Dark fermentative hydrogen production from simple sugars and various wastewaters by a newly isolated Thermoanaerobacterium thermosaccharolyticum SP-H2

The hydrogen-producing bacterium SP-H2 was isolated from a thermophilic acidogenic reactor inoculated with municipal sewage sludge and processing a carbohydrate-rich simulated food waste. Based on the 16S rRNA gene sequence, the bacterium was identified as Thermoanaerobacterium thermosaccharolyticum. The maximum growth rate was observed at 55–60 °C and pH 7.5. The H₂-producing activity of the bacterium was studied using mono-, di- and tri-saccharides related to both hexoses (maltose, glucose, mannose, fructose, lactose, galactose, sucrose, raffinose, cellobiose) and pentoses (xylose and arabinose), as well as using real wastewaters (cheese whey, confectionery wastewater, sugar-beet processing wastewater). The highest H₂ yield was observed during dark fermentation (DF) of maltose (1.91 mol H₂/mol hexose or 77.8 mmol H₂/L). The maximum H₂ production rate was observed during DF of xylose (13.3 ml H₂/g COD/h) and cellobiose (2.47 mmol H₂/L/h). The main soluble metabolite products were acetate, ethanol and butyrate. The acetate concentration had a statistically significant positive correlation with the H₂ content in biogas and the specific H₂ yield. Based on the results of the correlation analysis, it was tentatively assumed that in the formic acid (mixed-acid) type fermentation, the rate of H₂ production was higher than in the butyric acid type fermentation. With regard to real wastewater, cheese whey and confectionery wastewater were distinguished by a higher H₂ yield (152 ml H₂/g COD) and H₂ production rate (0.57 mmol H₂/L/h), respectively. The highest concentrations of confectionery wastewater and cheese whey, at which the DF process took place, were 5915 and 7311 mg COD/L, respectively. At the same time, SP-H2 dominated in the microbial community, despite the presence of indigenous microorganisms in wastewater. Thus, T. thermosaccharolyticum SP-H2 is a promising strain for DF of carbohydrate-rich unsterile wastewater under thermophilic conditions.     

Fermentative hydrogen production from wastewaters: A review and prognosis

Biohydrogen is a promising candidate which can replace a part of our fossil fuels need in day-to-day life due its perceived environmental benefits and availability through dark fermentation of organic substrates. Moreover, advances in biohydrogen production technologies based on organic wastewater conversion could solve the issues related to food security, climate change, energy security and clean development in the future. An evaluation of studies reported on biohydrogen production from different wastewaters will be of immense importance in economizing production technologies. Here we have reviewed biohydrogen production yields and rates from different wastewaters using sludges and microbial consortiums and evaluated the feasibility of biohydrogen production from unexplored wastewaters and development of integrated bioenergy process. Biohydrogen production has been observed in the range of substrate concentration 0.25–160 g COD/L, pH 4–8, temperature 23–60 °C, HRT 0.5–72 h with various types of reactor configuration. The most efficient hydrogen production has been obtained at an organic loading rate (OLR) 320 g COD/L/d, substrate concentration 40 g COD/L, HRT 3 h, pH 5.5–6.0, temperature 35 °C in a continuously-stirred tank reactor system using mixed cultures and fed with condensed molasses fermentation soluble wastewater. The net energy efficiency analysis showed vinasse wastewater has the highest positive net energy gain followed by glycerin wastewater and domestic sewage as 140.39, 68.65, 51.84 kJ/g COD feedstock with the hydrogen yield (HY) of 10 mmol/g COD respectively.     

Approbation of an innovative method of pretreatment of dark fermentation feedstocks

According to the International Energy Agency, only a small part of the full potential of biomass energy is currently used in the world. The annual amount of agricultural waste in the Russian Federation is estimated at about 152 million tons, and the energy potential of animal waste is 201 PJ/year. Anaerobic digestion is an efficient method of converting organic waste into renewable energy sources. Previously, the positive effect of pretreatment of various organic feedstocks in vortex layer apparatus (VLA) on the characteristics of anaerobic digestion and energy efficiency was shown. Currently, there is a significant interest in the world in obtaining biohydrogen from organic waste using the dark fermentation (DF) process. During pretreatment in the VLA, the iron working bodies are abraded and iron particles are introduced into the feedstock of the DF reactor. This may have a positive effect on the production rate and yield of hydrogen, which has not been previously studied. This work is aimed at evaluating the possibility of using the VLA as a method for pretreatment of a dark fermentation feedstock for the intensification of biohydrogen production. To achieve this goal, an experimental setup was constructed. It consisted of a 45 L DF reactor, a VLA and a process control system to collect data on the DF process parameters every 5 min. At a hydraulic retention time in the DF reactor of 24 h and in the VLA of 30 s, the hydrogen content in the biogas increased from 51.1% to 52.2%. At the same time, the pH increased from 3.85 to 4.8–4.9, and the hydrogen production rate increased by 16% to 1.941 L/(L day). The hydrogen yield was 80.9 ml/g VS. Thus, pretreatment of the feedstock in VLA can be an effective way to intensify the DF process; however, further study of the VLA operating modes is required in order to optimize the concentrations of iron particles introduced into the feedstock for the most efficient continuous production of dark fermentative biohydrogen.     

Acetate and butyrate as substrates for hydrogen production through photo-fermentation: Process optimization and combined performance evaluation

Organic acids viz., acetate and butyrate were evaluated as primary substrates for the production of biohydrogen (H₂) through photo-fermentation process using mixed culture at mesophilic temperature (34 °C). Experiments were performed by varying parameters like operating pH, presence/absence of initiator substrate (glucose) and vitamin solution, type of nitrogen source (mono sodium salt of glutamic acid and amino glutamic acid) and gas (nitrogen/argon) used to create anaerobic microenvironment. Experimental data showed the feasibility of H₂ production along with substrate degradation utilizing organic acids as metabolic substrate but was found to be dependent on the process parameters evaluated. Maximum specific H₂ production and substrate degradation were observed with acetic acid [3.51 mol/Kg COD_R-day; 1.22 Kg COD_R/m³-day (92.96%)] compared to butyric acid [3.33 mol/Kg COD_R-day; 1.19 Kg COD_R/m³-day (88%)]. Higher H₂ yield was observed under acidophilic microenvironment in the presence of glucose (co-substrate), mono sodium salt of glutamic acid (nitrogen source) and vitamins. Argon induced microenvironment was observed to be effective compared to nitrogen induced microenvironment. Combined process efficiency viz., H₂ production and substrate degradation was evaluated employing data enveloping analysis (DEA) methodology based on the relative efficiency. Integration of dark fermentation with photo-fermentation appears to be an economically viable route for sustainable biohydrogen production if wastewater is used as substrate.     

Anaerobic mesophilic co-digestion of sewage sludge with glycerol: Enhanced biohydrogen production

Anaerobic mesophilic co-digestion of mixed sewage sludge from wastewater treatment plants, WWTP, with crude glycerol, the major byproduct of the biodiesel industry, has been examined using a two-phase digestion process in a semi-continuous CSTR at laboratory scale. The objective was to determine the operational conditions that enhanced biohydrogen and methane production and to evaluate the effect of the organic loading rate (OLR) applied to the process. It was concluded that the Hydraulic Retention Time HRT of the methanogenic stage did not have an important influence on the operational process of co-digestion of sewage sludge and glycerol in terms of efficiency of organic removal and biogas yield. Hence, the results obtained were 73–77% organic matter removal (as CODr) with 0.032 LH₂/gCODr and 0.16 LCH₄/gCODr when the system operated at OLRs in the range of 15.33–17.90 gCODs/L d with HRTs of 3 days in the acidogenic digester and 6, 8, and 10 days in the methanogenic digester. In terms of volatile solids, the results obtained were 92–88% organic matter removal (as VSr) with 0.20 LH₂/gVSr and 1.27 LCH₄/gVSr when the system operated at OLRs in the range of 1.94–2.79 gVS/L d.     

Biohydrogen production,sludge granulation,and microbial community in an anaerobic inner cycle biohydrogen production (AICHP) reactor at different hydraulic retention times

This study investigated the continuous biohydrogen production in an anaerobic inner cycle biohydrogen production (AICHP) reactor fed with synthetic molasses wastewater as the model substrate under mesophilic conditions (37 ± 1 °C). The hydraulic retention times (HRTs) were set as 6.12, 4.90, 4.08, 3.50, and 3.06 h. Both maximum hydrogen production rate (HPR) (8.08 ± 0.48 L/L/d) and maximum granule formation were achieved at the HRT of 3.50 h. Acetic acid and butyric acid were the dominant metabolites in all tested HRTs throughout the experiment. Microbial community analysis showed that shortening the HRT promoted hydrogen production. This was mainly achieved by enhancing the growth of acetogenic bacteria in the AICHP reactor, rather than the growth of hydrogen-producing bacteria.     

Anaerobic acidification of sugar-beet processing wastes: Effect of operational parameters

The objective of this study was to maximize the hydrolysis and acidification of sugar-beet processing wastewater and beet pulp for volatile fatty acid (VFA) production through acidogenic anaerobic metabolism. Experiments were conducted to determine the optimum operational conditions (HRT, waste-mixing ratio and pH) for effective acidification in daily-fed, continuously mixed anaerobic reactors. For this purpose, reactors were operated at 35 ± 1 °C with different combinations of HRT (2-4 days), wastewater-pulp mixing ratios (1:0-1:1, in terms of COD) and pH ranges (5.7-7.5). Increased OLRs, resulting from pulp addition, increased the amount of acidification products (VFAs) which led to relatively low operational pH values (5.7-6.8). In this pH range, methanogenic activity was successfully inhibited and the lowest methane percentages (5.6-16.3%) were observed in the produced biogas. The optimum operational conditions were determined to be 2-day HRT and 1:1 waste mixing ratio (in terms of COD) without external alkalinity addition. These operational conditions led to the highest tVFA concentration (3635 ± 209 mg/L as H-Ac) with the acidification degree of 46.9 ± 2.1%.     

Green biohydrogen production in a Co-digestion process from mixture of high carbohydrate food waste and cattle/chicken manure digestate

This study was investigated biohydrogen production on the effects of different ratio of food waste to seed digestate and pH value from co-digestion process in anaerobic reactor. The seed digestate was mixture of cattle manure 45%, corn silage 25%, chicken manure 15%, and olive pomace 15% which was collected from the biogas plant in central Italy. It was found that the peaks of total biogas and the biohydrogen productions were 1355 ± 26 and 436 ± 10 mL whereas the biohydrogen yield was 50.4 mL/g-VS (45.8 mL/g-COD) with 43.33% COD removal rate, the bacteria to substrate volatile solids (VS) ratio was 2:1 where seed digestate to food waste was 6:4 under pH 6.5. As a consequence, food waste with a high COD concentration can be adapted C/N ratio by the cattle manure and chicken manure in the seed digestate which resulted in a high biohydrogen production. The food waste co-digestion system mixed with biogas plant digestate is one of approach to increase total biogas production.     

An integrated system development including PEM fuel cell/biogas purification during acidogenic biohydrogen production from dairy wastewater

Biohydrogen production from dairy wastewater with subsequent biogas purification by hollow fiber membrane module was investigated in this study. The purified and not purified (raw) biohydrogen were used as fuel in polymer electrolyte membrane (PEM) fuel cell. Furthermore, the effect of CO₂ on the performance of PEM fuel cell was evaluated considering cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and polarization curves. The maximum H₂ production rate was 0.015 mmol H₂/mol glucose and the biohydrogen concentration in biogas was ranged 33%–60% (v/v). CO₂/H₂ selectivity decreased with increasing pressure and maximum selectivity was obtained as 4.4 at feed pressure of 1.5 bar. The electrochemical active surface (EASA) areas were decreased with increasing CO₂ ratio. The maximum power densities were 0.2, 0.08 and 0.045 W cm⁻² for 100%, 80% and 60% (v/v) H₂, respectively. The results indicated that integrated PEM fuel cell/biogas purification system can be used as a potential clean energy sources during acidogenic biohydrogen production from dairy wastewater.     

UASFF start-up for biohydrogen and biomethane production from treatment of Palm Oil Mill Effluent

A start-up study was conducted to produce biohydrogen and biomethane from Palm Oil Mill Effluent (POME) using a two-stage up-flow anaerobic sludge fixed-film (UASFF) bioreactor. 100% molasses was used to start the system, and POME was added at 10% increments until it reached 100% after 59 days. During this period of continuous operation, the HRT and temperature were adjusted in order to optimize the condition for biogas production. Hydrogen and methane gas production fluctuated between 53–70% and 90–95%, respectively, in the last four days of operation (days 56–59), with POME percentage being increased from 70% to 100% (30%–0% molasses). Using 100% raw POME led to a total COD removal of 83.70%, average gas production rates of 5.29 L H₂ d⁻¹ (57.11% H₂) and 9.60 L CH₄ d⁻¹ (94.08% CH₄), in their respective units. This output is comparable to, if not better than using 100% molasses as substrate. This work concludes that based on the relative consistency of biogas production on days 56–59, the two-stage UASFF bioreactor operating at a final HRT of 4 h and temperature of 43 °C has taken a period of two months for start-up.     

Biological hydrogen production via dark fermentation: A holistic approach from lab-scale to pilot-scale

Biohydrogen is considered as fuel of future owing to its distinctive attribute for clean energy generation, waste management and high energy content. Suitable feedstock play important role for achieving high rate hydrogen production via dark fermentation process. In this regard, different organic wastes such as cane molasses, distillery effluent and starchy wastewater were examined as potential substrates for biohydrogen production by Enterobacter cloacae IIT-BT 08. Groundnut deoiled cake (GDOC) was considered as additional nutritional supplement to enhance biohydrogen yields. The maximum hydrogen yield of 12.2 mol H₂ kg⁻¹ COD_removed was obtained using cane molasses and GDOC as co-substrates. To further ensure reliability of the process, bench (50 L) and pilot scale (10000 L) bioreactors were customized and operated. The pilot scale study achieved 76.2 m³ hydrogen with a COD removal and energy conversion efficiency of 18.1 kg m⁻³ and 37.9%, respectively. This study provides an extensive strategy in moving from lab to pilot scale biohydrogen production thereby, providing further opportunity for commercial exploitation.     

Application of immobilized upflow anaerobic sludge blanket reactor using Clostridium LS2 for enhanced biohydrogen production and treatment efficiency of palm oil mill effluent

Polyethylene glycol (PEG) gel was used to immobilize hydrogen producing Clostridium LS2 bacteria for hydrogen production in an upflow anaerobic sludge blanket (UASB) reactor. The UASB reactor with a PEG-immobilized cell packing ratio of 10% weight to volume ratio (w/v) was optimal for dark hydrogen production. The performance of the UASB reactor fed with palm oil mill effluent (POME) as a carbon source was examined under various hydraulic retention time (HRT) and POME concentration. The best volumetric hydrogen production rate of 365 mL H₂/L/h (or 16.2 mmol/L/h) with a hydrogen yield of 0.38 L H₂/g COD_added was obtained at POME concentration of 30 g COD/L and HRT of 16 h. The average hydrogen content of biogas and COD reduction were 68% and 65%, respectively. The primary soluble metabolites were butyric acid and acetic acid with smaller quantities of other volatile fatty acid and alcohols formed during hydrogen fermentation. More importantly, the feasibility of PEG-immobilized cell UASB reactor for the enhancement of the dark-hydrogen production and treatment of wastewater is demonstrated.     

Effects of the reduction of the hydraulic retention time to 1.5 days at constant organic loading in CSTR,ASBR, and fixed-bed reactors – Performance and methanogenic community composition

The hydraulic retention time (HRT) is one of the key parameters in biogas processes and often it is postulated that a minimum HRT of 10–25 days is obligatory in continuous stirred tank reactors (CSTR) to prevent a washout of slow growing methanogens. In this study the effects of the reduction of the HRT from 6 to 1.5 days on performance and methanogenic community composition in different systems with and without immobilization operated with simulated thin stillage (STS) at mesophilic conditions and constant organic loading rates (OLR) of 10 g L⁻¹d⁻¹ of volatile solids were investigated. With the reduction of the HRT process instability was first observed in the anaerobic sequencing batch reactor (ASBR) (at HRT of 3 days) followed by the CSTR (at HRT of 2 days). The fixed bed reactor (FBR) was stable until the end of the experiment, but the reduction of the HRT to 1.5 days caused a decrease of the specific biogas production to about 450 L kg⁻¹ of VS compared to about 600 L kg⁻¹ of VS at HRTs of 4–5 days. Methanoculleus and Methanosarcina were the dominant genera under stable process conditions in the CSTR and the ASBR and members of Methanosaeta and Methanospirillum were only present at HRT of 4 days and lower. In the effluent of the FBR Methanosarcina spp. were not detected and Methanosaeta spp. were more abundant then in the other reactors.     

Continuous thermophilic hydrogen production and microbial community analysis from anaerobic digestion of diluted sugar cane stillage

The aim of this study was to promote biohydrogen production in an thermophilic anaerobic fluidized bed reactor (AFBR) at 55 °C using a mixture of sugar cane stillage and glucose at approximately 5000–5300 mg COD L⁻¹. During a reduction in the hydraulic retention time (HRT) from 8, 6, 4, 2 and 1 h, H₂ yields of 5.73 mmol g COD_added⁻¹ were achieved (at HRT of 4 h, with organic loading rate of 52.7 kg COD m⁻³ d⁻¹). The maximum volumetric H₂ production of 0.78 L H₂ h⁻¹ L⁻¹ was achieved using stillage as carbon source. In all operational phases, the H₂ average content in the biogas was between 31.4 and 52.0%. Butyric fermentation was the predominant metabolic pathway. The microbial community in accordance with the DGGE bands profile was found similarity coefficient between 91 and 95% without significant changes in bacterial populations after co-substrate removal. Bacteria like Thermoanaerobacterium sp. and Clostridium sp. were identified.     

Continuous fermentative hydrogen production from cheese whey wastewater under thermophilic anaerobic conditions

Hydrogen (H₂) production from cheese processing wastewater via dark anaerobic fermentation was conducted using mixed microbial communities under thermophilic conditions. The effects of varying hydraulic retention time (HRT: 1, 2 and 3.5 days) and especially high organic load rates (OLR: 21, 35 and 47 g chemical oxygen demand (COD)/l/day) on biohydrogen production in a continuous stirred tank reactor were investigated. The biogas contained 5–82% (45% on average) hydrogen and the hydrogen production rate ranged from 0.3 to 7.9 l H₂/l/day (2.5 l/l/day on average). H₂ yields of 22, 15 and 5 mmol/g COD (at a constant influent COD of 40 g/l) were achieved at HRT values of 3.5, 2, and 1 days, respectively. On the other hand, H₂ yields were monitored to be 3, 9 and 6 mmol/g COD, for OLR values of 47, 35 and 21 g COD/l/day, when HRT was kept constant at 1 day. The total measurable volatile fatty acid concentration in the effluent (as a function of influent COD) ranged between 118 and 27,012 mg/l, which was mainly composed of acetic acid, iso-butyric acid, butyric acid, propionic acid, formate and lactate. Ethanol and acetone production was also monitored from time to time.To characterize the microbial community in the bioreactor at different HRTs, DNA in mixed liquor samples was extracted immediately for PCR amplification of 16S RNA gene using eubacterial primers corresponding to 8F and 518R. The PCR product was cloned and subjected to DNA sequencing. The sequencing results were analyzed by using MegaBlast available on NCBI website which showed 99% identity to uncultured Thermoanaerobacteriaceae bacterium.     

Steady-state and dynamic modeling of biohydrogen production in an integrated biohydrogen reactor clarifier system

Steady-state operational data from the integrated biohydrogen reactor clarifier system (IBRCS) during anaerobic treatment of glucose-based synthetic wastewater at HRT of 8 h and SRT ranging from 26 to 50 h and organic loading rates of 6.5–206 gCOD/L-d were used to calibrate and verify a process model of the system developed using BioWin. The model accurately predicted biomass concentrations in both the bioreactor and the clarifier supernatant with average percentage errors (APEs) of 4.6% and 10%, respectively. Hydrogen production rates and hydrogen yields predicted by the model were in close agreement with the observed experimental results as reflected by an APE of less than 4%, while the hydrogen content was well correlated with an APE of 10%. The successful modeling culminated in the accurate prediction of soluble metabolites, i.e. volatile fatty acids in the reactor with an APE of 14%. The calibrated model confirmed the advantages of decoupling of the solids retention time (SRT) from the hydraulic retention time (HRT) in biohydrogen production, with the average hydrogen yield decreasing from 3.0 mol H₂/mol glucose to 0.8 mol H₂/mol glucose upon elimination of the clarifier. Dynamic modeling showed that the system responds favorably to short-term hydraulic and organic surges, recovering back to the original condition. Furthermore, the dynamic simulation revealed that with a prolonged startup periods of 10 and 30 days, the IBRCS can be operated at an HRT of 4 h and OLR as high as 206 gCOD/L-d without inhibition and/or marked performance deterioration.     

Extreme-thermophilic biohydrogen production from lignocellulosic bioethanol distillery wastewater with community analysis of hydrogen-producing microflora

Extreme-thermophilic biohydrogen production from distillery wastewater was investigated in batch and continuous-mode operation. Hydrogen-producing mixed culture was enriched by repeated batch cultivations. Effect of temperature and pH on biohydrogen yield was investigated in batch experiments. The highest hydrogen yield of 196. mL/g-volatile solids_addded (VS_added) was obtained at 70 °C and pH 7.0 in batch culture. Continuous biohydrogen production was performed in CSTR reactor with yield of 172.0 mL/g-VS_added at HRT (hydraulic retention time) of 4 days. The main metabolic products were acetate, lactate, and ethanol. Community structure of hydrogen-producing microflora was investigated by 16S rRNA gene sequence analysis. The microorganisms involved in both batch and continuous-mode operation were similar and hydrogen production was carried out by a group of extreme-thermophilic bacterial species related to Thermotoga, Coprothermobacter, Caldanaerobacter, Thermobrachium, and Caldicellulosiruptor.     

Start up study of UASB reactor treating press mud for biohydrogen production

Anaerobic digestion of press mud mixed with water for biohydrogen production was performed in continuous fed UASB bioreactor for 120 days. Experiment was conducted by maintaining constant HRT of 30 h and the volume of biohydrogen evolved daily was monitored. Various parameters like COD, VFA, Alkalinity, EC, Volatile solids, pH with respect to biohydrogen production were monitored at regular interval of time. SBPR was 10.98 ml g⁻¹ COD reduced d⁻¹ and 12.77 ml g⁻¹ VS reduced d⁻¹ on peak yield of biohydrogen. COD reduction was above 70 ± 7%. Maximum gas yield was on the 78th day to 2240 ml d⁻¹. The aim of the experiment is to study the startup process of UASB reactor for biohydrogen production by anaerobic fermentation of press mud. The inoculum for the process is cow dung and water digested in anaerobic condition for 30 days with municipal sewage sludge. The study explores the viability of biohydrogen production from press mud which is a renewable form of energy to supplement the global energy crisis.     

Biohydrogen production using horizontal and vertical continuous stirred tank reactor- a numerical optimization

This paper illustrates the method to predict the production of biohydrogen and biogas from the horizontal and vertical continuous mixed tank reactor using a numerical approach. Utilization of computational fluid dynamics on the estimation of bioreactor performance is very crucial owing to the uncertainty in the numerical results. Since there has been little work on CFD to determine the influence of hydraulic retention time, impeller speeds, vortex growth, and pH on biohydrogen yield rate the effort had been made. A series of simulations are done with optimal boundary conditions to ensure maximum hydrogen and biogas production rate. Two types of reactors HCSTR and VCSTR are operated at different impeller speed from 40 rpm to 120 rpm. Further, the rate of HRT and organic loading are varied. As the rpm of the impeller increase the rate of hydrogen and biomass production increases not later than 80 rpm. Meanwhile, the optimal range of HRT and pH are 4–8 h and 6.0–8.0. Running the impeller at optimized rate with derived conditions leads to high hydrogen and biogas production of 6 LH₂/Ld and 30 L/d. The obtained results are validated with the experimental findings to compare the uncertainty formed due to the numerical simulations. The optimum boundary condition obtained from the study is expected to provide the essential knowledge in establishing the full-scaled reactors.     

Biogas production in low-cost household digesters at the Peruvian Andes

Low-cost tubular digesters originally developed in tropical regions have been adapted to the extreme weather conditions of the Andean Plateau (3000-4000 m.a.s.l.). The aim of this study was to characterise biogas production in household digesters located at high altitude, operating under psychrophilic conditions. To this end, two pilot digesters were monitored and field campaigns were carried out in two representative digesters of rural communities. Digesters’ useful volume ranged between 2.4 and 7.5 m³, and hydraulic residence time (HRT) between 60 and 90 days. The temperature inside the digester’s greenhouse ranged between 20 and 25 °C. Treating cow manure, a specific biogas production around 0.35 m³ kg_VS⁻¹ was obtained, with some 65% CH₄ in biogas. In order to fulfil daily requirements for cooking and lighting, biogas production should be enhanced without increasing implementation costs as not to impede the expansion of this technology at household scale. In this sense, HRT below 60 days and OLR above 1 kg_VS m⁻³ day⁻¹ should be investigated to decrease digesters’ volume (i.e. costs) and increase biogas production rate. The adaptation of conventional gas burners to biogas characteristics can also contribute in improving the efficiency of the system.