Full-scale anaerobic reactor samples would be more suitable than lab-scale anaerobic reactor and natural samples to inoculate the wheat straw batch anaerobic digesters

Correlations and impact of anaerobic digestion operating parameters on the start-up duration: Database construction for robust start-up guidelines

Anaerobic digestion (AD) has become a popular technique for organic waste management while offering economic and environmental advantages. As AD becomes increasingly prevalent worldwide, research efforts are primarily focused on optimizing its processes. During the operation of AD systems, the occurrence of unstable events is inevitable. So far, numerous conclusions have been drawn from full and lab-scale studies regarding the driving factors of start-up perturbations. However, the lack of standardized practices reported in start-up studies raises concerns about the comparability and reliability of obtained data. This study aims to develop a knowledge database and investigate the possibility of applying machine learning techniques on experimentation-extracted data to assist start-up planning and monitoring. Thus, a standardized database referencing 75 cases of start-up of one-stage wet continuously-stirred tank reactors (CSTR) processing agricultural, industrial, or municipal organic effluent in mono-digestion from 31 studies was constructed. 10 % of the total observations included in this database concern failed start-up experiments. Then, correlations between the parameters and their impacts on the start-up duration were studied using multivariate analysis and a model-based ranking methodology. Insights into trends of choices were highlighted through the correlation analysis of the database. As such, scenarios favoring short start-up duration were found to involve relatively low retention times (average initial and final hydraulic retention times, (HRTi) and (HRTf) of 26.25 and 20.6 days, respectively), high mean organic loading rates (average OLRmean of 5.24 g VS·d-1·L -1) and the processing of highly fermentable substrates (average feed volatile solids (VSfeed) of 81.35 g L-1). The model-based ranking of AD parameters demonstrated that the HRTf, the VSfeed, and the target temperature (Tf) have the strongest impact on the start-up duration, receiving the highest relative scores among the evaluated AD parameters. The database could serve as a reference for comparison purposes of future start-up studies allowing the identification of factors that should be closely controlled.

Integrated genome-centric metagenomic and metaproteomic analyses unravel the responses of the microbial community to ammonia stress

Ammonia is a major inhibitor in anaerobic digestion of nitrogen-rich organic wastes. In this study, integrated genome-centric metagenomic and metaproteomic analyses were used to identify the key microorganisms and metabolic links causing instability by characterizing the process performance, microbial community, and metabolic responses of key microorganisms during endogenous ammonia accumulation. The identification of 89 metagenome-assembled genomes and analysis of their abundance profile in different operational phases permitted the identification of key taxa (Firmicutes and Proteobacteria) causing poor performance. Metabolic reconstruction indicated that the key taxa had the genetic potential to participate in the metabolism of C2C5 volatile fatty acids (VFAs). Further investigation suggested that during Phase I, the total ammonia nitrogen (TAN) level was maintained below 2000 mg N/L, and the reactor showed a high methane yield (478.30 ± 33.35 mL/g VS) and low VFAs concentration. When the TAN accumulated to > 2000 mg N/L, acid accumulation, mainly of acetate, began to occur, and the methane yield gradually decreased to 330.44 mL/g VS (Phase II). During this phase, the VFA degradation functions of the community were mainly mediated by Firmicutes. Approximately 61.54% of significant differentially expressed proteins (DEPs) related to acetate metabolism in Firmicutes were down-regulated, which led to an increase in acetate concentration to 4897.91 ± 1558.96 mg/L. However, the reactor performance showed spontaneous recovery without any interference (Phase III), during which Firmicutes gradually adapted to the high ammonia conditions. Approximately 75% of the significant DEPs related to acetate metabolism of Proteobacteria were also up-regulated in Phase III compared with Phase II; thus, VFA-related metabolic functions of the community were enhanced, which resulted in a decrease in the total VFA concentration to 195.39 mg/L. When the TAN increased above 4000 mg N/L, the system gradually showed acid accumulation dominated by propionate, accompanied by a second decrease in methane yield (Phase IV). During this phase, the number of up-regulated and down-regulated proteins related to acetate metabolism of Firmicutes and butyrate/valerate metabolism of Proteobacteria was comparable with that of Phase III, indicating that the metabolic functions related to acetate, butyrate, and valerate of the microbial community were not significantly affected. However, for propionate metabolism, the expression activity of fumarate hydratase from Firmicutes and Proteobacteria was severely inhibited by ammonia, as shown by down-regulation ratios of 63.64% and 85.71%, respectively. No protein with the same function that was not inhibited by ammonia could be detected, and the fumarate degradation function of the microbial community was severely damaged, leading to blocked propionate metabolism and irreversible deterioration of reactor performance. This study has provided a new perspective on the microecological mechanisms of ammonia inhibition.

Promoting the overall energy profit through using the liquid hydrolysate during microwave hydrothermal pretreatment of wheat straw as co-substrate for anaerobic digestion

Liquid hydrolysate (LH) derived from the microwave hydrothermal pretreatment (MHP) of wheat straw (WS) was anaerobically digested together with the solid residual to promote the overall energy profit. Different MHP temperatures (90, 120, 150, 180 °C) and retention times (10, 20, 40 min) were investigated. Increased MHP intensity generated plenty of VFAs (mainly acetate) and phenols in the LH, implying the double-side effect of LH on AD. The highest methane production of 227.92 mL CH₄·gVS^-1 Raw was obtained with MHP at 120 °C for 10 min, 21.53 % higher than the control. While, MHP at 180 °C for 40 min exhibited 29.02 % lower methane production (113.13 mL CH₄·gVS^-1 Raw) and 115.86 % longer lag phase (3.13 days) than the control. Butyrate fermentation endowed the treatment groups of 180 °C with resilience from the overload and inhibition. Methanosarcina was largely enriched by the abundant acetate in LH on the early stage of anaerobic digestion (AD), especially when with high MHP intensity. Increased abundance of Methanosaeta and Methanobacterium played a crucial role in maintaining methane production at the middle and later stage. The high number of species and evenness in methanogens community were beneficial for the startup of batch AD. Although negative net energy was obtained, the lower ratio of energy input and output compared with the most researches using the solid residual after MHP as the sole substrate for AD demonstrated the contribution of LH to the overall energy profit.

Effect of Alkaline and Mechanical Pretreatment of Wheat Straw on Enrichment Cultures from Pachnoda marginata Larva Gut

In order to partially mimic the efficient lignocellulose pretreatment process performed naturally in the gut system of Pachnoda marginata larvae, two wheat straw pretreatments were evaluated: a mechanical pretreatment via cutting the straw into two different sizes and an alkaline pretreatment with calcium hydroxide. After pretreatment, gut enrichment cultures on wheat straw at alkaline pH were inoculated and kept at mesophilic conditions over 45 days. The methanogenic community was composed mainly of the Methanomicrobiaceae and Methanosarcinaceae families. The combined pretreatment, size reduction and alkaline pretreatment, was the best condition for methane production. The positive effect of the straw pretreatment was higher in the midgut cultures, increasing the methane production by 192%, while for hindgut cultures the methane production increased only by 149% when compared to non-pretreated straw. Scanning electron microscopy (SEM) showed that the alkaline pretreatment modified the surface of the wheat straw fibers, which promoted biofilm formation and microbial growth. The enrichment cultures derived from larva gut microbiome were able to degrade larger 1 mm alkaline treated and smaller 250 µm but non-pretreated straw at the same efficiency. The combination of mechanical and alkaline pretreatments resulted in increased, yet not superimposed, methane yield.

Influence of particle scattering on photo biochemical transformation process of direct absorption methane digester

An outdoor anaerobic fermentation reactor loses a significant amount of energy due to heat dissipation to the surrounding environment. The digester of direct absorption biogas can effectively utilize solar energy and scattering of the medium to enhance reaction temperature, which can promote anaerobic fermentation of microorganisms. A numerical model for the direct absorption methane digester was established to investigate the mechanism of photo biochemical transformation. The average relative values of simulated results were 4.1% and 9.6%, indicating that the model can effectively simulate the heat transfer process of biogas slurry under solar irradiation. Decreasing the albedo and increasing the effect of forward scattering of small particles can improve the regenerative performance and biogas production of digester. Increasing the backward scattering effect of small particles limited biogas fermentation. Scattering distribution had bigger effects on the rates of biogas and propionic acid production than those of albedo.

Post-treatment of dewatered digested sewage sludge by thermophilic high-solid digestion for pasteurization with positive energy output

This study investigated the possibility to use thermophilic anaerobic high solid digestion of dewatered digested sewage sludge (DDS) at a wastewater treatment plant (WWTP) as a measure to increase total methane yield, achieve pasteurization and reduce risk for methane emissions during storage of the digestate. A pilot-scale plug-flow reactor was used to mimic thermophilic post-treatment of DDS from a WWTP in Linköping, Sweden. Process operation was evaluated with respect to biogas process performance, using both chemical and microbiological parameters. Initially, the process showed disturbance, with low methane yields and high volatile fatty acid (VFA) accumulation. However, after initiation of digestate recirculation performance improved and the specific methane production reached 46 mL CH₄/g VS. Plug flow conditions were assessed with lithium chloride and the hydraulic retention time (HRT) was determined to be 19-29 days, sufficient to reach successful pasteurization. Degradation rate of raw protein was high and resulted in ammonia-nitrogen levels of up to 2.0 g/L and a 30% lower protein content in the digestate as compared to DDS. Microbial analysis suggested a shift in the methane producing pathway, with dominance of syntrophic acetate oxidation and the candidate methanogen family WSA2 by the end of the experiment. Energy balance calculations based on annual DDS production of 10000 ton/year showed that introduction of high-solid digestion as a post-treatment and pasteurization method would result in a positive energy output of 340 MWh/year. Post-digestion of DDS also decreased residual methane potential (RMP) by>96% compared with fresh DDS.