Description
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, 0.02 – 0.13 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-2
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.
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