The variation of odor characteristics of wastewater sludge treated by advanced anaerobic digestion (AAD) and the contribution pattern of key odorants

Identification of the odor characteristics of wastewater sludge is important in the evaluation of sludge quality and disposal options considering that sludge odor nuisance may cause major environmental issues. In this study, raw sludge and sludge cake were collected from five WWTPs applied advanced anaerobic digestion (AAD) sludge treatment process to clarify the variation of odor characteristics using sensory analysis and instrumental analysis. The electronic nose, gas chromatography-ion mobility spectrometry (GC-IMS) and gas chromatography-triple quadrupole tandem mass spectrometry (GC–MS/MS) were used to profile and identify the chemical composition of key odorants. A total of 20 odorants were identified and quantified, including 6 groups of chemicals, among which volatile sulfur compounds (VSCs), indole, 3-methylindole and geosmin were identified as key odorants. The odor of the dewatered digested sludge was improved by means of changing the odor character from fecal/sulfide to earthy odor due to the reduction in VSCs concentration. The AAD and subsequent dewatering process resulted in effective removal of VSCs, which are important constituents that impact the sludge odor characteristics through synergistic effect on fecal odorants and masking effect on earthy odorants. Moreover, due to the variation of sludge quality after AAD treatment, the emission capacity of indole, 3-methylindole, and other volatiles increased, which could not be neglected for the formation of unique sludge odor.