WATER POLLUTION CONTROL RESEARCH SERIES • 17070ESJ 01/70
ULTIMATE DISPOSAL OF PHOSPHATE
FROM WASTE WATER
BY RECOVERY AS FERTILIZER
U.S. DEPARTMENT OF THE INTERIOR • FEDERAL WATER QUALITY ADMINISTRATION
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POLLUTION CONTROL RESEARCH SERIES
The Water Pollution Control Research Reports describe the results
and progress in the control and abatement of pollution of our
Nation's waters. They provide a central source of information on
the research, development, and demonstration activities of the
Federal Water Quality Administration, Department of the Interior,
through in-house research and grants and contracts with Federal,
State, and local agencies, research institutions, and industrial
organizations.
Water Pollution Control Research Reports will be distributed to
requesters as supplies permit. Requests should be sent to the
Planning and Resources Office, Office of Research and Development,
Federal Water Quality Administration, Department of the Interior,
Washington, D. C. 20242.
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ULTIMATE DISPOSAL OF PHOSPHATE FROM
WASTE WATER BY RECOVERY AS FERTILIZER
by
Maria G. Dunseth
Murrell L. Salutsky
Kenneth M. Ries
Joseph J. Shapiro
Dearborn Chemical Division
W. R. Grace & Company
Chicago, Illinois 60654
for the
FEDERAL WATER QUALITY ADMINISTRATION
DEPARTMENT OF THE INTERIOR
Program #17070 ESJ
Contract #14-12-171
FWQA Project Officer, Edwin F. Barth
Advanced Waste Treatment Research Laboratory
Cincinnati, Ohio
January, 1970
For sale by the Superintendent o? Documents, U.S. Government Printing Office
Washington, D.C. 20402 - Price 70 cents
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FWQA Review Notice
This report has been reviewed by the Federal
Water Quality Administration and approved for
publication. Approval does not signify that
the contents necessarily reflect the views
and policies of the Federal Water Quality
Administration, nor does mention of trade
names or commercial products constitute
endorsement or recommendation for use.
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TABLE OF CONTENTS
Page
INTRODUCTION 1
EXPERIMENTAL 3
BACKGROUND 4
DIGESTER SUPERNATANT STUDIES 6
PROCESS DESCRIPTION 6
WASTE CHARACTERIZATION AND PLANT SELECTION 9
PRELIMINARY PRECIPITATION STUDIES 9
PRECIPITATION BY CHEMICAL ADDITION 15
Bench Scale Experiments 15
Pilot Plant Studies 19
PRECIPITATION BY HEAT DECOMPOSITION 24
Bench Scale Studies 27
Pilot Plant Studies 35
Batch Experiments 35
Continuous Experiments 41
EFFLUENT QUALITY 47
PRODUCT CHARACTERIZATION , 56
SUMMARY AND CONCLUSIONS 64
PATENT DISCLOSURES 66
REFERENCES 68
APPENDIX 70
ANALYTICAL METHODS 71
iii
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ABSTRACT
Many of the proposed processes that reduce orthophosphate in the
effluents from sewage treatment plants result in the extracted phosphate
being concentrated in the digester supernatant. This phosphate must
be removed prior to disposal of the supernatant or its recycle back to
the head of the treatment plant.
Digester supernatant was treated in two ways. The first was to
add magnesia and elevate the pH to 9 to cause precipitation of the phosphorus,
The other technique was to apply heat and/or vacuum to the digester super-
natant which caused precipitation of the soluble orthophosphate. A 90%
removal of orthophosphate can be achieved by either approach. The heat
and/or vacuum process also yields a supernatant substantially lower in
BOD, COD, and nitrogen concentration.
The precipitated phosphorous (primarily a mixture of calcium phosphate
and magnesium ammonium phosphate) was found available for plant food
according to AOAC (i.e., Association of Official Agricultural Chemists)
procedures.
Work was conducted by bench and pilot plant studies on digester
supernatant from the Lake Zurich, Illinois sewage treatment plant
and from the Libertyville, Illinois sewage treatment plant.
This report was submitted in fulfillment of Program No. 17070 ESJ,
Contract No. 14-12-171, between the Federal Water Quality Administration
and the Dearborn Chemical Division of W. R. Grace and Company.
iv
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ACKNOWLEDGMENTS
Acknowledgment is extended to the following personnel of Dearborn
Chemical Division, W. R. Grace & Co. who contributed their talents on
this project.
J. B rinkman, Te chnic ian
J. Burg, Technician
A. Dick, Secretary
L. Force, Librarian
B, Fosner, Research Engineer
P. Olsson, Laboratory Manager
T. Tutein, Technician
The direction and suggestions offered by Mr. E. F. Earth,
Project Officer, for the FWPCA was greatly appreciated. Acknowledgment
is also extended to Mr. Henry Paulus, Mayor of the Village of Lake
Zurich; Mr. Glen Holland, Chairman of Sewer Water; Mr. Walter Herkera,
Superintendent of Public Works and Mr. Ira Ernst, Plant Operator; all
of the Village of Lake Zurich, Illinois, for their cooperation while
studies were conducted at their plant. Thanks are also due to
Mr. Allen Schertz, Village Administrator; Mr. Les Sitz, Public Works
Superintendent; and Mr. Bob Barley, Plant Operator; all of the Village
of Libertyville, Illinois, for their permission and assistance of work
carried out in the Libertyville plant.
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INTRODUCTION
-1-
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A conventional activated sludge or trickling filter sewage treatment
plant with an anaerobic digester, produces a stream of digester super-
natant which contains a high concentration of COD, BOD, nitrogen and
phosphorus.
Digester supernatant is a troublesome stream because it normally
is returned to the head of the plant, usually intermittently, causing
a severe overloading of the plant. A process for the removal of
phosphate from digester supernatant in a form that could be sold as
a fertilizer is appealing because of the -promise of sufficient monetary
return to pay for the treatment costs and because ultimate disposal of
the phosphorus results when it is returned to the soil. One approach
which seems plausible is to precipitate and recover the phosphate from
the digester supernatant as magnesium ammonium phosphate. This compound
has a low solubility in water, thus assuring high recovery of phosphorus
present in the waste streams as orthophosphate. Because of its low
solubility, magnesium ammonium phosphate has been used in analytical
chemistry for many years for the quantitative precipitation of phosphate.
Its precipitation is the basis for a process for descaling sea water
(1,2), i.e., removing magnesium and calcium from sea water prior to
desalination and the production of fresh water. Therefore, with proper
condition of pH, phosphate concentrations, etc., the precipitation and
recovery of magnesium ammonium phosphate from digester supernatant seems
feasible.
The agronomic properties of magnesium ammonium phosphate are
excellent (3,4). It has a high percentage of highly available plant
nutrients. Because of its low water solubility and, hence, "non-
burning" properties, magnesium ammonium phosphate can command a premium
price in certain fertilizer applications. If magnesium ammonium phosphate
could be produced from the phosphate and ammonia in the digester super-
natant, not only would a pollution problem be solved but the economics
should be favorable for production of the fertilizer due to savings in
raw material costs. It is postulated that revenue from the sale of the
fertilizer would pay for the waste treatment costs and perhaps even
return a small profit.
The research summarized in this report is concerned with a bench
and pilot plant study to determine the feasibility of recovering
phosphorus from digester supernatant in a form suitable for sale as
a fertilizer ingredient.
Phase I of this project concerned the leaching of orthophosphate
from waste activated sludge and formation of magnesium ammonium
phosphate from this leachate. Details of that study have been presented
in a Phase I Final Report. Due to the more practical considerations
of the Phase II study the report of this phase will serve as the final
report of the contract.
—2—
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EXPERIMENTAL
-3-
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BACKGROUND
Digester supernatant is a natural by-product of the conventional
sewage sludge digestion process. The recent interest in nutrient control
has resulted in many studies on the levels of phosphorus in various
digester supernatants. The amount of total phosphorus in digester super-
natant is primarily a function of the suspended solids present; whereas
the orthophosphate concentration is more closely related to the type
of sludge digested and the nature of the digester operation. For
example, Shea (5) reports a digester supernatant having 17,000 mg/1
suspended solids, contained 282 mg/1 total phosphorus as P, of which
208 mg/1 was in the orthophosphate form. Fisher (6) in a study of
several treatment plants found total phosphorus concentrations ranging
from 45 to 123 rag/1 and found 66 to 84 percent of the phosphorus was
in the orthophosphate form.
As new or "raw" sludge is pumped into a digester and mixes with the
older digesting sludge, an equivalent volume of the liquid in the digester
that contains the least amount of suspended solids must be pumped out.
Kappe (7) discusses the troublesome nature of this material and points
out the disadvantage of returning supernatant to the head of the waste
treatment plant (which is still widely practiced today).
Many of the proposed processes that reduce orthophosphate in sewage
treatment plant affluents result in the extracted phosphorus being
concentrated in the digester. (Anaerobic release occurs in the digester
and converts much of the phosphorus to soluble orthophosphate.) Unless
this increased orthophosphate can be removed prior to recycle of the
supernatant back to the treatment plant, the phosphate removal schemes
presently under study would not be continually effective. Therefore,
precipitation of phosphorus from digester supernatant has become extremely
important. Earth (8) describes various treatment schemes to minimize
the problem.
Precipitation of phosphate from the digester supernatant of an
efficient activated sludge sewage treatment plant, is particularly
interesting because most of the phosphate is concentrated in a volume
lower than 1% of the total stream of the plant.
Rawn (9) in studying digestlm in 1939, found crystalline material
in a supernatant line, which was identified as magnesium ammonium phosphate.
He felt this compound was too soluble to normally form under conditions
found in a digester, and its presence was left unexplained. However,
it appears that Rawn was wrong with regard to this solubility limitation.
The literature values shown in the following table indicate that
solubility of magnesium ammonium phosphate hexahydrate (MgNH/P04*6H20) in
water at 25°C averages about 0.016 g/100 ml (i.e., 160 mg/1).
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g/100 ml
Bridger, et al. (3) 0.018
Bube (10) 0.017
Szekeres and Rady (11) 0.016
Uncles and Smith (12) 0.014
Average 0.016
All solubility determinations in the table were based on phosphate analyses
except that of Uncles and Smith which was based on magnesium analysis.
Expressed as phosphorus (P) the solubility is about 20 mg/1.
Since the concentration of orthophosphate P in digester supernatant
generally exceeds 20 mg/1,precipitation of magnesium ammonium phosphate
can normally be expected. Other conditions usually associated with a
well-operated digester (i.e., a buffered system containing a high con-
centration of ammonium bicarbonate and other ammonium salts) further
decrease the solubility of the compound.
The solubility of magnesium ammonium phosphate is dependent on pH.
It increases rapidly as the pH is lowered but decreases as the pH is
raised above 7. Very high pH (i.e., greater than 10) should be avoided
because the compound decomposes to form trimagnesium phosphate,
Mg3(P04>2'4H20. The solubility of magnesium ammonium phosphate is
greatly reduced by precipitation in an ammoniacal solution containing
an excess of soluble magnesium and ammonium salts. This is the basis
of a well-established analytical procedure for phosphate determination
in which the solubility losses of phosphorus are reduced to only a few
milligrams per liter.
If calcium salts are present,calcium will coprecipitate with the
magnesium ammonium phosphate in various forms of calcium phosphate.
Although phosphate removal is achieved by precipitation with calcium,
the fertilizer value of the calcium phosphates is less than that of
magnesium ammonium phosphate. Thus, the calcium concentration during
precipitation of the magnesium ammonium phosphate should be as low as
possible to prevent dilution of the product with less desirable
phosphates.
-5-
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DIGESTER SUPERNATANT STUDIES
PROCESS DESCRIPTION
Two processes were studied for the removal of phosphate from
digester supernatant (DS); (1) precipitation by addition of magnesia
or magnesium carbonate and a soluble base for pH adjustment, and (2)
precipitation by heat and/or vacuum decomposition of ammonium
bicarbonate for pH adjustment. The flow sheets for the two processes
are shown in Figures 1 and 2.
In the first process, a slurry containing 5% to 10% magnesium
hydroxide solids or magnesium carbonate is continuously metered into
a pipe premixer where it prereacts with the DS. This mixture is further
neutralized with sodium hydroxide to reach a pH of 9.0, in a reactor
fitted with a high speed agitator. Continuous monitoring of pH is used
as a control for the process. The retention time in the reactor should
be approximately 10 minutes. The treated DS is discharged by gravity,
directly to a continuous centrifuge or a conventional settler. The
underflow from the settler is centrifuged or filtered. The solids are
dried in a conventional rotary dryer.
The second phosphate precipitation process is based on the use of
heat and/or vacuum to destroy the bicarbonate which is present in the
DS. Evolution of C02 raises the pH effecting the precipitation of
magnesium and calcium phosphates without the addition of chemicals.
This process does not require additional chemicals when the DS of an
area contains a high concentration of magnesium and calcium ions. In
areas where the water is soft, (i.e., where the hardness is below that
stoichiometrically needed to precipitate phosphate), then supplemental
magnesia will be required.
A continuous flow of DS is pumped into a reactor where the temperature
is raised to 65°-75°C. The reactor may be operated under a slight vacuum
to enhance the stripping of C02. The retention time necessary for complete
precipitation of phosphate is determined by the initial concentration of
ammonium bicarbonate. The pH is monitored continuously and it should
raise to 8.5 to 8,9 for optimum phosphate removal. A high degree of
agitation is desirable to expose a large surface area and enhance C02
evolution. The treated DS is pumped into a continuous centrifuge or a
conventional settler. The solids are dried in a conventional rotary
dryer. As an alternative, a fraction of the solids prior to drying may
be recycled to the reactor to increase the percent solids in the DS and
enhance crystal growth. These two processes may be operated on a batch
or a continuous basis.
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MgO
or
MgC03
DS
PIPE
PRE-MIXEft
NaOH
REACTOR
i
CENTRIFUGE
CENTRATE
SOLIDS
DRYER
I
PRODUCT
FIGURE I. RECOVERY OF PHOSPHATE FROM DIGESTER
SUPERNATANT BY ADDITION OF MAGNESIA
OR MAGNESIUM CARBONATE AND SODIUM HYDROXIDE
-7-
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HEAT
AND/OR
VACUUM
DS
i
REACTOR
SOLIDS
CENTRIFUGE
SOLIDS
DRYER
•*- CO,
->• CENTRATE
PRODUCT
FIGURE 2
RECOVERY OF PHOSPHATE FROM DIGESTER
SUPERNATANT BY DECOMPOSITION OF
AMMONIUM BICARBONATE
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WASTE CHARACTERISTICS AND PLANT SELECTION
A study of local plants was made to obtain a suitable source of
digester supernatant. A list of the plants surveyed follows:
Harrington, Illinois
Crystal Lake, Illinois
Deerfield, Illinois
East Dundee, Illinois
Highland Park, Illinois
Lake Zurich, Illinois
Libertyville, Illinois
Mundelein, Illinois
Wauconda, Illinois
West Dundee, Illinois
In Table I can be seen the analytical results for the DS samples
obtained at the ten locations. All the research work was performed
primarily with samples obtained from the Lake Zurich, Illinois and
Libertyville, Illinois plants.
The Lake Zurich, Illinois plant is a trickling filter plant with a
500,000 gal/day capacity. The plant is operated at design load. The
plant has a two-stage digestion system. The primary digester is circulated
and heated; the secondary tank is under quiescent conditions to settle
out the solids. The DS is returned from the secondary tank to the head
of the plant. All the experiments were performed with samples from the
secondary digester.
The Libertyville plant is a 1.5 mgd capacity conventional sludge plant
which is operating at design capacity. Seventeen small manufacturing
facilities contribute industrial waste to the sewage system. The plant
has a two-stage digestion system, the primary digester which is heated,
and a secondary tank for storage and further digestion under quiescent
conditions. The DS is returned from the secondary tank to the head
of the plant.
In Table II can be seen the average composition of the DS obtained
from the secondary digester from the Lake Zurich and Libertyville sewage
treatment plants. Samples were analyzed for a period of three months
and one year respectively.
A schematic of the Lake Zurich and Libertyville sewage treatment plant
can be seen in Figures 3 and A.
PRELIMINARY PRECIPITATION STUDIES
The main objective of this work was to develop optimum conditions
for the precipitation of magnesium ammonium phosphate hexahydrate from
a waste stream containing a phosphate concentration in the range of
50-100 mg/1 P as orthophosphate. The precipitation was achieved by
adding either a soluble salt of magnesium, solid magnesium oxide, a
slurry of magnesium hydroxide or a solid maghesium carbonate. Final
pH adjustment, when required, was achieved by adding a soluble
base such as sodium hydroxide or ammonia.
-9-
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TABLE I
o
DIGESTER SUPERNATANT
AT TEN LOCAL SEWAGE
Crystal
Plant Location Harrington. Lake Deerfield
SS mg/1
COD mg/1
PH
C02 mg/1
TKN mg/1
Amnonla N mg/1
TP mg/1
OP mg/1
Ca mg/1
Mg mg/1
7,060 - 11,500
7,720 4,550 13,386
7.2 7.3 7.1
1,685
624 494 934
404
170 46 227
62
136 150 496
78 7 128
CHARACTERIZATION
TREATMENT PLANTS
East Highland
Dundee Park
47,600
45r590
7.2
3,484
175
65
1,150
—
2,077
398
11,220
2,907
7,2
2,736
925
768
91
73
163
58
Lake
Zurich Libertyville
. 960
2,632
7.0
2,094
621
509
115
51
190
73
790
1,900
7.1
2,323
524
445
80
61
142
79
Mundelein
19,900
17,860
7.1
2,974
139
64
750
—
1,112
262
Wauconda
660
6,203
6,9
1,496
378
332
96
57
203
100
West
Dundee
^
35,200
6.9
.
18
—
212
•V
115
30
SS - Suspended Solids
COD - Chemical Oxygen Demand
TKN - Total Kjeldahl Nitrogen
TP - Total Phosphorus
OP - Orthophosphate Phosphorus
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TABLE II
AVERAGE COMPOSITION OF DIGESTER SUPERNATANT
FROM THE LAKE ZURICH AND LIBERTYVILLE
SEWAGE TREATMENT PLANTS
Suspended Solids (mg/1)
Total Phosphorus (mg/1 as P)
Orthophosphate (mg/1 as P)
Chemical Oxygen Demand (mg/1)
PH
C02 (mg/1)
TKN (mg/1 as N)
Lake Zurich
1,518
85
66
2,210
7,2
2,209
678
Libertyville
740
100
60
1,230
7.0
-
360
TKN - Total Kjeldahl Nitrogen
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SECONDARY
CLARIFIER
PLANT
EFFLUENT
PRIMARY
> .
"— CLARIFIERS
PRIMARY
DIGESTER
SECONDARY
DIGESTER
PRIMARY
SLUDGE
COMMINUTOR--
GRIT CHAMBER-
DIGESTER SUPERNATANT
RAW SEWAGE
DIGESTED
SLUDGE
FIGURE 3. SCHEMATIC OF THE LAKE ZURICH, ILLINOIS SEWAGE TREATMENT PLANT
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u
SECONDARY
CLARIFIERS
LRETJJRN _§ LUDGE _ I
WASTE I
ACTIVATED |
SLUDGE
(WAS)
I SECONDARY
J SLUDGE
PLANT
EFFLUENT
I
IPWMARY
ISLUDGE
DIGESTER
y
DIGESTER
SUPERNATANT
GRIT
CHAMBER
COMMINUTOR
RAW SEWAGE
FIGURE 4 SCHEMATIC OF THE LIBERTYVILLE, ILLINOIS, SEWAGE TREATMENT PLANT
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An evaluation of the waste streams from the Lake Zurich and
Libertyville sewage plants indicated that the water contained a large
concentration of calcium, over 100 mg/1. Calcium reacts with ortho-*
phosphate to form either dicalcium phosphate, tricalcium phosphate
or hydroxyapatite. In view of this condition, it became apparent
that it was desirable to study the effect of calcium, at different
concentrations, on the precipitation of magnesium ammonium phosphate.
A synthetic mixture containing Ca++, Mg"1"1" and PO^ ions in concentrations
similar to those expected in a waste stream was used for the preliminary
investigation.
The work on precipitation was not only concerned with the efficiency
of phosphate removal from the stream but also with the type and quality
of the precipitated solid because a process was visualized which would
yield a solid by-product with economic value.
The general procedure used in these experiments is as follows:
To 900 ml of deLonized water was added a predetermined volume of stock
solution containing (1) 10,000 mg/1 of P prepared with phosphoric
acid; (2) 10,000 mg/1 of Mg prepared with magnesium sulfate and (3)
5,000 mg/1 of Ca prepared with calcium chloride. Finally the volume
of the sample was brought up to 1 liter. The sample was agitated at
400 rpm in a beaker fitted with pH electrodes. A soluble base, either
a 10% solution of ammonium hydroxide or a IN sodium hydroxide solution
was used to bring the pH up to a predetermined level. Neutralization
was performed in approximately 15 minutes. After reaching the desired
pH level, the suspension was stirred for one hour to allow crystal
growth and finally settled for a period of approximately one hour. The
settled solids were filtered in a Buchner funnel using #5 Whatman paper.
In some cases the solids recovered were difficult to filter, and then
a fine fritted glass funnel or a centrifuge were used for the solid-
liquid separation. The filtrates were analyzed for residual Ca, Mg, and
P. The calcium and magnesium were determined by EDTA titration and the
phosphate by the aminonaphtholsulfonic acid method. The solids were
analyzed by X-ray and in some cases by standard AOAC (Association of
Official Agricultural Chemists) procedures for total phosphate, available
phosphate, nitrogen, calcium and magnesium.
Conclusions reached as a result of the preliminary precipitation
studies (13) are as follows:
1. In a system free of calcium the recovery of phosphate depends on
pH, increasing from 41% at pH 8.5 to 93% at 10.0. The precipitated
solid is predominantly MgNH4P04.6H20.
2. In a system containing calcium, phosphate recovery is higher than
that obtained at the same pH in its absence. The optimum pH is
9.5 yielding a phosphate recovery of about 98%, but the solid
contains calcium phosphates in addition to the magnesium phosphates.
3. A high phosphate recovery (95%) is obtained at pH 9.0 when the
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magnesium ion concentration is in excess, i.e., 25% or more
above stoichiometry.
4. Calcium precipitates preferentially as the phosphate when the
pH is maintained at 8.5.
5. There is a linear relationship between the initial calcium
concentration and the residual magnesium in solution after phosphate
precipitation.
6. At higher pH levels only a small amount of calcium precipitates
when the initial calcium concentration is low.
7. The recycle of magnesium ammonium phosphate as seed did not increase
phosphate recovery at any given pH level.
8. Increasing the retention time during neutralization, digestion
and settling did not increase phosphate recovery although it did
enhance the precipitation of magnesium.
9. A high ammonium ion concentration, such as that found in digester
supernatant, enhances the precipitation of MgNH4P04•6H20 and phosphate
recovery. At pH 9.0, a recovery of 96% was obtained. However, the
concentration of nitrogen in the precipitated solids decreases with
increasing pH above 9.0, thus indicating the precipitation of
Mg3(P04)2'4H20 at the higher pH levels.
10. The precipitation can be carried out continuously (rather than
batch-wise) without serious problems in maintaining the systems
equilibrium.
PRECIPITATION BY CHEMICAL ADDITION
All experiments were performed using samples of digester supernatant
from the Lake Zurich or Libertyville, Illinois sewage plants. Whenever
possible, the same sample of DS was used for individual studies to
insure uniformity of raw material. Some experiments were performed
using DS from the primary digester. Experiments were performed at
a bench and pilot plant scale.
Bench Scale Experiments
The general procedure used in these experiments is similar to that
described in the preliminary precipitation studies. Samples of primary
DS from the Libertyville plant containing from 82 to 91 mg/1 P as ortho-
phosphate were used in these experiments. Five liter samples were treated
either with (1) sodium hydroxide alone to raise the pH to the desired level,
(2) with a stoichiometric amount of magnesium sulfate followed by ammonium
hydroxide or sodium hydroxide for final pH adjustment, or (3) with a
stoichiometric amount of magnesia, MgO, followed by sodium hydroxide or
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ammonium hydroxide for final pH adjustment. Table III is a summary
of the data collected. When MgO (calcined magnesite) is used, final
pH of 8.0 may be obtained without addition of a soluble base. The
phosphate recovery was only 56% because this type material is not as
reactive as the precipitated magnesia. Under similar conditions,
when magnesium sulfate is used, a recovery of 76% of phosphate was
obtained. At pH 9.0 and 9.5 the phosphate recovery is high even when
no magnesium compounds are added. This is due to the fact that DS from
the Libertyville Plant contains a high concentration of calcium and
magnesium salts. At the higher pH levels, magnesium sulfate plus
ammonia gave the lowest residual phosphate. It was observed that liquid
solid separation became increasingly difficult with decreases in final
pH. A good separation was possible at pH range 9.0-9.5.
Additional experiments were performed using 40% of the stoichio-
metric amount of magnesia required followed by sodium hydroxide to
pH 9.0 and 9.5. Both experiments gave a high degree of phosphate
removal similar to that obtained with only sodium hydroxide neutralization.
Table IV shows the data collected when a sample of DS from the
secondary digester of the Libertyville sewage treatment plant was used.
The DS tested in these experiments was collected from the mid-
point valve of the settler tank of the anaerobic digestion process.
During the period when these samples were collected, it was found that
practically all the phosphate present in this stream was orthophosphate.
Most of the samples of DS contained between 55 and 87 mg/1 P. The
initial pH of the DS was 7.0.
Three sources of magnesia were used to precipitate the phosphate;
(1) solid magnesium oxide (calcined magnesite), (2) precipitated
magnesium hydroxide, and (3) precipitated magnesium carbonate. Three
levels of magnesia concentration were used because from previous work
it had been determined that all the streams from the Libertyville sewage
plant contain a surplus of calcium and magnesium ions. Sodium hydroxide
or ammonium hydroxide was used for final pH adjustment when the calcined
magnesite was used as a source of magnesium ion.
All the experiments were performed using the standard procedure
described previously, i.e., 15 minutes for neutralization, 1.0 or 1.5
hours for digestion, and 1 hour settling. The agitation rate was
approximately 500 rpm, except for the experiments in which Mg(OH>2
and MgC03 were used when the agitation was 1,000 rpm. Liquid-solid
separation was performed with a batch-type centrifuge. It was observed,
qualitatively, that gravity settling could be used if necessary to
reduce the volume prior to centrifugation. Solids obtained in
experiments wheze precipitation of phosphate was achieved by the use of
ammonia or sodium hydroxide alone were very difficult to settle, prior
to centrifugation, or to filter. This type treatment yielded suspensions
extremely difficult to handle.
The data in Tables III and IV indicate that similar phosphate
recovery could be obtained from the two types of digester supernatant
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TABLE III
PRECIPITATION OF PHOSPHATE FROM PRIMARY
DIGESTER SUPERNATANT, COLLECTED
AT THE LIBERTYVILLE, ILL.
SEWAGE TREATMENT PLANT
Chemicals Original P
Final pH Added (mg/1)
8.0
8.0
8.0
9.0
9.0
9.0
9.5
9.5
9.5
9.5
9.5
NaOH
MgO
MgS04+NaOH
NaOH
MgO+NaOH
MgS04+NaOH
NaOH
MgO+NaOH
MgS04+NaOH
MgS04+NH40H
MgO+NH40H
87
91
91
87
91
91
87
81
91
86
91
Residual P
(mg/l)
43
41
22
6
3
5
6
4
3
2
3
% Recovery
P
50
56
76
93
96
95
93
96
97
98
97
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TABLE IV
PRECIPITATION OF PHOSPHATE FROM SECONDARY
DIGESTER SUPERNATANT COLLECTED
AT THE LIBERTYVILLE, ILL.
SEWAGE TREATMENT PLANT
Final
pH
9.0
9.0
9.0
9.0
9.0
9.0
8.9
8.9
8.9
8.7
8.7
8.7
Chemical
Added
MgO+NaOH
MgO+NaOH
MgO+NaOH
MgO+NH40H
MgO+NH40H
MgO+NltyOH
Mg(OH)2
Mg(OH)2
Mg(OH)2
MgC03
MgC03
MgCOa
Amount Mg-H-
Added(l)
0.2
0.4
1.0
0.2
0.4
1.0
0.2
0.4
1.0
0.2
0.4
1.0
Original P
(rog/1)
82
82
82
82
71
71
73
73
73
73
73
73
Residual P
(mg/1)
8
5
5
8
7
5
6
6
6
10
8
7
% Recovery
P
90
94
94
90
90
93
92
92
92
86
89
90
(1) 1.0 means that a stoichiometric amount of Mg+*- with
respect to P was added.
-18-
-------
studied. The final pH adjustment when calcined magnesite is used
may be performed with either ammonia or sodium hydroxide. An amount of
magnesia less than stoichiometric can be used in this system without
greatly affecting the net phosphate removal. Precipitated magnesium
hydroxide or magnesium carbonate may be used without final pH adjustment
with a soluble base. A slightly higher recovery was observed when the
magnesium hydroxide was used.
Four experiments were p-erformed using digester supernatant from
the Lake Zurich sewage treatment plant. The sample of DS was obtained
from the top of the secondary digester. The objective of these experiments
was to determine the pH required for optimum phosphate removal at two
temperatures 70°F and 80°F. In these experiments no additional magnesium
was added. It was assumed that sufficient cations were present to
precipitate the orthophosphate in solution. Either sodium hydroxide
or ammonium hydroxide was used as a neutralizing agent. All the exper-
iments were performed using a volume of 5 litets of DS. The neutralization
step was performed in approximately 30 minutes. Samples were obtained
at pH 8.0, 8.5, 9.0, 9.5 and 10,0. The samples were centrifuged and
analyzed for orthophosphate. Figures 5 and 6 show the residual ortho-
phosphate in mg/1 P as a function of pH at 70°F and 80°F respectively
when sodium hydroxide is used as the neutralizing agent. Figures 7 and
8 show the residual orthophosphate in mg/1 P as a function of pH at
70°F and 80°F respectively when ammonium hydroxide is used as the
neutralizing agent. In all cases the orthophosphate concentration at
the lowest pH represents the concentration present originally in the
sample.
When sodium hydroxide is used as a neutralizing agent the lowest
residual orthophosphate was obtained at pH 9.0. There is a tendency
for the phosphate to go back into solution when the pH is 10.0. When
ammonium hydroxide is used as a neutralizing agent the lowest residual
concentration of orthophosphate was obtained at pH 10.0 when the
experiment was performed at 70°F and pH 9.0 when the experiment was
performed at 80°F.
At the optimum pH, a phosphate recovery of 88% was obtained when
using either neutralizing agent, i.e., sodium hydroxide or ammonia,
at 80°F. At the lower temperature studied, ammonia neutralization
at optimum pH, yielded 91% orthophosphate recovery as against 81%
for sodium hydroxide. The phosphate recovery under similar experimental
conditions, when Libertyville DS was treated with sodium hydroxide
is approximately 10% higher than that observed when samples of Lake
Zurich DS were used.
Pilot Plant Studies
Precipitation of phosphate by addition of chemicals was studied
at the pilot plant located at the Lake Zurich sewage treatment plant.
All these experiments were performed on a batch basis using a volume
of digester supernatant ranging from 10 gallons to 40 gallons. In
all cases the digester supernatant was transferred on a daily basis
-19-
-------
100
CO
<
80
I +
Q.
uT 60
O 40
20
7.5
ao
as
pH
9.O
10.0
FIGURE 5
RESIDUAL ORTHOPHOSPHATE AS A FUNCTION OF pH.
LAKE ZURICH DIGESTER SUPERNATANT TREATED
WITH SODIUM HYDROXIDE /TT 70* F
-20-
-------
100
80
I +
60l
& 401
x
o:
o
20
70
FIGURE 6
7.5
ao
as
pH
as
KXO
RESIDUAL ORTHOPHOSPHATE AS A FUNCTION OF
pH. LAKE ZURICH DIGESTER SUPERNATANT
TREATED WITH SODIUM HYDROXIDE AT 80* F
-21-
-------
looL
Q.
9
a
<
£ 60
40
20L
7.0
7.5
8X)
as
PH
IQO
FIGURE 7
RESIDUAL ORTHOPHOSPHATE AS A FUNCTION OF pH
LAKE ZURICH DIGESTER SUPERNATANT TREATED
WITH AMMONIUM HYDROXIDE AT 70» F
-22-
-------
100
80
60
20
7.0
7.5
8.0
as
pH
9.0
10.0
FIGURE 8 RESIDUAL ORTHOPHOSPHATE AS A FUNCTION OF
pK LAKE ZURICH DIGESTER SUPERNATANT
TREATED WITH AMMONIUM HYDROXIDE AT dO*F
-------
from the plant's secondary digester to & storage tank of approximately
300 gallons capacity. The storage tank was fitted with a slow moving
stirrer to maintain the DS solids in suspension.
The chemicals tested were: (1) magnesium hydroxide (precipitated
sea water magnesium hydroxide used as a slurry containing 33% solids),
(2) magnesium carbonate (solid MgC(>3'3H20), (3) sodium hydroxide, and
(4) ammonium hydroxide.
The procedure used was the same in all cases. A predetermined
volume of DS was transferred to a reactor fitted with a stirrer. The
chemical treatment used was added either as a solution or a slurry
containing approximately 33% solids. The chemical treatment was
added in a slug in a few minutes. The treated DS was stirred for
approximately two hours. Samples were obtained for analysis
after the digestion period. The solid-liquid separation was performed
on a batch centrifuge. The data for these experiments is summarized
in Table V. The highest phosphate removal was observed when magnesium
hydroxide was used in combination with sodium hydroxide at pH 9 or
when the magnesium hydroxide was used in combination with a cationic
polymer to enhance coagulation. Both experiments indicated a phosphate
recovery slightly higher than 90%. Magnesium carbonate at a very low
dose, i.e., 35 mg/1 as Mg and without need of additional base, yielded
a high orthophosphate removal, i.e., 87%. Ammonium hydroxide as a
neutralizing agent is more efficient, for phosphate removal, than sodium
hydroxide. At 85°F which is the temperature at which the DS is
discharged from the secondary digester, the orthophosphate removal was
89%. This is interesting because ammonia is present in the DS and
it is a volatile material. This suggests a process whereby ammonia
can be used to raise the pH of raw DS to 10.0. Then the precipitated
solids containing phosphate are separated from the liquid phase. The
treated DS, now free of phosphate and other impurities is flashed
under vacuum or air stripped, thus releasing ammonia which is recycled
to the incoming raw DS.
PRECIPITATION BY HEAT DECOMPOSITION
Temperature as a variable was studied using the same operating
conditions described on page 16. The DS sample was obtained at
Libertyville, Illinois. The experiments were performed at either
50°C, 60°C, or 70°C. Two sources of magnesium ion were studied,
precipitated magnesium hydroxide and magnesium carbonate. The levels
of magnesium ion concentration used were the same as in the previous
section. In all cases, a soluble base was not used and the increase
in pH was due only to magnesia neutralization. Table VI summarizes
the data for these experiments. From the data in the table it became
apparent that temperature had a more pronounced effect on the recovery
of phosphate than magnesium stoichiometry and/or source of magnesium
ion added. The lowest residual phosphate concentration was obtained
at the higher temperature studied. It is also significant that a
very high phosphate recovery was achieved at a pH below 9.0.
-24-
-------
TABLE V
N>
Ln
PRECIPITATION OF PHOSPHATE FROM SECONDARY DIGESTER SUPERNATANT
PILOT PLANT STUDY AT THE LAKE ZURICH SEWAGE TREATMENT PLANT
mg/1
Final Chemical Added
pH Added as Mg
8.7
9.0
7.9
7.9
8.7
9.4
9.0
9.0
10
10
Mg(OH)2+ 70
NaOH
Mg(OH)2+ 70
NaOH
Mg(OH>2+ 50
NaOH
Mg(OH)2+ 100
50 mg/1
Polvmer(l)
MgC03 35
MgCOji 35
NaOH
NaOH
NH40H
NHAOH
Temp.
°F
75
75
75
75
80
149
75
85
85
149
Vol. of
Batch
(gal.)
40
40
30
30
10
10
20
10
10
10
Original
mg/1
pH
7.2
7.2
7.4
7.1
7.2
7.2
7.1
7.1
7.0
7.2
TP
93
93
63
68
78
78
-
87
85
80
OP
76
76
56
56
69
69
72
66
66
71
COD SS
2580 1640
2580 1640
1660
1660
2035 1180
2035 1180
-
2535 1680
2275 1103
1908 1360
Treated
mg/1 %
TP
16
-
20
9
12
13
9
14
15
11
OP
10
5
-
5
9
11
-
14
7
10
COD
1416
-
864
960
1087
1122
-
1048
1103
937
TP
83
-
68
87
85
83
-
84
82
86
Removal
OP
87
93
-
91
87
84
87
79
89
86
COD
45
-
48
42
47
45
-
59
52
51
(1) Cationic polyethyleneimine polymer
TP - Total Phosphorus
OP - Orthophosphate Phosphorus
COD - Chemical Oxygen Demand
SS - Suspended Solids
-------
TABLE VI
PRECIPITATION OF PHOSPHATE FROM SETTLED
DIGESTER SUPERNATANT WITH
MAGNESIA AT HIGH TEMPERATURE
Reaction
Temp.°C
50
50
50
50
50
50
60
60
60
60
60
60
70
70
70
70
70
70
Final
PH
8.8
8.9
8.8
8.4
9.1
9.2
8.9
8.8
8.9
9.0
8.8
9.1
8.9
8.8
8.8
8.6
8.8
8.8
Chemical
Added
Mg(OH)2
Mg(OH)2
Mg(OH)2
MgC03
MgC03
MgC03
Mg(OH)2
Mg(OH)2
Mg(OH)2
MgC03
MgC03
MgC03
Mg(OH)2
Mg(OH)2
Mg(OH)2
MgC03
MgC03
MgC03
Amount
Mg-H-
Added(l)
0.2
0.4
1.0
0.2
0.4
1.0
0.2
0.4
1.0
0.2
0.4
1.0
0.2
0.4
1.0
0.2
0.4
1.0
Original
P
(mg/1)
73
73
73
71
71
73
73
73
73
71
71
73
73
73
73
71
73
73
Residual
P
(mg/1)
6
6
6
10
7
6
5
5
4
6
4
6
2
2
3
2
2
2
%
Recovery
P
92
92
92
86
90
92
93
93
94
92
94
92
97
97
96
97
97
97
(1) 1.0 means that a stoichiometric amount of Mg++ with
respect to P was added.
-26-
-------
Bench Scale Studies
During the evaluation of temperature as a variable, it became
apparent that by raising the temperature above 60 °C and by using
high speed agitation of approximately 1,000 rpm, the pH of the
digester supernatant liquor increased without need of adding a
neutralizing base. This is due to the decomposition of ammonium
bicarbonate, which is present in the liquor, according to the
following equation:
NH4HC03 - ^fyOH + C02
It was also found that phosphate would precipitate without the
addition of magnesium compounds because the DS used already contains
the required cations. In areas where the water is much softer,
supplemental magnesium might be required.
Table VII shows the data for experiments performed at 65°C, 70°C and
75°C on a batch process for varied retention periods. The material used
in these experiments is secondary settled digester supernatant obtained
from the Libertyville sewage treatment plant. The two variables,
temperature and retention, are important for maximum phosphate precipi-
tation. It was observed that the best results were obtained when a
high rate of agitation was used.
Several experiments were performed using a combination of heat and
vacuum to expedite the decomposition of ammonium bicarbonate and thus
obtain precipitation of phosphate at a lower operational temperature
or a shorter retention time. Again, all these experiments were performed
using settled digester supernatant from the Libertyville sewage plant.
Table VIII indicates that a substantial amount of phosphate may be
precipitated at 40°C when a vacuum of 27" Hg is applied. Practically
all the orthophosphate was removed from the system when the temperature
was increased to 65 °C.
It should be noted that in these experiments the temperature was
maintained constant by applying an external source of heat during the
specified period of time. The vacuum would lower the initial temperature
of the system if heat was not applied.
As mentioned earlier, a high agitation during heating was required
to obtain a high level of phosphate removal from the digester supernatant.
This means that exposure of a large surface area would expedite evolution
of CC>2 and thus obtain a high yield of phosphate using shorter retention
periods. To verify this hypothesis, experiments were performed using
a thin film flash evaporator manufactured by Buchler, Inc. The apparatus
can be seen in Figure 9. Samples of digester supernatant from the
Libertyville or Lake Zurich plant were introduced into the heating
chamber of the flash evaporator. The heating chamber rotates in a
constant temperature bath. The experiments were performed using 125 rpm.
-27-
-------
TABLE VII
PRECIPITATION OF PHOSPHATE
BY HEAT TREATMENT
FROM SETTLED DIGESTER SUPERNATANT
OBTAINED AT THE LIBERTYVILLE PLANT
Retention
Final Temp. Time Original P Residual P % Recovery
pH CC (Min.) (mg/1) (mg/1) P
8.9
8.4
8.4
8.9
8.9
65
65
70
70
75
60
120
120
180
60
58
71
71
73
71
4
2
4
3
3
94
97
94
95
96
-28-
-------
TABLE VIII
PRECIPITATION OF PHOSPHATE
BY HEAT AND VACUUM FROM
DIGESTER SUPERNATANT OBTAINED
AT THE LIBERTYVILLE PLANT
Final Temp. Retention Original P Residual P % Recovery
pH °C Time(Min.) (mg/1) (mg/1) P_
8.9
8.4
8.5
9.3
9.3
40
40
50
65
65
60
60
30
60
60
71
58
71
60
56
14
9
12
1
1
80
84
82
98
98
-29-
-------
-
FIG'JRE 9 FLASH EVAPORATOR APPARATUS
-------
The cooling chamber, which also rotates, collects the evolving gases
and also water which evaporates from the heated system. The experimental
runs were performed using heat alone or a combination of heat and vacuum.
Table IX contains the information on these experiments. In all cases
the temperature of the water bath was maintained at 65°C. The operating
temperature of the DS inside the heating chamber was not monitored
continuously but a spot check indicated a temperature of 60°C. The
vacuum was 27" of mercury. After the heat or vacuum and heat treatment
period, the entire contents of the flash chamber were transferred
into a graduate cylinder to determine any change in volume. When needed,
the volume was adjusted to its original level with distilled water.
Solid-liquid separation was obtained by centrifugation on a batch basis.
The conclusions which may be reached from the information in Table IX
are as follows: (1) Heat treatment followed by a period of heat plus
vacuum yields a higher phosphate recovery than when the whole period
of treatment involves heat plus vacuum. Samples 1 and 4 had a total
treatment period of 10 minutes; samples 2 and 6 had a total treatment
period of 20 minutes. In both cases the percent recovery of ortho-
phosphate is higher when the samples were heated prior to application
of a vacuum. (2) Heat alone, for similar pretreatment periods, yields
a higher orthophosphate removal than heat plus vacuum; i.e., samples
2 and 8. (3) Longer retention periods increase the orthophosphate
recovery yield under all conditions. (4) Orthophosphate precipitation
in a thin film flash evaporator is not substantially higher than when
the experiments are performed in a vessel with high speed agitation.
(5) Percent nitrogen removal was substantially higher when a short
period of heat plus vacuum was used after a period of heating the DS;
i.e., experiments 6, 7 and 8.
To understand the relationship between C02 and volatile nitrogen
evolution (probably as ammonia), a series of experiments were performed
using a synthetic solution of ammonium bicarbonate in distilled water.
The concentration of ammonium bicarbonate was similar to that present
in the digester supernatant of Libertyville. All the experiments were
performed in the flash evaporator following the procedure outlined in
the previous section. Figure 10 shows the rate at which C02 and N
are removed from the liquid phase as a function of time, when only heat
is applied to the system. Figure 11 shows the rate at which C02 and
N are removed from the liquid phase when heat and vacuum are applied.
From the information in Figures 10 and 11 it becomes clear that heat
alone encourages a faster evolution of C0£ than ammonia. Under heat
and vacuum the C02 evolves at a higher but parallel rate with ammonia.
An evaluation of the pH behavior for the two systems indicates that
the pH goes up as a function of time when heat alone is used and it
decreases as a function of time in the heat-vacuum system. These
experiments suggest that the precipitation of orthophosphate from DS
should be more efficient if performed in two steps. The DS should first
be heated under high agitation and then the heated DS treated under heat
and a slight vacuum in a separate reactor.
The phosphate precipitation process by heat decomposition was also
tested in a continuous system. A four liter reactor fitted with a
high speed agitator and a heating mantle was used. The DS was pumped
continuously at a rate calculated to give a 2-hour retention time.
-31-
-------
TABLE IX
PRECIPITATION OF PHOSPHATE BY HEAT & VACUUM
IN A THIN FILM FLASH EVAPORATOR
Sample
No.
1
2
3
4
5
6
7
8
Source of
Sample
Libertyville
Libertyville
Libertyville
Libertyville
Libertyville
Libertyville
Lake Zurich
Libertyville
Heat Treatment
Time(Min.)
0
0
0
5
10
15
15
20
Heat & Vacuum
Treatment
Tine (Min.)
10
20
30
5
5
5
5
O
% Removal
OP TN
69
73
91
74
84
85 44
85 42
81 12
PH
Original
7.3
7.2
7.2
7.4
7.4
7.3
7.0
7.2
Final
8.4
8.2
8.2
8.3
8.4
8.6
8.5
8.9
OP - Orthophosphate Phosphorus
TN - Total Nitrogen
-------
60
50
40
o
Ul
QC
5? 30
20
10
CO,
4 6
TIME, MINUTES
8
10
FIGURE 10 CARBON DIOXIDE AND NITROGEN REMOVAL AS
A FUNCTION OF TIME AT 65° C
-33-
-------
6€
50
30
20
10
J-
_L
4 «
TIME, MINUTES
8
10
FIGURE II
CARBON DIOXIDE AND NITROGEN REMOVAL AS A
FUNCTION OF TIME AT 65°C AND A VACUUM
OF 27" OF MERCURY
-34-
-------
Two continuous runs were performed, one at 70°C and the other at 65°C.
The percent recovery of phosphate averaged 90% and 87% respectively.
It was difficult to maintain a steady temperature because the heater
control was not adequate. These two experiments were performed with
digester supernatant from the Libertyville sewage treatment plant.
Pilot Plant Studies
To verify the results obtained with bench scale laboratory equipment,
and in most cases, performed with DS from the Libertyville sewage
treatment plant; larger scale pilot plant studies were performed using
digester supernatant from the Lake Zurich sewage treatment plant,
Figure 12 shows the schematic of the equipment used in the pilot
plant studies. In Figure 13 can be seen a photograph of the pilot plant
mounted in a trailer at the Lake Zurich sewage treatment plant location.
Figure 14 shows a close up of the reactor, the storage tanks and the
flashing chamber.
Batch Experiments. All these experiments were performed in a reactor of
30 gallon capacity manufactured by Groen, Inc. The reactor has a double
jacket which contains water that can be heated by a sealed electrical
element. Temperature can be accurately controlled by a built-in
thermostat which allows variation of + 2°C. The digester supernatant
was transferred on a daily basis from the sewage treatment plant's
secondary digester to the pilot plant storage tank which can hold up
to 300 gallons. The digester supernatant was maintained under slight
agitation to ensure a uniform raw material. A centrifugal pump was
used to transfer DS from the storage tank to the reactor. The reactor
was fitted with a lightning mixer with a motor of 1/12 hp. The stirrer
was capable of operation at 1700 rpm. A centrifugal pump was used to
discharge from the reactor. The rate of discharge could be adjusted
with a gate valve located in front of the pump. Samples of DS heated
with and without agitation were collected at several time intervals.
The samples were centrifuged in a batch centrifuge for approximately
10 minutes. The liquid phase was analyzed for residual orthophosphate,
C02, N and COD.
The effect of agitation on the precipitation of phosphate from
digester supernatant was studied at 65°C. In Figure 15 can be seen the
residual orthophosphate in mg/1 P as a function of time for experiments
with and without stirring at 65°C. Obviously stirring does play an
important role in the precipitation of orthophosphate. Table X summarizes
the data on pH, residual CCfc.N and COD. Evaluation of the C02 residual
and pH behavior explains the higher phosphate recovery when agitation is
used; i.e., a higher pH which means a lower carbon dioxide concentration,
decreases the solubility of phosphate compounds and thus a higher
orthophosphate recovery is possible. Nitrogen concentration decreased
by approximately 80 mg/1 and 60 mg/1 respectively after 30 minutes
when stirring and heat alone were used.
-35-
-------
REACTOR
I .. .1
TRANSFER
PUMP
STORAGE TANKS
FLOW
METER
TO VACUUM SYSTEM
FLASH
CHAMBER
FLASHED EFFLUENT
PUMP
FIGURE 12 SCHEMATIC OF THE PILOT PLANT EQUIPMENT
-36-
-------
FIGURE 13 PHOTOGRAPH OF THE PILOT PLANT TRAILER
-37-
-------
FIGURE 14 PHOTOGRAPH OF THE REACTOR, STORAGE TANKS,
AND FLASHING CHAMBER
-38-
-------
7CL_
CO
<
9
CO
UJ
QL
60
50
Ul
§ 40
Q.
CO
O
I
a.
o
fe 30
o
20
10
(I) ZERO TIME IS THE TIME AT
WHICH DESIRED TEMPERATURE
WAS REACHED.
HEAT AND MIXING
J_
ORIGINAL
D9
(I)
15 30
45
60
TIME IN MINUTES
FIGURE 15
THE EFFECT OF AGITATION ON PRECIPITATION
OF ORTHOPHOSPHATE FROM DIGESTER SUPERNATANT
BY HEAT TREATMENT AT 65° C
-39-
-------
TABLE X
RESIDUAL CARBON DIOXIDE, COD AND TOTAL NITROGEN IN
DIGESTER SUPERNATANT HEATED AT 659C WITH AND WITHOUT STIRRING
Concentration
mg/1
C02
COD
TKN
PH
Original
2156
1782
626
7.0
Heat Only
0 Time (1)
1980
1686
615
7.5
Heat and Stirring
15
min
1980
1004
583
7.9
30
min
1936
875
563
7.9
Original
1980
1700
604
7.6
0 Time (1)
1768
976
551
8.9
15
min
1108
968
537
9.3
30
min
915
772
518
9.3
(1) Time at which temperature was reached.
COD - Chemical Oxygen Demand
TKN - 3btalKjeldahl Nitrogen
-------
A series of batch experiments were designed to study the effect of
temperature and retention time on the precipitation of orthophosphate
from digester supernatant by heat. All the experiments were performed
under the same conditions. A volume of approximately 14 gallons of
digester supernatant was transferred to the reactor. The stirrer was
set at 1700 rpm and the temperature was raised to reach the desired
level. Usually, approximately 15 minutes was required to reach
equilibrium. Samples were taken at predetermined time intervals. The
samples were centrifuged in a batch type centrifuge and analyzed for
orthophosphate. Three temperature levels were studied 56°C, 65°C,
and 75°C. Figure 16 shows the residual orthophosphate for each
temperature as a function of time. A residual orthophosphate below
10 mg/1 P was obtained at either 65°C or 75°C regardless of the
retention time. The residual orthophosphate was higher than 10 mg/1 P
and remained fairly constant up to 2 hours when the temperature was 55°C.
In all cases the highest reduction of orthophosphate was observed during
the first 15 minutes of heating from the original temperature of the
DS to the desired temperature.
Continuous Experiments. During the preliminary work, all the equipment
was calibrated and the reactor was tested under varied operating conditions.
Some difficulty was experienced with the flow meters and the centrifugal
pumps. In both cases, when operating at low flows, there was a tendency
to plug with the suspended solids present in the digester supernatant.
This type problem should not be present when operating with larger size
equipment since the suspended solids present in the digester supernatant
have very small particle sizes.
A total of twenty seven continuous runs were performed. Equilibrium
in the system was reached quickly because of the size of equipment and type
of operation. The length of the continuous runs was from 2 to 6 hours.
Retention time varied from 15 to 120 minutes. The temperatures studied
were 65, 70, 75 and 80°C.
The operation in all cases was similar. Digester supernatant was
collected on a daily basis from the secondary digester of the Lake Zurich
sewage treatment plant. The digester supernatant was stored in a tank
which can hold up to 300 gallons. The digester supernatant was constantly
stirred at a very low rate to ensure uniformity. A predetermined Irolume
of DS was transferred to the reactor where it was heated up to the desired
temperature. A stirrer set at 1700 rpm was used during the operation.
Once the temperature desired was reached, digester supernatant from the
storage vessel was continuously pumped into the reactor at a predetermined
rate. The flow was regulated by a gate or needle valve depending on the
flow desired. The flow was monitored on a regular time basis. A flow
meter was installed between the pump and the reactor. The incoming
steam was directed through a pipe to the wall of the reactor opposite to
the outlet pipe to minimize by-passing. The treated DS was pumped out
of the reactor into a 40 gallon drum which was calibrated. The volume
-41-
-------
60.
CO
<
40L
30
20L
IOL
ZERO TIME IS THE TIME AT
WHICH DESIRED TEMPERATURE
WAS REACHED.
55°C
_L
J.
X
-L
ORIGINAL
DS
15 30 60
TIME IN MINUTES
90
120
FIGURE 16
THE EFFECT OF TEMPERATURE AND RETENTION
TIME ON ORTHOPHOSPHATE REMOVAL FROM DS
-42-
-------
of DS received from the reactor was monitored on a time basis. Hourly
spot samples were collected and centrifuged in a batch centrifuge for
analysis.
In Table XI can be seen the data for runs at 65°C. The conclusions
which may be drawn from the data are as follows:
1. Orthophosphate may be precipitated without addition of chemicals from
DS in a continuous process.
2. An average of 80% removal may be obtained at 65°C when the retention
time is held from 45 to 60 minutes.
3. A removal of 80-90% orthophosphate is possible at a retention higher
than 90 minutes.
4. Digester supernatant from the Lake Zurich and the Libertyville plants
behave equally under similar operating conditions.
5. The removal of total phosphate parallels the behavior observed for
the soluble orthophosphate. In general cations do not precipitate
non-orthophosphate P and therefore differences may occur when a large
amount of soluble non-orthophosphate P is present in the DS.
6. A substantial amount of COD is removed by the heat treatment of DS.
On the average a 50% removal can be anticipated under these operating
conditions.
Information on the continuous runs performed at 70°C is summarized in
Table XII. A shorter retention time than that used at 65°C was studied.
Two runs with a 15 minute retention time verified that the reactor was
too small to reach equilibrium when 1 gallon per minute was continuously
pumped into the reactor with a volume of 15 gallons. In both cases the
residual orthophosphate of 10 mg/1 obtained in the first two samples
increased to 16 mg/1 as a function of time. In both cases the first
two samples were taken in 15 minute intervals; the other samples were
taken after 1 hour and 2 hour operation. The residual C02 is also low
in the first two samples but then increases in the latter part of the
experiment.
A 30 minute retention time yielded an 80% removal of orthophosphate
and total phosphate. Again, the percent COD removed was close to 50%.
Table XIII contains the data for continuous runs performed at 75°C.
These results indicate that at 75°C a removal of 85% to 90% of the ortho-
phosphate may be expected. The percent removal of total phosphate was also
in the same range. The COD removal was slightly higher than the previous
50% average obtained at the lower temperatures. It appears that a longer
retention time does not improve the percent removal of any of the
impurities. This may be due to variations in the characteristics of the
digester supernatant which was used in the different experiments.
-------
TABLE XI
PRECIPITATION OF PHOSPHATE FROM DIGESTER
SUPERNATANT BY HEAT TREATMENT - 65°C CONTINUOUS RUNS
(LAKE ZURICH SEWAGE TREATMENT PLANT)
Original
Exp. Volume in Retention OP TP C02 COD SS
No. Reactor (gal) Time(min) pH fmg/1") pH
1 16 45 7.2 71 85 2200 2166 - 8.5.
8.5
8.5
8.6
8.6
8.7
8.7
2(1) 10 60 7.2 64 67 - 1930 480 8,7
9.5
9.0
8.9
9.1
3 16 60 7.2 78 103 2700 2850 2240
4 18 90 7.2 67 69 2217 2790 1980 8.5
8.8
8.8
8.9
8.8
5 16 120 7.1 71 - - - 8.6
9.0
(1) Sample from the Libertyville Waste Treatment Plant 9.1
9.1
OP - Orthophosphate Phosphorus 9,2
TP - Total Phosphorus 9.2
Treated
OP
18
17
17
15
12
14
12
8
12
14
13
12
15
14
9
9
8
9
7
11
9
9
8
8
8
TP C02
fmg/1}
21
19
20
18
18
19
18
11
16
17
16
14
16
15
18
16
14
14
13
_
-
-
-
-
—
1900
1800
1870
1730
1720
1640
1660
«•
—
—
-
-
1800
- .
1840
1630
1500
1440
1500
_
—
-
-
-
-
COD
1150
1190
1190
1160
1140
1120
1150
700
1050
1070
1030
870
1360
1280
1210
1350
1370
1380
1350
_
-
-
-
-
-
OP
75
76
76
79
83
80
83
87
81
78
80
81
81
82
87
87
88
87
90
85
87
87
89
89
90
% Removal
TP
75
78
76
79
79
78
79
84
76
75
76
79
84
85
74
77
80
80
81
. —
-
-
-
—
co2
14
18
15
21
22
25
24
_
-
—
—
-
33
—
17
26
32
35
32
_
-
—
-
-
COD
47
45
45
46
47
48
47
64
45
44
47
55
52
55
56
51
51
50
53
_
—
-
-
-
-
COD - Chemical Oxygen Demand
SS - Suspended Solids
-------
TABLE XII
PRECIPITATION OF PHOSPHATE FROM DIGESTER
SUPERNATANT BY HEAT TREATMENT - 70°C CONTINUOUS RUNS
i
•P-
tn
(LAKE ZURICH SEWAGE TREATMENT PLANT)
Original
Exp. Volume in Retention OP TP C02 COD
No. Reactor (gal) Time(min) pH (mg/1)
1 15 15 7.1 64 - 2190
2 15 15 7.1 62 - 2190
3 13 30 7.4 54 53 2112 1930
PH
8.9
8.9
8.5
8.4
8.9
8.9
8.5
8.5
8.7
8.8
8.6
OP
10
10
16
16
10
10
16
16
14
9
11
Treated
TP C02 COD
(tng/1)
- 1460
- 1340
- 1790
- 1812
- 1460
- 1340
- 1790
- 1812
13 1640 1030
11 1340 1020
10 1610 1020
OP
84
84
75
75
84
84
74
74
74
81
80
% Removal
TP C02
- 33
- 39
18
- 17
33
39
- 18
17
75 22
79 35
81 24
COD
^
-
-
-
-
" *
47
47
47
OP - Orthophosphate Phosphorus
TP - Total Phosphorus
COD - Chemical Oxygen Demand
SS - Suspended Solids
-------
TABLE XIII
PRECIPITATION OF PHOSPHATE FROM DIGESTER
SUPERNATANT BY HEAT TREATMENT - 75 °C CONTINUOUS RUNS
CLAKE ZURICH SEWAGE TREATMENT PLANTS
Original
Exp. Volume in Retention OP TP C0£ COD SS
No. Reactor (gal) Time(min) pH fmg/1) pH
1 8 15 7.2 62 102 2200 2340 1700 9.0
8.9
8.9
8.8
8.9
9.5
8.9
2 15 30 7.2 61 92 2200 2068 1100 8.7
8.7
8.5
8.5
8.8
8.9
9.0
3 15 45 7.1 63 80 2217 1990 - 9.0
8.8
8.8
OP - Orthophosphate Phosphorus
TP - Total Phosphorus
COD - Chemical Oxygen Demand
SS - Suspended Solids
Treated
OP
9
8
9
10
9
5
8
9
8
10
11
7
8
7
9
9
9
TP
(mg
12
13
10
12
9
7
9
14
13
14
14
11
11
11
12
11
11
C02
/I)
1280
1320
1460
1510
1440
-
1350
1190
1510
1740
1810
1450
1370
1300
1090
1450
1480
COD
1070
1100
1090
1060
1010
101Q
1010
1150
1050
1050
1070
1010
1010
990
1130
1080
1070
OP
85
87
85
84
85
90
87
85
87
84
82
89
87
89
86
86
86
% Removal
TP
88
87
90
88
91
93
91
85
86
85
85
88
88
88
85
86
86
C02
42
40
33
31
34
-
38
28
31
21
18
3'4
38
41
51
35
33
COD
54
53
53
55
57
57
57
49
49
49
48
51
51
52
43
45
46
-------
The last group of continuous experiments was performed at 80°C
to determine if a higher operating temperature could decrease the solublity
of the phosphates which precipitate from digester supernatant without
addition of chemicals. Table XIV summarizes the data for these experiments.
At 80°C it appears that the lower retention time of 15 minutes is
optimum. Therefore, at the higher temperatures, higher retention is not
recommended. The residual orthophosphate found in the experiment using
Libertyville DS is similar to that obtained with the Lake Zurich material,
but the calculated percent recoveries are lower because the original
Libertyville DS contained a very low concentration of orthophosphate.
A few experiments were performed in the continuous flash chamber to
determine if the CC>2 present in DS could be removed at a faster rate and
to a lower residual concentration than that possible by heat treatment
alone. All the experiments were performed following the same basic
procedure. Digester supernatant was transferred to the reactor where it
was heated to 65°C. In one example, the DS was immediately flashed after
it reached 65°C. In the other two cases, the DS was maintained at 65°C
for either 15 minutes or two hours. The heated DS was transferred to the
vacuum chamber by suction, as a continuous stream. A schematic of the
vacuum chamber is shown in figure 17. The volume maintained in the vacuum
chamber was one half gallon. The DS was discharged through a sparger
into the chamber which was operated at a vacuum 23" of mercury. The
volume in the chamber was maintained constant by pumping out the DS
at a predetermined rate. The average retention time within the chamber
was approximately 2 minutes. All the experiments demonstrated that the
residual concentration of CC>2 could not be significantly lowered by
flashing the DS under these operating conditions.
It was observed, however, that the nitrogen concentration was
affected by such treatment. In Table XV can be seen nitrogen analyses
for samples treated by heat alone and by heat and vacuum. The data
confirms the observations made during the bench scale experiments.
EFFLUENT QUALITY
The quality of the process effluent depends on the original con-
centration of impurities present in the digester supernatant and the
efficiency of the liquid-solid separation after treatment by either
chemicals or heat.
In Table XVI can be seen some examples of the differences in original
composition of digester supernatant samples used in our research. They
represent samples from the Libertyville and the Lake Zurich sewage treatment
plants.
The quality of the effluent was first determined under optimum
conditions in the laboratory scale equipment. All the experiments in
this section were performed with digester supernatant obtained at the
-47-
-------
TABLE XIV
PRECIPITATION 0! PHOSPHATE FROM DIGESTER
SUPERNATANT BY HEAT TREATMENT - 80'C CONTINUOUS RUNS
(LAKE ZnHTTH SEWAGE TREATMENT PLANTS
Exp. Volume in Retention
No. Reactor (gal) Time(min) pH
1(1) 6 15 7.4
2 6 15 7.5
3 15 45 7.4
Original
OP TP C02 COD SS
Treated
OP
(mg/1) oH
46 67 - 1020 720 9
9
9
85 109 2165 2333 2220 8
9
9
9
9
57 87 2200 2180 1440 8
8
9
9
8
.0
.3
.3
.6
.1
.1
.1
,1
.9
.9
.0
.0
.9
11
7
6
11
8
8
8
8
10
9
9
8
9
TP
C02
COD
OP
% Removal
TP
C02
COD
fmg/1}
13
9
8
16
11
11
11
11
13
11
11
11
11
_
-
—
1120
1040
1050
1070
1030
1370
1320
1140
1170
1320
540
480
450
1240
1190
1140
1120
1100
1090
1060
1110
1090
1040
76
85
87
87
91
91
91
91
82
84
84
86
84
81
87
88
85
90
90
90
90
85
87
87
87
87
_
-
—
48
52
51
5Q
52
38
40
48
46
40
47
52
56
47
49
51
52
53
50
51
49
50
52
(1) Sample of DS obtained from the Libertyville Sewage Treatment Plant
OP - Orthophosphate Phosphorus
TP - Total Phosphorus
COD - Chemical Oxygen Demand
SS - Suspended Solids
-------
INFLUENT
TO VACUUM SYSTEM
BAFFLE
FLASHED
EFFLUENT
FIGURE 17 SCHEMATIC OF THE FLASH CHAMBER
-49-
-------
TABLE XV
RESIDUAL CONCENTRATION OF NITROGEN IN DIGESTER
SUPERNATANT TREATED BY HEAT AND/OR HEAT AND VACUUM
(LAKE ZURICH SEWAGE TREATMENT PLANT)
Temp.
(0CC)
65
70-
80
65
Vacuum Original
(inches of Hg) TN NH3 as N
(mg/1)
684 532
642
656
23 684 532
Treated
TN NH3 as N
(mg/1)
586 517
544
492
369 290
% Removal
TN NH3 as N
(mg/1)
15 3
16
25
47 46
TN - Total Nitrogen
-50-
-------
TABLE XVI
COMPOSITION OF THREE TYPICAL DIGESTER SUPERNATANT
SAMPLES FROM THE LIBERTYVILLE
AND LAKE ZURICH SEWAGE TREATMENT PLANTS
Libertyville
Total P (mg/1) 70 82 67
Orthophosphate P (mg/1) 71 73 46
COD (mg/1) 673 560 1020
Total N (mg/1) 378 330 452
Suspended Solids (mg/1) - - 720
Lake Zurich
87 92 102
57 61 62
2180 2060 2340
656 590
1440 1100 1700
-51-
-------
Libertyville plant. Analyses of the effluents were studied after treatment
of the DS by 3 methods, namely (1) with magnesite and sodium hydroxide,
(2) with magnesium carbonate, and (3) with heat or heat and vacuum. In
all cases, the solids were separated from the liquid phase by centrifugation
in a batch type centrifuge.
In Table XVII can be seen the analyses for treated DS after precipitation
of phosphate was obtained by addition of calcined magnesite and sodium
hydroxide (for final pH adjustment). It is obvious that the quality of
the digester supernatant liquor can be greatly improved by precipitation
with magnesia. Phosphate concentration is reduced together with the
concentration of BOD, COD and the nitrogen. The residual total nitrogen
decreases with increases of magnesium ion concentration.
Analysis of a sample of DS treated with magnesium carbonate without
final pH adjustment with a soluble base can be seen in Table XVIII. The
magnesium concentration used was stoichiometric with respect to phosphate.
The treated DS contained a substantially reduced concentration of BOD
and COD as well as phosphate. In this instance, however, only a small
amount of nitrogen was removed.
In Table XIX can be seen the analyses of DS treated with heat or
heat and vacuum under different operating conditions. From the data it
may be concluded that a treatment at 65°C with or without vacuum yields
an effluent substantially cleaner than the original digester liquor. Two
of the most deterimental impurities, phosphorus and nitrogen, can be
reduced to very low concentrations.
Additional experiments were performed using the treated digester liquor
from the Lake Zurich sewage treatment plant. The DS from this plant was
always higher in suspended solids, nitrogen, COD and total phosphorus than
the Libertyville DS. All the treated samples for the solid-liquid
separation experiments were obtained from the pilot plant runs.
First, filtration was tested for the liquid-solid separation. An
Eimco filtration kit and procedure were used in these experiments. The
preliminary observations demonstrated that filtration is not a practical
method to obtain the desired effluent quality. The very fine organic
and inorganic solids present in the DS blind the filter cloth quite rapidly.
This approach was abandoned because it would be extremely expensive.
Clarification by settling appears to be a practical solution to the
liquid-solid separation problem. A solids contact clarifier as manufactured
by the Eimco or Dorr Oliver Companies should be quite suitable for this
application. Addition of polymers to improve the speed and efficiency of
the liquid-solid separation should be evaluated. The size of the equipment
used in the pilot plant studies, precluded any serious evaluation of
liquid-solid separation by settling on a pilot scale.
Jar test experiments were performed to determine the type of polymer
most suitable for this application. It was found that the best results
-52-
-------
TABLE XVII
ANALYSIS OF CHEMICALLY TREATED DIGESTER SUPERNATANT
FROM THE LIBERTYVILLE PLANT - pH 9.0
BOD (mg/1)
COD (mg/1)
(mg/1)
sphate P (mg/1)
1)
1)
(mg/1)
Original
82
315
360
413
MgO+NaOH
0.2 (1)
6
7
-
280
326
Treatment
MgO+NaOH
0.4 (2)
5
5
-
281
318
MgO+NaOH
1.0 (3)
6
5
64
341
298
(1) Amount of Mg^4" added - 20% of stoichiometry
(2) Amount of Mg"^ added = 40% of stoichiometry
(3) Amount of Mg++ added = stoichiometry
-53-
-------
TABLE XVIII
ANALYSIS OF DIGESTER SUPERNATANT BEFORE AND AFTER
PHOSPHATE PRECIPITATION WITH MAGNESIUM CARBONATE
(LIBERTYVILLE PLANT)
Total P (mg/1)
Orthophosphate P (mg/1)
BOD (mg/1)
COD (mg/1)
Total N (mg/1)
Original
70
71
234
673
378
Treated
5(1)
9
53
280
336
(1) The analysis for P was made several days
after the experiment was performed. It is
lower than the orthophosphate concentration
probably due to phosphate post precipitation
upon standing.
-54-
-------
TABLE XIX
ANALYSIS OF DIGESTER SUPERNATANT TREATED WITH
HEAT OR HEAT AND VACUUM
(LIBERTYVILLE PLANT)
Experiment No. 1
Temperature (°C) 40 65 65 65
Vacuum (27" Hg) yes no yes yes
Retention Time (min.) 60 120 60 60
Total P (mg/1) 9422
Orthophosphate P (mg/1) 9411
BOD (mg/1) 134 86
COD (mg/1) 268 340 139 214
Total N (mg/1) 319 45 8 16
-55-
-------
were obtained with a cationic polyethylene inline polymer. When a dose of
6 mg/1 of active polymer was added 89% of the suspended solids were
removed from the liquid phase. Visual observation of the polymer treated
sample of DS was similar in quality to the centrifuged DS sample.
Centrifugation was tested because it appears to be a good method
for high speed liquid-solid separation. Initial centrifugation tests
were made using a Sharpies high speed centrifuge operating at 13,200 g.
The unit, which was a small bench type model was fed at increasing
rates ranging from 280 ml/min to 2,500 ml/min. Table XX shows the
analytical results of two runs with treated DS from the Lake Zurich
plant. As shown, this centrifuge performs more effectively when
magnesia is used to treat the digester liquor. The density of the
solids is higher and therefore, they are easier to separate from the
liquid phase. Lower flow rates are required when the DS is only heat
treated. The magnesia treated samples indicate a dramatic reduction
in total phosphate, orthophosphate and suspended solids.
A Sharpies-Fletcher Mark III continuous centrifuge was also tested.
A photograph of the pilot scale centrifuge is shown in Figure 18. All
the experiments were performed at a rotation of 2,550 rpm. The flow
rate varied from 300 to 730 ml/min. Table XXI shows the results of these
tests. The quality of the effluent produced by the Mark III centrifuge
is acceptable. Centrifugation should be considered as a means of liquid-
solid separation and should be evaluated for longer periods to establish
reliability of performance.
PRODUCT CHARACTERIZATION
The solids precipitated from a waste stream of a sewage plant may
vary in composition depending on the impurities present in solution and
in suspension. If the waste stream under consideration contains a low
concentration of calcium, then it is anticipated that the ammonia nitrogen
and orthophosphate initially present will precipitate with magnesia as
a mixture of MgNH4P04'6H20 and Mg32'4H20 and 033^04)2.
The water of the Libertyville and Lake Zurich sewage plants contain
a high concentration of both calcium and magnesium. In order to predict
the type of product that might be precipitated from such a stream, phosphate
precipitation was first performed in a synthetic mixture containing 100
mg/1 P, 117 mg/1 Mg, and 114 mg/1 Ca. The analysis of the precipitate is
shown in Table XXII. Calculated composition of this precipitate is as
follows: 15.6% MgNH4P04«6H20, 45.0% Mg3(P04)2'4H20 and 36.8% Ca3(P04)2.
Approximately 10% of the total phosphate is chemically associated with
the nitrogen.
Analysis of several solids precipitated from digester supernatant
collected at the Libertyville plant are shown in Table XXIII. These
-56-
-------
TABLE XX
Cn
~j
I
CENTRIFUGATION OF DIGESTER SUPERNATANT WITH A SHARPIES
HIGH SPEED (13,200 g) CENTRIFUGE
(LAKE ZURICH SEWAGE TREATMENT PLANT)
Treatment
Mg(OH)2
(70 mg/1 Mg,
Heat
65°C
Original
Flow Rate (mg/1)
(ml/min.) TP OP SS
280 93 76 1640
pH 9.0)
1560 93 76 1640
2480 93 76 1640
660 100 80 2500
750 100 80 2500
TP - Total Phosphorus
OP - Orthophosphate Phosphorus
SS - Suspended Solids
Effluent % Recovery
(mg/1)
TP OP SS TP OP SS
5 2 80 97 95 95
10 5 211 93 89 87
11 5 560 93 88 66
20 11 520 80 86 79
31 24 860 79 70 66
-------
FIGURE 18 PHOTOGRAPH OF THE PILOT SCALE CONTINUOUS CENTRIFUGE
-------
TABLE XXI
VO
I
Treatment
Heat
(65°C)
CENTRIFUGATION OF DIGESTER SUPERNATANT WITH
A CONTINUOUS SHARPLES-FLEfTCHER MARK III
CENTRIFUGE (2,550 rpm)
(LAKE ZURICH SEWAGE TREATMENT PLANT)
Original Effluent % Recovery
Flow Rate (mg/1) (mg/1)
(ml/min.) TP OP SS TP OP SS TP OP SS
300 121 76 3160 19 15 60 84 80 98
730 121 76 3160 24 21 460 80 72 85
Mg(OH)2
(70 mg/1 Mg, pH 9.0)
450
93
76 1640
16
10 280
83
87
83
TP - Total Phosphorus
OP - Orthophosphate Phosphorus
SS - Suspended Solids
-------
TABLE XXII
ANALYSIS OF SOLIDS PRECIPITATED IN A
SYNTHETIC SYSTEM (pH 9.5)
% Composition
Total P205 43.3
Available ?205* 43.3
Total N 0.9
Ca 14.3
Mg 11.3
*Available to growing plants according to
tests of Association of Official Agricultural
Chemists (AOAC).
-60-
-------
TABLE XXIII
ANALYSIS OF SOLIDS PRECIPITATED FROM DIGESTER SUPERNATANT
OBTAINED AT THE LIBERTYVILLE SEWAGE TREATMENT PLANT
Treatment
MgO+NaOH
MgO+NaOH
MgO+NfyOH
MgO+NH40H
MgO+NaOH
70°C
65°C
Stoichio-
metry*
0.4
0.4
0.4
1.0
1.0
-
_
Final
pH
9.0
10.0
9.5
9.5
10.0
8.9
8.9
% Total
P205
21.48
23.47
29.48
21.70
20.65
19.51
17.58
% Avail.
P20<;**
21.08
23.08
28.60
20.50
20.38
19.41
17.40
% Total
N
3.35
2.73
2.44
2.60
1.86
3.61
2.46
% NHs
N
-
0.62
-
1.00
-
-
—
*Magnesium relative to phosphorus
**Available to growing plants according to AOAC test
-61-
-------
solids are approximately 50% lower in phosphate concentration than those
recovered from the synthetic system. This is expected because the
concentration of suspended solids present in the DS used was approximately
500 mg/1 and the phosphate solids recovered from a synthetic system are
also approximately 500 mg/1. Therefore, there is a 50% dilution of the
phosphates obtained from the DS with extraneous solids. The nitrogen
content of the solids precipitated from the DS is much higher than the
concentration found in the synthetic system i.e., from 2.0% - 3.0% N in
the DS solids compared to 0.9% N in the synthetic system. This is due to
the fact that organic nitrogen compounds which are suspended in the DS
are precipitated together with the phosphate.
It is significant to note that most of the phosphate is present in a
form available to plants, i.e., passes the solubility test established
by the AOAC procedure (Association of Official Agricultural Chemists).
An attempt to identify the phosphate mixture by X-ray techniques indicated
that the solids are amorphous. It was observed, however, that during the
precipitation procedure, a crystalline phase could be. separated from the
bulk of the solids. This crystalline material identified by X-ray as
MgNH4P04'6H20.
Tablfc XXIV summarizes the data on the solids collected from pilot plant
runs performed at the Lake Zurich sewage treatment plant. The digester
supernatant tested at Lake Zurich contained on an average 1500 mg/1
suspended solids. If the DS did not contain any inert solids, then the
theoretical concentration of total P205 would be 43% as shown in Table XXII.
The theoretical amount of phosphate solids which would be precipitated is
500 mg/1. Suspended solids present in the digester should dilute the
phosphate precipitate approximately by a factor of 4 and therefore the
average percent ?205 expected is 10.7%. The results in Table XXIV indicate
that solids containing from 10% to 15% P2(>5 may be recovered from Lake
Zurich digester supernatant. In all cases the phosphate is present in
a form available to plants. The nitrogen content is 3% in most cases.
-62-
-------
TABLE XXIV
ANALYSIS OF SOLIDS PRECIPITATED FROM DIGESTER SUPERNATANT
OBTAINED AT THE LAKE ZURICH SEWAGE TREATMENT PLANT
% Total
Treatment P?0^
NaOH (pH 8.9) 10.5
NH40H (pH 10.0) 10.7
65°C (60 min.
retention time) 13.5
65 °C (90 min.
retention time) 10.9
65 °C (30 min.
retention time) 12.9
70°C (45 min.
retention time) 14.0
75°C (retention time) 15.2
% Available (1)
10.1
10.3
13.3
10.7
12.6
13.6
14.7
% Total
N
2.8
4.0
3.3
3.1
3.5
3.3
3.5
(1) Available to growing plants according to AOAC test.
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SUMMARY AND CONCLUSIONS
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1. Precipitation of orthophosphate from digester supernatant may be
accomplished by adding either a soluble salt of magnesium, magnesium
oxide, magnesium hydroxide or magnesium carbonate. The efficiency
with which phosphate is removed depends on the reactivity of the
magnesium source, when an insoluble compound is used, and on the
final pH of precipitation. A pH range between 9.0 and 9.5 yields
the highest phosphate removal. When a final pH adjustment is
required either ammonium hydroxide or sodium hydroxide may be used
for final neutralization. Variables such as seed recycle, retention
time and speed of agitation do not have a significant effect when
precipitation is obtained by addition of magnesium ion and pH
adjustment.
2. Precipitation of orthophosphate from digester supernatant may be
accomplished by addition of a soluble neutralizing base such as
ammonium hydroxide or sodium hydroxide. In areas where hard water
exists, the digester supernatant contains sufficient cations such
as calcium and magnesium to precipitate the orthophosphate.
3. A new process for the precipitation of orthophosphate from digester
supernatant, based on the heat decomposition of ammonium bicarbonate
was tested on a bench and pilot scale. As the ammonium bicarbonate
decomposes, C02 evolves from the system causing a raise in pH.
Precipitation of calcium and magnesium phosphate is obtained without
need of chemical additives.
4. Phosphate precipitation by all the processes described improves the
quality of the treated stream because it removes a very high
percentage of phosphate and substantial amounts of nitrogen and COD.
5. All the processes were tested at both bench and pilot plant levels.
6. On the average a removal of 85 to 90 percent of the phosphate present
in digester supernatant may be expected, without addition of chemicals.
7. A mixture of MgNltyPO^• 61^0 and Mg3(P04>2'4H20 will precipitate from
a stream substantially free of calcium. If calcium is present in
high concentrations, then a mixture of MgNH4POA«6H20, Mg3(P04)*4H20
and Ca3(P04)2 is precipitated.
8. The percent composition of the solids is dependent on the amount of inert
impurities suspended in the original digester supernatant and in the
relative concentrations of magnesium and calcium present or added to the
system.
9. From the Libertyville sewage plant streams, it is anticipated that the
solids precipitated will contain approximately 20% ?2Q5 and 2%
nitrogen. From the Lake Zurich sewage plant, solids containing
12% ?20s and 3% nitrogen may be recovered.
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PATENTS
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During the course of the work on Contract No. 14-12-171, several
technical discoveries were made which may be novel and patentable. In
accordance with the Contract, disclosures of invention on three of these
were submitted to FWPCA.
1. Disclosure Number FWP-1533. The novel features of this invention
are as follows: A high total phosphate removal and substantial
removal of nitrogen,. COD, and BOD can be accomplished by decomposition
of the ammonium bicarbonate present in the digester supernatant.
The decomposition may be achieved by increasing the temperature,
by reducing the pressure or by a combination of these two variables.
A high degree of agitation or other means of exposing a large
surface (i.e., spraging) increases the speed of decomposition.
The resulting solids are high in phosphate concentration in a form
available to growing plants. The process may be operated continuously.
2. Disclosure Number FWP-1669. The novel features of this invention are
as follows: A high total phosphate removal can be accomplished by
increasing the pH of digester supernatant with ammonium hydroxide to
approximately 10.0. Phosphate precipitates as calcium phosphate and/or
magnesium ammonium phosphate. The precipitated solids are separated
from the liquid phase. The treated DS now free of phosphate is flashed
under vacuum or air stripped, thus releasing ammonia which is recycled
back to the incoming raw DS.
3. Disclosure Number FWP-1679. The novel features of this invention
are as follows: Chemical purification of sewage is based on the
precipitation of orthophosphate and ammonia nitrogen as magnesium
ammonium phosphate. In sewage water the ratio of nitrogen as N
to orthophosphate as F is approximately 4 to 1 on a mol basis. In
order to remove all the phosphate and nitrogen, an amount of
phosphate is added to the sewage, in the form of triple super-
phosphate (a low cost phosphate source), to be stoichiometrically
equivalent to the excess nitrogen. When the sewage containing
the balanced amount of phosphate and ammonia nitrogen is neutralized
with magnesia, precipitation of MgNItyPOA removes the undesirable
impurities N and P in an insoluble form. The solid product recovered
has fertilizer value and contains in addition to the magnesium
ammonium phosphate, calcium pnOsphate, and solid organic impurities
of varied composition. The liquid phase which contains water
soluble organic materials and polyphosphate is percolated through
a charcoal bed prior to discharge. The spent charcoal may be
regenerated for reuse by known processes.
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REFERENCES
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1. Dunseth, M. G. and Salutsky, M. L., "Removal of Scale Forming
Elements from Sea Water," Jnd. Eng. Chem. 56, 56-61 <1964).
2. Salutsky, M. L. and Messina, P., "Removal of Scale Formers with By-
Product Recovery," Proc. First Internat'l. Sym Water Desaln.,
Vol. 2, Washington, D. C., 1965, pp 695-707.
3. Bridger, G. L., Salutsky, M. L., and Starostka, R. W., "Metal
Ammonium Phosphates as Fertilizer," J. Agr. Food Chem. 10, 181-8
(1962).
4. Lunt, 0. R. , I^ofranek, A. M., and Clark, S. B., "Availability of
Minerals from Magnesium Ammonium Phosphates," J. Agr. Food Chem.,
12_, 497-504 (1964).
5. Shea, T., "Progress Report on Studies on Removal of Carbonaceous
Nitrogenous and Phosphorus Materials from Concentrated Waste Streams,"
pending FWPCA Report (May 1969).
6. Fisher, R. A., "Progress Report, Phase I, Development of a Process to
Remove Carbonaceous, Nitrogenous and Phosphorus Materials from
Anaerobic Digester Supernatant and Related Process Streams,"
FWPCA Report (May 1969).
7. Kappe, S. E., "Digester Supernatant Problems, Characteristics and
Treatment," Sew. and Indust. Wastes, 30, 7, 937, July (1958).
8. Barth, E. F., et al., "Chemical Control of Nitrogen and Phosphorus
in Wastewater Effluent," Jour. WPCF, 40, 12, 2040 (Dec. 1968).
9. Rawn, A. M., et al., "Multiple-Stage Sewage Sludge Digestion,"
Trans. Amer. Soc. Civil Eng., 104. 93 (1939).
10. Bube, K., "Uber Magnesiumammonium phosphat," Z. Anal. Chem., 49,
525-96 (1910).
11. Szekeres, L. and Rady, M., "The Solubility of Some Calcium Phosphates,"
Ind. Eng. Chem., Anal. Ed. 18_, 699-702 (1946).
12. Uncles, R. F. and Smith, G. B. L., "Solubility of Magnesium Ammonium
Phosphate Hexahydrate," Ind. Eng. Chem., Anal. Ed. 18, 699-702 (1946).
13. Ries, K. M., Dunseth, M. G., Salutsky, M. L., and Shapiro, J. J.,
"Ultimate Disposal of Phosphate from Waste Water by Recovery as
Fertilizer, Phase I - Final Report," FWPCA Report, Contract
No. 14-12-171 (July 15, 1969).
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APPENDIX
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ANALYTICAL METHODS
Orthophosphate (OP)
Orthophosphate concentration was determined using aminonaphtholsulfonic
acid method in accordance with Standard Methods. The sample was centrifuged
3474 gravity-minutes, and the centrate was then filtered through an 8u
membrane filter, 47 mm diameter. The filtrate was diluted with deionized
water from 40:1 to 200:1 depending on the suspected concentration. A blue
color was developed by using ammonium molybdate-acid reagent and amino-
naphtholsulfonic acid. The blank used was the diluted filtrate treated
only with the aminonaphtholsulfonic acid reducing reagent. The color was
allowed to develop for five minutes, and the color intensity was measured
on a Hach Direct Read colorimeter using a Hach #5330 color filter.
Total Phosphorus (TP)
Total phosphorus was determined using the nitric acid wet ash procedure.
The sample was digested at a low temperature in nitric acid until the sample
almost attained dryness. The resulting slurry was redissolved in hydrochloric
acid and boiled for 15 minutes. The resulting solution was diluted with
deionized water from 40:1 to 400:1 depending on the suspected concentration.
Color was developed according to the above procedure for Orthophosphate.
Total Solids (TS)
Total solids were determined in accordance with Standard Methods by
weighing the residue after 100 ml of the sample had been dried at 103°C
for approximately 20 hours. All gravimetric determinations were made using
a Mettler H-6 four place analytical balance.
Suspended Solids (SS)
Suspended solids were determined by filtering from 10 to 100 ml of the
sample, depending on the suspended solids concentration, through a 0.45
micron membrane filter. The suspended matter removed by filter was
dried at 103°C for two hours and weighed. This procedure is in accordance
with Standard Methods.
Total Kjeldahl Nitrogen (TKN)
Total Kjeldahl nitrogen was determined by digesting 100 ml of the
sample in sulfuric acid, in the presence of mercury, until the sample was
clear. The digested sample was made alkaline with sodium hydroxide in the
presence of sodium thiosulfate and the ammonia was distilled into 50 ml
0.1N sulfuric acid. The residual sulfuric acid was titrated with 0.1N
sodium hydroxide to the methyl red endpoint. The distillation was
performed in a Kjeldahl distillation unit.
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Chemical Oxygen Demand (COD)
Chemical oxygen demand was determined in accordance with Standard
Methods. On the average, the sample was analyzed 36 hours after it
was collected. The sample was refluxed for 2 hours in a known amount
of potassium dichromate and sulfuric acid. Mercuric sulfate was used
to eliminate chloride interference. Silver sulfate was added as a
catalyst in the refluxing. The excess dichromate was titrated with
ferrous ammonium sulfate.
Total Alkalinity as CaC03
Total alkalinity was determined in accordance with Standard Methods.
50 ml of sample was titrated with N/50 sulfuric acid to the methyl orange
endpoint. The alkalinity was expressed as mg/1 CaC03.
Carbon Dioxide (C02)
Carbon dioxide was determined by the titrimetric method in accordance
with Standard Methods. The sample was titrated with N/100 sodium hydroxide
to pH 8.3, using a pH meter.
Biochemical Oxygen Demand (BOD)
The method used in the determination of BOD was in accordance with
Standard Methods. The incubation of sample for BOD started within 2
hours after collection. All samples were seeded and dilutions were
incubated for 5 days at 20°C. Initial and final dissolved oxygen
determinations were made using the azide modification of the Winkler
Method.
Ammonia Nitrogen
Ammonia nitrogen was determined similar to the TKN except that prior
to distillation the sample was not digested.
pH was measured using a Beckman Expandomatic pH meter. A general
purpose glass electrode and a fiber junction calomel electrode were used.
Oxidation-Reduction Potential (ORP)
All oxidation-reduction potential measurements were made on a
Beckman Expandomatic pH meter using a fiber junction calomel electrode
and a platinum electrode. ORP corrected for calomel reference.
Disssolved Oxygen (DO)
All dissolved oxygen measurements, except those used in BOD
calculations were measured on a Precision Scientific galvanic cell.
oxygen analyzer.
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Percent Solids
Percent solids was determined on an Ohaus moisture balance.
Calcium (Ca) and Magnesium (Mg)
Calcium and magnesium concentrations were determined using atomic
absorption. The sample was digested in hydrochloric acid. Lanthanum
chloride was added to aask the phosphate. Hydrogen was used as a fuel
and air was the oxidant. The calcium was determined at 4227 angstroms
and the magnesium was determined at 2852 angstroms.
X-ray Diffraction
All X-ray diffraction used the powder diffraction technique.
Association of Official Agricultural Chemists (AOAC) Analytical
Procedures - (These procedures were used for the analysis of solid
products.)
Analyses in accordance with these procedures were run only on
precipitated solids recovered from the precipitation research.
Total Nitrogen by Kjeldahl
Sample containing nitrogen is digested with concentrated sulfuric
acid. The digested clear sample is distilled in the presence of excess
sodium hydroxide. The liberated ammonia in the distillate is collected
in a standard acid. The excess of acid is titrated with a standard
base. Methyl red indicator is used to detect the endpoint.
Total Phosphorus
Total phosphorus was determined by heating the sample in potassium
nitrate and nitric acid until dry. The evaporated residue is ignited and
then redissolved in deionized water and hydrochloric acid. The phosphorus
concentration of this solution is determined by using the aminonaphthol-
sulfonic acid reducing reagent method of orthophosphate determination.
Available Phosphorus
Available phosphorus was determined by placing the sample on filter
paper and washing it with deionized water. The residue on the paper is
added to hot ammonium citrate. The ammonium citrate solution is then
filtered and washed with deionized water. The two filtrates are combined
and the phosphorus concentration is determined according to the procedure
for total phosphorus.
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Water Soluble Phosphorus
The procedure for determining water soluble phosphorus was the same
as that used for available phosphorus except that the residue on the
filter paper was not digested in ammonium citrate.
Calcium
Calcium was determined by heating the sample in potassium nitrate and
nitric acid until dry. The evaporated residue was ignited and then
redissolved in deionized water and hydrochloric acid. Ferric chloride
was added and the sample was then neutralized with ammonium hydroxide.
Sodium acetate was added and the sample was digested and filtered. The
calcium concentration in the filtrate was determined by titration with
0.1 M EDTA to the Erichrom Black T endpoint.
Magnesium
Magnesium was determined by the same method as calcium, except that
the filtrate was digested with ammonium oxalate. The solution is buffered
to pH 10.0 and titrated with 0.1 M EDTA to the Erichrom Black T endpoint.
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