruct than a gravity flow vessel but it requires
less land area, and provides greater ability to handle fluctuations in flow
rate.
Provision must be made to periodically backwash downflow carbon beds because
even if they are preceded by a filter they gradually collect suspended solids.
In addition, biological growth takes place on the carbon granules and tends to
clog the bed. It is advisable to include a surface wash and air scour to be
assured of removal of the gelatinous biological growth.
Backwash of the carbon beds satisfactorily relieves clogging but does not
completely remove the biological growth. Consequently, significant biolog-
ical activity is manifest in the carbon beds at most times. This leads to
the development of anaerobic conditions in the carbon bed and generation of
sulfides. Aeration of the column feed has been utilized to prevent anaerobic
conditions, but this produced so much biological growth that excessive back-
wash was required.
In an attempt to overcome these difficulties upflow carbon columns have been
operated in a slightly expanded mode (about 10% expansion). This allows for
significant accumulation of biological activity on the carbon granule with
little increase in head loss. Consequently, aerobic conditions can be main-
tained and sulfide generation prevented.
If the expanded bed system is utilized backwash facilities must still be
provided as it has been found necessary to occasionally remove excess growth.
With this system the flow rate range which can be used is more restricted
than with packed bed systems. With the commercial sizes of carbon available
(8x30 mesh or 16x40 mesh) flow rates above 4 to 5 gpm/ft^ are required to
achieve the proper degree of expansion. In addition, care must be exercised
to avoid hydraulic surges which could wash carbon out of the system. The
filtration section of the physical-chemical treatment plant must follow
carbon contact when the expanded bed system is used.
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The latest carbon contacting scheme to be applied to waste treatment
utilizes powdered carbon (particle size below 200 mesh). This procedure
provides for a mixture of a carbon slurry and the wastewater in a Reactor-
Clarifier. Polymer addition is generally required to achieve a good gravity
separation of the carbon from the wastewater following contact. The
potential advantages of this system are the use of a cheaper carbon (IOC per
pound vs. 30C per pound for granular carbon) and a simpler type of contacting
system.
A critical aspect of the design of any carbon contacting system is the
expected capacity of the carbon for the organics. In the chemical processing
industries this is evaluated by running adsorption isotherm tests. In the
waste treatment field isotherms are of limited utility because the biological
activity which develops on the carbon tends to greatly enhance its apparent
capacity for organic removal.
The role of the design engineer is to utilize a system which makes greatest
utility of the capacity of the carbon regardless of what the capacity is.
In order to provide a good effluent and utilize most of the available capacity,
countercurrent contact is required. This is achieved by having the waste flow
through a number of contactors or stages in series in one direction, while the
carbon moves in the opposite direction. In the powdered carbon contacting
system this is exactly the procedure used. With granular carbon this procedure
cannot be used as undesirable attrition losses will take place. Rather when
an undesirable effluent is obtained the lead contactor is removed from service
and a spare contactor with fresh carbon placed at the end of the line. Each
contactor is then moved up one position in the line. This is accomplished
by piping and valving a series of columns to shift the inflow and outflow
points of the series accordingly rather than physically moving the columns.
As the number of stages increases, the piping and valving arrangement becomes
more complex and costly. In design, a compromise between the advantage of
adding another stage to more closely approach the highest use of the carbon
capacity, and the cost of each additional stage must be achieved.
An alternate arrangement which is used in some plants provides parallel flow
through a number of identical contactors. Each is at a different stage of
exhaustion and produces a slightly different effluent quality. These
individual effluents are blended to produce the final product.
Table 4 reviews the various factors which must be considered in designing
any carbon removal system.
In Table 5 carbon capacities obtained in field operation at various pilot
plants are given. In view of the fact that the waste, effluent criteria,
number of contact stages etc., varied from plant to plant it is not sur-
prising that some spread in the results is observed. These capacities are
expressed as pounds of organics removed (either as COD or TOO per pound of
carbon. For general planning purposes a capacity of 0.5 pounds of COD per
pound of carbon is reasonable. This is approximately equivalent to a
requirement of 500 pounds of activated carbon per million gallons of sewage
treated.
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A carbon requirement at this level would be prohibitively expensive if
regeneration and reuse of the exhausted carbon were not possible. At
present a technically and economically feasible method is available for
the regeneration of granular activated carbon,, This method requires
heating of the carbon in a multi-hearth furnace in the presence of steam
to ~ 1750°F. This treatment burns away the adsorbed and trapped organics.
During a regeneration cycle some of the carbon is physically lost by burning
and attrition, and some of each particle's capacity is lost by alteration of
surface properties. The overall loss expressed as percent by weight of
virgin carbon required to restore the total original capacity of the batch
ranges from 5 to 10 percent (10). Thus, for planning purposes carbon make-
up requirements at a plant should range from 25 to 50 Ibs per million
gallons of sewage treated.
At present experiments on a fluidized bed regeneration system for powdered
carbon are moving into the large-scale pilot stage. Successful completion
of these tests will be a big step forward in making a powdered carbon system
a technical and economic reality. The key factor will be maintaining the
carbon loss at a low enough level during the regeneration,
DESCRIPTION & PERFORMANCE OF SOME PHYSICAL-CHEMICAL PILOT PLANTS
In the previous sections of this paper summaries of performance from a variety
of physical-chemical treatment plants were presented. In this section a
somewhat more detailed look will be taken at a selection of these plants.
Ewing-Lawrence (4)
For the last several years FMC Corporation has been conducting studies of
physical-chemical treatment at the Ewing-Lawrence Treatment Plant near
Princeton, N.J. Figure 2 illustrates the clarification section of the plant.
These units were somewhat overdesigned to insure a good feed to the adsorption
section of the plant, as the major purpose of the research was to evaluate
carbon performance. Figure 3 is a schematic of the overall plant. Packed
bed versus expanded bed carbon contact in parallel was studied at this plant.
Figure 4 indicates that the performance of the packed bed contactor was
slightly superior to that of the expanded bed contactors. However, the
investigators indicated that the advantage was not sufficient to offset the
need to backwash more frequently and the potential difficulties from anaerobic
conditions in the carbon beds. Figure 5 illustrates the BOD removal obtained
over a 3-1/2 month period. It can be seen that despite considerable variation
in influent strength a consistently good effluent was produced. This figure
also reiterates that the bulk of the treatment is accomplished in the chemical
clarification step.
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Blue Plains (7)
At the Blue Plains treatment plant in Washington, D.C., a pilot physical-
chemical treatment system is being operated as part of a joint effort of
EPA and the District of Columbia. Figure 6 is a schematic of the treatment
system. Two-stage high lime clarification is provided with a small dose of
ferric chloride in the second stage for flocculation of calcium carbonate.
This stage of the plant has a capacity of 100,000 gal/day. The filtration,
ion exchange and downflow two-stage carbon beds have a capacity of 50,000
gal/day. This plant is highly automated and thus runs with a miniumum of
manual control. For example, the lime dose is automatically set by monitoring
the pH and alkalinity in the clarifiers, and backwash of the filters, ion
exchange columns, and carbon columns is automatically initiated by pressure
drop and/or a time cycle.
Figure 7 illustrates the treatment performance obtained at this plant for a
variety of pollution control parameters. Nitrogen removal was obtained by the
use of an ion exchange process with a zeolite which has a high affinity for
ammonium ion. Again, these data indicate that the bulk of the work is done
by the clarification step with the filter and carbon columns serving as
polishing devices. Table 6 gives the average effluent characteristics for
this installation. This effluent is far better in quality than normal
secondary effluent.
Salt Lake City, Utah (8)
Under contract between EPA and Eimco Corporation an evaluation is being
conducted of a physical-chemical treatment system employing powdered carbon
contact after chemical clarification. Figure 8 illustrates the pilot plant.
Two countercurrent stages of carbon contact in Reactor-Clarifier are provided
following chemical clarification. Some results with lime as the coagulant
are given in Table 7. These results are not quite as good as in the plants
using granular carbon discussed previously. The effluent, however, must
still be classified as of good quality. One potential advantage of this
contacting system is the ability to pace the carbon dose to the organic
demand. Evaluation of powdered carbon regeneration will take place at this
installation. The spent carbon will be dewatered on a vacuum filter prior
to being fed to the fluid bed furnace.
Rocky River, Ohio (3)
Several studies have been made utilizing polymer addition to existing primary
plants followed by small-scale pilot carbon adsorption. One such study was
done at the 10 MGD primary facility at Rocky River, Ohio. An anionic polymer
was added to the existing primary clarifier at a dosage of 0.3 mg/1, and a
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side stream of clarified effluent was applied to small carbon columns for a
period of about one month. The summary of the data is shown in Table 8. It
can be seen that, even with less than optimum clarification, effluent
comparable to good secondary effluent was produced with 33 minutes carbon
contact time. On the basis of this preliminary work the City of Rocky River
applied for and was awarded a Research and Development Grant from EPA to
help support a full-scale investigation of the clarification-adsorption
process for secondary treatment.
One of the principal motivations for the city to use physical-chemical treat-
ment is shown in Figure 9. The installation of conventional activated sludge
facilities would necessitate the condemnation of a considerable area of
expensive property, whereas a carbon adsorption system could easily fit into
the existing site. Testing has been undertaken to determine what coagulant
or combination of chemicals will be used in the clarification system. If
phosphorus removal is required an inorganic coagulant will necessarily be
the choice.
ADVANTAGES OF PHYSICAL-CHEMICAL TREATMENT VS. CONVENTIONAL TREATMENT
Several times in this discussion advantages of physical-chemical treatment
have been referred to. Perhaps the most important is the stability of
operation provided by a treatment system based on physical and chemical
technology. Biological systems are notoriously sensitive to changes in
environmental conditions. If a toxic material gains even temporary entrance
to the plant or a hydraulic peak occurs not only will the efficacy of the
biological plant drop off but recovery may take several days to several
weeks. In a physical-chemical plant the filtration system backs up the
clarifier and the carbon system backs up the first two thus upsets should
be unlikely. In addition, it can be expected that an immediate recovery of
the plant will take place once the source of upset is eliminated. This
inherent stability of performance is also reflected in greater design and
operational flexibility. Whole sections of a physical-chemical plant can
be cut in or out of the process stream as required, and a temporary overload
can be absorbed with little effect. A list of many of the major advantages
of a physical-chemical system is given in Table 9. Most have been discussed
at some point in this paper.
COST ESTIMATES
One of the uncertain factors concerning physical-chemical treatment is the
cost of the process. Definitive data will not be available until large-scale
plants have been built and are in operation for several years. Even then
local conditions may significantly affect the actual costs. Smith (11) has
taken the available information from pilot plants and preliminary designs of
several proposed large-scale plants and has made cost estimates for various
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size plants referenced to October 1970. Amortization was 6 percent for
24 years. These are presented in Table 10. Note that ranges are given
for each plant size. These ranges represent the spread of data available.
As a comparison Smith's estimate for primary and secondary treatment with
sludge incineration is 16.5 cents per 1000 gallons at the 10 MGD level.
With the addition of single stage lime for phosphorus removal the cost
would rise to 23.5 cents per 1000 gallons which is essentially the same
as physical-chemical treatment.
FUTURE DEVELOPMENTS
As a result of the advantages of physical-chemical treatment discussed
above and the favorable .economic comparison, a number of full-scale
treatment plants are being planned or designed. These are listed in
Table 11. Only the first two, Rocky River and Painesville, Ohio, will
receive Federal research funds. All the others are being planned with
no expectation of Federal funds other than the normal construction grant.
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REFERENCES
1. Gulp. G., "Chemical Treatment of Raw Sewage - 1"
Water & Wastes Engineering, h_, 61 (July 1967).
2. Gulp, G., "Chemical Treatment of Raw Sewage - 2"
Water & Wastes Engineering, ^, 54 (Oct. 1967).
3. Rizzo, J. L., Schade, R. F.,
"Secondary Treatment with Granular Activated Carbon,"
Water & Sewage Works, 116, 307 (Aug. 1967).
4. Weber, W. J., Hopkins, C. B., and Bloom, R.
"Physio-Chemical Treatment of Wastewater,"
JWPCF, 42, 83 (Jan. 1970).
5. Hannah, S. A., "Chemical Precipitation of Phosphorus,"
Paper presented at the Advanced Waste Treatment & Reuse
Symposium, Dallas, Texas, January 12-14, 1971,
EPA sponsored.
6. Villiers, R. V., Berg, E. L., Brunner, C. A., and Masse, A. N.
paper presented at ACS meeting, Toronto, Canada, May 1970.
7. Bishop, D. F., O'Farrell, T. P., and Stamberg, J. B.,
"Physical-Chemical Treatment of Municipal Wastewater,"
paper presented at the 43rd Annual Conference WPCF,
Boston, Mass., October 1970.
8. MonthlyProgress Reports - Contract No. 14-12-585
between Eimco Corporation, Salt, Lake City, Utah & EPA.
9. Smith, C. E., "Recovery of Coagulant, Nitrogen Removal and
Carbon Regeneration in Wastewater Reclamation," Final Report,
FWPCA Grant, WPD-85 (June 1967).
10. Appraisal of Granular Carbon Contacting," TWRC Report 11,
USDI-FWPCA, May 1969.
11. Smith, Robert, Internal Report, R. A. Taft Water Research
Center - EPA, February 1971.
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TABLE 10
PRELIMINARY COST ESTIMATE PHYSIGAL-CHEMICAL TREATMENT
Total Amortization + O&M, cents per 1000 gallons
Plant Size, MGD
Chemical Clarification
Carbon Adsorption
Filtration
TOTAL
5
9.5-13.5
11.5-18.0
2.9-4.5
23.9-36.0
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2.1-3.3
18.5-26.4
100
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4.5-7.8
1.0-1.4
9.5-14.5
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Two-stage lime recalcination of sludge.
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