Increased Recovery of Solid Waste by Improving Scalability for Anaerobic Co-Digestion of Organic Solid Wastes June 2014 P.I. Benjamin T.W. Bocher, Ph.D. Co-P.I. Philip J. Parker, Ph.D., P.E. Contributing Authors: Dylan Friss Trevor Rundhaug Marc Stern Civil & Environmental Engineering University of Wisconsin-Platteville UNIVERSITY OF WISCONSIN SYSTEM SOLID WASTE RESEARCH PROGRAM
25
Embed
UNIVERSITY OF WISCONSIN SYSTEM SOLID WASTE RESEARCH … · SOLID WASTE RESEARCH PROGRAM . 2 Abstract Anaerobic digestion of currently landfilled solid wastes was demonstrated as a
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Increased Recovery of Solid Waste by Improving Scalability for Anaerobic Co-Digestion of Organic Solid Wastes
June 2014
P.I. Benjamin T.W. Bocher, Ph.D. Co-P.I. Philip J. Parker, Ph.D., P.E.
Contributing Authors: Dylan Friss Trevor Rundhaug Marc Stern
Civil & Environmental Engineering
University of Wisconsin-Platteville
UNIVERSITY OF WISCONSIN SYSTEM SOLID WASTE RESEARCH PROGRAM
2
Abstract
Anaerobic digestion of currently landfilled solid wastes was demonstrated as a
technologically viable and environmental advantageous alternative to landfilling three
currently landfilled solid wastes (cafeteria waste, organic fraction of municipal solid waste, and
screenings from a cheese factory).
Anaerobic co-digestion of currently landfilled organic solid wastes removed between 75
and 80% of total solid material. Use of existing municipal digesters could make anaerobic co-
digestion a low-capitol alternative to landfilling. Utilization of excess capacity, as is common,
would directly translate into diversion from landfills, meaning anaerobic digestion has the
potential to divert 80% to 100% of readily biodegradable substrates if biosolids are
landfilled or disposed of via alternative methods, respectively.
Addition of certain substrates may lead to beneficial synergistic effects—degrading
more solids, for example, when two substrates are added compared to the sum of anaerobically
degrading each waste separately. Pilot-scale testing is recommended for wastewater treatment
facilities located near consistent feedstocks of readily biodegradable organic solid wastes that are
currently landfilled.
Anaerobic digestion of organic solid wastes throughout Wisconsin represents low
hanging fruit in the ongoing discussion of how to divert more solid waste from landfills.
Legislative action may be the needed impetus for implementation of this viable treatment
technology.
4
1.0 INTRODUCTION
In 2011, the United States generated about 250 million tons of municipal solid waste
(MSW) and recycled 87 million tons of this material, a 34.7 percent recycling rate. (Municipal
Solid Waste, 2012). For the more than 90 million tons (56 percent) of landfilled MSW that is
organic, a viable alternative to landfilling might be anaerobic digestion. Use of anaerobic
digestion has the potential to decrease total solids in landfills as well as mitigate negative impacts
on the environment.
Supplying the required volume of landfill space is becoming increasingly challenging due
to the difficulties of siting a landfill; this ever-increasing land required by landfills stresses the
development of homes, businesses, and crops (Mercury, 1998). Thus, all diversion is valuable.
Landfilling poses potentially serious problems for human and environmental health.
Although landfill liners are designed to remain impermeable “forever,” eventually, all liners will
fail; when this happens, there are various potential pathways of environmental contamination
(e.g., groundwater) (Schiopu and Gavrilescu, 2010). If landfill gas is not captured, methane
(CH4), a greenhouse gas that is more than twenty times as harmful as CO2, is released untreated
into the atmosphere. While technologies to capture landfill biogas and treat the leachate have
improved, the limitations of mass transfer in landfills cap the extent to which organics can be
converted into these resources. Thus, there is a need for solutions that not only reduce the
negative impacts of landfills on the environment and the health of the general public, but also
better utilize the fraction of solid waste that can be transformed into a valuable resource.
1.1 Anaerobic Digestion Pathway
Anaerobic digestion is one means to convert a greater fraction of this currently landfilled
organic solid waste into valuable products like CH4. Anaerobic digestion is an interdependent
process consisting of four general biological reactions, each carried out by a different guild of
microorganisms: hydrolysis, acidogenesis, acetogenesis, and methanogensis (Figure 1) (Batstone
et al., 2002). Hydrolysis is the enzymatic breakdown of complex polymers into monomers like
glucose and amino acids. As such, this step is crucial to successful digestion of solid wastes. For
example, MSW is broken down from large organic polymers such as proteins, fats and
carbohydrates into amino acids, fatty acids, and simple sugars. After hydrolysis breaks down the
large polymers, acidogenic bacteria further convert the organic matter into volatile fatty acids
(VFA) via a fermentative reaction wherein some carbonic acids, alcohols, ammonia, and reduced
sulfur are also produced (Batstone et al., 2002). The subsequent conversion of VFA into acetate,
5
hydrogen gas (H2), and carbon dioxide (CO2) from the metabolized biomass products of
acidogenesis is termed acetogenesis. A specialized trophic group of Archaea then generates CH4
from either acetate (CH3CH2CHOO-), or H2 and CO2 (Batstone et al., 2002).
Figure 1. Anaerobic Degradation pathway: four general reactions (hydrolysis, acidogenesis, acetogenesis, and methanogenesis) and corresponding microbes (Figure Courtesy of Prince Mathai).
1.2 Statement of Objectives
The aim of this research was to study a novel method of reducing the total mass of solid
waste that is currently disposed of in landfills. This could be accomplished by diverting readily
biodegradable organics into anaerobic digesters where this waste could have additional positive
effects on the environment through the generation of renewable energy in the form of CH4 gas.
Many anaerobic digesters are operated under capacity—sometimes significantly so (Gray et al.,
2008); thus, solid wastes could only be added to the extent that they only utilize currently unused
digester volume (i.e., no reduction in current treatment of municipal biosolids would occur).
This project was designed to allow for the utilization of existing excess capacity in
municipal anaerobic digesters for co-digestion (degradation of two or more substrates in the same
anaerobic digester at the same time) of municipal wastewater with additional industrial wastes or
6
MSW. Hence, the ultimate goal (though beyond the scope of this initial study) of this research is
greater implementation of anaerobic co-digesters treating municipal wastewater and organic solid
wastes, thereby diverting more solid wastes from landfills and increasing CH4 production at
Continuously mixed flow reactors (CMFR) were constructed using acrylic cylinders (4.5
in. I.D., 0.25 in. thick walls, and 12 in. tall) with acrylic disks (5.125 in. diameter, 0.25 in. thick)
used for the tops and bottoms of the reactors (3.1 L total volume) (Figure 2) (Ridout Plastics, San
Diego, CA). Methylene chloride was used to attach the bottom to the cylinder. The top was
connected by cutting a 0.25 in. inset groove and using two-part epoxy to bond the top to the
cylinders. Two 0.5 in. diameter holes were cut in the top where an acrylic gas port and feeding
tube were fixed using methylene chloride. A 3 in. piece of Norprene® tubing was attached to the
feeding tube and clamped off while not in use. Also, a 3 in. piece of Norprene® tubing was
connected to the gas port on one end and, on the other end, to a gas valve that was connected to a
gas bag.
8
Figure 2. Schematic of 2-L CMFRs that were fed solid waste and synthetic municipal wastewater. All measurements have units of inches, including fractions.
Lab-scale experiments were carried out in CMFRs with a 1.5-L working volume and a
15-day hydraulic retention time (HRT). CMFRs were maintained at 35±1 °C and continuously
mixed at 150 rpm in a shaker incubator (New Brunswick, I2500 series, Edison, NJ). Each day,
100 mL of effluent was removed from each CMFR vessel and replaced by 100 mL of “feed”
substrate to supply a feeding rate of 1g TCOD/(LCMFR*d); a nutrient solution (described by
Schauer-Gimenez et al., 2010 except that Na2MoO4.2H2O was used instead of NaMoO4.2H2O)
was added daily with the feed. The control CMFRs (CMFRs #1-3) were fed only a synthetic
municipal wastewater (SMW). As has been done previously, Diamond Adult dog food (990 g/L
TS and 920 g/L VS) was used by blending 10.8 g dog food with tap water for a total volume of
333 mL (32 ± 1 g/L total solids, TS, and 30 ± 0.5 g/L volatile solids, VS) to more accurately
mimic municipal wastewater (Bocher, 2012); 100 mL of this mixture was fed to the three control
CMFRs. The remaining nine CMFRs were co-digesters, as they were fed a combination of SMW
and one of the following organic solid wastes in an 80/20 ratio (based on TCOD): OFMSW
screenings) also exhibited toxicity in a sharp decrease in pH (Figure 4). It is likely that these
changes in SCOD concentration and pH were accompanied by VFA accumulation, which was
caused by the inhibition of methanogensis resulting from oxygen toxicity.
Figure 3. Soluble chemical oxygen demand of CMFRs 5, 6, 8, and 12 as a function of time.
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
28 30 32 34 36 38 40 42 Soluble Ch
emical Oxygen De
man
d, SCO
D (m
g/L)
Time (Days)
Reactor 5
Reactor 6
Reactor 8
Reactor 12
13
Figure 4. pH of CMFRs 8 and 12 as function of time.
3.2 Steady State CMFR Characteristics
The steady state metadata of the CMFRs are reported in
Table 7. The CH4 content was measured for three consecutive days (39 to 42) at steady
state. CH4 production statistically increased in OFMSW and Cafeteria waste as compared to
the control.
Table 7). For CMFRs 1-3 (Synthetic Wastewater), 4 (OFMSW), 9 (Cafeteria), and 11
(Cheese Screenings), respectively, the initial TCOD concentrations (33.6, 43.9, 43.9, and 57.5
g/L TCOD, respectively) (Table 4), decreased to steady state concentrations of 9.6, 9.4, 8.6, and
9.0 g/L TCOD during days 40 to 42.
Table 7; Figure 5). From this data, the TCOD removal was calculated for each substrate.
Table 7). A statistical increase in TCOD removal over the control was calculated in the
cafeteria waste and cheese screenings waste.
Table 7). Steady state SCOD concentrations in the effluent were statistically similar for
all substrates .
6.4 6.5 6.6 6.7 6.8 6.9 7
7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8
0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45
pH
Time (Days)
Reactor 8
Reactor 12
14
Table 7). The VS concentrations in CMFRs 1-3 (SMW), CMFR 4 (OFMSW), CMFR 9
(cafeteria), and CMFR 12 (cheese screenings) all decreased until steady state conditions.
Table 7). The trend to steady state depicted the data asymptotically approaching a steady
state condition near 9.0 g/L (Figure 6). In addition, Figure 6 exhibits how the variability in the
initial VS concentration (Table 4) for each replicate CMFR had little effect on the steady state VS
concentration.
Table 7). The TS results exhibited a similar trend as the VS results, but with higher
steady state concentrations.
Table 7; Figure 7).
Table 7. Steady State Characteristics of CMFRs (a) CH4 Production (b) Solids Concentrations (c) COD results. All values are presented as average ± standard error. (a)
Figure 5. Total chemical oxygen demand (TCOD) as a function of time.
Figure 6. Volatile solids (VS) as a function of time. Note: Because the loading rate was based on TCOD, there was as slight difference in the VS loading rates among the four substrate types. OFMSW is seen below the cafeteria substrate.
8
10
12
14
16
18
20
21 26 31 36 41
Total Che
mical Oxygen De
man
d, COD (g/L)
Time (Days)
OFMSW
Cafeteria
Cheese Screenings
SMW
Loading Rate
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40
Volatile Solids, VS (g/L)
Time (Days)
SMW Loading Rate
SMW (g/L)
OFMSW Loading Rate
OFMSW (g/L)
Cafeteria Loading Rate
Cafeteria (g/L)
Cheese Screenings Loading Rate
16
Figure 7. Total solids as a function of time. Note: Because the loading rate was based on TCOD, there was as slight difference in the TS loading rates among the four substrate types. OFMSW is seen below the cafeteria substrate.
3.3 CMFR Discussion
TS removals ranging from 76 to 80% during continuous operation indicate that anaerobic
co-digestion of readily biodegradable substrates with domestic wastewater could divert up to
80% more biodegradable solid waste from landfills if biosolids are landfilled and 100%
more biodegradable solid waste from landfills if biosolids are disposed of via alternative
methods.
Table 7). Given that co-digestion would use excess capacity in existing anaerobic
digesters, there would be more degradation, more CH4 generation, and less landfilled solid
wastes.
While the data show that the greatest TS removal was in the SMW CMFRs, all of the
solid wastes studied resulted in at least 76% TS removal, and, thus, could be co-digested in the
many municipal digesters have unused capacity, thereby diverting these wastes from landfills. For
example, the East Bay Municipal Utility District in San Francisco demonstrated that addition of
0
20
40
60
80
100
0 10 20 30 40
Total Solids, TS (g/L)
Time (Days)
SMW Loading Rate
SMW (g/L)
OFMSW Loading Rate
OFMSW (g/L)
Cafeteria Loading Rate
Cafeteria (g/L)
Cheese Screenings Loading Rate
Cheese Screening (g/L)
17
OFMSW actually increased the methane yield by 300% (Gary et al., 2008). Furthermore, this
increase was present at 15- and 10-day retention times, which are at or below typical minimum
operating retention times. Therefore, these substrates represent a vast, untapped potential for
diversion of significant quantities of currently landfilled solid wastes AND for production of
valuable, renewable energy.
Another example that affirms the findings of these lab-scale CMFRs is that anaerobic
digestion of MSW has been in use at full-scale in Toronto, ON, Canada for nearly a decade
(Barclay, 2012). The research performed at UW Platteville and described in this report points in
the same direction as the early research that allowed implementation in Toronto (Allen Kani,
2001). Therefore, pilot scale tests and an economic feasibility study would be appropriate at
this time if various waste producers, haulers, and municipalities are interested in pursuing this
work.
CMFRs treating cheese screening substrate and cafeteria substrate demonstrated a
statistically greater TCOD removal (40 and 43%, respectively) compared to the control (36%).
Table 7). This may indicate the possibility of synergistic effects from the addition of
the cheese screenings substrate and cafeteria substrate to the SMW. The percent of CH4 in the biogas was similar in CMFRs treating all four substrates, but,
compared to the control, CH4 production (mL CH4/gVS-d) was 20% greater in the CMFRs
treating cafeteria substrates and 40% greater in the CMFR treating OFMSW.
Table 7). The CH4 production results of cheese substrates show similar data as the
synthetic wastewater.
Table 7). The increases in CH4 production may be indicative of a synergistic
relationship between cafeteria or OFMSW and the SMW in the CMFRs (Table 6).
3.4 ADP Results
ADP assays were conducted using F/M ratios of 1.4, 2.0, and 2.8 g TCOD /g VS,
resulting in three different sets of data for each substrate (Table 8). As expected, blank ADP
assays resulted in negligible ADP values, solids removal and TCOD removal (Table 8),
demonstrating minimal activity in the microbial community from endogenous decay. The
18
heterogeneous nature (i.e., some large particulates were present) of the solid waste substrates was
one source of error in the ADP assay set up.
Table 8. ADP Results for (a) SMW, (b) OFMSW, (c) Cafeteria, and (d) Cheese Screenings (Values are Presented as the Average of Three Physical Replicates ± One Standard Error) (a)
substrate demonstrated a statistical increase in TCOD removal (40 and 43%, respectively)
compared to the control (36%).
Table 7). Compared to the control, CH4 production (mL CH4/gVS-d) was 20% greater in
the CMFRs treating cafeteria substrates and 40% greater in the CMFR treating OFMSW.
Table 7). The increases in CH4 production may be indicative of a synergistic relationship
between cafeteria or OFMSW and the SMW in the CMFRs.
Table 7).
Anaerobic digestion of organic solid wastes throughout Wisconsin represent low
hanging fruit in the ongoing discussion of how to divert more waste from landfills.
Legislative action may be the needed impetus for implementation of this viable treatment
technology.
4.1 Correlation of Batch and Continuous Operation
The ADP assays (batch process) were not representative of continuous operation. In fact,
the TS removals present in the ADPs for solid waste substrates were, at most, just over one-third
of the TS removal measured in the CMFRs.
24
5.0 RECOMMENDATIONS
Based on the results of this research, pilot scale studies that include economic
feasibility (e.g., tipping and hauling fees as appropriate) are recommended at this time because
anaerobic co-digestion was a feasible disposal method for these currently landfilled wastes. All
substrates demonstrated similar TS removals, and the cafeteria substrate yielded the greatest
maximum ADP value (116 mL CH4/g VS-d) among all substrates (25 mL CH4/g VS-d for SMW,
95 mL CH4/g VS-d for OFMSW, and 105 for cheese screenings). The cafeteria and cheese
screenings were also the two greatest in terms of TCOD removal. Cafeteria waste is produced
about one mile from the Platteville wastewater treatment facility and is currently pulped before
disposal; therefore, cafeteria waste is likely the optimal substrate among those three examined
herein and co-digestion using this waste is recommended for pilot-scale operation. That said,
there might be reasons outside the technical scope of this research (e.g., greater quantity, more
consistent production, greater support from the waste producer) that make one of the other wastes
a more optimal substrate.
A next broader step in this study is to survey municipal digesters around Wisconsin to
determine the available capacity (i.e., the difference between typical OLR and design OLR).
Then, identify and characterize (i.e., determine the quantity and ADPs descriptors like CH4
generation rate, TS removal) the currently landfilled, readily degradable, organic solid wastes
produced and their various proximities from anaerobic digesters with excess capacity. Those
municipal anaerobic digesters with excess capacity that are located near sources of readily
biodegradable organic solid wastes currently being landfilled can then be prioritized as the
most suitable candidates to aid in diversion of solid wastes.
25
6.0 REFERENCES
Allen Kani Associates with Enviros RIS Ltd. (2001) WDO Study: Implications of Different Waste Feed Streams (Source-Separated Organics and Mixed Waste) On Collection Options and Anaerobic Digestion Processing Facility Design, Equipment and Costs. http://nerc.org/documents/toronto_report.pdf.
APHA: Standard Methods for the Examination of Water and Wastewater (1998). 20th ed., American Public Health Association/American Water Works Association/Water Environment Federation, Washington D.C., USA. Barclay, Jody. 2012-01-23. City of Toronto Anaerobic Digester and Municipal. Solid Waste Dryer and
Homogenizer. Canadian Biomass Innovation Network. <http://cbin.gc.ca/projects/22>.
Batstone, D.J., Keller, J., Angelidaki, R.I., Kalyuzhnyi, S.V., Pavlostathis, S.G., Rozzi, A., Sanders, W.T.M., Siegrist, H. and Vavilin, V.A. (2002). Anaerobic Digestion Model No.1. STR No. 13, IWA Publishing, London, UK.
Bocher, B. T. W. (2012a). Relating Methanogen Community Structure and Function in Anaerobic Digesters. Marquette University.
Bocher, 2012: Bocher, Benjamin. (2012) Relating Methanogen Community Structure and Function in Anaerobic Digesters. Marquette University, Ph.D. Thesis.
Bocher, B. and Zitomer, D. (2012). Staged Anaerobic Digestion Alters Microbial Community Structure and Increases Methanogenic Activity WEFTEC, 2012: New Orleans, LA.
Bocher, B. and Zitomer, D. Staged Anaerobic Digestion as a Means to Increase Specific Methanogenic Activity in International Water Association: 12th World Congress on Anaerobic Digestion. 2010: Guadalajara, Mexico.
Bocher, B. Maki, J., Johnson, M, and Zitomer, D. (in press). Relating Methanogen Community Structure and Anaerobic Digester Function.
Brown, D., Shi, J., & Li, Y. (2012). Comparison of solid-state to liquid anaerobic digestion of lignocellulosic feedstocks for biogas production. Bioresource Technology, 124, 379–86. doi:10.1016/j.biortech.2012.08.051
Fernandez, J., Pérez, M., and Romero, L.I., (2008) Effect of substrate concentration on dry mesophilic anaerobic digestion of organic fraction of municipal solid waste (OFMSW). Bioresource Technology, 99, 6075-80. doi:10.1016/j.biortech.2012.08.051
Gray, D., Suto, P., and Peck, C. (2008) Anaerobic Digestion of Food Waste. U.S. ENVIRONMENTAL PROTECTION AGENCY REGION 9. East Bay Municipal Utility District. <http://www.epa.gov/region9/organics/ad/EBMUDFinalReport.pdf>.
Komilis, D. P., Ham, R. K., & Park, J. K. (2004). Emission of volatile organic compounds during composting of municipal solid wastes. Water research, 38(7), 1707–14. doi:10.1016/j.watres.2003.12.039
Li, Y., Park, S. Y., & Zhu, J. (2011). Solid-state anaerobic digestion for methane production from organic waste. Renewable and Sustainable Energy Reviews, 15(1), 821–826. doi:10.1016/j.rser.2010.07.042
Mar, L. E., & VanDuinen, M. (2011). Final Report Life Cycle Assessment of Systems for the Management and Disposal of Food Waste. Prepared for InSinkerator,<http://www.insinkerator.com/en-us/Documents/Disposer/LCA-Executive-Summary.pdf >.
Mata-Alvarez, J., Macé, S., & Llabrés, P. (2000). Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives. Bioresource Technology, 74, 3–16. Mercury. (1998). Toxicological Profiles. <http://www.atsdr.cdc.gov/toxprofiles/tp46-c5.pdf> (April 25, 2014). Mercury Compounds. (2013, October 18). EPA. <http://www.epa.gov/ttnatw01/hlthef/mercury.html> (March 13, 2014). Municipal Solid Waste. (2014, February 28). EPA. <http://www.epa.gov/osw/nonhaz/municipal > (March 12, 2014).
26
Navaratnam, N. (2012). Anaerobic co-digestion for enhanced renewable energy and green house gas emission reduction, Ph.D. Thesis, Civil, Construction, and Environmental Engineering. Marquette University, Milwaukee, USARapport, J. (University of California-Davis), Zhang, R., Jenkins, B. M., & Williams, R. B. (2008). Current Anaerobic Digestion Technologies Used for Treatment of Municipal Organic Solid Waste. (pp. 1–90). Sacramento, CA.
Rapport, J. (University of C.-D., Zhang, R., Jenkins, B. M., & Williams, R. B. (2008). Current Anaerobic Digestion Technologies Used for Treatment of Municipal Organic Solid Waste. Environmental Protection (pp. 1–90). Sacramento, CA.
Salminen, E., Rintala, J., Lokshina, L. Y., & Vavilin, V. a. (2000). Anaerobic batch degradation of solid poultry slaughterhouse waste. Water Science and Technology, 41(3), 33–41. <http://www.ncbi.nlm.nih.gov/pubmed/11386301>
Schauer-Gimenez AE, Zitomer DH, Maki JS, Struble CA. (2010). Bioaugmentation for improved recovery of anaerobic digesters after toxicant exposure. Water Res. 44:3555–3564.
Schiopu, Ana-Maria and Gavrilescu, Maria. (2010) Options for the Treatment and Management of Municipal Landfill Leachate: Common and Specific Issues. CLEAN – Soil, Air, Water, 38(12), 1101 – 1110. doi: 10.1002/clen.200900184. http://dx.doi.org/10.1002/clen.200900184
Sell, S. T. (2011). A scale-up procedure for substrate co-digestion in anaerobic digesters through the use of substrate characterization , BMPs , ATAs , and sub pilot-scale digesters by. Iowa state University.
Shahriari, H., Warith, M., Hamoda, M., and Kennedy, K.J. (2012). Anaerobic digestion of organic fraction of municipal solid waste combining two pretreatment modalities, high temperature microwave and hydrogen peroxide. Waste Management, 32(1), 41-52. doi: 10.1016/j.wasman.2011.08.012. <http://www.ncbi.nlm.nih.gov/pubmed/21945550>.
Snyder, R., & Hedli, C. C. (1996). An overview of benzene metabolism. Environmental health perspectives, 104 Suppl 6(December), 1165–71. <http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1469747&tool=pmcentrez&rendertype=abstract>.
Trzcinski, A. P., & Stuckey, D. C. (2012). Denaturing Gradient Gel Electrophoresis Analysis of Archaeal and Bacterial Populations in a Submerged Anaerobic Membrane Bioreactor Treating Landfill Leachate at Low Temperatures. Environmental Engineering Science, 29(4), 219–226. doi:10.1089/ees.2011.0003 U.S. EPA Solid Waste and Emergency Response. (2009). Municipal Solid Waste Generation, Recycling,
and Disposal in the United States: Facts and Figures for 2009 (pp. 1–12). Washington, D.C. <http://www.epa.gov/wastes/nonhaz/municipal/pubs/msw2009-fs.pDF>.
Vinyl Chloride. (2013, October 18). EPA. <http://www.epa.gov/ttnatw01/hlthef/vinylchl.html> (March 13, 2014). WI DNR (2011). Landfill Annual Tonnage Capacity Report. <http://dnr.wi.gov/topic/Landfills/Fees.html>.
Yadvika, S., Sreekrishnan, T. R., Kohli, S., & Rana, V. (2004). Enhancement of biogas production from solid substrates using different techniques –– a review. Bioresource Technology, 95(1), 1–10.
Wittebolle L., Marzorati M., Clement L., Balloi A., Daffonchio D., Heylen K., et al. (2009). Initial community evenness favours functionality under selective stress. Nature 458 623–626.