determined after manual compression. All values are .... paper was shredded using a paper shredder to a 2 cm particle size (Fellows model PS-70). Dog food ...
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 600day 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
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
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
misc. Total
days. The process is very stable, does not require 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.
Stage 2
Pump A
Pretreatment
Reservoir
Activated Reactor
0.7 100
treated 1-2 days with air to oxidize reduced residues, and 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.
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 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%
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
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 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/m3 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
