United States
Environmental Protection
Agency
Water Engineering Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-85/001 Mar. 1985
Project Summary
Determining the Stability of
Treated Municipal
Wastewater Sludges
John S. Jeris, Daniel Ciarcia, Edward Chen, and Miguel Mena
A study was conducted to determine
*he potential for further biological degra-
dation of municipal wastewater sludges
that may already have undergone some
degree of treatment by a sludge stabil-
ization process. A literature survey was
carried out to determine the most
fruitful study approaches, and labor-
atory-scale studies followed.
The literature survey comprehensively
summarizes available information on
the characteristics of sludges stabilized
by anaerobic, aerobic, or thermal condi-
tioning processes. The feed materials
considered were raw primary sludge,
activated sludge, and mixtures of the
two. The sludges produced by treatment
had a broad range of instability because
design factors varied widely for the
treatment processes that generated
them. Many of the parameters consid-
ered useful in determining sludge stab-
ility were also reviewed in the literature
survey.
The laboratory study built on methods
described in the literature for evaluating
sludge stability. Sludges studied includ-
ed primary, trickling filter, and activated
sludges, as well as sludges from full-
scale anaerobic digesters, heat treat-
ment processes, and aerobic digestion.
Stability of these as-received sludges
was evaluated by measuring their re-
sponse to additional aerobic or anaero-
bic digestion, and by cumulative gen-
eration of hydrogen sulfide. Responses
to aerobic digestion of the as-received
sludges were generally similar and
showed substantial reductions in param-
eters such as biological oxygen demand
(BOD) and chemical oxygen demand
(COD). Oxygen uptake eventually
reached a low stable value for all sludg-
es. The same kind of reduction in param-
eters occurred in the as-received sludg-
es with anaerobic digestion. The hydro-
gen sulfide generation test generally
showed well-defined points at which
generation of the gas virtually ceased as
sludge storage increased. With addi-
tional development, the test shows
promise as a method for comparing
sludges to determine their potential for
further biological decomposition.
Though much has been learned about
the response of various sludge stability
parameters to further digestion, a sim-
ple measurement indicating sludge sta-
bility was not developed.
This Project Summary was developed
by EPA's Water Engineering Research
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 major and most offensive byproduct
of municipal wastewater treatment is
sludge. Before ultimate disposal, munic-
ipal sludges undergo varying amounts of
treatment by stabilization processes to
reduce their odor and their potential for
adverse environmental effects. The stabil-
ity of these sludges varies with their
origin and final treatment (if any) before
disposal. This project analyzed municipal
sludges originating from aerobic and
anaerobic biological processes and from
thermal processes.
A sludge should be stable before final
disposal. Unfortunately, the term "stable"
is not clearly defined with respect to
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municipal sludges. Before disposal, a
sludge should be stabilized enough that
no adverse environmental effect can be
easily observed upon disposal. Sludges
disposed of by means of land application
should be sufficiently stable so that odor
and health problems do not develop.
Ocean disposal of sludges should not
have adverse effects on the marine eco-
system. This report does not determine
how stable a sludge should be before
disposal; rather it defines stability of any
given sludge with respect to equilibrium
concentrations of various parameters
achieved after long-term biological diges-
tion.
The objects of this investigation were
therefore (1) to review the literature for
the best ideas on sludge stability and
methods for measuring it, (2) to search
experimentally for the most promising
measures and to test them, and (3) to
recommend the best methods or avenues
for further research.
The primary experimental activity was
concerned with anaerobic and aerobic
digestion of sludges that had been stabi-
lized at a municipal wastewater treatment
plant. The object of the experimental
program was to determine how much
biological activity remained in the sludge
and if possible, to relate this instability to
some sludge parameters or group of
parameters.
Literature Survey
The scope of the literature review was
confined to the stabilization of municipal
sludge originating from aerobic and an-
aerobic biological processes and from
thermal processes. Primary objectives
were to review the literature, gather
information on sludge stability, make the
data available to the scientific community,
and provide ideas for the laboratory work.
To define the direction of the review,
conclusions had to be drawn about what
constitutes a stable sludge. Probably the
best indicator of stability is the inability of
the sludge to degrade further biologically.
Consequently, the processes of anaerobic
and aerobic digestion were carefully
reviewed, and all parameters that might
be related to stability were examined. For
anaerobically digested sludge, param-
eters such as gas production, methane-
to-carbon-dioxide ratio, volatile solids
reduction, adenosine triphosphate (ATP)
concentration, BOD, COD, and others
may be important for measuring the
potential for further biological degrada-
tion. Odor production potential could be
very important in determining the stable
state of a sludge. For example, a sludge
digested for 30 days is less odorous than
a sludge digested for 15 days. Potential
appeared to exist for establishing a simpli-
fied method for measuring sludge odor
and relating it with other parameters to
obtain a sludge stability index.
For aerobically digested sludges, oper-
ating parameters such as temperature,
degree of mixing, organic loadings, solids
retention time, and feed sludge character-
istics affect the finished sludge's stability,
but they are unlikely candidates as meas-
ures of stability. Parameters such as
specific oxygen uptake rate, ATP, BOD,
COD, organic nitrogen, ammonia nitro-
gen, nitrate, and others are better indi-
cators of the sludge's biological activity.
The available literature on thermal
treatment was reviewed primarily to
examine how changes in severity of the
thermal conditions (time and tempera-
ture) affect the chemical and dewatering
properties of the sludge.
Experimental Procedures
Sludges were obtained from 11 munic-
ipal treatment plants in New York State
and New Jersey. These sludges were
primarily mixtures of primary and acti-
vated sludges that had been digested
aerobically or anaerobically. They includ-
ed the following:
Extended aeration activated sludge
Aerobically digested activated
sludge
Anaerobically digested primary
sludge
Anaerobically digested activated
sludge
Anaerobically digested primary and
activated sludge
Anaerobically digested primary and
trickling filter sludge
Thermally treated primary and acti-
vated sludges
The source and pretreatment of these
sludges may be important in their initial
instability. Brief descriptions of the treat-
ment plants are included in the text of the
full report.
Seven anaerobic sludges from five
municipal wastewater treatment facilities
were studied in this investigation. Sludge
retention times in the digesters ranged
from 13 to 37 days; volatile solids reduc-
tions ranged from 27 to 56 percent. Six
different aerobic sludges were studied
from six municipalities. Plant sizes ranged
from 0.4 mgd flow for a small extended
aeration plant to 85 mgd. Aerobic diges-
tion times ranged from 6 to 21 days. One •
extended aeration plant did not digest its ^
sludge.
The major portion of the anaerobic
digestion work was carried out in 18-liter
digesters that were continuously stirred
and maintained at 35°C. Provision was
made for monitoring gas production and
composition and for periodic sampling.
Some work was also carried out in 1.5-
liter digesters that were mixed by hand
once daily. Their only difference other
than size was the manner of mixing.
Aerobic digesters were 19 liters in
capacity and were mixed and provided
with oxygen by a diffused air system.
Temperatures ranged from 20° to 26°C.
Evaporation losses were made up by
addition of distilled water.
Special analytical procedures were
used for ATP measurements and centri-
fuge button tests. The ATP tests, which
might have produced interesting results,
have not been reported because of diffi-
culties with a new but faulty photometer.
The centrifuge button test followed a
procedure reported by Hartman et al.
(Journal of the Water Pollution Control
Federation, 51, 2353, 1979). Hydrogen
sulfide is generated from a button of
sludge cake centrifuged from a 50-ml
sample of sludge. The test measures the
time it takes for the hydrogen sulfide to
discolor 50 percent of a strip of lead
acetate paper.
The objective of the experiments was to
extend anaerobic or aerobic digestion far
beyond its normal termination point to
reach a stable condition. The various
parameters used to follow the course of
digestion (e.g., gas production for an-
aerobic digestion and oxygen uptake rate
for aerobic digestion) were determined
along with special measures such as the
centrifuge button test and specific resist-
ance to filtration. For both anaerobic and
aerobic digestion, the residence time was
infinite, that is, there was no daily feed.
This batch digestion was continued for as
long as 100 days to reach a stable
condition.
Results and Discussion
The Search for Indicators of
Stability
The data collected(1) permit a multitude
of comparisons between sludges, (2)
show the effects of digestion time on
parameters indicating sludge quality or
degree of stabilization, and (3) permit an
assessment of these parameters as abso-
lute or relative indicators of stability.
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Table 1 describes the feed stocks for the
anaerobic and aerobic digestion experi-
ments by their source and stabilization
history. The data collected require graphic
presentation for best understanding. Ex-
amples in Figures 1,2, and 3 illustrate the
data comparisons that are possible. They
are drawn from the aerobic digestion
runs of the Phase 1 experiments.
The effect of additional aerobic diges-
tion time on Stony Point aerobically
digested sludge (Run 4A) is illustrated in
Figure 1. The data indicate that it takes 30
to 40 days for the sludge to reach a state
Table 1. Sludge Charged to Digesters in Phase 1 and 2 Experiments
Sludge Being Tested
Phase
and
Run % Seed %
Phase 1.
anaerobic
digestion:
1
2
3
4
5
6
Phase 1,
aerobic
digestion:
1A
2A
3A
4A
5A
6A
Phase 2.
anaerobic
digestion:
7
a
9
10
11
12
13
Phase 2.
aerobic
digestion:
7A
8A
9A
75
75
65
100
75
75
0
0
0
100
84
98.4
0
0
0
0
0
0
0
100
65
65
25
25
35
+
25
25
100
100
100
t
16
1.6
100
100
100
100
100
100
100
§
35
35
Type of Sludge
from This Previous
Plant Treatment Steptsf Stabilization
Cedar Creek
Cedar Creek
Stony Point
Cedar Creek
Rockland County
Poughkeepsie
Beacon
Cold Springs
Musconetcong
Stony Point
Rockland County
Poughkeepsie
Stony Point
26th Ward
Coney Island
Cedar Creek
Oyster Bay
Yonkers
Yonkers
Stony Point
26th Ward
Jamaica
P
SA
EA
P + SA
P + SA
P + A
A
EA
CS
EA
P + SA
P+A
EA
P + SA
A
P + SA
P + TF
P
SA
EA
P + SA
P+A
None
None
aerobic digestion
anaerobic digestion
high pressure HT
low pressure HT
aerobic digestion, 6 day
None
aerobic digestion. 21 day
aerobic digestion, 14 day
high pressure HT
low pressure HT
aerobic digestion
anaerobic digestion
anaerobic digestion
anaerobic digestion
anaerobic digestion
anaerobic digestion
anaerobic digestion
aerobic digestion
anaerobic digestion
anaerobic digestion
*Abbreviations: P, primary treatment; A. activated sludge; SA, step aeration; EA, extended
aeration; CS, contact stabilization; TF, trickling filter; HT, heat treatment.
+This sludge was the seed for Runs 1 -6.
\This sludge was the seed for Runs 4A-6A.
§This sludge was the seed for Runs 7A-9A.
of equilibrium in which parameters are
not changing. Parameters such as sus-
pended solids (SS), volatile suspended
solids (VSS), COD, and specific oxygen
uptake rate (SOUR) level out after this
time interval. Of these parameters, SS,
VSS, and COD are not useful unless
related to their initial values. At best,
then, their absolute values are of no
utility, but their percentage reductions
might be relative indicators of stability.
Experience indicates that the SOUR falls
to approximately the same value for all
aerobically digested sludges after pro-
longed digestion. The data in Figure 1
indicate that it reaches a steady value at
about the same time as other parameters.
Thus the absolute value for SOUR seems
to be a good indicator of sludge stability.
Figure 2 presents the same type of data
for aerobic digestion of a heat-treated
sludge (Rockland County, Run 5A). A
substantial seed of Stony Point sludge
(see Table 1) was added to ensure that the
original would digest aerobically. Note
that SS, VSS, COD, and BOD fell sub-
stantially in the first 40 days, but SOUR
remained constant. Clearly, the sludge
was undergoing stabilization, but the
SOUR did not indicate it. Thus although
the SOUR might be useful as an indicator
of stability for aerobically digested sludg-
es, it does not appear to be useful for
heat-treated sludges.
Figure 3 shows the change in specific
resistance and the capillary suction test
(CST) for the Stony Point and Musconet-
cong (Run 3A) aerobic sludges after
additional aerobic digestion. Changes in
these parameters did not follow the
known increase in stability that takes
place, so these parameters are not good
indicators of stabilization.
The lead acetate test for detecting
hydrogen sulfide emissions on Phase 1
sludges showed promise as a general
indicator of stability for all types of
sludges. Changes in the test during
Phase 2 were intended to improve reli-
ability. They included using a constant
sludge mass and reducing the percent of
blackened paper for a positive indication
of hydrogen sulfide. The changes actually
reduced reliability, however.
Parameter Changes
Resulting from Long-Term
Digestion
Aerobic Digestion of Aerobic Sludge—
Five sludges that had been aerobically
treated in the wastewater treatment
plants were subjected to further aerobic
digestion. Similar patterns of change in
-------
10
10 20 30 40 SO 60 70 80
Days
(b)
COD(mg/LS 103)
A
BOD (mg/L S 102)
10 20 30 40 50 60 70 80
Days
16
14
I
12
X).
(0
^•OConductivity Vmhos S 10*
cm
0.0-00-
pH
Alkalinity (mg/L S102)
' i i
Organic Nitrogen (mg/L)
20
Ammonia
Nitrogen (mg/L)
10 20 30 40 50 60 70 80 10 20 30 40 50 60
Days Days
Figure 1. Aerobic stability parameters for aerobic sludge in Run 4A, Stony Point, New York (summer).
70 80
parameters were observed for all of the
aerobic sludges (Beacon, Cold Springs,
Musconetcong, Stony Point summer and
Stony Point winter). In most cases, all
forms of solids decreased relatively quick-
ly over the first 20 to 30 days and rather
slowly thereafter. Oxygen demand exhib-
ited a similar pattern, with higher frac-
tional decreases in BOD and SOUR than
COD. Only one sludge showed anoma-
lous results, and there seemed to be a
reasonable explanation for this behavior.
All changes in pH, alkalinity, and con-
ductivity were similar except for the
anomalous sludge. Alkalinity rapidly de-
creased to very low levels in about 20
days. The pH first decreased to a minimum
(usually into the range of 4 to 5) over the
first 20 days and typically returned to the
original value. Conductivity exhibited the
reverse pattern, first rising and then
falling. The conductivity peak occurred a
few days after the pH minimum. The shift
in conductivity is a response to the
increased hydrogen ion concentration
(low pH) because of the high specific
conductance of this ion.
The nitrogen forms pattern was again
similar for all units except for the anom-
alous sludge unit. Organic nitrogen de-
creased rapidly as a result of the endo-
genous metabolism of the cellular mate-
rial. The nitrogen is released into the
liquor in the form of ammonia, resulting
in the initial increase of this nitrogen
form. Ammonia removal takes place
concurrently, primarily by nitrification
and to a minor extent by stripping. Once
the organic nitrogen breakdown is com-
plete, no ammonia is fed to the system, so
the ammonia level peaks and starts to
decrease. Note that this ammonia peak
correlates well with the stabilization of
SOUR, BOD, organic nitrogen and (to a
smaller degree) with alkalinity. Undoubt-
edly the pH changes observed are due to
the changes in nitrogen forms and strip-
ping of ammonia and carbon dioxide. The
specific resistance and CST measure-
ments did not generally indicate signif-
icant change as digestion proceeded, but
there was a tendency for a rise and fall
pattern.
In all cases, significant grease removal
(greater than 50%) took place over the
first 20 to 30 days for all sludges. This
removal along with nitrate formation may
account for the drop in pH observed
during the initial period of stabilization.
Aerobic Digestion of Heat-Treated
Sludges—The heat-treated sludges (Rock-
land and Poughkeepsie) that were further
aerobically digested exhibited patterns of
-------
10 20 30 40 50 60 70 80
12
1C
8
6,
4
2
(c)
pH
Alkalinity (mg/LS102)
Conductivity //m/)os S /0s
•D
10 20 30 40 50 60 70 80
Days
Figure 2. Aerobic stability parameters for thermal sludges in Run SA (compare with 4A), Kockland County, New York.
COD (mg/L S 103)
BOD (mg/L S 10')
10 20 30 40 50 60 70 80
Ammonia
Nitrogen (mg/L)
Organic
0 Nitrogen (mg/L S 10'
10 20 30 40 50 60 70
change in solids, oxygen demand, nitro-
gen forms, and grease that were similar
to those of the aerobic sludges discussed
above. The pH pattern was different,
however. For both heat-treated sludges,
the pH first rose and then returned to near
the original level. This rise and fall is the
normal pattern expected in batch aeration
of sludge, not the pattern observed pre-
viously with the aerobic sludge. As am-
monia is released during protein break-
down, ammonium bicarbonate tends to
be formed first, followed by nitric acid as
nitrification of the ammonia occurs. In the
five aerobic sludges, a large population of
nitrifiers may have been present initially.
Their presence would foster rapid initial
nitrification with consequent fall in pH,
whereas heat treatment would retard
nitrifiers, delaying the onset of nitrifica-
tion and allowing pH to rise. Specific
resistance and CST showed no consistent
trends.
Aerobic Digestion of Anaerobically
Digested Sludges—Two anaerobically
digested sludges were further aerobically
digested. The pattern of parameter change
with time of aeration was similar to that
observed previously in aerobic stabiliza-
tion. The main area of difference was in
the pH-conductivity pattern and in the
ammonia nitrogen pattern. The pH rapidly
decreased and then remained constant,
and conductivity rose as pH fell. The
ammonia nitrogen at time zero was
already at a high level because of protein
breakdown under aerobic conditions with
no possibility of nitrate formation. Both
the organic and ammonia nitrogen de-
creased with time as aerobic stabilization
converted ammonia to nitrate. Thus no
ammonia peak was observed. The specific
resistance rose, and the CST remained
constant.
Anaerobic Digestion of Sludges—Thir-
teen si udges were subjected to prolonged
anaerobic stabilization. These included
raw sludges, mixtures of raw sludge and
waste-activated or trickling filter sludge;
waste-activated sludge alone, heat-
treated sludge, sludge that had already
been anaerobically digested, and sludge
that had already been aerobically diges-
ted. A similar pattern of changes in
measured parameters was observed for
all of these units, regardless of the source
of the sludge. Some departures from the
general patterns occurred, but these were
probably due to experimental difficulties
rather than to basic differences in the
pattern of stabilization. Quantitative dif-
ferences between units can be traced to
differences in the degree of stabilization
-------
30
20
10
(a)
m
^ I Specific Resistance
0—^ (M/Kgt
1
CST(sec)
2x10'*
1x10" 20
15(
10
5
Grease %
10 20 30 40 50 60 70 80 90 JQ 20 30 40 50 60 70 80 90
Stony Point, NY (Summer)
20
15
to
5
0 0
^ Specific Resistance \ "(M/Kgj
m
0
0
**] © CST(sec)
o o
1 1 1 1 1 1 1 I
2x10"
20
1x10" 15
10
<
5
(d)
„
. ^^~^
-------
6. The centrifuge button test appears
to be a reasonable indicator of
potential for hydrogen sulfide odor
formation and is generally complet-
ed in 15 to 30 days for -most
sludges.
7. TS.VS, BOD, and COD continuously
decreased for all sludges until
steady state was typically reached
in 20 to 40 days. These measure-
ments are not particularly sensitive
indicators of stabilization.
8. Alkalinity, pH, and conductivity did
not generally appear useful as
stability indicators; but they may
serve as such in conjunction with
nitrification and nitrate formation.
9. During anaerobic treatment of an-
aerobic sludges, the rate of gas
formation and the concentration of
volatile acids decrease rapidly in 5
to 10 days, but their relationship to
stability is not evident.
Recommendations
1. Continue testing the most promis-
ing indicators of sludge stability
using various municipal sludges to
confirm their effectiveness.
2. Extend testing to include sludges
from rotating biological contactors,
trickling filters, and solids from
composting operations.
3. Develop and extend the applications
of the centrifuge button technique
for predicting potential for hydrogen
sulfide, odor generation, especially
rn conjunction with treatment plant
operation.
4. Continue the development of the
ATP and crude fiber analyses.
5. Confirm the use of the SOUR anal-
yses as a stability indicator for
aerobically treated sludges.
6. Develop stability and drainability
relationships using the CST.
The full report was submitted in fulfill-
ment of Cooperative Agreement No.
CR806809 by Manhattan College, under
the sponsorship of the U.S. Environmen-
tal Protection Agency.
JohnS. Jeris, Daniel Ciarcia, EdwardChen, and Miguel Mena are with/Manhattan
College, Bronx, NY 10471.
R. V. Villiers was the EPA Project Officer (see below).
The complete report, entitled "Determining the Stability of Treated Municipal
Wastewater Sludges," (Order No. PB 85-147 189/AS; Cost: $19.00, subject to
change} will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
For further information, contact J. B. Farrell:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati. OH 45268
, US GOVERNMENT PRINTING OFf ICE 1985. 559-111/10797
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Environmental Protection
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