2002-01-2351
Anaerobic Digestion for Reduction and Stabilization of Organic
Solid Wastes During Space Missions: Laboratory Studies
David Chynoweth, Patrick Haley, John Owens, Art Teixeira, Bruce Welt, and Elana Rich
Ag. and Biol. Eng., University of Florida
Tim Townsend
Envir. Eng. Sci., University of Florida
Hong-Lim Choi
Animal Sci. and Tech., Seoul National University
Copyright © 2001 Society of Automotive Engineers, Inc.
ABSTRACT
The technical feasibility of applying anaerobic digestion
for reduction and stabilization of the organic fraction of
solid wastes generated during space missions was
investigated. This process has the advantages of not
requiring oxygen or high temperature and pressure while
producing methane, carbon dioxide, nutrients, and
compost as valuable products. High-solids leachbed
anaerobic digestion employed here involves a solidphase fermentation with leachate recycle between new
and old reactors for inoculation, wetting, and removal of
volatile organic acids during startup. After anaerobic
conversion is complete, the compost bed may be used
for biofiltration and plant growth medium. The nutrientrich leachate may also be used as a vehicle for nutrient
recycle. Physical properties of representative waste
feedstocks were determined to evaluate their space
requirements and hydraulic leachability in the selected
digester design.
Anaerobic biochemical methane
potential assays were run on several feedstocks to
determine extent and rates of bioconversion.
Modifications for operation of a leachbed anaerobic
digestion process in space environments were
incorporated into a modified design, including flooded
operation to force leachate through feedstock beds and
separation of biogas from leachate in a gas collection
reservoir. The results of runs in a prototype laboratoryscale reactor system operated on simulated solid waste
blends are presented.
INTRODUCTION
This paper presents preliminary operational information
of a proposed solid waste management system based on
high-solids leachbed anaerobic digestion (HSLAD). The
function of the process is to reduce volume and weight
of, stabilize, and recover inorganic nutrients, stabilized
compost, carbon dioxide, and methane from
biodegradable waste fractions. Focus was on a 600-day
exploratory mission (e.g., to Mars) based on this
emphasis at a recent NASA-sponsored solid waste
workshop (Verostko et al. 2001). This type of mission
would require growth of plants as a food supplement as
well as for oxygen regeneration. As shown in Table 1, a
6-person crew would generate about 10.5 kg/d dw (7.5
kg organic matter) solid wastes, including dry human
wastes, inedible plant residues, trash, packaging
material, paper tape, filters, and other miscellaneous
wastes (Verostko et al. 2001). These estimates are
constantly being revised based on actual International
Space Station (ISS) data and revised scenarios for food
and packaging materials as well as other factors
contributing to solid wastes.
The most significant
components are inedible plant wastes, paper, and other
trash. Other solid wastes may be expected from the air
and water processing operations.
The focus of this work is to evaluate a new version of the
patented high-solids process sequential batch anaerobic
composting (SEBAC) (Chynoweth and Legrand 1993)
which has been modified to operate under hypo- and
micro-gravity environments of space missions. The
SEBAC process uses a combination of solid-phase
fermentation and leachate recycle to provide a simple,
reliable process that inoculates new batches, removes
volatile organic acids, and concentrates nutrients and
buffer. The process has been tested on a variety of
high-solids feedstocks, including woody biomass, the
organic fraction of municipal solid waste, yard wastes,
and blends of yard wastes and biosolids (Chynoweth et
al. 1992; Chynoweth and Legrand 1993).
Organic
matter is decomposed primarily to methane, carbon
dioxide, and compost over a residence time of 10-30
days. The process is very stable, does not require
Table 1. Estimates of daily solid waste streams for a 6-person crew
during a 600-day exploratory mission (Verostko et al.2001)
trash
packaging
materials
paper
tape
filters
misc.
Total
Dry Wt.,
kg
Ash, %
dw*
Organic
Matter, kg
Moisture,
%
Percent of
total
0.72
5
0.68
85
9.4
5.45
0.56
5
5
5.2
0.53
75
10
51.4
5.3
19.0
2.02
1.16
0.25
0.33
5
5
1.1
10
10
0.07
10.6
Stage 2
Reservoir
For space applications, a five-reactor system is
envisioned, including one for feed collection and
compaction, three for anaerobic composting, and one for
post-treatment processing (Figure 1). Feed would be
collected, coarsely shredded, mixed with station
wastewater to give the desired <35% solids, and
compacted to a density of 300 kgdw/m3. This collection
pre-treatment step would require 5 days and be
conducted in the same reactor used for the entire
treatment process. The anaerobic digestion process
would proceed for 15 days. Biogas from anaerobic
composting would be treated to recover carbon dioxide
and remove hydrogen sulfide and other contaminants.
The methane may be used for energy (e.g. in a fuel cell)
or discarded. The final compost would be dewatered,
treated 1-2 days with air to oxidize reduced residues, and
Activated
Reactor
0.7
100
heated for 1 hour at 70oC to insure inactivation of
pathogens. Pathogens would also be inactivated during
the anaerobic process and aerobic post-treatment step
(Bendixen 1994; Engeli 1993). The final compost and
associated nutrient-rich water would be used as solid
substrate and source of nutrients for plant growth.
mixing or oxygen, and is resilient after months of being
idle without feedstock addition. Since the reactors may
be operated at low (ambient) pressures, bulky, high
pressure vessels are not needed.
Filling
Reactor
10.9
2.4
3.1
7.5
New
Reactor
Stage 1
This project is being conducted in three parts, including
laboratory-scale feasibility, systems analysis, and
prototype digester design and optimization. The scope
of this paper summarizes the results to-date from the
laboratory-scale feasibility studies. For this work, several
food crop residues were obtained, including wheat,
potato, sweat potato, tomato, peanut, and rice. Physical
properties of several paper types and crop residues were
measured under dry and wet saturated conditions to
predict their behavior in a laboratory-scale digester
designed for space applications. Biochemical methane
potential (BMP) assays were run to estimate the extent
and rate of anaerobic conversion. A laboratory-scale
reactor was designed, built, shaken down and started up
Mature
Reactor
Reservoir
Waste
Component
dry human
waste
inedible plant
biomass
Aerobic
Reactor
Stage 3
Pump C
Pump A
Pretreatment
Pump B
Anaerobic Digestion
Post-treatment
Figure 1. Sequential batch anaerobic composting system for space missions
on wheat crop residue.
The systems analysis work, presented in a separate
companion paper at this conference by Xu et al. (2002),
addresses the current design of the anaerobic digestion
system, its integration with other processes and
estimates of mass balances and equivalent systems
mass. For the third task, a prototype digestion system is
being built that is designed and sized for processing the
wastes generated during an extended mission with a
crew of six. This system will be used to further refine the
design and optimization of the process.
METHODS
FEEDSTOCK SELECTION AND ANALYSIS - Based on
recent publications (Drysdale et al. 2001; Wheeler 2001),
the following feedstocks were selected for biochemical
methane potential (BMP) assays and other digester runs:
ideal for anaerobic decomposition, i.e., broad spectrum
inoculum, excess inoculum, excess nutrients, substrate
concentration below inhibitory levels, excess buffering
capacity, moderate temperature, and strict anaerobic
conditions.
A 10-L inoculum (for BMP assays)
semicontinuously-fed stirred digester (Figure 2) was
operated with dog food (Science diet Large Canine
Growth Formula, Hill Pet Nutrition, Inc.) as a feedstock.
This feed has been used in past projects because it is a
reproducible multi-substrate feedstock. The digester
was operated at a loading rate of 1.6 g VS per L per day,
and hydraulic retention time of 20 days. At steady state,
it exhibited a methane gas content of 57%, methane
yield of 0.33 L per g VS added, and methane production
rate of 0.53 LCH4/Lreactor/d. The digester was quite
stable with a pH of 7.3 and volatile fatty acids
concentration of 155 mg/L, well below inhibitory levels.
The conditions for BMP assays included 100 mL culture
space vehicle: processing wastes from tomato,
carrot, cabbage, spinach, chard, lettuce, radish,
onion
planetary: processing wastes from wheat, white
potato, sweet potato, soybean, peanut, rice
paper: high grade paper and paper products
human feces: simulated (with dog food)
Several feedstocks were obtained from various NASA
laboratories and contractors as well as a University of
Florida researcher (Table 2). Subsamples were dried
and milled to the millimeter size. Total solids (TS) was
determined by drying overnight at 105oC. Volatile solids
(VS), a measure of organic matter, was determined by
ashing at 550oC for two hours and determining the ashfree dry weight.
Preliminary estimates of bulk densities were determined
on several paper and plant residue samples. One value
was determined by filling a 1-L beaker to the 1-L mark
using hand compression. The same weight of sample
was saturated with water and volume occupied by solids
determined after manual compression. All values are
reported as dry weight per volume.
The BMP assay was conducted on several
representative solid waste feedstocks to determine the
ultimate biodegradability (and associated methane yield)
and conversion kinetics during the anaerobic
methanogenic fermentation of organic substrates as
described by Owen et al. 1979. This standard and
routine method (Owen et al. 1979 and Turick et al. 1992)
involves batch incubation of a substrate under conditions
Figure 2. BMP inoculum digester
volume in 250-mL serum bottles (Figure 3), inoculum
from the inoculum digester (described above), an
inoculum-to feed ratio of 2:1 (volatile solids basis), feed
concentration of 2 g/L (VS basis), and incubation
temperature of 35oC. Total gas and methane production
were measured several times per week during the initial
stages of the assay, and less frequently for the final
stages. Gas production was measured via a graduated
syringe and methane content by thermal conductivity gas
chromatography. Samples were run in triplicate and
controls included inoculum and inoculum plus Avicel
cellulose. These batch serum bottle reactors were
incubated until no further gas production could be
detected (typically 30 days).
Table 2. Biomass residue feedstocks
Feedstock
wheat
tomato
peanut
potato
sweet potato
Description
hydroponic
field grown
hydroponic
hydroponic
hydroponic
Contact Person
Keith Henderson, JSC
Agronomy Department, UF
Desmond Mortley, Tuskegee
Neil Yurio, Dynamak, KSC
Desmond Mortley, Tuskegee
rice-Italica
temperate Japonica dwarf-medium Italica
Hartwell Allen, UF
rice-Koshi
temperate Japonica dwarf Koshi
Hartwell Allen, UF
rice-K204
tropical Japonica dwarf L204
Hartwell Allen, UF
rice-Labelle
tropical Japonica tall Labelle
Hartwell Allen, UF
rice-M103
temperate Japonica dwarf M103
Hartwell Allen, UF
rice-M202
temperate Japonica dwarf M202
Hartwell Allen, UF
rice-N22
tropical Indica tall N22
Hartwell Allen, UF
rice-S102
terperate Japonica dwarf S102
Hartwell Allen, UF
Figure 3. BMP assay bottles
LABORATORY DIGESTER SYSTEM - The terrestrial
SEBAC design (Chynoweth et al. 1992; Chynoweth and
Legrand 1993) depends on gravity for leachate recycle
and gas collection. For space applications (including
hypo- and micro-gravity), modifications included noheadspace flooded operation and gas separation in an
external vessel.
Flooded operation permits forced
pumping of leachate between reactors without
dependence upon gravity.
Only two reactors were required to validate operation in
the flooded mode without headspace, and external gas
collection. Figure 4 shows a schematic of the set-up
after several modifications were made to overcome
several operational problems.
The reactors were
fabricated from clear PVC pipe, 102-mm id (4-in), with an
overall height of 72.9 cm; the total working volume was
5.9 L. The bottom of each reactor a PVC cap was
drilled and tapped in the center for a 3.2 mm (1/8)-in
NPT fitting. The top of each reactor was made of a 12.7
mm thick clear Lexan blank-flange, which was drilled to
accept the bolts from a glued 102 mm (4-in) PVC flange
fitting. The clear Lexan top was also drilled and tapped
(3.2 mm NPT) for a sampling port and a biogas/leachate
outlet. The top flange was connected to the base of the
reactor with a Neoprene connector with stainless steel
pipe clamps. A steel frame supported the reactors by
the top flange along with chain clamps attached to the
frame. A 4-L glass aspirator bottle served as a common
leachate reservoir and biogas/leachate separator.
The PVC reactors and glass reservoir were wrapped with
2.4 m (8-ft) of 416-watt flexible electric heating tape
(Thermolyne Corporation), which was powered by a
Thermolyne 45500 input control to maintain leachate
temperature at 34-37oC. Flexible plastic tubing was
connected to 12.7 mm (½-in) barbed/NPT couplings
threaded into the top and bottom of the reactors.
Leachate was pumped at around 128 mL/min using a
peristaltic pump (Cole-Parmer model 7553-30 pump with
two 7018 pump heads). Schedule-80 12.7 mm (½-in)
PVC ball valves allowed isolation of the reactors from the
influent leachate lines. Leachate was drawn from the
bottom of the reservoir into the bottom of both reactors
via the peristaltic pump.
4
3
5
1
2
2
6
Figure 4. Schematic of SEBAC laboratory digester set-up
modified for flooded operation. Components include: (1) 4 L leachate reservoir, (2) - timer controlled peristaltic
pumps set at 128 mL/min, (3) - 5.9 L PVC reactors with
screen baskets containing solid waste heated by electrical
heat tape, (4) - leachate and biogas outlet tubing, (5) biogas line, and (6) - tipping bucket biogas meter.
After passing up through the solid waste beds and
reactors, the leachate and biogas flowed out of the top of
the reactors and into the top of the reservoir through a
No. 10 black rubber stopper. Separated biogas flowed
out of the top of the reservoir, through natural rubber
tubing, through a check valve, to a wet tip gas meter
(submerged inverted tipping bucket triggered by 110 mL
of gas), which controlled the gas pressure in the
reservoir at around 10 cm H2O. Sampling points for
biogas and leachate were placed using 12.7 mm (½-in)
barbed T’s and septums. An additional barbed-T with
septum on the reactor inlet lines allowed measurement
of hydraulic pressure using a model 05-2 pressure
transducer (Setra Systems, Inc.).
Shredded feedstock was placed into a basket (10 cm OD
and 60.96 cm length) fashioned from aluminum
hardware cloth (3.2 mm) and lowered into the reactor on
top of 51 mm PVC spacer rings and a 102-mm round
wire screen (1-cm openings). An additional screen and
spacer ring was placed above the screen basket.
INITIAL START-UP - The laboratory reactor system was
started by placing 540 g of milled wheat stems (wet
basis) in a screen basket in one reactor, along with 540
mL of inoculum from the 10-L inoculum digester
(described above), 23.5 g NaHCO3, and 4960 mL of dechlorinated tap water. The second reactor contained
only de-chlorinated tap water (~5.9 L). Initially, two
leachate reservoirs made from 500-mL sidearm
Erlenmeyer flasks were connected to each of the reactor
outlet tubes and contained an additional 100 mL of tap
water in each. Also, two individually-controlled peristaltic
pumps recirculated leachate continuously from each
reservoir into the bottom of the other reactor.
The pH of the leachate was monitored and an attempt
was made to keep it above 6.5 by the addition of
NaHCO3 and/or additional inoculum. On days 5, 7, and
11, 27.6 g of NaHCO3 were added. On day 4, 500-mL
of leachate was replaced with fresh inoculum and on
days 11, 19 and 20, 200-mL of leachate was also
replaced with inoculum. On day 32, another 500 mL of
leachate was replaced by inoculum.
NORMAL START-UP AND OPERATION - After the
initial start-up of the first reactor, the second digester
was capable of start-up without additional inoculum or
NaHCO3. When a run was completed the pumping
system was shut-off and the ball valves at the bottom of
each reactor were closed. The tubing from the top of the
reactor to the leachate reservoir was clamped off and
detached. Also, the tubing was disconnected from the
ball valve at the bottom of the reactor and the leachate
was drained out of the reactor into a storage container.
The lid of the reactor was taken off and the basket
removed. Remaining biomass was removed from the
basket, weighed and frozen to await analysis of VS and
TS. The basket was then rinsed with deionized water
and placed back into the empty reactor.
Approximately 500 g of shredded feedstock were placed
into the wire mesh basket and compacted using a 5.1 cm
(OD) solid plastic tamper. The wheat straw for the initial
runs was received pre-shredded to a particle size of
>1cm. Rice straw (obtained as whole grass) was
shredded using a garden shredder to a particle size of
3.1-7.6 cm (Black and Decker model 8051). Office
paper was shredded using a paper shredder to a 2 cm
particle size (Fellows model PS-70). Dog food was
placed into the reactor in its unaltered pellet state of 1.3
cm (Science Diet Large Canine Growth formulated by
Hill’s Pet Nutrition, Inc). For the third run, portions of
each feedstock were placed into the reactor and then
compacted to create a layering effect inside the basket.
After the reactor was filled, the screen and spacers were
replaced on top of the basket and the previously
removed leachate was poured into the reactor onto the
contents. Additional de-chlorinated tap water was added
to fill the reactor. De-chlorinated tap water was also
added to the leachate reservoir to achieve a 3000 mL
volume. The top was then sealed and the system tubing
was reconnected. Leachate was pumped every other
hour for a 20-minute interval at a flow rate of 128mL/min.
The reactor system was run until the gas production rate
peaked and then dropped below 1 L of gas per day. At
this time, the process of empting and filling the reactor
was repeated.
Total solids (TS) and volatile solids (VS) were performed
as described above. Leachate pH was determined on a
model 805MP pH meter (Fisher Scientific). Methane in
the biogas was measured on a gas partitioning gas
chromatograph with a thermoconductivity (TC) detector
(Fisher Scientific) and compared to an external standard
containing N2:CH4:CO2 in a volume ratio 15:55:30 (the
detector response is linear in the range used). Methane
volumes were converted to dry gas at STP.
Volatile organic acids (VOA) in the leachate were
assayed on a gas chromatograph (Shimadzu) with a
flame ionization detector (FID).
Samples were
centrifuged at 10,000 rpm for 10 min and the resulting
supernatant was acidified with 1:9 v/v parts sample to
20% H3PO4. Two μL of sample were injected on to a 2m long 3.2 mm id glass column packed with 10%
SP1000 and 1% H3PO4-coated 100/120 Chromosorb
WAW. Carrier gas was N2 at a flow rate of 60 mL/min.
Conditions were: inlet - 180o C, column - 155o C, and
detector – 200o C. Quantification was determined on a
LC-100 integrator (Perkin Elmer) using an external
Table 3. Comparison of bulk densities of several types of
paper under dry and saturated conditions
Material
shredded paper
legal pad paper
toilet paper
brown towel paper
domestic towel paper
wheat residue
Bulk Density, kg (d.w./m3
Dry Hand
Wet
Compacted
(saturated)
Hand
Compacted
67.2
336
44.2
354
53.1
193
32.5
186
52.6
202
136
166
standard containing acetate, propionate, butyrate,
isobutyrate, valerate and iso-valerate at 100 mg/L each
(the detector response is linear in the range employed).
RESULTS
FEEDSTOCK PROPERTIES AND PROCESSING Preliminary studies were conducted at the 1-L scale in
beakers to determine the influence of wetting on
reduction of bulk density. The results (Table 3) showed
that wetting resulted in significant reduction in the
volumes required for given dry weights of several types
of paper. Bulk densities exceeding 300 kg/m 3 were
obtained. The limited effect on the wheat sample may
be attributed to the fact that this sample was ground to a
particle size lower than will be used in full-scale systems.
Two devices are being constructed to more
systematically evaluate this important parameter as well
as the influence of compaction on hydraulic conductivity.
FEEDSTOCK BIODEGRADABILITY – Biochemical
methane potential assays were conducted on several
representative solid waste components to determine the
conversion efficiency and ultimate methane yield. These
data, shown in Table 4, with sample plots in Figure 5,
indicate that conversion was complete in about 10 days
which is significantly lower than the 21 days projected at
the start of the research project. Based on the final
methane yields, the highest conversion was observed for
residues from peanut and the lowest for residues from
wheat. These data along with those conducted on paper
types in a study by Owens and Chynoweth (1993)
provided a reasonable spectrum of the biodegradability
of the feed types expected during space missions. Data
for a variety of different feedstocks from Chynoweth et al.
(1993) are included in Table 4 for comparison purposes.
For interpreting these data, it is important to realize that
the ultimate methane yield is influenced by the
biodegradability and the hydrogen-to-carbon ratio of the
feedstock. Carbohydrates, the major component of plant
residues, have a theoretical methane yield of 0.36 L/g
VS. Using this value, it was possible to estimate the
conversion efficiencies of tested materials, which ranged
from 50 – 83%. In general, conversion of peanut and
rice residues exceeded 75% and was higher than that of
other residues tested. Some plant components (e.g.
lignin) are not biodegradable under anaerobic conditions
( Chynoweth and Pullammanappallil 1996).
Kinetic constants (Table 5) obtained from the logarithmic
plots of the BMP data (e.g. in Figure 5) varied by about
2-fold. These data can provide an estimate of the
potential influence on the kinetics of conversion for a
blend of feedstocks and ultimately an estimate of the
reactor size and operating conditions. In general peanut
and rice residues exhibited more rapid conversion
kinetics that other residues and paper types.
LABORATORY DIGESTER STUDIES – The laboratory
digester design, construction, and modification were
completed and one startup and two shakedown runs
(Runs 1 and 2) were conducted using wheat stem
residues. Run 3 was completed with a feedstock blend
consisting of rice residue, paper, and dog food. Chronic
mechanical problems related to leachate pumping and
gas collection required frequent redesign of the system
during start-up and Run 1, but no similar problems were
encountered in Runs 2 and 3. A reliable design was
finally developed which performed well without leaking,
clogging, and pump failure. Data from the three poststartup runs are shown in Figures 6 – 10 and Tables 5
and 6.
Runs 1 and 2, which received wheat stem residues only,
exhibited similar performance. The calculated methane
yields for these two runs were 93 and 96%, respectively,
of the ultimate yields observed in the BMP assay and the
reduction in organic matter (volatile solids reduction) was
70 and 77%, respectively. Both runs had final biogas
methane contents of ~60%.; the balance (~40%) was
carbon dioxide. Conversion was more or less complete
after 25 days.
The volatile organic acids (VOA)
concentration in the re-circulating leachate increased
during the first 6 days of these runs, but then decreased
by the end of the runs to <100 mg/L. VOA levels of <500
mg/L are indicative of stable performance (Chynoweth
and Pullammanappallil 1996). The principal volatile
acids formed were acetic and propionic (Figure 9). The
pH dropped slightly in both runs, corresponding to the
transient accumulation of volatile acids, but then
increased as the VOAs were converted to methane.
During the first few days, VOAs are conveyed by
leachate recirculation through the leachate reservoir and
then into the neighboring active reactor. This removes
the VOAs from the site of formation and facilitates their
conversion to methane and carbon dioxide.
Run 3 was conducted with a blend of feedstocks, which
included rice residue, shredded paper, and dog food at
particle sizes representative of that anticipated in a
mission-scale system. These feedstocks simulate the
types of materials (crop residue, paper, and feces)
expected during a space mission. Performance of this
run exceeded that of the previous runs which used only
wheat residues (at a much finer particle size) in terms of
methane yield, organic matter (VS) reduction, and
kinetics. The methane yield was 0.30 L/g VS added and
the VS reduction was 85%. Data in Figure 8 indicate that
the conversion was more rapid and was more or less
complete in 15 days compared to 25 days for wheat. As
a consequence of faster kinetics, the accumulation of a
higher concentration of VOAs was observed, but again
the VOAs decreased to low levels by the end of the run.
The principal volatile acids formed were again acetic and
propionic acid (Figure 9).
F eed sto ck C H 4 Yield
W heat S tem s
W heat R oots
0.40
C e llu lo s e
0.35
CH4 Yie ld (L/g VS)
S lu d g e C o n t ro l
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
5
10
15
20
25
30
35
10
15
20
25
30
35
10
15
20
25
30
35
20
25
30
To m a t o
0.4
C e llu lo s e
0.35
S lu d g e C o n t ro l
CH4 Yie ld (L/g VS)
0.3
0.25
0.2
0.15
0.1
0.05
0
0
5
P eanut
0.4
C e llu lo s e
0.35
S lu d g e C o n t ro l
CH4 Yie ld (L/g VS)
0.3
0.25
0.2
0.15
0.1
0.05
0
0
5
S w eet P otato
C e llu lo s e
0.4
S lu d g e C o n t ro l
CH4 Yie ld (L/g VS)
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0
5
10
15
35
T i m e (d a y s)
Figure 5. Example biochemical methane potential runs
Mass balances were conducted for volatile solids,
methane, and carbon dioxide for all three runs (Figure
10). A mass balance for water was conducted for Run 3
only (Figure 11 and Table 6). The mass balance for
volatile solids and biogas resulted in recoveries of 87 to
99%, respectively, where some losses can be attributed
to CO2 and residual VOA, which were dissolved in the
leachate and therefore not included in the mass balance
calculations. For Run 3, 382g (VS) of feed blend
produced 82 g CH4, 202 g CO2, and 57.4 g effluent
solids, representing a 89% recovery. The mass balance
for water in Run 3 gave a recovery of >99% (Table 6).
The waste blend contained 29.6 g of H2O when it was
placed in the reactor, while the digested residue
contained 548 g when it was removed from the reactor.
The difference is an estimate of the amount of process
water consumed (which can come from other process
wastewater), since the leachate drained from the reactor
Table 4. Biochemical methane potential assays of feedstocks
Feedstock
CH4 Yield,
Standard
conversion,
L/g VS added
Deviation
% of cellulose
-1
k, d
Standard
Deviation
This Study
celllulose
control
wheat stems
wheat roots
tomato
peanut
sweet potato
potato
This Study
0.36
0.27
0.18
0.23
0.30
0.24
0.28
0.0150
0.0150
0.0007
0.0097
0.0024
0.0228
75
50
64
83
67
78
0.109
0.112
0.095
0.224
0.175
0.212
0.0036
0.0032
0.0047
0.0164
0.0089
0.0095
rice-Italica
rice-Koshi
rice-L204
rice-Labelle
rice-M103
rice-M202
rice-N22
rice-S102
Owens and
Chynoweth
(1993)
office paper
0.30
0.28
0.29
0.27
0.30
0.30
0.29
0.28
0.0042
0.0006
0.0010
0.0030
0.0053
0.0010
0.0219
0.0007
83
78
81
75
83
83
81
78
0.224
0.171
0.201
0.173
0.220
0.208
0.189
0.205
0.0196
0.0059
0.0125
0.0066
0.0170
0.0093
0.0057
0.0051
0.37
0.140
food board
0.34
0.120
wax paper
0.34
0.083
magazine
paper
news paper
0.20
0.112
0.10
0.069
Chynoweth et
al. (1993)
grasses
woods
seaweeds
0.16 - 039
0.014 – 0.32
0.26 – 0.4
vegetable oil
0.94
primary sludge
0.59
food wastes
0.54
MSW
0.22
at the end of this run is used in the following run. The
process water consumption measures was 518 g H2O
required for 383 g VS of waste processed. In previous
work with this system (Chynoweth et al. 1992) the
kinetics of degradation improved over the first 3-5 runs,
as the population of organisms increases and adapts to
the feedstock material (Chynoweth et al. 1991), so these
results are promising.
DISCUSSION
Research presented here is supportive of the suitability
of high-solids leachbed anaerobic digestion for
bioregenerative reduction and stabilization of the organic
components of solid wastes during extended planetary
space missions. It was shown that the volume of
representative feedstocks (on a dry weight basis) can be
reduced significantly (2-4 fold or more) by saturation with
0.13 – 0.16
water followed by compaction. Water for this purpose
may be obtained from process recycle water or the
station wastewater pool; fresh water is not required. This
significantly reduces the reactor size (volume) and mass
required for conversion.
This work has enabled
reduction of the reactor system size from the originally
estimated 3.2 m3 (Verotsko et al. 2001) to ~1 m 3.
Compaction to higher densities (~300 gk/m 3) is a major
parameter influencing the size of this system and giving it
an advantage over aerobic composting.
Further
verification of compaction
Biochemical methane potential assays indicated that
expected feedstocks vary in their biodegradability. Since
inedible crop residues represent the largest fraction of
solid wastes during extended missions, it may
Table 5. Summary of current data from flooded operation of laboratory SEBAC reactors
Units
Run 1
Input solids
Type
milled wheat stems
Size
cm
<1
Initial feedstock weight
g
500
Total weight
g
500
TS
%
92.6
VS
% of TS
91.7
TS
g
463
VS
g
424
Reactor volume
L
5.91
Basket volume
L
4.94
Bulk density
g TS/L
93.7
Output solids
Wet weight
g
1350
TS
%
10.1
VS
%VS
88.7
Weight VS out
g VS
121
Conversion data
TS reduction
%
70.5
VS reduction
%
71.4
Volume Reduction
%
na
Methane yield
dry L@STP/g VS
0.26
Weight of Methane
Produced
g VS
79
% of the methane yield
measured by BMP assay
%
96.3
Carbon Dioxide Yield
dry L@STP/g VS
0.27
Weight of Carbon Dioxide
Produced
g VS
222
Final biogas Methane
% CH4
59.0
Final biogas Carbon
Dioxide
%CO2
41.8
Maximum CH4 production
rate
L CH4/L reactors/d
0.54
Maximum VOA in leachate
Final VOA in leachate
Minimum pH
Final pH
Temperature
mg/L
mg/L
pH units
pH units
o
C
3580
24
7.11
7.47
35
Run 2
Run 3
milled wheat stems
<1
500
500
92.6
91.7
463
424
5.91
4.94
93.7
Rice, Paper, Dog food
5-7, 2, 1
241, 159, 38
437
91.9, 95.4, 92.4
95.1, 92.7, 94.8
407
382
5.91
4.94
82.4
1130
9.3
82.1
86.3
630
13.0
70.1
57.4
77.3
79.7
na
0.25
80.2
85
86.4
0.30
76
82
93.0
0.25
na
0.27
208
60.4
202
59.4
34.9
32
0.51
1.02
4270
91.9
7.13
7.77
35
7860
13.7
6.79
7.8
35
Methane Yield Run 3
Wheat Stems / Mixed Blend
90
Methane Yield
Percent Methane
Percent CO2
0.3
Methane Yield Run 2
Wheat Stems
80
0.35
80
Methane Yield
Percent Methane
Percent CO2
0.3
0.25
0.2
50
0.15
40
30
0.1
20
0.05
Methane Yield (L/g VS)
60
% CH4 %CO2
Methane Yield (L/g VS)
70
70
60
0.25
50
0.2
40
0.15
30
0.1
% CH4 %CO2
0.35
20
10
0.05
0
10
0
0
5
10
15
20
25
0
Time (days)
0
0
5
10
15
20
25
Time (days)
Total VOA for Run 3
New Reactor (Rice/Paper/Dog food)
Total VOA for Run 2
(Wheat Stems)
8000
8
8000
7.8
7000
7.6
7000
7.4
6000
7.2
5000
8
7.8
7.6
7.4
pH
4000
7
6.8
3000
7.2
Total VOA
4000
7
pH
6.8
3000
6.6
6.6
2000
2000
6.4
1000
6.4
1000
6.2
0
6
0
5
10
15
20
pH
Total VOA
VOA (mg/L)
5000
pH
VOA (mg/L)
6000
6.2
0
25
6
0
5
10
Time (days)
15
20
25
Time (days)
Figure 6. Performance data on laboratory digester
on wheat stems (Run 1)
Figure 7. Performance data on laboratory digester
on wheat stems (Run 2)
Table 6. Summary of water balance data for run 3
Units
Initial Weight of waste
TS Rice
TS Paper
TS Dog Food
Water in Rice
Water in paper
Water in Dog Food
Water in waste at
beginning
Weight of full Reactor
Weight of Reactor
with no waste
Water added
Water in digested
biomass at finish
Water drained from
reactor ar finish
Total Water in
Total Water out
%
%
%
g
g
g
Run 3
436.6
91.9
95.4
92.4
19.5
7.33
2.8
mL
29.6
g
9389.4
g
mL
3709
5680.4
mL
547.9
mL
mL
mL
5136.4
5710
5684.3
VOA Analysis for Run 1
VOA (mg/L)
4000
Methane Yield Run 3
Wheat Stems / Mixed Blend
0.35
Propionate
3000
Isobutyrate and Butyrate
Isovalerate and Valerate
2500
2000
1500
90
1000
Methane Yield
Percent Methane
Percent CO2
0.3
80
500
70
0
0.25
0
5
10
60
0.2
50
0.15
40
15
Acetate
30
3500
Propionate
20
3000
Isobutyrate and Butyrate
VOA (mg/L)
10
0
0
0
5
10
15
20
25
Time (days)
Isovalerate and Valerate
2500
2000
1500
1000
500
Total VOA for Run 3
New Reactor (Rice/Paper/Dog food)
0
0
8000
5
10
8
15
4000
Acetate
7.4
Total VOA
3500
Propionate
3000
Isobutyrate and Butyrate
7.2
6.8
3000
6.6
2000
VOA (mg/L)
7
pH
pH
4000
25
VOA Analysis for Run 3
New Reactor (Rice/Paper/Dog food)
7.6
6000
5000
20
Time (date)
7.8
7000
VOA (mg/L)
25
VOA Analysis for Run 2
4000
0.1
0.05
20
Time (date)
% CH4 %CO2
Methane Yield (L/g VS)
Acetate
3500
Isovalerate and Valerate
2500
2000
1500
6.4
1000
6.2
0
6
0
5
10
15
20
25
1000
500
0
0
Time (days)
5
10
15
20
25
Time (date)
Figure 8. Performance data on laboratory digester
on rice, paper, and dog food (Run 3)
Figure 9. Volatile organic acid analysis for runs 1, 2, and 3
be desirable to give preference to crops (or development
Mass Balance of Runs 1, 2, and 3
g CO2
450
g CH4
400
g VS
350
Grams
300
250
200
150
100
50
0
Initial VS Run 1 Final VS Run1 Initial VS Run 2 Final VS Run 2 Initial VS Run 3 Final VS Run 3
Figure 10. Mass Balances for Runs 1, 2, and 3
of varieties) that have highly biodegradable residues.
With respect to other solid wastes (e.g. packaging,
filters, etc.), biodegradability should be given emphasis in
materials selection. Rates of biodegradation determined
by this method are also relevant as they directly influence
the conversion kinetics of feed blends and the reactor
volume and weight requirements.
BMPs provide a
simple but valuable method for comparing and screening
several different feedstocks for methane yield and
conversion efficiency and kinetics under standard ideal
conditions for aerobic digestion. Actual performance in a
digester is dependent upon design and operating
conditions such as residence time and temperature
The flooded, no-headspace reactor design appears to be
working well. This design can be easily adapted to hypoand micro-gravity conditions.
The flooded design
permits leachate recycle through leachbeds without
dependence upon gravity. It also permits use of pump
pressure to encourage leachate to pass through high
density beds with limited hydraulic conductivity. The
current reactor design for anaerobic digestion has been
performing without problems for several months. This
design requires an external vessel for gas liquid
separation. Under hypogravity conditions, gas would
separate from the leachate by gravity.
Under
microgravity conditions, a gas-liquid separation process
(e.g., centrifugal) could be employed.
Performance of the HSLAD system has exceeded
expectations. Conversion efficiencies of 75% and 85%
have been obtained at residence times ranging from 1525 days, for wheat and a blend of rice residue, paper,
and dog food. Performance has been stable without
requirement for pH control. In the three runs reported,
volatile organic acids accumulated to high values, in one
case exceeding 8,000 mg/L. Although the process
proceeded without detectable inhibition, process kinetics
might be improved by increasing leachate recycle rates
to reduce VOA accumulation.
This work is preliminary and should proceed to address
the following issues:
1. Additional feedstocks need to be analyzed effects of
water saturation on compressibility, hydraulic and
conductivity.
2. Biochemical methane potential assays should be
conducted of prospective crop residues, food
wastes, and proposed materials for packaging,
clothing, etc.
3. Balances for N, P, K, and other nutrients need to be
determined, with identification of their concentrations
in feeds and effluent liquid, solid, and gas streams.
4. Plant growth potential and phytotoxicity studies
should be conducted on digester effluent solid and
liquid streams.
5. Numerous pre- and post-treatment options need to
be evaluated, including feed shredding, effluent
dewatering, and aerobic post-treatment of solids.
6. Pathogen reduction during this process should be
assessed.
7. Systems analysis and its integration with other space
station operations. A companion paper at this
conference (Xu et al. 2002) is presenting a
preliminary systems analysis of this anaerobic
digestion option.
ACKNOWLEDGMENTS
This research was funded by the NASA/University of
Florida Environment Systems Commercial Space
Technology Center
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DEFINITIONS, ACRONYMS, ABBREVIATIONS
BMP
biochemical methane potential
d
day
DO
dissolved oxygen
dw
dry weight
PVC
polyvinyl chloride
SEBAC
sequential batch anaerobic composting
TS
total solids
TC
thermal conductivity
VOA
volatile organic acids
VS
volatile solids (ash free dry weight)
GC
gas chromatograph
HSLBAD
high solids leachbed anaerobic digestion
ISS
International Space Station
FID
flame ionization detector