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Energy and the Environment
Composition and Toxicity of Biogas Produced from Different Feedstocks in California Yin Li, Christopher Alaimo, Minji Kim, Norman Y. Kado, Joshua Peppers, Jian Xue, Chao Wan, Peter G. Green, Ruihong Zhang, Bryan M. Jenkins, Christoph Franz Adam Vogel, Stefan Wuertz, Thomas M. Young, and Michael J. Kleeman Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b03003 • Publication Date (Web): 03 Sep 2019 Downloaded from pubs.acs.org on September 4, 2019
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Composition and Toxicity of Biogas Produced from Different Feedstocks
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in California
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Yin Lia, Christopher P. Alaimoa, Minji Kima, Norman Y. Kadoc, Joshua Peppersb, Jian Xuea, Chao
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Wana, Peter G. Greena, Ruihong Zhangb, Bryan M. Jenkinsb, Christoph F.A. Vogelc, Stefan Wuertzd,
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Thomas M. Younga, and Michael J. Kleemana,*
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a
Department of Civil and Environmental Engineering, University of California – Davis, Davis,
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California 95616, USA
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b
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California 95616, USA
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c Department
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California – Davis, Davis, California 95616, USA
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d
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Singapore 637551
Department of Biological and Agricultural Engineering, University of California – Davis, Davis,
of Environmental Toxicology and Center for Health and Environment, University of
Singapore Center for Environmental Life Sciences Engineering, Nanyang Technical University,
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*corresponding author:
[email protected] 20
Department of Civil and Environmental Engineering
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University of California
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Davis, CA 95616
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Phone: 530 752 8386
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Abstract
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Biogas is a renewable energy source composed of methane, carbon dioxide, and other trace
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compounds produced from anaerobic digestion of organic matter. A variety of feedstocks can be
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combined with different digestion techniques that each yield biogas with different trace
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composition. California is expanding biogas production systems to help meet greenhouse gas
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reduction goals. Here we report the composition of six California biogas streams from three
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different feedstocks (dairy manure, food waste and municipal solid waste). The chemical and
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biological composition of raw biogas is reported and the toxicity of combusted biogas is tested
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under fresh and photo-chemically aged conditions. Results show that municipal waste biogas
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contained elevated levels of chemicals associated with volatile consumer products (VCPs) such as
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aromatic hydrocarbons, siloxanes, and certain halogenated hydrocarbons. Food waste biogas
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contained elevated levels of sulfur-containing compounds including hydrogen sulfide, mercaptans,
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and sulfur dioxide. Biogas produced from dairy manure generally had lower concentrations of
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trace chemicals, but the combustion products had slightly higher toxicity response compared to
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the other feedstocks. Atmospheric aging performed in a photochemical smog chamber did not
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strongly change the toxicity (oxidative capacity or mutagenicity) of biogas combustion exhaust.
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Key words: Biogas, California, chemical composition, bacteria, toxicity
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Abstract Art
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1. Introduction
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Biogas is a renewable fuel produced from the anaerobic digestion of organic feed stocks
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including municipal waste, farm waste, food waste and energy crops. Raw biogas typically consists
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of methane (50-75%), carbon dioxide (25-50%) and smaller amounts of nitrogen (2-8%). Trace
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levels of hydrogen sulfide, ammonia, hydrogen and various volatile organic compounds are also
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present in biogas depending on the feedstock1. Life cycle assessment studies have shown that
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deploying biogas technologies can effectively reduce greenhouse gas (GHG) emissions and
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therefore reduce the climate impact of energy consumption2–4. Biogas production and utilization
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practices also help diversify energy systems while simultaneously promoting sustainable waste
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management practices 1,5. California is promoting biogas utilization by mandating the low carbon
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fuels, offering grants to develop biogas production facilities, and providing assistance in accessing
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pipeline infrastructure6–8.
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There are many environmental factors to consider when developing biogas energy sources
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including the potential for air quality impacts. California is home to eight of the ten most polluted
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cities in the United States9 and so the widespread utilization of any new fuel must be carefully
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analyzed for effects on air quality and human health. The concentrations of minor chemical and
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biological components in biogas differ from those found in other fuels. Some of these components
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have the potential to be toxic to human health and the environment, to form toxic substances during
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the combustion process, or to form toxic substances after photochemical aging in the atmosphere.
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The California Air Resources Board (CARB) and the Office of Environmental Health
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Hazzard Assessment (OEHHA) compiled a list of twelve trace components potentially present in
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biogas at levels significantly above traditional fossil natural gas including carcinogens (arsenic, p-
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dichlorobenzene, ethylbenzene, n-nitroso-di-n-propylamine, vinyl chloride) and non-carcinogens
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(antimony, copper, hydrogen sulfide, lead, methacrolein, mercaptans, toluene). A limited dataset
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of measurements is available to characterize levels of these biogas components in California.
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Measurements of landfill biogas composition have been made over many decades around the world
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to identify sources of odor, to reduce ground level volatile organic compound (VOC)
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contamination, and to optimally recover biogas as an energy source10. Trace components identified
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in landfill biogas include halocarbons, aromatic hydrocarbons and siloxanes11–16. Animal waste
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has significant biogas potential in California but often contains sulfur compounds that must be
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removed prior to use17,18. Food waste is a relatively new feedstock that has only been analyzed for
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biogas plant performance19,20. Previous studies have tested biogas or simulated biogas burning in
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engines or turbines, focusing on engine/turbine performance, NOx and small hydrocarbon
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emissions21–26, but these studies did not examine trace chemical compounds in the engine
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combustion exhaust that could pose environmental and human health concerns.
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Here we report the composition and toxicity of biogas produced and directly used for
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electricity production at five different facilities in California. Samples at each site were collected
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over three separate days spanning a range of environmental conditions. Comprehensive
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measurements were performed for 273 different features including major biogas chemical
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components, a variety of different organic and inorganic trace components, trace elements, and
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microorganisms. Concentrations were compared to previously reported measurements and to the
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regulatory limits specified by OEHHA and the California Division of Occupational Safety and
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Health (Cal/OSHA). A standard biomarker assay was used to evaluate the oxidative capacity of
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biogas combustion exhaust and the associated short-term inflammatory response. A carcinogen
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screening mutagenicity bioassay was used to evaluate the probability that biogas combustion
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exhaust will damage DNA, leading to increased cancer risk over longer time periods. These
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comprehensive measurements help to understand the potential air quality impacts of widespread
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biogas production and combustion for electricity generation across California.
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2．Materials and Methods
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2.1 Biogas sources
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A total of eighteen sets of samples were collected from five biogas facilities: 1) dairy waste
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biogas produced by a flushed manure collection and covered lagoon system, 2) dairy waste biogas
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produced by a scraped manure collection and digester system, 3) food waste biogas, 4) food waste
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biogas mixed with nearby landfill gas, 5a) biogas produced by the core portion of a regional landfill,
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and 5b) biogas produced from perimeter of the same regional landfill. The biogas production and
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utilization technologies used at each site are summarized in Table 1. A map showing the locations
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of all biogas facilities studied is present in Figure S1. All of the facilities generate electricity on-
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site using engines or turbines tuned to operate on biogas.
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2.2 Chemical analysis
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Biogas is a complex matrix containing hundreds of trace chemical compounds that cover a
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broad range of functional groups with different volatility.
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techniques are employed to measure the full range of compound classes. Common sampling
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methods include collecting high volatility compounds in Tedlar (polyvinyl fluoride) bags or in
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metal canisters, enriching lower volatility compounds onto solid sorbent tubes, and stripping polar
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compounds using liquid sorbents in glass impingers. The widely used analysis procedures include
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compound separation using gas or liquid chromatography optionally coupled with a desorption
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unit followed by detectors that may be compound-specific or general mass spectrometers.
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Detection limits are typically tens to hundreds of parts per billion by volume for different
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compounds10,11,13–15,27–29.
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The current study employed sampling and analysis techniques following the practices
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summarized above as published by the EPA (TO-1529, 8081b30, 8270d31, 8082a32, 2933) and ASTM
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(D194534, D622835) standard laboratory methods. Tedlar® sample bags were collected under
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positive system pressure or using a “Vac-U-Chamber” (SKC-West, Inc.) vacuum sampling
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apparatus if biogas pressure was negative. Each Tedlar bag sample analysis included a pure
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nitrogen system blank and calibration standards. Tedlar sample bags were directly connected to
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the instruments summarized in Table S1 and analyzed for the 119 compounds listed in Table S2.
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Semi-volatile and/or reactive chemical compounds were collected on three different types of
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sorbent tubes: XAD-2 sorbent tubes, coconut charcoal sorbent tubes and DNPH-treated silica gel
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tubes. Flow through each sorbent tube was controlled at 1.0 L٠min-1 using a calibrated variable
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area flow meter with built-in stainless steel valve followed by a downstream Teflon diaphragm
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pump (R202-FP-RA1, Air Dimensions Inc.). All sorbent tubes were sealed until just prior to
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sampling, and immediately capped at the conclusion of sampling. Each sample analysis run
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included a system blank, two sample blanks, and calibration standards. A multi-point calibration
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curve generated from the calibration standard was used to quantify the target compounds. Table
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S3 lists collection times, extraction methods and analysis methods for each sorbent tube. A
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comprehensive list of target chemical compounds in each sampling/analysis pathway is presented
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in Tables S4-S6 (102+33+13=148 compounds total). Biogas samples for metals analysis were
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collected using three serial glass impingers that each contained 20 mL of 5% nitric acid and 10%
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hydrogen peroxide in double deionized (18.2 MOhm*cm) water. Liquid solutions from each of
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the impingers were transferred into separate capped vials and then analyzed with inductively
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coupled plasma mass spectrometry (Agilent 7500i ICP-MS, operated with a Glass Expansion
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AR50 MicroMist nebulizer).
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2.3 Biological analysis
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Samples for biological analysis were collected on two 47 mm polycarbonate filters (0.4
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µm pore-size) to support analysis for cultivable microorganisms and corrosion inducing bacteria
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DNA. Sample flow rates ranged from 1-5 L٠ min-1 over times ranging from 2-4 hours. Condensate
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transported along with biogas was also collected. Individual filters were placed in 50 ml Falcon
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tubes containing 15 ml phosphate buffered saline (PBS). Filters were eluted in PBS by vortexing
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the Falcon tube for 5s followed by manual shaking for 2 min in a biosafety cabinet. Cultivable
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aerobic and anaerobic bacteria in eluates and condensates were enumerated by propagation in
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different growth media using the most probable number (MPN) tests36,37. Positive samples in the
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MPN tests were further characterized using DNA sequencing by conducting polymerase chain
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reaction (PCR) targeting 16S rRNA38. This allowed simultaneous identification of different
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bacteria species in each sample. Nucleic acids in eluates and condensates were extracted using the
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FAST DNA® SPIN KIT for Soil (MP Biomedicals, Irvine, CA) following the manufacturer’s
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protocol. Five qPCR assays targeting total bacteria and corrosion inducing bacteria including
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sulfate reducing bacteria (SRB), iron oxidizing bacteria (IOB) and acid producing bacteria (APB)
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were selected from the literature39–43.
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2.4 Toxicity analysis
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Samples of biogas combustion exhaust were collected from five different electricity
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generators summarized in Table 1 after dilution with pre-cleaned background air. A 5.5 m3 Teflon
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photochemical reaction chamber (0.051 mm NORTON FEP fluoropolymer film) installed in a 24-
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foot mobile trailer was used to simulate atmospheric aging of diluted exhaust under both light
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(daytime UV=50 W m-2) and dark (nighttime) conditions at each test facility. The reaction chamber
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was flushed three times before each test with air that was pre-cleaned using granulated activated
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carbon to remove background gases followed by a High Efficiency Particulate Arrestance (HEPA)
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filter to remove background particles. Dark aging tests started by filling the reaction chamber to
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50% capacity with clean air, followed by injecting combustion exhaust through a 1/2-inch diameter
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insulated stainless steel transfer line at a flow rate of 26 L٠min-1 (50~55oC) for 255 seconds. The
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reaction chamber was then filled to 100% capacity with clean air within 90 seconds, yielding a
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well-mixed system at a dilution ratio of approximately 50:1. The diluted exhaust was aged in the
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chamber for 3 hours before collection onto 47 mm Teflon filters (Zefluor, 2 µm pore size) at 20
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L٠min-1 for 3.5 hours. Light aging tests followed the same experimental protocol with the
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exception that 100 liters of VOC surrogate gas (1.125±0.022 ppmv m-xylene and 3.29±0.07 ppmv
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n-hexane in air, Scott Marrin, Inc.) was injected into the chamber immediately after the combustion
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exhaust, creating a final VOC concentration of 90 ppbv. Hydroxyl radical concentrations during
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the light aging tests were calculated to be 5-6 x 106 molecules cm-3 based on the decay rate of the
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m-xylene and final ozone concentrations at the end of the 3 hour experiment were measured to be
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110~125 ppb.
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The expression of in-vitro pro-inflammatory markers was measured in human U937
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macrophage cells (American Tissue Culture Collection, Manassas, VA). Macrophages are the first
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line of defense in human lungs, and substances in engine exhaust sample may interact with the
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macrophage cells though the Toll-Like Receptors (TLR), Aryl hydrocarbon Receptor (AhR) and
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the NF-KappaB protein complex to induce inflammatory responses. Biomarkers checked in this
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study include Cytochrome P450 monooxygenase (CYP1A1: marker for polycyclic aromatic
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hydrocarbons), Interleukin 8 (IL-8: marker for inflammation), and Cyclooxygenase (COX-2: a key
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enzyme for production of prostaglandins mediating pain and inflammation). After a 6-hr treatment
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of exhaust filter extract, mRNA was isolated from U937 macrophage cells and reverse-transcribed
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into cDNA for quantitative expression analysis using qPCR. Results were normalized to
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housekeeping gene β–Actin expression and expressed as fold increase of mRNA in treated cells
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relative to untreated cells 44. The mutagenicity bioassay was carried out via a micro-suspension
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modification of the Salmonella/microsome Ames assay45,46 that is ten times more sensitive than
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the standard plate incorporation test. Frame-shift mutations of Salmonella typhimurium tester
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strain TA98 were observed. TA98 requires exogenous histidine (His-) for growth, however
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substances in exhaust samples could cause deletion or addition of nucleotides in the DNA sequence
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of TA98 histidinegene (frame-shift), resulting in TA98’s ability to manufacture histidine (His+)
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The resultant colonies are referred to as “Revertants”as the DNA sequence is changed back to its
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original correct form. Liver homogenate (S9) from male Aroclor-induced Sprague Dawley rats
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(Mol Tox, Boone, NC) was added to the assay to provide metabolic activation of the sample.
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3. Results and discussion
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3.1 Biogas composition
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Concentrations of major biogas components (methane, carbon dioxide, nitrogen and
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oxygen) are shown in Figure 1. Methane (CH4) content of the different biogas streams varied from
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49.5% to 70.5% with the exception of perimeter landfill biogas (biogas 6) which contained only
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35.4% methane due to high levels of air intrusion into the gas extraction system. Biogas produced
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from flushed dairy waste using a covered lagoon (biogas 1) had the highest measured methane
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concentration. In contrast, biogas produced from scraped dairy waste in an anaerobic digester
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(biogas 2) had a much lower methane concentration of 51.3%. Similar trends were reported by
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Saber et al. who measured the average methane concentration in a covered lagoon dairy biogas
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facility in the western US to be 67.6%, while the methane concentration in a complete mixed dairy
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biogas digester in the western US was only 60.5% 47. In addition, the biogas 2 facility adds iron
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chloride to digester slurry. Iron chloride is known to inhibit anaerobic digestion processes,
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resulting in lower biogas methane content48. Core landfill biogas had an average methane
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concentration of 49.5%, which fell into the range reported by previous studies conducted in US
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and Europe 11,12,14. The carbon dioxide content in the biogas ranged from 20.2% in lagoon dairy
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biogas (biogas 1) to 46.9% in food waste/landfill biogas (biogas 4). Concentrations of CH4 and
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CO2 observed in this study fall in the range commonly reported for biogas. Another important
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GHG formed during the life cycle of organic waste management is nitrous oxide (N2O). N2O is
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known to account for more than 20% of the total global warming potential (100-yr scale)
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associated with GHGs emissions from organic waste storage practices49,50, but N2O is unlikely to
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form in the anaerobic digestion process that produces biogas51.
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Small amounts of nitrogen (N2, < 8%) and oxygen (O2,
