Increase In Organic Loading Rate Research Articles

Effect of Organic Loading Rate (OLR) on Biogas Yield Using a Single and Three-Stages Continuous Anaerobic Digestion Reactors

The rate at which feedstock is added to the anaerobic digester (AD) reactor has to be adjusted for the growth rate of methanogens bacteria. Increase in biogas yield is as a result of improved mathanogens forming bacteria. Under loading and over loading of feedstock in the AD reactor has effect on methanogens forming bacteria. If more feedstock is added than the bacteria are able to degrade, the process will become acidic. Feedstock has to been fed to the reactor at a uniform rate and volume. If feeding pattern has to change, this must be done gradually so that bacteria can adapt to the new conditions. For optimum biogas yield, required amount of feedstock must be added to the AD reactor. The aim of this research work is to determine the effect of organic loading rate (OLR) on biogas yield from food waste, water hyacinth, cow dung, waste water from abattoir, poultry dropping and pig dung. The experimental set up comprises of single stage and three-stage continuous AD reactors. The same quantity and composition of feedstock were used and this was subjected to a variation of OLR 0.5 kg/m3(1.5 kg/m3, 2 kg/m3, 2.5 kg/m3, and 3 kg/m3). The experiment was conducted within a mesophilic temperature range of 36°C-37°C, percentage total solid (%TS) of 9.98% and percentage volatile solid (%VS) of 78%. pH meter was used to monitored the daily pH reading of the slurry. It was observed that the quantity of biogas yield from the feedstock increases with increasing organic load rate to the optimum value of 1.5 kg/m3and started decreasing above the optimum value for a single stage AD reactor but this was not the case for the three-stages continuous AD reactors that experienced continuous increase in biogas yield with a successive increase in OLR from 1-5 kg/m3-3.0 kg/m3.

Novel insight of spatial mass transfer conditions of upflow anaerobic reactor

Refractory organics in industrial wastewater are difficult to treat and pose a serious threat to the environment. Upflow anaerobic reactors such as upflow anaerobic sludge blanket (UASB) and internal circulation (IC) reactors are most widely used in the world due to high efficiency, but the inside process remains unknown like a black box. In this study, spatial mass transfer conditions of a pilot-scale UASB reactor, being decomposed as the bottom, middle and top zones, treating pharmaceutical wastewater at organic loading rates (OLR) of 4.32–8.30 kg COD/(m3.d) were investigated. In addition, the roles of external and internal factors such as organic loading rate and granule properties including size, density, porosity and permeability on mass transfer were investigated for the first time. The average molecular diffusion rates (FMD) of reactor were 7.43 × 10−8, 1.4 × 10−7, 2.0 × 10−7 and 2.73 × 10−7 mg/s respectively, while, average convective diffusion rates (FCD) were 5.39 × 10−1, 1.09, 1.32 and 1.42 mg/s at OLRs of 4.32, 5.70, 6.92 and 8.30 kg COD/(m3.d) respectively. Interestingly, mass transfer rates increased with the increase of OLR and decreased with a height of the reactor, also, it is directly dependent on granule size. The novel results reveal that the bottom zone is of the fastest mass transfer as compared to middle and top zones of the reactor; furthermore, it is the main degradation zone due to not only high mass transfer but also low feed/sludge and high granule level. The performance of upflow reactors could be enhanced for higher biogas production by installation of biofilm zone in the top zone to replace granules of that zone. The results could facilitate understanding bioprocess in upflow reactors.

Long-term performance of UASB in treating N, N-dimethylformamide-containing wastewater with a rapid start-up by inoculating mixed sludge

Wastewater containing N, N-dimethylformamide (DMF) was treated by artificially mixing the anaerobic granular sludge (AGS) with DMF-degrading activated sludge (DAS) in this study. An up-flow anaerobic sludge blanket (UASB) successfully treated wastewater containing approximately 2000 mg L−1 DMF during an operation period of 215 days. An inoculation of DAS brought about remarkable results: a rapid start-up with effective DMF methanogenic degradation on the first day, and under a low organic loading rate (OLR) of 1.63–4.22 g COD L−1 day−1, the UASB maintained excellent DMF removal efficiency at over 96% along with the high methane production rate (MPR). However, when the OLR increased to 9.24 g COD L−1 day−1, DMF removal efficiency and MPR dropped to 47.36% and 1.05 L L−1 day−1. A further increase in the OLR to 13.25 g COD L−1 day−1 resulted in a sharp deterioration in the DMF-degrading ability, at merely 19.19% and a low MPR of 0.38 L L−1 day−1. The excessive elevation of OLR resulted in the insufficient hydrolysis of the DMF, and the further weakening of the conversion from DMF to intermediates and an acceleration the decaying of DMF-hydrolyzing bacteria. Methane-producing archaea was starved of intermediates when hydrolysis was inadequate. Since the DAS can be massively domesticated, and the OLR should be kept lower than 6.17 g COD L−1 day−1, the timely replenishing of the DAS to the UASB may be a solution to maintain a stable and effective DMF hydrolysis for long-term operation. The results of this study provide insight for the development of a new concept and an improved method for the effective treatment of wastewater containing degradation-resistant organics.

Anaerobic digestion of soft drink beverage waste and sewage sludge

Soft drink beverage waste (BW) was evaluated as a potential substrate for anaerobic co-digestion with sewage sludge to increase biogas production. Results from this study show that the increase in biogas production is proportional to the increase in organic loading rate (OLR) rate due to BW addition. The OLR increase of 86 and 171% corresponding to 10 and 20% BW by volume in the feed resulted in 89 and 191% increase in biogas production, respectively. Under a stable condition, anaerobic co-digestion with BW did not lead to any significant impact on digestate quality (in terms of COD removal and biosolids odour) and biogas composition. The results suggest that existing nutrients in sewage sludge can support an increase in OLR by about 2 kg COD/m3/d from a carbon rich substrate such as soft drink BW without inhibition or excessive impact on subsequent handling of the digestate.

Assessment of continuous fermentative hydrogen and methane co-production using macro- and micro-algae with increasing organic loading rate

A two-stage continuous fermentative hydrogen and methane co-production using macro-algae (Laminaria digitata) and micro-algae (Arthrospira platensis) at a C/N ratio of 20 was established. The hydraulic retention time (HRT) of first-stage H2 reactor was 4 days. The highest specific hydrogen yield of 55.3 mL/g volatile solids (VS) was obtained at an organic loading rate (OLR) of 6.0 gVS/L/d. In the second-stage CH4 reactor at a short HRT of 12 days, a specific methane yield of 245.0 mL/gVS was achieved at a corresponding OLR of 2.0 gVS/L/d. At these loading rates, the two-stage continuous system offered process stability and effected an energy yield of 9.4 kJ/gVS, equivalent to 77.7% of that in an idealised batch system. However, further increases in OLR led to reduced hydrogen and methane yields in both reactors. The process was compared to a one-stage anaerobic co-digestion of algal mixtures at an HRT of 16 days. A remarkably high salinity level of 13.3 g/kg was recorded and volatile fatty acid accumulations were encountered in the one-stage CH4 reactor. The two-stage system offered better performances in both energy return and process stability. The gross energy potential of the advanced gaseous biofuels from this algal mixture may reach 213 GJ/ha/yr.

Organic loading rate shock impact on extracellular polymeric substances and physicochemical characteristics of nitrifying sludge treating high-strength ammonia wastewater under unsteady-state conditions.

Laboratory experimentation was used to investigate the impact of the organic loading rate shock on extracellular polymeric substances (EPSs) and the physicochemical characteristics of nitrifying sludge (NS) treating high-strength ammonia wastewater. The increased organic loading rates (OLRs) strongly influenced the stability of the NS with regard to nutrient removal, biomass–liquid separation, and surface properties, leading to the sludge system collapse at the OLR of 0.75 kg COD per kg MLVSS d. However, an incomplete recovery of the NS after the high OLRs shock was observed when decreasing the OLRs. In addition, the variations of OLRs resulted in relatively stable amounts of tightly bound EPS (TB-EPS), but a significant change in loosely bound EPS (LB-EPS). Both in LB-EPS and TB-EPS, the proteins (PN) contents and proteins to polysaccharides (PN/PS) ratios decreased with the increase in OLRs. Results from the excitation emission matrix spectra implied that the tryptophan PN-like substances were the major components in EPS at low OLRs, while the humic acid-like and fulvic acid-like substrates increased markedly at high OLRs. Furthermore, correlation analysis demonstrated that PN and the PN/PS ratio were the most important factors in determining the physicochemical properties of the NS. It was indicated that the PN could accurately reflect the sludge properties of the NS, and thus effectively change the surface properties of the sludge, contributing to the cohesion between the aggregates to maintain a stable structure.

Mesophilic anaerobic co-digestion of sewage sludge with glycerine: Effect of solids retention time

The main objective of this paper is to examine the effect of the increase in organic loading rates (OLRs), by reducing the solids retention time (SRT) from 20 d to 5 d, in single-phase mesophilic anaerobic co-digestion of sewage sludge with glycerine (1% v/v). It was experimentally confirmed that anaerobic co-digestion of these biowastes under steady-state conditions can achieve an 85 ± 5% reduction in volatile fatty acids (VFA) at SRTs of between 20 and 9 d, with a methane production yield of around 0.8 l CH4/l/d. Decreases in the SRT not only allow the sludge stability and biogas production to be maintained, but also lead to an increase in the waste that could be treated and lower operating costs. Therefore, mesophilic anaerobic co-digestion of sewage sludge and glycerin at a SRT lower than 20 d is possible and preferable due to being more economical and environmentally friendly.

Effect of organic loading rates on biogas production and anaerobic biodegradation of composting leachate in the anaerobic series bioreactors

This study investigated the biodegradability of composting leachate and biogas production in different Organic Loading Rates (OLRs) in lab-scale series AMBR-ASBR bioreactors, Anaerobic Migrating Blanket Reactor (AMBR) and Anaerobic Sequencing Batch Reactor (ASBR). The raw Leachate with OLRs of 1.04–19.65gCOD/L.d went into the AMBR reactor and then into the ASBR reactor with hydraulic retention time of 24h. The produced methane and biogas were measured by GC-TCD and gas meter. The results showed that the best removal efficiency for BOD5, COD and readily biodegradable COD (rbCOD) in AMBR-ASBR hybrid reactor were 99.43%, 97.35% and 99.79%, respectively. The maximum amounts of biogas and methane production were 16.37 and 9.99L in OLR of 10.08gCOD/L.d. With the increase in OLRs, biogas production also increased, but in higher OLRs (18.52gCOD/L.d) with a sudden increase in OLR, methane and biogas production decreased. Moreover, the ratio of the biogas production to inlet COD was 1.35L (R2=0.994), that is equal to the ratio of produced biogas (L) to inlet Volatile Suspended Solids (VSS) (g) (R2=0.972). Thus, considering OLRs effects, it was observed that the AMBR-ASBR hybrid reactor as a novel series system could be remarkably efficient in compost leachate treatment and increasing biogas production.

Influence of ammonia in the anaerobic digestion of food waste

Ammonia toxicity was investigated in mesophilic (37°C) and thermophilic (55°C) digesters using high and low-nitrogen food wastes (FW). Mesophilic inoculum was successfully acclimated to thermophilic conditions by a step change in temperature followed by incremental increases in organic loading rate (OLR). Digestion performance and stability were monitored via volatile fatty acid (VFA) profiles, alkalinity, specific methane production (SMP) and volatile solids (VS) destruction. High-nitrogen mesophilic digesters stabilised by day 70 and responded well to increases in OLR, with a SMP of 0.45L CH4 g−1 VS, stable pH, and VFA <0.2gL−1. Thermophilic digesters fed on high and low-nitrogen FWs showed almost identical responses to acclimatisation. Behaviour then deviated, with high-nitrogen digesters accumulating VFA. Stable pH could be maintained for up to 310days before eventual failure at total ammonia nitrogen (TAN) concentration >5.0gNL−1, although accumulation of propionic and other longer-chain VFA began at TAN ∼3.5gNL−1. The low-nitrogen digesters showed no VFA accumulation, and had a SMP of ∼0.39L CH4 g VS L−1day−1 with 91% VS destruction. After 384days the TAN concentration was increased from ∼0.7gNL−1 by urea addition. This resulted in progressive VFA accumulation in one digester when TAN reached ∼3.5gNL−1, while stable operation at very low VFA was possible at up to ∼2.5gNL−1 in the second digester. The results confirmed acclimatisation to thermophilic conditions was possible on a far shorter timescale than to high TAN concentrations.

The effect of the solids and hydraulic retention time on moving bed membrane bioreactor performance

The aim of the present paper was to investigate the effect of solids (SRT) and hydraulic (HRT) retention time on Integrated Fixed Film Activated Sludge (IFAS) University of Cape Town (UCT) membrane Bioreactor (MBR). In particular, three different pairs of SRT and HRT values were analysed, namely, Phase I 56 d/30 h, Phase II 31 d/15 h and Phase III 7 d/13 h. The short-term effect of these three SRT/HRT conditions was assessed by analysing several system performance indicators: organic carbon and biological nutrient (nitrogen and phosphorus) removal, biomass respiratory activity, activated sludge filtration properties and membrane fouling. The results showed that the decrease of SRT/HRT had a positive influence on system performance. Specifically, the IFAS-UCT-MBR showed excellent removal of organic matter - highest value (99%) at the shortest SRT/HRT (7 d/13 h). Also, the increase in organic loading rate resulting from the decrease of SRT and HRT led to improved nitrogen removal due to higher N removal by sludge wasting requiring less N removal (as N2) by denitrification. Complete nitrification of influent ammonia was achieved at all three SRT/HRT phases, guaranteed by the presence of biofilm carriers in the aerobic reactor, which ensured a higher media SRT than suspended biomass SRT. The increase of the organic loading rate and decrease in SRT led also to a higher heterotrophic activity as demonstrated by the respirometric batch tests, which is due to the increasing active biomass fraction of the volatile suspended solids as SRT decreases. The SRT/HRT decrease over the three phases resulted in an overall increase of the Extracellular Polymeric Substance concentration, which caused an increase in membrane fouling.

Effect of feeding mode and dilution on the performance and microbial community population in anaerobic digestion of food waste

The effect of feeding mode and dilution was studied in anaerobic digestion of food waste. An upflow anaerobic digester with a settler was fed at six different organic loading rates (OLRs) from 4.6 to 8.6kgCOD/m3/d for 200days. The highest methane productivity of 2.78LCH4/L/d was achieved at 8.6kgCOD/m3/d during continuous feeding of diluted FW. Continuous feeding of diluted food waste showed more stable and efficient performance than stepwise feeding of undiluted food waste. Sharp increase in propionate concentration attributed towards deterioration of the digester performances in stepwise feeding of undiluted food waste. Microbial communities at various OLRs divulged that the microbial distribution in the continuous feeding of diluted food waste was not significantly perturbed despite the increase of OLR up to 8.6kgCOD/m3/d, which was contrast to the unstable distribution in stepwise feeding of undiluted food waste at 6.1kgCOD/m3/d.

Ulva biomass as a co-substrate for stable anaerobic digestion of spent coffee grounds in continuous mode

Ulva biomass was evaluated as a co-substrate for anaerobic digestion of spent coffee grounds at varying organic loads (0.7–1.6g chemical oxygen demand (COD)/Ld) and substrate compositions. Co-digestion with Ulva (25%, COD basis) proved beneficial for SCG biomethanation in both terms of process performance and stability. The beneficial effect is much more pronounced at higher organic and hydraulic loads, with the highest COD removal and methane yield being 51.8% and 0.19L/g COD fed, respectively. The reactor microbial community structure changed dynamically during the experiment, and a dominance shift from hydrogenotrophic to aceticlastic methanogens occurred with increase in organic loading rate. Network analysis provides a comprehensive view of the microbial interactions involved in the system and confirms a direct positive correlation between Ulva input and methane productivity. A group of populations, including Methanobacterium- and Methanoculleus-related methanogens, was identified as a possible indicator for monitoring the biomethanation performance.

Performance of modified anaerobic inclining-baffled reactor treating recycled paper mill effluent: effects of influent chemical oxygen demand concentration and hydraulic retention time

ABSTRACTThe performance of modified anaerobic inclining-baffled reactor (MAI-BR) treating recycled paper mill effluent (RPME) was investigated by varying the influent chemical oxygen demand (CODin) concentration from 1000 to 4000 mg/L, and the hydraulic retention time (HRT) from 3 to 1 day, corresponding to an organic loading rate increase from 0.33 to 4 g COD/L day. Throughout 126 days of operation, a maximum removal efficiency of up to 96% of chemical oxygen demand (COD) and 99% of biological oxygen demand, methane (CH4) yield of 0.259 L CH4/g COD, and a stable effluent pH of 6.5 were achieved. Furthermore, the compartmental performance showed that most of the organic substrates were removed in the initial two compartments, resulting in low pH and alkalinity levels and a high concentration of volatile fatty acids. Overall, the results showed that the MAI-BR successfully treated RPME, and the performance was affected by the variation of HRT more than the CODin.

Insights into quorum quenching mechanisms to control membrane biofouling under changing organic loading rates

A quorum quenching (QQ) consortium comprised of both acyl homoserine lactones (AHLs)- and autoinducer-2 (AI-2)-degrading bacteria, either immobilized in polymer-coated alginate beads or in liquid suspension, was examined for fouling control in lab-scale MBRs under both steady and changing organic loading rates (OLRs). Under steady conditions the QQ consortium retarded biofouling by a factor of 3. However, a continuous increase in OLR vastly reduced the effectiveness of QQ bacteria; the biofouling was retarded only by factors of 1.4–1.8. A significant increase in extracellular polymeric substance (EPS), especially loosely-bound EPS in mixed liquor together with an increase in polysaccharide content up to 4 times in EPS resulted from the increase in OLR, was attributed to the impaired QQ efficacy. In control MBRs, cake layer resistance was the major factor (>60%) contributing to the increased trans-membrane pressure, as compared with pore blockage resistance and intrinsic membrane resistance. In contrast, the pore blockage resistance became dominant in QQ MBRs (>40%).

Process stability and microbial community composition in pig manure and food waste anaerobic co-digesters operated at low HRTs

This study assessed the effects of reducing hydraulic retention times (HRTs) from 21 days to 10.5 days when anaerobically co-digesting pig manure and food waste. Continuously stirred tank reactors of 3.75 L working volume were operated in triplicate at 42°C. Digester HRT was progressively decreased from 21 to 15 days to 10.5 days, with an associated increase in organic loading rate (OLR) from 3.1 kg volatile solids (VS)·m–3·day–1 to 5.1 kg VS·m–3·day–1 to 7.25 kg VS·m–3·day–1. Reducing HRT from 21 days to 15 days caused a decrease in specific methane yields and VS removal rates. Operation at a HRT of 10.5 days initially resulted in the accumulation of isobutyric acid in each reactor. High throughput 16S rRNA gene sequencing revealed that this increase coincided with a shift in acidogenic bacterial populations, which most likely resulted in the increased isobutyric acid concentrations. This may in turn have caused the increase in relative abundance of Clocamonaceae bacteria, which syntrophically degrade non-acetate volatile fatty acids (VFAs) into H2 and CO2. This, along with the increase in abundance of other syntrophic VFA oxidizers, such as Spiorchatetes, suggests that VFA oxidation plays a role in digester operation at low HRTs. Reducing the HRT to below 21 days compromised the ability of the anaerobic digestion system to reduce enteric indicator organism counts below regulatory limits.

Process stability and microbial response of anaerobic membrane bioreactor treating high-strength kitchen waste slurry under different organic loading rates

Anaerobic membrane bioreactor (AnMBR) was applied for treating high-strength kitchen waste slurry obtained after the pretreatment of kitchen waste for oil recovery and solid separation. The process stability and microbial community structure of the AnMBR were investigated under different organic loading rates (OLRs). When the OLR increased from 4.7 kg-COD m−3 d−1 to 5.9 kg-COD m−3 d−1, 7.5 kg-COD m−3 d−1 and 9.3 kg-COD m−3 d−1, the digestion efficiency (i.e. conversion efficiency of COD to methane) increased from 68.8% to 78.1%, 81.5% and 84.4%, respectively, while the permeate COD all kept below 1500 mg L−1. Additionally, the low VFA concentration (<300 mg L−1) and stable pH value around 7.5 under the four OLRs confirmed the robust stability of the AnMBR system. Furthermore, with the increase of OLR, the hydrolytic enzyme activity was enhanced, especially when the OLR increased from 4.7 kg-COD m−3 d−1 to 9.3 kg-COD m−3 d−1, the amylase, protease and lipase activities increased by 1.9-fold, 2.0-fold and 2.4-fold, respectively. Meanwhile, the increase of OLR had no obvious effects on the bacterial community but remarkable influence on the archaeal community as the dominant archaea shifted from Methanobacterium to Methanosaeta.

Effects of organic loading rate on biogas production from macroalgae: Performance and microbial community structure

Macroalgae biomass has been considered as a promising feedstock for biogas production. In order to improve the efficiency of anaerobic digestion (AD) of macroalgae, semi-continuous fermentation was conducted to examine the effects of organic loading rate (OLR) on biogas production from Macrocystis pyrifer. Results showed that, under OLRs of 1.37, 2.74, 4.12 and 6.85kgVSsubstrate/(m3·d), the average unit biogas yields were 438.9, 477.3, 480.1 and 188.7mL/(gVSsubstrated), respectively. It indicated that biogas production was promoted by the increased OLR in an appropriate range while inhibited by the OLR beyond the appropriate range. The investigation on physical-chemical parameters revealed that unfavorable VFAs concentration, pH and salinity might be the main causes for system failure due to the overrange OLR, while the total phenols failed to reach the inhibitory concentration. Microbial community analysis demonstrated that several bacterial and archaeal phyla altered with increase in OLR apparently.

Fouling behaviour of soluble microbial products and extracellular polymeric substances in a submerged anaerobic membrane bioreactor treating low-strength wastewater at room temperature

The properties of soluble microbial products (SMP) and extracellular polymeric substances (EPS) were determined and their role in membrane fouling behaviour was considered in an anaerobic membrane bioreactor (AnMBR) treating low-strength wastewater at room temperature. The SMP/EPS properties of interest were specific production, molecular weight distribution and adhesion force. The results showed the largest factor affecting the SMP/EPS properties and their diverse membrane fouling performances was the organic loading rate (OLR). An increase in the OLR resulted in an increase in the production of specific EPS and macromolecules in the SMP/EPS fractions, in effect exacerbating the flocculation ability of the mixed liquor in the AnMBR and thus facilitating the fast formation of cake layers. Furthermore, the EPS tended to be more viscoelastic and hydrophobic at a higher OLR and because the adhesion forces of the EPS-membrane and EPS-EPS were significantly enhanced as the OLR increased, cake fouling was significantly accelerated. The results indicated that the main cause of fouling was SMP-induced pore blockages, and that membrane resistance increased gradually until an OLR of 0.7 gCOD/L/d, but increased remarkably when the OLR was higher than 1.4 gCOD/L/d, caused by the EPS-induced fast-growth and compact cake layer on the membrane surface.

Effects of organic loading rate on hydrogen and volatile fatty acid production and microbial community during acidogenic hydrogenesis in a continuous stirred tank reactor using molasses wastewater.

Microbial community associated with hydrogen production and volatile fatty acids (VFAs) accumulation was characterized in acidogenic hydrogenesis using molasses wastewater as a feedstock. Hydrogen and VFAs production were measured under an organic loading rate (OLR) from 19 to 35g-CODl-1 day-1 . The active microbial community was analysed using RNA-based massively parallel sequencing technique, and their correlation patterns were analysed using networking analysis. The continuous stirred tank reactor achieved stable hydrogen production at different OLR conditions, and the maximum hydrogen production rate (HPR) was 1·02L-H2 l-1 day-1 at 31·0g-CODl-1 day-1 . Butyrate (50%) and acetate (38%) positively increased with increase in OLR. Total VFA production stayed around 7135mgl-1 during the operation period. Although Clostridiales and Lactobacillales were relatively abundant, the HPR was positively associated with Pseudomonadaceae and Micrococcineae. Total VFA and acetate, butyrate and propionate concentrations were positively correlated with lactic acid bacteria (LAB) such as Bacillales, Sporolactobacillus and Lactobacillus. The close relationship between Pseudomonadaceae and Micrococcineae, and LAB play important roles for stable hydrogen and VFA production from molasses wastewater. Microbial information on hydrogen and VFA production can be useful to design and operate for acidogenic hydrogenesis using high strength molasses wastewater.

Dry fermentation of manure with straw in continuous plug flow reactor: Reactor development and process stability at different loading rates

In this work, a plug flow reactor was developed for continuous dry digestion processes and its efficiency was investigated using untreated manure bedded with straw at 22% total solids content. This newly developed reactor worked successfully for 230days at increasing organic loading rates of 2.8, 4.2 and 6gVS/L/d and retention times of 60, 40 and 28days, respectively. Organic loading rates up to 4.2gVS/L/d gave a better process stability, with methane yields up to 0.163LCH4/gVSadded/d which is 56% of the theoretical yield. Further increase of organic loading rate to 6gVS/L/d caused process instability with lower volatile solid removal efficiency and cellulose degradation.