ROBERT A. TAFT WATER RESEARCH CENTER
REPORT NO. TWRC- 8
ALUMINA COLUMNS
FOR
SELECTIVE REMOVAL
OF
PHOSPHORUS FROM WASTEWATER
"»
•
ADVANCED WASTE TREATMENT RESEARCH LABORATORY -VIM
U.S. DEPARTMENT OF THE INTERIOR
FEDERAL WATER POLLUTION CONTROL ADMINISTRATION
OHIO BASIN REGION
Cincinnati, Ohio
-------
EVALUATION OF OPERATING PARAMETERS OF ALUMINA
COLUMNS FOR THE SELECTIVE REMOVAL OF PHOSPHORUS FROM
WASTEWATERS AND THE ULTIMATE DISPOSAL OF PHOSPHORUS
AS CALCIUM PHOSPHATE
by
L. L. Ames
for
The Advanced Waste Treatment Research Laboratory
Robert A. Taft Water Research Center
This report is submitted in
fulfillment of Contract No.
14-12-413 between the Federal
Water Pollution Control Ad-
ministration and Battelle-
Northwest.
U. S. Department of the Interior
Federal Water Pollution Control Administration
Cincinnati, Ohio
March 1969
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FOREWORD
In its assigned function as the Nation's principal na-
tural resource agency, the United States Department of the
Interior bears a special obligation to ensure that our ex-
pendable resources are conserved, that renewable resources
are managed to produce optimum yields, and that all resources
contribute their full measure to the progress, prosperity,
and security of America -- now and in the future.
This series of reports has been established to present
the results of intramural and contract research studies car-
ried out under the guidance of the technical staff of the
FWPCA Robert A. Taft Water Research Center for the purpose
of developing new or improved wastewater treatment methods.
Included is work conducted under cooperative and contractual
agreements with Federal, state, and local agencies, research
institutions, and industrial organizations. The reports are
published essentially as submitted by the investigators. The
ideas and conclusions presented are, therefore, those of the
investigators and not necessarily those of the FWPCA.
Reports in this series will be distributed as supplies
permit. Requests should be sent to the Office of Information,
Ohio Basin Region, Federal Water Pollution Control Administra-
tion, 4676 Columbia Parkway, Cincinnati, Ohio 45226.
11
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TABLE OF CONTENTS
ABSTRACT iv
INTRODUCTION !
LABORATORY STUDIES 5
Small Alumina Columns 5
One Liter Alumina Columns 21
CONCLUSIONS AND RECOMMENDATIONS 31
REFERENCES 32
ill
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The use of high surface area alumina for phosphorus removal from
wastewaters vas investigated on a laboratory scale.
A synthetic secondary sewage effluent containing 10 to 30 mg POj/1,
O _!_
300 mg HC03~/1, 112 mg Cl~/l ana 50 mg SO^/l as anions and 130 mg Na ,/l,
60 mg Ca /I, 25 mg each of K and Mg and 20 mg NK, /I as cations was
composited. With the above synthetic sevage effluent and 7.6 ml alumina
columns, the effects of various compositional changes on phosphorus
removal were ascertained.
+ 2 +2
The concentration of Mg and Ca showed marked effects on alumina
column phosphorus capacity, with the phosphorus capacity about half for
+ 2 + 2
the solution containing no Ca or Mg as compared to solutions containing
+ 2 +2
the above Ca and Mg concentrations. Probably adsorption onto the
alumina occurs partly as a calcium or magnesium phosphorus complex rather
than as a simple phosphorus anion. This notion is supported by the fact
that reducing the phosphorus content of the influent solution by one-
third only increases column capacity by a factor of two rather than three.
Increasing the competing sulfate concentration in the influent solution
_2
to h800 mg SO, /I showed little effect on phosphorus adsorption by
alumina. Raising the pH of the synthetic sewage effluent to greater than
8.0 led to precipitation of calcium carbonate on the alumina grains which
progressively lowered alumina phosphorus capacity. This problem was not
encountered when using actual sewage effluent. It was found that alumina
columns could be satisfactorily regenerated with NaOH. An acidic wash
was not required.
Sewage effluent from the Richland trickling filter plant was used
in scaleup to one liter alumina columns. Approximately UOO column volumes
of Richland effluent were required for a 20 to 50 mesh, 275 nrVg alumina
column to reach 10% phosphorus breakthrough. If two columns were operated
in series, more than 600 column volumes could be loaded on the first
column before elution would be required.
IV
-------
Elution of phosphorus was accomplished with eight column volumes of
2M NaOH, followed by 20 column volumes of washwater. The column was then
ready for reloading with phosphorus. If the column is drained at the end
of the elution, NaOH losses are about 5% of a column volume per elution.
The washwater containing phosphorus and NaOH washings can be recycled back
to the sewage plant headend. The eight plus column volumes of NaOH con-
taminated with HCO and phosphorus, as well as organic coloring matter,
can be cleaned by addition of 1.25 to 6.25 g/1 of lime. The phosphorus is
removed as Ca (PO,)OH (hydroxyapatite) and the HCO ~ as CaCO (Calcite).
The coloring matter settles out with the above solids, allowing reuse of
the NaOH. Thus only a small volume of solid waste is produced by the
phosphorus removal process.
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INTRODUCTION
(12)
Sawyer has stated that phosphorus may represent the major
uncontrolled pollutant now released to the environment in wastewater.
(17)
According to Stephan , the average soluble phosphorus content of
municipal secondary effluent is 25 mg PO./l, measured as orthophosphate.
Other anions include Cl~ at 130 mg/1, NO ~ at 15 mg/1, HCO ~ at 300 mg/1,
—2
and SO) at 100 mg/1. Combined chemical-biological sewage treatment and
phosphorus removal systems are now under study and development ' '
(2 k 9 16)
as well as tertiary chemical-physical removal processes ' ' ' . The
above treatment systems, however, operate most economically as large-
scale processes. A simple, economical phosphorus removal process would
be desirable for the smaller, one to ten million gallons /day trickling
filter sewage treatment plants. A phosphorus polishing process may
also be required for the larger, combined treatment plants.
Few, if any, of the suggested or proven chemical-physical processes
for phosphorus removal have contemplated the use of ion exchange. There
is little economic incentive for employing an ion exchange process for
phosphorus removal as Table 1 illustrates. Dowex 1 is a typical, strongly
_2
basic polystyrene matrix resin. The SO. constant was not given, but
could be expected to be considerably higher than that of any of the above
univalent anions. Use of an organic resin, then, requires removal of
nearly all the other anions in addition to phosphorus, resulting in a
liter of resin treating ^0 liters or less, of average secondary sewage
effluent containing approximately 600 mg/1 of assorted anions.
Table 1
Anion exchange equilibrium constant t-,-,\
(k) measurements on chloride Dowex 1 resin
k_ Anion k_
8.7 ' HC03~ 0.32
l+.l HgPO^' 0.25
3.8 OH~ 0.09
1.0
-------
In addition to low phosphorus removal efficiency, the problems of
disposal of large regenerant waste volumes and use of relatively large
process equipment are encountered with nonselective processes. Plainly,
a more phosphorus-selective adsorbent is required if adsorption removal
processes are to be economically attractive. Such a potentially economical
process is available by utilizing alumina in adsorption columns.
Alumina ore, or bauxite, characteristically occurs in nature as an
impure weathering product of aluminous rocks, and contains a mixture of
gibbsite, boehmite and diaspore. According to the 196? Minerals Yearbook,
production of bauxite in 1966 was ^0 million long tons with the United
States consuming about one-third of world production. About six million
tons of 99% purity alumina are produced annually in the United States,
chiefly for use in the aluminum metal industry, but also for use in the
abrasive, refractory and chemical industries.
As the composition of bauxite implies, several crystalline modifica-
tions of hydrated and nonhydrated aluminas exist. Nomenclature differences
present some difficulties, however, so that only a few names will be given
as an example in Table 2 along with physical properties. All of the
aluminas in Table 2 show reproducible X-ray diffraction patterns. The
Table 2
Some alumina crystal modifications
Comparable
Structure
Alpha Alumina
Trihydrate
Beta Alumina
Trihydrate
Alpha Alumina
Monohydrate
Beta Alumina
Monohydrate
Density
Nomenclature
Gibbsite
Baverite
Boehmite
Diaspore
Formula
Al(OH)
Al(OH)
AIO(OH)
2.53
3.01
AIO(OH)
Alpha Alumina
Corundum
3.98
-------
formulae are given in the form which emphasizes that these are hydroxide
(OH~-containing) compounds and not hydrated (H 0-containing) compounds.
(13)
Anion exchange reactions on aluminas then become probable. Schwab
stated, for example, that the adsorption of ions, both cations and anions,
on alumina takes place by displacement of ions of equal sign contained in
the alumina surface. Shuvaeva and Gapon ' also interpreted alumina
chromatographic separations to mean that ion exchange is the primary
mechanism involved in ion removal. They also showed that secondary
(lU)
adsorption occurs during removal of multivalent ions. Shibata gives
a cation adsorption series on alumina, in decreasing order of preference
as Fe+3, Hg+2, Pb+2, Cu+ , Zn+ , Ca+ , Ni+ , Fe+ and Mn ", approximately
the adsorption order for a normal hydrated ion series. Kubli found
the anion alumina adsorption series, in order of decreasing preference,
to be OH~, POU~3, C^'2, F~, S03~2, CrO^'2, N02~, Cl~, N03", MnO^" and
so,~2.
l±
Yee first made use of the chromatographic properties of high
surface area alumina for the selective removal of phosphorus from waste-
water. He reported that a downflow, 2 ml, alumina column could process
about 1000 column volumes of a synthetic waste containing 25 mg PO^/1 as
orthophosphates only to one percent breakthrough. Upflow tests were run
on larger 200 ml columns using- low-level radioactive process wastewater
containing about 1 to 3 mg P0,/l and suspended solids. Yee estimated
that about 20,000 column volumes of the above waste could be processed
to one percent phosphate breakthrough in the effluent. Higher waste
throughputs were reported for forms of phosphorus other than orthophos-
phates. However, since biological treatment hydrolyzes 90% or more of
the phosphorus, orthophosphates will predominate.
In Yee's work, the adsorbed phosphorus was stripped from the alumina
column with 1 M NaOH and reacidified with 1 M HNO for a total of from 12
— -3
to 16 column.volumes of regenerant waste produced. Alumina losses during
regeneration were given as from 5 to 35 percent, depending on the type of
alumina used. No continuous cycling data was given for phosphorus removal
on alumina columns. Yee estimated that phosphorus removal from wastewaters
-------
on alumina columns would be $39.00/million gal in the case of a 1^ mg
PO, /I influent to $6U.OO/million gal for a 23 mg PO^/1 influent. Eight
percent/cycle alumina losses were assumed in the above cost estimate.
Laboratory research at Battelle-Northwest has extended the work of
Yee, and has demonstrated that the use of alumina columns represents a
potentially economical phosphorus removal process.
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LABORATORY STUDIES
Small Alumina Columns
A synthetic secondary sewage effluent was used in the small column
studies. The composition of the synthetic, to be referred to as SSE, is
given in Table 3. The SSE was used as a standard influent and was
Table 3
Synthetic Secondary Sewage Effluent (pH = l.h)
Constituent mg/1 Constituent mg/1
Na+ 130 Cl~ 112
K+ 25 HC03~ 300
NH^+ 20 SO)-2 50
Ca+2 60 P 3.5-10.3
Mg+ 25
compositionally altered to determine the effects of the changes on phos-
phorus removal by alumina.
Small, 1.1 cm diameter by 8.0 cm height columns were used with the
above SSE to determine alumina phosphorus removal. The SSE was pumped
through the .column at a predetermined rate. Column effluent was sampled
at intervals and the phosphorus content of the sample determined by the
standard ammonium molybdate-aminodisulfonic acid method. The phosphorus
removal values were plotted as C/CO vs. volume of throughput where C
equals the phosphorus concentration in the effluent and Co is the phos-
phorus concentration in the influent.
Several investigators of alumina adsorption phenomena have considered
that the adsorption is essentially a highly selective ion exchange process.
With that assumption, increasing the surface area of the alumina should
increase alumina phosphorus adsorption capacity. Three different columns
containing aluminas of known surface area were compared as to their phos-
phorus capacities with the results shown in Figure 1. A simple and direct
relationship between alumina surface area and phosphorus capacity was
indicated. Surface area effects probably overshadowed any effects due to
differences in the crystalline structure of the alumina. The boehmite
-------
0.2 -
1000 2000
COLUMN VOLUMES (7.6 ml)
3000
Figure 1. Phosphorus loading of aluminas of different surface areas.
Q
Columns - A, boenmite, 22 m /g
B, "boehmite, 200 m2/g
C, gel, 292 m2/g
Influent - SSE (7.0 mg P/l)
Flov Rate - 30 column volumes/hr
Temperature - 23°C
-------
columns, for example, treated less SSE than the noncrystalline alumina,
and also vere less in surface area.
A rough determination of the reaction enthalpy change "between 23°C
and 60°C yielded a AH° of 15 ± ^ Kcal/mole. The AH° was obtained by
measuring the effects of heat on the equilibrium phosphorus distribution
between solution and alumina. Generally speaking, chemical reactions
range upward from 20 Kcal/mole, while ion exchange reactions usually vary
from 2 to 10 Kcal/mole. It may be concluded that phosphorus adsorption
represents an unusually high bond strength type of ion exchange reaction.
As reported in the introduction, polyvalent anions such as phosphorus
are involved in secondary adsorption reactions. Small alumina columns
with varying phosphorus concentrations in the standard influent were com-
pared as to phosphorus removal with the results shown in Figure 2. At 10%
breakthrough, the throughputs for 10.3, 7.0, and 3.5 mg P/l in the standard
influent were 900, lUOQ, and 1900 column volumes, respectively. About the
same loading differences were maintained between the isotherms for other
breakthrough values. If the relationship between phosphorus removal and
alumina surface area were a simple one, halving the phosphorus concentration
without changing the surface area should give twice the influent throughput
volume to the same percent breakthrough. Since the relationship between
alumina surface area and phosphorus removal obviously is not a simple
inverse one, other factors such as the formation of complexes with other
ions contained in the solution must be considered. The divalent cations
of calcium and magnesium would be most apt to enter into complex formation
+'P
with phosphorus. The effect on phosphorus removal of deleting Mg (25 mg/l)
+2
and Ca (60 mg/l) from the standard influent was measured with the results
given in Figure 3. Phosphorus is removed from the influent containing no
calcium or magnesium but the phosphorus loading is about halved when com-
pared to a standard influent containing the usual calcium and magnesium
concentrations. Most of the original phosphorus capacity is restored by
+2
the 25 mg/l of Mg . At least part of the phosphorus is bonded onto the
+2 +2
alumina surface by a complex involving Mg and Ca . As the phosphorus
concentration increases, the portion of the phosphorus removed by this
-------
1. 0|
1 - • - r
0. 8
0.6
0.4
0.2
J 1 L.
1000
2000
COLUMN VOLUMES (7. 6 ml'
Figure 2. Effects of phosphorus concentration on phosphorus removal by
alumina.
Columns - Boehmite, 100 to 200 mesh
Influent - SSE with P content as indicated
Flow Rate - 30 column volumes/hr
Temperature - 23°C
-------
1.0
0. 8 -
0. 6 -
1000 2000
COLUMN VOLUMES (7.6 ml)
3000
Figure 3. Effects of calcium and magnesium ions on phosphorus removal
by alumina.
Columns - Boehmite, 100 to 200 mesh
Influent - SSE (3.5 mg P/l)
Flow Rate - 30 column volumes/hr
Temperature-23°C
-------
complex may increase, giving larger phosphorus removal values than could
be expected from a consideration of alumina surface area alone.
The competition, or effects of other anions on phosphorus removal
was of interest because large concentrations of other anions could signifi-
cantly lower the alumina phosphorus capacity. Sulfate ion was chosen to
demonstrate phosphorus capacity effects due to some similarities in the
-2 -2
behavior of SOv and HPO, . The phosphorus loading curves from a
simulated secondary sewage effluent containing 3.5 mg P/l are shown in
Figure k. Unusually large concentrations of sulfate ions are necessary
to adversely affect phosphorus removal by the boehmite. This conclusion
_2
is probably valid for other anions with the exception of arsenate. HAsO,
_2
could be expected to substitute for HPO, in the adsorption reaction with
little difference in alumina selectivity between the two anions.
Though the aluminas used in the study were all high in surface area,
grain size still had an effect on the efficiency of phosphorus removal.
Assuming a normal distribution within size ranges and a spherical
shape, the 10 to 20 mesh range should have a surface area of less than 1
m^/g and the 1*8 to 100 mesh somewhat larger. A B.E.T. surface area, however,
2
results in values of 290 m /g for both size ranges of alumina gel. The
same high surface area is available for all gel size ranges and all sizes
would remove the same amount of phosphorus if equilibrium between phosphorus
and solution were attained. The effects of size range on phosphorus loading
characteristics seen in Figure 5 are due to the kinetics of phosphorus
diffusion from the solution surrounding the alumina particle, through the
particle to the adsorption site.
Diffusion kinetics for exchange reactions are of two types; particle
diffusion and film diffusion. Particle diffusion is so-called because
diffusion of the exchanging ions from alumina particle surface into the
particle is the rate-determining step. Particle diffusion assumes a high
concentration of the exchanging ions in the solution surrounding the alumina
particle. If the size of the exchanging ions is very large, however, the
concentration of ions in the solution can be quite low and still have
particle diffusion. Film diffusion is so-named because diffusion of the
exchanging ions within the particle is very fast, leading to depletion
10
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1.0
0. 8
0.6
0.4
0.2
0.05M Na.SO,
2 4
0. 5M Na9SO.
2 4
NO Na2S04
1000 2000
COLUMN VOLUMES (7. 6 ml)
Figurj k. Phosphorus loading curves from standard secondary sewage effluent
containing varying amounts of sodium sulfate.
Columns - Boehmite, 8g
Influent - SSE (3.5 mg P/l)
Flow Rate - 30 column volumes/hr
Temperature - 23°C
11
-------
1.0
0.8
0.6
o
0.4
0.2
0
20 TO 48 MESH
10 TO 20 MESH
0 100 MESH
1000
2000
COLUMN VOLUMES (7.6 ml)
Figure 5- Effect of grain size range on phosphorus adsorption kinetics,
Sieve dimensions are U.S. Standard Series.
Columns- Alumina gel, mesh sizes as indicated, surface area =
290 m2/g
Flow Rate - 35 column volumes/hr
Temperature - 23°C
Influent - SSE (6.7 mg P/l)
12
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of these ions in a film of solution surrounding the alumina particle.
Diffusion of ions across the film surrounding the particle is the rate-
determining step. The concentration of exchanging ions in the solution is
usually low. The diffusion time necessary to attain a given fraction of
equilibrium loading varies inversely as the square of the particle radius
in the case of particle diffusion, and "by a simple inverse ratio to ^article
radius in the case of film diffusion. From a consideration of the curves
shown in Figure 5, it is probable that particle diffusion is rate controlling,
despite the low concentrations of phosphorus. More elaborate techniques
are required, however, to verify the rate controlling step.
Elution of SSE phosphorus loaded columns with various strength acidic
solutions and sodium sulfate is shown in Figure 6. Elution rates fell off
rapidly and were generally inefficiently slow. In addition, acidic elutions
left the column in a condition that was detrimental to phosphorus reloading.
A 1N_ HpSO, alumina column prewash resulted in 2 to 5$ phosphorus leakage
and reduced phosphorus loading as shown in Figure 7.
Basic elutions are recommended both because they are more efficient
and do not reduce the following phosphorus reloading. Sodium hydroxide
and sodium carbonate phosphorus elution curves are given in Figure 8.
The sodium carbonate elution rate is slower than that of sodium hydroxide.
Both of the curves of Figure 8.represent elution of a first cycle loading
alumina column, and illustrate that diffusion of hydroxide ion into the
alumina particle, and phosphorus out of the particle requires considerable
time. More time is required to regenerate the last 20% of the alumina
phosphorus capacity than the duration of the elution afforded.
A leaching experiment with 1M NaOH and boehmite and alumina gel was
conducted to obtain a feeling for alumina losses during sodium hydroxide
elutions. Losses would occur mainly during elution because alumina solu-
bility in solutions such as SSE is very low. Excessive losses of alumina
during elution would moreover, adversely affect wastewater treatment costs.
Eight gram columns of 16 to 60 mesh alumina gel and 100 to 200 mesh boehmite
were leached for several days at a flow rate of 30 column volumes/hr with
1M NaOH. The dissolution rate of alumina gel was 6.3 vt%/2k hrs and for
boehmite, 1.2 vt%/2k hrs. The above weight losses may be compared to Yee's
13
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X
X
20
24
COLUMN VOLUMES (25 ml)
Figure 6. Alumina phosphorus elution curves. X/XO is the fraction of P
removed from the column by the elution treatment.
Column - Boehmite, 100 to 200 mesh
Flow Rate - 35 column volumes/hr
Temperature - 23°C
I - eluted with LOW H2SOi|, C0 = 257 mg P
II - eluted with 0.5N H2S01|, Co = 296 mg P
III - eluted with 0.1N HgSO^, Co = 269 mg P
IV - eluted with 2.ON NagSO^, Co = 2?2 mg P
-------
0.8
0.6
0.4
0. 2
0
ACID-WASHED AS-RECEIVED
(1N_ H2S04>
1000 2000
COLUMN VOLUMES (7. 6 ml)
Figure 7. Effect of acid washing of alumina on subsequent phosphorus
reloading.
Column - Boehmite, minus 100 mesh
Flow Rate - 30 column volumes/hr
Temperature - 23°C
Influent - SSE (7.5 mg P/l)
15
-------
X
X
8 12 16 20
COLUMN VOLUMES (7.6 ml)
24
Figure 8. Sodium carbonate and sodium hydroxide phosphorus elution curves.
X/XQ is the fraction of P removed from the column by the elution treatment.
Columns - Boehmite, 100 to 200 mesh, 200 m2/g
Flow Rate - 30 column volumes/hr
Elution Solutions - I. 1M NaOH, Co = 7^.6 mg P
II. 1M Na2C03, C0 = 5^.7 mg P
16
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reported losses of Q% per elution for boehmite and up to three or four
times the 8% loss for alumina glass. Although the weight loss ratio is
the same as that of this study, the lower losses here probably reflect
leaving out the acid step in regeneration.
If sodium hydroxide is to be used as an alumina column regenerant,
some method of disposal or reuse must be found for the spent regenerant
solution. Lime represented a possible cheap solution to the problem by
removing phosphorus and carbonate from the spent regenerant sodium hydroxide.
Equilibrium results with phosphorus removal from one and five molar sodium
hydroxide solutions by lime were ascertained as shown in Figures 9 and 10.
A 0.1137 molar disodium acid phosphate was used because the most concentrated
phosphorus solution obtained early in the elution step is approximately
of this composition. The hypothetical tricalcium phosphate line was added
for comparative purposes only. The phosphorus is precipitated as a
hydroxyapatite. An apatite is also the raw natural material used in
making fertilizer and phosphorus chemicals. The monetary value of the
apatite produced would probably be very small.
In any case, increments of lime were added to 100 ml solutions
containing the milligrams of phosphorus indicated in Figures 9 and 10.
Some of the'Samples were shaken for 15 minutes and some for an hour,
with the same concentration of phosphorus remaining in the solution. A
rapid lime-solution reaction was indicated. The solids filtered from
these mixtures were identified by X-ray diffraction as lime and hydroxy-
apatite. No extraneous anions were added to the sodium hydroxide
-2
regenerant solution by the lime, and C03 , originally HCO^ before elution,
p _
also was removed. Less than 30 mg SO, /I and 1 mg Cl /I were found in
a regenerant solution that had been reused 10 times.
Some alumina column fouling with calcite (CaCO.,) occurred during
loading with high pH synthetic secondary sewage effluents. For several
consecutive cycles of loading and sodium hydroxide elution, the results
are seen in Figure 11. Wote that alumina column phosphorus capacity
steadily declines for each successive cycle. An alumina sample removed
from the column for X-ray diffraction study showed the presence of calcite
IT
-------
1000
en
E
O
Q_
800
600
400
200
0
I I
\ \ACTUAL REMOVAL
\ \ CURVE
\
\
\ "X .
LINE FOR '
STOICHIOMETRIC
REMOVAL
0
1.0
2.0
GRAMS Ca(OH)2 ADDED
Figure 9. Equilibrium phosphorus removal results vhen Ca(OH)2 was added
to 100 ml of a l.OM NaOH + 0.1137M Na PO, solution.
A = 15 minutes equilibration time
• = 1 hr equilibration time
18
-------
1000 -
en
O
D_
800
600
400
200
0
0
\
\
\
\
ACTUAL REMOVAL
CURVE
\
\
\
\
\
\
LINE FOR
Ca3(P04)2 \
STOICHIOMETR1C \
REMOVAL \
\
. \
1.0
GRAMS OF Ca(OH)2 ADDED
2. 0
Figure 10. Equilibrium phosphorus removal results when Ca(OH)p was
added to 100 ml of a 5.0M NaOH + 9.1137M Na3 PO^ solution.
• = 1 hr equilibration time
19
-------
coating the alumina grains. This precipitation of calcite on the alumina
lowers the alumina surface area available for phosphorus adsorption, and
calcite is not removed from the column during elution with sodium hydroxide.
As alumina grains became calcite-coated, phosphorus surface adsorption
was effectively blocked. Several ways are available to treat the calcite
problem. One way is to leach the column with a bicarbonate solution. The
restoration of a calcitic alumina column is shown in Figure 12. Nearly
all of the phosphorus capacity was regained after NaHCO solution was
recirculated through the column. Lime can be added to the phosphorus-
containing liaHCO to precipitate apatite, if an excess of lime is avoided.
The 15aIiCO solution can then be reused. It is still necessary to remove
any alumina-adsorbed phosphorus with NaOH. The problem of calcite imbalance
was not encountered with actual secondary sewage effluent from the Richland
trickling filter plant.
If Richland effluent is used to load both the large (one liter) and
small (6.7 ml) columns, the resulting phosphorus breakthrough curves are
very similar. However, if the Richland effluent breakthrough curve is
compared to the phosphorus breakthrough curve of the SSE on the same size
column, the results are considerably different as shown in Figure 13.
The SSE can not really represent a synthetic secondary sewage effluent,
in terms of total alumina phosphorus capacity at least. Even though the
phosphorus capacity of the alumina is reduced when Richland effluent is
used for loading, the characteristic reactions of alumina are the same in
each case, as will be shown later. The small column-SSE work is therefore
useful in predicting how the alumina will react to given loading and
elution conditions. The small column-SSE work is not useful for predicting
phosphorus capacities of alumina when actual sewage effluents are used
for loading.
One Liter Alumina Columns
The composition of Richland trickling filter effluent used as the
influent in the one liter column studies varied over the range given in
Table 4. One liter alumina columns (5.1 cm diameter by ^9 cm high) were
used with Richland effluent to determine the effects of elution method on
21
-------
o
o
IV
0
1000
2000
COLUMN VOLUMES (7.6 ml)
Figare 12. Restoration of calcitic alumina column phosphorus capacity
with NaHCO following loading Cycle III.
Column - Boehmite, 100 to 200 mesh
Flow Rate - 30 column volumes/hr
Temperature - 23°C
Cycles - As indicated
NaHCO^ Concentration - l.OM NaHCO
22
-------
o
1.0
0. 8
0. 6
0.4
0. 2
R ICHLAND
EFFLUENT
200 400
600
COLUMN VOLUMES (7. 6 ml)
800 1000 1200
Figure 13. Comparison of SSE (10 mg P/l) and Richland effluent (10 mg P/l)
phosphorus loading curves.
Columns - Alumina, 20 to U8 mesh
Flow Rate - 15 column volumes/hr
Temperature - 23°C
23
-------
Table U
Richland Secondary Effluent Composition
Constituent mg/1 range
Na+ 1*0-60
K+ 12-15
N\ 10-lU
Mg++ 6-9
Ca+ 25-1+0
NO 3- 11
POU-3 25-35
Alkalinity as CaCO-^ equivalent 180-190
PH 7.2-7.8
subsequent phosphorus reloading. The Richland sewage effluent was stored
outside in a covered 500 gallon polyethylene tank. The effluent was
pumped inside and degassed by heating to room temperature and passed
through a 7.6 cm diameter by 60 cm granulated charcoal column before going
to the alumina.
The one liter alumina column loading data are given in Figure lU.
Note that there is an initial drop in capacity going from Cycle I to
Cycle II, as there was when SSE was used as an influent. Various elution
treatments were tried following each cycle to ascertain the effects, if
any, on the following loading cycle. Ten column volumes of 1M NaOH were
used to elute the column after loading Cycles I and II. Saturated Ca(OK)
solution was tried as an eluting agent following loading Cycles III and
IV. The phosphorus capacity fell steadily for the following loading Cycles
IV and V. The precipitation of solids in the alumina bed was observed.
Recovery and X-ray diffraction of a sample of the solids showed that they
were mostly calcite and a little apatite. A gram of the precipitated
solid contained 23 mg of phosphorus. As the alkalinity of the Richland
effluent implies, it must contain about 115 mg HCO ~/l. Since the alumina
can adsorb part of this HCO^~, as well as the phosphorus, the Ca(OH)
solution must immediately precipitate calcite (and apatite) within the
column and on the alumina grains resulting in reduced phosphorus adsorption
capacity.
2k
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0.2-
0 100 200 300 400 500 600
COLUMN VOLUMES (ONE LITER)
Figure ik.
Comparison of consecutive phosphorus loading curves for a
variously-regenerated two inch alumina column.
Column - Alumina, 20 to U8 mesh
Influent - Richland secondary effluent (10.3 mg P/l)
Flow Rates - Cycles I, II, and III - 15 column volumes/hr;
Cycles IV through IX - 5 column volumes/hr
Temperature - 23°C
Elution Treatment - I, 10 1 1M NaOH; II, 10 1 1M NaOH; III,
50 1 sat. Ca(OH)2 solution; IV, 50 1 sat.
Ca(OH)9 solution; V, 15 1 10# HC1; VI, 12
1 2M NaOH; VII, l6 1 2M NaOH; VIII, 8 1 2M
NaOH. Elution treatments followed above-
associated loading cycle numbers.
-------
An elution curve after Cycle III loading is given in Figure 15. The
hydroxide from the saturated Ca(OH)2 solution is able to elute the adsorbed
phosphorus, and is removed from the solution in the process. The effluents
from the elution contained very little phosphorus because the bulk of it
was precipitated in the column along with calcite.
An acid wash after Cycle V contained 8.4 g of phosphorus in 15 liters.
The following loading, Cycle VI, showed low capacity because acid is a poor
phosphorus eluting agent and acid treatment of alumina, as shown by previous
work, results in lowered phosphorus capacity. A regeneration with 2M NaOH
following loading Cycle VI resulted in restoration of the column to normal
phosphorus operating capacity (1+00 column volumes of Richland effluent to
10% breakthrough). Nearly twice as much phosphorus was eluted after loading
Cycle VI as vas loaded on the column by Cycle VI. The extra phosphorus
was left over from an incomplete elution with HC1 after loading Cycle V.
Nearly all of the phosphorus loaded during Cycle VII was removed by the
2M NaOH elution following Cycle VII. The two molar solution was used
because all of the biodegradable material was not removed by the charcoal
column, resulting in formation of a bacterial colony about 2 cm deep on the
top of the downflow column. This cap seemed to be more easily removed by
the 2M NaOH. The 1M NaOH was made up to 2M NaOH and reused a total of four
times with good results.
Elution of Cycle VIII loading was accomplished with eight column volumes.
of 2M llaOK. The column was drained before elution to avoid contamination of
the NaOH by Richland effluent, and drained after elution to cut NaOH losses.
The column then was washed with tap water. The phosphorus content of the
various wash solutions is given in greater detail in Figures 16 and 17. The
phosphorus content is indicated on one scale and pH on an opposite scale.
The high phosphorus washwater through 20 column volumes varies over the
range 83 mg to 10 mg F/l. The rest of the wash ranges between 10 mg and
1.2 mg P/l and is relatively low in pH.
For the first eight phosphorus loading cycles on the one liter column,
charcoal-filtered Richland effluent was used as the influent to reduce
the biodegradable materials applied which might interfere with phosphorus
removal.
26
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14.0
12.0
10.0
8.0
0
ELUTION FOLLOWING CYCLE
II I LOADING
10
20
30
40
50
COLUMN VOLUMES (ONE LITER)
Figure 15. Elution of the two inch alumina column with saturated Ca(OH),
solution.
Column - 20 to it8 mesh alumina loaded with P from Richland
secondary effluent
Flow Rate - 6 column volumes/hr
Temperature - 25°C
27
-------
14.0
- 13.0
10 12 14 16 18 20 22 24 26 28 30 32
COLUMN VOLUMES (LITERS)
Figure 16. Elution of the two inch alumina column with NaOH following
loadings VIII and IX.
Column - 20 to U8 mesh alumina loaded with Mchland effluent
Flow Rate - 6.1 column volumes/hr
Temperature - 23°C
28
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cr>
80 J
60
40
20
0
I 1 I
10 12 14 16 18 20 22 24 26 28 30 32
COLUMN VOLUMES (LITERS)
Figure 17. Phosphorus concentrations in the column wash-water following
elution of VIII and IX. Column, flow rate and temperature
are the same as those given in Figure 16.
29
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The charcoal column was 7.6 cm in diameter by 75 cm in height. The
charcoal column vas replaced on the ninth loading cycle "by a 20 to 50
mesh sand filter 7.6 cm in diameter by 20 cm in height. The resulting
phosphorus loading curve is given in Figure 1^. The alumina and the
influent solution were somewhat more colored than with the charcoal
filtration, but the loading cycle was otherwise normal. Carbon treat-
ment is probably not justifiable if phosphorus removal is the only goal.
The elution data of cycle IX are compared to that of cycle VIII
in Figures 16 and 17. There was little difference between the sand
and charcoal filtered elution data.
The NaOH solutions, after use, contained the color units shown in
Table 5. The colors matched fairly well when determined by the standard
platinum-cobalt method.
Table 5
Color test results on 2M NaOH elution
following loading Cycle VIII
Cumulative
Column Volumes Color Units
2 5
3 125
k 175
5 ' ^5
6 35
7 25
8 15
9 5
Stirring the used NaOH with 10 to 20 g of Ca(OH) resulted in
—2
removal of all color and phosphorus (and also C0_ ) from the solution.
The resulting solids settled well and the NaOH could be reused.
The preliminary laboratory work with phosphorus removal by adsorp-
tion on alumina columns, and subsequent treatment and reuse of eluting
solutions, has shown that the process is technically feasible. Process
scaleup to obtain engineering performance data is suggested as a means
of substantiating economic feasibility.
30
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CONCLUSIONS MD RECOMMENDATIONS
The removal of phosphorus from secondary sewage treatment effluent
was successfully demonstrated with one liter alumina columns on a
laboratory scale.
Ninety-five to plus 99% of the phosphorus was removed from a
synthetic secondary sewage effluent by alumina beds producing an effluent
that was 0.03 mg P/l, or less, at column flow rates of less than 10
column volumes/hr. The same phosphorus removal results were obtained
with up to 500 column volumes of Richland trickling filter effluent.
Regeneration of the alumina column was accomplished with 8 column volumes
of 2 M NaCH. This regenerant solution was restored by the addition of
a small amount of lime, removing the phosphorus as hydroxyapatite and
carbonate as calcite. Thus, only a small volume, solid waste is produced
by the process. Alumina losses of about 0.1$ per elution cycle were
indicated.
It is recommended that this promising phosphorus removal process be
scaled up to a portable pilot plant size. More operating data is required
at sites that can furnish different composition influents because treatment
costs and phosphorus removal efficiencies probably will vary from site to
site. The process should be demonstrated at several sites, in any case,
for the benefit of interested personnel in the field. Some points that
should be determined more precisely because of their effects on process
costs include:
l) Measurement of alumina attrition and dissolution losses over
several loading and eluting cycles,
2) More precise measurement of the quantity of lime that is
necessary to render the spent regenerant solution reuseable,
3) Measure solid waste product volume per elution cycle,
It) Measure effect of column flow rate on phosphorus leakage, and
5) Measure effect of type of sewage treatment on subsequent
phosphorus removal.
31
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