Ethanol and biogas production from birch by NMMO pretreatment

Birch wood was pretreated with N-methylmorpholine-N-oxide (NMMO or NMO) followed by enzymatic hydrolysis and fermentation to ethanol or digestion to biogas. The pretreatments were carried out with NMMO (w_NMMO = 85%) at 130 °C for 3 h, and the effects of drying after the pretreatment were investigated. Enzymatic hydrolysis of the untreated wood resulted in 8%–10% of theoretical glucose yield after 4 days hydrolysis, while the NMMO pretreatment improved this yield to 91%. Consequently, ethanol production yield from NMMO-pretreated materials resulted in around 9-fold improvement compared to the untreated wood. On the other hand, drying of the pretreated wood had a negative impact and decreased the yield of enzymatic hydrolysis by 4%–10%. Digestion of the untreated wood with thermophilic bacteria resulted in maximum methane yield of 158 cm³ g⁻¹ of VS in 30 days, while the NMMO pretreatment improved the methane yield up to 232 cm³ g⁻¹ of VS (80% of the theoretical biogas yield) in just 9 days.     

Semi-continuous co-digestion of solid slaughterhouse waste, manure, and fruit and vegetable waste

The potential of semi-continuous mesophilic anaerobic digestion (AD) for the treatment of solid slaughterhouse waste, fruit-vegetable wastes, and manure in a co-digestion process has been experimentally evaluated. A study was made at laboratory scale using four 2 L reactors working semi-continuously at 35 °C. The effect of the organic loading rate (OLR) was initially examined (using equal proportion of the three components on a volatile solids, VS, basis). Anaerobic co-digestion with OLRs in the range 0.3–1.3 kg VS m⁻³ d⁻¹ resulted in methane yields of 0.3 m³ kg⁻¹ VS added, with a methane content in the biogas of 54–56%. However, at a further increased loading, the biogas production decreased and there was a reduction in the methane yield indicating organic overload or insufficient buffering capacity in the digester.In the second part of the investigation, co-digestion was studied in a mixture experiment using 10 different feed compositions. The digestion of mixed substrates was in all cases better than that of the pure substrates, with the exception of the mixture of equal amounts of (VS/VS) solid cattle–swine slaughterhouse waste (SCSSW) with fruit and vegetable waste (FVW). For all other mixtures, the steady-state biogas production for the mixture was in the range 1.1–1.6 L d⁻¹, with a methane content of 50–57% after 60 days of operation. The methane yields were in the range 0.27–0.35 m³ kg⁻¹ VS added and VS reductions of more than 50% and up to 67% were obtained.     

Kinetic study of thermophilic anaerobic digestion of solid wastes from potato processing

Anaerobic treatment of solid wastes from potato processing was studied in completely stirred tank reactors (CSTR) at 55 °C. Special attention was paid to the effect of increased organic loading rate (OLR) on the biogas yield in long-term experiments. Both biogas yield and CH₄ in the biogas decreased with the increase in OLR. For OLR in the range of 0.8 gl⁻¹ d⁻¹–3.4 gl⁻¹ d⁻¹, biogas yield and CH₄ obtained were 0.85 l g⁻¹–0.65 l g⁻¹ and 58%–50%, respectively. Biogas yield y as a function of maximum biogas yield y_m, reaction rate constant k and HRT are described on the basis of a mass balance in a CSTR and a first order kinetic. The value of y_m can be obtained from curve fitting or a simple batch test and k results from plotting y/(y_m−y) against 1/OLR from long-term experiments. In the present study values for y_m and k were obtained as 0.88 l g⁻¹ and 0.089 d⁻¹, respectively. The simple model equations can apply for dimensioning completely stirred tank reactors (CSTR) digesting organic wastes from food processing industries, animal waste slurries or biogas crops.     

Enhanced substrate degradation and methane yield with maleic acid pre-treatments in biomass crops and residues

Organic acids are envisaged as alternative catalysts to strong mineral acids, in pre-treatment of ligno-cellulosic biomass for anaerobic digestion (AD). To evaluate this hypothesis, an untreated control and four pre-treatments (25 °C for 24 h) involving two levels of maleic acid (34.8 and 69.6 kg m⁻³), alone and combined with sulphuric acid (4 kg m⁻³), were studied in three agricultural substrates: Arundo (aka giant reed), Barley straw and B133 fibre sorghum. Methane production was assessed in a batch AD assay (35 °C for 51 days) with 4 g L⁻¹ of volatile solid (VS) load. Fibre composition and structure were investigated through chemical analysis and Fourier transform infrared (FTIR) spectrometry. Arundo and B133 that were the most and least recalcitrant substrate, respectively, staged the highest and lowest increase in methane with high maleic acid: +62% over 218 cm³ g⁻¹ of VS in untreated Arundo; +36% over 284 cm³ g⁻¹ of VS in untreated B133. Barley straw showed an intermediate behaviour (+41% over 269 cm³ g⁻¹ of VS). H₂SO₄ addition to maleic acid did not improve CH₄ output. The large increase in methane yield determined by pre-treatments was reflected in the concurrent decrease of fibre (between 14 and 39% depending on fibrous component). Based on FTIR spectra, bands assigned to hemicellulose and cellulose displayed lower absorbance after pre-treatment, supporting the hypothesis of solubilisation of structural carbohydrates and change in fibre structure. Hence, maleic acid was shown a suitable catalyst to improve biodegradability of ligno-cellulosic biomass, especially in recalcitrant substrates as Arundo.     

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.     

Production of hydrogen and methane by one and two stage fermentation of food waste

Anaerobic digestion is an attractive process for generation of hydrogen and methane, which involves complex microbial processes on decomposition of organic wastes and subsequent conversion of metabolic intermediates to hydrogen and methane. Comparative performance of a sequential hydrogen and methane fermentation in two stage process and methane fermentation in one stage process were tested in batch reactor at varying ratios of feedstock to microbial inoculum (F/M) under mesophilic incubation. F/M ratios influence biogas yield, production rate, and potential. The highest H₂ and CH₄ yields of 55 and 94 mL g⁻¹ VS were achieved at F/M of 7.5 in two stage process, while the highest CH₄ yield of 82 mL g⁻¹ VS in one stage process was observed at the same F/M. Acetic and butyric acids are the main volatile fatty acids (VFAs) produced in the hydrogen fermentation stage with the concentration range 10–25 mmol L⁻¹. Little concentrations of VFAs were accumulated in methane fermentation in both stage processes. Total energy recovery in two stage process is higher than that in one stage by 18%. This work demonstrated two stage fermentation achieved a better performance than one stage process.     

Mild alkaline pre-treatments loosen fibre structure enhancing methane production from biomass crops and residues

Three ligno-cellulosic substrates representing varying levels of biodegradability (giant reed, GR; fibre sorghum, FS; barley straw, BS) were combined with mild alkaline pre-treatments (NaOH 0.05, 0.10 and 0.15 N at 25 °C for 24 h) plus untreated controls, to study pre-treatment effects on physical-chemical structure, anaerobic digestibility and methane output of the three substrates. In a batch anaerobic digestion (AD) assay (58 days; 35 °C; 4 g VS l⁻¹), the most recalcitrant substrate (GR) staged the highest increase in cumulative methane yield: +30% with NaOH 0.15 N over 190 ml CH₄ g⁻¹ VS in untreated GR. Conversely, the least recalcitrant substrate (FS) exhibited the lowest gain (+10% over 248 ml CH₄ g⁻¹ VS), while an intermediate behaviour was shown by BS (+15% over 232 ml CH₄ g⁻¹ VS). Pre-treatments speeded AD kinetics and reduced technical digestion time (i.e., the time needed to achieve 80% methane potential), which are the premises for increased production capacity of full scale AD plants. Fibre components (cellulose, hemicellulose and acid insoluble lignin determined after acid hydrolysis) and substrate structure (Fourier transform infra-red spectroscopy and scanning electron microscopy) outlined reductions of the three fibre components after pre-treatments, supporting claims of loosened binding of lignin with cellulose and hemicellulose. Hence, mild alkaline pre-treatments were shown to improve the biodegradability of ligno-cellulosic substrates to an extent proportional to their recalcitrance. In turn, this contributes to mitigate the food vs. fuel controversy raised by the use of whole plant cereals (namely, maize) as feedstocks for biogas production.     

The effect of waste paper on the kinetics of biogas yield from the co-digestion of cow dung and water hyacinth

The effect of waste paper on biogas yield produced by co-digesting fixed amount of cow dung and water hyacinth in five digesters A-E was studied at room temperature. Waste paper was observed to improve biogas yield in digesters B-E with digester A acting as the control. However, as the amount of waste paper increased the biogas yield was observed to decrease. Kinetic model based on first order kinetic was derived to estimate the maximum, ultimate, biogas yield and also the ultimate methane yield from these biomass mixtures. The maximum biogas yield estimated using this model for digesters B-E were 0.282, 0.262, 0.233, and 0.217 lg⁻¹ VS fed with goodness of fit (R²) of 0.995, 0.99, 0.889, and 0.925 respectively, which were obtained by fitting the experimental biogas yield (y_t) against (exp(kt)−1)/exp(kt). The ultimate biogas and methane yield at very low batch solid load were extrapolated to be 0.34 and 0.204 lg⁻¹ VS fed respectively. In essence, the addition of waste paper in the co-digestion of cow dung and water hyacinth can be a feasible means of improving biogas yield and also alternative means of recycling waste paper. Furthermore, the kinetic model developed can compliment other models used in anaerobic digestion of agricultural and solid waste.     

Pretreatment of energy crops with sodium hydroxide and cellulolytic enzymes to increase biogas production

This work presents the influence of alkali pretreatment on the enzymatic hydrolysis and efficiency of anaerobic digestion of lignocellulosic biomass pretreated both in a one- (chemical or enzymatic) and two-step (chemical and enzymatic) process. In this study two species of energy crops were used Miscanthus giganteus and Sida hermaphrodita. The aim of this work was to compare biogas production and methane yield during fermentation of pretreated and untreated energy crops. The results show that alkali pretreatment is necessary for the effective biogas generation from plant material due to high delignification level and significant hemicellulose degradation. The two-step hydrolysis process consisting on the alkali and enzymatic step leads to the release of high concentrations of glucose (about 20 g L⁻¹). The best results were achieved for M. giganteus with biogas production yield of 421.5 Ndm³ kg TS⁻¹ and with methane production yield of 257 Ndm³ kg TS⁻¹.     

Effects of solid retention time on anaerobic digestion of dewatered-sewage sludge in mesophilic and thermophilic conditions

Anaerobic digestion of dewatered-sewage sludge using continuous stirred tank reactors (CSTRs) in duplicates was evaluated under thermophilic (50 °C) and mesophilic (37 °C) conditions over a range of nine solid retention times (SRTs). The 35- and 30-day SRTs were designed to simulate a full-scale plant operation while 25-, 20-, 15- and 12-day SRTs were planned to evaluate process performance at the various SRTs. The 9-, 5- and 3-day SRTs were performed to push the reactors to extend their degradation capacity and test the threshold for process imbalance. The corresponding organic loading rates (OLR) varied from 1.6 to 20.5 kg VS m^?3 day^?1. Biogas production rate could be tripled when the SRT was shortened from 30 to 12 days and more than doubled from 35- to 15-day SRT because of a concomitant increase in OLR. In general, higher biogas productivity was realized under thermophilic, but methane yields were comparable due to the higher methane content in the biogas under mesophilic digestion. The methane content in biogas fluctuated between 55 and 65% and the methane yield ranged from 0.314 to 0.348 Nm³ CH₄ kg VS_added^?1 day^?1 for both thermophilic and mesophilic digestion. The VS-reduction at 12- and 15-day SRT ranged from 45 to 52% and there was no accumulation of VFAs. Increasing concentrations of VFAs, decreasing concentration of partial alkalinity and decrease in pH were noted as signs of reactor instability. Process imbalance started at 9-day SRT, souring of the reactors, cell wash-out and foaming was noted as the principal causes of process failure under both thermophilic and mesophilic conditions. This study projected the possibility of using CSTRs in treating dewatered-sewage sludge at a shorter SRT to achieve reasonable biogas production and VS-reduction without encountering adverse operation conditions as foaming and wash-out of cells.     

Effect of chitosan on reactor performance and population of specific methanogens in a modified CSTR treating raw POME

In this study, chitosan was periodically added to a CSTR treating raw POME in order to retain more working microorganisms at high OLRs. The data indicated that the CSTR with chitosan addition can be operated with reactor stability at an OLR of 26.5 kg m⁻³ d⁻¹ which is approximately 7.5 kg m⁻³ d⁻¹ higher than that for the control CSTR without chitosan addition. In the control CSTR, the biogas production did not increase with increased OLR, and the overall process was limited by slow methanogenic rates. For the CSTR with chitosan addition, the biogas production was 9.39 m³ m⁻³ d⁻¹ with a methane volume fraction of 68% at an OLR of 26.5 kg m⁻³ d⁻¹. Corresponding to this increase in methane production, it was found that Methanosarcinales numbers were significantly higher (P < 0.05) in the CSTR with chitosan addition than in the control CSTR.     

The kinetics of Scenedesmus obliquus microalgae growth utilizing carbon dioxide gas from biogas

Microalgae Scenedesmus obliquus was cultured in a laboratory photobioreactor to determine the efficacy of using biogas as a carbon source for the microalgae's growth. The biogas contained ∼60% CH₄ and ∼40% CO₂, and was derived from an anaerobic digester operating from animal wastes, and an anaerobic reactor utilizing high strength wastewater. The results showed that biogas is a viable carbon source for microalgae growth and that significant portions of the biogas' CO₂ can be utilized for algae growth, resulting in a biogas having a high concentration of methane. This paper develops the kinetic expressions for the algae's growth by assuming an autocatalytic reaction between carbon substrate and microalgae. The maximum specific growth rate and biomass productivity of S. obliquus were 0.56 d⁻¹ and 0.145 g L⁻¹d⁻¹ respectively. The biomass contained 51.8% carbon and higher heating value (HHV) was 22.9 MJ kg⁻¹.     

Using a food and paper-cardboard waste blend as a novel feedstock for hydrogen production: Influence of key process parameters on microbial diversity

In this study, fermentation of a thermally treated simulated organic solid waste into hydrogen (H₂) was examined using a pretreated anaerobic mixed culture. The culture was fed a steam exploded food waste plus paper-cardboard waste blend liquor with and without linoleic acid (LA). The individual and interaction effects of the initial pH, LA concentration and the initial chemical oxygen demand (COD) concentration on H₂ and methane (CH₄) production was assessed using a Box–Behnken design (BBD). The BBD model predicted a maximum H₂ yield of 87 mL g⁻¹ COD or 98 mL H₂ g⁻¹ VS with 1.6 g L⁻¹ LA, an initial pH of 5.93 and an initial COD of 9.34 g COD L⁻¹. The major microbial populations detected in cultures at pH 5.5 with and without LA included Clostridium sp., Enterococcus asini, Enterococcus faecalis, and Lactobacillus gallinarum. The dendrogram for the 16S rRNA gene T-RFs profiles showed four major groups with a similarity index of 72–75% for Clade III. The major H₂-producing populations were grouped in Clade I with a similarity index range of 55–75%.     

Optimization of mechanical pre-treatment of Laminariaceae spp. biomass-derived biogas

Macroalgae have not met their full potential to date as biomass for the production of energy. One reason is the high cost associated with the pretreatment which breaks the biomass's crystalline structure and better exposes the fermentable sugars to anaerobes. In the attempt to overcome this technological barrier, the performance of a Hollander beater mechanical pretreatment is assessed in this paper. This pretreatment has been applied to a batch of Laminariaceae biomass and inoculated with sludge from a wastewater treatment plant. The derived biogas and methane yields were used as the responses of a complex system in order to identify the optimal system input variables by using the response surface methodology (RSM). The system's inputs considered are the mechanical pretreatment time (5–15 min range), the machine's chopping gap (76–836 μm) and the mesophilic to thermophilic range of temperatures (30–50 °C). The mechanical pretreatment was carried out with the purpose of enhancing the biodegradability of the macroalgal feedstock by increasing the specific surface area available during the anaerobic co-digestion. The pretreatment effects on the two considered responses are estimated, discussed and optimized using the tools provided by the statistical software Design-Expert v.8. The best biogas yield of treated macroalgae was found at 50 °C after 10 min of treatment, providing 52% extra biogas and 53% extra methane yield when compared to untreated samples at the same temperature conditions. The highest biogas rate achieved by treating the biomass was 685 cc gTS⁻¹, which is 430 cc gTS⁻¹ in terms of CH₄ yield.     

Coproduction of hydrogen,ethanol and 2,3-butanediol from agro-industrial residues by the Antarctic psychrophilic GA0F bacterium

In this study, the simultaneous production of hydrogen, ethanol, and 2,3-butanediol was assessed using three agro-industrial residues: cheese whey powder (CWP), wheat straw hydrolysate (WSH) and sugarcane molasses (SCM), by the Antarctic psychrophilic GA0F bacterium [EU636050], which is closely related to Pseudomonas antarctica [KX186936.1]. The main soluble metabolites produced in all the fermentations were ethanol and 2,3-butanediol. CWP demonstrated to be the most effective carbon source, since fermentation of this substrate resulted in the highest yields of H₂ (73.5 ± 10 cm³ g⁻¹), ethanol (0.24 ± 0.03 g g⁻¹) and 2,3-butanediol (0.42 ± 0.04 g g⁻¹), followed by the use of SCM, whereas WSH showed to have an inhibitory effect during the fermentation process, showing the lowest production values. Our results demonstrated the ability of the Antarctic psychrophilic GA0F bacterium to produce valuable products using low-cost substrates at room temperature conditions.     

The ability of macroalgae to stabilise and optimise the anaerobic digestion of household food waste

This study investigated the potential of seaweed waste (SW) as a sustainable feedstock for anaerobic co-digestion with food waste (FW). The study was conducted at laboratory scale using a batch test approach run over 34 days. Methane (CH₄) potential assays were conducted at the following FW to SW dry mass ratios: 100:0, 90:10, 75:25, 50:50 and 0:100. Results indicated that anaerobic co-digestion of FW and SW at a mixture ratio of 90:10 produced the highest methane yield (252 cm³ g⁻¹ of volatile solids (VS)), rates of reaction (0.08 d⁻¹) and resulted in a better stability of the process. Predictions based on the Buswell formula suggested that all reactors were performing below the theoretical (maximum) with a greater disparity at increasing levels of seaweed in the feed, likely due to high levels of sulphur in the SW (1.73% mass fraction). The analysis of heavy metals in SW and final digestate indicated that using SW for anaerobic co-digestion with FW enhanced the process by providing trace nutrients without impacting the heavy metal content of the digestate. The analysis of carbon (C) and nitrogen (N) indicated that by using SW for co-digestion with FW, C:N optimal mass ratios were achieved. It was concluded that the addition of SW for anaerobic co-digestion of FW can be used to accelerate the bioenergy production from FW. An additional benefit will be the abatement of the negative impacts of SW in coastal areas, making the overall process more sustainable.     

Scaling up self-stratifying supercapacitive microbial fuel cell

Self-stratifying microbial fuel cells with three different electrodes sizes and volumes were operated in supercapacitive mode. As the electrodes size increased, the equivalent series resistance decreased, and the overall power was enhanced (small: ESR = 7.2 Ω and P_max = 13 mW; large: ESR = 4.2 Ω and P_max = 22 mW). Power density referred to cathode geometric surface area and displacement volume of the electrolyte in the reactors. With regards to the electrode wet surface area, the large size electrodes (L-MFC) displayed the lowest power density (460 μW cm⁻²) whilst the small and medium size electrodes (S-MFC, M-MFC) showed higher densities (668 μW cm⁻² and 633 μW cm⁻², respectively). With regard to the volumetric power densities the S-MFC, the M-MFC and the L-MFC had similar values (264 μW mL⁻¹, 265 μW mL⁻¹ and 249 μW cm⁻¹, respectively). Power density normalised in terms of carbon weight utilised for fabricating MFC cathodes-electrodes showed high output for smaller electrode size MFC (5811 μW g⁻¹-C- and 3270 μW g⁻¹-C- for the S-MFC and L-MFC, respectively) due to the fact that electrodes were optimised for MFC operations and not supercapacitive discharges. Apparent capacitance was high at lower current pulses suggesting high faradaic contribution. The electrostatic contribution detected at high current pulses was quite low. The results obtained give rise to important possibilities of performance improvements by optimising the device design and the electrode fabrication.     

Response surface design to study the influence of inoculum,particle size and inoculum to substrate ratio on the methane production from Ulex sp.

Ulex europaeus is one of the world worst invaders vegetal species and its suitability for biogas production is significant. The effect of three factors affecting the Biochemical Methane Potential (BMP, expressed as volume of CH₄ per mass of volatile solids of waste) and the biodegradability rate (k, expressed in volume of CH₄ per mass of VS and time) of U. europaeus was assessed by a Central Composite Face Centred Design. The BMP varied from 153 L kg⁻¹ to 308 L kg⁻¹. Inoculum to substrate ratio (ISR) and the type of inoculum had high influence on the final results. k varied from 14 L kg⁻¹ d⁻¹ to 49 L kg⁻¹ d⁻¹. The conditions that simultaneously maximized the BMP and k were an inoculum consisting in 55% (v) of granular sludge and 45 % (v) of suspended sludge from a sludge digester, an ISR of 4 g g⁻¹, and a particle size of 1.9 mm. Considering the average biomass production in shrub land areas, the potential energy production from U. europaeus is estimated in (36.9 ± 19.3) GJ ha⁻¹ yr⁻¹. For example, in Europe, a maximum energy supply of 7 EJ yr⁻¹ could be achieved from potentially harvestable shrub land areas.     

Bio-methane potential test (BMP) using inert gas sampling bags with macroalgae feedstock

An approach to Bio-methane potential test (BMP) was carried out at mesophilic temperature of 35 °C with Supel™ inert gas sampling bags as biogas collection and storage bags, using selected seaweed (macroalgae) as substrate. Samples were given a range of pre-treatments from washing, drying and macerating. Dried laminaria digitata (DD) with 68.14% VS (%TS) produced the highest BMP of 141 ± 5.77 L CH₄/kg VS, with methane content increasing to about 70%, while the lowest BMP of 93.35 ± 5.03 L CH₄/kg VS with methane content of about 65% was obtained for fresh laminaria digitata (FD) with 72.03% VS (%TS). Methane yields of 97.66 and 67.24 m³ CH₄/t wet weight based on BMP results were obtained for DD and FD. Both DD and FD achieved within 28% and 38% of the theoretical BMP value based on the Buswell equation, respectively. The total methane (V) produced was computed based on;V = X₁ + X₂ – X₃ corrected to Standard temperature and pressure (STP).where X₁ = daily calculated headspace methane volume, X₂ = daily measured volume of methane in gas bags, X₃ = previous day headspace methane volume. An advantage of this approach is the volumetric measurement of gas produced directly from the gas bags, hence it does not require liquid displacement or pressure transducers. Results from a second set of freshly collected sample seaweed sample showed it was in agreement with published BMP values. All analysis were carried out without mineral supplementation.     

Kinetic study of mesophilic anaerobic digestion of pulp & paper sludge

Anaerobic digestion of pulp and paper sludge (PPS) and monosodium glutamate waste liquor (MGWL) was studied in completely stirred tank reactors (CSTR) at 37 ± 2 °C. This work focused on the effect of increased organic loading rate (OLR) on the methane production in long-term experiments. For OLR in the range of (1.5-5.0) kg m⁻³ d⁻¹ based on VS fed, VFA and SCOD concentrations decreased for the first 10 days and then kept stable at about 2.3 kg m⁻³ and 4.0 kg m⁻³ respectively until to the critical OLR of 5.0 kg m⁻³ d⁻¹; and the methane generation rate enhanced to 0.838 m³ m⁻³ d⁻¹ during this period until to the reactor failure. Additionally, reaction rate constant k and sludge retention time (SRT) are described on the basis of a mass balance in a CSTR followed a first order kinetic equation. In the present study, values for y_m and k were obtained as 0.733 m³ kg⁻¹ of removed VS and 0.07 d⁻¹, respectively. The simple model can apply for dimensioning a CSTR digesting of organic wastes from pulp and paper industries, food processing industries, sewage treatment plants or biogas crops.