Development of Methods to Measure the Hydrogen Sulfide Production Potential of Sulfur-Containing Wastes FINAL REPORT

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1 Development of Methods to Measure the Hydrogen Sulfide Production Potential of Sulfur-Containing Wastes Mei Sun, Wenjie Sun and Morton A. Barlaz Dept. of Civil, Construction, and Environmental Engineering, North Carolina State University Raleigh, North Carolina, KEYWORDS: hydrogen sulfide, biochemical sulfide potential, microbial sulfate reduction, solid wastes, landfills FINAL REPORT Prepared for the Environmental Research and Education Foundation 2018

2 Executive summary There are approximately 2000 landfills in the U.S that are permitted to receive municipal solid waste (MSW) (U.S. EPA, 2014b). In addition to MSW, many of these landfills receive a variety of non-hazardous industrial wastes. Examples of such wastes include (1) construction and demolition (C&D) waste that contains gypsum wallboard (i.e., CaSO4), (2) the fines fraction from C&D recycling facilities that contains small pieces of wallboard, (3) flue gas desulfurization (FGD) residue that is generated from processes to remove SOx from combustion off-gas at both coal-fired power plants and MSW combustion facilities, and (4) fly ash that may or may not be mixed with FGD. A common feature of these wastes is that they contain sulfate, which, when co-disposed with MSW in landfills, can be biologically reduced to hydrogen sulfide (H2S). The presence of H2S in landfill gas (LFG) has been reported to occur at landfills throughout the U.S. and globally at concentrations as high as 12,000 ppm (Eun et al., 2007; Ko et al., 2015; Lee et al., 2006). The presence of H2S in LFG is problematic for several reasons: (1) its low odor threshold, ppm, may result in odors due to fugitive LFG emissions (Beauchamp et al., 1984; OSHA, 2005), (2) it is toxic to humans and presents challenges for occupational safety in enclosed areas at landfills such as subgrade elements of the leachate collection system (Selene and Chou, 2003; WHO, 2000), and (3) it is corrosive to LFG collection and control systems. In addition to these well-recognized problems, the toxicity of H2S to anaerobic microbial activity is often overlooked. The biological formation of H2S via sulfate reduction has been reported to inhibit the activity of both sulfate-reducing (O'Flaherty et al., 1998; Reis et al., 1992) and methanogenic microorganisms (McDonald and Parkin, 2009). Sulfide toxicity has implications for the manner in which the H2S production potential of sulfur-containing wastes is assessed. In traditional biodegradability testing, the material of interest is incubated in a reactor system and its biodegradability is measured, often by the measurement of the end products (e.g., methane (CH4) or H2S). For example, the biodegradability of a packaging material could be assessed by measurement of the biochemical methane potential (BMP) (Ress et al., 1998). By analogy, the H2S production potential of a waste would be assessed by measurement of H2S production after incubation in a test system. If, however, the accumulation of H2S limits H2S production, ES-1

3 then use of a traditional system will provide an artificially low estimate of the H2S production potential of a sulfur-containing waste. The overall objective of this research was to develop and document a protocol to assess the H2S production potential of wastes that contain sulfur. While sulfate is the primary form of sulfur that is considered to be problematic, this may be overly simplistic as fly ash contains at least two forms of sulfur (sulfate and sulfite) and other wastes may contain solid phase sulfide that can be released during testing. Two systems to measure the H2S production potential of a waste were developed and demonstrated in this research. The first system is analogous to the BMP system and is termed the biochemical sulfide potential (BSP) test. The BSP system involves testing in a 125 ml serum bottle that includes the waste of interest (~1 g), biological growth medium (50 ml), an inoculum (10 ml) that contains microorganisms that will convert cellulosic wastes to methane and reduce sulfate to H2S, and copy paper which served as a source of organic carbon. A source of organic carbon is required for the biological reaction in which sulfate is converted to H2S. Tests were conducted to evaluate whether removal of the H2S during testing had an influence on the measured H2S production potential of a waste. The results showed that H2S removal was important and that the amount of sulfate converted increased when H2S was removed during testing (Figure ES-1). H2S was removed by including a base trap in each serum bottle. The base trap is a test tube that contains 3 ml of 2 N NaOH. The tube is inserted into the serum bottle prior to sealing. Because H2S is volatile, its production results in its presence in the serum bottle headspace. In the presence of a base trap, the H2S dissolves into the base trap, thus lowering the gaseous H2S concentration in the serum bottle. The base trap was shown to alleviate H2S toxicity, and H2S yields were consistently higher in the presence of a base trap relative to a system in which H2S was allowed to accumulate in the serum bottle headspace. ES-2

4 Figure ES-1. H2S inhibition from Na2SO4 reduction in biochemical sulfide potential test. The figure shows that the percent of initial sulfate recovered as H2S increases when a trap is used to remove H2S from the system headspace. Columns show the means of triplicates and the error bars represent one standard deviation. The second system that was used to measure the H2S production potential was a high solids reactor (8-L) system. Reactors were filled with newsprint that served as the source of organic carbon (500 g), and a sulfur-containing waste (125 g), and were inoculated with leachate from a laboratory-scale reactor that contained decomposing residential MSW. Duplicate reactors were operated with and without continuous sparging of the reactor headspace with nitrogen (N2) gas to remove H2S. The results showed H2S production was consistently higher in reactors that were sparged with N2 (illustrative example shown in Figure ES-2). ES-3

5 Figure ES-2. H2S yield in sparged and non-sparged reactors for C&D fines. The filled symbols represent sparged reactors and the empty symbols represent non-sparged reactors. SW stands for sulfur-containing waste. The H2S production potential of 8 wastes that contain sulfate was measured in both the serum bottle (BSP test) and reactor systems under the conditions demonstrated to alleviate H2S toxicity: a base trap in the serum bottles and N2 sparging in the reactors. The results of the serum bottle and reactor tests are presented in Figure ES-3. Also shown in Figure ES-3 is the H2S production potential that was calculated based on chemical leaching tests. In leaching tests, the total sulfate plus sulfide in a sample is measured and the H2S production/release potential is calculated from the stoichiometry of sulfate conversion to H2S plus the release of solid phase sulfide to the gas phase. It was expected that the calculated H2S production potential based on the leaching tests would consistently be the highest and this is largely consistent with the results presented in Figure ES-3. However, there was not a consistent trend between the BSP and reactor tests as the BSP test resulted in higher estimates of H2S production in only 5 of 8 samples. The results presented in Figure ES-3 show that with one exception, the use of a leaching test to estimate H2S production potential results in the highest estimate of H2S production and is likely greater than what will occur in a landfill. The results also suggest that there is value to a laboratory test in getting a better estimate of H2S production potential as ES-4

6 there is not a quantitative correlation between chemical composition and H2S production potential. While the BSP test was lower than the reactor system in 3 of 8 tests, it is nonetheless recommended as an appropriate tradeoff between the information provided, time, and cost. Fly ash C&D Fines Trona A Trona B Lime A Lime B MSW A MSW B FGD-Gypsum CaSO4 2H2O Theoretic al H 2 S production (ml/g) S-containing waste Figure ES-3. Comparison of H2S production from solid wastes by chemical composition analysis, biochemical sulfide potential tests and reactor tests. The data show the means of triplicates and the error bars represent one standard deviation. FGD- Gypsum and CaSO4 2H2O control were not tested in reactors. Finally, the reactor data were used to estimate laboratory-scale decay rates. These rates were then used to estimate a decay rate for H2S applicable at field-scale by assuming that the ratio of the decay rate constants of newsprint conversion to methane and that of a sulfur-containing waste to H2S are constant between the lab and the field as expressed by eqn.es-1. CH 4 decay rate of newsprint at lab-scale = CH 4decay rate of newsprint at field-scale H 2 S decay rate at lab-scale H 2 S decay rate at field-scale eqn. ES-1 Newsprint was used because it was the source of organic matter in the reactors and its methane production rate constant was measured in parallel with H2S production from ES-5

7 the sulfur-containing wastes (Table ES-1). With reference to eqn. ES-1, the decay rate constant of newsprint at field-scale was adopted from a published estimate of yr -1 (De la Cruz and Barlaz, 2010). This newsprint decay rate constant corresponds to a landfill with an MSW decay rate constant of 0.04 yr -1. Using eqn.es-1, estimates for decay rate constants (ksfield) for the tested sulfur-containing wastes are presented in Table ES-1 and the values range from yr -1. In previous research on U.S. landfills, it has been reported that a field-scale MSW methane decay rate constant of yr -1 may be more appropriate (Wang et al., 2013). If a field-scale bulk MSW decay rate constant of 0.1 yr -1 were assumed, then the field-scale H2S decay rate constants for the wastes given in Table ES-1 would increase by a factor of 2.5. There is considerable uncertainty in the decay rate estimates presented in Table ES-1. In addition to the assumption implicit in eqn. ES-1, it is assumed that the first order decay model for methane generation is applicable to H2S and that toxicity is not a controlling factor. Nonetheless, it is noteworthy that the laboratory-scale decay rate constant for H2S production from C&D fines (18.4 yr -1 ) is higher than the methane production decay rate constant (5.6 yr -1 ). The implication of this is that H2S production from C&D fines will decrease faster than methane generation from MSW. Table ES-1. Estimation of Field-Scale H2S Decay Rate Constants for Sulfur-Containing Wastes. Standard deviations are given parenthetically. Treatment H2S decay rate constant (Lab scale) (yr -1 ) CH4 decay rate constant (Lab scale) (yr -1 ) H2S decay rate constant (Field scale) (yr -1 ) Control Reactor Not applicable 5.6 (0.3) Not applicable Fly Ash 30.8 (1.3) 5.7 (0.1) 0.18 (0.01) C&D Fines 18.4 (6.7) 5.4 (0.2) 0.11 (0.04) Trona Ash A 25.6 (0.2) 5.5 (0.2) 0.15 (0.01) Trona Ash B 35.5 (1.9) 5.6 (0.1) 0.21 (0.01) Lime Ash A 39.0 (0.1) 5.6 (0.1) 0.23 (0.00) Lime Ash B 37.3 (2.4) 5.6 (0.1) 0.22 (0.01) MSW Ash A 61.4 (0.6) 5.6 (0.1) 0.36 (0.00) MSW Ash B 64.0 (0.2) 5.6 (0.1) 0.38 (0.00) ES-6

8 ES-7

9 Table of Contents Table of Contents... i List of Figures... ii List of Tables... ii Introduction... 1 Results and Conclusions Serum bottle assays Reactor Tests Comparison of sulfide production measured by different methods Conclusions Materials and Methods Serum bottle assays Reactor Tests Acknowledgements References Appendix List of related publications at the time of final report submission i

10 List of Figures Figure ES-1. H2S inhibition from Na2SO4 reduction in biochemical sulfide potential test Figure ES-2. H2S yield in sparged and non-sparged reactors for C&D fines Figure ES-3. Comparison of H2S production of solid wastes by chemical composition analysis, biochemical sulfide potential tests and reactor tests Figure 4. H2S inhibition to CH4 production from copy paper Figure 5. H2S inhibition from Na2SO4 reduction in biochemical sulfide potential test: (A) Percent of initial sulfur recovered as H2S; (B) Gas phase H2S concentration; (C) Percent of initial sulfur recovered as residual SO4 2-, and (D) Percent of initial sulfur recovered as the sum of H2S and SO Figure 6. Relationship between H2S production and chemical composition of solid wastes: (A) Observed production in BSP assays vs. theoretical production based on the S content; (B) BSP attributable to sulfate reduction vs. sulfate contents of solid wastes Figure 7. H2S concentration in the sparged and non-sparged reactors for sulfurcontaining wastes in this study Figure 8. H2S yield in the sparged and non-sparged reactors for sulfur-containing wastes in this study Figure 9. CH4 yield in the sparged and non-sparged reactors for sulfur-containing wastes in this study Figure 10. Comparison of H2S production of solid wastes by chemical composition analysis, biochemical sulfide potential tests and reactor tests List of Tables Table ES-1. Estimation of Field-Scale H2S Decay Rate Constants for Sulfur-Containing Wastes. Standard deviations are given parenthetically Table 2. Sulfur content and sulfide production of solid wastes using biochemical sulfide potential tests. Results are from 30 day incubations in treatments with base traps ii

11 and no ph neutralization. The data show the means of triplicates, and standard deviations are given in parentheses Table 3. H2S yield and recovery in laboratory-scale reactors Table 4. CH4 production in laboratory-scale reactors Table 5. Estimation of Field-Scale H2S Decay Rate Constants for Sulfur-Containing Wastes. Standard deviations are given parenthetically Table 6. Medium composition for the biochemical sulfide potential tests iii

12 Introduction The disposal of solid waste in landfills remains a common approach for both municipal solid waste (MSW) and a variety of non-hazardous industrial wastes in the U.S. (U.S. EPA, 2014b). When biodegradable wastes are disposed in landfills, a series of biological reactions occur, and the main end products, methane (CH4) and carbon dioxide (CO2), make up the major fraction of landfill gas. In addition to MSW, landfills often receive industrial wastes, some of which contain sulfur. When the sulfur-containing wastes are co-disposed with MSW in landfills, the oxidized form of sulfur, mainly sulfate, can be reduced to hydrogen sulfide (H2S) as described by eqn. 1 (Fairweather and Barlaz, 1998; Kijjanapanich et al., 2014; U.S. EPA, 2014a; Yang et al., 2006; Zhang et al., 2013; Zhang et al., 2014). Substantial concentrations of H2S have been reported in LFG at landfills that have accepted sulfate-containing wastes (Lee et al., 2006). Organic matter (MSW of MSW leachate) + SO H2O H2S + CO2 eqn. 1 The presence of H2S in landfill gas is problematic for several reasons: (1) it is toxic to humans (Selene and Chou, 2003; WHO, 2000), (2) it is corrosive to landfill gas treatment systems, and (3) it has a low odor threshold ( ppmv), such that fugitive emissions may result in odor problems (Beauchamp et al., 1984; OSHA, 2005). A fourth problem that is often overlooked is that H2S is toxic to the microorganisms that generate both CH4 and H2S. Although no data are available for H2S toxicity to microbial activity in landfills, studies conducted in other anaerobic systems (e.g., waste stream digesters, granular sludge treatment reactors, etc.) suggest that H2S can depress microbial CH4 production, with inhibitory concentrations in the range of mg/l (3-25 mm) (Parkin et al., 1990). Thus, H2S generation may affect both H2S production in laboratory-scale testing and CH4 production in landfills. Despite our understanding of the microbiology of H2S production, the H2S production potential of sulfur-containing wastes is not currently predictable with simple tests. In the absence of an understanding of the H2S production potential of a waste, it is difficult for landfill owners to evaluate how to manage a specific waste. To solve this problem, this study was conducted to develop and document a protocol to assess the H2S production potential of wastes that contain sulfur. 1

13 This study includes the development of two tests to measure H2S production potential; serum bottle tests and reactor tests. The serum bottle test is analogous to the biochemical methane potential (BMP) system and is termed the biochemical sulfide potential (BSP) test. The BSP system involves testing in a 125 ml serum bottle that includes the waste of interest (~1 g), biological growth medium (50 ml), an inoculum (10 ml) that contains microorganisms that will convert cellulosic wastes to CH4 and reduce sulfate to H2S, and copy paper which serves as a source of organic carbon. In the reactor test, 8-L reactors were filled with newsprint that served as the source of organic carbon (500 g), and a sulfur-containing waste (125 g), and were inoculated with leachate from a laboratoryscale reactor that contained decomposing residential MSW. Development and demonstration of both serum bottle and reactor tests are described separately in the following sections. Results and Conclusions 1. Serum bottle assays Sulfide toxicity to methane production The presence of sulfate will decrease CH4 production by either competition between sulfate-reducing bacteria and methanogens for electron donors/carbon source, or by the inhibition of microbial activity due to H2S toxicity. In both BSP tests and in landfills, electron donors are present in excess, but H2S can accumulate and result in inhibition of CH4 production. To assess the toxicity of H2S to CH4 production, H2S gas was injected into serum bottles to achieve varying headspace H2S concentrations after equilibration with the liquid phase. The CH4 yield (ml/g copy paper) at each H2S concentration was compared to a no H2S control (Figure 4). There was no significant difference (p>0.2) in CH4 production between the control and the H2S amended treatments at up to 2.5% H2S. However, there was a decrease in the CH4 yield (p<0.05) at higher concentrations. The 2.5% threshold concentration corresponds to a total aqueous sulfide concentration of 3.7 mm (or 126 mg/l) based on the ph at the end of the BSP incubation. This threshold concentration is near the lower end of the inhibitory range (3-25 mm) reported previously (Parkin et al., 2

14 1990). At a headspace H2S concentration of 5%, the CH4 yield decreased to 75% of the control. The average gaseous H2S concentration measured in landfills is typically well below the 2.5% toxicity threshold observed in this study, suggesting that widespread H2S inhibition should not be a universal concern for landfill management. However, local H2S concentrations in a small area enriched with sulfur-containing waste could be elevated and not detected in bulk gas compositions measured in a header pipe. While H2S accumulation may or may not be a problem in field-scale landfills, these data demonstrate the importance of controlling its accumulation in batch test systems. Figure 4. H2S inhibition to CH4 production from copy paper. The horizontal line is the CH4 yield per gm of copy paper in the absence of H2S. The results were corrected for a blank with no copy paper added. The data show the means of triplicates after 30 day incubations at 37 C, and the error bars represent one standard deviation. Sulfide production self-inhibition and toxicity alleviation: BSP test with Base Trap In addition to inhibition of CH4 generation (Figure 4), high concentrations of H2S may also inhibit its own production. Thus, tests were conducted to evaluate the importance of H2S accumulation on sulfate reduction by using base traps to sequester H2S after it was produced. In addition to H2S, the base traps also remove CO2, which resulted in a ph increase in the growth medium in preliminary work. Thus, tests with base traps were 3

15 conducted with and without ph neutralization. In the tests with ph neutralization, serum bottles were opened and the ph was adjusted to 7.0 twice a week during the 30- day incubation period. Tests were conducted with a soluble source of sulfate, Na2SO4, to demonstrate that base traps could sequester produced H2S and H2S alleviate the toxicity. As presented in Figure 5, when the initial sulfate concentration was low (<10 mm), all sulfate was converted to sulfide and no inhibition was observed in any of the treatments. At higher initial sulfate concentrations, sulfate was only partially transformed and the residual sulfate increased as the initial sulfate concentration increased (Figure 5C). In the absence of base traps, H2S concentrations exceeded 3% while the use of traps resulted in decreased gas (and concurrent liquid) phase H2S concentrations (Figure 5B). As a result of the H2S sequestration by base traps, H2S production was promoted (Figure 5A) and sulfate residual was minimized (Figure 5C) relative to the no trap treatments. Figure 5. H2S inhibition from Na2SO4 reduction in biochemical sulfide potential test: (A) Percent of initial sulfur recovered as H2S; (B) Gas phase H2S concentration; (C) Percent 4

16 of initial sulfur recovered as residual SO4 2-, and (D) Percent of initial sulfur recovered as the sum of H2S and SO4 2-. Results are from 30 day incubations in treatments with or without base traps, and with base traps plus ph neutralization. Columns show the means of triplicates and the error bars represent one standard deviation. Note different scales on y-axis. Previous studies have reported that ph control is essential for accurate assessment of H2S toxicity, not only because the activity of both methanogens and sulfate-reducers are phdependent, but also because the sensitivity of both populations varies with ph (Al- Zuhair et al., 2008; Azabou et al., 2005; O'Flaherty et al., 1998). In tests with soluble sulfate, the ph of the growth medium after incubation with base traps was , while bottles without base traps had a ph of Although ph neutralization resulted in increased sulfate reduction (less residual sulfate, Figure 5C), the final mass balance was weaker than that in the treatment with base traps but no ph neutralization (Figure 5D). A likely explanation is that although the base trap adsorbed most of the H2S produced, some remained in the gas phase (Figure 5B) and was lost from the system when the bottles were opened for ph neutralization. The H2S loss, the relatively modest improvement in sulfate conversion, and the labor-intensive nature suggest that ph neutralization may not be beneficial in BSP tests. Sulfide production with different sulfate containing solid wastes Although complete sulfate reduction was achievable when Na2SO4 was used as the sulfate source, of more interest is the conversion of actual sulfur-containing wastes with potentially lower bioavailability. For example, Karnachuk et al (2002) examined microbial H2S production from hannebachite (CaSO3 0.5H2O), gypsum (CaSO4 2H2O), anglesite (PbSO4), and barite (BaSO4), and reported that only gypsum exhibited H2S production comparable to soluble Na2SO4, and the limited solubility for the other materials resulted in less sulfate conversion. To demonstrate the protocol developed in this study on actual wastes, the H2S production potential of nine sulfur-containing solid wastes was measured in BSP assays with base traps but no ph neutralization. Sulfide production as well as initial and residual sulfate 5

17 and sulfide concentrations are presented in Table 2. The BSP for the nine samples varied from 0.8 ml/g to 58.8 ml/g. Interestingly, even for waste samples of the same type (i.e., Lime A & B, or Trona A & B), their BSPs were considerably different. These differences are consistent with the different chemical compositions of these wastes. Both Trona A and Lime B contained solid phase sulfide in the initial sample whereas Trona B and Lime A did not. After incubation, essentially all of the solid phase sulfide present in Trona A and Lime B initially was released to the gaseous or aqueous phase. This result confirms that sample characterization should include both the sulfate and sulfide contents as both may influence the total H2S release. For most of the wastes as well as the calcium sulfate control, over 80% of the initial sulfate was converted to sulfide after incubation (sulfate removal in Table 2). However, residual sulfate was surprisingly high for the FGD-gypsum and the C&D fines, considering that they are both essentially pure CaSO4. These two samples had the highest initial sulfate contents amongst the nine wastes tested but the sulfate concentrations were comparable to that of the CaSO4 control. The aqueous and gaseous H2S concentrations at the end of incubation in these two samples were comparable to those in the other samples tested (data not shown), and much lower than the inhibitory level observed in Figure 5, precluding H2S toxicity as an explanation. The relationship between the measured BSP and the chemical composition of the samples is presented in Figure 6. In Figure 6A, theoretical H2S production is calculated from the stoichiometric conversion of the initial sulfate to sulfide plus liberation of the initial solid phase sulfide to the liquid and gas. While there is an increasing trend between the theoretical and observed BSP (Figure 6A), the ratio of observed to theoretical H2S ranges from 11 to 97%. This range however does not represent sulfate conversion since liberated solid phase sulfide was also included. To further evaluate sulfate reduction, the volume of H2S that was released by the difference between the initial and residual solid phase sulfide in each sample was calculated and subtracted from the observed BSP to calculate a BSP attributable to sulfate reduction only. This corrected BSP is compared to the initial sulfate content in Figure 6B. Here, an increasing trend is also observed when the three outliers with low S mass balance recoveries are excluded. As presented in Table 1, sulfate removal ranged from 48 to 99%. 6

18 Table 2. Sulfur content and sulfide production of solid wastes using biochemical sulfide potential tests. Results are from 30 day incubations in treatments with base traps and no ph neutralization. The data show the means of triplicates, and standard deviations are given in parentheses. Sample Description NCSU ID Fly ash C&D Fines Trona A Trona B Lime A Lime B MSW A MSW B FGD- Gypsum CaSO4 2H2O control Before incubation (dry mass % as S) Sulfide 0.0% 0.1% (0.1%) 7.5% (0.2%) 0.0% 0.0% 6.9% (1.2%) 0.2% (0.1%) 0.1% (0.1%) 0.4% (0.1%) 0.0% Sulfate 0.4% 8.0% (0.2%) 3.6% (0.7%) 1.1% 1.0% 2.3% (0.3%) 2.0% 1.6% 22.8% (0.1%) 18.6% Released Sulfide a 0.1% 0.9% (0.1%) 7.4% (0.2%) 0.8% 0.5% 7.7% (1.0%) 1.1% (0.1%) 0.8% 5.3% (0.7%) 18.1% (0.2%) After incubation (dry mass % as S) Residual sulfide b 0.0% 0.0% 0.2% (0.3%) 0.2% 0.1% 0.0% 0.4% (0.1%) 0.3% 0.0% 0.0% (0.2%) Residual sulfate c 0.0% 4.1% (0.4%) 0.4% (0.2%) 0.0% 0.2% 0.0% 0.1% 0.1% 7.8% (2.2%) 1.2% (1.7%) BSP d (ml/g S- containin g waste) 0.8 (0.0) 6.6 (0.8) 56.6 (1.9) 6.3 (0.0) 4.1 (0.2) 58.8 (7.3) 8.7 (1.0) 5.8 (0.2) 40.6 (5.2) (1.7) S mass recovery (%) e 33% (1%) 62% (6%) 71% (3%) 100% (3%) 86% (2%) 84% (10%) 74% (3%) 83% (0%) 57% (7%) 104% (8%) Sulfate removal (%) f 99% (0%) 48% (5%) 89% (5%) 99% (0%) 81% (1%) 98% (1%) 96% (1%) 91% (1%) 66% (9%) 93% (9%) 7

19 a. Released sulfide is gaseous plus aqueous phase sulfide b. Residual sulfide is solid phase sulfide measured by acid extraction c. Residual sulfate is the sum of aqueous phase sulfate in the medium plus that recovered from the residual solids after acid extraction d. BSP = Released sulfide converted to gaseous H 2S equivalent (ml) Solid waste added (g) Released sulfide + Residual sulfide + Residual sulfate e. S mass recovery = f. Sulfate removal = (1 Initial sulfide + Initial sulfate Residual sulfate% Initial sulfate% ) 100% 8

20 Figure 6. Relationship between H2S production and chemical composition of solid wastes: (A) Observed production in BSP assays vs. theoretical production based on the S content; (B) BSP attributable to sulfate reduction vs. sulfate contents of solid wastes. The solid line in (A) represents a complete conversion of sulfur in the wastes. The inset of (B) is an expansion of the lower range data, and the circled points are outliners with low S mass recovery. 2. Reactor Tests Hydrogen Sulfide and Methane Production in Landfill Simulation Reactors The H2S yield of each waste is summarized in Table 3 and the H2S concentration and yield data are presented in Figure 7 and Figure 8. H2S yields in the sparged reactors were significantly higher (p<0.05) than those in the non-sparged reactors for all eight materials tested, which indicates that sparging enhanced the conversion of sulfate to sulfide for all wastes. While controlled toxicity tests were not conducted in the reactor system, the H2S concentration was as high as 7% (70,000 ppm) in the non-sparged reactors and orders of magnitude higher than in the sparged reactors, suggesting the H2S inhibition was responsible for the reduced H2S yields and reduced sulfate conversion in the non-sparged reactors. The effect of sparging was minimal in the reactors containing fly ash, as calculated sulfate conversion was near 100% in both treatments (Figure 8A). Notably, the H2S concentration only remained elevated for a relatively short time (Figure 7A). 9

21 Table 3. H2S yield and recovery in laboratory-scale reactors Solid waste H2S yield a (ml H2S / g dry S-containing waste) H2S recovery b (%) Sparged Non-sparged Sparged Non-sparged Fly Ash 2.8 (0.2) c 2.4 (0.4) (6.0) 88.1 (15.4) C&D Fines 22.3 (0.6) 8.6 (0.9) 39.8 (1.1) 15.5 (1.6) Trona Ash A 30.9 (1.1) 1.9 (0.3) (4.2) d 7.6 (1.1) Trona Ash B 1.0 (0.2) 0.1 (0.0) 13.3 (3.1) 1.8 (0.2) Lime Ash A 0.6 (0.1) 0.2 (0.0) 9.2 (0.8) 2.7 (0.2) Lime Ash B 16.1 (0.4) 3.7 (1.0) (2.6) d 23.5 (6.5) MSW Ash A 1.6 (0.2) 0.3 (0.2) 11.3 (1.7) 2.0 (1.1) MSW Ash B 2.5 (0.4) 0.2 (0.1) 22.6 (3.5) 1.7 (0.9) a. H2S generation was corrected for background H2S in the control (newsprint only) reactors which ranged from to 0.02 ml H2S/g newsprint. The reported yield is based on the period before sparging was initiated in some of the non-sparged reactors as described in the text. b. H2S recovery is the fraction of the initial sulfate recovered as H2S. c. Standard deviations are given parenthetically. d. If it is assumed that 100% of the initial solid phase sulfide was released to the gas phase, then the H2S recovery is 48.3 and 40% in Trona Ash A and Lime Ash B, respectively. Methane yields are summarized in Table 4 and cumulative yield curves are presented in Figure 9. A separate set of control reactors containing newsprint and inoculum was initiated each time that a set of reactors with a sulfur-containing waste was initiated, which is why three sets of control reactors are presented in Table 4 and Figure 9. In control reactors, 500 g of dry, shredded newsprint was added to each reactor in the absence of a sulfur-containing waste. As noted in the Methods, all reactor sets were monitored until H2S production was complete so that a conversion percentage and decay rate constant could be calculated. However, control reactors in Sets B and C were terminated before CH4 production was complete. 10

22 11

23 Table 4. CH4 production in laboratory-scale reactors Solid waste a CH4 yield (ml CH4/ g dry newsprint) Sparged Non-sparged Control Reactor (Set A) (3.4) (4.7) Fly Ash (1.1) (0.3) Trona Ash A (1.0) 1.8 (0.1) C&D Fines (17.1) 69.7 (15.6) Control Reactor (Set B) 86.3 (0.6) 67.5 (4.0) Trona Ash B 72.7 (4.1) 25.9 (12.6) Lime Ash A 77.3 (2.0) 71.9 (6.7) Lime Ash B 80.5 (1.4) 6.1 (0.8) Control Reactor (Set C) 84.4 (2.1) 70.1 (1.9) MSW Ash A 81.4 (3.6) 27.6 (3.1) MSW Ash B 84.8 (0.7) 22.9 (3.8) a: Three control reactors are presented because a new set of control reactors was set up each time a new set of sulfur-containing wastes was tested. Standard deviations are given parenthetically. In Sets 2 and 3, reactors were terminated before CH4 production was complete which is why the control reactor CH4 yields are lower in Sets B and C With the exception of Lime Ash A (Figure 9G), CH4 yields were significantly higher in the sparged reactors relative to the non-sparged reactors (p<0.05), which is also consistent with H2S inhibition in the non-sparged reactors. The explanation for the absence of an effect attributable to sparging in the Lime Ash A reactors is unclear, though the behavior in these reactors was unique. Sulfate conversion in Lime Ash A reactors was only 9.2 and 2.7% in the sparged and non-sparged reactors, respectively, and there was little sulfate-reduction activity as evidenced by the low gaseous H2S concentrations and yield (Table 3). 12

24 A B C D Figure 7. H2S concentration in the sparged and non-sparged reactors for sulfur-containing wastes in this study. The filled symbols represent sparged reactors and the empty symbols represent non-sparged reactors. 13

25 E F one-time sparging start of continuous sparging G H start of continuous sparging start of continuous sparging Figure 7 (continued). H2S concentration in the sparged and non-sparged reactors for sulfur-containing wastes in this study. The filled symbols represent sparged reactors and the empty symbols represent non-sparged reactors. 14

26 Figure 8. H2S yield in the sparged and non-sparged reactors for sulfur-containing wastes in this study. The filled symbols represent sparged reactors and the empty symbols represent non-sparged reactors. SW stands for sulfur-containing waste. 15

27 H Figure 8 (continued). H2S yield in the sparged and non-sparged reactors for sulfur-containing wastes in this study. The filled symbols represent sparged reactors and the empty symbols represent non-sparged reactors. SW stands for sulfurcontaining waste. 16

28 Figure 9. CH4 yield in the sparged and non-sparged reactors for sulfur-containing wastes in this study. The filled symbols represent sparged reactors and the empty symbols represent non-sparged reactors. NP stands for newsprint. 17

29 Figure 9 (continued). CH4 yield in the sparged and non-sparged reactors for sulfur-containing wastes in this study. The filled symbols represent sparged reactors and the empty symbols represent non-sparged reactors. NP stands for dry newsprint. 18

30 I J K Figure 9 (continued). CH4 yield in the sparged and non-sparged reactors for sulfur-containing wastes in this study. The filled symbols represent sparged reactors and the empty symbols represent non-sparged reactors. NP stands for newsprint. 19

31 Interestingly, the sparged control reactors produced significantly more CH4 than the non-sparged controls in each set, despite the absence of sulfate in the controls (Table 4). Increased CH4 in the non-sparged control reactors was most pronounced in the first set of controls. Thus, although the CH4 yield in the Fly Ash reactors was higher in the sparged treatment (Figure 9B), the ratio of the CH4 yields in the sparged and non-sparged treatments in the Fly Ash reactors was comparable to that in the controls (Figure 9A), so no effect can be attributed to sparging. H2S concentrations of up to 30,000 ppm were present in the non-sparged Fly Ash reactors on day 75 (Figure 7A) at which time CH4 production was equivalent in the sparged and non-sparged Fly Ash reactors (Figure 9A). Sulfate conversion was essentially complete in the Fly Ash reactors within days (Figure 8A) while CH4 production continued for about 300 days. Once the stimulatory effect of sparging was established, selected non-sparged reactors were sparged with N2 to evaluate whether either the one time or continuous removal of H2S would stimulate sulfate reduction and/or CH4 production. For example, the duplicate Trona Ash A non-sparged reactors were sparged one time on days 100 and 175, respectively. While there was no substantial effect of H2S production (Figure 8E), CH4 generation was stimulated after the onetime sparging (Figure 9C) although the cumulative CH4 yield in the continuously sparged reactor treatment was still higher. Continuous N2 sparging was initiated in Lime Ash A and B and Trona Ash B after days of operation. Both H2S and CH4 generation were stimulated in Lime Ash B and Trona Ash B but not Lime Ash A (Figure 8 and Figure 9). The absence of an effect for Lime Ash A could be attributed to the fact that H2S accumulations were relatively low (Figure 7G) so the effect of sparging on H2S concentrations was not as meaningful. This contrasts with Lime Ash B where sparging significantly reduced H2S concentrations (Figure 7H). The explanation for the behavior of Trona Ash B reactors is not clear as continuous sparging of the previously non-sparged reactors stimulated both H2S (Figure 8F) and CH4 (Figure 9F) production despite the low accumulation of H2S in these reactors (Figure 7F). Despite some inconsistencies, it appears that H2S toxicity on sulfate reduction and CH4 production is reversible if an appropriate strategy is applied to alleviate the accumulated H2S. When the measured yield is 20

32 plotted against total S or sulfate there is only a general trend of increasing H2S with total S. There is however, no consistent behavior amongst the samples H2S Recovery The fraction of the initial sulfate that was recovered as H2S is presented in Table 3. The recovery ranged from 9.2% to 121.2% in the sparged reactors and was lower in the non-sparged reactors. The calculated recovery is greater than 100% for Fly Ash, Trona Ash A and Lime Ash B. While the average Fly Ash recovery is slightly above 100%, this is likely a result of measurement error and the standard deviation of the averages suggests that the calculated average recovery of 102.9% is not different from 100%. In the case of Trona Ash A and Lime Ash B, these samples contained 7.5% and 6.9% solid phase sulfide, respectively. Apparently, some or all of the sulfide was liberated during the decomposition experiment. If the theoretical H2S recovery is calculated as the measured H2S divided by the sum of the stoichiometric conversion of the initial sulfate to gaseous H2S plus the release of the initial solid phase sulfide to the gas phase, then H2S recoveries for Trona Ash A and Lime Ash B would be 48.3 ± 1.7% and 40.0 ± 1.0%, respectively. Although the recoveries are higher in the sparged treatments relative to the nonsparged treatments, the overall conversions are relatively low. Once the correction for initial solid phase sulfide is considered, the conversions range from 9.2 to 48.3% for the samples tested excluding the fly ash as discussed above. Two samples of Trona Ash were tested, and both their initial characterization and their biodegradation behavior were different. Trona Ash A had more sulfate but also contained solid phase sulfide initially. While Trona Ash A exhibited a high conversion, when the conversion is recalculated assuming that all of the solid phase sulfide was released as gaseous H2S, the conversion is only 48.3%, still well above the 13.3% conversion recorded for Trona Ash B. The effect of one time sparging on H2S production in the non-sparged reactors was minimal for Trona Ash A (Figure 8E) but significant for Trona Ash B (Figure 8F), even though its overall conversion was still relatively low. 21

33 Two samples of Lime Ash were also tested and here too, their initial characterization and their biodegradation behavior were different. Lime Ash B had more sulfate but also contained solid phase sulfide initially. Sulfate conversions in Lime Ashes A and B, after correction for the initial sulfide in Lime Ash B, were 9.2 and 40%, respectively. Continuous sparging had a significant effect on H2S (Figure 8H) and CH4 (Figure 9H) generation in Lime Ash B but no effect on CH4 in Lime Ash A (Figure 8G and Figure 9G). In the C&D fines reactors, H2S accumulated within about 14 days of reactor initiation (Figure 7B), and the concentrations remained elevated (up to 30,000 to 35,000 ppm) for almost 200 days, which appeared to impact both CH4 (Figure 9D) and H2S production (Figure 8B). While both CH4 and H2S production were stimulated by sparging and the shorter duration of H2S accumulation in the sparged reactors (about 100 days), overall H2S conversion was still relatively low, even in the sparged. The low H2S yield in the sparged C&D fines reactors is surprising given the solubility of gypsum. In previous work, Yang et al. (2006) reported H2S generation was reduced by the presence of wood and concrete which were also present in our samples. The mechanism for this reduction in H2S production was not clear. Sulfate conversion in both MSW Ashes was relatively low. While sparging stimulated both CH4 (Figure 9J&K) and H2S (Figure 8C&D) production, measured H2S concentrations never exceeded 9000 ppm (Figure 7C&D) in any of the reactors, a concentration well below those observed in the Fly Ash reactors which had much higher conversions. The effect of the elevated H2S concentrations on sulfate conversion and CH4 generation appeared to vary between wastes. For example, among the sparged reactors, H2S concentrations were highest in Trona Ash A and Lime Ash B (Figure 7E&H), yet these two materials had among the highest sulfate conversions. Similarly, CH4 production in sparged reactors containing these wastes is comparable to that in the controls (Figure 9A&C, E&H). Thus, general statements about the extent of sulfate conversion based on the initial sulfate content are not possible for the wastes tested. 22

34 The measured H2S yields are based on H2S recovery in the reactor gas phase. Aqueous sulfide was minimal in the final leachate sample from the sparged reactors. Although solid phase sulfide was measured in the residual solids at the end of the monitoring period, the results were so variable that they were not useful and hence not reported. Nonetheless, some solid phase sulfide was present. It is also possible that the extent of sulfide precipitation varied by waste which would influence gaseous H2S recovery. Estimated H2S decay rate in laboratory and field scales The reactor data were used to develop estimates of a field-scale decay rate constant for the sulfur-containing wastes. To begin, it was assumed that both lab- and fieldscale H2S production could be estimated by using a first order decay rate equation as described in the Methods. To estimate a decay rate constant for H2S that is applicable at field-scale, it was assumed that the ratio of the decay rate constants for newsprint conversion to CH4 and a sulfur-containing waste s conversion to H2S are constant between the lab and the field as expressed by eqn. 2. CH 4 decay rate of newsprint at lab-scale = CH 4decay rate of newsprint at field-scale H 2 S decay rate at lab-scale H 2 S decay rate at field-scale eqn. 2 Newsprint was used because it was the source of organic matter and its CH4 production rate constant was measured in parallel with H2S production from sulfate-containing wastes (Table 5). The average newsprint decay rate constant for Control Set A, Fly Ash, Trona Ash A and C&D fines was 5.6 yr -1 (range ) and this average was used in eqn. 2. With reference to eqn. 2, the decay rate constant of newsprint at field-scale was adopted from a published estimate of yr -1 (De la Cruz and Barlaz, 2010). This newsprint decay rate constant corresponds to a landfill with an MSW decay rate constant of 0.04 yr -1. Using eqn. 2, estimates for decay rate constants (ksfield) for the tested sulfur-containing wastes are presented in Table 5. In previous research on U.S. landfills, it has been reported that a field-scale MSW CH4 decay rate constant of yr -1 may be more appropriate (Wang et al., 2013). If a field-scale decay rate of 0.1 yr -1 were assumed, then the calculated ksfield values given in Table 5 would increase by a factor of

35 Table 5. Estimation of Field-Scale H2S Decay Rate Constants for Sulfur- Containing Wastes. Standard deviations are given parenthetically. Treatment H2S decay rate constant (Lab scale) (yr -1 ) CH4 decay rate constant (Lab scale) (yr -1 ) H2S decay rate constant (Field scale) (yr - 1 ) a Control Reactor Not applicable 5.6 (0.3) Not applicable Fly Ash 30.8 (1.3) 5.7 (0.1) 0.18 (0.01) C&D Fines 18.4 (6.7) 5.4 (0.2) 0.11 (0.04) Trona Ash A 25.6 (0.2) 5.5 (0.2) 0.15 (0.01) Trona Ash B 35.5 (1.9) 5.6 (0.1) 0.21 (0.01) Lime Ash A 39.0 (0.1) 5.6 (0.1) 0.23 (0.00) Lime Ash B 37.3 (2.4) 5.6 (0.1) 0.22 (0.01) MSW Ash A 61.4 (0.6) 5.6 (0.1) 0.36 (0.00) MSW Ash B 64.0 (0.2) 5.6 (0.1) 0.38 (0.00) a. The H2S decay rate constants are based on an assumed MSW decay rate constant of 0.04 yr -1. The constants will vary proportionately with the assumed MSW decay rate constant. The only published estimate of field-scale rate constants of H2S based on field measurements is that presented in Anderson et al. (2010) who estimated a decay rate constant of 0.7 yr -1 for landfills that used C&D fines as daily cover. There are, however, so many differences between the values in Anderson et al. (2010) and the value estimated here that a direct comparison is flawed. To estimate a decay rate from field data, Anderson et al. (2010) allowed ksfield and the ultimate sulfide generation potential (Ls0) to vary simultaneously to get the best fit between the model and measured data. For the estimates developed here, Ls0 was treated as an intrinsic property of the waste and was not allowed to vary when calculating kslab. In addition, Anderson et al. (2010) had minimal data on the actual H2S generation rate and did not consider a time-varying collection efficiency, which is a confounding factor in working with data from field-scale landfills. Finally, there was uncertainty in both the mass of sulfur actually buried over time and the mass 24

36 of H2S recovered through the gas collection and control system in Anderson et al. (2010). The estimated decay rate constant for C&D fines derived in this study was 0.11 yr -1 and this would increase to 0.27 yr -1 at a bulk MSW decay rate constant of 0.1 yr -1, which is likely more representative of a wet landfill in the northeastern U.S. While there is considerable uncertainty in the decay rate constant estimates, it is noteworthy that both the values in Table 5 and the values proposed by Anderson et al. (2010) are consistent in that the decay rate constant for H2S production from C&D fines (18.4 yr -1 in Table 5) is higher than the CH4 production decay rate constant (5.4 yr -1 in Table 5). The implication of this is that H2S production from C&D fines will decrease faster than CH4 generation from MSW. 3. Comparison of sulfide production measured by different methods The H2S production potential of a waste was estimated in three ways. First, theoretical H2S production potential was calculated from the measured sulfate plus sulfide content of a waste (chemical extraction). In addition, H2S production potential was measured using a BSP assay and a reactor test. The chemical test is the fastest and most conservative as it measures maximum possible H2S production. However, to the extent that not all of the sulfur in a waste reacts, the chemical test will over predict H2S production. Both the BSP and reactor tests were used to better simulate reactive sulfate and microbiological conversion. BSP tests require at least 30 days while the reactor tests had incubation periods of days. Figure 10 summarizes the results of the three approaches. Theoretical (or chemical) H2S production is always the upper limit of the amount that could be generated, but there is no consistent trend between the BSP and reactor tests as the BSP test resulted in higher estimates of H2S production in only 5 of 8 samples. While H2S production predicted by chemical composition was in general higher than the value measured through both BSP assays and reactors, the degree to which measured values were lower varied. 25

37 Fly ash C&D Fines Trona A Trona B Lime A Lime B MSW A MSW B FGD-Gypsum CaSO4 2H2O Theoreti cal H 2 S production (ml/g) S-containing waste Figure 10. Comparison of H2S production from solid wastes by chemical composition analysis, biochemical sulfide potential tests and reactor tests. The data show the means of triplicates and the error bars represent one standard deviation. FGD-Gypsum and CaSO4 2H2O controls were not tested in reactors. 4. Conclusions The objective of this research was to develop and demonstrate a laboratory-scale protocol to measure the H2S production potential of a sulfur-containing waste. Two tests were developed. The first test is referred to as the biochemical sulfide potential (BSP) test. The BSP system involves testing in a 125 ml serum bottle that includes the waste of interest, biological growth medium, an inoculum, and copy paper that served as a source of organic carbon. The results showed higher sulfate conversion in a system when base traps were added to sequester H2S. The second system that was used to measure the H2S production potential was a high solids reactor that was filled with newsprint as the source of organic carbon, a sulfurcontaining waste and an inoculum. The results showed that sparging the reactors with N2 to remove H2S was important to measure the H2S production potential of a waste. Sulfate conversion to H2S varied considerably and there was only a qualitative relationship between the initial sulfur content of a waste and recovered H2S. The 26

38 results were complicated by the fact that two of the wastes tested contained solid phase sulfide that was released during testing. The potential presence of solid phase sulfide emphasizes the importance of measuring either the total S or the sulfate and sulfide content of a waste for proper characterization. The reactor data were used to estimate field-scale decay rate constants for each S- containing waste tested and the estimated decay rates varied from yr -1. These estimates are based on a CH4 production decay rate constant of 0.04 yr -1. While the H2S decay rate constants are uncertain, they are considerably higher than the CH4 production decay rate constant, which suggests that H2S production will decrease faster than CH4 generation for all of the wastes tested. The objective of this research was to recommend an appropriate test method to assess the H2S production potential of a waste. While the results suggest that estimates based on chemical composition over predict the H2S production measured in experimental systems, the estimate based on chemical composition is nonetheless recommended for wastes that contain sulfate. This is because sulfate that did not react in the batch experimental systems used in this research may nonetheless leach from the waste and be transported in the aqueous phase (i.e. leachate) to another area of decomposing waste where conversion to H2S does occur. Materials and Methods 1. Serum bottle assays Test assays for H2S production A laboratory-scale batch test protocol was developed using serum bottles to measure the H2S production potential of sulfur-containing wastes. This protocol will be referred to as a biochemical sulfide potential (BSP) test, analogous to the biochemical methane potential (BMP) test (Shelton and Tiedje, 1984). Compared to larger scale reactor tests, the BSP test is economical, well-controlled, and can be 27