Decreased Biogas Production Research Articles

Fixing collapsed dry anaerobic digestion system of kitchen waste caused by severe VFAs accumulation

Severe inhibition by volatile fatty acids (VFAs) frequently occurs during the dry anaerobic digestion of kitchen waste at high organic load rate (OLR), leading to decreased biogas production and even the collapse of the anaerobic digestion system. In this study, VFAs accumulation was significantly reduced by adding powder-activated carbon (PAC) or alkaline additives (AA), thus restoring the collapsed system to a stable operating state. With the addition of PAC or AA, the maximal OLR reached 11.14 g VS/(L·d), which was 30 % higher than observed without any additives (control group), while the VFAs concentration was maintained below 3000 mg/L. At this OLR, the volumetric biogas production rate stabilized at 5.1 L/(L·d) in the presence of PAC and AA, while that of the control group gradually decreased to zero. The VFAs concentration with PAC addition was 86 % lower than that of the control group, possibly because PAC might stimulate the formation of direct interspecies electron transfer between syntrophic bacteria and methanogens (Methanosarcina), thereby promoting VFAs degradation. The addition of AA, which resulted in a 95 % decrease in ammonia-nitrogen concentration, can provide a good growth environment for the microorganisms involved in acidogenesis and hydrogenotrophic methanogenesis.

Development of an innovative MFC-biosensor for real-time monitoring of anaerobic digestion for biogas production: Controlled substrate feeding strategy

A microbial fuel cell (MFC) based biosensor was designed to monitor volatile fatty acids (VFAs) in anaerobic digestion (AD) to control substrate feeding. The VFAs concentrations 7–40.83 mM showed a linear relationship with biosensor voltage (18–67 mV) indicating that voltage < 18 mV was due to low organic matter. Likewise, VFAs concentration lower than 7.87 mM resulted in decreased biogas production rate (BPR) and required substrate feeding. In a continuous food waste AD, the biosensor guided feeding produced 558 mL/g VS with the highest biomethane (62.17%), comparing with 344 and 387 mL/g VS produced by conventional 3- and 7-days intervals feeding, respectively. The microbial community was also more stable under the new strategy, where Prevotella and Methanosarcina were ~6-fold and 1.5-fold higher than conventional groups. The developed portable biosensor (~$200) worked effectively during outdoor monitoring of a domestic biogas plant. The VFAs based MFC biosensors can enhance the biogas yield with minimum substrate feeding while maintaining a suitable microbial community.

Effect of the ozonation pretreatment on biogas production from waste activated sludge of tehran wastewater treatment plant

In this study, the ozonation impact on anaerobic digestion of wasted thickened activated sludge of Tehran wastewater treatment plant has been investigated. Regarding that, the thickened activated sludge, a final solid waste of treatment plants, was subjected to ozonation pretreatment to evaluate its characteristics. Three major sludge characteristics were considered for investigating ozonation effects on anaerobic digestion. A comparison of two different ozone doses, i.e., 0.05 and 0.1 gO3 g−1TS was performed for short-term (10 days) and long-term (30 days) sludge retention times. Furthermore, biogas production, sludge composition, dewaterability, and energy balancing, were studied during the periods mentioned above. The results show that the lower ozone dose has an acceptable performance in a short-term operation for all parameters except energy balance which was negative in all the scenarios. Higher ozone dosage not only decreased biogas production and volatile solids reduction but also significantly increased the energy demand of the system. In addition, a longer retention time reduced the effectiveness of the ozone pretreatment during that time. Eventually, the feasibility of ozone pretreatment capacity for sustainable management of different sludge from economic and technological perspectives can be evaluated by further assessing this case study.

The Effects of Hot Water and Ultrasonication Pretreatment of Microalgae (Nannochloropsis oculata) on Biogas Production in Anaerobic Co-Digestion with Cow Manure

Anaerobic co-digestion provides a promising solution for converting inexpensive carbon from wastes to biogenic methane. We used microalgae (Nannochloropsis oculata) with cow manure and sludge to produce a better quantity and quality of biogas. To further improve the gas production, microalgae were pretreated with ultrasonication, hot water, and a combination of both. Interestingly, the results showed that the pretreatment of microalgae decreased biogas production by 5 to 30%. The no-pretreatment runs produced a maximum of 118 L of biogas. The relative content of biogenic methane was higher in the pretreated feedstock (48 to 52%) in comparison with the no-pretreatment runs (44%). The conversion of volatile suspended solids present in the feedstock to total biogenic methane production was highest in hot-water-treated runs. The carbon content in the gas produced by the pretreated microalgae peaked (38%) in the middle of the experiment (i.e., at 45 days), whereas for no-pretreatment runs, the content remained constant from the start to the middle and declined (from 36 to 34%) at the end of the experiment (i.e., at 90 days). We also report the chemical structure of microalgae with and without pretreatments.

Biogas Production From Co Digestion of Automotive Wastewater And Synthetic Wastewater in Anaerobic Reactor

The purpose of this study is to investigate the biogas potential from automotive wastewater codigested with Synthetic wastewater treated in anaerobic reactor. This experiment was run by continuously mixed with mechanical Agitator stirring tank reactor and were established under mesophilic condition at temperature of 38°C. The source of inoculum used in this study were obtained from sewage treatment plant. The experiment was analyzed based on COD removal and biogas production in relation with parameters such as pH, organic loading rate, alkalinity and effect of Zinc and Copper which runs on automotive wastewater. From the result obtains, the reactor condition was unstable during mono digestion of automotive wastewater which aligned with indication of decreasing biogas production, low pH value below 6.0 and low COD removal with value of 63.4%. However, the reactors become stable during co digestion of automotive wastewater with Synthetic wastewater with 0.12 g/L/d OLR and with highest methane production 0.140 L of CH4 / day. The findings illustrate the nutrient available in Synthetic wastewater could enhance the synergistic effect in co digestion with automotive wastewater and therefore resulted with the highest COD removal rates.

Opportunities and challenges: Experimental and kinetic analysis of anaerobic co-digestion of food waste and rendering industry streams for biogas production

Large amounts of food waste and sewage sludge exert a hazardous environmental impact in several countries. Producing biogas and digestate from food and industrial waste is one of the solutions for waste management, stabilization of sludge, resource and energy recovery and reductions in the amount of waste. However, biogas production from such substrates has challenges in degradation efficiency, inhibitory effects and other challenges, and thus co-digestion and pretreatment techniques could be applied to enhance biogas production. The aim of this study is to explore the effects of co-digestion of food waste, meat and bone meal and rendering wastewater sludge. First, thermal pretreatment was performed (35°C, 5 days) by adding the rendering-industry streams to food waste in the amounts of 0, 5, 10 and 15% on a total solid basis, and further anaerobic digestion (40.5°C, ca. 40 days) was then performed. Both experimental and kinetic analysis were conducted, and the major factors regarding opportunities and challenges in the two-stage process are discussed. Results have shown that both co-substrates from rendering industry decreased the biogas yield of food waste. When 5% of them was added to food waste, meat and bone meal decreased biogas production by 12%, and wastewater sludge decreased it by 23%. Both co-substrates, on the other side, increased the rate of reaction of food waste digestion when applying different common kinetic models.

Reversibility of propionic acid inhibition to anaerobic digestion: Inhibition kinetics and microbial mechanism

Anaerobic digestion is a technology that simultaneously treats waste and generates energy in the form of biogas. Unfortunately, when a high organic loading rate is applied, anaerobic digestion can suffer from volatile fatty acid accumulation that results in pH drop and decreased biogas production. In particular, propionic acid has shown to inhibit biogas production even at a very low concentration. Therefore, the kinetics of biogas production in relation to propionic acid concentration needs to be investigated. In batch experiments on anaerobic co-digestion of food waste and dairy manure in the present study, cumulative biogas production showed little inhibition by propionic acid in the concentration range of 6.5–14.6 mM, but a lag phase of 9.4, 16.3 and 60.8 d was detected in the digesters with initial propionic acid concentrations of 22.7, 36.2, and 56.4 mM, respectively. After the lag phase, these digesters accelerated to specific biogas yields of 0.59–0.70 L g-VS−1. The similar specific biogas yields across all of the digesters at initial propionic acid concentrations of 6.5–56.4 mM indicated reversibility of the inhibition. The reversibility was made possible by microbial acclimation and the shift to hydrogenotrophic methanogenesis in syntrophy with acetogenic bacteria. Evidently, an increase of hydrogenotrophic Methanobacterium and Methanoculleus abundances was found at 36.2 and 56.4 mM. Batch digestion experiments must be extended beyond the lag phase in order to fully reveal the inhibition kinetics. This paper highlights the need for a standard protocol that experimentally evaluates inhibition in anaerobic digestion.

The influence of selected pharmaceuticals on biogas production from laboratory and real anaerobic sludge.

The presented study summarizes laboratory tests results to define the inhibition effect of selected pharmaceuticals on biogas production under anaerobic digestion conditions. Two sets of inhibition tests were realized: (i) with real anaerobic sludge (from municipal wastewater treatment plant (WWTP) where sludge is present and includes a wide spectrum of pharmaceuticals over a long period) and (ii) with laboratory sludge (sludge without pharmaceuticals). Methanogenic tests lasting 20 days were performed with three analgesics (diclofenac, ibuprofen, and tramadol), two antibiotics (amoxicillin and ciprofloxacin), β-blocker (atenolol), three psychoactive compounds (carbamazepine, caffeine, and cotinine), and a mixture of these compounds. All tests were performed with two concentrations of pharmaceuticals (10 μg/L and 500 μg/L). Results of the methanogenic tests showed the different behaviors of the investigated sludges in the presence of individual pharmaceuticals. Stimulation of anaerobic digestion was mostly detected for laboratory (unadapted) sludge (e.g., the addition of ibuprofen at a concentration of 500 μg/L increased biogas production by 61%). On the other hand, pharmaceuticals inhibited biogas production for real sludge (e.g., the addition of ciprofloxacin 500 μg/L decreased biogas production by 52%).

Aeration to Improve Biogas Production by Recalcitrant Feedstock

Digestion of wastes to produce biogas is complicated by poor degradation of feedstocks. Research has shown that waste digestion can be enhanced by the addition of low levels of aeration without harming the microbes responsible for methane production. This research has been done at small scales and without provision to retain the aeration in the digestate. In this paper, low levels of aeration were provided to poultry litter slurry through a sub-surface manifold that retained air in the sludge. Digestate (133 L) was supplied 0, 200, 800, or 2000 mL/day air in 200 mL increments throughout the day via a manifold with a volume of 380 mL. Digesters were fed 400 g of poultry litter once weekly until day 84 and then 600 g thereafter. Aeration at 200 and 800 mL/day increased biogas production by 14 and 73% compared to anaerobic digestion while aeration at 2000 mL/day decreased biogas production by 19%. Biogas quality was similar in all digesters albeit carbon dioxide and methane were lowest in the 2000 mL/day treatment. Increasing feed to 600 g/week decreased gas production without affecting biogas quality. Degradation of wood disks placed within the digesters was enhanced by aeration.

What kind of effects do Fe2O3 and Al2O3 nanoparticles have on anaerobic digestion, inhibition or enhancement?

Fe2O3 and Al2O3 nanoparticles are widely used in products and find their way to wastewater treatment plants through the contact of water with these products. In this study, impacts of Fe2O3 and Al2O3 nanoparticles on methane potential of waste activated sludge (WAS) were investigated by comparing long and short term toxicity test results, modelling and FISH analysis. Methane production from the samples treated with the maximum concentration of Fe2O3 nanoparticles decreased 28.9% at the end of the long term BMP test. EC50 value for BMP test of the Fe2O3 nanoparticles was calculated as 901.94 mg/gTS with high coefficient of determination. Methane production from the samples treated with Al2O3 nanoparticles increased up to 14.8% (p > 0.05) at the end of the BMP test. However, short term toxicity tests for Fe2O3 and Al2O3 nanoparticles showed no impact on anaerobic digestion of WAS. Kinetic parameters obtained from models and captured FISH images were consistent with these results. Different impacts of nanoparticles on methane production suggested that anaerobic microorganisms can be affected from nanoparticles in various mechanisms. Hydrolysis (kH) and overall reaction rates (kR) values were determined as 0.0277 and 0.1441 d−1, respectively for each concentration of Al2O3 nanoparticles and raw WAS. Similarly, methane production from WAS containing 5, 50, 150 and 250 mgFe2O3/gTS were modeled with same kinetic values. However, kH constant was calculated as 0.0149 d−1 for 500 mgFe2O3/gTS. This means that Fe2O3 nanoparticles starting from this concentration inhibited the methanogenic consortium and caused decreased biogas production and spesific methane production rate.

Inter-stage thermophilic aerobic digestion may increase organic matter removal from wastewater sludge without decreasing biogas production.

Combining aerobic and anaerobic digestion in a two-stage system can improve the degradation of wastewater sludge over the use of either technology alone. But use of aerobic digestion as a pre-treatment before anaerobic digestion generally reduces methane production due to loss of substrate through oxidation. An inter-stage configuration may avoid this reduction in methane production. Here, we evaluated the use of thermophilic aerobic digestion (TAD) as an inter-stage treatment for wastewater sludge using laboratory-scale semi-continuous reactors. A single anaerobic digester was compared to an inter-stage system, where a thermophilic aerobic digester (55 °C) was used between two mesophilic anaerobic digesters (37 °C). Both systems had retention times of approximately 30 days, and the comparison was based on measurements made over 97 days. Results showed that the inter-stage system provided better sludge destruction (52% volatile solids (VS) removal vs. 40% for the single-stage system, 44% chemical oxygen demand (COD) removal vs. 34%) without a decrease in total biogas production (methane yield per g VS added was 0.22-0.24 L g-1 for both systems).

Effect of nanoscale zero-valent iron on sludge anaerobic digestion

Nanoscale zero-valent iron(NZVI) was prepared with a method of reduction in liquid phase by the reaction of FeSO4 and NaBH4, and characterized by SEM. This study investigated the effect of different concentration of NZVI on biogas production and methane content during the anaerobic digestion process at the medium temperature of 35°C. The results indicated that the group with 1000mg/L NZVI maintained ammonia at the level of 600–800mg/L, pH between 6.5-8.0, and improved CODcr degradation rates at 2.91%; this concentration of NZVI produced maximum concentration of VFAs, which can reach 3846mg/L, and strengthen the use of acetic acid. Relative to the control group, the system increased cumulative biogas production of 18.11%, decreased biogas production cycles of three days and increased the methane content by 6.93%.

Lignocellulosic waste valorisation strategy through enzyme and biogas production

Lignocellulosic wastes are generally pre-treated to facilitate the hydrolysis stage during the anaerobic digestion process. A process consisting of solid state fermentation carried out by white rot fungi and anaerobic digestion was evaluated on corn stover to produce ligninolytic enzymes and biogas. The enzyme production was quantified every 3d for a month at 30°C, and three fungal strains and two particle sizes of waste were compared. Of the main outcomes, Pleurotus eryngii produced the highest laccase enzyme activity compared with Pleurotus ostreatus and Trametes versicolor. Furthermore, this activity was improved by 16% when copper was used as an enzyme inducer. On the other hand, most of the conditions studied showed a decrease in maximum biogas production compared with untreated waste, the addition of copper decreased biogas production by 20%. Despite the above, Pleurotus eryngii showed promising results allowing a 19% increase of biogas production and high enzyme production values.

Evaluation of the effects of low energetic microwave irradiation on anaerobic digestion

The present study investigates the effects of microwave irradiation on the performance of anaerobic digestion processes. A first set of experiments is performed to distinguish the upper limit of the applied energy levels. Secondly, the effects of these treatments on the performance of the digestion process are evaluated in 3 experimental setups: (i) monitoring the acetic acid degradation, (ii) performing a biological methane potential (BMP) assay and (iii) conducting a specific methanogenic activity (SMA) test. The solubilisation experiment reveals a limited degree of disintegration of anaerobic biomass up to a microwave treatment of 10000 kJ/kg TS. Above this threshold value the soluble COD level started to rise, with up to 350% at 30000 kJ/kg TS regardless of the microwave output power. Because solubilisation of the biomass increases the easily degradable content, this would lead to false observations regarding increased activity. Therefore, solubilisation is minimized by limiting the microwave treatment to a maximum of 6000 kJ/kg TS during the second part of the experiments. Monitoring the degradation of acetic acid after a low intensity microwave treatment, reveals that microwave irradiation shortens the lag phase, e.g., from 21 to 3 h after a microwave treatment of 1000 kJ/kg TS at 100 W. However most treatments also result in a decrease of the maximum degradation and of the degradation rate of acetic acid. BMP assays are performed to evaluate the activity and performance of the entire anaerobic community. Every treatment results in a decreased biogas production potential and decreased biogas production rate. Moreover, each treatment induced an increase of the lag phase. The SMA experiments show no influence of the microwave irradiation in terms of biogas or methane production.

Improving biogas production from continuous co-digestion of oily wastewater and waste-activated sludge by hydrodynamic cavitation pre-treatment

ABSTRACTHydrodynamic cavitation (HC) was evaluated as a pretreatment for synthetic oily wastewater (OWW) to be co-digested with waste-activated sludge (WAS). The main objective of the present research was the enhancement of biogas production by the application of HC pretreatment. HC was applied to the OWW, and the OWW and WAS were added to a 50 L continuous digestion reactor. As a control system, an identical digestion reactor was set up for co-digestion of the WAS and the OWW without pretreatment. The reactors were initially filled with inoculum and the hydraulic retention time (HRT) was set to 22 d. The HRT was gradually reduced to 19, 16, and finally 13 d, but the substrate quality was kept constant. The loading rate, accordingly, increased from 0.86 to 1.46 g TVS/(L d). The biogas volume was recorded online and its quality was analyzed regularly. The HC improved biogas production up to 43% at 22 d of HRT. Reducing the HRT decreased biogas production from the main reactor while that of the control reactor was more or less constant. HC also increased the biogas methane content; the methane concentration of the main reactor was about 3% higher than the methane concentration of the control reactor. The main reactor experienced no clogging or accumulation of fatty materials.

Proteotyping of laboratory-scale biogas plants reveals multiple steady-states in community composition

Complex microbial communities are the functional core of anaerobic digestion processes taking place in biogas plants (BGP). So far, however, a comprehensive characterization of the microbiomes involved in methane formation is technically challenging. As an alternative, enriched communities from laboratory-scale experiments can be investigated that have a reduced number of organisms and are easier to characterize by state of the art mass spectrometric-based (MS) metaproteomic workflows.Six parallel laboratory digesters were inoculated with sludge from a full-scale BGP to study the development of enriched microbial communities under defined conditions. During the first three month of cultivation, all reactors (R1-R6) were functionally comparable regarding biogas productions (375–625 NL Lreactor volume−1 d−1), methane yields (50–60%), pH values (7.1–7.3), and volatile fatty acids (VFA, <5 mM). Nevertheless, a clear impact of the temperature (R3, R4) and ammonia (R5, R6) shifts was observed for the respective reactors. In both reactors operated under thermophilic regime, acetic and propionic acid (10–20 mM) began to accumulate. While R4 recovered quickly from acidification, the levels of VFA remained to be high in R3 resulting in low pH values of 6.5–6.9. The digesters R5 and R6 operated under the high ammonia regime (>1 gNH3 L−1) showed an increase to pH 7.5–8.0, accumulation of acetate (>10 mM), and decreasing biogas production (<125 NL Lreactor volume−1 d−1).Tandem MS (MS/MS)-based proteotyping allowed the identification of taxonomic abundances and biological processes. Although all reactors showed similar performances, proteotyping and terminal restriction fragment length polymorphisms (T-RFLP) fingerprinting revealed significant differences in the composition of individual microbial communities, indicating multiple steady-states. Furthermore, cellulolytic enzymes and cellulosomal proteins of Clostridium thermocellum were identified to be specific markers for the thermophilic reactors (R3, R4). Metaproteins found in R3 indicated hydrogenothrophic methanogenesis, whereas metaproteins of acetoclastic methanogenesis were identified in R4. This suggests not only an individual evolution of microbial communities even for the case that BGPs are started at the same initial conditions under well controlled environmental conditions, but also a high compositional variance of microbiomes under extreme conditions.

Addressing problems at small-scale biogas plants: a case study from central Vietnam

The anaerobic digestion process is an important technology in improving the environment because it solves organic waste management problems and simultaneously produces both biogas and fertiliser. The use of biogas plants has been spreading in many developing countries, bringing various operational problems with their widening use. This study attempts to identify the problems with this technology at the level of owners of biogas plants (n = 141) and local facilitators (n = 9) in central Vietnam. A survey was conducted from July to September 2012. The methods of data collection included focus group discussions, semi-structured personal interviews and a questionnaire survey. The survey revealed that 29% of biogas plant owners have experienced at least one problem with this technology. The most frequently encountered problem is linked to leakages from reactors leading to undesired CH4 emissions, which sometimes stopped the biogas plants from functioning. Other problems concern the failure of biogas cookers to properly function with solid digestate incrustation floating in the main tank, resulting in decreased biogas production. The respondents call for better-trained builders and facilitators, who are often unable to solve difficulties encountered with BGPs. The importance of a working information flow between actors is demonstrated. The study also involves the calculation of the payback period of biogas plants. The findings show a linear relationship between the payback period and biogas plant-owners' satisfaction with biogas technology, biogas production and the biogas programme. In addition, the study recommends improvements in the skills of facilitators because they have a direct impact on the quality of training of BGP owners and builders. In conclusion, this study provided an innovative problem analysis of biogas technology along with appropriate recommendations. It demonstrated the need for further research on the eradication of problems associated with biogas technology.

Kajian Teknologi Produksi Biogas Dari Sampah Basah Rumah Tangga

The investigation about study the biogas production Technology from household waste has been done. This research aims to determine the starter concentration, water and household waste ratio to produc highest biogas, and also to find the fermentation time produce biogas with hight rendement. It was reache by applying 5 levels of cow dung starter concentration and 5 levels of water and household waste ratio i.e 0%, 6,25%, 12,5%, 18,75% and 25% and 1:2, 1:2,5, 1:3, 1:3,5 and 1:4 (b/v) respectively. The fermentation time was 3 days. The result showed that the higher cow dung concentration the lower biogas production. The highis biogas production (971,4 mg/L) was found with 6,25% of the starter. The increasing water and household waste ratio would decreased biogas production, and the using of 1:3,5 and 1:4 didn’t produce any biogas. The highest biogas production was found with 1:2 of water/household ratio i.e (631.29 mg/L). the fermentation time which produce the highest biogas rendement was found with 1 day of fermentation time

Impact of nickel and cobalt on biogas production and process stability during semi-continuous anaerobic fermentation of a model substrate for maize silage

The importance of nickel and cobalt on anaerobic degradation of a defined model substrate for maize was demonstrated. Five semi-continuous reactors were operated for 250 days at 35 °C and a well-defined trace metal solution was added to all reactors. Two reactors each were limited regarding the concentration of Ni2+ and Co2+, respectively, for certain time intervals. The required nickel concentration was depending on the organic loading rates (OLR) while, for example, above 2.6 g ODM L−1 d−1 nickel concentrations below 0.06 mg kg−1 FM in the process significantly decreased biogas production by up to 25% compared to a control reactor containing 0.8 mg Ni2+ kg−1 FM. Similarly, limitation of cobalt to 0.02 mg kg−1 FM decreased biogas production by about 10%. Limitations of nickel as well as cobalt lead to process instability. However, after gradual addition of nickel till 0.6 mg and cobalt till 0.05 mg kg−1 FM the OLR was again increased to 4.3 g ODM L−1 d−1 while process stability was recovered and a fast metabolisation of acetic and propionic acid was detected. An increase of nickel to 0.88 mg kg−1 FM did not enhance biogas performance. Furthermore, the increase of cobalt from 0.05 mg kg−1 FM up to 0.07 mg kg−1 FM did not exhibit a change in anaerobic fermentation and biogas production.

Co-digestion of onion juice and wastewater sludge using an anaerobic mixed biofilm reactor

The co-digestion of onion juice and aerobic wastewater sludge produced from an onion processor using an anaerobic mixed biofilm reactor (AMBR) was investigated for biogas energy production potential and waste treatment. Two experiments were conducted to study the performance of an AMBR at different organic loading rates (OLRs) using different mixtures of onion juice and aerobic sludge. In the first experiment, the OLR was increased from 1.24 to 4.37 gVS/L/d by increasing the amount of onion juice in the feed mixture while maintaining a constant amount of aerobic sludge. When the OLR reached 4.37 gVS/L/d, the AMBR failed as indicated by decreased biogas production and pH. Increase of carbon to nitrogen ratio (C/N) from 13.7 to 20.3 and lack of proper alkalinity were suspected to be the causes for the failure. In the second experiment, the C/N of the feed mixture was maintained at about 15 while the OLR was increased from 1.40 to 3.60 gVS/L/d. The digester showed stable performance. The average biogas and methane yields of the two experiments were 0.62 ± 0.05 L/gVS and 0.37 ± 0.08 L/gVS, respectively. It was concluded that the C/N of about 15 was recommended for treating the mixture of onion juice and aerobic sludge.