Biochemical Methane Potential Tests Research Articles

Effect of Mechanical Solids Separators on Potential Reduction of Methane Emissions From Dairy Manure Storage

Highlights Four mechanical manure separators were evaluated in on-farm studies. Mass balances and biochemical methane potential tests were conducted to measure on-farm separation efficiencies. Separator performance depended on environmental, material, and design factors. Over 70% solids separation and methane emission reduction potentials were achieved. Abstract. The separation efficiencies of four mechanical solid-liquid separation technologies and their effect on the reduction of methane production potential from dairy lagoons were studied using on-farm separator measurements and laboratory biomethane potential tests. The studied technologies included 1-stage sloped dual-screen, 2-stage sloped dual-screen, 1-stage horizontal scraped screen, and 1-stage sloped single-screen separators. The technologies were evaluated across various seasons on four dairies in California. On-farm sampling and measurements of the influent of flushed manure and the mass of the solids separated, as well as in-laboratory measurements of methane (CH4) production potential, were carried out. A mass balance approach was employed to determine the removal efficiencies of total and volatile solids (TS and VS) and nutrients, as well as methane emission reduction potential. The performance of the separators depended on manure characteristics, system design (e.g., screen size and orientation), separator operation and management (e.g., manure flow rate), and manure processing pit type and configuration. Among the four studied separator systems, the 2-stage sloped dual-screen separator showed the highest average TS and VS removal efficiencies, and methane emission potential reduction of 52.5%, 59.7%, and 55.8%, respectively. The 1-stage horizontal scraped screen separator had the lowest TS and VS removal efficiencies, and methane emission reduction potential of 6.3%, 9.3%, and 4.9%, respectively. The results have potential implications for nutrient and greenhouse gas management strategies and/or policies in agriculture. Keywords: Greenhouse gas, Lagoon, Manure management, Separation efficiency, Settling basin, Solid-liquid separation, Sustainable agriculture.

Methane Production of Sargassum spp. Biomass from the Mexican Caribbean: Solid-Liquid Separation and Component Distribution.

In the last decade, Sargassum spp. seaweed species have caused massive flooding on the Caribbean Sea coasts. These seaweed species have a high content of recalcitrant compounds, such as insoluble fibers and polyphenols, which generate low methane yields in anaerobic digestion (AD). This study investigated the effect of solid-liquid separation of Sargassum biomass on biodegradability and methane yield. A biochemical methane potential (BMP) test was conducted with both fractions and raw biomass (RB). A mass balance was developed to assess the distribution of the components. The obtained liquid fraction (LF) showed high biodegradability and a high methane production rate, and it generated a methane yield of 159.7 ± 7.1 N L kg VS-1, a value that corresponds to approximately twice that achieved with RB and the solid fraction (SF). The component distribution analysis showed that about 90% of total solids (TS), volatile solids (VS), ash, carbon, and cellulose were retained in the SF. In conclusion, the LF had high biodegradability and methane yield. This suggests the potential for LFs of Sargassum biomass to be treated in large-scale high-load reactors; however, studies applied to SFs are needed because they retain a large amount of organic matter with low biodegradability.

Energy Production from Cattle Manure within a Life Cycle Assessment Framework: Statistical Optimization of Co-Digestion, Pretreatment, and Thermal Conditions

Waste to energy processes from anaerobic digestion (WtE-AD) from cattle manure (CM) have low CH4 yields due to CM’s structural composition. The search for alternatives to increase the energy yields of these processes must consider the optimization of operating parameters within a framework of mitigating the environmental footprint. The goal of this paper is to provide a statistical optimization strategy based on experimental designs to improve CH4 yields and reduce the environmental profile of CM valorization through a WtE-AD process. Biochemical methane potential tests were conducted to determine the energetic and environmental effects that alkaline pretreatments, different AD temperatures, and co-digestion formulations with fruit and vegetable waste (FVW) have on the WtE-AD process from CM. The evaluation was conducted following a life cycle assessment approach through energy balances. The results indicate that the highest CH4 yield (384.3 mL CH4 g VS−1) and the lowest environmental impact (−0.06 kg CO2 eq kWh−1 of electricity production) were achieved with the co-digestion of CM with FVW 1:1, pretreatment with 10 g NaOH 100 g−1 of VS of CM, and a temperature of 45 °C. It was found that the CM pretreatment with NaOH substantially increases the energy profile of the WtE-AD process without compromising the environmental impact since greenhouse gas emissions in chemical production are negligible.

Effects of synthesized AlFe2O4 and MgFe2O4 nanoparticles on biogas production from anaerobically digested sugar refinery wastewater.

The application of magnetic nanoparticles (MNPs) toward sustainable water economy and bioenergy production has become a subject of great interest. Anaerobic digestion (AD) has been widely exploited in wastewater treatment settings, whereby utilization of MNP additives by microorganism response for degradation of organics into biogas is seen to be eco-friendly and economically viable. The present study investigated the impact of two synthesized MNPs such as aluminum ferrite (AlFe2O4) and magnesium ferrite (MgFe2O4) on biogas production via biochemical methane potential (BMP) tests. A BMP set-up of three (3) bioreactors was tested with a working volume of 0.8 L which comprises 0.5 L wastewater (WW) and 0.3 L activated sludge (AS) with 1.5g of the MNPs and a control system without MNPs. The degradation of chemical oxygen demand (COD) was increased with set-ups that contained MNPs as compared to the control system, MgFe2O4 (93.96%) > AlFe2O4 (85.95%) > control (68.83%). In terms of biogas production, the methane yield was also recorded as MgFe2O4 (85.7%) > AlFe2O4 (84.3%) > control (65.7%). The physical and chemical stability of MNPs makes them more advantageous for application in biogas production. In the prospects of biogas enhancement and biodegradability, integrating MNPs in an anaerobic digestion system will result in a more efficient anaerobic process performance for biogas production.

Novel free nitrous acid and ultrasonication pretreatment enhanced sludge biodegradability

ABSTRACT The main goal of this study was to investigate the novel combined Ultrasonication and Free Nitrous Acid (FNA) pretreatment on biodegradability and kinetics of thickened waste-activated sludge (TWAS). Partial factorial design with four levels of (0, 600, 1500, and 3000 KJ/Kg) for ultrasonication and 0, 0.7, 1.4, and 2.8 mg HNO2-N/L for FNA dose were examined creating 16 different combinations. Results revealed that combined pretreatment could significantly improve solubilization and solid destruction compared to solo pretreatments. The highest organic matter solubilization of 25.6% and volatile suspended solids destruction of 21.7% were observed when 2.8 mg HNO2-N/L and 1500 KJ/Kg were combined. Moreover, combining the pretreatments further enhanced biodegradability up to the highest percentage of 50.3% when pretreatment of 3000 KJ/Kg and 2.8 mg HNO2-N/L was applied. Also, the experimental data from a biochemical methane potential test was fitted well into First Order Kinetic and Modified Gompertz models, given that the coefficients of determination, R2, for models at all treatment levels were above 99%.

Effects of Oxidative Treatments on Biomethane Potential of Solid Olive Residues

As energy systems transition toward renewable resources, anaerobic digestion (AD) is actually receiving growing attention. AD relies on biochemical methane potential (BMP) tests to determine the methane potential of by-products of carbonious nature. This investigation aims to understand how an oxidative treatment, like the Fenton reaction, influences the BMP, starting from solid residues of olive oil production, coming from the two-phase extraction systems (TPES). We compared two different olive pomaces (with and without stones), both from TPES. The Fenton treatment here proposed is able to produce three effects in the employed matrices: improving the speed of BMP decreasing the bacteriostatic effect of phenols, reducing the H2S content in the produced biogas (precipitating it as FeS) and enhancing the production of methane in the first four weeks of the test.Graphical

Effect of nitrogen and iron supplementation on the process performance and microbial community structure of a hydrogen-producing reactor continuously fed with tequila vinasse

Dark fermentation is gaining importance as a bioprocess able to valorize different organic wastes/wastewaters through the co-production of organic acids and biogenic hydrogen (bioH2). In this study, the process performance and microbial community structure of a mesophilic lab-scale fermenter inoculated with an enriched hydrogenogenic consortium and fed with undiluted tequila vinasse (TV) were evaluated for 28 days equivalent to 112 hydraulic retention times (HRT). The influence of external nutrients supply on process performance was carefully evaluated. The fermenter was operated at 6 h HRT and fed with i) TV without additional nutrients, ii) TV with additional nitrogen (0.3 g/L), and iii) TV supplemented with nitrogen (0.3 g/L) and iron (0.01 g/L) chemical sources. Despite the inherent presence of macro- and micronutrients in TV, the use of raw TV with no additional nutrients resulted in a very low hydrogen production rate of 24 NmL-H2 L−1h−1. Contrarily, a nitrogen dosing of 1.2 g-NH4Cl L−1 led to a sudden increase in the bioH2 production, reaching productivity and yield values of 283 NmL-H2 L−1h−1 and 82 NmL-H2 g−1-volatile solids added, respectively. The further addition of FeSO4·7H2O (0.05 g/L) did not improve the TV-to-bioH2 conversion. Bridging hydrogen production and organic acids with molecular analysis, 16S rRNA gene sequencing, indicated that maintaining a suitable C/N ratio in the fermenter could ensure balance and syntrophy between lactic acid bacteria (e.g., Sporolactobacillus, Lactobacillus) and bioH2 producers (Clostridium). Further biochemical methane potential tests with the fermentation broth demonstrated that biohythane process (bioH2 + CH4) resulted in 5 % higher bioenergy recovery than that derived from the single-stage anaerobic digestion.

Biogas generation potential of discarded food waste residue from ultra-processing activities at food manufacturing and packaging industry

There is a tremendous increase in the demand for efficient and productive food supply chain due to rapid increase in the world population. At the same time, anaerobic digestion (AD) of discarded food waste residue (FWR) along with buffer substrate (BS) could be a sustainable way to transform waste to energy. In this study, substrate combinations (SCs) were prepared using pre (OFPre), and post (OFPost) processed FWR; anaerobic sludge (AS) as BS. The Biochemical methane potential test (BMPT) was performed for SCs prepared at varying proportion. The decline in substrate biodegradability ratio (SBR) and C/N ratio was also determined. The study was performed under three different temperature condition ranging from 35 °C to 73 °C over 90 days. It was found that SC of C5 and C6 had maximum biogas generation potential (BGP) under thermophilic (45 °C to 52 °C) temperature condition, and the cumulative CH4 production was 997 and 880 ml/g of VS. The artificial neural network (ANN) and random forest regression (RFR) were the two-machine learning (ML) tools used to predict and correlated the CH4 yield using input data series of SCs, SBR and C/N ratio. The ANN-CH4 prediction model was found to have best possible prediction efficiency.

Comparison of machine learning methods for predicting the methane production from anaerobic digestion of lignocellulosic biomass

Biogas derived from the anaerobic digestion of biomass can provide a carbon-neutral resource for green energy supply in the future. The biochemical methane potential (BMP) test has been widely applied to assess the characteristics of methane production from anaerobic digestion in batch mode. However, the determination of key parameters in the BMP test, such as specific methane yield (SMY), usually requires long-term experiments, especially for lignocellulosic feedstocks with slow degradation rates. This study aims to propose an appropriate data-driven model for the efficient prediction of the SMY using data from 277 samples of various lignocellulosic biomass materials by evaluating ten different machine learning (ML) methods. The Pearson coefficient matrix indicates that the chemical components are more relevant as attributes for the ML models, compared to element compositions, and the content of lignin has a strong linear correlation with SMY. Classic nonlinear ML methods (R2 ≥ 0.61) perform better than linear methods (R2 ≤ 0.56), and an ensemble learning model (R2 = 0.71) is better than a single learner (R2 ≤ 0.67). A K-nearest neighbor (KNN) model using leave-one-out cross-validation (LOOCV) obtains the best performance (R2 = 0.75, MAE = 30.2 mL/gVS). The generalization performance of the best model is found to have an average relative error of 10.05%.

Post-treatment Efficiency Evaluation of Anaerobic Digestion Effluent through Bio-electrochemical Biochemical Methane Potential Test

Post-treatment Efficiency Evaluation of Anaerobic Digestion Effluent through Bio-electrochemical Biochemical Methane Potential Test

Enhanced anaerobic digestion of waste activated sludge with periodate-based pretreatment

The potential of periodate (PI) in sludge anaerobic digestion is not tapped, although it has recently attracted great research interest in organic contaminants removal and pathogens inactivation in wastewater treatment. This is the first work to demonstrate significant improvement in methane generation from waste activated sludge (WAS) with PI pretreatment and to provide underlying mechanisms. Biochemical methane potential tests indicated that methane yield enhanced from 100.2 to 146.3 L per kg VS (VS, volatile solids) with PI dosages from 0 to 100 mg per g TS (TS, total solids). Electron spin resonance showed PI could be activated without extra activator addition, which might be attributed to the native transition metals (e.g., Fe2+) in WAS, thereby generating hydroxyl radical (•OH), superoxide radicals (•O2−), and singlet oxygen (1O2). Further scavenging tests demonstrated all of them synergistically promoted WAS disintegration, and their contributions were in the order of •O2− > •OH > 1O2, leading to the release of substantial biodegradable substances (i.e., proteins and polysaccharides) into the liquid phase for subsequent biotransformation. Moreover, fluorescence and ultraviolet spectroscopy analyses indicated the recalcitrant organics (especially lignocellulose and humus) could be degraded by reducing their aromaticity under oxidative stress of PI, thus readily for methanogenesis. Microbial community analysis revealed some microorganisms participating in hydrolysis, acidogenesis, and acetoclastic methanogenesis were enriched after PI pretreatment. The improved key enzyme activities and up-regulated metabolic pathways further provided direct evidence for enhanced methane production. This research was expected to broaden the application scope of PI and provide more diverse pretreatment choices for energy recovery through anaerobic digestion.

Hydrothermal pretreatment of protein-rich substrate: Modified physicochemical properties and consequent responses in its anaerobic digestion

Wasted tofu rich in protein was subject to hydrothermal pretreatment (HTPT) under different conditions (at 120, 140, 160 and 180 ℃; for 0, 30, 60 and 90 min) followed by biochemical methane potential (BMP) tests, and 140 ℃ and 0 min were found to be respectively the best temperature and duration for HTPT of tofu in terms of its biogas production. Under the under the optimal conditions (140 ℃, 0 min) the accumulative methane yield reached up to 510.9 mL·(gVS)-1, which was 26.98 % higher than that without HTPT (402.3 mL·(gVS)-1). The start-up process of continuous anerobic digestion (AD) of the tofu before and after hydrothermal treated (HT) at the optimal HTPT conditions (140 ℃, 0 min) was examined, to investigate and compare how their consequent AD responded to HTPT. It was found that, for start-up of continuous AD, the HT tofu delivered more balanced nutrients and thus led to more stable AD and quicker biogas production. Unavoidably, HTPT generated products refractory to biodegradation, to slightly decrease the total biogas production. During AD of HT tofu some weak ammonia-tolerant microbes, such as methylotrophic methanogens, survived and played indispensable roles. Analyses of living microbial community structure indicated that, some hydrolytic acidification bacteria intolerant to ammonia nitrogen (such as Proteobacteria) were always active and appeared at high proportion. The viable methylotrophic methanogens, e. g. RumEn M2, took obvious responsibilities in start-up of the AD for HT tofu.

Biochemical methane potential and active microbial communities during anaerobic digestion of biodegradable plastics at different inoculum-substrate ratios

The influence of the inoculum-substrate ratio (ISR) on the mesophilic and thermophilic biochemical methane potential test of two biodegradable plastics was evaluated. Poly(lactic acid) (PLA) and polyhydroxybutyrate (PHB) were selected for this study, the first for being recalcitrant to mesophilic anaerobic digestion (AD) and the second, by contrast, for being readily biodegradable. Several ISRs, calculated on the basis of volatile solids (VS), were tested: 1, 2, 2.85, 4, and 10 g(VS of inoculum).g(VS of substrate)−1. A high ISR was associated with an enhanced methane production rate (i.e., biodegradation kinetics). However, the ultimate methane production did not change, except when inhibition was observed. Indeed, applying the lowest ISR to readily biodegradable plastics such as PHB resulted in inhibition of methane production. Based on these experiments, in order to have reproducible degradation kinetics and optimal methane production, an ISR between 2.85 and 4 is recommended for biodegradable plastics. The active microbial communities were analyzed, and the active bacteria differed depending on the plastic digested (PLA versus PHB) and the temperature of the process (mesophilic versus thermophilic). Previously identified PHB degraders (Ilyobacter delafieldii and Enterobacter) were detected in PHB-fed reactors. Thermogutta and Tepidanaerobacter were detected during the thermophilic AD of PLA, and they are probably involved in PLA hydrolysis and lactate conversion, respectively.

Fate and effects of polyethylene terephthalate (PET) microplastics during anaerobic digestion of alkaline-thermal pretreated sludge

Plastics are resilient, hard to degrade materials that can persist in nature for centuries. Microplastics (MPs) exhibit similar tough character and hold the potential to harm marine and terrestrial ecosystems upon their release into the environment. Most modern wastewater treatment plants remove MPs from wastewater with over 90% efficiency but unfortunately concentrate them in sludge. Recent studies have reported MPs’ impact on the performance of sludge treatment systems, including anaerobic digesters. Despite its resilience, polyethylene terephthalate (PET) has inherent weaknesses against alkaline and thermal conditions and becomes more prone to further degradation if exposed to such stress conditions. Sludge pretreatment practices aiming to increase biogas production by disrupting floc structure show great similarity with the stress factors mentioned. Thus, this study aims to integrate pretreatment with anaerobic digestion and investigate the fate and effects of PET MPs during these processes. For this purpose, waste activated sludge samples spiked with different doses of PET (0, 1, 3, 6 mg/g TS) in sizes of 250–500 µm were pretreated by 0.5 M alkali for two days and then thermally hydrolyzed at 127 °C for 120 min. Pretreated and unpretreated sludges were digested in a 60-day biochemical methane potential test. The results showed that the spiking of PET MPs into sludge posed a positive impact on the methane yield of unpretreated reactors at statistically significant levels. Integrating pretreatment increased the methane yield by 22.0% and made the impact of MPs on digester efficiency no longer observable. Also, PET exposed to pretreatment and 60-day digestion experienced remarkable changes in surface morphology, crystallinity and carbonyl index, which can further impact their fate and effects on the environment.

Assessing the value of kinetic results from biochemical methane potential tests: Reproducibility from a large inter-laboratory study

Kinetic information extracted from biochemical methane potential (BMP) tests is often reported but its value is unclear. Inter-laboratory reproducibility provides a useful indication of its value. Here we extracted estimates of the first-order rate constant k from 1259 methane production curves collected in a large inter-laboratory study on BMP in order to quantify reproducibility. Reproducibility in k was poor; relative standard deviation was 50–140%. Substrate comparisons (k for one substrate compared to another) also had low reproducibility, regardless of low p values from inferential statistical tests. The use of a shared inoculum did not improve reproducibility in k. We conclude that k estimates from BMP tests only partially reflect intrinsic substrate properties. Therefore, interpretation and application of batch kinetic results should be done cautiously.

Biochemical Methane Potential of a Biorefinery’s Process-Wastewater and its Components at Different Concentrations and Temperatures

A sustainable circular bioeconomy requires the side streams and byproducts of biorefineries to be assimilated into bioprocesses to produce value-added products. The present study endeavored to utilize such a byproduct generated during the synthesis of 5-hydroxymethylfurfural as a potential feedstock for biogas production. For this purpose, biochemical methane potential tests for the full process-wastewater, its components (5-hydroxymethylfurfural, furfural, levulinic acid, and glycolic acid), together with furfural’s metabolites (furfuryl alcohol and furoic acid), and phenols (syringaldehyde, vanillin, and phenol), were conducted at mesophilic and thermophilic temperatures to assess their biodegradability and gas production kinetics. 0.1, 0.2, 0.3, and 0.4 g COD of the test components were added separately into assays containing 35 mL of inoculum. At their lowest concentrations, the test components, other than the process-wastewater, exhibited a stimulatory effect on methane production at 37 °C, whereas their increased concentrations returned a lower mean specific methane yield at either temperature. For similar component loads, the mesophilic assays outperformed the thermophilic assays for the mean measured specific methane yields. Components that impaired the anaerobic process with their elevated concentrations were phenol, vanillin, and 5-hydroxymethylfurfural. Poor degradation of the process-wastewater was deduced to be linked to the considerable share of 5-hydroxymethylfurfural in the process-wastewater governing its overall characteristics. With excessive recalcitrant components, it is recommended to use such waste streams and byproducts as a substrate for biogas plants operating at moderate temperatures, but at low rates.

Anaerobic Co-Digestion to Enhance Waste Management Sustainability at Yosemite National Park

This study evaluated the co-digestion of domestic wastewater solids (WWS) and food waste (FW) at the bench-scale for Yosemite National Park, California, which operates a 1900 m3/d wastewater treatment plant in El Portal, California. A 35-day biochemical methane potential test was performed on varying amounts of FW as a percentage of total waste (WWS plus FW) on a volatile solids basis (%FW). Specific methane yield and volumetric methane yield increased substantially with increasing %FW. A higher %FW was also associated with slower degradation kinetics but higher methane content in biogas. The 75 %FW treatment had relatively rapid kinetics, a high cumulative specific methane yield (453 mL CH4/g VS), and an elevated methane content in biogas, and is suggested as an upper limit %FW mixture for full-scale co-digestion. This, coincidently, is near the estimated ratio of WWS and FW production at the Park (70 %FW). Co-digesting the Park’s feedstock of FW with WWS in existing anaerobic digestion facilities could increase methane production five-fold. Combusting this methane in a combined heat and power system would produce about twice the energy needed to heat anaerobic digestors and power the treatment plant.

What Is the Right Rate? Determining Digestibility Kinetics of Pretreated Waste Activated Sludge During Anaerobic Digestion

Pretreatment of municipal sludge has the potential to improve anaerobic digestion by increasing the ultimate biodegradability of particulate chemical oxygen demand (PCOD) to methane (CH4) gas. However, the effect of pretreatment, that is, increased soluble COD from solubilization of PCOD, makes it difficult to correctly estimate hydrolysis kinetics in biochemical methane potential tests. In this study, batch digestion tests were used to evaluate the general impact of alkaline and thermal pretreatment of waste activated sludge (WAS) and to determine a different method to estimate hydrolysis rates from pretreated streams. Throughout the 60-day batch experiments, liquid and gas samples were collected and analyzed for total (TCOD) and semi-soluble (SSCOD) COD, total (TSS) and volatile (VSS) suspended solids, pH, volatile fatty acids (VFAs), and ammonium concentration. Pretreatments significantly decreased VSS concentrations at the start of digestion, but all samples showed similar VSS destruction to the control at the end of the experiments. Ultimate CH4 production was significantly increased by alkaline and thermal pretreatments, which produced 203 and 219 mL CH4 gVSS−1, respectively, versus 175 mL CH4 gVSS−1 in the control. Although CH4 generation was a good proxy for solids hydrolysis of untreated WAS, CH4 generation significantly overestimated the rate of hydrolysis of pretreated solids. The overestimation was due to the abundance of SSCOD available at the start of the experiment being converting to CH4. Incorporating SSCOD or VFAs into the hydrolysis kinetics resulted in much slower hydrolysis rates that were similar to those measured by VSS destruction directly (e.g., 0.08 d−1 with SSCOD + CH4 vs. 0.19 d−1 from CH4 alone for thermal pretreatment vs. 0.05 d−1 with VSS alone). The slower kinetics indicate that the necessity to understand the trade-offs between the performance advantages of pretreatment (i.e., solids destruction and energy conversion) versus the operational and financial impacts on large-scale digestion.

Urine pretreatment significantly promotes methane production in anaerobic waste activated sludge digestion

Methane production of waste activated sludge (WAS) in anaerobic digestion is hindered due to the rate-limited hydrolysis process and the low methane potential of WAS. Pretreatment of WAS is a common and appealing strategy to improve methane production in anaerobic digestion. In this study, we proposed to use urine, an easily obtained human waste with high ammonium concentration and pH, as a novel pretreatment strategy for anaerobic WAS digestion. Urine pretreatment at levels of 5–30 % (Vurine/Vurine+WAS) could substantially enhance methane production by 5–35 % in biochemical methane potential (BMP) tests, with the highest methane production of 179.6 ± 3.3 mL/g volatile solids (VS) achieved under the highest level of urine (i.e. 30 % urine addition). Based on the model analysis, the biochemical methane potential (B0) and hydrolysis rate of WAS (k) rose from 131.9 mL/g VS and 0.19 d−1 in the control without pretreatment to 136.3–178.2 mL/g VS and 0.22–0.30 d−1, respectively, after the urine pretreatment (5–30 % addition). Urine pretreatment with 5–30 % addition also improved the degradation extent (Y) of WAS by 3–35 %. The promising results indicate that urine pretreatment in anaerobic digestion is a promising technology to improve the efficiency of anaerobic digestion with environmental and economic benefits.

Combined ultrasonic–hydrothermal pretreatment to improve the biomethane potential of hazelnut shell

This work evaluated for the first time the employment of hydrothermal alone (i.e. at 60, 80 and 100 °C) and combined ultrasonic–hydrothermal pretreatments on hazelnut shell (HS) to promote the energetic valorization of HS through anaerobic digestion. The highest cumulative biomethane yield of 137 mL CH4·g VS−1 was achieved performing biochemical methane potential tests under wet–mesophilic conditions with the hydrothermally (i.e. at 100 °C) and ultrasonically–pretreated HS. This CH4 yield was 2.3–fold higher than that obtained with the raw HS due to an enhanced hemicellulose polymerization and delignification after the sequential hydrothermal and ultrasonic pretreatment. Under the same pretreatment conditions, total volatile fatty acids peaked at 755 mg HAc L−1. The biomethane production followed the modified Gompertz model (R2 = 0.993–0.996) and a Pearson correlation test showed that it was mainly influenced by the soluble chemical oxygen demand (i.e. 0.983). A positive energy balance revealed that the produced biomethane can offset the energy needed for the pretreatment.