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

Effect of incubation temperature on identification of key odorants of sewage sludge using headspace GC analysis

Currently, headspace gas chromatography-mass spectrometry is a widely used method to identify the key odorants of sludge. However, the effect of incubation temperature on the generation and emission of key odorants from sludge was still uncertain. Thus, in this paper, headspace solid-phase microextraction gas chromatography-mass spectrometry (HS-SPME-GC-MS) and headspace gas chromatography-coupled ion mobility spectrometry (HS-GC-IMS) were carried out to analyze the volatiles emitted from the sludge incubated at different temperatures (30 °C, 50 °C, 60 °C, and 80 °C). The results indicated that the total volatile concentration of the sludge increased with temperatures, which affected the identified proportion of sludge key odorants to a certain extent. Differently from the aqueous solutions, the variation of volatile emission from the sludge was inconsistent with temperature changes, suggesting a multifactorial influence of incubation temperature on the identification of sludge odorants. The microbial community structure and adenosine triphosphate (ATP) metabolic activity of the sludge samples were analyzed at the initial state, 30 °C, and 80 °C. Although no significant effect of incubation temperature on the microbial community structure of the sludge, the incubation at 80 °C led to a noticeable decrease in microbial ATP metabolic activity, accompanied by a significant change in the proportion of odor-related microorganisms with low relative abundances. Changes in the composition and activity of these communities jointly contributed to the differences in odor emission from sludge at different temperatures. In summary, the incubation temperature affects the production and emission of volatiles from sludge through physicochemical and biochemical mechanisms, by which the microbial metabolism playing a crucial role. Therefore, when analyzing the key odorants of sludge, these factors should be considered.

Comprehensively assessing priority odorants emitted from swine slurry combining nontarget screening with olfactory threshold prediction

The lack of one-to-one olfactory thresholds (OTs) poses an obstacle to the comprehensive assessment of priority odorants emitted from swine slurry using mass spectrometric nontarget screening. This study screened out highly performing quantitative structure-activity relationship (QSAR) models of OT prediction to complement nontarget screening in olfactory perception evaluation. A total of 27 compounds emitted at different slurry removal frequencies were identified and quantified using gas chromatography-mass spectrometry (GC-MS), including thiirane, dimethyl trisulfide (DMTS), and dimethyl tetrasulfide (DMQS) without OT records. Ridge regression (RR, R2 = 0.77, RMSE = 0.93, MAE = 0.73) and random forest regression (RFR, R2 = 0.76, RMSE = 0.97, MAE = 0.69) rather than the commonly used principal component regression (PCR) and partial least squares regression (PLSR) were used to assign OTs and assess the contributions of emerging volatile sulfur compounds (VSCs) to the sum of odor activity value (SOAV). Priority odorants were p-cresol (25.0-58.9 %) > valeric acid (8.3-31.7 %) > isovaleric acid (6.7-19.0 %) > dimethyl disulfide (4.7-15.7 %) > methanethiol (0-13.6 %) > isobutyric acid (0-8.6 %), whereas the contributions of three emerging VSCs were below 10 %. Vital olfactory active structures were identified by QSAR models as having high molecular polarity, high hydrophilicity, high charge quantity, flexible structure, high reactivity, and a high number of sulfur atoms. This protocol can be further extended to evaluate odor pollution levels for distinct odor sources and guide the development of pertinent deodorization technologies.

Insights into the role of ·OH generated in Fe2+/CaO2/coal slime system for efficient extracellular polymeric substances degradation to improve dewaterability of sewage sludge

The disposal of massive sewage sludge and coal slime is a problem facing municipalities in China. A hypothesis for the co-disposal of sludge and coal slime is proposed to improve dewaterability by utilizing the beneficial role of coal slime as a filter assist and CaO₂ enhanced system in this research. Results showed that capillary suction time, specific resistance to filtration and water content decreased dramatically from 49.3 s, 13.2 × 10¹² m/kg and 84.85% to 19.1 s, 1.0 × 10¹² m/kg and 50.07%, respectively, under the optimal conditions with 0.3/0.1/0.3-Fe²⁺/CaO₂/coal slime g/g DS. The hydroxyl radicals generated in the Fe²⁺/CaO₂ process acted on extracellular polymeric substances (EPS), resulting in a drop in the ratio of α-helix/(β-sheet + random coil) in the secondary structure of EPS proteins and a reduction in the concentration of aromatic proteins and tryptophan-like substances in TB-EPS, thereby enhancing the sludge dewaterability. Furthermore, coal slime as the skeleton building material induced a rise in sludge particle size and contact angle, lowering the hydrophilicity, compressibility of sludge and providing more channels to facilitate water flow. This work verified the promising application prospect of the Fe²⁺/CaO₂/coal slime combined system in the enhancement of sludge dewaterability.

Development of an electronic nose to characterize water quality parameters and odor concentration of wastewater emitted from different phases in a wastewater treatment plant

For public health consideration, it is important to ensure the wastewater discharged from wastewater treatment plant is within the regulatory limits. This problem can be effectively solved by improving the accuracy and rapid characterization of water quality parameters and odor concentration of wastewater. In this paper, we proposed a novel solution to realize the precisive analysis of water quality parameters and odor concentration of wastewater by the electronic nose device. The main work of this paper was divided into three steps: 1) recognizing wastewater samples qualitatively from different sampling points, 2) analyzing the correlation between electronic nose response signals and water quality parameters and odor concentration, and 3) predicting the odor concentration and water quality parameters quantitatively. Combined with different feature extraction methods, support vector machine and linear discriminant analysis were applied as classifiers to recognize samples at different sampling points, which reported the best recognition rate of 98.83%. Partial least squares regression was applied to complete the second step, and R² was reaching 0.992. As for the third step, ridge regression was used to predict water quality parameters and odor concentration with the RMSE less than 0.9476. Thus, electronic noses can be applied to determine water quality parameters and odor concentrations in the effluent discharged from wastewater plants.

Malodorous volatile organic compounds (MVOCs) formation after dewatering of wastewater sludge: Correlation with the extracellular polymeric substances (EPS) and microbial communities

Malodorous volatile organic compounds (MVOCs) are often the key odorants in determining sludge odor character and odor impact. However, the emission characterization and generation mechanisms of MVOCs from various dewatered sludge have not been sufficiently understood partly due to the diverse and complex composition and low concentration of odorants. In this study, waste activated sludge (WAS) was collected to examine the variation of MVOC emission from sludge after different dewatering treatment in lab-scale trials. The MVOCs were measured using the electronic nose (eNose), headspace gas chromatography-coupled ion mobility spectrometry (HS-GC-IMS), and headspace solid-phase microextraction gas chromatography-mass spectrometry (HS-SPME-GC-MS). The results showed that centrifugation treatment promoted the generation of various odorous volatiles. The identified key odorants included dimethyl sulfide (DMS), dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS), geosmin, and p-cresol according to their odor activity values (OAVs). The effects of the enhanced dewatering on volatile production were greater than thickening, however, the key odorants of dewatered sludge using gravity thickening varied more greatly than sludges from centrifuge thickening. The distribution of extracellular polymeric substances (EPS) and variation of microbial community showed correlations with the production of key odorants. Tryptophan-like substances in the inner layer of EPS (LB-EPS and TB-EPS) were better correlated with the key odorants. The bound EPS released by centrifugation may play the role of precursor for odorous microbial volatiles. According to the predicted functions of differential microbial genera, Desulfobulbus (Desulfobacterota), Gordonia (Actinobacteriota), and Hyphomicrobium (Proteobacteria) were associated with the production of DMS, DMDS, and DMTS, while Gordonia and Hyphomicrobium were related to p-cresol production.