Characterization of digestate composting stability using fluorescence EEM spectroscopy combining with PARAFAC

Effects of phosphate-containing additives and zeolite on maturity and heavy metal passivation during pig manure composting

This study investigated the effects of the combination of phosphogypsum with calcium oxide (PPG + CaO), superphosphate with calcium oxide (SSP + CaO) and zeolite (Zeolite) on composting maturity and heavy metal passivation in pig manure composting. The results showed that all treatments reached the maturity requirements and the phosphorus-containing additive treatments had higher final germination indices (GIs). Compared with CK, additive treatments enhanced the compost maturity by promoting volatile fatty acids (VFAs) decomposition (26.4%-30.5%) and formation of stable humus substances. All additive amendment treatments increased humic acid-like substances by over 20%, and the PPG + CaO treatment had the highest level of humus. Composting process reduced the bioavailability of Cu (49.2%), Cd (5.0%), Cr (54.3%), and Pb (26.6%). Correlation analysis found that the heavy mental passivation rate was significantly negatively correlated with the contents of VFAs and nitrogenous substances, and positively correlated with the pH, GI, humic acid content and the ratio of humic acid to fulvic acid (HA/FA). Therefore, the PPG + CaO treatment further increased the passivation rates of Cu (65.6%), Cd (21.7%), and Pb (48.7%) and decreased the mobilization of Zn by promoting maturity and humification during composting.

Fluorescence characteristics of dissolved organic matter during anaerobic digestion of oil crop straw inoculated with rumen liquid

Fluorescence excitation-emission matrix (EEM) spectroscopy is a powerful tool for characterizing dissolved organic matter (DOM), a key component of anaerobic digestion. In this study, the fluorescence characteristics of DOM during 55 days of anaerobic digestion of oil crop straw inoculated with rumen liquid were investigated. EEM spectroscopy coupled with parallel factor analysis (PARAFAC) showed that three major fluorescence components, tyrosine- (C₁), humic- (C₂) and tryptophan-like substances (C₃), were identified in all DOM samples. The F _max values of C₁ and C₃ increased rapidly during the first 5 d, decreased dramatically from day 5 to day 35, and then remained stable, while C₂ was not biodegraded. The changes in the F _max values of the fluorescence components reflected the biodegradation of lignin and/or embedded cellulose by rumen microorganisms. The changes in the Stokes shift of the fluorescence peak were readily explained by the variation in the hydrophobic/hydrophilic fraction distribution. The humidification index (HIX) and A : T ratio of the DOM decreased after 5 d and then increased gradually. Compared with the McKnight fluorescence index (MFI), the Y fluorescence index (YFI) was better able to track the evolution of the DOM. Correlation analysis of the different fluorescence indices (intensities) and absorbance indices was also carried out. The EEM-PARAFAC individual components, HIX and A : T ratio were conveniently used to characterize the degree of anaerobic conversion of the organic matter, and the peak at the Stokes shift of ∼1.0 μm^-1 was used as one of the indicators showing the stabilization of anaerobic digestion. These findings may assist in developing fluorescence technology for monitoring the anaerobic digestion of crop straw.

Humic-like substances extracted from different digestates: First trials of lettuce biostimulation in hydroponic culture

Digestate valorization is a key challenge for the feasibility of Anaerobic Digestion plants. In this study, humic-like substances (HLS) extracted from two digestates (sewage sludge and manure) were used for the biostimulation of hydroponic cultures of Lactuca sativa (lettuce) at different doses and compared to a commercial reference of leonardite HLS. Aerial biomass increase averages ranged from 7 to 30 %, but the results presented a high coefficient of variation (around 20 %). The commercial reference did not present statistically significant biomass yield improvement. The application of manure digestate extract at the higher fulvic-like acids dose (4.6 mg/L of dissolved organic carbon) presented the best and most significant results compared to the blank (Hoagland's solution only). However, this result cannot be strictly dissociated from the supplementary amounts of nutrients brought by the extracts. Additionally, all the products presented low heavy metal content compared to the recent EU regulation for biostimulants (2019/1009). This preliminary study confirmed the interest of extracting HLS from two digestates for application as biostimulants, shedding light on a new perspective for digestate valorization.