For safe land application, biosolids are stabilized and treated to reduce pathogen levels. Fecal indicator levels are usually measured with culture-based methods, which are time consuming and laborious. Thus, there is a need for alternative indicators that could expedite the classification of biosolids. This could be accomplished through the quantification of humic acids (HAs) as a surrogate, since the natural degradation of organic matter simultaneously breaks down and mineralizes, while also synthesizing humus, which includes the stable polyphenolics humic and fulvic acids and humin. Therefore, this study aimed to investigate whether HAs and fecal bacteria exhibit an inverse correlation, supporting the notion that the level of HAs could potentially serve as an indicator for stabilized biosolids. This entailed using laboratory-scale anaerobic digesters at two temperatures (25°C and 37°C), with levels of pathogenic (Salmonella spp.) and fecal indicator bacteria (Escherichia coli, Enterococci, and Fecal Coliforms) determined, along with extracted HAs being quantified by chemical luminescence (acidic Ce(IV) and rhodamine 6G) over a 34-day study period. Results suggest that an inverse correlation exists between levels of HA and the bacteria studied, with a Pearson correlation coefficient of at least −0.85 for all three fecal bacteria and the extracted HAs. The inverse correlation was stronger at greater temperatures as the 37°C digesters had reduced levels of microbes and increased levels of HA compared with the 25°C digesters. These findings support the idea that humic acids could potentially be used as a chemical indicator for biosolids stabilization.
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