United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-046 Mar. 1984
Project Summary
Characterization of Sewage
Sludge and Sewage
Sludge-Soil Systems
Lee E. Sommers, Darrell W. Nelson, and Allen W. Kirleis
Field and laboratory studies were
conducted to characterize the chemical
properties of municipal sewage sludges.
to evaluate the fate of sludge compo-
nents in soils, and to determine the
distribution of trace metals in milling
fractions of grains grown on sludge-
treated soils. Specific studies included
(1) characterizing the organic compo-
nents in sewage sludges, (2) separating
sludges into organic and inorganic
fractions, (3) examining the capacity of
sludge to retain metals, (4) evaluating
sludge through spectroscopic, infrared,
and electron spin resonance studies, (5)
conducting laboratory soil incubation
studies to characterize mineralization
and nitrification of nitrogen and decom-
position of organic carbon in sludge-
amended soils, (6) determining metal
speciation with analytical data for soil
solution samples, and (7) evaluating the
uptake of Cd, Zn, Ni, Cu, Fe, and Mn in
oats, corn, wheat, and soybeans grown
in soils treated with sewage sludge.
This Project Summary was developed
by EPA's Municipal Environmental Re-
search Laboratory, Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The recent emphasis on applying
municipal sewage sludge to agricultural
cropland necessitates a knowledge of the
sludge constituents and their fate in soils.
Sludges can be used as an alternative
fertilizer because they contain not only
macronutrients required by plants (i.e., N,
P, and K), but also essential micronutri-
ents such as Cu, Zn, and Mn. Studies
have shown that comparable crop yields
can be obtained with conventional fertil-
izer materials and sewage sludge, but
sludges may also have constituents that
can be toxic to plants or to animals or
humans consuming products grown on
sludge treated soils. Trace metals, or-
ganics, and pathogens are the three broad
groups of sludge constituents that have
received the greatest emphasis in re-
search and regulatory activities. Of the
metals contained in sludge, Cd has
receivedthe most attention asa potential
threat to human health, and Zn, Cu, and
Ni have been implicated in decreasing
crop yields. The polychlorinated biphenyls
(PCB's) have received more attention than
other sludge-borne organics.
This series of studies was conducted to
characterize the chemical composition of
sewage sludges, to study the fate of
sludge components in soils, and to evalu-
ate the distribution of metals in crops
grown on sludge-treated soils.
Chemical Composition of
Sewage Sludges
The organic components in sludge were
characterized by several extraction, chro-
matographic, and spectroscopic tech-
niques. Fatty acid esters (fats and waxes)
and phthalic acid esters (plasticizers)
constituted 2% to 10% and <1 % of sludge
oven-dry weight, respectively. Palmitic
acid esters and dibutyl phthalate were the
predominant components in the respec-
tive fractions. Upon H2SO4 hydrolysis of
the sludges and subsequent analysis of
the hydrolyzates by high pressure liquid
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chromatography and colorimetric proce-
dures, hexuronic acids, glucose, and
xylose were detected. The sugars were
likely monomers of hemicellulose and
cellulose polymers, which together made
up 0.99% to 2.55% of sludge oven-dry
weight. Organic N in the sludges was
present primarily as a-amino acid N,
constituting 31% to 36.5% of the organic
N. The amino acid distribution in sludge
hydrolyzates was determined by an amino
acid analyzer and revealed that amino
acids of bacterial origin were present (i.e.,
a-amino pimelic, a,e-diaminopimelic, /8-
alanine, y-amino butyric, and ornithine)
as well as those commonly found in plant
and animal protein. Sodium-amalgam
reduction of the sludges produced few or
no phenolic compounds, indicating small
concentrations of humic-type materials.
Infrared and X-ray analyses of residues
from HF-treated sludges and NaOH ex-
tracts (humic acids) of sludges indicated
the presence of condensed aromatic rings
or quinone-like compounds and little or
no inorganic crystalline material.
Sludges were separated into light and
heavy fractions by an ultrasonic disper-
sion and heavy liquid separation tech-
nique to provide information on trace
metal forms in sludges. The light fraction
(organics) was 8% to 26% of the sludge
dry weight, and the heavy fraction (inor-
ganics) constituted 69% to 88% of the
sludge weight. Most of the metals were
associated with the sludge light fraction
(organic).
Infrared studies were conducted to
characterize qualitatively the sewage
sludge light (organic) and heavy (inor-
ganic) fractions obtained using the heavy
liquid separation technique. Infrared
spectra of sludge organic fractions re-
vealed the presence of proteinaceous and
polysaccharide-like materials. Infrared
absorptions characteristic of carboxyl
groups and aromatic compounds were
not observed in the spectra of sludge light
fractions. The position of the amide-l
band in the infrared spectra of the sludge
light fractions was used to obtain direct
evidence of metal binding sites involving
amide N and possibly amide O. A discrete
shift in the position of the amide-l band
was initiated by extracting indigenous
metal cations from the sludge organic
fraction. This shift was reversed by satu-
rating the extracted sludge organic frac-
tion with Cu2+. Absorption bands of quartz
and calcite dominated the infrared spectra
of sludge inorganic fractions.
Electron spin resonance (ESR) spectros-
copy was used to study the mechanism of
Cu2+ binding by humic acid (HA) extracted
from soil and sewage sludge. The ESR
data showed that the addition of pyridine
(py) and 1,10-phenanthroline (phen) to
the soil and sludge Cu-HA complexes
resulted in the added nitrogen ligands
replacing water coordinated to Cu2*. The
added ligand did not displace HA oxygen
donor ligands also coordinated to the
Cu2+. The addition of a more basic nitrogen
donor, ethylenediamine (en), resulted in
the formation of Cu(en)l+ by displacement
from Cu2+ or both coordinated aquo
ligands and HA oxygen donors. ESR
parameters revealed that two HA oxygen
atoms were coordinated equatorially to
Cu2+ in the original soil and sludge Cu-HA
complexes. In the sludge Cu2+-HA com-
plex, the Cu2+ ion also appeared to form
axial bonds with HA nitrogen donor
ligands originating from proteinaceous
materials associated with the sludge HA
fraction. Characterization of an adduct
with glycylglycine showed that the dipep-
tide formed coordinate bonds with Cu2+
bound to sludge HA.
The metal retention capacities'of calcar-
eous and acidic sludges and sludge
fractions were determined. The results
obtained from the equilibration of Cd, Cu,
Ni, or Zn salts with calcareous and acidic
sludges indicated that calcareous sludges
have a higher capacity to retain added
metals than noncalcareous sludges and
that metal retention was directly related
to sludge pH. As solution pH increased
above 5.0, chemical precipitation of
metals, most likely as carbonates, became
the dominant mechanism in the removal
of Cd, Cu, Ni, or Zn from solution. Metal
retention was reduced when pH values of
sludge and metal solutions were below
5.0. Most of the Cd, Cu, Ni, and Zn
retained by sludge at low pH values was
likely the result of complexation by organic
matter and sorption by amorphous Fe, Al,
and Mn hydrous oxides. The retention
capacity for Cd and Cu with heavy and
light sludge fractions decreased in the
following order:
light fraction>heavy fraction>whole sludge
This order indicates the formation of
stable complexes between sludge organ-
ics and metal ions. The heavy fraction,
which is composed largely of inorganic
materials, also retained large proportions
of added Cd and Cu. The sludge compo-
nents responsible for Cd, Cu, Ni, or Zn
retention were estimated by selective
extraction procedures. The results indi-
cated that three sludge components are
primarily involved in Cd, Cu, Ni, or Zi
retention. Sludge components are rankei
below in decreasing order of their impor
tance for retention of each metal:
Cd: carbonate>organic matter>hydrou
oxides;
Cu: carbonate>organic matter = hydrous
oxides;
Ni: carbonate>organic matter = hydrous
oxides;
Zn: carbonate>organic matter>hydrous
oxides.
Fate of Sludge
Components in Soils
A laboratory soil incubation system
was used to study metal extractability
(with DTPA), nitrogen mineralization,
nitrification, and organic C decomposi-
tion in soils amended with municipal and
synthetic sludges. The synthetic sludges
were prepared to contain varying concen-
trations of Cu, Cd, Zn, Pb, and Ni at a
constant level of organic C and N. The
extent of decomposition of municipal
sewage sludges (as measured by C02
evolution) appeared to be a function of the
organic C content of the sludges. As
much as 30% to 47% of the C02-C evolved
from the sludge-treated soils was due to
dissolution of calcium carbonate present
in the sludges. Significant inhibition of
nitrification was found in soils amended
with synthetic sludges at rates greater
than 60 metric tons/ha, whereas stabi-
lized municipal sludges showed rapid
nitrification of all application rates.
Generally, the percent of sludge organic
N mineralized increased with sludge appli-
cation rate. Metals (Zn, Cu, Ni, and Cd)
extracted from soil with DTPA generally
increased with incubation time and were
directly related to the amount of sludge-
borne metal added to the soil. Similar
correlations were obtained between
added and DTPA-extractable metals for
synthetic and municipal sludges. In gen-
eral, synthetic sludges offer promise for
studying the decomposition rate of munic-
ipal sludges in soils.
The decomposition of liquid and dried
municipal sewage sludges was deter-
mined in soils under laboratory and field
conditions. Evolved COs was continuously
monitored in soil + sludge mixtures
incubated in the laboratory. Primary, raw,
and waste-activated sludges showed the
greatest degree of decomposition, where-
as digested and composted sludges had
the lowest decomposition values during a
112-day incubation. The data indicate
that the percentage of sludge organic C
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composed in soil will be less for those
idges subjected to intensive stabiliza-
n processes at the treatment plant.
rbon dioxide evol ution i n the field study
is monitored for a 24-hr period by
jaling cylinders containing soil-sludge
nixtures. The cylinders remained open to
he environment between sampling
>eriods. Soil moisture and temperature
were also monitored. Evolution of C02
was initially rapid in all soil and sludge
;ombinations, but it decreased with time
swing to consumption of easily decom-
posable organic substrates in the sludge
md gradual lowering of soil temperature
n the late fall and winter. Carbon dioxide
evolution rates for all treatments were
affected by temperature and increased in
sarly spring. Dewatering an anaerobically
digested sludge by air-drying reduced the
extent of decomposition in soil. Because
of C02 release from carbonate dissolution,
he presence of inorganic C in sludges
can interfere with organic C decomposi-
ion calculations based on COj evolution.
Data from both the laboratory incubation
md field study show good agreement
loncerning relative order and magnitude
>f sludge decomposition. A first-order
i odel adequately predicts decomposition
>' most sewage sludges under field
; >nditions. The model failed to predict an
ncrease in the rate of C02 evolution in
jarly spring. This increase is possibly due
o increased organic C availability induced
>y freezing and thawing cycles during the
)revious winter.
Incubation studies using carboxyl-
abeled 14C dibutyl phthalate (DBF) added
o soils at 0.1% (w/w) indicate that 90%
>f the DBF was degraded within 80 days.
Decomposition of DBF was shown to be
nicrobially mediated and was a function
jf soil type, pH, and temperature. Applica-
ion of DBF at rates of 0.4% (w/w) and the
addition of ammonium and CaCOa or
iewage sludge had little effect on the
sxtentof DBF degradation. Dibutyl phthal-
ate contained in sewage sludge or other
waste materials to soils should not pose a
long-term persistence problem in soils.
Humic acids (HA) extracted from siudge,
soil, and sludge-amended soil were char-
acterized by elemental and functional
group analyses, and by visible and infra-
red spectroscopy. When compared with a
soil HA, the sludge HA appeared to be a
more aliphatic, N-enriched polymer with
decreased amounts of carboxyls and
phenolic hydroxyls. Proteinaceous mate-
rials associated with the sludge HA
appeared to be the major source of N
enrichment. Sludge organic matter was
apparently recovered in HA extracted from
soil 2 and 14 months after sludge applica-
tion. The association of sludge-borne
proteinaceous and aliphatic materials
with the sludge and sludge-amended soil
HA fractions was indicated by increased
amide and C-H stretch infrared absorp-
tions, as well as by the increased N
contents and decreased C:H ratios. The
observation of proteinaceous materials
associated with HA extracted from sludge-
amended soil 14 months after application
suggested that these proteinaceous
materials were resistant to decomposi-
tion, possibly as a result of their incorpo-
ration into the HA structure.
Infrared spectroscopy was used to
identify the mode by which carboxylates
of a soil HA complexed Cu2+ and Fe3+.
Carboxylic acid groups of soil HA were
converted to carboxylates (CO-?) by form-
ing coordinate bonds with Cu2+ and Fe3+.
The separation of the antisymmetrical, va
(COi), and symmetrical, vs (COi), stretch-
ing vibrations of COI obtained from
spectra of metal-HA complexes indicated
that COz formed a unidentate complex
(i.e., a single M-O bond) with Cuz+ and
Fe3+. The formation of bidentate or
bridging complexes between COi of HA
and Cu or Fe3* was not observed.
Metal-chelate stability diagrams were
developed as a function of pH and redoxto
predict the behavior of five synthetic
chelating agents in soils. The metals
evaluated include H+, Fe3*, Fe2+, AI3+,
Mn2*, Ca2*, Mg2+, Zn2+, Cu2+, Cd2+, Pb2*,
and Ni2*. In acid systems, Ni2+ at 10 5 to
10~7 M was the predominant ligand
species for all chelating agents. Similar
results were obtained in alkaline systems
when NiCO3 controlled Ni2* solubility. In
the absence of Ni2*, either Cda* or Pb2*
ligand species dominated at alkaline pH
for all chelating agents. In acid systems,
Cu, Pb, Fe, and Zn are the major chelated
species. These chelating agents deserve
further study in acid soils as extractants
for metals and as reagents for determining
metal ion activities through competitive
equilibria. The results identify ligands
that may be useful in determining various
metal ion activities in soils. The diethyl-
enetriamine pentaacetic acid (OPTA) soil
test appears to have a sound theoretical
basis for evaluating the plant-available
Pb, Cd, and Ni in soils.
A cooperative study was conducted
with the Tennessee Valley Authority to
determine the effects of sewage sludge
applications on the heavy metal status of
a soil. Sewage sludge from Tuscumbia,
Alabama, was applied to an acid soil (pH
~ 5) as a single treatment in 1971 or as
annual treatments from 1971 through
1974 in amounts totaling 50 to 800
metric tons/ha. Soil samples (0- to 15-cm
depth) obtained from the plots in 1977
were moistened to -0.33 bar water
potential and equilibrated for 7 days
before displacement of the soil solution.
The soil solutions were analyzed for Zn,
Cd, Cu, Ni, Pb, Mn, Fe, Ca, Mg, Al, Na, K,
PO*, SO4, Cl, conductivity, pH, and organic
C. The metal species present were calcu-
lated with a computer program (GEO-
CHEM). Sludge application increased soil
solution concentrations of Zn, Mn, and
Cd, but Cu, Ni, and Pb levels in most
solutions were below the detection limits
of the analytical methods used. Cd, Zn,
and Mn in the soil solution appeared to
exist predominately as the free ion.
Depending on the model used to represent
metal interaction with soluble organic C,
complexation by inorganic and organic
ligands ranged from 9% to 33%. 3% to
22%, and 3% to 31 % of the total soluble
Cd, Zn, and Mn, respectively. Cd was
complexed with organic ligands to a
greater extent than were Zn or Mn.
Sulfate, phosphate, and chloride formed
inorganic complexes with Cd, Zn, and Mn,
but they constituted less than 10% of the
total metal present. The general degree of
metal complexation by organic and inor-
ganic ligands increased in the following
order:
Zn = Mn = Ni < Cd < Cu
Speciation of Zn and Cd into free metal
ion and complexed forms was essentially
the same in the soil solution, regardless
of metal concentrations in solution and
rate or frequency of sludge application.
Calculated activities of Cd2* indicated
undersaturation with respect to known
Cd solid phases, whereas Zn2* activities
were comparable with those of ZnFezCu,
soil Zn, and Zn2SiO.t. Soluble Zn and Cd
were directly related to the amount of
metal added and to the extractability of
added metal with DTPA. The solubility
and DTPA-extractability of Cd were in-
creased more by a single application of
si udge than by multiple, annual additions.
This relationship was not as consistent
for added Zn.
Distribution of Trace Metals in
Milling Fractions of Grains
Grown on Sludge-Treated Soils
Oats, corn, wheat and soybeans were
grown on soils treated with stabilized
sewage sludge from three Indiana cities.
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Sludges were applied to a Chalmers silty
clay loam soil at rates of 56 to 448 metric
tons/ha. Oat samples were separated
into groats and hulls, and concentrations
of Cd, Zn, Cu, Fe, and Mn were determined
in each fraction by atomic absorption
spectrophotometry. The effect of sludge
application rates on the concentrations of
trace metals in whole oats, groats, and
hulls was similar for all three sludges
used, and they generally decreased in the
following order:
Zn = Fe > Mn > Ni > Cu > Cd
Except for Mn, the concentrations of trace
metals increased with increasing rates of
sludges applied, but the groats contained
greater concentrations of Cu, Ni, Zn, Cd,
and Mn than the hulls.
The metal concentrations in whole-
kernel corn were not significantly in-
creased by sewage sludge applications of
56 to 448 metric tons/ha. Except for one
Cd-enriched sludge, Cd concentrations in
whole corn and fractions from dry-milling
were less than 0.05 mg/kg. As expected,
the germ fraction contained the greatest
proportion of the fat, ash, and trace
metals, and most of the protein was found
in the grits fraction. Both the concentra-
tion and the percent distribution of metals
in dry-milling fractions decreased in the
following order:
germ > hulls > flour > meal > grits
The metal concentrations in both wheat
and soybean grain were increased by
sludge applications. For wheat, the bran
contained significantly higherconcentra-
tions of all metals than did the flour.
Metal concentrations were similar in
whole soybean gram and the resulting
meal.
Conclusions
1. Characterization of the organic com-
ponents in sewage sludges indica-
ted that fatty acid esters and
phthalates were predominant forms
present in nonaqueous extracts of
sludge.
2. A heavy liquid extraction procedure
developed to separate sewage
sludges into organic and inorganic
fractions indicated that most metals
naturally present in the sludge were
associated with the fraction en-
riched in organic matter.
3. Municipal sludges had the capacity
to retain appreciable amounts of
Cd, Cu, Ni, and Zn. The most likely
primary mechanisms involved in
sludge retention of added metal
salts were precipitation as carbon-
ates and sorption by organic matter
and hydrous oxides.
4. Studies conducted to evaluate the
spectroscopic properties of sludge
organic matter indicated that sludge
humic acids were more aliphatic
and contained fewer carboxyl and
phenolic hydroxyl functional groups
than soil humic acids.
5. Infrared investigations indicated
that the amide group in residual
proteinaceous materials could be
involved in the retention of Cu by
sludge.
6. Electron spin resonance studies
indicated that Cu (II) was bound to
oxygen-containing donor ligands in
the humic acid fraction of sewage
sludge.
7. In laboratory incubation experi-
ments, evolution of COz from
sludge-treated soils indicated that
elevated amounts of trace metals
may temporarily inhibit decomposi-
tion, but they do not do so over an
extended period. Some temporary
inhibition of nitrification was ob-
served in sludge-amended soils.
8. Studies conducted u nder laboratory
and field conditions indicated that
the extent of sludge decompositic
in soils was inversely related to tr
intensity of treatment and proces
ing within the sewage treatmei
plant.
9. Laboratory incubation studies using
dibutyl phthalate indicated that
more than 90% of this compound
will be degraded within 80 days
after application to soils.
10. The free metal ion was the primary
species of Zn and Cd found in soil
solutions, regardless of total metal
concentrations or the rate or fre-
quency of sludge application.
11. For oats, metal concentrations were
significantly greater in the groats
than in the hulls.
12. Minimal increases in grain metal
content were observed when corn
was grown on sludge-amended
soils.
13. Concentrations of Zn, Cu, Cd, and
Ni in wheat and soybean grain were
increased by sludge applications.
Bran and meal contained the high-
est metal concentrations for wheat
and soybeans, respectively.
The firll report was submitted in fulfill-
ment of Grant No. R804547-02 by Purdue
University under the sponsorship of the
U.S. Environmental Protection Agency.
L. E. Sommers. D. W. Nelson, A. W. Kirleis. S. D. Strachan. J. C. In/nan, S. A. Boyd,
J. G. Graveel, and A. D. Behel are with Purdue Unversity, West Lafayette. IN
47907.
James A. Ryan is the EPA Project Officer (see below).
The complete report, entitled "Characterization of Sewage Sludge and Sewage
Sludge-Soil Systems." (Order No. PB 84-158 898; Cost: $28.00. subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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