The Effect of Digester Temperature on the Production of Volatile Organic Sulfur Compounds Associated with Thermophilic Anaerobic Biosolids

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ABSTRACT
Volatile organic sulfur compounds (VOSC) are recognized as major contributors to malodors associated with dewatered and land applied biosolids. Recent research in the area of VOSC production and control has revealed that microbial degradation of sulfur containing amino acids is the likely mechanism in the formation of hydrogen sulfide (H2S) and methanethiol (MT). The objective of this research was to determine whether increased digestion temperatures within the thermophilic range (49°C-57.5°C) could have an impact on the microbial production of VOSC, and in doing so, decrease biosolids odor. In addition, observations of digester performance and stability were made in order to determine the feasibility of operating at these high temperatures. Results of this research suggest that high thermophilic temperatures are an effective means of biosolids odor abatement. Biosolids associated with digestion temperatures greater than 53°C yielded an 83% reduction in total headspace sulfur when compared with mesophilic biosolids and a 71% reduction in total headspace sulfur when compared to low-temperature thermophilic (49°C) biosolids. However, treatment at higher temperatures (55°C, 57.5°C) showed evidence of inhibition of methanogenic activity. These data suggest that simple operational parameters such as temperature can be utilized to manipulate the activity of various microbial communities of interest within an anaerobic digester, and that increased digestion temperature can indeed be effective in the control of biosolids odor.

INTRODUCTION
The control of odors associated with dewatered biosolids is of practical concern to wastewater utilities looking to utilize biosolids land application. Encroachment of residential development into rural and agricultural areas requires that biosolids are not only safe, but also aesthetically pleasing to the general public. In response to these concerns, considerable research in recent years has been focused both on the mechanism of biosolids odor production and methods for odor abatement.

Volatile organic sulfur compounds, including methanethiol (MT), dimethyl sulfide (DMS), dimethyl disulfide (DMDS), have been shown to correlate well with the human perception of “biosolids odor,” and thus, headspace VOSC concentrations are the focus of this research (Witherspoon et al. 2004). The production of these compounds has been linked to biological activity within post-dewatered biosolids. Specifically, the production of hydrogen sulfide (H2S) and MT are attributable to the activity of protein degrading bacteria and the biodegradation of the sulfur containing amino acids cysteine and methionine, respectively. The production of DMS and DMDS are the result of methylation and oxidative dimerization of MT, respectively (Segal et al. 1969; Oho et al. 2000; Higgins et al. 2006).

The degradation of methylated sulfur compounds by methylotrophic methanogens has also been shown to be an important mechanism in the assessment of VOSC associated biosolids odor. Previous work in the area of biosolids odor has shown that when dewatered biosolids are stored in anaerobic serum bottles, the headspace VOSC concentration increases over time to a particular peak concentration, and then decays to below detectable levels (Muller et al. 2004). In contrast, when a methanogenic inhibitor such as 2-bromoethanesulfonic acid is added to dewatered biosolids, the decay of VOSC does not occur (Chen et al. 2005; Higgins et al. 2006). Though the methanogenic species responsible for VOSC degradation have not yet been identified within an anaerobic digester, supporting evidence for the mechanism of VOSC degradation has been previously provided (Lomans et al. 1999; Lomans et al. 1999). This indicates that the activity of methanogenic organisms capable of degrading methylated sulfur compounds is a key mechanism in the observed decay of biosolids VOSC. For this reason, methanogenic activity must be considered when developing control strategies for biosolids odor.

Cysteine and methionine, the parent compounds for the biological production of H2S and MT have previously shown to be present in both anaerobic digester feed and anaerobically digested sludges (Higgins et al. 1997; Dignac et al. 1998; Higgins et al. 2004). Though the specific microbial populations within anaerobic sludge digesters that are responsible for VOSC production are not well established, previous studies have shown that sulfur-containing amino acids can be broken down within an anaerobic digester environment resulting in the formation of H2S and MT (Segal et al. 1969). This suggests that for feeds containing cysteine and methionine (e.g. waste activated sludge), VOSC production is likely tied to a community of protein metabolizing organisms within the anaerobic digester. This manipulation of this population within the anaerobic digester could result in an advantageous odor control method for treatment plant operators.

The manipulation of a specific microbial community through easy to control operational parameters is not new to anaerobic digestion. The potential deactivation of various human pathogens has recently been recognized as an advantage of various biological sludge stabilization technologies. The United States Environmental Protection Agency (EPA) currently lists thermophilic aerobic digestion (ATADTM), in-vessel anaerobic composting (En-vesselTM, IPS ProcessTM) and various multi-phased anaerobic digestion processes (CBI ATPTM Process, Two-Phased Thermo-Meso Feed Sequencing Anaerobic Digestion) as Processes to Further Reduce Pathogens (EPA 2003). In this case, the specific microbial community of interest is a group of enteric human pathogens accustomed to life at standard body temperature (37°C), or mesophilic conditions. Previous research has shown that the viability of pathogenic organisms can be significantly affected by high temperature incubation and/or treatment (Berg et al. 1980; Watanabe et al. 1997; Ghosh 1998; Iranpour et al. 2006). The aforementioned processes each use increased temperatures between 50°C-65°C to deactivate non-thermostable components of the pathogenic cells and viruses.

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