Stability Of Anaerobic Digestion Process Research Articles

An integrated evaluation strategy for anaerobic digestion monitoring based on acid-base balance and thermodynamics of volatile fatty acid degradation

Process monitoring is a top priority for anaerobic digestion (AD), yet it poses a significant challenge due to its inherent inaccuracies and case-specific nature in diagnosing reactor status. Herein, a long-term instability simulation experiment using lignocellulosic biomass was conducted with varying organic loading rate (OLR) and hydraulic retention time (HRT) levels. The increase in OLR (0.75 to 20 kg VS m−3·d−1) and decrease in HRT (20 to 4 d) led to substantial total volatile fatty acids (TVFA) accumulation (>14 g/L) with a concomitant decrease in methane yield and organics removal. Full processing capacity with considerable conversion efficiency at four steps (hydrolysis: 60.43 ± 2.82 %, acidogenesis: 56.13 ± 3.27 %, acetogenesis: 56.03 ± 3.31 %, methanogenesis: 55.93 ± 3.32 %) and optimal methane yield (3.13 ± 0.17 L/(L·d)) was achieved at an OLR of 18 Kg VS·m−3·d−1 and HRT of 4 days. An integrated evaluation strategy for AD process stability is proposed based on the acid-base balance and thermodynamics of VFA degradation. Propionate, iso-valerate, TVFA/bicarbonate alkalinity (BA), and BA/total alkalinity (TA) were identified as potential auxiliary indicators for diagnosing AD imbalances, while ΔG for propionate and valerate degradation served as the early warning signs. To enhance the efficacy and universality of early warnings, relative variations for acid-base indicators based on steady-state (∼15 %) and the absolute threshold value for ΔGPropionate (-40 KJ/mol) and ΔGValerate (-55 KJ/mol) were adopted. This approach integrates considerations for microbial adaption, thereby enhancing an assessment of whether a given signal reflects irreversible acidification or is merely a temporary fluctuation induced by reversible acidification. The proposed strategy enables a reliable and accurate early warning, holding great promise for practical applications.

Acclimation of microbial communities to low and moderate salinities in anaerobic digestion

In recent years anaerobic digestion (AD) has been investigated as suitable biotechnology to treat wastewater at elevated salinities. However, when starting up AD reactors with inocula that are not adapted to salinity, low concentrations of sodium (Na+) in the influent can already cause disintegration of microbial aggregates and wash-out. This study investigated biomass acclimation to 5 g Na+/L of two different non-adapted inocula in two lab-scale hybrid expanded granular sludge bed (EGSB)-anaerobic filter (AF) reactors fed with synthetic wastewater. After an initial biomass disintegration, new aggregates were formed relatively fast (i.e., after 95 days of operation), indicating microbial community adaptation. The newly formed microbial aggregates accumulated Na+ at the expense of calcium (Ca2+), but this did not hamper biomass retention or process performance. The hybrid reactor configuration, including a pumice stone filter in the upper section, and the low up-flow velocities applied, were key features for retaining the biomass within the system. This reactor configuration can be easily applied and represents a low-cost alternative for acclimating biomass to saline effluents, even in existing digesters. When the acclimated biomass was transferred from EGSB to an up-flow anaerobic sludge blanket (UASB) reactor configuration also fed with saline synthetic wastewater, more dense aggregates in the form of granules were obtained. The performances of the UASB inoculated with the acclimated biomass were comparable to another reactor seeded with saline-adapted granular sludge from a full-scale plant. Regardless of the inoculum origin, a defined core microbiome of Bacteria (Thermovirga, Bacteroidetes vadinHA17, Blvii28 wastewater-sludge group, Mesotoga, and Synergistaceae) and Archaea (Methanosaeta and Methanobacterium) was detected, highlighting the importance of these microbial groups in developing halotolerance and maintaining AD process stability.

The role of microbial ecology in improving the performance of anaerobic digestion of sewage sludge.

The use of next-generation diagnostic tools to optimise the anaerobic digestion of municipal sewage sludge has the potential to increase renewable natural gas recovery, improve the reuse of biosolid fertilisers and help operators expand circular economies globally. This review aims to provide perspectives on the role of microbial ecology in improving digester performance in wastewater treatment plants, highlighting that a systems biology approach is fundamental for monitoring mesophilic anaerobic sewage sludge in continuously stirred reactor tanks. We further highlight the potential applications arising from investigations into sludge ecology. The principal limitation for improvements in methane recoveries or in process stability of anaerobic digestion, especially after pre-treatment or during co-digestion, are ecological knowledge gaps related to the front-end metabolism (hydrolysis and fermentation). Operational problems such as stable biological foaming are a key problem, for which ecological markers are a suitable approach. However, no biomarkers exist yet to assist in monitoring and management of clade-specific foaming potentials along with other risks, such as pollutants and pathogens. Fundamental ecological principles apply to anaerobic digestion, which presents opportunities to predict and manipulate reactor functions. The path ahead for mapping ecological markers on process endpoints and risk factors of anaerobic digestion will involve numerical ecology, an expanding field that employs metrics derived from alpha, beta, phylogenetic, taxonomic, and functional diversity, as well as from phenotypes or life strategies derived from genetic potentials. In contrast to addressing operational issues (as noted above), which are effectively addressed by whole population or individual biomarkers, broad improvement and optimisation of function will require enhancement of hydrolysis and acidogenic processes. This will require a discovery-based approach, which will involve integrative research involving the proteome and metabolome. This will utilise, but overcome current limitations of DNA-centric approaches, and likely have broad application outside the specific field of anaerobic digestion.

Enhanced biogas biological upgrading from kitchen wastewater by in-situ hydrogen supply through nano zero-valent iron corrosion

The in-situ hydrogen supply by nano zero-valent iron (nZVI, nFe0) corrosion provided a feasible way to improve the efficiency of biogas biological upgrading. This work studied the effects of nZVI at different dosages (0, 2, 4, 6, 8 and 10 g/L) on anaerobic digestion of kitchen wastewater by two buffer systems 2-[4-(2-hydroxyethyl) piperazin-1-yl] ethanesulfonic acid (HEPES) and sodium hydrogen carbonate (NaHCO3). The addition of nZVI improved the content of methane (CH4) and stability of anaerobic digestion process. In HEPES buffer system, the CH4 was all increased and the maximum reached 90.51% with 10 g/L nZVI, higher than 32.25% compared to the control. The maximum hydrogen enrichment (HE) was 113 ppb after nZVI addition, indicating the mass transfer efficiency of hydrogen (H2) was improved. Microbial community analysis showed that the total relative abundance of Methanobacterium and Methanolinea at 10 g/L nZVI was 53.72%, which was 1.62 times of the control group. However, in the NaHCO3 buffer system with 10 g/L nZVI addition, the content of CH4 and the loosely bound extracellular polymeric substances (LB-EPS) was lower than the control. The results indicated that the addition of nZVI was feasible for biogas upgrading, and the bidirectional effect of nZVI on the promotion or inhibition of bio-methanation might be related to the buffer system of the anaerobic process.

Thermodynamics of volatile fatty acid degradation during anaerobic digestion under organic overload stress: The potential to better identify process stability

Anaerobic digestion (AD) operating under organic overload stress usually increases the potential for process instability, leading to significant economic and ecological consequences. Volatile fatty acids (VFAs) accumulation is regularly considered a major factor during AD and their degradation is subject to thermodynamic constraints. To date, no study has systematically investigated the mechanisms of VFA degradation on process stability from the perspective of thermodynamics. Hence, increased substrate-to-inoculum ratio was applied in this study to simulate organic overload stress using batch tests with Hybrid Pennisetum. As a result, VFAs accumulation increased, accompanied by decreased methane yield, slower methane production kinetics and even severe process instability. Metagenomic analysis demonstrated that the accumulated propionate and butyrate were degraded by methyl-malonyl-CoA and the β-oxidation pathway while syntrophic acetate oxidation was preferred during acetate degradation. The deviation of stability parameters to varying degrees from the recommended threshold values was observed. However, a subsequent thermodynamic analysis revealed that moderate organic overload stress merely retarded the syntrophic oxidation of propionate, butyrate, and acetate. As a result, the methanogenic activity decreased, and the lag phase of AD was extended, but no adverse thermodynamic effects actually occurred. Changes in the Gibbs free energy for syntrophic propionate and acetate oxidation have the potential to better identify process stability. This study provided novel insights into the underlying thermodynamic mechanisms of VFA degradation and may have important implications for improving the current diagnostic mode for AD process stability.

Application of biomass-derived hydrochar in process stability of anaerobic digestion

Stability and in-situ upgrading of biogas in anaerobic digestion (AD) processes using biomass-derived hydrochar as a cheaper alternative is presented. The influence of hydrochar on AD of organic fractions of municipal solid wastes (OFMSWs) was conducted at OLR of 1 kg VS/[m3.d]). The biochemical methane potential of hydrochar was insignificant while digester amended with 20% hydrochar had a longer lag phase, producing biogas after the third day of AD. The total volume of biogas of 464.29 NI/kg oTS in digester containing both OFMSWs and hydrochar was higher when compared with digester without hydrochar (257.62 NI/kg oTS). Likewise, the methane content of digester containing hydrochar was 10% higher. The pH and electrical conductivity in amended digester declined daily from 8.36 to 7.31 and 17.12 to 8.15 mS/cm, respectively. The stability indicators responded positively to the addition of hydrochar to OFMSWs, outperforming other digesters treated with only hydrochar or OFMSWs.

Ozonation of sewage sludge prior to anaerobic digestion led to Methanosaeta dominated biomethanation

Anaerobic digestion (AD) of sewage sludge is commonly employed for sludge stabilization and energy generation at wastewater treatment plants. Of pretreatment techniques used to accelerate rate-limiting hydrolysis step in AD, economically feasible ozonation is prevalent and permits sludge reduction via oxidation. The stability of AD process during ozonation is crucial for successful energy recovery and needs optimal dosage that encourages methanogenic consortium efficient in methane production. This study, therefore, aimed at investigating the impact of 0.03, 0.06 and 0.09 g O3/g TSS ozone doses on the productivity of mesophilic semi-batch lab-scale 5 L AD operated at a hydraulic retention time of 15 d with organic loading rates of 1.45–1.80 g VS/L.d and on its methanogenic consortium. Measurements of biogas, methane and quantification of methanogenic consortium were done by water displacement, gas chromatography, and fluorescence in situ hybridization techniques, respectively. Although all applied ozone doses improved sludge characteristics, the highest average daily removal efficiencies with 35% COD, 32% TS, 42% VS, 60% TSS and 69% VSS were achieved through ozonation with 0.06 g O3/g TSS ozone dose. The same ozone dose also yielded 48% more methane in the biogas recovered and this biomethanation was correlated to the abundance of acetotrophic Methanosaeta species.

Simultaneous effect of cathode potentials and magnetite concentrations on methanogenesis of acetic acid under different ammonia conditions

Electromethanogenesis (EM) is a system that facilitates direct interspecies electron transfer (DIET) in anaerobic digestion (AD) by providing an external power supply to favor desired reactions. Substrates of AD commonly contain ammonia (NH3) as biodegradation product of nitrogen-rich compounds that can deteriorate the stability of AD process. Optimized cathode potential (VCAT) and magnetite (Mag) concentration ([Mag]) are expected to improve AD efficiency in the presence of NH3. Response surface analysis with central composite face-centered design was used in this study to investigate the effect of VCAT and [Mag] under different total ammonia nitrogen concentration ([TAN]). Highest cumulative methane production was achieved at VCAT = -737.4 mV, [Mag] = 18.2 mM, and [TAN] = 1.5 g/L; highest acetate degradation rate was achieved at VCAT = 757.6 mV, [Mag] = 21.4 mM, and [TAN] = 1.5 g/L. The study demonstrated that VCAT promotes either microbial growth or electrochemical NH3 removal. A Shift from acetoclastic to hydrogenotrophic pathway was also observed by the increase of hydrogenotrophic methanogen populations at the end of experiment. This study is beneficial for process control of AD under different NH3 conditions.

Effect of the Organic Loading Rates Increase on Process Stability and Microbial Community Composition during the Anaerobic Digestion of Fresh Vinegar Residue

Anaerobic digestion (AD) was employed to utilize fresh vinegar residue (FVR) in this study. The performance and microbial community structure at increasing organic loading rates (OLRs) were evaluated. The results showed that the mono-fermentation of FVR was unstable at high OLRs due to the high acidity and the high cellulose content of the raw material. The optimum OLR was 2.0 g VS/(L·d), with the maximum average methane yield at 216 mL /g VS. High OLRs (2.5 g VS/(L·d)) led to the irreversible inhibition of AD process, although digester ceased feeding during the recovering stage. The diversity of microbial community decreased at high OLRs. The relative abundance of major bacterial community at different OLRs also varied. The syntrophic volatile fatty acids (VFAs) oxidizing bacteria declined while the acid-producing bacteria became dominant. The close correlation between the AD process stability and the microbial community structure suggested that maintaining a rich and diverse microbial community was essential for the practical application of AD on vinegar residue (VR) utilization.

Substrate-induced pH changes and process stability of anaerobic digestion of shea waste

Shea butter extraction involves the generation of waste of environmental concern. To help find a solution to the management of this waste, and to search for an alternative source of energy, this study was carried out to investigate the potential of shea waste in generating methane gas through anaerobic digestion. At high concentrations, volatile fatty acids produce a low pH, which may inhibit hydrolysis, thereby affecting the stability of the anaerobic digestion process. The influence of pH changes on process stability (or otherwise) was also investigated. The study involved two fermentation processes: mono-fermentation and co-fermentation involving a mix of shea waste and cattle dung. The mono-fermentation investigations comprised six treatments - three organic dry matter concentrations of 7%, 5% and 3%, combined with two hydraulic retention times of 30 and 60 days. The co-fermentation investigations comprised three treatments of shea waste and cattle manure mix in proportions (by volume) of 50:50, 75:25 and 90:10. The results showed that changes in pH were a good parameter for indicating process instability. The results also showed that monofermentation of shea waste was not a viable option in anaerobic digestion for biogas production, whilst only the substrate with 50% cattle manure in the co-fermentation trials showed process stability, producing biogas with adequate methane content.

Efficient anaerobic digestion of a mild wet air pretreated molasses ethanol distillery stillage: A comparative approach

The effect of a mild, wet air pretreatment and the subsequent anaerobic digestion (AD) was examined on the recovery of a complex and toxic molasses ethanol distillery stillage. The biogas yield and organics removal due to pretreatment were compared with the raw stillage AD. The application of a scoria support in this industrial residue AD process stability was also assessed. Consequently, a statistically significant cumulative specific methane recovery difference (p-value = 0.000) with an almost complete biological oxygen demand (BOD) removal and a significant chemical oxygen demand (COD) reduction, which were 100% and 92% respectively were achieved. Additionally, the biogas recovery rate was hastened due to pretreatment. The application of scoria, whose property has been instrumentally inspected, has helped stabilize the pH in the AD systems. In a comparative approach, this study suggests the energy benefit and an ecofriendly discharge of stillage by the ethanol industry towards sustainability.

Promoting anaerobic digestion by algae-based hydrochars in a continuous reactor

The microalgae and macroalgae-based hydrochars produced by hydrothermal carbonization were mainly used as biofuels, however, their application in anaerobic digestion (AD) was little known. This study investigated the effects of microalgae Chlorella-based hydrochar (HC-C) and macroalgae Laminaria-based hydrochar (HC-L) on a continuous AD reactor under different organic loading rates (OLR). The AD process stability of hydrochars supplemented reactors were performed well under the increase of OLR from 2.6 to 6.5 g COD/L/d, and HC-C and HC-L addition could significantly enhance the daily methane yield by 36.0% and 31.4%, respectively. Interestingly, the possible mechanisms of HC-C and HC-L on the enhanced AD were similar, namely increasing sludge granulation, promoting the Methanothrix relative abundance and key enzyme activities, and further facilitating potential direct interspecies electron transfer between methanogens and organic-degrading bacteria. This study provided an implication on the potential application of algae-based hydrochars in wastewater treatment and energy recovery.

Effect of organic loading rate on anaerobic digestion of raw cheese whey: experimental evaluation and mathematical modeling

AbstractWhey is a high strength waste product of cheese manufacture. Anaerobic digestion of cheese allows pollution potential reduction and simultaneously energy production. Organic loading rate (OLR) is an important operating variable for anaerobic digestion (AD) process related to system stability, waste treatment capacity and biogas production. The actual OLR depends on the types of wastes (i.e., content of chemical oxygen demand [COD]) fed into a digester. In this paper, the effect of OLR on the AD process of the raw cheese whey in a semi-continuous up-flow system is studied experimentally and with numerical simulations using a simple dynamical model calibrated with experimental data. The digester operation was performed for 90 days, doubling the OLR every 30 days, from OLR of 2.5–10 gCOD L−1 d−1. Experimental results show that the increase in OLR favors the production of biogas. However, the proportion of methane may decrease. The highest methane yield and the most considerable substrate degradation were obtained at OLR of 5 gCOD L−1 d−1 and 10 gCOD L−1 d−1, respectively. The proposed mathematical model is used to describe the dynamic behavior of key variables as COD, volatile fatty acids (VFA) and methane production. A good fit between the variables estimated by the mathematical model and experimental data was obtained, reaching determination coefficients (R2) greater than 0.8. Therefore, this model might be beneficial in predicting the maximum methane production rate and the maximum OLR that could be used without risking the AD process stability.

Effect of carbon-rich-waste addition as co-substrate on the performance and stability of anaerobic digestion of abattoir wastewater without agitation

Multiple wastes’ co-digestion is one of the alternatives for improved anaerobic digestion (AD) process of industrial and municipal wastes. The present work investigated the influence of fruit–vegetable solid waste (FVW) addition as a co-substrate on the performance of AD of abattoir wastewater (AWW). The co-digestion was done at a lab-scale-based experiment under mesophilic condition using a two-phase anaerobic sequencing batch reactor without agitation. It was tested at different mixing ratios (100%AWW; 75%AWW:25%FVW; 50%AWW:50%FVW; 25%AWW:75%FVW; 100%FVW) with the intention of looking for the best mixing ratio with the best performance. It was fed on a semi-continuous basis and operated for 18 days (d) total retention time (HRT): 3 days for the acidogenesis reactor and 15 days for methanogenesis reactor. The addition of FVW enhanced biogas yield and VS removal by 70.26% and 57.11%, respectively, at optimum mixing ratio. Moreover, to some extent improvement of AD process stability verified by the decreased TVFA:TAlk ratio and free ammonia nitrogen was observed upon progressive addition of FVW. Finally, this co-digestion process should further be studied for its performance at different HRTs with agitation.

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.

Pilot Scale Solid State Anaerobic Co-digestion of Dairy Manure and Milking Parlor Wastewater

A pilot scale solid-state anaerobic co-digestion study was conducted to elucidate the feasibility of anaerobic digestion as an effective treatment method for dairy manure (DM) and milking parlor wastewater (MPW), and it focusing mainly on the stability of anaerobic digestion process and methane production potential. Dairy manure and milking parlor wastewater were co-digested (1/0.48, w/w) in 11m3 volume reactor for 63days at the mesophilic condition (36°C). In addition, pH, volatile fatty acids and ammonia were periodically analyzed to monitor anaerobic digestion inhibitions. The results showed that the co-digestion of DM and MPW was successfully performed without any inhibition caused by ammonia and volatile fatty acids accumulation. The potential methane production (P) from a modified Gompertz model was about 130.5 mL g-VS-1, and approximately 80% of the methane production was obtained during the 40 days digestion test and the lag phase was 14 days. The pilot scale SSAD (solid-state anaerobic digestion) system showed about 1.3-fold higher methane production than lab-scale system. The results of solid-state anaerobic co-digestion of bedded pack barn dairy manure and milking parlor wastewater using pilot-scale anaerobic digester can be used as a reference for the design of farm-scale anaerobic digester. The pilot scale SSAD system showed about 1.3-fold higher methane production than lab-scale system. The potential methane production (P) was about 130.5 mL g-VS-1, and approximately 80% of the methane production was obtained during the 40 days digestion test and the lag phase was 14 days.

Effect of biogas recirculation strategy on biogas upgrading and process stability of anaerobic digestion of sewage sludge under slightly alkaline condition

In this study, in-situ biogas upgrading was examined and compared in lab-scale semi-continuous anaerobic digestion (AD) systems coupled with biogas recirculation under neutral and slightly alkaline conditions. Results showed that neutral pH (~7.5) condition yielded 6–7% higher CH4 content and 52–53% less H2S production, while slightly alkaline condition (pH ~8.0) achieved 14–15% higher CH4 content and 76–77% less H2S production under biogas recirculation in comparison to the control (no biogas recirculation). Little effect was noticed on daily CH4 production rate at slightly alkaline condition under biogas recirculation. Endogenous free ammonia nitrogen-dissolved inorganic carbon-volatile fatty acids (FAN-DIC-VFAs) buffer system was found to regulate the pH and alkalinity in the bulk liquor. Therefore, biogas recirculation under slightly alkaline condition is proposed as an efficient and sustainable strategy for biogas upgrading during AD of sewage sludge.

Anaerobic Conversion of Biodegradable Municipal Solid Waste to Biogas: A Review

Huge quantity of Municipal Solid Waste (MSW) is generated daily. This waste comprises a biodegradable portion which can be converted into biogas (bioenergy) by anaerobic digestion (AD). This study reviews MSW and its management, AD feedstock and their characteristics, factors affecting biogas production in a biodigester and anaerobic co-digestion of Organic Fraction of MSW (OFMSW) with other substrates. Municipal solid waste is managed through waste diversion (reduction, reuse, recycling and recovery) and waste disposal (controlled incineration, landfilling and controlled dumping). AD feedstock includes agricultural waste/residues, animal wastes, energy crops, food waste, forestry crops and residues, organic industrial waste and wastewater, weeds, aquatic algae, sewage and OFMSW. The essential factors that influence the production of biogas are temperature, pH, mixing rate, carbon/nitrogen ratio, organic loading rate, micro and macro-nutrient availability, retention time, nature of the feedstock and digester type. Anaerobic co-digestion of OFMSW with other substrates results in improved AD process stability, enhanced biogas productivity, maximization of the capacity of available feedstock for anaerobic digestion. It is also a cost-effective and improved technique to optimize anaerobic digestion process via the increase in nutrients and bacterial variety in substrates. The generation rate and composition of MSW, as well as the characteristics of OFMSW feedstock for anaerobic digestion, are required for the design of a full-scale biodigester for municipal use. The information provided in this review is invaluable to researchers, governments, industries and other stakeholders interested in anaerobic conversion of biodegradable solids to bioenergy.

Impact of biochar-supported zerovalent iron nanocomposite on the anaerobic digestion of sewage sludge

Anaerobic digestion (AD) is an attractive technology for sludge treatment as it stabilizes sludge and produce renewable energy. However, problems such as low organic matter content and high heavy metals level are often encountered which severely limits the effectiveness of AD. In this study, the biochar-supported nanoscale zerovalent iron (nZVI-BC) was synthesized and used as additives during AD of sewage sludge to investigate the enhancement effects for methane production and its impacts on microbial structure at mesophilic temperature. nZVI-BC addition enhanced process stability by improving the generation and degradation of intermediate organic acids, but inhibitory effects were observed at high dosage. The methane content and cumulative methane yields were increased by 29.56% and 115.39%, respectively. Compared with AD without nZVI-BC, the application of nZVI-BC showed positive effect on improvement of metals (Cu, Cd, Ni, Cr, and Zn) stabilization in the digestate. Microbial community analysis illustrated that nZVI-BC addition could significantly increase the Shannon diversity index and Chao1 richness index of archaea, and meanwhile archaea were more diverse in nZVI-BC amended digesters than in control. It was notable that Methanosaeta dominated in all the digesters at genera level, while the relative abundance of hydrogenotrophic methanogens (Methanobacterium and methanospirillum) increased 35.39% in nZVI-BC amended digesters compared to the control, resulting in higher methane production. The results will guide development of microbial management methods to enhance the stability of AD process.

Bioconversion of Dried Leaves from Algerian Date Palm (<i>Phoenix dactylifera</i> L.) to Biogas by Anaerobic Digestion

This work aimed to study anaerobic digestion of a new substrate made from the waste of Algerian date palm (Phoenix dactyliferaL.) in order to evaluate its biogas production capacity. The lignocellulosic component biodegradability and the process stability were also tested. Two reactors were launched with the same concentration of 20g total solid (TS)/L. A physical pre-treatment (grinding) of dried leaves lowered the substrate size below 2 mm. The theoretical methane yield (499 - 538 mL CH4/g of Volatile Solids (VS)) indicated a high substrate methanogen potential. The tested methane and biogas yield reached 130±6 mL CH4/gVS and 197±6 mL/g VS respectively. The biodegradability through the volatiles solid reduction was 29%. The total alkalinity (TA) value increased from the beginning to the end, due to the substrate nature which gave a high buffering capacity of the medium. During the stable process, a higher Chemical Oxygen Demand (COD) removal was reached with a value of 77.58%. The majority of the biogas yield was generated during the same period, which confirmed that the anaerobic digestion process stability is indispensable. Finally, this study was an opportunity to better exploit local by-products for the bioenergy production.