2002-01-2521
Anaerobic Digestion for Reduction and
Stabilization of Organic Solid Waste During
Space Missions: Systems Analysis
Qiyong Xu and Tim Townsend
Environmental Engineering &. Science University of Florida
David Chynoweth, Patrick Haley, John Owens, and Elana Rich
Agricultural and Biological. Engineering University of Florida
Sabrina Maxwell
Boeing
Hong-Lim Choi
Animal Science and Technology Seoul National University
safe treatment of the wastes produced by the
crew. The waste stream not only can be
deleterious to the crew because of the
existence of pathogens or the concentration
of toxic substances, but also includes many
resources (water, nutrients, etc.) vital to the
life support of the crew. So the purpose of
treating wastes includes the reduction of
waste mass, volume, odor, and toxic
materials and the regeneration of inorganic
nutrients. Because of the resupply
constraints in long duration missions, waste
treatment and recycle become a critical
component to future long duration space
missions.
ABSTRACT
High Solids Leachbed Anaerobic Digestion
(HSLAD) is a biological waste treatment
system that has been successfully
demonstrated for solid waste treatment in
terrestrial applications. The process involves
a solid phase leach bed fermentation,
employing leachate recycle between new
and mature reactors for inoculation, wetting,
and removal of volatile organic acids during
startup. HSLAD also offers a potential option
for treatment of biodegradable wastes on
long-duration space missions and for
permanent planetary bases. This process
would produce 1.5 kg of methane, 4.1 kg of
carbon dioxide and 1.9 kg of compost from
7.5 kg of biodegradable solid wastes
generated daily from a crew of six. HSLAD
can operate at low temperature and pressure
and has the potential for being a net energy
producer. A detailed analysis of this process
was conducted to design the system size
required for a space mission with a 6-person
crew. The mass, energy and water balance
of the process and an equivalent system
mass (ESM) analyses are presented.
In general, solid waste treatment includes
collection, size reduction, conversion, and
post-treatment. Processing technologies can
be further
divided
into
pre- and
post-processing (PPP), and physicochemical
(PC) and biological primary (BC) processing
[14].
PPP processes are used to separate and
size-reduce waste, reduce volume, prepare
for primary processing systems and further
refine products. Some PC processes,
including
incineration,
electrochemical
oxidation, thermal destruction and pyrolysis,
can deal with most solid waste stream
components and can accommodate very
high feed rates, converting the wastes
INTRODUCTION
One of the prerequisites to a successful long
duration space mission is the efficient and
1
Reactor
Reactor
Stage 2
New
Mature
Reactor
Reactor
Stage 1
Reservoir
Activated
Reservoir
Filling
Aerobic
Reactor
Stage 3
Pump C
Pump A
Pretreatment
Pump B
Anaerobic Digestion
Post-treatment
Figure 1. Schematic of HSLAD System
almost entirely to end products [1]. However,
HIGH SOLIDS LEACHBED
PC techniques often require significant
ANAEROBIC DIGESTION (HSLAD)
power and heat rejection capabilities. Thus,
as has been pointed out by numerous
PROCESS DESCRIPTION
authors, PC is expected to be more suitable
for intermediate durations (up to several
years)[7], but not necessarily for long
Like other composting methods, HSLAD is a
duration space missions (of the order of
solid state microbial waste treatment process
decades) where nutrients contained in waste
that yields a stabilized organic residue and
need to be recycled.
recycles nutrients. Compared with other
biological
technologies,
HSLAD
has
In contrast, biological processes (BC) can
advantages, including simple operation, low
efficiently recapture valuable nutrients
energy requirements, low temperature and
contained in solid waste while providing a
pressure working conditions, and is a
number of secondary functions such as
potential energy producer. [2]
oxygen
production,
carbon
dioxide
absorption and water purification. However,
HSLAD involves three main phases:
biological processes require high mass and
pretreatment, anaerobic digestion, and
volume investment to maintain adequate
post-treatment.
growth conditions for microorganisms and
plants. Also, retention times for biological
Pretreatment
systems are typically longer than PC
systems. These characteristics make
To improve treatment efficiency of HSLAD, it
biological processes most appropriate for
is necessary to pretreat the solid waste
longer missions [7].
generated by a 6-person crew. The
biodegradable solid wastes, such as inedible
This work is part of a project consisting of
plant biomass and paper, are collected,
three parts: laboratory scale feasibility,
coarsely shredded to 2-5 cm, and then
system analysis, and prototype digester
compacted to 300 kg-ash free dw/m3.
design and optimization. The laboratory
scale feasibility work focusing on digester
Anaerobic Digestion
design and optimization is presented in a
companion paper by Chynoweth et al.
(2002). This paper addresses the current
As shown in Figure 1, the AD process
design of the anaerobic digestion system,
involves three stages of digestion that occur
the integration with other station operations,
sequentially as conversion proceeds. The
and calculation of mass balance and
waste does not get removed, but passes
equivalent system mass.
through the different stages. In Stage 1, after
the shredded waste is placed into the new
reactor, leachate will be recirculated
2
between the mature reactor and the new
reactor, providing nutrients and bacteria from
the mature reactor to the new reactor and
removing volatile organic acids from the new
reactor. Fermentation products, such as
volatile acids formed during start-up, are
removed to the old reactor where they are
converted to methane. In Stage 2, the
reactor is activated and leachate is recycled
to itself. In stage 3, the reactor is recycled
with a new reactor for startup. The residence
time of all three anaerobic digestion stages is
15 days [15].
Future long-duration space missions could
require crews to go beyond Earth-orbit for
periods on the order of two to three years or
more. Whether wastes generated by the
crew can be treated safely and recycled
efficiently or not is one of the limiting factors
for long-duration space missions. The
HSLAD
process
may
provide
bioregenerative solutions to other technical
challenges of space missions including water
reclamation, reformation of hydrogen and
carbon, and air revitalization.
APPLIED WASTE STREAM
Post-treatment
Wastes produced during space missions can
be classified into crew waste, life support
system waste, and payload waste [14]. Crew
waste includes metabolic waste and related
materials such as packaging, food
containers, and wipes for housekeeping and
personal hygiene, and trash. Life support
system waste is waste generated by the
Environmental Control and Life Support
System (ECLSS) itself, and payload waste
are any waste generated specific to a
payload, such as animal metabolic wastes
and plant residues. Table 1 shows the
estimated results of a model for daily solid
waste streams generated for a 6-person
crew in Mars exploration mission [17]. As
shown in Figure 2, the most significant
components of wastes are inedible plant
waste, packing materials and paper. Other
solid wastes may be expected from both the
air and water processing operation.
After anaerobic digestion is complete, the
remain solid residues which have been
separated, are aerated for 1 day to remove
lingering reduced compounds (NH3, H2S and
remaining VOA) and dewatered to 30-35%
moisture. During this aerobic step, the
compost may be heated to 70oC for one hour
to ensure removal of pathogens.
In practice, HSLAD is a very stable waste
management system, which has been
proven by successful demonstration on a
variety of high-solids feedstocks, including
woody biomass, the organic fraction of
municipal solid waste, and yard waste. [3].
The conversion efficiency is a function of the
biodegradability of the feed components,
ranging from 50-90% and the organic matter
is converted to methane, carbon dioxide, and
compost with a residence time of less than
15 days. The process is resilient and can
start up rapidly after being dormant for
Table 1. Estimates of daily solid waste streams for a 6-person crew during a 600-day
exploratory mission (Adapted from Verostko et al. 2001)
Organic
Dry Wt.,
Percent of
Ash, %
Waste Component
Matter, dry
Moisture, %
Kg
total
dw
kg
Dry human waste
0.72
9.4
5
0.68
85
Inedible plant biomass
5.45
51.4
5
5.2
75
Trash
0.56
5.3
5
0.53
10
Paper
1.16
10.9
5
1.1
10
Packaging materials
2.02
19.0
Tape
0.25
2.4
Filters
0.33
3.1
Misc.
0.07
0.7
Total
10.6
100
30
7.5
61
several months.
3
Filters 3.1%
Misc. 0.7%
Dry Human
Waste 6.8%
Tape 2.4%
Paper 11.0%
Packaging
Materials 19.1%
Inedible Plant
Biomass 51.6%
Trash 5.3%
Figure 2. Space Mission Waste Composition Dry Wt. Basis
Assuming the reactor is rectangular tank,
with a height to side ratio of 2 and it is
necessary to add an additional 25% to the
height for leachate distribution and
collection, the practical dimension of the
reactor can be calculated as follows:
SIZING OF HSLAD
REACTOR VOLUME
For a typical 15-day anaerobic digestion
cycle (5 days for one anaerobic stage), each
reactor contains 5-days worth of solid waste.
So, the amount of ash free waste in each
reactor is:
V  L2  H ………………………….………(3)
7.5 kg waste/day  5 days = 37.5 kg waste
………………………..………………(1)
H  H ' / 1.25 …………..……………………(4)
H ' / L  2 ………………………..…………(5)
After compaction, the density of the
biodegradable waste is 300 kg (ash free
dw)/m3.
If the sides of the reactors are L= 0.43 m, the
height of the reactors is H’ = 0.86 m, the
waste height is H = 0.69 m, then the practical
reactor volume is VP = 0.16 m3
Reactor volume (V) needed for wastes is:
V
37 .5 kg
300 kg / m 3
 0.125 m3…………………(2)
Table 2. Design Parameters of HSLAD for Space Mission (6-person crew)
Total
Side
Volume
Pressure
Pump
Height (m)
Leachate
length (m)
(m3)
(kPa)
Energy (kJ)
3
Volume (m )
Reactor
0.86
0.43
0.16
7
Water
Reservoir
0.86
0.54
0.25
7
Pump A
145
101.5
Pump B
145
101.5
Pump C
145
101.5
4
According to Darcy’s Law, the hydraulic head
of leachate (h) can be calculated as follows:
WATER RESERVOIR VOLUME
As shown in Figure 3, two water reservoirs
are used in the HSLAD system, which mainly
provide the necessary leachate equal to
amount to optimal moisture content of 70%.
In addition, the reservoirs can serve as gas
separators and replace water lost due to
evaporation and due to removal of entrapped
biogas bubbles in solid leach-bed.
q  K i  K 
hH
H
……………………..(9)
Q Q
q  2
A L ……………………….………(10)
h = 14.8 m
After the dry waste is compacted to 300 kg
ash free dry wt/m3, some water and leachate
must be added to achieve a wet density of
approximately 1000 kg/m3.
This is the hydraulic head of water and can
be converted to pressure:
For one reactor, the total amount of water
required will be the sum of required water
and the head space water:
So, 145 kPa (21 psi) pressure should be
provided by pump. And the energy required
by pump can be calculated as follows:
0.125 m 3  (1000  300 ) kg / m 3  (0.16  0.125 )
ET  P  Q  101 .5 kJ…………..……..….(12)
P   water  g  h  145 kPa ………...……...(11)
 0.123 kg  0.123 m 3 H 2 O
And the results of pump design are listed in
table 2.
……………………………………..………(6)
MASS AND WATER BALANCE
ANALYSIS
There are two reservoirs in HSLAD system
and it is assumed that 25% volume of each
reservoir is always filled with leachate. So
the total amount of water needed is:
As mentioned before, a 6-person crew would
generate about 10.6 kg-dw/d, including 7.5
kg ash free organic matter. The organic
matter can be biodegradated by bacteria into
methane, carbon dioxide, compost and other
trace biogases. Biochemical reactions
occurring in anaerobic composting include:
0.123m 3  3  2  0.25Vreservoir  2Vreservoir …(7)
The volume of reservoir is 0.25 m 3.
Assuming reservoir has the same height as
the reactor, 0.86m. the side length of
reservoir is 0.54 m.
C 6 H 12O6  3CH 4  3CO2 …………….(13)
PUMP DESIGN
Protein  CH 4  CO2  H 2 S  NH 3 ….(14)
To recirculate the leachate from the reservoir
to the reactor, a pump must be used to
provide energy. As calculated above, for one
reactor, the required leachate volume is
0.0875 m3. Based on the operational
experience, it is assumed that the total
leachate recirculation flow rate is 8 times the
required leachate volume per day, (0.0875
m3). However, the pumps operate only 30
minutes every 2 hours. So the average
leachate recirculation flow rate is:
Q  8  0.0875
Fats  CH 4  CO2 …………………….(15)
The conversion of organic matter in the
HSLAD systems is a function of feedstock,
ranging from 50-90%. Based on the
composition of solid wastes generated by a
6-person crew, it is assumed that about 75%
solid wastes can be biodegraded. According
to the mass balance law, the solid waste
balanced reaction can be written as follows:
m3
24
min
 (  30 )
day
2
day ………..(8)
 1.9 10 3 m 3 / min
5
Table 3. Mass and Energy Balance of HSLAD System
Balance
Input
Output
Mass(kg)
7.5 kg biodegradable waste
1.5 kg methane, 4.1 kg carbon dioxide
and 1.9 kg compost
Energy(kJ)
3.18104
5.8104
7.5 kg biodegradable solids  1.5 kg CH 4
ETotal  E heat  E pump  3.18 10 4 kJ …….(20)
(1.3 m 3 )  4.1kg CO2 (1.3 m 3 )  1.9 kg compost
……………………....(16)
Energy Potential
Moisture content plays an important role in
HSLAD operation and the optimal moisture
content is about 70%. After pretreatment, the
moisture content of solid waste is lower than
70%. So, it is necessary to add extra water
for HSLAD operation.
Biogas produced from the HSLAD process is
similar to “ natural” gas except for dilution by
CO2. The biogas has a lower calorific value
than natural gas, approximately 22,248
kJ/cubic meter for biogas versus 40,789
kJ/cubic meter of natural gas. According to
(16), the volume of biogas produced by the
solid waste is 1.3+1.3 =2.6 m 3. So, the
energy containing in the biogas can be
calculated as follows:
As mentioned above, the total amount of
water needed to operate the HSLAD system
is the total volume of the reservoirs, 0.5 m 3.
The only water consumption in the process is
loss via evaporation in the biogas (very small
amount) and the water that leaves in the final
compost. Because 75% solid waste can be
degraded in HSLAD system and the
moisture content of the final compost is 70%,
the amount water that should be added to
compensate water consumption is:
(1.3  1.3) m 3  22248 kJ/m3  5.8 10 4 kJ .per
day………………………………………..(21)
If additional energy conversion equipment
and enough oxygen can be provided, the
process energy requirements could be offset
by the fuel value of the biogas.
7.5  (1  75%)  70%  1.3 kg / day ………(17)
As shown in Table 3, the HSLAD process
would produce 1.5 kg of methane, 4.1 kg of
carbon dioxide, and 1.9 kg of compost daily
from 7.5 kg of biodegradable solid waste
generated daily from a crew of six. According
to the energy balance analysis, the energy
potential of the biogas produced by solid
waste is greater than the energy requirement
of HSLAD. In other words, the HSLAD
system would be used as energy producer if
additional energy conversion equipment and
enough oxygen are provided.
ENERGY BALANCE ANALYSIS
Although electrical energy will be required to
operate the pump and heat systems, the
HSLAD process has the potential for being a
net energy producer.
Energy Requirement
Because the anaerobic reactors are
self-heating systems and can keep
themselves at 350C without significant
additional heat input, the main energy
consumption is pump operation and water
heating from 200C to 350C
INTEGRATION POTENTIAL
ANALYSIS
WASTEWATER RECLAMATION
Eheat  m   w  T  3.1510 4 kJ ………..(18)
One of the potential advantages of HSLAD is
that it can pretreat wastewater through
reducing Biochemical Oxygen Demand
(BOD), volatile organics, and dissolved and
suspended solids in wastewater. For any
E pump  3  101 .5  10 4  305 kJ …………..(19)
6
biological system, moisture plays a vital role
in growth, metabolism, solute transport, and
other functions. Moisture content can range
from 55 to 70% for a solid-phase system
such as composting, and approximately 98%
for an aqueous system. The optimal moisture
content for the HSLAD operation is 70%;
however,
after
the
shredding
and
compaction process in pretreatment, the
moisture content of solid waste is less than
70%. The wastewater from clothes washing
and dish washing would be used as the
makeup water needed for the HSLAD
conversion of solid waste. As calculated
above, 1.3 kg waste water must be added
per day for HSLAD operation.
biomethanogenesis reaction and can be
accomplished in the same reactor used for
the proposed anaerobic stages of solid
waste conversion. H2 consumption could be
used for intermittent fine control of LSS
gases and CH2 could be disposed to space
to eliminate C without the loss of O.
BIOFILTRATION OF AMBIENT AIR
In the post-treatment of the HSLAD system,
the remaining solids are treated with ambient
air to oxidize reduced residues and control
pathogens in the compost. A possible use of
compost is to absorb air contaminants, such
as VOCs and NH3, because compost
contains organic material and has a large
surface area where microorganisms can
attach and grow. When the ambient air
passes through the compost, the compost
can absorb particles and chemicals in
ambient air and the microbial population in
compost can degrade a wide array of VOCs
Additionally, NH3 can be captured and
converted to nitrate through nitrification for
return to a plant growth system [11].
REFORMATION OF CARBON DIOXIDE
AND HYDROGEN
A closed Advanced Life Support (ALS)
system must include a CO2 removal
subsystem to reduce carbon dioxide
produced from crew respiration and
combustion. Since the oxygen supply from
plants is relative limited especially in the
space mission with low food closure, water
could be electrolyzed to provide oxygen, but
hydrogen would be produced as a byproduct.
So it is desirable to develop a subsystem to
reduce both CO2 and H2.
The compost biofilter can handle a wide
range of loading rates with low mass, water,
power and maintenance requirements and
simultaneously remove many trace air
contaminants.
HSLAD can provide a bioregenerative
alternative for hydrogen oxidation using
carbon dioxide as the electron acceptor.
PLANT GROWTH SUBSTRATE
CO2  4H 2  CH 4  2H 2 O …………….(22)
Another potential use of HSLAD compost is
as a nutrient-rich plant growth medium.
Compost contains substantial amounts of
inorganic nutrients vital for plant growth.
The
reaction
is
a
well-known
CO2
Reduction
Subsystem
Biogas
Processing
Subsystem
Wastewater
Treatment
Subsyste
Air
Cleaning
Subsystem
2
m
H
S
Waste
Biodegradable Size
Collection
Reduction
Material
Subsystem
Macerate
Leachate
Recirculation
Pretreatment
Wastewater
L
Anaerobic
Digestion
D
Aeration
Post-treatment
Ambient Air
CO +H
2
A
2
7
Figure 3. Schemic of HSLAD with other subsystems
Plant Growth
System
Plants themselves can effectively extract
nutrients that remain in the compost.
Additionally, for some nutrients that are
poorly extracted during short-term aqueous
extraction, it is possible from them to be
recovered over the long term through direct
extraction by the plant root system. [11].
parameters of HSLAD must be determined
MASS
Because of practical and budget limitations,
the mass of the full-scale test unit under
construction is expected to be around 1000
lb. However, an actual system for space
applications is anticipated to have a mass of
less than 400 pounds (181 kg) by taking
advantage of lighter, durable materials.
EQUIVALENT SYSTEM MASS (ESM)
CALCULATION OF HSLAD
Equivalent System Mass (ESM) is a
technique by which several physical
quantities that describe a system or
subsystem may be reduced to a single
physical parameter—mass [9]. In 1999, ESM
was selected as the basis of the NASA
Advanced Life Support (ALS) Project
Research and Technology Development
Metric. The advantage of ESM analysis is
that it allows the comparison of two life
support systems with different parameters
using a single scale.
VOLUME
The arrangement of five reactors and two
water reservoirs is shown in Figure 5. So, the
volume of HSLAD (V)
follows:
can be calculated as
V  2 11  2 m 3 …………………………(23)
POWER AND COOLING
As stated in the energy balance analysis, the
energy requirements of HSLAD is 3.18104
kJ. The power requirement is:
The computation of ESM depends on the
mission being considered. In the ALS Metric,
three missions are baselined; namely,
low-earth orbit (LEO), Mars Transit, and
Martian surface. The HSLAD would be
considered the Mars Surface mission. To
calculate the ESM of HSLAD, the operation
0.43 0.43 0.43 0.43
In general, the cooling requirement is 8 times
as the power requirement. So the cooling
requirement is 2.9 kW.
3.18 10 4 kJ / day
 0.37 kW ……………(24)
86400 sec/ day
CREW TIME
Based on practical operational
it
0.43
0.54 experience,
0.54
0.43
Reservoir
0.86
Reactor
1.00
0.54
1.00
0.43
ESM may be used to objectively evaluate
different systems based on their mass,
volume, power, cooling, crew time and
resupply requirements. The technology with
the lowest ESM value is the most cost
effective option for the mission being
considered, provided the options have the
same function reliability [13].
Reservoir
Reactor
1.00
2.00
(a) Plain view
(b) Elevation view
Figure 4 Arrangement of Reactors and Reservoirs
8
Table 4. ESM of HSLAD
Parameter of HSLAD
Cost Factors for Mars Surface
ESM (kg)
181 kg
1 kg/kg
181
Volume
2 m3
2.08 kg/m3
4.16
Power
0.37 kW
86.9 kg/kW
32
Cooling
2.9kW
66.7 kg/kW
193
0.417 hr/person-wk
4923 kg( hr/person-wk)
2053
0 kg
1 kg/kg
0
Mass
Crew Time
Logistics
Sum
2463
is assumed that the crew time for operating
the HSLAD system is 10 min per day for
regular operation, 2 hours per month for
inspection and maintenance, and 2 days per
year for parts replacement.
RESEARCH NEEDS
Evaluation of HSLAD for reduction and
stabilization of waste during space missions
is at the initial stage. Some areas that need
to be researched to refine this analysis
include:
So, crew time = 0.417 hr/person-wk.
The calculation of ESM for HSLAD is shown
in Table 4.

Designing
hypo-gravity
In Table 5, ESM assessments were made for
Mars Planetary Missions. These calculations
only represent the crew time independent
costs as insufficient data exists on the
technologies. As a result, the HSLAD system
has a relatively low ESM value, which
substantiates that HSLAD is one technology
that should be considered for further R&TD.

Testing proposed feeds
and
for

Designing for optimization, automation,
and safety controls

Determining the effectiveness
pretreatment of station wastewater

Evaluating the conversion yields and
kinetics of reformatting carbon dioxide
Table 5. ESM comparison of different waste technologies
Mass
Volume Power Cooling
Technology
(kg)
(m3)
(kW)
(kW)
storage
50
1.01
0
0
bulk compaction
50.1
0.81
0.35
0.35
pyrolysis
42.5
0
0.6
0.4
sterilization
85
1.29
0.3
0.3
pyrolysis/scw
6
0
2
0.4
batch incineration
220.2
0.64
0.38
2.01
drying
132.5
1.02
2
2
composting,7days
401.5
2.03
0.09
0.8
dry size reduction
135
4.21
2
2
scwo
633.2
0.36
0.73
3.81
lyophilization
342.1
0.74
0.95
7.79
composting, 21days
1046.5
4.24
0.1
0.8
continuous incineration
323.2
4.63
6.68
7.81
wet size reduction
223.5
5.15
16
14.4
plasma arc
1170.2
3.97
34.88
38.81
activated carbon production
26.7
0.12
31
110.7
single cell protein
113.5
80.68
80
80
electrochemical oxidation
3330.2
5.17
20.08
700.71
HSLAD
181
2
0.37
2.9
9
operating
ESM of
Mars Surface
71
121
121
157
207
401
461
505
530
959
960
1197
1522
2683
6877
10110
14090
51923
410
for
and hydrogen

Determining nutrient balance
2.

Evaluating finished anaerobic compost
as a medium for biofiltration

Determining the extent and kinetics of
conversion in aerobic post treatment
3.
The results of some of these needs are
presented in another paper at this
conference [18].
4.
CONCLUSIONS
High Solids Leachbed Anaerobic Digestion
(HSLAD) uses a combination of solid phase
fermentation and leachate recycle to provide
a simple, reliable process that inoculates the
new batch, removes volatile organic acids,
and concentrates nutrients. It not only
operates at low temperature and pressure,
but can also transform the biodegradable
waste into resources without production of
any noxious odors or pollution, and has the
potential for being a net energy producer.
5.
6.
7.
HSLAD has a potential ability to integrate
with other subsystems, such as wastewater
treatment and the carbon dioxide reduction
system. With proper integration with other
subsystems, it can effectively reduce the
total equipment mass and improve the
treatment efficiency of wastewater and air
purification.
8.
9.
Compared with other biological waste
processes, HSLAD system has a relative low
ESM value, which indicates to some extent
that HSLAD is a cost effective technology.
10.
ACKNOWLEDGEMENTS
11.
The authors would like to gratefully thank
Hwidong Kim, Belinda Grothpietz, Valerie
Paredeo and Heng Li for their aid in the
research. This work was supported by a
grant from the NASA/UFL Environmental
Systems Commercial Space Technology
Center.
12.
13.
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DEFINITIONS, ACRONYMS,
ABBREVIATIONS
HSLAD: High Solids Leachbed Anaerobic
Digestion
ESM: Equivalent System Mass
PPP: Pre- and Post-Processing
PC: Physicochemical
SEBAC:
Sequential
Batch
Anaerobic
Composting
ECLSS: Environmental Control and Life
Support System
ALS: Advanced Life Support
LEO: Low-Earth Orbit
BOD: Biochemical Oxygen Demand
VP: Practical Reactor Volume (m3)
H’: Height of Reactor (m)
H: Height of Waste (m)
L: Length of Reactor Side (m)
A: Area of Reactor (m2)
Q: Flow (m3/min)
q: Area flow rate (m/min)
i: Gradient
K: Hydraulic Conductivity,4.5103 m/min
h: Hydraulic Head (m)
P: Pressure (Pa)
w: Specific heat of water ,4.3 kJ/kg.0C
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