United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
EPA-600/2-80-037
June 1980
Research and Development
Removal and
Recovery of
Metals and
Phosphates from
Municipal Sewage
Sludge
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. "Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9, Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-80-037
June 1980
REMOVAL AND RECOVERY OF METALS AND PHOSPHATES
FROM MUNICIPAL SEWAGE SLUDGE
by
Donald S. Scott
Department of Chemical Engineering
University of Waterloo
Waterloo, Ontario, Canada N2L 3G1
Grant No. R-804669
Project Officer
R. V. Villiers
Wastewater Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for publication.
Approval does not signify that the contents necessarily reflect the views
and policies of the U.S. Environmental Protection Agency, nor does mention of
trade names or commercial products constitute endorsement or recommendation
for use.
ii
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FOREWORD
The Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health
and welfare of the American people. Noxious air, foul water, and spoiled
land are tragic testimony to the deterioration of our natural environment.
The complexity of that environment and the interplay between its components
require a concentrated and integrated attack on the problem
Research and development is that necessary first step in problem solution
and it involves defining the problem, measuring its impact, and searching for
solutions. The Municipal Environmental Research Laboratory develops new and
improved technology and systems for the prevention, treatment, and management
of wastewater and solid and hazardous waste pollutant discharges from municipal
and community sources, for the preservation and treatment of public drinking
water supplies, and to minimize the adverse economic, social, health, and
asthetic effects of pollution. This publication is one of the products of
that research; a most vital communications link between the researcher and
the user community.
This report presents the results of a study that looked at the technical
and economical feasibility of removing and then recovering the heavy metals
and phosphates present in phosphorus-laden chemical sludge. The results
should be valuable in considering future research effort in this area.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
iii
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ABSTRACT
This research program was undertaken to yield more extensive laboratory
data for the acid extraction process described by Scott and Horlings in 1975,
which was found to be a simple method of extracting metals and phosphate from
chemical type municipal sludges. The data were desired for different.types of
sludges in order to assess the benefits of this process and to estimate
probable costs at different conditions.
Two sludges were used, an anaerobically digested chemical sludge to
which ferric chloride had been added as a phosphate precipitant; and an aerobic
waste activated sludge to which alum had been added for phosphate control.
Both sludges were dewatered and incinerated in multiple hearth incinerators
at their respective plants, the aerobic sludge, however, being first combined
with settled primary sludge. Hence filter cake samples were also used as
starting materials.
Adequate extractions (about 80% or more) of phosphorus, iron, aluminum,
zinc, magnesium, nickel and manganese can be obtained from either sludge or
filter cake at a pH of 1.5, using sulfuric acid. Chromium extractions are
less predictable, and vary from 50%-90%. Copper cannot be extracted from
anaerobic sludge, but does extract from aerobic systems. Lead, which forms
an insoluble sulfate, is not extracted. Iron is in the ferrous form in an-
aerobic systems but in the ferric form in aerobic systems.
The resulting acid extracted residual solids show a removal of from 50%-
70% of the ash-forming inorganics. The calorific value increases from 30%-
40%, because only from 7%-10% of the organic matter or COD is extracted by
the acid.
Acid extraction by the method proposed here (20 minutes extraction
followed by five minutes of heating) appears to improve filterability of
anaerobic sludges somewhat, but has no significant effect on aerobic sludge.
The acidic extract solution can be separated into fractions by selective
precipitation using lime as a neutralizing agent. Extracts containing ferrous
iron can be separated into a solid containing most of the Al, Mg, Cr and P
and a liquid containing most of the iron by precipitation at pH 4.5. The
liquor can be recycled back to the plant for phosphate removal. Extracts rich
in aluminium or ferric ion can be separated into a solid containing essentially
all the JFe, Mg, Al, Ni, Mn and P by precipitation at pH 4.0 This solid can
then be further separated by successive treatment with ammonia and sodium
hydroxide.
iv
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Costs of the process were estimated for each of the two plants from
which samples were obtained. The larger plant (180 mgpd) producing an iron
rich anaerobic sludge gave a cost of $33 per .million gallons of influent,
after credits for iron and phosphate were taken, but without credit for savings
in incineration costs or ash disposal. The smaller plant (30 mgpd) gave a
cost of $50.70 per million gallons of influent on the same basis. Some of
this higher cost for the smaller plant results from the more extensive treat-
ment required for chemical recovery. The major cost factor in the total
process is the cost of reagents, with this being about 60% of the total cost.
This report was submitted in fulfillment of EPA Grant R-804669 by
Donald S. Scott under the sponsorship of the U.S. Environmental Protection
Agency. This report covers a period from September 1, 1976 to December 31,
1978 and work was completed as of October 1, 1978.
v
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CONTENTS
Foreword . . . .
Abstract . . . .
Figures
Tables
Acknowledgement.
1.
2.
3.
4.
5.
6.
7.
Introduction
Conclusions
Recommendations
Materials
Experimental Procedures.
Results and Discussion .
Estimates of Cost. . . .
References
. iii
. iv
.viii
. ix
x
1
4
5
6
8
. 11
. 28
. 33
vii
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Number
1.
2.
FIGURES
Amount of metals precipitated vs pH
Page
20
Residual fraction of metals in the extract as a function
of pH 21
3. Acid requirement of sludge to attain a given pH value .... 22
viii
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TABLES
Number Page
1. Characteristics of sludge samples . 11
2. Analysis of original samples 12
3. Extraction conditions 13
4. Analysis of extract solutions 13
5. Solids content of filtered extracts 14
6. Extraction results 15
7. TOG and COD analysis 16
8. Filtration tests 17
9. Precipitation test at pH 4.5 18
10. Extraction conditions 19
11. Percent extraction of metallic elements 23
12. Solids content of filtered extracts 24
13. Extraction residues : 25
14. COD analysis 25
15. Ash fusion points 27
xx
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ACKNOWLEDGEMENTS
The cooperation is gratefully acknowledged of the staff of the Ash-
bridge's Bay wastewater treatment plant of the Metropolitan District of
Toronto, and of the staff of the wastewater treatment plant of the city of
Warren, Michigan, both of whom supplied samples and plant operating data.
Most of the laboratory tests and analyses were done by, or under the
direction of, Mr. Harry Horlings, Research Associate in the Department of
Chemical Engineering, University of Waterloo. His skill and enthusiasm were
essential for the completion of this work.
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SECTION 1
INTRODUCTION
Sewage sludge is one of the end-products of the physical, chemical and
biological operations employed in modern municipal wastewater treatment
systems. Effective sludge management and disposal is a problem and a challenge
in treatment plants, particularly those of large capacity located in highly
urbanized areas. The upgrading of treatment plants, particularly the addition
of phosphorus removal practices, results in increased volumes of biological
or chemical sludges, which may be more difficult to handle than a simple
primary sludge. As treatment plants become more sophisticated, the sludge
handling and disposal problem grows.
There is at present no means of disposing of final sewage sludge at a
profit, and so sludge remains a growing liability for the plant operator. In
fact it has been estimated that treatment and disposal of sludges produced in
municipal wastewater treatment plants can account for up to 60% of the total
cost of operating these facilities (1). A number of methods of sludge disposal
exist which are presently viable for urban plants such as agricultural land
disposal, land reclamation, sanitary land filling and incineration. Recently,
however, there has been a growing concern that the heavy metals content of
either sludge or incinerator ash is not adequately fixed, and can re-enter
the ecology by leaching and/or plant uptake. Relatively little is known
concerning the uptake of heavy metals by plants from contaminated soils, and
it is only recently that systematic research into this potential problem has
been undertaken,for example (2-8). Preliminary results show that certain
combinations of the right type of plant, soil and metallic element can lead to
undesirable metal concentrations in plant foods, or to deleterious effects on
plant growth. In Canada, limits to the use of sludge for application to
agricultural land are now being considered, .and such limits already exist in
England.
Land for disposal of sewage sludge as sanitary land fill, drying beds,
or lagoons is becoming increasingly difficult to find and increasingly costly
in the vicinity of large urban centres. In addition, the sludge volume
increases annually, and as more chemical sludges are produced with higher
metal and phosphate contents, suitable land disposal will become more and more
costly and difficult.
Because of these considerations, incineration is growing in popularity
as a final disposal process for sludge. In fact, incineration is a final
disposal mechanism only for the organic part of the sludge, and the inorganic
constituents are left as an ash, high in metals and phosphorus content.
Recently, studies have been carried out and are currently underway to attempt
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to assess the nature of this ash and to explore ways of recovering metals or
phosphate from it (9-13). Presently available results suggest that the ash is
sufficiently refractory that extraction of metals and phosphate from it is
difficult in any reasonable yield or at reasonable cost. Ash quantities are
considerably increased in chemical sludges due to, the added metal salts and
the precipitation of phosphates. One study has shown that neutral water can
leach some metals, e.g., chromium, out of the ash in sufficient quantities to
cause groundwater contamination (9).
Despite possible problems with the quantity and nature of the ash,
incineration is the most rapidly growing method of sludge disposal (at present
over 200 installations in the United States). If it were possible to remove
metals and phosphate from sludge prior to incineration, the resulting ash
would be innocuous with a low metal content, the ash volumes would be reduced,
and the calorific value of the sludge feed to the incinerator would be en-
hanced. If the metals and phosphates could be recovered in usable form, their
value might help to offset the cost of removal. An anaerobically digested
sludge might profit the most from such treatment, as the metals and phosphates
(and ash content) are highest in these sludges because of the reduction in
organic mass during digestion.
The present work was undertaken to extend the initial preliminary work of
Scott and Horlings (14)(15) who found that brief extraction of the sludge at
20°C with acid in modest excess would successfully solubilize from 80%-100% of
most cations as well as the phosphate ion. Further, a rough separation of the
metals could be made by selective precipitation.
A follow-up study was carried out by Oliver and Carey (10) who examined
a number of sludges for their acid extractability using a titration technique
with sulfuric acid. They found that little metal extraction occurred after
reaching a pH of 1.5, and that extractability of a given metal varied widely
depending on the nature of the sludge. In general, they report somewhat
lower extraction yields than do Scott and Horlings for similar sludges. Most
of the sludges tested by Oliver and Carey had been anaerobically digested,
but only five were chemical sludges. In this latter group, generally 70%-
100% of the nickel, cadmium, iron, aluminum and phosphorus could be extracted
at a pH of 1.5 under their conditions. Copper and lead did not extract from
anaerobic sludges, and the behavior of chromium and zinc was unpredictable,
although generally better than 50% up to 90% extraction occurred. The failure
of copper to extract was assumed by Oliver and Carey to be due to organic
complexing, whereas Scott and Horlings assumed the copper to be in a univalent
state. Oliver and Carey also attempted to extract sludges with ammonia and
ammonium salt mixtures, but found that soluble metal yields were low.
Electrochemical plating of zinc, copper, cadmium and nickel was also
attempted from sludge and from clear acid extract solutions by Oliver and
Carey. The latter worked well, but attempts to plate from sludge did not
give useful yields. Cost estimates for the extraction and recovery process
were done only for reagent costs, and processing costs were not worked out
They concluded that incinerator ash might be
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a better starting material than sludge buf did not evaluate the economics here
either, except to conclude that they did not appear to be favorable. On the
other hand, the study done by Cambrian Process Ltd. (9) concludes that the
major metals, iron and aluminum, as well as most of the minor ones such as
zinc and chromium are so refractory in incinerator ash that a useful degree of
metal recovery can only be achieved by treatment before.incineration.
The economic estimates made by Oliver and Carey are not adequate to allow
any decision to be made concerning the costs of either the acid leaching of
sludge or the treatment of incinerator ash. Additional studies are now under-
way, or have been recently reported, concerning methods of recovery of metals
or phosphate from incinerator ash, and better economic estimates may soon be
available.
The objectives of the present study were to carry out evaluations of the
acid extraction process, as originally described by Scott and Horlings, for
chemical sludge from a plant employing anaerobic digestion, and for a second
sludge from a plant using aerobic (activated) sludge processes. In particular,
the effect of the acid extraction process on the quality of the sludge as an
incinerator feed was of interest. Data were also desired which would allow a
cost estimate to be made of the cost of acid extraction treatment, and an
estimate of the possible value of recovered chemicals.
Advantages of acid extraction of sludge prior to incineration result
from the reduction in ash mass and volume, the increased calorific value and
the absence of large amounts of metals in the ash. If the sludge is not
incinerated but is used for agricultural purposes, acid extraction would
yield a sludge from which most of the nickel, cadmium, chromium and zinc (all
known to be injurious to crops) had been removed. Although the phosphorous
content of the sludge would be reduced, the resulting extracted residual
solids would in fact probably show a better nitrogen to phosphorus ratio than
the original sludge, which is relatively top rich in phosphate.
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SECTION 2
CONCLUSIONS
This work has demonstrated the technical feasibility of the acid extrac-
tion, process for municipal chemical sludges of widely varying types. Depend-
ing on the nature of the sludge, it is equally feasible to recover chemicals
or metals from the extract solutions.
However, the cost of the extraction and recovery process is such that
the value of recovered chemicals or metals will only pay 20%-30% of estimated
annual total cost. The major cost factor is the expense of reagents for
recovery of chemicals, and cost reduction would be most readily achieved by
developing better methods for treating extract solutions.
For large plants the cost of removing metals and phosphates might be
warranted if fuel costs are high for incineration, or if ash disposal is a
major problem. In the present stage of development, the process does not
appear to be likely to show benefits commensurate with costs for medium size
or small treatment plants.
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SECTION 3
EECOMMENDATIONS
If a larger scale treatment plant can be identified which is having
problems with high costs for incineration, or difficulty in disposing of ash,
it would be worthwhile to assess the behavior of the sludge filter cake feed
to the incinerator in the acid extraction process. The process is most likely
to work well for anaerobically digested sludge, although it performs well on
a wide variety of sludge.
Some further experimentation might be warranted in assessing filterability
characteristics and means of improving these for acid treated sludges,
especially for aerobic sludge or primary sludge. Further work to reduce the
cost of separating iron or aluminum, and phosphate, from the extract solutions
might make the process nearly self-financing for larger plants.
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SECTION 4
MATERIALS
As the present project deals with the evaluation of an acid extraction
process for chemical sludges from municipal waste water treatment plants
(i.e., those sludges produced in plants in which a chemical precipitant has
been added, primarily for the removal of phosphate) typical materials were
sought for the initial evaluation. The anaerobically-digested sludge pro-
duced by the main treatment plant of the Metropolitan District of Toronto,
Canada at Ashbridge's Bay, Toronto, was one chosen for this purpose. A
second choice was the aerobic sludge, and mixed sludge incinerator feed,
produced by the municipal treatment plant of Warren, Michigan.
The Ashbridge's Bay plant is currently 180 million gallons/day treatment
capacity and is presently undergoing a substantial expansion. It incorporates
in the existing plant secondary activated sludge treatment, which since
December, 1976, has also included addition of ferric chloride as a phosphate
precipitant. Waste activated sludge is anaerobically digested, the underflow
from these digesters is treated with polymeric flocculants and then de-watered.
The de-watered sludge cake is incinerated in large multiple hearth furnaces.
On February 15th, 1977, samples were obtained from the Ashbridge's Bay
plant of the thickened underflow from the anaerobic digesters (Sludge A) and
of the de-watered sludge cake (Cake A) being fed to the incinerators. The
primary material for evaluation was the thickened underflow from the anaerobic
digesters, since the acid extraction process proposed in this project for
chemical sludges may alter their filtering characteristics as well as the
chemical composition, and hence the de-watered sludge presently produced might
not be a preferred starting material for acid extraction. On the other hand,
the sludge filter cake can be re-slurried to 10%-11% solids, and acid extrac-
tion, of this slurry yields a much richer extract and minimizes process equip-
ment volume.
It is interesting to note that the Ashbridge's Bay expansion will include
heat treating units using both the Porteous and the Zimpro methods, and hence
yet a third potential starting material for acid extraction studies may soon
be available.
All samples were stored at 0° - 1°C throughout the test period.
The sludge filter cake differs in composition from the digester under-
flow only in moisture content and by the addition of small amounts of a poly-
meric conditioner prior to de-watering.
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The second set of samples was obtained from the municipal wastewater
treatment plant of Warren, Michigan, a northern suburb of Detroit. This
plant is operated as two 18 mgpd plants, for a total capacity of 36 mgpd.
The plant serves a population of 180,000, and the area is the home of a number
of industries (General Motors Technical Center, Detroit Tank Arsenal, and
factories producing steel, electrical equipment and tools and dies). The
waste water treatment facility consists of mechanical screening, grit removal,
primary settling yielding a primary sludge, aeration, final settling, sand
bed filtration and chlorination. Phosphate reduction is accomplished by alum
addition prior to final settling at an average dosage of 71 ppm. Waste
activated sludge is concentrated to about 4% solids, together with backwashings
from the sand filters used for effluent final polishing, in a Komline-Sanderson
pressure flotation unit. Samples of this thickened sludge were one of the
primary materials studied in this work (Sludge B). The waste activated sludge
solids from the flotation step are mixed with primary sludge, a cationic floc-
ulant is added and the mixed sludge is dewatered by vacuum filtration, and
thenincinerated in a standard multiple hearth incinerator. Samples of the
incinerator feed filter cake (Cake B) were the second material used for acid
extraction tests. In addition, samples of incinerator ash were taken.
Sampling was done at 12:00 noon on December 13, 1977. All samples were
returned immediately to the laboratory and stored at 2°C. Experience has
shown that chemical behaviour is unchanged at this temperature. However, the
sludge ages rapidly with respect to the floes formed by the polymeric coagu-
lants, and so filtering behaviour deteriorates rapidly to that of an untreated
(-by polymeric filter aids) sludge.
In this plant, it is likely that the mixed sludge feed to the filters,
or the filter cake would be a suitable raw material for an acid extraction
step.
Thickened sludges and filter cakes were sampled by taking lots of several
gallons from process streams. In a micro sense, these materials were not
particularly homogeneous, and great care was found to be necessary in taking
small samples for laboratory analysis from these larger samples. Intense
agitation before sampling and duplicate analyses on different samples were
found to be necessary to obtain consistent results. (repeated values with
+ 2%).
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SECTION 5
EXPERIMENTAL PROCEDURES
Moisture Content and Elemental Analysis
For the, basic characterization of the sludge, Method I proposed by Van
Loon (16) was adopted. For moisture determinations, this requires drying of
the sludge at 110°C for 16 hours. Digestion of the sludge was done using
nitric acid - hydrochloric acid mixtures as recommended by Van Loon (16).
All metallic elemental analyses were done by atomic absorption spectroscopy
using a Parkin Elmer Model 303 spectrophotometer. For most determinations
the nitrous oxide-acetylene flame was found to give better sensitivity than
the acetylene-air flame.
Phosphorous was determined as orthophosphate using the method outlined by
Murphy and Riley (17). In this procedure ammonium molybdate and potassium
antimonyl tartrate are reacted in the presence of sulfuric acid with dilute
solutions of orthophosphate to produce the phosphomolybdate ion P04*12 MO033"~.
This complex ion is then reduced by the addition of ascorbic acid to produce
the intensely colored "molybdenum blue" which reached maximum color intensity
within ten minutes. The optical density of the samples was then read at a
wave length of 882 m in a calibrated spectrophotometer. Details of reagents
and procedures can be found in the original paper of Murphy and Riley.
Dissolved Solids, Volatile Solids and Ash Contents
Dissolved solids in extract solutions or in sludge were determined by
evaporating a known volume to dryness at 103°C for a minimum time of nine
hours.
Volatile solids were determined by firing a known weight of dry solids in
a muffle furnace at 600°C for 20 minutes. The residue was allowed to air cool,
and then desiccated and weighted.
Ash content was found by firing a known weight of dry dissolved solids
or dried sludge solids in a muffle furnace at 800*C for two hours, cooling,
desiccating and weighing.
Total Organic Carbon
These values were found in the usual way by the difference between total
carbon values and inorganic carbon values.
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Total carbon and inorganic carbon were measured using a Beckman Model
915 Total Organic Carbon Analyser. Total organic carbon was found by the
difference between two successive identical samples, total carbon being
measured for one and inorganic carbon for the other.
Chemical Oxygen Demand
The standard method of the Water Pollution Control Federation, 13th
edition (18) was used for this determination.
Calorific Values
Calorific values of sludges, filter cakes or residues from extraction
were determined in 'a standard Parr Oxygen Bomb Calorimeter.
Filterability
Filterability was measured by both the Buchner funnel filtration test as
described in Manual 20 of the Water Pollution Control Federation, (19) and by
the filter leaf test. The latter employed a Dorr-Oliver Test Leaf filter,
11 cm. diameter with a No. 114 fast filtering Whatman filter paper as filter-
ing medium. The test was done.with a vacuum of 15 inches of mercury, sub-
mergence time of one minute and a drying time of two minutes.
Ash Softening Point and Fusion Point
No standard procedure is available for the measurement of softening or
fusion points of ash from sewage sludge. Therefore, the ASTM procedure out-
lined for ash from coal or coke was adopted (20), using cones in the shape of
triangular pyramids 1/4" on the base and 3/4" high, prepared in a special
brass mold according to the ASTM specification. Heating was done in a
temperature-controlled muffle furnace with a mildly reducing atmosphere with
a heating rate of between 5° - 10°C per minute.
Acid Extraction of Sludge
Sludge extraction was performed using sulfuric acid and procedures found
by Scott and Horlings (2) to be nearly optimal. For the digester sludges,to
0.5 kg. of sludge "as-is", concentrated (97%) sulfuric acid was added at room
temperature slowly until a desired final pH value had been reached (final pH
values of either 1.5 or 0.8 were used as standard in these tests). Reaction
was allowed to proceed for 20 minutes with stirring, and the extract mixture
was then brought to a boil and boiled for five minutes. After cooling to
65°C, the mixture was vacuum filtered, the residue washed several times, and
filtrate and washing combined diluted to 1000 ml. It was this resulting
extract solution which was used for all analyses. In some tests with the
aerobic sludge, for which adequate washing was difficult, the undiluted extract
was analysed also.
Extraction of the de-watered sludge cake sample was similar except that
0.4 kg. of "as-is" cake were mixed before acidification with 385 mis. of
water to give a slurry of suitable consistency. The filter cake contained
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a polymeric conditioner which was not present in the digester underflow
samples. It was found that about 11% solids was the maximum concentration for
which slurry viscosity was still acceptable.
Precipitation of Metals from Extracts
To a portion of clear acid extract solution measured volumes of either
standard sodium hydroxide solution or known weights of calcium hydroxide, were
added slowly with agitation until a desired pH value was reached. Stirring
was continued for a further 10 minutes while adjusting the pH until a reason-
ably constant value was obtained. The solution was then filtered and the
filtrate analysed and the percentage precipitation of the metal calculated.
10
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SECTION 6
RESULTS AND DISCUSSION
Characterization of Sludge Samples
The original sludge sample was characterized to give basic information
by which to judge the effects of acid extraction. Results are shown in
Table I.
TABLE I. CHARACTERISTICS OF SLUDGE SAMPLES
Property
% Dry Solids
% Ash
PH
% Volatile Solids
Sludge A
Digester
Underflow
3.16
47.62
7.9
54
Cake A
De-watered
Sludge Cake
14.51
46.40
7.6
53
Sludge B
Flotation
Waste
Act. Sludge
4.09
34.9
6.7
57.2
Cake B
Mixed
Sludge
13.4
32.5
6.7
61.3
Elemental analysis was carried out on three of the above samples (Sludge
A, Sludge B and Cake B) after digestion in an HCI-HNOQ mixture, and the
results are given in Table 2. Cake B was not analysed since it had essentially
the same composition as Sludge A. The incinerator ash from the Warren, Mich.
plant was also analysed and the results are also given in Table 2 for informa-
tion4although no further work was done with this ash.
11
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TABLE 2. ANALYSIS OF ORIGINAL SAMPLES
Element
Sludge A Sludge B Cake B
gins/kg, dry solids
Incinerator Ash
Ca
P
Fe
Cu
Pb
Ni
Zn
Al
Cr
Mh
Mg
40.1
32.8
52.4
1.10
2.04
.082
3.33
9.27
3.30
0.53
5.20
4.42
39.8
31.0
0.83
0.42
1.28
5.9
35.9
3.79
0.95
4.24
5.77
32.5
27.7
0.81
0.43
1.28
5.9
29.0
3.49
0.89
3.49
12.9
88.7
61.5
1.80
Nil
2.38
8.0
74.0
6.07
1.59
8.24
The results for metals and phosphate content are within normal ranges
for most elements. Calcium is high in Sludge A, indicating some lime addition
for pH control. The high iron in Sludge A and the high aluminum in Sludge B
are the direct result of ferric chloride and alum additions, respectively,
for phosphorus precipitation. High values of lead in Sludge A and of nickel
in Sludge B and Cake B are indicators of specific industrial activities in
the area served by the plants. For these amounts of lead arid nickel, the
level would be of concern if the sludge were used on agricultural land. The
incinerator ash is correspondingly rich in nickel. Lead may or may not appear
in incinerator ash depending on the incineration temperature since much of it
is readily volatilized.
Extraction Tests - Anaerobic Sludge A and Filter Cake A
Three extraction tests Were performed, two on digester underflow, each
at different acid dosages, and one on the reslurried sludge filter cake.
Experimental conditions are summarized in Table 3.
12
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TABLE 3. EXTRACTION CONDITIONS
Test No. Final
pH
1 1.5
2 0.8
Grams Sample
Used
Digester
Underflow
500 grams
Digester
Underflow
500 grams
mis . cone .
H2S04 (97%)
4.3
9.5
Acid/ Sludge Solids
Weight Ratio
0.483
1.068
1.5 Sludge Filter
Cake
400 grams
10.4
0.318
Extract solutions were analysed for important metallic elements.
results are shown in Table 4.
TABLE 4. ANALYSIS OF EXTRACT SOLUTIONS
These
Element
P
Fe
Cu
Al
Pb
Cr
Zn
Mn
Extract 1 Extract 2 Extract 3
Amount Extracted Amount Extracted Amount Extracted
gms/kg solids % gms/kg solids % gms/kg solids %
32.9
51.6
0
6.81
0.66
2.53
2.97
0.54
100
98.6
0
73.5
32.3
76.6
89.2
100
32.0
54.2
.03
7.43
0.68
3.23
3.23
0.55
97.5
100
2.7
80.2
33.3
97.9
97.0
100
25.6
50.8
0
5.45
0.19
1.83
2.33
0.54
78.0
96.9
0
58.9
9.3
55.5
70.0
100
From these results, the degree of extraction of metals and phosphates is
generally linked, up to a point, with the amount of acid used per unit weight
of sludge solids. Phosphate, aluminum, chromium and zinc all show improved
extractions at higher acid usage levels. Iron and magnanese are fully
13
-------
extracted at all levels. All the iron is in the ferrous state, and hence all
the copper is in the cuprous form, which explains the negligible extraction
of copper. The lead forms the sparingly soluble sulfate, and the degree of
extraction of this element therefore depends on the concentration of the PbSO/
in the solution. Since Extract 3 was much more concentrated than the other
two (almost four times the sludge solids concentration) the reduced lead
extraction for this case is probably only a reflection of the limited solu-
bility of lead sulfate. The generally poorer extraction from the sludge filter
cake is due primarily to the inadequate acid addition (only about two thirds
of that required for good extraction). Because of the very small nickel
content, it is not reported. Similarly, calcium is not reported because it
forms the slightly soluble calcium sulfate.
Total dissolved solids, volatile siilids and ash contents were determined
for the extract solutions, with results as shown in Table 5.
TABLE 5. SOLIDS CONTENT OF FILTERED EXTRACTS
Extract 1
Extract 2
Extract 3
Dissolved Solids
kg/m3
kg/kg orig. sludge solids
Volatile Solids
kg/m3
kg/kg orig. sludge solids
Ash
kg/kg dissolved solids
kg/kg orig. sludge solids
11.67
.775
6.05
.402
.309
.240
24.13
1.603
6.27
.417
.156
.250
27.13
.467
11.62
.200
.381
.178
The dissolved solids from extract 2 contained a considerable amount of
unreacted sulfuric acid, and hence this value was difficult to obtain
accurately and is much higher than that for Extract 1. This suggests that
the correct amount'of acid needed for reasonably complete extraction of those
elements readily soluble in sulfuric acid is just a little more than that used
to prepare Extract 1, that is, of the order of 0.5 kg. of acid per kg. of
sludge solids. The ash content based on original sludge solids is abnormally
low for Extract 3 because of the incomplete extraction. As expected, the
inorganic salt content (as reflected by ash) of the extract increases as more
acid was used for the extraction, although the difference between Extract 1
and Extract 2 is small.
14
-------
The washed residual solids from the extraction tests were also character-
ized with respect to fraction of the original sludge solids as insoluble
residue, ash content of residual solids, calorific value of dry residual
solids and moisture content of the cake as obtained from a standard filter
leaf test. Results are shown in Table 6, together with the values for the
original sludges.
TABLE 6. EXTRACTION RESIDUES
Sample % Residual* % Ash % Moisture Calorific % %
Solids in in Sample Value Volatile Ash
Residue or joules/kg Solids Removed
Filter Cake (kcal/kg) in
Dry Residue
Residual
Solids
Sludge
Filter Cake
.
Digester ___—
Underflow
Extract 1 59,5
Residue
Extract 2 51.1
Residue
Extract 3 76.8
Residue
46.40
A 7 fi9
H- / « DZ
30.50
27.40
39.04
85.49
QA RA
yo * OH
60.62
61.88 ,
65.6
12.32 x 106
(2942)
15.88 x 106 67.7
(3792)
17.40 x 106 70.3
(4155)
14.48 x 106 59.5
(3458)
61.9
70.6
37.0
^Because of the addition of sulfate ion, the sum of dissolved solids
in the extract plus residual, solids is greater than 100% of original
sludge solids.
All of the extraction residues have been substantially reduced in ash
content by the extraction, more or less in proportion to the amount of acid
used, up to a weight ratio of acid/sludge solids of about 0.6. A cqrrespond-
ing increase in calorific value occurs, with the best case, Extract Residue 2,
showing an increase of 41.2%. The recommended acid usage as shown by Extract
Residue 1 gives an increase of 28.9% in calorific value, and the incomplete
extraction in Residue 3 shows an increase of 17.5%. The filter cakes formed,
which were of much lower moisture content than the sludge filter cake present-
ly used as incinerator feed, showed improved filtration rates, but did not
wash well.
Total organic carbon analyses and COD analyses were carried out on the
extracts and extract residues. Results are shown in Table 7.
15
-------
TABLE 7.' TOG AND COD ANALYSES
Sample
TOG COD
gm/kg original sludge solids
Extract 1
Extract 2
Extract 3
Extract Residue 1
Extract Residue 2
Extract Residue 3
Digester Underflow
Sludge Filter Cake
22.5
32.0
172.7
138.4
214.3
62.8
81.7
79.5
631.9
600.4
731.8
729.9
812.5
The total organic carbon and COD both show somewhat less organic carbon
reporting in the extract and residues than appeared to be present in the
original sludge solids. It is possible that the values for TOG or COD are
somewhat high for the original sludge because of thepresence of carbonate and
sulfide ions in this anaerobic product-, and both of these species are destroyed
by the acid extraction step. However, the most important feature of these
results is the conclusion that the extraction with acid removes only about
10% of the total organic carbon or COD and the remaining 90% is in the resi-
due. Since much of this "soluble" organic carbon is probably associated with
the sludge liquid-all soluble solids in the sludge liquid become part of the
extract - then it can be concluded that the extraction with sulfuric acid is
quite highly specific for the inorganic constituents of the sludge.
Filterability of Sludges and Acidified Sludges
Original digester underflow and re-slurried sludge filter cakes were both
tested with respect to filterability the same day as sampling was done, in
order to minimize aging effects in storage. After acid extraction, in each
of the three tests done, the extract solution was subjected also to filter-
ability tests. Both Buchner funnel and filter leaf tests were done^as ex-
plained in Section 5. Filter leaf performance is expressed as kg/m hr.
based on a 3 minute dewatering + washing cycle and 20 cycles/hour. The
specific cake resistance as measured by the Buchner funnel test is given as
m/kg, calculated from the formula
2 b P A2
cy
16
-------
where b - slope of a glot of 6/V vs V where 9 is the time and V the filtrate
volume, s/m"
2
P - differential filtration pressure, N/m
2
A - filtering area, m
c - solids concentration in the slurry, kg/m
2
y - viscosity of the filtrate, N s/m
Results of filtration tests are shown in Table 8.
TABLE 8. FILTRATION TESTS
Sample
Digester Underflow
Filter Cake
(Re-slurried)
Extraction 1 Slurry
Extraction 2 Slurry
Extraction 3 Slurry
Temp.
°C
15
30
21
21
65
65
65
Filter Cake
Resistance, r
m/kg
6.62 x 1013
9.45 x 1013
1.29 x lo}3
1.20 x 101J
2.10 x 1013
2.10 x 1013
0.92 x 1013
Filter Leaf
Yield
kg/m~ hr
0.859
0.983
0.497
0.497
1.242
0.695
6.41
Cake
Moisture
%
60.6
61.9
65.7
It is apparent from these results that there is only a rough correlation
between the Buchner funnel results and the filter leaf results. The filter
leaf results for extractions 1 and 2, in which digester underflow sludge was
used, suggest only a little improvement in filterability following the acid
extraction and heating. However, in every case when measured as filter cake
resistance on the Buchner funnel, the acid treatment gave a somewhat improved
filterability. This is in agreement with results published previously by
Scott and Horlings (15), and found to be generally true for anaerobically
digested chemical sludges, whether iron or alum has been added as a phosphorus
precipitant (e.g., (21)(22)).
The more concentrated slurry used in extraction 3 (reslurried sludge
filter cake with about four times the solids content of extractions 1 and 2)
suggests a marked improvement in filterability might occur in these cases.
However, the moisture contents of the cakes formed on the filter test leaf are
much lower than that of the sludge filter cake from the plant sample.
17
-------
The specific cake resistance values for the digester underflow without
the addition of polymeric coagulants are well within the range quoted by.
Metcalf and Eddy (23) for anaerobically digested sludges.
Precipitation from Extract Solutions
According to the results reported by Scott and Horlings (15), when the
iron exists in solution largely in the ferrous form, a partial separation
between iron and heavy metals can be made by bringing the pH to the range of
4.0-5.0. The heavy metal precipitate (largely metal hydroxides and phosphates)
can then be filtered off with the possibility of recovering and recycling
soluble iron from the extract.
As recommended by Scott and Horlings, lime was added to each of the
three extract solutions until the pH was 4.5. The precipitate was then
filtered off and the filtrate analysed. Lime additions were 2.54 kg/m Of
extract, 12.6 kg/m3 and 5.4 kg/m3 for Extracts 1, 2 and 3 respectively.
Results are shown in Table 9.
TABLE 9. PRECIPITATION TEST AT pH 4.5
Element
P
Fe
Zn
Cr
Al
Pb
Extract 1
% Removal
49
22.8
31.8
75.8
94.9
100
Extract 2
% Removal
47.5
14.1
29.1
52.3
97.5
100
Extract 3
% Removal
45.9
18
51.1
77.1
96.4
100
It can be seen from the tabulated results that about half the phosphate
was removed and much of the chromium, aluminum and lead, with the zinc having
only a minority removed. Theseresults are in agreement with those reported
by Scott and Horlings (15). Filterability of precipitates was poor for
Extracts 1 and 3, but good for Extract 2, presumably because of the additional
calcium sulfate formation which acted, as a filter aid. Results are reasonably
comparable among the various extracts, although Extract 3 is nearly four times
as concentrated as the other two. (Extracts used in this work contained from
1-5 kg/m3 of metallic cations, and had been diluted by a factor of two from
their maximum feasible strength).
The filtrate obtained from this treatment with lime in a plant in which
extracts were not unreasonably diluted would contain about 1.2-1.5 kg/m of
ferrous iron, about 0.48 kg/m3 of phosphorous, 0.064 kg/m3 of zinc and small
18
-------
amounts of other metals. The filtrate could be brought to a higher pH
(approximately 7.0) which would then cause a precipitate to form consisting
almost entirely of ferrous phosphate.
Some work was also carried out as part of a Masters program (21) on an
anaerobically digested chemical sludge from a small treatment plant (Guelph,
Ontario, Canada) to which alum had been added as a phosphate precipitant.
Ah extract solution was made by the same procedure as used here (extraction
at pH 1.5) and then this extract solution was subjected to fractional precipi-
tation by the -addition of sodium hydroxide. Figures 1 and 2 show the change
in extract composition as a function of pH and the percentage precipitation
of the various elements. In this solution, larger than usual amounts of
magnesium and aluminum occurred (9.8 gms/kg of dry sludge solids for Mg and
34.1 gms/kg of dry sludge solids for Al). The behaviour of the phosphate,
aluminum and magnesium shown is very interesting, and suggests that magnesium
and aluminum ions can be used as specific agents for the removal of phosphates
in the presence of other metals,particularly ferrous iron,by adjusting the pH
to no higher than 3.0 with adequate amounts of Mg and Al present.
Extraction Tests - Waste Activated Sludge B and Mixed Sludge Filter Cake B
Five extractions tests were performed, three on the waste activated
flotation sludge and two on the reslurried filter cake from the mixed aerobic
and primary sludge. Experimental conditions are summarized in Table 10.
TABLE 10. EXTRACTION CONDITIONS
Test No. Final pH Grains sample mis cone. Remarks kg. acid/
used H2S04 (97%) kS- dry sludge
solids
IB 1.5 flotation 4 no wash-water .360
sludge 500
gms
2A 0.8 flotation 15 no wash-water 1.348
sludge 500
gms
3A 0.8 sludge filter 12 no wash-water .797
cake 200 gms '•
5 1.5 flotation 4 : wash water .360
sludge 500 i added
gms
6 .1.5 sludge filter 4 wash water .365
cake 150 gms added
19
-------
2500
2000
Q
LU
— 1500
O
LL)
OC
O.
£
a. 1000
Q.
500
250
Figure 1. Amount of metals precipitated vs pH.
20
-------
-START
1-0 r-
-Ca
0
Figure 2. Residual fraction of metals in the extract as a function of pH.
21
-------
• WARREN, MICHIGAN, FLOTATION SLUDGE
0 ASHBRIDGES BAY, TORONTO,
DIGESTER UNDERFLOW
8
10
12
14
16
mis 97 % H2S04 ADDED
to 500 gms OF SLUDGE
Figure 3. Acid requirement of sludge to attain a given pH value.
The variation of the final pH of the extract slurry of sludge and acid is
shown in Figure 3, and the amount of acid used was selected from this chart
to give the desired final pH. The curve also describes well the acid require-
ment of the anaerobic digester underflow from the Toronto plant, although the
Warren flotation waste activated sludge was about 4.0% solids and the Toronto
anaerobic sludge was only 3.2% solids, but the latter had a significantly
higher metals content (25% more). -In fact, the acid usage can be made compar-
able among different sludges by proportioning it to the sum of the iron,
aluminum, calcium, magnesium and zinc contents, per unit of dry sludge solids,
these elements accounting for the great majority of cationic equivalents in
the sludge which consume acid. Hence, the two sludges shown in Figure 3 fall
on the same line because they happen, fortuitously, to contain the same amount
of these cations per unit volume of total sludge. Figure 3 also shows the
buffering effect of the sludge solids at lower pH values. This effect makes
22
-------
it difficult to precisely predict acid requirements for lower pH, and is
mainly responsible for the different amounts of acid per unit of solids appar-
ently needed to achieve a pH of 0.8 for the' flotation sludge and the mixed
sludge filter cake.
In Table 10 the comment "no wash-water" means that the extract was
analyzed and the percent solubilization of a species calculated based on the
known total liquid volume. The phrase "wash-water added" means that the
extracted residue was washed, and the percent extraction based on the 'amount
of the particular species found in the filtrate plus washings. Since the
residual solids washed poorly, this method gives a lower extraction yield.
Extraction yields for the five tests are given in Table 11, and the lower
extraction yields shown in tests 5 and 6 compared to test IB, for example, are
indicative of the difficulty in washing residual solids. Apparently, up to
25% of the solubilized material may be retained in the residual solids unless
they are adequately washed.
TABLE 11. PERCENT EXTRACTION OF METALLIC ELEMENTS
Element
P
Fe
Al
Zn
Mg
Ca
Ni
Cu
Cr
Mn
Pb
IB
78
81
96.8
100
100
39
100
94.5
100
100
0
2A
92
98
100
100
100
74
100
100
98.8
100
0
Extract No.
3A
83.0
99.6
93.9
97.9
96.3
66
100
73.1
91.9
100
0
5
66.5
72.6
76.0
67.5
78.5
20.8
78.1
7.7
60.9
100
0
6
70.5
74.7
74.1
64.2
78.8
25.6
75.8
0.7
66.7
90.9
0
The above results indicate that adequate extraction, about 80%, of iron
and phosphorus can be achieved at an acid usage of 0.4 kg/kg, original sludge
solids, that is, at a final pH of 1.5 in the extract. At this level of acid
usage, essentially complete extraction of all minor metals having soluble
sulfates is achieved. This is an important result, for it shows that a
23
-------
minimum acid usage will completely solubilize injurious heavy metalsjwith the
exception of lead,in both the waste activated sludge and the mixed sludge.
For this sludge, in particular, it is encouraging to note the high level of
extraction of zinc, nickel and chromium. Copper appears to be soluble in
acidic solution but re-precipitates at higher pH, that is, during washing with
neutral water (compare tests IB and 5).
In addition to the degree of extraction, the total dissolved solids, the
fraction of volatile solids and the ash content of the dissolved solids from
the washed residue solids (tests 5 and 6) were determined and are given in
Table 12.
TABLE 12. SOLIDS CONTENT OF FILTERED EXTRACTS
Extract 5
Extract 6
Dissolved Solids
kg/kg orig. sludge solids
Volatile Solids
kg/kg orig. sludge solids
kg/kg extract solids
Ash
kg/kg orig. sludge solids
kg/kg extract solids
0.560
0.24
0.429
0.251
0.448
0.519
0.261
0.503
0.196
0.378
The washed residual solids from extraction tests 5 and 6 were also
characterized with respect to total dry solids, volatile solids fraction and
ash content and these values are given in Table 13. Calorific values are
also given in Table 13, expressed per unit weight of dry solids for the solid
specified.
24
-------
TABLE 13. EXTRACTION BESIDUES
Sample % residual
solids
Flotation.
sludge
De-watered —
filter cake
Extract 5 73.1
Residue
Extract 6 70.2
Residue
% volatile
solids in
residual
solids
57.24
61.28
62.20
73.79
% ash
in
residual
solids
34.89
32.5
21.00
23.15
Calorific value
joules /kg
(kcal/kg)dry residual
solids
18.03 x 106
(4306)
21.08 x 106
(5033)
23.51 x 106
(5614)
22.26 x 106
(5315)
% ash
removal
56.0
50.0
COD analyses were carried out on extracts and original sludge samples.
Results are shown below in Table 14.
TABLE 14. COD ANALYSIS
Sample
COD (gms/kg dry orig. sludge solids)
Extract 5
Extract 6
Flotation sludge
De-watered filter cake
157
138
970
1157
The larger COD value of the de-watered sludge compared to that of the
flotation sludge can partly be accounted for by the COD of the polymeric
floculant added prior to filtration.
A solids material balance based on results in Tables V and VI cannot
really be carried out because of the loss of some volatile material, for
example, carbon dioxide, on acidification. In general, however, the sum of
(original sludge solids + sulfate added) is only 2% - 5% higher than the sum
of (dissolved extracted solids + residual solids).
Results in Tables 11, 12 and 13 show clearly the selective nature of the
extraction, that is, organics remain largely with the residue while the
.25
-------
extract solids have a high proportion of inorganic material. The higher vola-
tile solids content and calorific value of the filter cake is due to the
content of primary sludge in this material. The smaller increase in calorific
value of the filter cake after extraction appears to be due to the less
effective extraction of inorganics in Test #6. The degree of extraction
of inorganics could be significantly improved with a
resulting decrease in ash and increase in calorific value with adequate wash-
ing of the residual solids.
Even with the poor washing in these tests, the results show that the
residual solids would yield only 44% (for the waste flotation sludge) to 50%
(for the filter cake) of the ash weight after incineration that would result
from the present sludge feed. Calorific values of the sludge solids would
increase from 30% (flotation sludge) to 6% (filter cake). The COD appears to
remain to a very large degree in the residual solids.
Hence, the beneficial results of acid extraction are of the same degree
for these aerobic and mixed sludges treated with alum as was found for thicken-
ed anaerobic sludge treated with ferric chloride.
Filtration Behaviour
Filtration tests using the Buchner funnel technique were carried out on
the waste activated sludge and on re-slurried filter cake. In the plant,
polymeric additives are used to attain adequate filtering or settling rates.
The beneficial effects of polymer additives are transitory, and hence by the
time samples were tested in the laboratory, aging had occurred to the point
where filtration results were not comparable to operating plant values.
Hence, laboratory numbers have no absolute significance. Filtration tests did
show clearly, however, that for the aerobic waste activated sludge, or for
the mixed sludge, acid treatment had no significant effect on filterability,
neither increasing nor decreasing filtration rates. Tests on fresh plant
samples would be necessary to determine if the lowered pH after acid treatment
significantly affected the effectiveness of polymeric floculants. Because
it would be preferable in plant practice, although probably not essential, to
have reasonably good washing of extract residues after acid treatment, this
aspect of filterability and washing effectiveness would need to be further
investigated.
Filter cake resistance, r, as measured by the Buchner funnel test was
3.46 x lO-^ m/kg, well within the usual range of values for aerobic or mixed
sludges as given by Metcalf and Eddy (23).
Ash Fusion Points
Harkness et al (24) present data for both the softening point and the
fusion point of ash from incinerated digested sludges, some of which contained
iron as a phosphate precipitant, and some aluminum (added as aluminum chloro-
hydrate). Wide variations in iron content were found to cause essentially no
change in fusion temperature and little in softening point. Similarly,
aluminum content up to 26% in the ash tended to raise the softening point and
the fusion point very slighly in oxidizing conditions. In reducing conditions,
,26
-------
high aluminum content appeared to decrease the softening and fusion points
noticeably.
Some tests were carried out in earlier work (21) on an anaerobically
digested sludge which contained a fairly high iron content (Iron had been
used as a phosphate precipitant). Aluminum content was low. Ash softening
points and fusion points were measured in a mildly reducing atmosphere for
the ash produced by the residual acid-extracted solids as well as for the ash
from the unextracted sludge. In addition, an unextracted filter cake which
had been heavily conditioned with lime and ferric chloride prior to de-watering
was ashed, and softening and fusion points determined. Results are given
below.
TABLE 15. ASH FUSION,POINTS
Sample
Softening Point °C
Fusion Point °C
Extracted Residual Solids
Unextracted Sludge Solids
Conditioned Filter Cake
1170
1240
1160
1200
1280
1210
Experience of others suggests that limited amounts of some metals in the
ash may be beneficial in raising the softening point, but excessive lime or
iron can be detrimental, that is, they can reverse the gain obtained by small
contents. However, the results in Table 15 indicate, in agreement with the
results of Harkness et al, that wide ranges of variation in iron or aluminum
contents in the ash do not lead to any but minor changes in softening or
fusion points.
27
-------
SECTION 7
ESTIMATES OF COST
Basis of Cost Estimate
Two preliminary cost estimates were carried out, one for each of the
types of sludge studied in this work. The iron-rich anaerobic sludge was
assumed to be available as the filter cake, and the amount was taken to be
equal to that produced by the Ashbridge's Bay plant of the city of Toronto
(180 mgpd input). Hence, this case represents a large scale of operations.
The second estimate was made for the filter cake pro'duced from the mixed
sludge at the Warren, Michigan, plant (30 mgpd). Hence, this second case
represents a medium size plant producing waste activated plus primary sludge
without anaerobic digestion and rich in aluminum.
Filter cake was selected as a starting material since it is readily
available, and the more concentrated solutions possible when starting with
filter cake minimizes the process plant size for acid extraction. In both
plants, only polymeric flocculants were added to the sludge to aid in de-
watering, and hence sludges and filter cakes have essentially the same
compositions.
The process envisaged consists of the units listed below for each case,
together with important operating parameters. All process units are of plastic
or lined steel construction.
Large Plant
180 mgpd (Imperial), iron rich anaerobically digested sludge.
Basis - 1200 Ibs. of sludge for 10 Ibs. of influent
Filter Cake slurried to 10% solids
Acids usage 0.48 kgs/kg. dry sludge solids
Plant operates 330 days per year, 24 hours per day
Sludge solids produced - 32400 tonnes/year
98181 kg/day
Acid (100% H SO ) - 15550 tonnes/year
—— 48576 kg/day
28
-------
1. Extraction Tank - residence time 20 minutes, 20°C
Volume 4000 U.S. gals..
2. Hot Tank - residence time 5 minutes, 95°C
Volume 1000 U.S. gals.
3. Heat Exchangers, one exchanging effluent from hot tank with
influent 68 m area, second steam heated with low pressure
steam 23 m area.
4. Filter for extract slurry, rotary vacuum, 65°C
58420 kgs.2solids/day = 2434 kgs solids/hr.
Area 200 m
Wash water 29000 kgs/hr.
Cake to incineration
Extract to precipitation tank
5. Precipitation tank and,agitator 3000 gals.
Lime added 5 kg/m (pH 4.0)
Kg** added
2
6. Rotary vacuum filter, 20 m
Cake to phosphate recovery
Filtrate to phosphate removal section of treatment plant
Metals in Extract Solution before Precipitation (estimated from Table 4)
kg/day tonnes/yr.
P 3063 1011
Fe 4880 1610
Al 638 211
Mg 511 168
Zn 275 : 91
Cr 236 : 78
Pb 65 21
29
-------
Metals in recycle to treatment plant*
kg/day tonnes/yr.
Fe 4291 1416
Zn 206 68
Cr 118 39
Al
153
32
51
11
* Fractional precipitation estimated from Table 9^ and Figure 5 of reference
(15) .
Total fixed capital investment required with process equipment costs
from Woods (25) adjusted to 1979 and using Lang factors for a solids-fluid
plant is $1,220,000.
Annual cost of production with 1^304 at $50/tonne, Ca(OH)2 at $40/tonne,
10% depreciation per year = $2,460,000/yr = $41.41 per million gallons.
Credit for Fe produced = $350/tonne of Fe = $496,000/yr = $8.35/million
gallons. No operating costs have been included.
Net cost of treatment is $33.06 per million gallons.
Depending on the treatment of the precipitate cake, the credit for the
phosphate produced will just about pay for the cost of further processing.
Hence, the value of the phosphate and metals in the cake (Al, Mg, Pb, Zn,
Fa"1"*, Cr) is just about enough to allow further refining so they can be
marketed.
The above conclusion is based on a scheme whereby the cake is extracted
with ammonia to solubilize Zn, Cr, Ni, Cu. The extract is boiled to recover
ammonia and the metal'hydroxides and oxides precipitated,recoveredjand sold.
The residue containing P, Mg, Al, Pb, Fe1'' is treated with sodium hydroxide
solution to a pH of 12 which removes the Al and Pb. The resulting solid,
which is mainly magnesium or ferric phosphate, can then be marketed.
No credit has been taken for the decreased incinerator ash volume which
must be handled, or the increased .calorific value of the sludge.
Medium Plant
30 mgpd (U.S.), aluminum rich mixed sludge filter cake.
Basis - 1500 Ibs. of sludge per million U.S. gallons of influent
Filter cake slurried to 10% solids
Acid usage 0.36 kg/kg, dry sludge solids
Plant operates 330 days/year; 24 hours/day
30
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Process is identical to that proposed for the large plant, except
filtrate from precipitation tank goes to waste.
Estimated metals in extract solution before precipitation, assuming average
washing (from Table 1).
P
Fe
Al
Zn
Mg
Ni
Cu
Cr
Mn
Pb
Assumed % extraction
75
78
85
80
90
80
10
80
95
0
kg/day
497
442
503
97
64.2
20.9
1.5
57.0
17.3
0
tonnes/yr.
164
146
166
32
21.2
6.9
0.5
18.8
5.7
0
Assuming precipitation at pH 4.0, based on Figure 2, metals and phos-
phates in cake allowing for the fact that iron is in ferric form,
P
Fe
Al
Zn
Mg
Ni
Cr
kg/ day
473
442
503
48.5
64.2
20.9
57.0
tonnes /yr.
156
146
166
16
21.2
6.9
18.8
31
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To recover aluminum salts for recycle in the waste water treatment plant
the precipitated cake is treated with ammonia to solubilize and remove Zn,
Ni and Cr. These are then precipitated by boiling and can be sold as a mixed
metal oxide. The residual cake is then treated with sodium hydroxide solution
at pH 12 to solubilize the aluminum. The residue, containing Fe, Mg and
phosphate primarily, can be sold for its phosphate content.
Fixed Capital investment cost (using six-tenths rule) = $420,000
Cost of operations, including depreciation at 10% per year = $824,210 per year.
Credit for recycle of aluminum salt $160,000/year
Credit for phosphate and metals $162,000/year
Net cost of process - $50.70 per million gallons of influent.
Discussion
The capital costs for this process are quite reasonable, and fixed costs
are only a small part of the annual total costs. The major cost is for
chemical reagents - 64% of the total cost of production for the large plant
and 58% for the medium plant. Hence, less expensive techniques to separate
iron or aluminum and phosphate from the extract liquor would be most effective
in reducing costs. Solvent extraction has been suggested as one possibility.
In carrying out the cost estimates, it was assumed that adequate washing
of residual extracted solids was possible with reasonable amounts of wash
water. This can be achieved with anaerobic sludges, but the mixed aerobic-
primary sludge would need some conditioning prior to de-watering, for adequate
washing.
In general, costs for an acid extraction process might be within reason
for large plants if the benefits gained in the incineration process and the
reduced cost of ash disposal were deemed to be of comparable value. For
medium size plants, the process is unlikely to be economical unless some
inexpensive way of processing the extract solution is developed.
32
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REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
Smith, R. and Eilers, R.G., "Computer Evaluation of Sludge Handling and
Disposal Costs," Proc. 2nd Nat. Conf. on Municipal Sludge Management and
Disposal, Anaheim, Calif., August 18-20 (1975).
Bates, I.E., "Land Application of Sewage Sludge," Research Report No. 1,
Environment Canada, Ottawa, September (1972).
"Report of the Land Disposal Subcommittee - Projects Conducted 1971-78,"
Research Report No. 70, Environment Canada, Ottawa,, (1977).
Bates, I.E. and Beauchamp, E.G., "Land Disposal of Sewage Sludge, Vol.
V," Research Report No. 73, Environment Canada, Ottawa, (1978).
Cohen, D.B. and Bryant, D.N., "Chemical Sewage Sludge Disposal on Land
(Lysimeter Studies), Vol. II," Research Report No. 79, Environment
Canada, Ottawa, (1978).
Cohen, D.B. and Bryant, D.N., "Air-Dried Chemical Sewage Sludge Disposal
on Agricultural Land," Tech. Devel. Report EPS 4-WP-78-3, Environment
Canada, Ottawa, April (1978).
Webber, J., "Effects of Toxic Metals in Sewage on Crops," Wat. Poll.
Control, 71, 404-12, (1972).
Dowdy, R.H. and Larson, W.E., "The Availability of Sludge-Borne Metals
to Various Vegetable Crops," J. Environ. Qual. , 4^, 278, (1975).
Cambrian Processes Ltd., "Recycling of Incinerator Ash," Research Report
No. 18, Environment Canada, Ottawa, (1975).
Oliver, B.C. and Carey, J.H., "The Removal and Recovery of Metals from
Sludge and Sludge Incinerator Ash," Research Report No. 33, Environment
Canada, Ottawa, (1976).
Fowlie, P.J.A. and Stepko, W.E., "Sludge Incineration and Precipitant
Recovery, Vol II," Research Report No. 74, Environment Canada, Ottawa,
C1978).
Schroeder, W.H. and Cohen, D.B., "Sludge Incineration and Precipitant
Recovery, Vol. Ill," Research Report No. 75, Environment Canada, Ottawa
(1978).
33
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13. Gabler, R.C. and Neyland, D.L., "Extraction of Metals and Phosphorus
from Incinerated Municipal Sewage Sludge," 'Proc. 32nd Industrial Waste
Conf., May 1977, Ann Arbor Publishers Inc., Ann Arbor, Mich., p. 39-49,
(1978).
14. Scott, D.S. and Horlings, H. , "Removal of Phosphates and Metals from
Sewage Sludge," Research Report No. 28, Environment Canada, Ottawa,
(1974).
15. Scott, D.S. and Horlings, H., "Removal of Phosphates and Metals from
Sewage Sludges," Env. Sci. Tech., 9., 849-55, (1975).
16. Van Loon, J.C., "Heavy Metals in Agricultural Land Receiving Chemical
Sewage Sludges, Vol. IV., Analytical Methods," Research Report No. 51,
Environment Canada, Ottawa, (1976).
17. Murphy, J. and Riley, J.P., "A Modified Single Solution Method for the
Determination of Phosphate in Natural Waters," Anal. Chimica Acta,
31-36, (1962).
18. A.P.H.A. , A.W.W.A. , and W.P.C.F., "Standard Methods for the Examination
of Waste and Wastewater," 13th Edition, Water Poll. Control Federation,
Washington, B.C., (1971).
19. "Sludge Dewatering," Water Pollution Control Federation, WPCF Manual of
Practice No. 20, Washington, D.C., (1969).
20. "Sampling and Analysis of Coal and Coke," A.S.T.M. Standard D27—46,
Amer. Soc. Testing Materials, Baltimore, p. 38-40, (1946).
21. Jha, S., "Removal of Metals and Phosphates from Unconditioned Anaerobic
Sewage Sludge," M.A.Sc. Thesis, Dept. of Chemical Engineering, Univ. of
Waterloo, Ontario, (1975).
22. Soupilas, A., "Removal and Recovery of Heavy Metals and Phosphates from
Alum Treated Sewage Sludge," M.A.Sc. Thesis, Dept. of Chemical
Engineering, Univ. of Waterloo, Waterloo, Ontario, (1977).
23. Metcalf and Eddy Inc., "Wastewater Engineering, Treatment, Disposal,
Reuse," 2nd Edition McGraw-Hill Book Co., N.Y., (1979).
24. Harkness, N., et al, "Some Observations on the Incineration of Sewage
Sludge," J. Water Poll. Control, _71, 16-33, (1972).
25. Woods, D.R. , "Financial Decision Making in the Process Industries,"
Prentice-Hall, Englewood Cliffs, N.J., (1975).
34
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1.
4.
1.
y.
REPORT NO. 2
EPA-600/2-80-037
3. RECIPIENT'S ACCESSION-NO.
TITLE AND SUBTITLE 5. REPORT DATE
REMOVAL AND RECOVERY OF METALS AND PHOSPHATES FROM June 1980 (Issuing Date)
MUNICIPAL SEWAGE SLUDGE
AUTHOR(S)
Donald S. Scott
PERFORMING ORGANIZATION NAME AND ADDRESS
Department of Chemical Engineering
University of Waterloo
Waterloo, Ontario, Canada N2L 3G1
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratoi
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
15
16
17.
6. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
10. PROGRAM ELEMENT NO.
SOS #1 - 1BC821
11. CONTRACT/GRANT NO.
Grant No. R-804669
13. TYPE OF REPORT AND PERIOD COVERED
ry — Gin., OH Research 9/76 - 10/78
14. SPONSORING AGENCY CODE
EPA/600/14
. SUPPLEMENTARY NOTES
Project Officer: R.V. Villiers (513) 684-7664
. ABSTRACT
The purpose of this work was to look at the technical and economical aspects
of acid extracting heavy metals and phosphates from municipal chemical sludges
and subsequently recovering them by lime neutralization. The results showed that
such a process was technically feasible, but the cost of the process was
economically unattractive. The results should be valuable in assessing similar
technology for removing heavy metals from municipal sludges.
KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS
Sludge
Heavy Metals Removal
Acid Extracting
Costs
18.
DISTRIBUTION STATEMENT
Release to Public
b.lDENTIFIERS/OPEN ENDED TERMS
Sludge Treatment
Sludge Processing
19. SECURITY CLASS (This Report)'
Unclassified
20. SECURITY CLASS (This page)
Unclassified
c. GOSATi Field/Group
13B
21. NO. OF PAGES
45
22. PRICE
EPA Form 2220-1 (9-73)
35
ft U.S. GOVERNMENT PRINTING OFFICE: 1980 -657-146/5696
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