United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
^
Research and Development
EPA/600/S2-86/118 May 1987
&EPA Project Summary
Destruction and Stabilization of
Sludge by Multiple-Stage
Digestion and Thermal Treatment
Yeun C. Wu
A study was conducted to compare
the performance of conventional single-
stage anaerobic sludge digestion with
three-stage anaerobic sludge digestion.
The conventional digester and the first
two stages of the multiple-stage system
were operated at 35°C; the third stage
was maintained at 49°C. The effluent
sludges from both processes were fur-
ther treated by wet air oxidation (WAO).
Study results indicated that multiple-
stage digestion outperformed conven-
tional single-stage digestion in terms of
reducing chemical oxygen demand
(COD) and volatile solids (VS) and pro-
ducing methane. The multiple-stage
system's first two stages alone achieved
a greater degree of sludge stabilization,
COD and VS destruction, and methane
production than did the single-stage
system. The WAO process achieved 86%
VS reduction at 250°C.
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
Recent research on the anaerobic di-
gestion of wastewater sludge has sug-
gested that separation of the digestion
process into stages can improve per-
formance. Two methods of separation
have been used: either a mesophilic stage
followed by a thermophilic stage, or an
acid-forming stage followed by a
methane-forming stage. This project
combined these two methods by using
the following three stages: a hydrolyzing
(mesophilic) stage, followed by an acid-
forming (mesophilic) stage, followed by a
methane-forming (thermophilic) stage.
Materials and Methods
The multiple-stage system consisted of
three Plexiglas* digesters in series with
effective volumes of 23, 114, and 68 L.
The digesters were well mixed with
mechanical stirrers and were heated to
maintain temperatures of 35°, 35°, and
49°C, respectively. The feed rates to the
multiple-stage system were 0.42, 0.57,
and 0.95 L/hr over three operating
periods, producing overall solids retention
times (SRT's) of 20, 15, and 9 days.
Chemical oxygen demand (COD) loadings
ranged from 2.4 to 8.8 kg COD/m3-day,
and volatile solids (VS) loadings ranged
from 1.3 to 4.0 kg VS/m3-day.
A102-L, single-stage, Plexiglas digester
was also run at 35°C to act as a control.
The feed rate to this digester was con-
trolled to yield the same SRT and loadings
as the overall multiple-stage system for
each operating period.
The feed sludge for both the multiple-
stage and control systems was a mixture
of primary and secondary sludges from
the Allegheny County Sanitary Authority
in Pittsburgh, Pennsylvania (Table 1).
The digested sludges were further
treated by a wet air oxidation (WAO)
process. The WAO apparatus was a 3.78-
L titanium unit supplied by an autoclave
vendor. Tests were run at 1.4 x 107
pascal (Pa) (2100 psi) and at temperatures
up to 300°C.
"Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use
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Table 1. Feed Sludge Characteristics
Parameter Mean
COD (mg/L) 68.000
Total solids (%) 5.1
Volatile solids {%) 3.2
Lipids f% of TS) 18
Carbohydrates (% of TS) 23
Protein (% of TS) 44
pH (range) 5.2 to 6.7
Alkalinity (mg/L as CaCO3 250
NH3-N(mg/L) 410
Organic-N (mg/L) 1800
Soluble-P (mg/L) 30
Volatile acids (mg/L) 2800
COD/N/P ratio 1000/6/0.5
Results
Reductions in COD, VS, lipids, carbo-
hydrates, and protein are shown in Tables
2 through 6, respectively. The reductions
shown are based on quantities entering
the first stage. Mean coliform reductions
were 1 .3, 1 .6, 2.0, and 4.9 logs for Stages
1, 2, 3, and the overall multiple-stage
system, respectively. A 2.3-log coliform
reduction was achieved in the control
system. Metals (Cd, Cu, Mn, Ni, Pb, and
Zn) partitioned to the sludge rather than
the supernatant in the effluents from
both systems.
Table 4. Lipid Reduction (%)
SRT(days)
Type of System
Three-Stage:
Stage 1
Stage 2
Stage 3
Overall
Control
Sum of Stages 1 and 2
Table 5. Carbohydrate
Type of System
Three-Stage:
Stage 1
Stage 2
Stage 3
Overall
Control
Sum of Stages 1 and 2
9 15 20
2 26 22
39 34 35
15 11 13
56 71 70
45 57 61
41 60 57
Reduction (%)
SRT(days)
9 15 20
19 28 35
40 35 23
10 13 12
69 75 70
42 43 34
59 63 58
Table 6. Protein Reduction (%)
SRT(days)
Type of System
Three-Stage:
Stage 1
Stage 2
Stage 3
Overall
9 15 20
17 15 17
13 16 6
7115
37 41 28
was always found in the filtrate (<= 1 1
mg/L). Mn (<= 1.3 mg/L), Ni (<= 2.0
occasionally found
and Pb never were.
Table 7. Methane
Type of System
o.u my/ i-/ weie
in the filtrate, but Cd
Yield (L/day)
SRT(days)
9 15 20
Three-Stage:
Stage 1 3 9 17
Stage 2 196 160 103
Stage3 47 37 21
Overall 247 206 141
Control 81 60 49
Sum of Stages 1 and 2 199 169 120
Table 8. Methane Production Rate
(L/day per L of digester)
SRT(days)
Type of System
Three-Stage:
Stage 1
Stage 2
Stage 3
Overall
Control
9 15 20
0. 1 0.3 0.8
1.7 1.5 0.9
0.7 0.4 0.3
1.2 1.0 0.7
0.8 0.6 0.5
Table 2. COD Reduction (%)
SRT(days)
Type of System
15 20
Three-Stage:
Stage 1
Stage 2
Stage 3
Overall
Control
9
28
6
44
13
38
8
59
20
27
7
54
32 42 44
Sum of Stages 1 and 2 37 51 47
Table 3.
Volatile Solids Reduction (%)
SRT(days)
Type of System
Three-Stage:
Stage 1
Stage 2
Stage 3
Overall
9
12
41
8
60
15
17
40
10
67
20
25
27
8
59
Control 44 51 49
Sum of Stages 1 and 2 53 57 52
Control 20 25 21
Sum of Stages 1 and 2 30 31 23
Total methane yield and normalized
methane production (i.e., rate per unit
volume of the digester) are presented in
Tables 7 and 8. The percentage of
methane in the digester gas ranged from
22% to 46%, 69% to 71%, 65% to 69%,
and 65% to 69% for the Stages 1, 2, 3,
and control digesters, respectively.
The WAO studies showed that little
volatile solids destruction occurred below
180°C. Destruction rates increased
rapidly for temperatures from 180° to
250°C, with little further increase in
destruction above 250°C. A constant 86%
VS destruction was achieved at 250°C.
Dewatered sludge filter cake (>4% VS)
was not destroyed as effectively as wet
sludge, probably because of an oxygen-
limiting condition.
Most of the metals in the WAO effluent
were associated with the solids, but Cu
Conclusions
The multiple-stage system outperformed
the single-stage system as the parameters
in Tables 2 through 8 indicate. In fact, the
first two stages of the multiple-stage
system were together more efficient than
the entire control system, even though
the total retention time was 35% shorter.
Although the original intent was to
maximize methane production in the third
stage, the second stage of the multiple-
stage system had a greater total methane
yield and production rate than both the
third stage and the control system. The
retention time in the second-stage di-
gester was only 55% that in the control
digester.
The need for the third stage of the
multiple-stage system was questionable.
The third stage gave several benefits that
would have to be weighed against the
costs of adding a third stage: an additional
2.0-log reduction in coliforms, a 15% to
19% increase in methane, a 6% to 9%
further reduction of COD, and an 8% to
11 % further reduction of VS (percentage
of reduction based on quantities entering
the first stage).
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Separation of the digestion process into
stages did not create a need for chemical
addition to control alkalinity and pH.
Although the digestion process was
separated into stages, hydrolytic and
liquefying reactions occurred in all three
digesters of the multiple-stage system,
as evidenced by reductions in lipids, pro-
teins, and carbohydrates in all three
digesters. Overall reduction of these
compounds was greater in the multiple-
stage system than in the control system.
At longer retention times in the first-
stage digester, excessive foaming became
a problem. Thus a first-stage retention
time of about 1 day should be used.
Dewatering and settling characteristics
of the multiple-stage digested sludge were
only slightly better than those of the
control digested sludge.
The principal products from the WAO
of the digested sludge were volatile acids,
nitrogen compounds, carbon dioxide, and
water. Organic nitrogen was broken into
ammonia nitrogen, nitrate, and nitrite,
but some reformation of organic nitrogen
compounds from these products and
organic acids appeared to occur at higher
temperatures. The effluent from the WAO
was usually slightly basic.
If the multiple-stage system is used in
conjunction with WAO, total VS destruc-
tion can be as high as 95.5%. However,
use of a two-stage system should be
considered if WAO is used, because the
savings in digester volume, heating, and
mixing may offset the costs of additional
WAO capacity without sacrificing the ef-
ficiency of VS destruction.
The solids in the effluent of the in-
tegrated treatment system (three-stage
digestion followed by WAO) possessed
excellent settling characteristics, which
improved with increasing temperature.
Sludge volume index values of less than
18 mL/g were possible. The final effluent
had a pH of about 7.5 and contained high
levels of nutrients, such as ammonia,
phosphorus, and organic nitrogen. How-
ever, since almost none of the original
metal content was solubilized, the final
sludge was highly contaminated with
metals, and metal stabilization and de-
toxification could be required.
The full report was submitted in ful-
fillment of Interagency Agreement No.
DW-930244011 by the University of
Pittsburgh under the partial sponsorship
of the U.S Environmental Protection
Agency.
Yeun C. Wu was with the University of Pittsburgh. Pittsburgh, PA 15261.
Donald S. Brown and John N. English are the EPA Project Officers (see below).
The complete report, entitled "Destruction and Stabilization of Sludge by
Multiple-Stage Digestion and Thermal Treatment," (Order No. PB 87-140
505/AS; Cost: $18.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, V'A 22161
Telephone: 703-487-4650
For further information Donald S. Brown can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
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