Classification of pores, water diffusivity and penetration characteristics of waste materials, and role of water as electron carrier in landfills: A review.

Coupling of biogeochemical processes occurs between different waste components and waste layers during decomposition of wastes materials deposited in landfills by mechanisms similar to those occurring in marine sediments (i.e., sediment batteries). In landfills, moisture serves as a medium for transfer of electrons and protons under anaerobic conditions for decomposition reactions to proceed spontaneously, although some reactions occur very slowly. However, the role of moisture in landfills in view of pore sizes and pore size distributions, time dependent changes in pore volumes, heterogeneity of waste layers, and associated impacts on moisture retention and transport characteristics in landfills are not well understood. The moisture transport models developed for granular materials (e.g., soils) are not appropriate to describe the conditions at landfills due compressible and dynamic conditions in landfills. During waste decomposition processes, absorbed water and water of hydration can be transformed to free water and/or become mobilized as liquid or vapor, creating a medium for transfer of electrons and protons between waste components and waste layers. The characteristics of different municipal waste components were compiled and analyzed for pore size, surface energy, and moisture retention and penetration for electron-proton transfer for continuance of decomposition reactions in landfills over time. Categorization of pore sizes appropriate for waste components and a representative water retention curve for conditions in landfills were developed to clarify the terminology and highlight the differences between the landfill conditions and granular materials (e.g., soils) for use of appropriate terminologies. Water saturation profile and water mobility were analyzed by considering water as a transfer medium for carrying electrons and protons for sustaining long-term decomposition reactions.

Thermal properties of municipal solid waste components and their relative significance for heat retention, conduction, and thermal diffusion in landfills.

Significant amounts of heat can be generated during the initial stages after wastes are deposited in landfills, primarily due to decomposition of food waste. Objectives of this study are to compile, examine and compare thermal properties of municipal solid waste (MSW) components, and liquid and gas phases in MSW landfills and their thermal responses that effect temperature increases in gas and leachate. Specific thermal properties examined include thermal conductivity, thermal diffusivity, and specific heat of waste materials deposited in landfills, liquids (water), and gases present. Compilation of these properties will allow in depth thermal analyses to evaluate heat transfer dynamics in landfills with different waste compositions. Examination of thermal characteristics of MSW components indicate that heat generated during decomposition of waste components would primarily be transferred to liquid (leachate) due to formation of water and gaseous components and their high specific heats. As a result, both the leachate and gases released from a landfill during the initial stages after wastes are deposited and when some oxygen is present as an electron acceptor will be warmer. Except for the metals and construction waste, it is likely that most waste components will have a significant temperature gradient during warming up and cooling off stages due to their low thermal conductivities and low thermal diffusivities. Even when the gas phase is at higher temperatures, it will take long time for waste materials (other than food waste and metals) to come to a uniform temperature during the heat generation (primarily due to decomposition of food waste) in a landfill.

PFAS use in electronic products and exposure risks during handling and processing of e-waste: A review.

Poly- and perfluorinated alkyl substances (PFAS) have been and are used in electronic products due to their unique properties that improve product quality and performance. Ubiquities and persistence of some PFAS detected in environmental samples (water, soil, air) have attracted much attention and regulatory actions in recent years. This review provides an overview of PFAS use in electronic components; trends in quantities of e-waste generation; PFAS exposure pathways during e-waste handling and processing; reported PFAS in environmental samples and samples of serum, blood, and hair collected from people living near and working at e-waste processing sites. Processes used for manufacturing electronic components (e.g., embedded processes, additive manufacturing) make recycling or materials recovery from discarded electronic units and components very difficult and unfeasible. Exposure during numerous processing steps for materials recovery and scavenging at disposal sites can result in PFAS intake through inhalation, ingestion, and dermal routes. Chemical risk assessment approaches have been continuously evolving to consider chemical-specific dosimetric and mechanistic information. While the metabolic fate of PFAS is not well understood, some PFAS bioaccumulate and bind to proteins (but not to lipids) in biota and humans due to their surface-active characteristics and very low solubility in water and fat. It is difficult to associate the adverse health effects due to exposure to e-waste directly to PFAS as there are other factors that could contribute to the observed adverse effects. However, PFAS have been detected in the samples collected from different environmental compartments (e.g., water, soil, leachate, blood sera, rainwater) at and near e-waste processing sites, landfills, and near electronics and optoelectronics industries indicating that e-waste collection, processing, and disposal sites are potential PFAS exposure locations. Better monitoring of e-waste handling sites and detailed epidemiological studies for at risk populations are needed for assessing potential health risks due to PFAS exposure at these sites.

Effects of saltwater intrusion and sea level rise on aging and corrosion rates of iron pipes in water distribution and wastewater collection systems in coastal areas.

Failure of water distribution and wastewater collection pipes can impact human health and environment due to potential contamination of water and soil in the surrounding areas, as well as service interruptions and road closures. This paper provides a quantitative assessment of projected shifts in failure rates of iron pipes due to saltwater intrusion and sea level rise in coastal areas. Probability of pipe failure depends on pipe characteristics, environmental parameters, and systems conditions. Analyses showed that saltwater intrusion and sea level rise can significantly affect the integrity of water and wastewater collection pipes due to potential changes in soil and groundwater characteristics that increase corrosion rates. Corrosion rate is intensified by increased conductivity of groundwater due to salinity. Coastal areas with subtropical climate have higher corrosion rates. The estimated lifetimes of ductile iron pipes are between 80 and 100 years. However, the actual service lifetimes can be significantly shorter. Pipes in subtropical areas corrode significantly faster than those in temperate climates. Presence of dissolved oxygen and chloride ion can significantly intensify the corrosion rates, shortening service times of iron pipes. Projections for pipe corrosion can help development of effective asset management strategies and for infrastructure maintenance planning in coastal areas that are vulnerable to sea level rise.

PFAS risk propagation terminology in spatial and temporal scales: Risk intensification, risk attenuation, and risk amplification.

Poly- and per fluorinated alkyl substances (PFAS) are man-made chemicals that are used in many industrial applications to improve performance and durability of products. The CF bond is one of the strongest bonds in organic chemistry which makes PFAS highly persistent in the environment. Therefore, PFAS levels have increased in different environmental compartments (air, water, soil) at global scale over time since the early 1950s. Terminology used for describing potential risks and those used in risk communication can be confusing as different disciplines use risk concepts and vocabulary that are different from those used for exposure and/or health risk assessment for hazardous materials. For example, terms such as emergent risk, emerging risk, risk intensification, risk awareness, risk perception, risk attenuation, risk amplification, and risk absorption are often misused or misinterpreted, especially in describing and communicating risks associated with PFAS in the environment or PFAS exposure. In addition, appropriate risk terms associated with psychological, social, institutional, and cultural elements are often misused. Here, appropriate risk terminology for describing and quantifying risks and health risks in reference to PFAS exposure and PFAS related risk propagation in spatial and temporal scales are explained with examples.

Materials and energy recovery from oily sludges removed from crude oil storage tanks (tank bottoms): A review of technologies.

Sludge and solids accumulating in crude oil storage tanks (referred as tank bottoms) reduces tank volume and requires periodic removal and disposal. Effective management of tank bottoms require considerations to reduce the toxicity of wastes and reduce potential environment impacts. This review compares alternative technologies for economical and environmentally beneficial management of oily sludges for recovery of hydrocarbons and energy with and without oil recovery. Management options with oil recovery include solvent extraction, centrifugation, surfactant oil recovery, and pyrolysis. Management options without oil recovery include incineration and anaerobic co-digestion. The selection of the appropriate technology depends on the characteristics of oily sludge, treatment capacity, as well as operation and maintenance costs. An efficient treatment can involve integration of different technologies for recovery of different oil fractions and to reduce energy demand. Technologies that utilize renewable energy (e.g., solar pyrolysis) can offset the high energy demand of pyrolysis process while recovering marketable products.

Volatile organic contaminants (VOCs) emitted from sewer networks during wastewater collection and transport.

Presence of volatile organic compounds (VOCs) in sewer networks is a concern due to exposure of workers during maintenance at manholes and repairs of wastewater pipes, and hazard potential for gas explosion. Occurrence, types, and levels VOCs present at the wastewater treatment plant influents in municipalities in Florida (USA) were compared. Gas phase concentrations were estimated by the Henry's law. In addition, gas samples were collected from the sewer lines at one municipality (City of Hallandale Beach, Florida). Comparison of the gas phase concentrations estimated from the liquid influent samples at the wastewater treatment plants with the gas samples collected from the sewer lines showed that gas concentrations estimated by the Henry's law from the influent liquid concentrations underestimate the gas phase VOC levels. The VOCs detected in gas samples collected at the manholes (City of Hallandale Beach) were acetone (11-75.5 µg/m3), chloroform (15-117 µg/m3), chloromethane (1.6-5.6 µg/m3), dichlorodifluoromethane (2.5-4.5 µg/m3), 1,4-dichlorobenzene (2.5-57 µg/m3), ethanol (7.5-329 µg/m3), methylene chloride (0.6-3.2 µg/m3), pentane (4.7-43.9 µg/m3), propane (1.0-2.7 µg/m3), tetrachloroethene (0.88-2410 µg/m3), trichloroethene (0.23-4.4 µg/m3), toluene (5.3-43 µg/m3), and total xylenes (0.48-4 µg/m3).

Increasing gaps between materials demand and materials recycling rates: A historical perspective for evolution of consumer products and waste quantities.

Advancements in product design towards increasingly more compact and efficient systems have created challenges for recycling and materials recovery due to lack of appropriate infrastructure and mechanisms for collection as well as lack of appropriate materials recovery mechanisms and processes for discarded consumer goods. During the period from 1960 to 2015, the world population has increased from about 3 billion to over 6 billion. During the same period, the municipal solid waste quantities have more than doubled globally (from 1.5 million tons/day in 1960 to 4.0 million tons/day in 2015). Due to relatively short use times, especially for high tech consumer products, the number of obsolete products and waste quantities have been increasing exponentially. The American Chemical Society has identified 44 of the 118 elements in the periodic table as endangered and to become extinct in their ore forms within the next 80 years. Nine elements are in serious threat of extinction in the next 100 years, 7 are in rising threat from increasing use elements, and 28 are in future risk of supply. Some elements used in high tech products and industrial applications (e.g., cordless power goods, LCD displays, wind turbines, magnets, rechargeable batteries, smartphones) have shown significant oscillations in their prices during the last decade. Price oscillations for some elements are likely to occur more frequently and with increasing amplitudes in the coming decades as their availability is jeopardized due to increasing demand as well as political and economic challenges for materials supply. Mechanisms for establishing effective waste management and recycling infrastructures for discarded products, similar to that of supply chain management, are needed for sustainable use and management of available material ores on Earth's crust that can support the infusion of technology and use in consumer products.

Managing siloxanes in biogas-to-energy facilities: Economic comparison of pre- vs post-combustion practices.

Siloxanes present in small concentrations in biogas interfere with the operation of biogas-to-energy facilities. During biogas combustion, siloxanes form white deposits on engine components (engine heads, spark plugs, valves) in crystals or amorphous forms depending on the temperature. The purpose of this study was to evaluate the economic feasibility of biogas-to-energy systems for managing the operational challenges due to siloxanes in biogas. The facility maintenance cost data were compiled by a survey of biogas-to-energy facilities in the United States. Economic analyses were performed to compare the operational costs due to increased maintenance for removing the white deposits forming on the engine components and the installation of a pretreatment system (carbon adsorption) to remove siloxanes prior to combustion. Numerical analyses showed that for the facilities with operating capacities less than 1300 m3/h (750 scfm), the costs for installation and operation of the carbon adsorption system exceeded the maintenance costs for removal of deposits from the engine components. The maintenance costs correlated well with the reported maintenance needs which were between 120 and 800 man hours per year. On the basis of siloxane removal costs alone, it is not economically feasible to install a carbon adsorption system for siloxane removal prior to combustion for small facilities processing less than 1300 m3/h (750 scfm) of biogas. However, using a process for siloxane removal prior to gas engines (e.g., carbon adsorption) would be improve the overall performance of the gas engines and reduce maintenance need at all facilities.

Persistence times of refractory materials in landfills: A review of rate limiting conditions by mass transfer and reaction kinetics.

Monitoring programs at closed landfills show that transformation of plastics, wood, and metals continue long after the active decomposition of the waste fractions are considered as complete. Studies conducted in natural anaerobic environments (e.g., marine sediments and rocks) provide insight for slow degradation mechanisms involving coupling of thermodynamically favorable and unfavorable reactions and biochemical transformations by microbial consortia. These transformations occur at much slower rates through more complex and less obvious mechanisms and are not evident until after the primary decomposition mechanisms become less significant. This study presents a review of the conditions that limit the mass transfer and reaction kinetics for anaerobic transformations in landfills and provides new insights for reaction mechanisms (e.g., anaerobic oxidation and anaerobic corrosion) that occur at relatively slow rates in mature landfills. Conditions and mechanisms of slow transformations by microbial and chemical activities with relatively small energy yields and availability of electron acceptors (e.g., inorganics, plastics) and/or diffusion of gas and moisture into the previously isolated areas in waste deposits were discussed. Time scales for mass transfer and reaction kinetics were compared under anaerobic conditions for different waste components deposited at municipal solid waste landfills. Half-lives of different materials under anaerobic conditions were estimated and compared. Emergence of syntrophic methanogenic communities and conditions for triboelectric effects were evaluated as possible electron transfer mechanisms between waste layers for occurrence of extremely slow transformations of wastes deposited in landfills.

Removal of crude oil from highly contaminated natural surfaces with corexit dispersants.

Dispersants are used to reduce the impact of oil spills in marine environment. Experiments were conducted with natural materials which were contaminated by direct application of fresh Louisiana crude oil. The natural materials evaluated included sea sand (South Beach in Miami, Florida), red mangrove leaves (Rhizophora mangle), and sea shells (Donax variabili). Salt water at two different salinities (17 and 34 ppt) was used with two types of Corexit dispersant solutions (9500A and 9527A) in concentrations ranging from 100 to 3500 mg/L. Washing of the contaminated samples was conducted by a three-step mixing procedure (salt water only, then with the addition of the dispersant solution to the salt water, and salt water) to simulate oil-saltwater-dispersant interactions. In general, increasing dispersant concentration increased the percentage of oil dispersed into the aqueous phase up to dispersant solutions containing 400 mg/L for Corexit 9500A and 300 mg/L Corexit 9527A. Increasing the dispersant concentration above these levels also decreased the dispersion of oil from the surfaces. At very high concentrations of dispersant solutions (above 1500 mg/L), the percentage of oil dispersed into the solution from the contaminated surfaces was about one half what was observed at 400 mg/L with Corexit 9500A and 300 mg/L Corexit 9527A. Although dispersants were most effective for removing the fresh Louisiana crude oil from sand particles and dispersing into the solution due to large surface area of the particles per unit weight; the residual oil remaining on the sand particles was relatively high in comparison to mangrove leaves and sea shells due to clustering of sand particle with oil. There was some oil penetration into the porous structure of the sea shells (at the microscopic level) which could not be removed.

Odor impact zones around landfills: Delineation based on atmospheric conditions and land use characteristics.

Odors emitted from landfills can result in complaints by the residents living near the landfills. The aim of this study was to develop an assessment and delineation tool to identify the areas which can be impacted by the odors released from landfills based on land use characteristics and atmospheric conditions; and estimate the number of people who may be impacted. Odor emissions and dispersion analyses were conducted for three case study landfills under different atmospheric conditions in view of the land use characteristics around each landfill. Odor emissions and odor intensity levels were estimated based on the total gas production and the level of odorous compounds present in the landfill gas using the Landfill Gas Emissions Model (LandGEM) software. To delineate the odor impact zones, air dispersion characteristics of the odorous gases were analyzed using the dispersion modeling software, Areal Locations of Hazardous Atmospheres (ALOHA), and mapped using ArcGIS. Impact zone analyses were conducted based on the odor perception thresholds. The methodology developed involved coupling landfill gas emissions model (LandGEM), dispersions model (ALOHA) and mapping software for land use and population density (ArcGIS) allows visualization of the potential impact zones for preliminary delineation of the buffer zones around landfills, developing appropriate mitigation measures in view of the changing land use characteristics and population density around the existing and planned landfills. The odor impact zone delineation methodology was named Land-OZ (short for Landfill Odor Impact Zone). Results of using the odor impact zone tool showed that atmospheric stability could increase the odor impact radius around the three landfills evaluated between 340 and 1100 percent depending on the land use characteristics of the surrounding areas.

Classification of oil-particle interactions in aqueous environments: Aggregate types depending on state of oil and particle characteristics.

There are significant differences in the aggregation mechanisms and types of aggregates that form by oil-particle interactions in marine and laboratory environments depending on the state of oil (i.e., dissolved, emulsified, floating), size and type of particles involved (i.e., colloidal, granular, organic, inorganic), oil-particle interaction mechanisms, and settling/suspension characteristics. Distinct characteristics of oil-particle aggregates that form by interaction of granular particles with floating oil separate them from the well-known oil-colloidal particle aggregates (OcPA), which are sometimes called Pickering emulsions. Unlike OcPA, which involve emulsified oil (entrained oil droplets suspended in the water column) and colloidal particles, the oil-granular particle aggregates (OgPA) involve the floating oil and granular particles. Here, to clarify the differences and similarities between the two types of aggregates (OcPA and OgPA), we present classification of oil aggregates, drawing attention to important characteristics of OcPA, marine oil snow (MOS), and OgPA.

Viscous effects on the interaction of granular particles with floating oils in water.

Light hydrophobic liquids (LHLs) can be submerged in water with granular particles by forming particle encapsulated liquid sacks. Formation and submergence of granular encapsulated LHL sacks can be an effective method for capturing and controlling the fate of floating oils. However, formation characteristics of the LHL sacks and effect of LHL viscosity on their behavior are not well understood. In this study, we examined the encapsulation characteristics of LHL sacks depending on liquid viscosity. Silicone oils with viscosities ranging from 10cSt to 1000cSt were used as the LHLs. Sand with two different particle sizes (40-100mesh and 20-30mesh) were used as the granular particles. The submerged LHL sacks were stable and remained separate from each other without collapsing or aggregating over time. They could be moved in water by sliding while keeping their encapsulation.

Morphology, composition and aggregation mechanisms of soft bioflocs in marine snow and activated sludge: A comparative review.

Conditions that lead to marine snow formation and aggregates that constitute the marine snow have similarities with the soft bioflocs that form during wastewater treatment by activated sludge process. Analysis of the conditions and similarities of the soft bioflocs in these two aquatic environments provide insight for the processes that lead to formation and growth of hydrated aggregates consisting of both living and nonliving particles, their chemical and biolocial composition, settling/suspension behavior, and contributing factors for their structure and morphology. This literature review provides a comparative analysis of the soft aggregates that form in marine and wastewater environments to characterize the conditions for formation and growth of highly hydrated aggregates consisting of microorganisms, suspended solids and large molecules. The marine snow and bioflocs that form in wastewater are visually similar and even contain microorganisms that are of similar type (i.e., Zoogloea, filamentous bacteria). During wastewater treatment, the microorganisms are not stressed and exopolymeric substances (EPS) produced have shorter molecules and higher protein content while EPS produced by the marine organisms are significantly larger in molecular size (by orders of magnitude) and have higher carbohydrate content.

Struvite formation and decomposition characteristics for ammonia and phosphorus recovery: A review of magnesium-ammonia-phosphate interactions.

Struvite (MgNH4PO4·6H2O) forms in aqueous systems with high ammonia and phosphate concentrations. However, conditions that result into struvite formation are highly dependent on the ionic compositions, temperature, pH, and ion speciation characteristics. The primary ions involved in struvite formation have complex interactions and can form different crystals depending on the ionic levels, pH and temperature. Struvite as well as struvite analogues (with substitution of monovalent cations for NH4+ or divalent cations for Mg2+) as well as other crystals can form simultaneously and result in changes in crystal morphology during crystal growth. This review provides the results from experimental and theoretical studies on struvite formation and decomposition studies. Characteristics of NH4+ or divalent cations for Mg2+ were evaluated in comparison to monovalent and divalent ions for formation of struvite and its analogues. Struvite crystals forming in wastewater systems are likely to contain crystals other than struvite due to ionic interactions, pH changes, temperature effects and clustering of ions during nucleation and crystal growth. Decomposition of struvite occurs following a series of reactions depending on the rate of heating, temperature and availability of water during heating.

Wicking of light hydrophobic liquid phase from water by pulverized rubber: Theoretical and experimental analyses.

Pulverized rubber (PR) can be utilized for capturing floating oils to prevent spreading and volatilization of hydrocarbons. Experiments were conducted using PR with four different particle sizes (ranging from 0.075 to 0.600mm) and South Louisiana crude oil. The oil capture performance of the PR particles was compared with that of powdered activated carbon (AC). Oil-particle interactions were analyzed using capillary theories for lateral aggregation and wicking processes, as well as sorption capacity in relation to particle size. The sorption capacity (as oil to sorbent ratio) for PR with particle size 0.115mm (4.41g/g) was comparable to that of AC with particle size 0.187mm (5.00g/g). Sorption efficiency (oil:powder ratio, g/g) of the PR increased with decreasing particle size. Sorption of oil by PR occurred rapidly (in less than 10min) which indicated strong capillary action. No additional sorption occurred after 30min. For the PR sample with larger particle size (0.600-0.400mm), lateral aggregation was clearly noticeable. The PR-oil aggregates could be easily removed from the water surface without breaking. The cost, availability and recycling potential of PR make it a feasible alternative material for oil spill response and industrial applications which require removal of floating oils.

Self assembly, mobilization, and flotation of crude oil contaminated sand particles as granular shells on gas bubbles in water.

Contaminant fate and transport studies and models include transport mechanisms for colloidal particles and dissolved ions which can be easily moved with water currents. However, mobilization of much larger contaminated granular particles (i.e., sand) in sediments have not been considered as a possible mechanism due to the relatively larger size of sand particles and their high bulk density. We conducted experiments to demonstrate that oil contaminated granular particles (which exhibit hydrophobic characteristics) can attach on gas bubbles to form granular shells and transfer from the sediment phase to the water column. The interactions and conditions necessary for the oil contaminated granular particles to self assemble as tightly packed granular shells on the gas bubbles which transfer from sediment phase to the water column were evaluated both experimentally and theoretically for South Louisiana crude oil and quartz sand particles. Analyses showed that buoyancy forces can be adequate to move the granular shell forming around the air bubbles if the bubble radius is above 0.001mm for the sand particles with 0.28mm diameter. Relatively high magnitude of the Hamaker constant for the oil film between sand and air (5.81×10-20J for air-oil-sand) indicates that air bubbles have high affinity to attach on the oil film that is on the sand particles in comparison to attaching to the sand particles without the oil film in water (1.60×10-20J for air-water-sand). The mobilization mechanism of the contaminated granular particles with gas bubbles can occur in natural environments resulting in transfer of granular particles from sediments to the water column.

From electronic consumer products to e-wastes: Global outlook, waste quantities, recycling challenges.

Advancements in technology, materials development, and manufacturing processes have changed the consumer products and composition of municipal solid waste (MSW) since 1960s. Increasing quantities of discarded consumer products remain a major challenge for recycling efforts, especially for discarded electronic products (also referred as e-waste). The growing demand for high tech products has increased the e-waste quantities and its cross boundary transport globally. This paper reviews the challenges associated with increasing e-waste quantities. The increasing need for raw materials (especially for rare earth and minor elements) and unregulated e-waste recycling operations in developing and underdeveloped counties contribute to the growing concerns for e-waste management. Although the markets for recycled materials are increasing; there are major challenges for development of the necessary infrastructure for e-waste management and accountability as well as development of effective materials recovery technologies and product design.

Selectivity and limitations of carbon sorption tubes for capturing siloxanes in biogas during field sampling.

Siloxane levels in biogas can jeopardize the warranties of the engines used at the biogas to energy facilities. The chemical structure of siloxanes consists of silicon and oxygen atoms, alternating in position, with hydrocarbon groups attached to the silicon side chain. Siloxanes can be either in cyclic (D) or linear (L) configuration and referred with a letter corresponding to their structure followed by a number corresponding to the number of silicon atoms present. When siloxanes are burned, the hydrocarbon fraction is lost and silicon is converted to silicates. The purpose of this study was to evaluate the adequacy of activated carbon gas samplers for quantitative analysis of siloxanes in biogas samples. Biogas samples were collected from a landfill and an anaerobic digester using multiple carbon sorbent tubes assembled in series. One set of samples was collected for 30min (sampling 6-L gas), and the second set was collected for 60min (sampling 12-L gas). Carbon particles were thermally desorbed and analyzed by Gas Chromatography Mass Spectrometry (GC/MS). The results showed that biogas sampling using a single tube would not adequately capture octamethyltrisiloxane (L3), hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5) and dodecamethylcyclohexasiloxane (D6). Even with 4 tubes were used in series, D5 was not captured effectively. The single sorbent tube sampling method was adequate only for capturing trimethylsilanol (TMS) and hexamethyldisiloxane (L2). Affinity of siloxanes for activated carbon decreased with increasing molecular weight. Using multiple carbon sorbent tubes in series can be an appropriate method for developing a standard procedure for determining siloxane levels for low molecular weight siloxanes (up to D3). Appropriate quality assurance and quality control procedures should be developed for adequately quantifying the levels of the higher molecular weight siloxanes in biogas with sorbent tubes.