Biosolids leachate variability, stabilization surrogates, and optical metric selection

Sludge and biosolids organic matter (OM) are increasingly assessed via optical measurements of associated leachates - especially at pilot and bench scales. Limited work has systematically characterized optical and size exclusion chromatography (SEC) based properties of leachable OM from full-scale solid stabilization processes, however. In this study, leachable OM of biosolids from nine full-scale facilities with lime treatment (LT), anaerobic digestion, or aerobic digestion (AeD) (n = 3 facilities per type), was sampled for three sampling dates per facility and multiple extractions per biosolid (n = 54 leachates). Leachates were characterized by high-performance size exclusion chromatography (HP-SEC-UV), ultraviolet (UV)-visible absorbance spectra, and excitation emission matrix (EEM) spectroscopies, pH, and leachable DOC/kg solid. AnD and AeD biosolids-DOM consistently exhibited higher molecular weight DOM (1360 Da) and fluorescence emissions >380 nm after digestion. This suggested higher molecular-weight, heterogeneous OM is released into soluble phases after biological treatment. Fluorescent emission peaks >450 nm were present only in AnD-biosolids leachates (n = 27), providing a consistent signature for anaerobically digested material. Given the agreement of these trends with pilot scale studies, strategies detecting fluorescence emissions >380 nm are proposed stabilization surrogates at full-scale facilities. Prior to this study, there was limited literature consensus for which optical metrics best quantified sludge and biosolids-DOM transformations, however. Critical analysis of ten optical metrics indicated that not all pre-established metrics captured unique spectral differences in biosolids-DOM spectra, as optical metrics were first developed for aquatic DOM. Absorption (E-2 : E-3, SUVA(254), S-R), and fluorescence metrics (HIX, BIX, and fluorescence regional integration (FRI)) were less adaptable to biosolids-DOM spectra. Principal component analysis (PCA) confirmed that targeted peak ratio assessment (i.e. B : T or A : C peak maxima analysis) best differentiated intrinsic fluorescent DOM changes by treatment. Three independent parallel factor analysis (PARAFAC) models for major treatment types of LT, AnD, and AeD biosolids-DOM also validated treatment differences captured by peak picking surrogates. PARAFAC models described new components in the OpenFluor spectral database. Overall, targeted fluorescence peak picking represents a surrogate strategy to monitor leachate quality changes across full-scale treatment trains. These findings may advance optical approaches for process engineering and advanced stabilization research.

Automated summary

This study systematically characterized the optical properties of leachable organic matter (OM) in biosolids from different full-scale treatment processes. The results showed that higher molecular weight, heterogeneous OM is released into soluble phases after biological treatment, and this can be monitored using targeted fluorescence peak picking. The study also highlighted the need for new optical metrics to accurately quantify sludge and biosolids-DOM transformations.