Prediction of organic matter accessibility and complexity in anaerobic digestates

The effect of a two-stage anaerobic digestion on digestates: Organic matter quality and microbial communities

The impact of a two-stage anaerobic digestion (AD) system integrating a dark fermentation (DF) step prior to AD, referred to as DF-AD2, on the organic matter (OM) quality and the microbial communities in anaerobic digestates was investigated. Two treatment routes (one-stage AD (AD1) and DF-AD2) were compared by advanced characterization using the same feedstock and treatment duration. The DF-AD2 improved the percentage of CH4 during AD2 by 8.3 % and the volatile solids removal by 6.8 % compared to AD1. The DF step increased the dissolved OM and mineralized nitrogen after AD despite similar OM complexity and predicted carbon mineralization in soils. Moreover, respirometry tests related the enhanced bioaccessibility of DF effluent to greater biological activity (126.3 ± 5.8 mg O2) compared to the substrate (51.1 ± 5.8 mg O2). Nonetheless, DF-AD2 did not impact the biological stability of digestates (32.09 ± 1.1 and 30.2 ± 1.5 mg O2 for AD1 and DF-AD2, respectively). Low-stress operational conditions of the tests might smooth the DF-AD2 effect on digestate biological stability and microbial communities. Archaea varied after DF but homogenized during AD2, with the genus Methanosarcina comprising 71-79 % of the relative abundance. Concurrently, the orders Bacteroidales, Spirochaetales and Cloacimonadales dominated Bacteria in both AD1 and AD2. Overall, this study evidence that a DF-AD2 system is a feasible way to improve both OM removal and the nitrogen fertilizing value of digestates, without hindering digestate biological stability or microbial communities. However, optimizing operational parameters and pre-treatment processes may be necessary to enhance the system's energy output.

Integrated modelling of anaerobic digestion process chain for scenario assessment

Anaerobic digestion (AD) expansion as a renewable energy source offers environmental benefits, such as reducing mineral fertilizer use and preserving soil organic matter. However, poor AD performance can cause greenhouse gas emissions and nutrient loss, impacting efficiency. To address this, an innovative AD process chain (ADPC) model was developed to dynamically simulate biogas production, organic matter bioaccessibility, digestate phase separation, storage, and soil application, focusing on carbon and nitrogen dynamics. Evaluated against 5 lab-scale experiments and C/N soil dynamics without specific calibration, the model accurately reproduced trends (bias: 11% for biogas, r2: 0.78 for soil N dynamics), despite discrepancies in digestate carbon speciation and soil C dynamics, emphasizing the importance of calibrating feedstock biodegradability and hydrolysis parameters to enhance accuracy. Additionally, feedstock seasonality scenarios demonstrated the importance of dynamic modeling in predicting agronomic digestate fate positioning ADPC as a valuable tool for scenario testing considering both energy and agronomic valorization.

Application of digestate from low-tech digesters for degraded soil restoration: Effects on soil fertility and carbon sequestration

The application of digestate to soil represents a common practice for its recycling, but its application to degraded lands to achieve their restoration and sequester organic C into the soil is still almost unexplored. In this context, this study describes a first attempt to use digestate from a low-tech digester for degraded soil restoration in Colombia. An experimental site (2700 m2) previously subjected to intensive mono-cultivation was treated with digestate application for 4 months (40 Mg ha-1 dry weight). Soil samples were collected (0, 4, 8, and 12 months after digestate application) to evaluate chemical and biochemical parameters, as well as total soil organic C stocks and their fractionation among different pools. Results showed that soil pH (from 5.3 to 6), total organic C (from 1.9 to 3 %), total N (from 0.17 to 0.27 %), available P (from 10 to 68 mg kg-1), exchangeable nutrients content (K, Mg, Ca, Fe), respiration rate, microbial biomass C and N, and metabolic activities exhibited an increasing trend after digestate application, leading to a recovery of the soil biological fertility (i.e. biological fertility index increased from 8 to 19 in a range from 1 to 20). Digestate promoted C sequestration in the more stable and recalcitrant pools. Soil application of digestate from low-tech digesters may thus represent a win-win resource recovery strategy to enhance degraded land recovery, contribute to climate change mitigation and support rural communities. In the circular bioeconomy context, afforestation appears as the most promising strategy to take advantage of the restored land.

Improved organic matter biodegradation through pulsed H2 injections during in situ biomethanation

During in situ biomethanation, microbial communities can convert complex Organic Matter (OM) and H2 into CH4. OM biodegradation was compared between Anaerobic Digestion (AD) and in situ biomethanation, in semi-continuous processes, using two inocula from the digester (D) and the post-digester (PoD) of an AD plant. The impact of H2 on OM degradation was assessed using a fractionation method. Operational parameters included 20 days of hydraulic retention time and 1.5 gVS.L-1.d-1 of organic loading rate. During in situ biomethanation, 485 NmL of H2 were injected for each feeding (3 times a week). Maximum organic COD removal was 0.6 gCOD in AD control and at least 1.6 gCOD for in situ biomethanation. Therefore, COD removal was 2.5 times higher with H2 injections. These results bring out the potential of H2 injections during AD, not only for CO2 consumption but also for better OM degradation.

Retention time and organic loading rate as anaerobic co-digestion key-factors for better digestate valorization practices: C and N dynamics in soils

The growing use of anaerobic co-digestion (AcoD) in processing organic waste has led to a significant digestate production. To effectively recycle digestate back into soils, it is crucial to understand how operational variables in the AcoD process influence the conversion of organic matter (OM). To address this, a combination of biochemical fractionation and various soil incubation tests were employed to assess the stability of OM in digestates generated from anaerobic continuous reactors fed with a food waste-hay mixture and operating at different hydraulic retention times (HRT) and organic loading rates (OLR). This study revealed that digester performance and operating parameters impacted carbon dynamics in soils. A decrease in the carbon mineralization in soils when increasing the HRT was reported (48 ± 4 % for 70 days compared to 59 ± 1 % for 42 days). Specific HRT and OLR values were found to be linked to carbon accessibility and complexity, confirming that longer HRT lead to higher OM removal and increased complexity in soluble OM, despite minor discrepancies in relative carbon distribution. Furthermore, comparable rates of nitrogen mineralization in soils were observed for all digestates, consistent with the accessibility of nitrogen from the particulate OM. Nevertheless, AcoD converted substrates with the potential to immobilize nitrogen in soils into fast-acting fertilizers. In summary, this study underscores the importance of controlling the AcoD performances to evaluate the suitability of digestates for sustainable agricultural practices.

Deciphering the contribution of microbial biomass to the properties of dissolved and particulate organic matter in anaerobic digestates

The recalcitrant structures either from substrate or microbial biomass contained in digestates after anaerobic digestion (AD) highly influence digestate valorization. To properly assess the microbial biomass contribution to the digested organic matter (OM), a combination of characterization methods and the use of various substrate types in anaerobic continuous reactors was required. The use of totally biodegradable substrates allowed detecting soluble microbial products via fluorescence spectroscopy at emission wavelengths of 420 and 460 nm while the protein-like signature was enhanced by the whey protein. During reactors' operation, a transfer of complex compounds to the dissolved OM from the particulate OM was observed through fluorescence applied on biochemical fractionation. Consequently, the fluorescence complexity index of the dissolved OM increased from 0.59-0.60 to 1.06-1.07, whereas it decreased inversely for the extractable soluble from the particulate OM from 1.16-1.19 to 0.42-0.54. Accordingly, fluorescence regional integration showed differences among reactors based on visual inspection and orthogonal partial latent structures (OPLS) analysis. Similarly, the impact of the substrate type and operation time on the particulate OM was revealed by ¹³C nuclear magnetic resonance using OPLS, providing a good model (R²X = 0.93 and Q² = 0.8) with a clear time-trend. A high signal resonated at ∼30 ppm attributed to CH₂-groups in the aliphatic chain of lipid-like structure besides carbohydrates intensities at 60-110 ppm distinguished the reactor fed with whey protein from the other, which was mostly biomass related. Indeed, this latter displayed a higher presence of peptidoglycan (δH/C: 1.6-2.0/20-25 ppm) derived from microbial biomass by ¹H-¹³C heteronuclear single-quantum coherence (HSQC) nuclear magnetic resonance. Interestingly, the sample distribution obtained by non-metric multidimensional scaling of bacterial communities resembled the attained using ¹³C NMR properties, opening new research perspectives. Overall, this study discloses the microbial biomass contribution to digestates composition to improve the OM transformation mechanism knowledge.

SoilFract: A mechanistic model accounting for the fate of exogenous organic matter in soil carbon and nitrogen cycles

As its use in agriculture grows, the fate of digestate in soil raises concerns on many different levels. In particular, the degradability of its organic matter when spread on soil is still an ongoing topic. In an effort to better understand the processes and dynamics of digestate soil incubation, C and N mineralization kinetics obtained in 358 days long laboratory incubations during decomposition of digestates were simulated using a dynamic model. The model includes twelve compartments related through processes including 18 parameters. The main novelty of this model is the use of accessibility-related variables to describe the fate of exogenous organic matter in soil, thus enabling a detailed understanding of its outcome in soil. Model calibration on cattle manure digestate incubation resulted in the estimation of parameter values. The newly calibrated model was then tested on an energy crop digestate incubation experiment. The model was able to reproduce accurately the experimental behavior of most variables.

Predicting the Stability of Organic Matter Originating from Different Waste Treatment Procedures

Recycling organic wastes into farmland faces a double challenge: increasing the carbon storage of soil while mitigating CO₂ emission from soil. Predicting the stability of organic matter (OM) in wastes and treatment products can be helpful in dealing with this contradiction. This work proposed a modeling approach integrating an OM characterization protocol into partial least squares (PLS) regression. A total of 31 organic wastes, and their products issued from anaerobic digestion, composting, and digestion-composting treatment were characterized using sequential extraction and three-dimension (3D) fluorescence spectroscopy. The apportionment of carbon in different fractions and fluorescence spectra revealed that the OM became less accessible and biodegradable after treatments, especially the composting. This was proven by the decrease in CO₂ emission from soil incubation. The PLS model successfully predicted the stability of solid digestate, compost, and compost of solid digestate in the soil by using only the characterized variables of non-treated wastes. The results suggested that it would be possible to predict the stability of OM from organic wastes after different treatment procedures. It is helpful to choose the most suitable and economic treatment procedure to stabilize labile organic carbon in wastes and hence minimize CO₂ emission after the application of treatment products to the soil.