PHOSPHORUS REMOVAL
TECHNOLOGY TRANSFER DESIGN SEMINAR
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND MONITORING
NATIONAL ENVIRONMENTAL RESEARCH CENTER
CINCINNATI, OHIO
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GENERAL INFORMATION ON PHOSPHORUS REMOVAL
Prepared for the
U. S. Environmental Protection Agency
Technology Transfer Design Seminar
Presented at
Chicago, Illinois, November 28-30, 1972
National Environmental Research Center
Advanced Waste Treatment Research Laboratory
Office of Research & Monitoring
Cincinnati, Ohio
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GENERAL INFORMATION ON PHOSPHORUS REMOVAL
I. J. Kugelman
The key role of phosphorus in the process of eutrophication (aging
of lakes and impoundments) has been known for many years. However, until
recently, eutrophication of the nation's waterways was not a significant
problem. Consequently, control of the phosphorus level in streams and lakes
was not considered an important pollution control problem. Since the con-
clusion of World War II, however, the rate of eutrophication has increased
to the point where it is a major water quality problem, Although o':her
(1 2)
nucriencs pi ay a role in eutrophication, recent articles by Sawyer '
have indicated that much of the recent increase in eutrophication rate is
linked to significant increases in phosphorus discharges to rivers and lakes.
Sawyer has shown that virtually all of the increase in phosphorus discharge
is due to the activities of man and has termed the resulting eutrophication,
cultural eutrophication.
The major sources of phosphorus contributing to eutrophication are
domestic sewage and agricultural runoff. Domestic sewage is the primary
source in critical areas, and only this source will be discussed here.
Phosphorus gains entrance to sewage from human body wastes (primarily urine)
and through the use of condensed inorganic phosphate compounds as builders
in detergent formulations. Each of these sources accounts for about half
of the phosphorus in domestic sewage. Thus, while elimination of phosphorus
from detergent formulations would be helpful, it would not be the total
answer to the eutrophication problem. Treatment of domestic sewage to re-
move a significant portion of the phosphorus contributed by human wastes
and detergent builders would, however, have a significant effect on
eutrophication rate.
Phosphorus Removal in Conventional Treatment
Removal of any pollutant from wastewater requires that it be converted
to either an insoluble gas or an insoluble solid. Because none of the
chemically stable forms of phosphorus is a gas at normal temperature and
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- 2 -
pressure, removal from wastewater is dependent on formation of an insoluble
solid. Less than 10 percent of the phosphorus discharged to municipal sewer-
age systems is insoluble and none of the conventional treatment techniques
is particularly effective in insolubilizing this nutrient. Thus, phosphorus
removal in conventional treatment systems is relatively poor. Primary treat-
ment can remove only the 10 percent of the phosphorus which is initially
insoluble. During secondary treatment, phosphorus removal is achieved by
synthesis into the biomass followed by sedimentation and sludge wasting.
However, municipal sewage contains a considerable excess of phosphorus over
that required for biomass synthesis during complete utilization of the
organic carbon present; thus, removals are generally limited to 20-40
percent . Studies indicate that biological systems have the capacity for
(4)
much higher removals through the mechanism of "luxury uptake" . However,
attempts to implement this phenomenon in actual plants have not been
successful . Indeed, a recent study has illustrated that luxuty uptake
may actually be a chemical precipitation rather than a biologically mediated
, (6)
phenomenon
Phosphorus Removal by Chemical Precipitation
Fortunately, phosphorus forms essentially completely insoluble precipitates
with a number of substances, thus high levels of removal can be obtained when
appropriate doses of the proper chemicals are applied. A large variety of
chemicals can be utilized for this purpose but economic factors dictate the
use of salts of iron, salts of aluminum or lime.
For both the salts of aluminum and iron, the required dose is set by
similar factors. The major control is the stoichiometric reaction between
the metal salt and the phosphorus compound as illustrated in equation 1.
M+3 + P0"3 ............ >MP0 (1)
This reaction indicates a requirement of 1 mole of the metallic cation for
each mole of phosphorus in the wastewater. However, a number of factors act
to increase the dose of metallic salt required. As indicated by equation 2,
the metal salt reacts with the alkalinity in the wastewater to form insoluble
hydroxide .
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R+3 + 3HC03"1 ............ > M(OH)3I + 3C02 (2)
Although this is a competing reaction with the phosphorus precipitation, the
metal consumed is not wasted as the hydroxide formed is an excellent flocculant
of the phosphorus precipitates of equation 1. Another factor which tends to
increase metal dosage over that theoretically required is pH. Figure l
illustrates that aluminum phosphate is least soluble at pH 6 and iron phos-
phate is best precipitated at pH 4 . It is usually best to use additional
metal salt rather than adjust the pH of the sewage from its normal range of
7-8. Another factor which influences coagulant dose is the presence of sus-
pended organic solids in the wastewater. A portion of the metal salt added
will be required for coagulation of these materials, again increasing the
dose requirement.
The dose required to achieve any specified degree of phosphorus removal
cannot be specified with great precision because of the factors discussed
above. It is best to conduct bench-scale jar tests on the wastewater to be
treated to establish the metal salt dose requirement . Average of results from
(3)
many installations which can serve as a rough guide are given in Table 1
TABLE 1
AVERAGE METAL SALT DOSE REQUIRED FOR
SPECIFIED PERCENT PHOSPHORUS REMOVAL
Dose Metal Salt
% P Removal Mole Ratio M /P
75 1.4
85 1.7
95 2.3
The reaction of lime with wastewater constituents is given in equations
3 and 4:
3HP04"2 + 5Ca+2 + 40H"1 ..... > Ca (OHMPO^) ^ + 3H 0 (3)
Ca(OH)2 + Ca(HC03)2 ......... > 2CaC03^ + 2^0 (4)
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A TOTAL PHOS.JF1LT.)
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12
FIGURE: 2
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Equation 3 represents the formation of the insoluble compound calcium
hydroxyapatite. The kinetics of this reaction as well as the stoichiometry
are pH dependent. Elevated pH values 9.5-11.5 are required to ensure good
phosphorus removal by precipitation and liquid solids separation techniques.
(8)
Figure 2 illustrates this pH effect . Thus, while pH was a secondary factor
in establishing the dose requirement with iron or aluminum as the precipitant,
it is the primary factor when calcium is utilized.
In order to reach the pH range which will ensure good phosphorus removals,
the alkalinity demand of the wastewater must be met, as illustrated in equation 4.
Because the bicarbonate concentration in wastewater is much higher than the
phosphorus concentration (several hundred mg/1 vs. =s 10 mg/1), therefore, most
of the lime dose required to achieve effective phosphorus removal is used to
meet the alkalinity demand. Here a situation exists similar to that occurring
when metal salts are used for phosphorus removal; i.e., the product of the
cation alkalinity reaction serves as a flocculant of the phosphorus pre-
cipitate. Thus, the reaction of the lime with the alkalinity does not
completely waste the lime.
When reaction 4 is complete, the pH of the wastewater is in the range
of 9.5-10.0. For wastewater with moderate to high levels of alkalinity
(> 200 mg/1), sufficient calcium carbonate is formed under these conditions
to effectively bring down the hydroxyapatite. After sedimentation, the pH
is reduced to < 8 by the addition of CO . This process is referred to as
single-stage lime precipitation. If the alkalinity of the wastewater is low
> 150 mg/1, however, insufficient calcium carbonate is formed. For this
situation, the pH must be raised above 11 which will ensure precipitation
of magnesium hydroxide according to equation 5:
Ca(OH)2 + Mg+2 > Ca+2 + MgtOH)^ (5)
Magnesium hydroxide is a gelatinous precipitate which will effectively
scavenge hydroxyapatite from solution. Adding excess lime to raise the pH
to high levels results in a wastewater containing excess calcium. The calcium
is reduced by carbonation with CO . As illustrated in equation 6, this re-
duces the pH and results in a second precipitation of calcium carbonate:
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- 7 -
C02 + Ca+2 + 2 OH" > CaC03^'+ ^0 (6)
The second precipitation is carried out in a sedimentation tank separate from
that in which hydroxyapatite, calcium carbonate and magnesium hydroxide pre-
cipitation takes place. The recarbonation reaction is complete at pH 9.5-10.0,
which is the same end point pH as in the single stage process. A second dose
of CO is then applied to reduce the pH below 8. The final pH reduction in
the two-stage and single-stage processes is utilized to prevent post precipi-
tation of CaCO., in other treatment units. Flow diagrams of the single-stage
and two-stage lime precipitation processes are given in Figure 3.
Table 2 gives a list of chemicals which are useful for phosphorus
precipitation. Small doses of organic polymer may also be required to aid
flocculation. In selecting the chemical for use at any particular site, the
factors listed in Table 3 should be taken into account. It is clear that
the cost of the chemical is only one factor among many which bear on the
ultimate cost of phosphorus removal. Of prime importance is dewatering and
ultimate disposal of sludges. Here, lime has an advantage because lime
sludges are more easily dewatered than iron or aluminum sludges. In addition,
lime sludge can be recalcined to reusable lime in an incinerator. However,
large capital expenditures such as an incinerator can only be justified at
large treatment plants.
Phosphorus removal is achieved by precipitation followed by liquid
solids separation. For the most part, the usual liquid-solids separation
equipment in a treatment plant can be utilized for phosphorus removal. This
results in a considerable savings in capital as well as integration of
phosphorus removal into conventional treatment plant operation. In addition,
it has been found that the use of chemical precipitants in conventional treat-
ment can markedly upgrade performance of the treatment plant. This results
from coagulation of organic suspended and colloidal solids by the chemicals
added to precipitate phosphorus.
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SINGLE STAGE LIME TREATMENT
LIME SLURRY
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FIGURE: 3
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- 9 -
TABLE 2
CHEMICALS FOR PHOSPHORUS REMOVAL
Ferric Chloride
Ferric Sulfate
Ferrous Chloride
Ferrous Sulfate
Alum
Sodium Aluminate
Steel Mill Pickling Liquor
Lime
Fed,
FeSO,
NaAlO,
FeCl. + FeSO.
2
Ca(OH),
TABLE 3
FACTORS AFFECTING CHOICE OF CHEMICAL
FOR PHOSPHORUS REMOVAL
Influent Phosphorus Level
Wastewater Suspended Solids and Alkalinity
Chemical Cost Including Transportation
Reliability of Chemical Supply
Sludge Handling Facilities
Ultimate Disposal Methods
Compatibility with Other Treatment Processes in Plant
Potential Adverse Environmental Effects
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Figure 4 illustrates a conventional treatment plant with the three
general sections in which phosphorus removal can be carried out. Chemical
can be added either:
a. Just before the primary tank with removal taking place in the primary.
b. In the secondary (biological) section of the plant with removal in
the secondary sedimentation tank.
c. In a tertiary stage as was discussed in the section on suspended
solids removal.
Table 4 illustrates typical results obtained with phosphorus removal
in the primary, secondary, or tertiary. As can be seen, good removals are
obtained in all sections, however, the lowest levels of phosphorus remaining
are achieved in the tertiary addition. One reason for this is that a filter
is usually included in the tertiary plant, thus better removal of fine preci-
pitates is achieved. In addition, when the flow reaches this section of the
plant all the complex phosphorus forms which are more difficult to precipitate
have been hydrolyzed to orthophosphate which is the easiest to precipitate.
In plants where removal in the primary was practiced a major effect to
note is the significant increase in BOD and suspended solids removal achieved
over the removal usually obtained in the primary tank. This may be important
in helping to meet water quality standards for BOD and suspended solids if the
treatment plant is overloaded.
In plants where the chemical is added in the secondary section, it has
been observed that much more stable operation of the activated sludge is
obtained than before chemical addition. The effect of the chemical is to
weigh the sludge down, preventing its loss when a filamentous or dispersed
growth predominates. Even in plants which have historically exhibited
excellent performance, chemical addition has improved performance by helping
maintain a higher concentration of activated sludge in the aeration tank.
This is illustrated by the data in Table 5 from parallel operation for one
( 9 )
year at Penn State University . It has been found best to add the
chemical between the boilogical reactor and the final sedimentation tank
rather than at the head end of the secondary tank.
Table 6 summarizes the advantages and disadvantages of carrying out
phosphorus removal in the various sections of a treatment plant.
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TABLE 5
COMPARISON OF ACTIVATICD SLUDGE PERFOR1-IANCE
WITH AND WITHOUT ALUM ADDITION
Parameter
S.S.
BOD
COD
Soluble P
Total P
Influent
mg/1
110
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10.0
Effluent, mg/1
Alum Normal
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The cost of adding phosphorus removal to existing treatment plants
is given in Table 7. The lowest cost option is addition in the secondary
because capital expenditure for chemical feed equipment and extra sludge
handling equipment are least in this mode. However, removals below 1 mg/1 P
are difficult. The most costly option is tertiary because of the high capital
cost. However, an excellent quality effluent is obtained which is very low in
phosphorus, BOD and suspended solids. Note the higher tertiary cost here than
for the upgrading system. This is due to higher chemical doses, and additional
sludge handling difficulties. Table 8 lists a number of plants where phosphorus
removal is being conducted or is in the planning stages.
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- 16 -
V.
TABLE 7
ECONOMICS OF PHOSPHORUS REMOVAL
Cost Cents per 1000 Gallons
Plant Size,
iGD
Chemical Addition
Point
Primary
Secondary
Tertiary
10
100
6-8 .
6-7
28 '
3-5
3.5-4.0
12
3-4
3.0-3
6
.5
TABLE 8
INSTALLATIONS 1MIACTICING OR PLANNING PHOSPHORUS RKMOVAL
Lor at 3 on
Chicago, Illinois
Seatt -1 e, Washington
Pomona, Ca J jfornia
S. Lake Talioc, California
PiscaLtaway > Mary.l and
Colorado SprJngs, Colorado
Richardson, Texas
Blue P.laJnr.
El Laju), Texas
Hal field l\vp., Pa.
Capacity, MOD
30
20
2
7.5
5
2
1
300
0.5
5.0
Status
Design
Design
In Operation
In Operat ion
Construct ion
In Operation
In Operation
In Operat. ion
Construe Lion
In Operation
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- 17 -
BIBLIOGRAPHY
1) Sawyer, C. N. "ABC's of Cultural Eutrophication Part I Cultural Changes,"
Water & Sewage Works 120, 278, Sept. (1971)
2) Sawyer, C. N. "ABC's of Cultural Eutrophication Part II Wastewaters ,"
Water & Sewage Works 120, 322 (Oct. 1971)
3) Process Design Manual for Phosphorus Removal, EPA Technology Transfer,
Washington, DC (Oct. 1971)
4) Levin, G. V., Shapiro, J. "Metabolic Uptake of Phosphorus by Wastewater
Organisms" Journal WPCF, 37, 800 (1965)
5) Mulbarger, M. C., Shifflett, D. G., Murphy, M. C., and Huffman, D. 0.,
"Phosphorus Removal by Luxury Uptake," Journal WPCF 43, 1617, (1971)
6) Menar, A. B., Jenkins, D. "Fate of Phosphorus in Waste Treatment
Processes; Enhanced Removal of Phosphate by Activated Sludge."
Environmental Science and Technology, 4, 1115 (1970)
7) Recht , H. L., Ghassemi, M., "Kinetics and Mechanism of Precipitation
and Nature of Precipitate Obtained in Phosphate Removal from Waste Water
Using Al III and Fe III Salts," EPA Report 17010 EKI (1970)
8) Stamberg, J. B0, Bishop, D0 F., Warner, H. P., Griggs, S. H., Lime
Precipitation in Municipal Wastewaters. Chemical Engineering Symposium
Series Water 1970 67, 310.
9) Op. Cit. (3) pages 7-13
10) Kreissl, F. F., "Phosphorus Removal Today," presented at the Sanitary
Engineering Institute, University of Wisconsin, Madison, Wisconsin,
March 9-10, 1971.
11) Bishop, D. F., O'Farrell, T. P., Stamberg, J. B. "Physical Chemical
Treatment of Minicipal Wastewater." Journal WPCF 44, 361, (1972)
12) Mdddleton, F. M., Convery, J. J., "Municipal Pollution Control Technology
in the United States of America." EPA-NERC, Cincinnati, Ohio (1971)
13) Mulbarger, M. C., Shifflett, D. G., "Combined Biological and Chemical
Treatment for Phosphorus Removal." Chemical Engineering Progress
Symposium Series, 67, 107 (1970)
14) Laughlin, J. E. , "Modification of a Trickling Filter Plant to Allow
Chemical Precipitation" presented at EPA Technology Transfer Seminar,
Dallas, Texas. July (1971)
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- 18 -
15) Gulp, R. L., Gulp, G. L. Advanced Wastewater Treatment, Van Nostrand
Reinhold Co. New York (1971)
16) Progress Reports EPA - Nassau County Advanced Waste Treatment Project
WPRD 61-01-67
17) Cohen, J. M.3 "Nutrient Removal from Wastewater by Physical-Chemical
Processes." EPA report, NERC, Cincinnati, Ohio, March (1971)
^TT
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