Volatile Solids Basis Research Articles

Process efficiency in relation to enzyme pre-treatment duration in black soldier fly larvae composting.

Black soldier fly larvae (BSFL) composting is a treatment in which biodegradable food waste is converted into animal-feed protein and organic fertiliser. BSFL composting has greatest potential for mixed food waste, but under European Union regulations only plant-based waste is permitted as feed for larvae. Biomass conversion efficiency (BCE) in BSFL composting is lower for plant-based waste than for mixed food waste. One way of improving BCE for plant-based waste is to add enzymes to make the waste more available to the larvae, but enzyme pre-treatment is not commonly applied prior to BSFL composting. Therefore this study examined the impact of enzyme pre-treatment duration on process efficiency in BSFL composting of lettuce-cabbage waste pre-treated with enzymes for 0-4days. The results showed that total solids (TS) in larvae decreased with longer enzyme pre-treatment. Direct addition of enzymes at the start of BSFL treatment (0day pre-treatment) resulted in 22% higher BCE on a volatile solids (VS) basis compared with the control, while longer pre-treatment did not improve BCE further. Much of the VS was respired in the 0-day pre-treatment, resulting in lower mass of residues at the end of treatment. Longer pre-treatment increased microbial respiration, suggesting that the microbial community consumed more easily available carbohydrates during the pre-treatment step, which counteracted the purpose of enzyme pre-treatment, i.e. increasing BCE during BSFL composting.

Anaerobic digestion of cornmeal – the effect of crude enzyme extract and co‐digestion with cow manure

AbstractThis study examined the effect of a crude enzyme extract, containing mainly starch‐degrading enzymes, on cornmeal (Zea mays) hydrolysis. This was followed by an investigation of the effect of enzymatic treatment for the anaerobic digestion of this biomass. Cornmeal and cow manure were also co‐digested, and both batch and semi‐continuous experiments were performed. The enzymatic pretreatment of cornmeal resulted in a yield of 65 ± 5% reducing sugars, with 1:10 w/v (grams of dry substrate per mililiters of enzyme extract) enzyme load at 45 °C for 48 h. There was an 8% enhancement of methane production observed during the batch assays, both when cornmeal hydrolysate was digested and when enzymes were added directly to the digester. Synergetic effects were found when co‐digesting cornmeal and cow manure, leading to higher methane yield (280 NmL gVS–1) than that (200 NmL gVS–1) calculated based on the methane potential of the individual substrates. Regarding long‐term effects, the laboratory‐scale semi‐continuous experiments also demonstrated that the co‐digestion of cornmeal and cow manure (1:1 volatile solid (VS) basis) led to a stable process reaching an organic loading rate of 3 g VS L day–1 and achieving a daily methane production of 1280.12 ± 99.4 NmL CH4/day. However, when cornmeal was investigated in mono‐digestion, and the enzyme extract was directly added during semi‐continuous digestion of cornmeal, volatile fatty acid (VFA) accumulation was observed, leading to a decrease in pH, and no significant enhancement of the conversion into methane was observed. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd

Tree-Based Automated Machine Learning to Predict Biogas Production for Anaerobic Co-digestion of Organic Waste

The dynamics of microbial communities involved in anaerobic digestion of mixed organic waste are notoriously complex and difficult to model, yet successful operation of anaerobic digestion is critical to the goals of diverting high-moisture organic waste from landfills. Machine learning (ML) is ideally suited to capturing complex and nonlinear behavior that cannot be modeled mechanistically. This study uses 8 years of data collected from an industrial-scale anaerobic co-digestion (AcoD) operation at a municipal wastewater treatment plant in Oakland, California, combined with a powerful automated ML method, Tree-based Pipeline Optimization Tool, to develop an improved understanding of how different waste inputs and operating conditions impact biogas yield. The model inputs included daily input volumes of 31 waste streams and 5 operating parameters. Because different wastes are broken down at varying rates, the model explored a range of time lags ascribed to each waste input ranging from 0 to 30 days. The results suggest that the waste types (including rendering waste, lactose, poultry waste, and fats, oils, and greases) differ considerably in their impact on biogas yield on both a per-gallon basis and a mass of volatile solids basis, while operating parameters were not good predictors of yield at this facility.

Co-composting of banana peel and orange peel waste with fish waste to improve conversion by black soldier fly (Hermetia illucens (L.), Diptera: Stratiomyidae) larvae

Black soldier fly (BSF) larvae composting is a promising waste treatment that can add value to available biodegradable waste. However, substrates that have low protein content and contain complex molecules (e.g. fruit peels) are not easily degraded by the larvae. This study evaluated the impact on the BSF larvae composting efficiency of co-composting different mixtures of banana and orange peels with incremental increase of fish waste. Mixtures (in total 50 distinct mixtures) of varying proportions of banana peels, orange peels and fish waste were evaluated. BFSL fed on orange peel and banana peel mixtures, containing no fish waste, resulted in a lower biomass conversion efficiency (4.5% ± 1.3) on a volatile solids (VS) basis (BCEvs). Co-composting the fruit peels with fish waste increased the biomass conversion efficiency and the highest BCEvs (25%) was attained when 75% fish waste was included. However, the BCEvs varied greatly (18.0% ± 5.8), likely due to varying fish waste composition. A 25% fish waste inclusion resulted in more than twice as high BCEvs (12.3% ± 2.1) compared to when no fish waste was included. As the conversion efficiency variance increased with increasing fish waste inclusion, it was recommended to keep the inclusions of the fish waste to around 25% of the total mixture, in order to increase the reliability of the BSF larvae composting efficiency.

Assessment of NIR spectroscopy for predicting biochemical methane potential of agro-residues – A biorefinery approach

Biorefinery approaches are suitable for agro-residues valorization. In this work, the integration of solid-liquid extraction followed by anaerobic digestion is explored. This study aims to assess the near-infrared (NIR) spectroscopy and multivariate regression models as reliable methods to predict the methane yield of raw and extracted agro-residues. Tomato residues (ripe semi-rotten tomato-RT, green (unripe) fruit-GT, tomato plant-TB) and grape pomace residues (GP) were extracted for phenolic compounds recovery. GP is the richest substrate in phenolic compounds (55.8 mg g−1, expressed gallic acid equivalents on a dry extract basis). The experimental values of biochemical methane potential (BMP) varied in the range128–307 NmL CH4 g−1, in volatile solid basis for tomato residues, while 115–177 NmL CH4 g−1 were determined for GP. The experimental BMP observed before and after extraction was statistically similar. The prediction based on the NIR-based model also exposes the same trend and shows a reasonable prediction error compared to other models. In conclusion, NIR spectroscopy and some multivariate regression models may be used for BMP prediction in a biorefinery context.

Biochemical Methane Potential of Cork Boiling Wastewater at Different Inoculum to Substrate Ratios

The present study evaluates the digestion of cork boiling wastewater (CBW) through a biochemical methane potential (BMP) test. BMP assays were carried out with a working volume of 600 mL at a constant mesophilic temperature (35 °C). The experiment bottles contained CBW and inoculum (digested sludge from a wastewater treatment plant (WWTP)), with a ratio of inoculum/substrate (Ino/CBW) of 1:1 and 2:1 on the basis of volatile solids (VSs); the codigestion with food waste (FW) had a ratio of 2/0.7:0.3 (Ino/CBW:FW) and the codigestion with cow manure (CM) had a ratio of 2/0.5:0.5 (Ino/CBW:CM). Biogas and methane production was proportional to the inoculum substrate ratio (ISR) used. BMP tests have proved to be valuable for inferring the adequacy of anaerobic digestion to treat wastewater from the cork industry. The results indicate that the biomethane potential of CBWs for Ino/CBW ratios 1:1 and 2:1 is very low compared to other organic substrates. For the codigestion tests, the test with the Ino/CBW:CM ratio of 2/0.7:0.3 showed better biomethane yields, being in the expected values. This demonstrated that it is possible to perform the anaerobic digestion (AD) of CBW using a cosubstrate to increase biogas production and biomethane and to improve the quality of the final digestate.

Anaerobic co-digestion of alkaline pre-treated grease trap waste: Laboratory-scale research to full-scale implementation

Anaerobic co-digestion (AcoD) of grease trap waste (GTW) and sewage sludge (SS) is an attractive management option for GTW valorisation and the transition to energy-neutral wastewater treatment plants (WWTPs). The main constraints that limit the direct use of GTW as a co-substrate in WWTP anaerobic digesters are the high content of non-disabled materials (floating plastics and fibres) and the low anaerobic biodegradability rates. This study assessed a single-step alkali and sieving pre-treatment of GTW at a low temperature (<25 °C) to improve the bioavailability of GTW and facilitate its use as a co-substrate. The experimental results showed that the alkaline pre-treatment of GTW at NaOH doses of 20–30 gNaOH·kgVS−1 allowed the recovery of approximately 55–62 % of organic matter (on a volatile solids basis) present in the GTW in the form of saponified fat and generated a small waste reject fraction. Full-scale anaerobic AcoD assays (SS and pre-treated GTW) did not show inhibitory or operational problems under the experimental conditions and tested concentrations. Finally, a preliminary economic assessment for a 160 000 population equivalent WWTP in a European scenario presented a payback period of 1.75 y.

Assessing the environmental sustainability of grass silage and cattle slurry for biogas production

Grass silage and cattle slurry have been identified as potential significant resources for biogas production. While a higher proportion of grass silage enables a higher specific methane yield to be achieved, there are concerns that using high shares of grass silage may have negative environmental impacts. Previous studies which consider grass as a feedstock have focused on environmental sustainability in the context of greenhouse gas mitigation. However, there is a potential risk of burden shifting occurring if other environmental impacts, such as eutrophication and terrestrial acidification, are not taken into account. A consequential life cycle assessment was conducted to examine mono-digestion of cattle slurry and co-digestion with grass silage in different ratios on a volatile solids (VS) basis. The prior uses of the feedstocks were considered, along with the processes displaced by the biogas and digestate produced. The net environmental impact varied according to the proportion of silage and slurry digested. Higher environmental burdens were observed for mixes with a greater ratio of grass silage to slurry. The optimum environmental performance for the baseline scenario was observed at a VS ratio of 0.4:0.6 for silage and slurry, where there is a net reduction for all impact categories considered. The choice of marginal technologies that are displaced has a significant influence on the results, as have the assumptions about how the grass silage is sourced. This study provides greater insight into the environmental impacts of co-digesting an energy crop with animal manure in varying proportions.

Anaerobic Co-Digestion of Sugarcane Leaves, Cow Dung, and Food Waste: Focus on Methane Yield and Synergistic Effects

Anaerobic co-digestion (AcoD) of food waste (FW) and lignocellulose waste for methane production is a promising technology. This work investigated the methane generation from AcoD of FW, sugarcane leaves (SL), and cow dung (CD) under mesophilic condition in a batch test. As for AcoD of two feedstocks (SL and FW or CD and FW), introduction of SL and CD (25%, volatile solid (VS) basis) showed slight improvement in methane production from FW. In contrast, positive synergistic effect (synergy index=1.03–1.14>1) was observed in all the AcoD reactors of the three feedstocks (SL, CD, and FW). The optimum mixing ratio of FW:SL:CD (VS basis) was 85:11.25:3.75 with a synergy index of 1.07, achieving a methane yield rate and methane content of 297.16 mL/g VS and 73.26%, respectively. This methane production rate was 110.45% and 444.72% higher than mono-digestion of SL and CD. The biodegradability, soluble chemical oxygen demand (SCOD), and VS removal rate are 56.44%, 44.55%, and 55.38%, respectively. The optimum results indicate that AcoD of FW, SL, and CD have higher potentials for energy recovery and providing a forceful scientific evidence for their energy utilization.

Pilot-scale production of poly-3-hydroxybutyrate-co-3-hydroxyvalerate from fermented dairy manure: Process performance, polymer characterization, and scale-up implications

Polyhydroxyalkanoate (PHA) production from waste feedstocks using mixed microbial consortia (MMCs) is a promising alternative to conventional commercial production from refined feedstocks with axenic cultures. Prior work has reinforced this potential with nutrient-limited waste feedstocks and stringently controlled operation. The process would be more competitive, however, without these restrictions. To that end, a pilot-scale three-stage feast-famine MMC PHA system using dairy manure was operated with relaxed control for 87 days. All three stages absorbed operational fluctuations and exhibited stable performance comparable to tightly controlled analogs. PHA mass fractions (0.29 to 0.40 g/g; volatile solids basis) approached commercial viability and the purified PHA (21 ± 3% 3-hydroxyvalerate; mole basis) exhibited similarly stable material properties. Finally, characterization of the MMC's PHA synthesizing capability suggested an innate potential for medium-chain-length monomers. Collectively, these results demonstrate the potential for PHA production from a feedstock without a strict nutrient limitation and the process's resilience against operational variability.

Batch anaerobic co-digestion of food waste and sludge: a multi criteria decision modelling (MCDM) approach

The present study investigates the application of Multi Criteria Decision Modelling (MCDM) on batch anaerobic co-digestion of kitchen food waste (FW) and the fresh septic tank sludge (STS) for robust output. The batch experiment was carried out at a temperature of 37 ± 2 °C. Different ratio of FW and STS (FW: STS:100:0; 75:25; 60:40; 50:50; 25:75; 0:100) were adopted based on volatile solid (VS) basis as co digestion strategy for 90 days digestion period. Results revealed that experimentally, mono-digestion of FW showed maximum biogas yield (544 ± 65 mL/g VS) followed by blending ratio of 75:25 (FW: STS). However, mono-digestion of STS showed only 150 ± 15 mL/g VS of biogas yield and also showed negative effect on co digestion with FW for every blending on VS basis as STS was high in minerals. MCDM revealed that the FW is the best possible alternative amongst the other blends with STS considering pH, chemical oxygen demand, VS reduction, alkalinity, C/N and biogas yield as output.

Synergistic effects of rice straw and rice bran on enhanced methane production and process stability of anaerobic digestion of food waste

This study investigated the synergistic effects of rice straw (RS) and rice bran (RB) addition on methane production and process stability of anaerobic digestion of food waste (FW). Positive synergistic effect (Synergy index (SI) = 1.03–1.24 > 1) was noticed in all the co-digestion reactors. The optimum mixing ratio of FW:RS:RB (volatile solid (VS) basis) was 60:10:30 with the maximum SI (1.24), achieving 27.4% increase in methane yield (235.4 mL/g-VS) and around 5 days shorter of λ (3.7 days) compared to the mono-digestion of FW (184.8 mL/g-VS and 8.2 days). Remarkably high concentration of volatile fatty acids (VFAs) was also accumulated in the mono-digestion of FW, especially propionic acid, which to a great extent caused the methane production to stagnate. Results from this study demonstrate that co-digestion of FW and RS with RB has high potentials for energy recovery from AD of the mixed feedstocks and its stable operation.

In search of the optimal inoculum to substrate ratio during anaerobic co-digestion of spent coffee grounds and cow manure.

The inoculum to substrate (I:S) ratio is a crucial operating parameter during the start-up period of anaerobic digestion (AD) processes and this ratio shows high differentiation with respect to substrate composition. While spent coffee grounds (SCG) have started to gain attraction in AD as a co-substrate due to their vast production and promising methane potential, there is still not enough information on the operative environment of SCG-based biogas reactors. This study investigated the optimal I:S ratio during anaerobic co-digestion of SCG and cow manure. Biochemical methane potential tests were conducted at mesophilic conditions and the influence of I:S ratio on methane production and digestion stability was evaluated at a wide range of I:S ratios from 0.5:1 to 4:1 (volatile solids (VS) basis). Methane yields increased gradually starting from the I:S ratio of 0.5:1 up to 3:1 and the highest methane yield (225 mlCH4 gVS-1) was achieved at the I:S ratio of 3:1. Comparatively lower methane yields were obtained at the ratios of 3.5:1 and 4:1. Instable AD conditions were established at the lowest I:S ratio examined (0.5:1), which caused volatile fatty acid (VFA) accumulation. The results highlighted that anaerobic co-digestion of SCG and cow manure is a promising approach, while the I:S ratio should be well-maintained due to the high potential risk of rapid and/or excess VFA production of these feedstocks.

Increased loading stress leads to convergence of microbial communities and high methane yields in adapted anaerobic co-digesters

Enhancing biogas production, while avoiding inhibition of methanogenesis during co-digestion of grease interceptor waste (GIW), can help water resource recovery facilities reduce their carbon footprint. Here we used pre-adapted and non-adapted digesters to link microbial community structure to digester function. Before disturbance, the pre-adapted and non-adapted digesters showed similar methane production and microbial community diversity but dissimilar community composition. When exposed to an identical disturbance, the pre-adapted digester achieved better performance, while the non-adapted digester was inhibited. When re-exposed to disturbance after recovery, communities and performance of both digesters converged, regardless of the temporal variations. Co-digestion of up to 75% GIW added on a volatile solids (VS) basis was achieved, increasing methane yield by 336% from 0.180 to 0.785 l-methane/g-VS-added, the highest methane yield reported to date for lipid-rich waste. Progressive perturbation substantially enriched fatty acid-degrading Syntrophomonas from less than 1% to 24.6% of total 16S rRNA gene sequences, acetoclastic Methanosaeta from 2.3% to 11.9%, and hydrogenotrophic Methanospirillum from less than 1% to 6.6% in the pre-adapted digester. Specific hydrolytic and fermentative populations also increased. These ecological insights demonstrated how progressive perturbation can be strategically used to influence methanogenic microbiomes and improve co-digestion of GIW.

Enhancement of Volatile Fatty Acids Production from Food Waste by Mature Compost Addition.

Food waste (FW) collected from a university canteen was treated in acidogenic fermenters to produce volatile fatty acids (VFA) under biological pretreatment with mature compost. Batch assays working at pH 6 revealed an increment of 9.0%, 7.9%, and 4.1% (on COD basis) of VFA concentration when adding 2.5%, 3.5%, and 4.5% w/w of mature compost, respectively, even though the volatile solids (VS) concentration of food waste was lower in the tests with increasing doses of mature compost. For batch tests at pH 7, this VFA generation improvement was lower, even though enhanced COD solubilization was recorded. Operating in semi-continuous conditions at 35 °C, pH of 6, and hydraulic retention time (HRT) of 3.5 days, the addition of 2.5% w/w of mature compost led to a VFA concentration up to 51.2 ± 12.3% more (on VS basis) when compared to a reference reactor without compost addition. Moreover, the percentage of butyric acid on VS basis in the fermentation broth working at a pH of 6 increased from up to 12.2 ± 1.9% (0% compost addition) to up to 23.5 ± 2.7% (2.5% compost addition). The VFA production was not improved when a higher percentage of mature compost was used (3.5% instead of 2.5% w/w), and it slightly decreased when mature compost addition was lowered to 1.5% w/w. When working at a pH of 7 in the semi-continuous fermenters with the addition of 2.5% w/w mature compost at an HRT of 3.5 days, an improvement of 79% and 104% of the VFA concentration (on VS basis) were recorded as compared to fermenters working at a pH of 6 with 2.5% and 0% w/w of mature compost addition, respectively. At a pH of 7, higher production of propionic and valeric acids was found with respect to the reactor working at a pH of 6. The effect of pH on VFA generation was estimated to have greater contribution than that of only biological pretreatment using mature compost. At a pH of 7, the VFA yield was higher for the fermenter working with 2.5% w/w mature compost but at a pH of 7 and HRT of 5 days, the effect of mature compost on VFA production improvement was lower than that obtained at a pH of 6. Moreover, higher solubilization in terms of soluble chemical oxygen demand and total ammonium was detected when biological pretreatment using mature compost was applied at both a pH of 6 and a pH of 7, which indicates enhanced hydrolysis in both conditions.

Dynamic behaviors of batch anaerobic systems of food waste for methane production under different organic loads, substrate to inoculum ratios and initial pH

This study characterized dynamic behaviors of batch anaerobic digesters treating food waste in terms of methane production, organics destruction and process stability under different organic loads (OLs), substrate to inoculum (S/I) ratios [on volatile solid (VS) basis] and initial pH. The results showed that OL, S/I ratio and initial pH significantly affected batch anaerobic process. Methane yield was proved to be inversely proportional to OL and S/I ratio. Digester with lowest OL (5g VS/L) obtained greatest methane yield (551.4mL/g VS), highest organics removal (94.1%) and good stability. Enhancing OL to 10g VS/L was recommended for satisfactory stability and higher volumetric methane productivity. When OL was designated as 10g VS/L, digester with low S/I ratio (1/2) achieved satisfactory methane yield (539.3mL/g VS), high organics removal (92.3%) and stable performance. When OL was relatively high (20g VS/L), adjusting initial pH to 7.5 contributed to stable performance via enhancing buffering capacity against volatile fatty acids (VFA) disturbance. Strong VFA inhibition occurred under high OL (40g VS/L) or great S/I ratio (2/1) or acidic initial pH (6.5). In this case, acetate was dominant VFA, followed by butyrate. However, when digester was stable, acetate was main VFA, followed by propionate. This study provided practical guidance on process configurations for batch digesters of food waste needed to achieve satisfactory performance and stability.

Co-feeding spent coffee grounds in anaerobic food waste digesters: Effects of co-substrate and stabilization strategy

Anaerobic digestion of spent coffee grounds (SCG) is considered disadvantageous, particularly under mono-digestion conditions, owing to slow degradation and nutrient imbalance. This study investigated the effect of co-feeding of SCG at a low ratio into food waste (FW) digesters, with the aim to determine whether SCG can be effectively treated and valorized using the spare capacity of existing digesters. Duplicate reactors showed stable performance under FW mono-digestion conditions but manifested severe deterioration in three volume turnovers after co-feeding of SCG (FW:SCG at 10:1 on a volatile solids basis). The reactors failed to recover despite repeated interrupted feeding and stabilization, and Ulva was added (FW:SCG:Ulva at 20:2:1) for nutrient supplementation. The two reactors subjected to different stabilization strategies (i.e., timing and intervals of interrupted feeding) responded differently to Ulva co-feeding: one recovered and maintained stable albeit suboptimal performance, whereas the other failed. Furthermore, the microbial communities developed differently in the reactors.

Production of volatile fatty acids and H2 for different ratio of inoculum to kitchen waste

ABSTRACT The aim of this study was to evaluate the effect of different inoculum ratio on the dark fermentation of kitchen waste in terms of volatile fatty acids (VFAs) and H2 production. The experiments were performed in batch bioreactors of effective volume 1 L without pH regulation. The ratio between the DS and KW was being increased from 0.11 to 0.51 on a volatile solids (VS) basis, while the initial content of KW was equal to 34.1 g VS/L. Increase of the DS/KW ratio from 0.11 to 0.28 resulted in the rise of VFAs and H2 production. Further increase in the amount of added DS did not cause a significant change in the production of VFAs and H2. In the bioreactor with the DS/KW ratio of 0.28, the production of VFAs and H2 was equal to 16.0 g/L and 68.1 mL/g VS, respectively. Acetic and butyric acids were produced in the largest amount and their content, for DS/KW ratio of 0.28, were equal 37% and 43%, respectively. At the ratio of DS/KW above 0.4, the caproic acid content attained the level of 25%. Based on the DS and KW microbiological analysis, it was observed that dominant bacteria were Bacteroidetes, Firmicutes, Proteobacteria, Spirochaetes and WWE1 at the phylum level.

Biohydrogen production from co-fermentation of fallen leaves and sewage sludge

The co-fermentation of fallen leaves and sewage sludge was performed for the production of hydrogen at different mixing ratios. The experimental results indicated that the optimal mixing ratio of sludge to leaves was 20:80 (volatile solids (VS) basis), and the co-fermentation process showed a synergistic effect on biohydrogen production at this mixing ratio. The biohydorgen yield reached 37.8 mL/g-VSadded at the mixing ratio of 20:80, which was higher compared to the mono-fermentation of sludge (10.3 mL/g-VSadded) or the leaves (30.5 mL/g-VSadded). The VS removal was also highest (15.7%) at the mixing ratio of 20:80, which was higher compared to sludge mono-fermentation (6.2%) or leaves mono-fermentation (12.8%). Meanwhile, the co-fermentation process enhanced the biohydrogen production rate and led to a more efficient fermentation pathway. Microbial community analysis showed that the co-fermentation system enriched much more Clostridium, Bacillus and Rummeliibacillus genera, which was responsible for the synergistic effect on biohydrogen production.

Solid-state anaerobic digestion of wheat straw: Impact of S/I ratio and pilot-scale fungal pretreatment

Solid State Anaerobic Digestion (SSAD) of fungal pretreated wheat straw was evaluated in a leach bed reactor. During a first experiment, the effect of Substrate/Inoculum (S/I) ratios on the start-up phase was investigated. High S/I increased methane productivity but also raised the risk of reactor failure due to Volatile Fatty Acid (VFA) accumulation. With S/I ratios between 1.2 and 3.6 (Volatile Solid (VS) basis), the SSAD start-up using wheat straw was successful. Moreover, reactors were able to recover from acidification when the Total VFA/alkalinity ratio was lower than 2 gHAc_eq/gCaCO3, with VFA concentrations lower than 10 g/L and a pH close to 5.5. The conventional threshold of 0.6 gHAc_eq/gCaCO3 for stable wet AD is therefore not adapted to SSAD.During a second experiment, after the wheat straw was submitted to a fungal pretreatment in a non-sterile pilot-scale reactor, it was digested with an S/I ratio of 2.8-2.9. Under batch SSAD conditions, the biodegradability of pretreated wheat straw was slightly improved in comparison to the control (254 versus 215 NmL/g VS, respectively). Considering mass losses occurring during the pretreatment step, suboptimal pretreatment conditions caused a slightly lower methane production (161 versus 171 NmL/gTSinitial after 60-days anaerobic digestion). Nevertheless, pretreatment improved the start-up phase with lower acidification relative to controls. It would be particularly beneficial to improve the methane production in reactors with short reaction times.