Relationship between ultraviolet-visible spectra and soluble species in the liquid phase of wastewater sludge during biological digestion under mesophilic and thermophilic conditions

Abstract

Absorption of ultraviolet (UV)-visible (Vis) light by the liquid phase of wastewater sludge during biological digestion was studied. Sludge samples were obtained from batch anaerobic and aerobic digestion experiments conducted at mesophilic and thermophilic temperatures, as well as influent and effluent of a full-scale anaerobic digester. In tandem with acquiring the UV-Vis spectra, the concentration of soluble parameters was measured. Integrated areas under the spectra in the UV and Vis regions were calculated as AUV and AVis, respectively. For the batch experiments, the computed areas increased with digestion time. In addition, a power-law function was able to describe the correlation between soluble chemical oxygen demand (SCOD) concentration and AUV for anaerobic digestion at 35 °C and 45 °C, and aerobic digestion at 55 °C. Coefficients of determination (R2) for the mentioned cases were between 0.812 and 0.933. Furthermore, power-law functions also described a correlation between SCOD and soluble ortho-phosphate concentrations during batch anaerobic digestion at 35 °C (R2 = 0.921) and 45 °C (R2 = 0.812). Moreover, analysis of the data obtained from the aerobic digestion experiment suggests that the mass of UV-absorbing matter linearly decreased over time during aerobic digestion at 45 °C (R2 = 0.925) and 55 °C (R2 = 0.996). The outcomes of the current study indicate that the integrated area under the UV spectrum of sludge filtrates reflects the progression of biological digestion of wastewater sludge under thermophilic and mesophilic conditions, which can be used for process monitoring and optimization.