A componential approach for evaluating the sources of trace metals in municipal solid waste.

Trace metals concentrations of 25 elements were determined for 22 subcomponents of biodegradable and non-biodegradable waste samples representing the United States municipal solid waste (MSW) stream collected during three separate waste sorts. The subcomponent trace metal concentrations and estimated composition results were used to predict trace metal concentrations present in the overall MSW stream along with MSW compost and waste to energy (WTE) ash, which were compared to health-based standards (i.e., US EPA regional screening levels) and to values previously reported in the literature. These estimates for potentially problematic elements like As and Sb could be attributed to abundant base materials in MSW, while other elements, such as Pb, were calculated at much lower concentrations than other published studies. This suggests that trace metals measured in actual MSW compost and WTE ash could originate not only from MSW base components but also from other sources, such as highly concentrated low-mass wastes (e.g., e-waste). While the removal of small quantity components with high metal concentrations may reduce concentrations of some potentially problematic metals (e.g., Pb), others (e.g., As and Sb) are likely to persist in quantities that impede reuse and recycling since they are present in the more abundant base MSW components (e.g., papers, plastics, organics). Promoting meaningful reductions in potentially problematic trace metals in MSW-derived materials may require reevaluating their presence in higher-volume, lower-concentrated MSW components such as paper, plastics, and organics.

Determination of as-discarded methane potential in residential and commercial municipal solid waste.

Methane generation potential, L0, is a primary parameter of the first-order decay (FOD) model used for prediction and regulation of landfill gas (LFG) generation in municipal solid waste (MSW) landfills. The current US EPA AP-42 default value for L0, which has been in place for almost 20 years, is 100 m3 CH4/Mg MSW as-discarded. Recent research suggests the yield of landfilled waste could be less than 60 m3 CH4/Mg MSW. This study aimed to measure the L0 of present-day residential and commercial as-discarded MSW. In doing so, 39 waste collection vehicles were sorted for composition before samples of each biodegradable fraction were analyzed for methane generation potential. Methane yields were determined for over 450 samples of 14 different biodegradable MSW fractions, later to be combined with moisture content and volatile solids data to calculate L0 values for each waste load. An average value of 80 m3 CH4/Mg MSW was determined for all samples with 95% of values in the interval 74-86 m3 CH4/Mg MSW as-discarded. While no statistically significant difference was observed, commercial MSW yields (mean 85, median 88 m3 CH4/Mg MSW) showed a higher average L0 than residential MSW (mean 75, median 71 m3 CH4/Mg MSW). Many methane potential values for individual fractions described in previous work were found within the range of values determined by BMP in this study.

Effects of temperature and particle size on the biochemical methane potential of municipal solid waste components.

The effects of temperature and substrate size on the biochemical methane potential (BMP) assay were tested using eight municipal solid waste components. Two sample sizes were tested; size-reduced particles (x < 2 mm) which are typically used for BMP assays and unground samples (x > 20-100 mm) more similar to an as-disposed condition. Two incubation temperatures (35 and 55 °C) were tested for each component. BMPs for office paper, newspaper, paperboard, and coated paper displayed little difference with regards to temperature or particle size. Mesophilic corrugated cardboard BMPs were significantly greater than their thermophilic counterparts. Hardwood, softwood, and cotton BMPs varied with particle size and temperature. Particle size reduction may increase the bioavailable carbon compounds for wood, but this step was not necessary to achieve similar methane yields for paper products. Extrapolating BMP results to predict landfill methane generation may have greater uncertainty for wood wastes and cotton textiles than paper products.

Translating landfill methane generation parameters among first-order decay models.

Landfill gas (LFG) generation is predicted by a first-order decay (FOD) equation that incorporates two parameters: a methane generation potential (L0) and a methane generation rate (k). Because non-hazardous waste landfills may accept many types of waste streams, multiphase models have been developed in an attempt to more accurately predict methane generation from heterogeneous waste streams. The ability of a single-phase FOD model to predict methane generation using weighted-average methane generation parameters and tonnages translated from multiphase models was assessed in two exercises. In the first exercise, waste composition from four Danish landfills represented by low-biodegradable waste streams was modeled in the Afvalzorg Multiphase Model and methane generation was compared to the single-phase Intergovernmental Panel on Climate Change (IPCC) Waste Model and LandGEM. In the second exercise, waste composition represented by IPCC waste components was modeled in the multiphase IPCC and compared to single-phase LandGEM and Australia's Solid Waste Calculator (SWC). In both cases, weight-averaging of methane generation parameters from waste composition data in single-phase models was effective in predicting cumulative methane generation from -7% to +6% of the multiphase models. The results underscore the understanding that multiphase models will not necessarily improve LFG generation prediction because the uncertainty of the method rests largely within the input parameters. A unique method of calculating the methane generation rate constant by mass of anaerobically degradable carbon was presented (kc) and compared to existing methods, providing a better fit in 3 of 8 scenarios. Generally, single phase models with weighted-average inputs can accurately predict methane generation from multiple waste streams with varied characteristics; weighted averages should therefore be used instead of regional default values when comparing models. IMPLICATIONS: Translating multiphase first-order decay model input parameters by weighted average shows that single-phase models can predict cumulative methane generation within the level of uncertainty of many of the input parameters as defined by the Intergovernmental Panel on Climate Change (IPCC), which indicates that decreasing the uncertainty of the input parameters will make the model more accurate rather than adding multiple phases or input parameters.