v>EPA
United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-81-031 Mar. 1981
Project Summary
Evaluation of Mixing
Systems for Biogasification of
Municipal Solid Waste
Stephen C. James, Carlton C. Wiles, Joseph T. Swartzbaugh, and Ralph B.
Smith
An investigation was conducted of
systems for mixing municipal solid
waste (MSW) with municipal sewage
sludge (MSS) in an anaerobic digester
to produce usable fuel (methane gas).
Adequate mixing is of paramount
importance to the success of this bio-
gasification process. Gas draft tubes
and mechanical agitators were evalu-
ated for use in a 387,500-L (100,000-
gal), 10.7-m-diameter digester. Feed
ratios of MSW to MSS were either 3:1
or 9:1. Loading rates of volatile solids
varied from 1.25 to 3.125 g/L per
day, and total solids in the feed were 4,
7, or 10 percent. Hydraulic retention
time was 22.5 days, except for one
11 -day study.
Problems that occurred during the
study were dense scum formation
(hard cellulose mats up to 1.5 m
thick), heavy wear on the mixing
systems as a result of the cellulose
fibers and grit from the MSW/MSS
mixture, and insufficient amounts of
volatile solids in the mixed zone. The
digester operated without problems
as long as the total solids level was 5
percent or below; higher concentra-
tions resulted in insufficient mixing
and operational problems.
Increased mixing power would im-
prove the distribution of volatile solids,
probably decrease scum formation,
and result in increased gas production.
But maintenance problems resulting
from the nature of the MSW/MSS
mixture and the increased energy
costs of a higher-powered mixer would
detract from system performance.
MSW/MSS mixtures with high cellu-
lose contents are therefore judged not
to be amenable to anaerobic digestion
using the same methods employed for
municipal wastewater.
This Project Summary was devel-
oped by EPA's Municipal Environmental
Research Laboratory, Cincinnati, OH,
to announce key findings of the research
project which is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Background
The production of usable fuels from
municipal solid waste (MSW) is a subject
of great interest and concern in view of
current costs and supplies of energy.
One possible method of fuel recovery
from MSW is the production of methane
gas during anaerobic digestion.
Anaerobic digestion has been associ-
ated with wastewater treatment plants
for many years, but the nature of munic-
ipal sewage sludge (MSS) is quite dif-
ferent from MSW. Incoming wastewater
solids of MSS are approximately 70
percent organic and 30 percent inor-
ganic. Much of the inorganic material is
removed in pretreatment units and thus
-------
\
is kept from the anaerobic digestion
units. Normally, these units receive
both raw and biological (secondary treat-
ment) sludge. Natural decomposition
then proceeds on this MSS.
MSW, on the other hand, contains a
much larger inorganic fraction than
MSS, and its organic fraction is com-
posed largely of water insoluble cellu-
losic materials. A large part of the
organic fraction of. MSS is also cellu-
losic, but it is made up mostly of toilet
paper that has been pretreated to im-
prove its solubility. (Insolubility limits
the rate of degradation because of
enzyme accessibility to certain areas.)
Also, the grit in the MSW feed is highly
abrasive to the system operation.
As part of a research program on fuel
recovery from municipal solid waste,
the U.S. Environmental Protection
Agency (EPA) sponsored this investiga-
tion of the feasibility of mixing MSW and
MSS in anaerobic digesters to produce
methane gas. This biogasification study
employed a 387,500-L, 10.7-m-diame-
ter digester to which was fed various
ratios of MSW'MSS at various loading
rates of volatile and total solids
Previous Investigations
Investigations dealing with the anaer-
obic digestion of the entire organic
fraction present in MSW have been
conducted. In most cases, MSS was
added to the reactors to provide nutri-
ents. The digestion studies of Golueke
and McGouhey1 showed that MSW
could be digested. Their organic loading
rates were the same as those used in
this study. Pfeffer2 studied the various
technical and economic aspects of
anaerobic digestion of a MSW/MSS
mixture. He reported that gas production
varied, depending on the loading rate,
detention time, and temperature. Klass
and Ghosh3'4 studied the anaerobic
digestion of MSS and shredded refuse
in which the organic materials (includ-
ing paper fiber) were separated from the
inorganics. The Dynatech Research and
Development Company6 conducted lab-
oratory experiments and made prelimi-
nary cost estimates for a refuse biogasi-
fication plant and concluded that the
process was feasible. But their estimates
of mixing requirements were based on
wastewater treatment practice and are
probably too low. Diaz et al.6 conducted
studies in which scum layer accumula-
tion was noticed. Addition of a mechani-
cal mixer to the laboratory-scale reactor
resolved this problem.
Methods and Materials
Methods
Before a large-scale study was under-
taken, a laboratory investigation was
conducted using two 208-L laboratory
digesters at several MSW:MSS ratios
and volatile solids loading rates. These
initial studies showed that cellulose
tended to accumulate and form a fibrous
mat at the top of the digester.
Following this laboratory study, a
378,500-L digester was used for a full-
scale 75-day study. The study was
terminated early as a result of the for-
mation of a scum layer 0.6 to 1.5 m
thick. Additional tests were then per-
formed with the same digester vessel to
compare mixing methods. Four tests
were conducted with gas draft tubes,
and five were done with mechanical
agitators. Operating parameters ob-
served were the mixing mode, feed
ratio, loading rate, and percent of total
solids in the feed.
Apparatus
The digester vessel was equipped
with a 10-hp (7.5 kW) Chemineer Model
4HTD10* mechanical mixer with 1.4-m
agitator blades, an Aerohydraulics
Model 3-12 expanding piston gas mixer,
and a 40-hp (30kW) Vaughan Model
300 scum breaker pump. The scum
breaker was added for startup operations
and for helping to breakup excessive
scum layers. The gas mixer used the
self-generated biogas as its mixing gas.
Figure 1 shows the location of the
Gas Gun /6
Sampling\
Ports
Mechanical
Agitator
Scum
Breaker
Pump
Gas Guns
Mechanical
i i; Agitator
D =
0
f
6
Scum
Breaker
Pump
Gas
Gun
apparatus and the sampling points.
Oiled-fired hot water coils at the perim-
eter provided temperature control of the
digester.
Feedstock
The feedstock used for this study was
a MSW/MSS mixture. The MSS came
from the regional wastewater treatment
plant in Franklin, Ohio, and the MSW
consisted of the organic reject stream
from the Black Clawson Fiberclaim
Corporation plant at Franklin, Ohio. The
organic reject is a finely pulped slurry of
MSW that has been cleaned of exces-
sively gritty organic and inorganic mate-
rials and typically has a solids concen-
tration of 4 percent.
Procedures
Evaluations were made of nine inde-
pendent tests at various loading rates
and ratios of MSW to MSS; total solids
concentrations varied from 4 to 10
percent. Table 1 provides a summary of
the operating conditions.
Total and volatile solids distributions
were measured for each test. Samples
were taken from five sampling ports at
the top, middle, and bottom of the |
digester vessel (Figure 1). Results pro- "
vided a profile of the digester contents
and were used to determine the effec-
tiveness of each test.
Digester Control
No process can be operated without
having adequate control and an indica-
tion of its progress. The total solids test
was used as an external control to
monitor what was coming into the
digester. Measurements of temperature,
volatile acids, alkalinity, and pH were
used as internal controls for assessing
the condition of the microbial culture
inside the digester.
The digester was heated to mesophilic
temperatures (32° to 35°C) and con-
trolled in this range for the nine inde-
pendent tests. To measure the microbial
condition in the digester, volatile acids,
pH, and alkalinity results for the digester
effluent and some random internal
samples were determined for each
independent test. Total and volatile
solids were measured each day at the
three different digester levels to access
the mixing modes.
Figure 1. Schematic view of
anaerobic digester.
"Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use
-------
Table
1.
2.
3.
4.
5.
5.
7.
8.
9.
1 . Summary
Mixing Mode
Gas
Mechanical
Gas
Gas
Mechanical
Mechanical
Gas
Gas
Mechanical
of Operating Conditions
Feed Ratio
(MSW:MSS)
3:1
3-1
3:1
9:1
9:1
9:1
9:1
9:1
9:1
Loading Rate (g
volatile solids/
L/day)
1.25
1.25
2.35
1.25
1.25
2.19
2.19
3.125
3.125
Total Solids
in Feed
(%)
4
4
4
4
4
7
7
10
10
ThepH of the digester was maintained
near neutral (6.8 to 7.2) for all nine
tests. Figures 2 and 3 show daily pH
values for the second (mechanical) and
eighth (gas) mixing tests. The pH is a
simple test, but it shouId not be depended
on as a process control parameter be-
cause of the alkalinity in the digester.
The volatile acids/alkalinity ratio is
the major internal process control mea-
surement and can vary from 0.05 to
0.35 without significant changes m
digestion. The volatile acids/alkalinity
relationship was generally maintained
in this range except for startup and the
74
72
.
6.8
6.6
10
20
25
Days
Figure 2. pH values over time for
second test (mechanical
mixing).
74
72
68
66
10 15
Days
2U 25
third test, which had a retention time of
11 days rather than 22.5 days. Figures 4
and 5 show the ratio over time for the
second (mechanical) and eighth (gas)
mixing tests.
Examination of the internal opera-
tional-control parameters indicated that
the digester was healthy. Even though
individual parameters sometimes ex-
ceeded the optimal ranges, there were
no signs that the digesters had become
sour. The optimal ranges quoted were
recommended for MSS digesters. Be-
.2
A-15
.05
10 15
Days
20 25
Figure 4. Volatile acids alkalinity
ratio over time for second test
(mechanical mixing).
.5
=S .4
I -2
.7
10 15
Days
20 25
Figure 3. pH values over time for
eight test (gas mixing).
Figure 5. Volatile acids alkalinity
ratio over time for eighth test
(gas mixing).
cause of its resistance to degradation,
the cellulose acts as a stabilizing param-
eter with regard to digester upset.
Results
The results of the first gas mixing test
using a 3:1 ratio of MSW to MSS at a
loading rate of 1.25 g of volatile solids/L/
day are presented in Tables 2 and 3.
For this first gas mixing test, total and
volatile solids distributions were higher
for the top layer than for the middle or
bottom layers. A cohesive mat started to
form on the top layer and randomly
floated throughout the digester. The
data in Table 4 show the average daily
mass flow and the accumulation of total
and volatile solids in the floating scum
layer.
The second test was conducted using
the mechanical mixer and was continued
immediately after the first study. Within
3 weeks, an extensive scum layer 0.6 m
thick with an average of 25 percent total
solids developed at the top of the di-
gester. At the end of the test, the scum
layer was 0.9 to 1.5 m thick at various
sampling points. As in the first test, high
total solids corresponded to high volatile
solids. In this and the following tests,
the differential between the highest
total solids percentages for the top level
and the lowest total solids percentages
for the middle and bottom levels in-
creased. Moreover, the volatile solids
percentages for the top level generally
became disproportionately higher than
those for the other two levels. Thus the
greater the total solids concentration
was at the top level, the greater was the
entrapment of volatile solids in the
scum layer. Consequently, the volatile
solids (substrate for the microorganisms)
were removed from the active level of
the digester.
Visual inspection of the digester
contents indicated that mixing was not
uniform in the digester. Movement was
turbulent near the center-mounted
agitator shaft and slower near the walls
of the digester. This observation and the
data trends confirmed the manufactur-
er's statement that for solids distribution
to be uniform, a more powerful motor
would be required. The small motor was
used in this study to provide a scale
comparable with the gas draft tubes. To
supplement mixing, the scum breaker
pump was operated intermittently in
these first two tests.
The third test (gas mixing) was an
attempt to lower the retention time from
22.5 to 11 days. High volatile a'cids/
-------
Table 2. Percent Total Solids Distribution for Test 1 *
Level Date
Top 8/0 1
8/04
8/08
8/11
fi /1 *»
o / / *j
8/18
8/22
8/25
Middle 8/01
8/04
8/08
8/11
8/15
8/18
8/22
8/25
Bottom 8/01
8/04
8/08
8/11
8/15
8/18
8/22
8/25
5
(wall)
6.14
6.01
3.92
2.48
q K?
j .\jf.
12.52
9.62
5.55
2.98
3.64
2.81
3.48
3.25
2.41
3.47
2.64
2.40
5.28
3.72
0.93
3.60
4.13
3.12
*Gas mixing mode; MSW:MSS,
4
8.68
6.29
3.88
10.29
311
.£. i
3.45
13.21
5.50
2.69
3.84
3.29
3.65
2.76
2.54
5.30
3.36
2.54
4.20
4.84
1.62
5.00
4.69
3.35
2.91
3:1; volatile
Port Number
3
3.87
4.38
3.16
5.55
A ?1
*T.£. 1
3.27
10.44
5.56
2.99
6.40
4.83
2 19
2.76
2.47
4.55
2.80
1.82
394
3.11
3.48
2.50
3.14
3.27
solids loading,
2
7.15
4.35
4.73
2.54
391
.^*J
4.20
6.59
7.94
2.92
3.93
3.23
2.30
2.65
2.30
2.83
3.21
2.77
4.43
3.06
2.89
3.77
3.88
5.42
3.63
1
(cGntGr)
1.93
3.23
3.04
2.04
9 #?
Z. OO
2.66
5.26
11.41
2.62
2.42
3.00
2.53
2.80
2.58
3.57
3.08
2.92
3.75
3.30
6.02
3.46
2.80
3.37
3.14
1.25g/L/day; total
solids in feed, 4 percent.
Table 3. Percent Volatile Solids Distribution
Level Date
Top 8/01
8/04
8/08
8/11
8/15
8/18
8/22
8/25
Middle 8/01
8/04
8/08
8/11
8/15
8/18
8/22
8/25
Bottom 8/01
8/04
8/08
8/11
8/15
8/18
8/22
8/25
*Gas mixing mode;
in feed, 4 percent.
5
(wall)
63.1
57.4
53.3
53.7
61.0
62.1
62.3
54.3
76.2
50.0
67.7
51.9
51.6
51.5
56.2
48.8
75.0
56.2
61.7
66.1
50.5
51.0
49.3
MSW:MSS,
4
64.1
55.0
51.5
62.5
52.8
56.7
61.0
56.6
60.4
53.5
53.1
47.6
53.2
52.4
57.3
47.4
58.6
54.6
57.1
60.0
52.2
58.4
54.2
53.4
for Test 1 *
n A/
Port Number
3
64.0
49.1
52.6
58.4
60.6
56.2
54.9
56.7
59.1
54.5
55.4
44.4
50.9
52.5
52.6
52.2
79.6
53.0
48.0
50.0
50.6
58.9
58.5
2
65.7
47.4
59.8
57.9
51.9
55.9
61.7
59.3
61.6
53.2
52.2
48.5
48.0
52.5
48.6
47.8
57.7
48.8
42.4
51.4
52.1
52.0
49.3
51.6
3:1; volatile solids. 1/25g/L/day;
1
(center)
83.7
54.7
48.7
54.0
56.0
52.0
55.6
56.2
56.6
56.5
48.4
51.0
52.1
51.4
54.7
50.0
69.4
47.4
47.5
50.5
51.6
50.0
50.5
52.9
total solids
Table 4. Average Daily Mass Flow
for Test 1 (Gas Mixing)
Total Volatile
Solids Solids
Parameter (kg/day) (kg/day)
Feed Blend 754 485
Effluent Liquid 580 295
Product Gas 77 77
Mass Out
Mass In .871 .767
alkalinity ratios (near 1 .0) indicated that
the digester was not operating near
optimal conditions.
The fourth and fifth tests (mechanical
mixing) were conducted using a 9:1
MSW to MSS ratio, a loading rate of
1.25 g/L/day, and a 4-percent total
solids feedstock. Results are presented
in Tables 5 and 6.
Total solids buildup was very high
within a week in the region near the
wall. The total solids concentration was
low around the center of the region of
maximum agitation. The volatile solids
increased with the total solids concen-
tration and was more than 70 percent at
many points in the too laveraftera short
period. Data indicated that the organic
fraction is not completely digested
when it is removed from the active
mixing region.
Data from Test No. 4 (gas mixing) also
indicate high total solids uniformly
dispersed in the top layer after a week of
operation. The volatile solids were also
high in the top layer. The data in Table 7
show the average daily mass balance
for the gas mixing test. Notice that more
than 50 percent of the volatile solids
have accumulated in the scum layer.
In the sixth (mechanical) and seventh
(gas) mixing tests, the loading rate was
increased to 2.2 g/L/day. In the me-
chanical testing, rapid buildup of solids
(greater than 30 percent) occurred
within 1 week. But the region around
the agitator shaft remained low in total
solids. The volatile solids concentration
was again very high in the scum layer
and low in the well-mixed region. Simi-
lar results were recorded with the gas
mixing test. Total and volatile solids
were evenly distributed throughout the
top layer of the digester, however.
The eighth (gas mixing) and ninth
(mechanical) tests were conducted
using a 9:1 ratio of MSW to MSS, a
loading rate of 3. 1 25 g/L/day, and a 1 0-
percent total solids feedstock. Results of
the gas mixing test No. 8 (Tables 8, 9,
and 10) indicate that the system allowed
-------
'Table 5. Percent Total So/ids Distribution for Test 5*
Port Number
Level Date 54321
(wall) (center)
Top 4/20 18.8 25.5 0.9 0.9 0.9
4/24 17.6 27.4 0.8 0.7 1.0
4/27 14.9 17.6 0.7 0.7 0.7
5/02 16.9 21.0 14.8 0.4 0.5
5/04 22.0 19.7 13.8 0.7 3.7
Middle 4/20 1.3 1.2 0.8 1.0
4/24 1.3 1.7 0.6 0.6 0.5
4/27 3.2 1.3 0.9 0.6 0.7
5/02 0.6 0.6 0.4 0.3 0.6
5/04 2.2 0.6 1.4 0.6 0.6
Bottom 4/20 1.5 1.3 1.3 1.3 1.0
4/24 3.6 1.2 0.8 0.8 0.6
4/27 1.2 2.1 0.9 0.8 0.9
5/02 1.4 1.8 0.9 1.0 0.7
5/04 1.3 0.9 1.4 1.7 0.8
* Mechanical mixing modes MSW: MSS, 9:1 ; volatile solids, 1 .25g/L/day, total solids
in feed, 4 percent.
Table 6. Percent Volatile Solids Distribution for Test 5*
Pf)rt Niimhpr
*L/I( / V UfllUd
Level Date 54321
\ (wall) (center)
Top 4/20 65.1 69.0 36.0 40.9 44.4
4/24 68.3 67.8 42.9 47.6 48.2
4/27 70.7 83.6 52.0 59.1 40.0
5/02 72.0 76.1 76.6 75.0 75.0
5/04 72.1 69.0 62.1 43.5 69.7
Middle 4/20 40.0 40.0 38.1 27.8
4/24 57.1 63.6 66.7 62.5 73.3
4/27 79.5 47.6 44.8 42.9 39.1
5/02 52.9 64.3 70.0 70.0 66.7
5/04 50.9 60.0 57.6 52.6 63.6
Bottom 4/20 47.9 46.7 43.8 36.7 47.8
4/24 50.0 68.3 61.5 69.6 66.7
4/27 50.0 61.4 42.9 50.0 5O.O
5/02 50.0 52.3 52.6 68.0 61.1
5/04 43.3 60.0 52.6 53.0 52.0
* Mechanical mixing mode; MSW: MSS, 9:1; volatile solids, 1.25g/L/day; total solids
in feed, 4 percent.
Table 7. Average Daily Mass Flow an increase at the top level of the
for Test 4 (Gas Mixing) digester in both total and volatile solids.
Tntai i/ lat'i Similar results for the mechanical
Solids Solids mixing test (Table 1 1 ' show that 80 to 90
Parameter? fkn/riftvi (kn/Havi percent of the volatile solids were being
Feed Blend 724 485 active mixing region.
Effluent Liquid 369 176
Product Gas 40 40 Discussion
I ooo i_/ui Tho Hinflctor cucto m ^vnopioni^fiiH
Mass ln i)&6 A4t> operational oroblems when the MSW:
MSS ratio was above 3:1 and the loading
rate and feedstock were above 1 .25
g/L/day and 4 percent total solids. This
condition was generally a result of poor
mixing because of the low-powered
motor, and it probably could be improved
through the use of a higher-powered
mixing system. But the improved mixing
results in increased energy use, and
thus the system does not appear feasible
from the standpoint of energy require-
ments.
The major problem associated with
the digestion process was the tendency
of the solids to coalesce into floating,
fibrous mats. Accompanying the forma-
tion of these fibrous mats was the
movement of the volatile solids, out of
the zone of digestion into the mat area.
This relocation resulted in a reduction in
the bioconversion process. A prime
cause of the coalescing and accumula-
tion of solids is the high cellulose
content of the MSW. Disintegration of
the cellulose fibers requires (1 ) separa-
tion and exposure of their fibrils, (2)
attack of fibrils by enzymes to break
their molecular bonds, and (3) digestion
of the resulting short-chained molecules
by the microbes. Though the mixing of
the MSW and MSS promotes these
three processes, it also has the opposite
effect of causing separated fibrils to
coalesce into stringers and mats. Though
the mats rise to the fluid surface in the
form of large scum accumulations, the
stringers interfere with the mixing
equipment and retard the fluid flow, and
consequently the enzyme and bacterial
movement.
However, there is also the aspect that
the strictly organic components (food
waste, yard waste, etc.) were completely
digested and that the cellulose fraction
was not being digested. It is reasonable
to assume this with the 20-to-30-day
duration periods and 22.5-day solids
retention time. Examination of the pH
and the volatile acids/alkalinity ratio
would support the above statement.
Extended duration and solids retention
time would provide further information
of the cellulose degradation.
Equipment problems also resulted
form the gritty cellulosic feedstock.
Excessive wear on the Moyno pump and
scum breaker pump was recorded. The
operating life of this equipment would
be likely to be short in a full-scale,
continuous-operation plant. Inspection
of the mechanical agitator showed
excessive buildup of cellulosic material.
Rooe-like strinaers became wound
-------
Table 8.
Percent Total Solids Distribution for Test 8"
10% TS Feed)
(Gas Mixing, 9:1, 3.125,
Port Number
Level
Top
Middle
Bottom
Date
7/24
7/27
7/31
8/03
8/07
8/10
8/14
7/24
7/27
7/31
8/03
8/07
8/10
8/14
7/24
7/27
7/31
8/03
8/07
8/10
8/14
5
(wall)
35.5
29.8
27.3
32.8
29.1
23.5
15.6
0.9
1.0
1.0
1.2
4
35.2
34.0
33.7
36.7
14.8
14.8
1.1
1.0
1.0
0.9
3
35.6
36. /
40.2
43.3
31.6
16.3
0.7
1.2
0.8
1.1
2
1.4
14.8
13.8
22.1
13.4
12.9
23.5
1.7
1.1
0.3
1.0
0.9
0.8
1.2
1.5
0.4
1.0
1.2
0.7
1
(center)
/./
18.6
14.0
13.7
22.3
1.1
0.3
/./
1.4
0.7
1.7
0.3
1.1
1.2
08
Gas mixing mode; MSW: MSS, 9:1; volatile solids, 3.125g/L/day; total solids in
feed, 10 percent.
Table 9. Percent Volatile Solids Distribution for Test 8*
Port Number
Level
Top
Middle
Bottom
Date
7/24
7/27
7/31
8/03
8/07
8/10
8/14
7/24
7/27
7/31
8/03
8/07
8/10
8/14
7/24
7/27
7/31
8/03
8/07
8/10
8/14
5
(wall)
61 9
60.4
59.0
58.8
56.0
69.3
78.4
48.3
51.2
53.6
56.3
4
61.6
57.3
63.7
52.5
76.8
80.9
43.5
50.3
47.6
52.6
3
51.5
53.9
58.2
51.7
61.7
785
46.;
56.0
48.2
54.3
2
57.7
74.8
70.0
63.2
78.4
73.5
67.4
56.7
54.7
27.9
48.1
49.3
49.0
54.0
54.9
38.4
49.1
50.7
50.2
1
(center)
57.5
57.4
74.7
81.5
86.4
53.9
27.3
50.7
67.6
49.0
54.7
26.0
49.3
56. 8
55.7
Table 10.
Parameter
A verage Daily Mass
Flow for Test 8, Gas
Mixing
Total Volatile
Solids Solids
(kg/day) (kg/day)
Feed Blend
Effluent Liquid
Product Gas
Mass Out
Mass In
3960
660
80
.187
2840
360
80
.155
Table 11.
Parameter
A verage Daily Mass
Flow for Test 9
(Mechanical Mixing)
Total Volatile
Solids Solids
(kg/day) (kg/day)
Feed Blend
Effluent Liquid
Product Gas
Mass Out
Mass In
4272
510
73
3012
261
73
.136
.111
Gas mixing mode; MSW: MSS, 9:1; volatile solids, 3.125g/L/day; total solids in feed,
10 percent.
around the shaft and agitator arms and
caused decreased mixing efficiency and
excessive wear on the agitator drive
mechanism.
The best methane production was
observed in Test No. 2, which produced
6600 ftVday (187,000 L/day) of biogas
at an average composition of 62 percent
methane. In this test, 16 ft3 (453 L) of
gas was produced per pound of volatile
solids destroyeda reasonable rate for
a healthy digester. An overall system
mass balance for Test No. 2 shows that
no solids accumulation took place. The
volatile solids destruction observed was
38 percent.
Mixing employed throughout this test
consisted of 24-hr/day operation of the
10-hp (7.5-kW) mixer and 4-hr/day
operation of the 40-hp (30-kW) scum
breaker pump. If full load operation is
assumed for both and power generation
efficiencies are ignored, the energy
used for mixing is equal to 400 hp-
hr/day (300-kW-hr/day). The methane
produced was 4092 ftVday (116,000
L/day), which has an energy content of
1600 hp-hr/day (1190 kW-hr/day),
which is only four times greater than the
direct energy usage of our admittedly
underpowered mixing systems.
To improve the mixing characteristics
of this system, a 50- to 100-hp (37.5- to
75-kW) mechanical mixer would be
-------
required. The electrical energy required
to operate this mixer would be approxi-
mately 350,000 to 700,000 kwh/yr7.
The energy produced in the current
system (1600 hp-hr/day) converts to
435,000 kwh/yr. Thus a 50-hp (37.5
kW) mixer would result in an overall 20-
percent energy gain, and the 100-hp
(75-kW) mixer would result in a 61-
percent energy loss. The use of a larger
mixer should produce an increase in
volatile solids destruction and thus a
subsequent increase in gas production.
A doubling in gas production would be
necessary to produce a net gain in
energy by the 100-hp (75-kW) system.
Note, however, that additional energy
expenditures (such as digester heating,
MSW processing, etc.) have not been
considered in these energy calculations.
Conclusion
Data analysis indicates that MSW/
MSS mixtures with high cellulose con-
tents are not very amenable to anaerobic
digestion, either with regard to operat-
ing procedures or energy recovery.
Increased mixing power would improve
the distribution of volatile solids, prob-
ably decrease scum formation, and
result in increased gas production. But
maintenance problems resulting from
the nature of the MSW/MSS mixture
and increased energy costs caused by
mixing requirements would detract
from system performance.
This study was performed for the U.S.
Environmental Protection Agency by
Systems Technology Corporation under
Contract No. 68-03-2105.
Solid Waste " Progress Report No.
1207, NSF/RANN/SE/C-872/PR/
74/2, Dynatech R/D Company,
Cambridge, Mass., 1974. 184 pp.
L. G. Diaz, F. Kurz, and G. J. Trezek.
Compost Science, 15 (3), 1974.
Innovative and Alternative Technol-
ogy Assessment Manual. EPA-430/
9-78-009, U.S. Environmental Pro-
tection Agency, Cincinnati, Ohio,
1978, p. D-32.
References
1. C. G. Golueke and P. H. McGouhey.
"Comprehensive Studies of Solid
Waste Management." 2nd Annual
Report, SERL Report No. 69-1, Sani-
tary Engineering Research Labora-
tory, University of California, Berkely,
1969.
2. J. T. Pfeffer, "Reclamation of Energy
from Organic Refuse." EPA-670/2-
74-016, U.S. Environmental Protec-
tion Agency, Cincinnati, Ohio, 1974.
3. D. L. Klass and S. Ghosh. Chemtech,
3, 689-698, 1973.
4. S. Ghosh and D. L. Klass. "Conver-
sion of Urban Refuse to Substitute
Natural Gas by the Biogas Process."
Fourth Mineral Waste Utilization
Symposium, Institute for Gas Re-
search, Chicago, III., 1974.
5. D. L. Wuse, S. E. Sadek, and R. G.
Kispert. "Fuel Gas Production from
Stephen C. James and Car/ton C. Wiles are with the Municipal Environmental
Research Laboratory, USEPA. Cincinnati, OH 45268 and Joseph T. Swartz-
baugh and Ralph Smith are with the Systems Technology Corporation, Xenia,
OH 45385.
Stephen C. James is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of Mixing Systems for Biogasification
of Municipal Solid Waste." (Order No. PB81-171 597; Cost: $9.50, 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
* US GOVERNMENT PRINTING OFFICE 1981-757-012/7043
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