Biomethane Potential Tests Research Articles

Anaerobic Digestion of Secondary Tannery Sludge: Optimisation of Initial pH and Temperature and Evaluation of Kinetics

The feasibility of retrofitting and operating a low cost anaerobic mesophilic batch reactor for the reduction of secondary tannery sludge (STNS) produced in tannery wastewater treatment plants at varying initial reactor pH (pHin) using acclimated inoculum was demonstrated using the bio-methane potential test protocol. Temperature had a significant effect on process indicators whilst the effect of pHin was negated by the STNS’ high alkalinity and its buffering capacity offered by ammonia/ammonium speciation, eliminating the need for pH control during digestion. Furthermore, there was an insignificant and significant variation in the biodegradability (Bo) and gas yields of STNS, respectively. This nullified the need for reactor heating if gas yields were not prioritised. Nonetheless, optimum conditions (35.5–37.0 °C and pHin 6.5) yielded 200–217 mL biogas/gVS (≈ 55% CH4), 108–118 mL CH4/gVS and 69–74% VS, 54–57% TS and 47–48% COD reduction. Cumulative CH4 generations were accurately modelled using the modified Gompertz, Logistic, First order, Cone (0.828 ≤ Adj R2 ≤ 0.986) and process kinetics were determined. However, the correlations between kinetics and process parameters were mostly non-monotonic. The positive effect of temperature on A (r = 0.84) coincided with a decrease in methanogenesis rate (k) and ratio of hydrolysis and methanogenesis (K/µm) at optimum pHin signifying an inhibited steady state reactor condition.

Storage of Food Waste: Variations of Physical\u2013Chemical Characteristics and Consequences on Biomethane Potential

Food waste (FW) storage influences its physical–chemical characteristics and anaerobic digestion (AD) performance. In this work we present the results of two weeks long experiment where two types of FW were stored in dedicated cells (10 L and 300 L). Air was evenly flushed on the top surface of the substrates and then analyzed to identify and quantify possible gaseous emissions. Solid and liquid fractions were also periodically sampled and analyzed for total solid, volatile solid, ammonia and VFA contents. Results showed that storage initiated a hydrolysis process that modified the physical structure of FW, leading to the production of gases (CH4, CO2 and ethanol) and a partly liquefied FW. Depending on experimental conditions, a fraction between 61 and 70% of the initial substrate remained solid at the end of the storage period. In the liquid phase, a large proportion of lactic acid was measured with maximum contents of 5.9 and 14.8 g/kgvs for the small-scale experiments with two different FW types and 3.0 g/kgvs for the large-scale experiment, leading to inhibition of the biomethane potential (BMP) tests. In conclusion, this work showed that when storage of FW is needed before AD, the optimal time recommended to keep a high methane yield is one week.

Effects of pre-treatments and co-digestion on biogas production from Okra waste

The effects of pre-treatment and co-digestion on the biogas yield of okra wastes are the focus of this paper. Okra wastes (O) were treated with 5% alkaline (NaOH) (OA), microwave (OMW), and microwave + alkaline. Sheep slurry (S) was co-digested with okra waste in varying proportions [25% (OS25), 50% (OS50), and 100% (S)]. Biomethane potential tests using 500 ml bioreactors were carried out in the batch mode at a hydraulic retention time of 25 days under mesophilic conditions (37 °C). The cumulative biogas yield from the alkaline treated sample (OA) increased by 45.87% when compared to the control (O), and other pre-treated samples were statistically the same with untreated okra waste at the 95% confidence level. Samples co-digested with sheep slurry recorded a low biogas yield. The results of the kinetic studies performed with the measured cumulative biogas yield showed that while the modified Gompertz model presented a better prediction range (0.66%–2.48%), the first-order kinetic model produced a better goodness of fit (<10) and R2 values when compared to the modified Gompertz model. The results of the Artificial neural network (ANN) performed were compared with the earlier used kinetic models, and it was observed that all the performance indicators (R2, RMSE and %prediction difference) from the ANN model best predicted the biomethane yields.

Macrophytes as a Digestion Substrate. Assessment of a Sonication Pretreatment

Biomethane potential (BMP) test was performed to access the anaerobic degradation of three macrophytes (Eichhornia crassipes, Pistia stratiotes and Salvinia molesta). Sonication pretreatment was evaluated in order to pre-hydrolyze the plants, aiming to remove the lag time and potentially increase the methane production. Two sonication energies were tested: 40,000 and 80,000 kJ kgTS− 1. Results indicated that not all the aquatic plants presented the same behavior. Raw biomass of Eichhornia and Pistia required a long lag-phase (11 and 8 days, respectively), while Salvinia did not, being the faster and most productive raw biomass (64% of biomethanization factor). Pretreated biomass was solubilized, however the results did not have a direct translation to the digestion performance. In fact, no substantial improvement on the amount of methane production occurred. The main experimental evidence was the full reduction of the lag-phase for Eichhornia and Pistia, while Salvinia did not exhibited a substantial change neither in methane production or in kinetics. The high biodegradability of Salvinia biomass without pretreatment indicated that ultrasound pretreatment is not required. Energetic Sustainability Index evidenced that sonication pretreatment process was not sustainable, due to the required high water content of the bulk.

Anaerobic Digestion of Three Microalgae Biomasses and Assessment of Digestates as Biofertilizer for Plant Growth

This study focused on the determination of biochemical methane potential of three different microalgae, Chlorella sp., Neochloris conjuncta, and Botryococcus braunii, and evaluation of the digestates as biofertilizers for corn growth. Biomethane Potential Test Unit consisting of twelve 2‐L flasks placed in temperature controlled water at 37°C was utilized. As Botryococcus braunii had the highest lipid content (9.54%), it also had the highest biochemical methane potential (290.63 N mL g−1 volatile solid). The digestates containing microalgae and inoculum were applied to corn plant with four application dosages (0 [control], 5, 10, and 15 t ha−1) based on dry mass. Results emphasized that the lowest dosage (5 t ha−1) of the digestates containing Chlorella sp. and inoculum (digested dairy manure with silage maize) increased plant dry weights by 9% and 23%, respectively. However, further increase in the dosages of the digestates containing Botryococcus braunii and Neochloris conjuncta resulted in a decrease of plant growth and nutrients uptake by corn plants from the soil. Results highlighted that increment of the dosages of the digestates containing Botryococcus braunii and Neochloris conjuncta led to a decrease of plant growth and nutrient uptake from soil. © 2018 American Institute of Chemical Engineers Environ Prog, 38:e13024, 2019

Synergistic co-digestion of wastewater grown algae-bacteria polyculture biomass and cellulose to optimize carbon-to-nitrogen ratio and application of kinetic models to predict anaerobic digestion energy balance

This study investigated enhancing methane production from algal-bacteria biomass by adjusting the C/N ratio through co-digestion with a nitrogen-poor co-substrate – cellulose. A biomethane potential test was used to determine cumulative biogas and methane production for pure and co-digested substrates. Four kinetic models were evaluated for their accuracy describing experimental data. These models were used to estimate the total energy output and net energy ratio (NER) for a scaled AD system. Increasing the algal C/N ratio from 5.7 to 20–30 (optimal algae:cellulose feedstock ratios of 35%:65% and 20%:80%) improved the ultimate methane yield by >10% and the first ten days production by >100%. The modified Gompertz kinetic model demonstrated highest accuracy, predicting that co-digestion improved methane production by reducing the time-lag by ∼50% and increasing rate by ∼35%. The synergistic effects increase the AD system energy efficiency and NER by 30–45%, suggesting potential for substantial enhancements from co-digestion at scale.

Comparison of biomethane potential lab tests for Latvian locally available algae

The aim of this study is to evaluate locally available algae Fucus vesiculosus, Ulva Intestinalis and Cerathophyllum demersum using biomethane potential laboratory tests. Mechanical pre-treatment (cutting) and washing are applied to study the effects on cumulative biogas yield. Fucus vesiculosus (cut and washed) produced 81.8 l CH4/kg VS, Ulva intestinalis (cut and washed) produced 92.1 l CH4/kg VS and Cerathophyllum demersum (untreated) produced 405.3 l CH4/kg VS. For marine algae, the effect of pre-treatments varies from 17 % to 44 %, for freshwater algae it varies between –12 % and 5 %. Best t-test and p values are for washed Ulva intestinalis. Results show that from three tested algae species, Cerathophyllum demersum is the most suitable algae for biogas production in Latvia, but more detailed analysis of environmental, economical and legal aspects is needed.

Evaluation of biomethane potential from by-products and agricultural residues co-digestion in southern Italy

The suitability of the co-digestion of feedstock-mixtures (by-products and agricultural residues) depends on their ability to produce biogas. In this study, the effects of mixing five feedstocks (citrus pulp, olive pomace, poultry manure, Italian sainfoin silage and opuntia fresh cladodes) on anaerobic digestion for biogas production have been investigated by carrying out biomethane potential (BMP) tests on six different mixing ratios of the selected five biomasses.The BMP test results demonstrated that all the six studied feedstock-mixtures could be potentially used for renewable energy generation by biogas plants. More in detail, two mixing ratios of the studied feedstock-mixtures showed the best biomethane potential of 249.9 and 260.1 Nm3CH4/tVS, respectively.Since this research study made it possible to screen the suitability and technical feasibility of the feedstock-mixtures analysed, the results provide the basis for subsequent pilot scale evaluation of anaerobic digestion in Mediterranean area, where by-products and agricultural residues are profuse and necessary to produce advanced biofuels.

Drinking water treatment sludge as an effective additive for biogas production from food waste; kinetic evaluation and biomethane potential test

The effect of drinking water treatment sludge (DWTS) as a mixture additive, on biogas and methane production from food waste was studied. Mesophilic anaerobic digestion of food waste with 5 concentrations of DWTS (0, 2, 6, 12, and 18 ppm) was carried out. It was found that DWTS can significantly enhance biogas and methane yield. The highest biogas (671 Nml/g VS) as well as methane yield (522 Nml/g VS) was observed when 6 mg/kg DWTS was added. This is equal to 65 and 58 percent increase in comparison with the control digester. The calculated lag time for methane was found to be in between 3.3 and 4.7 days. The DWTS also reduced the lag phase and retention time. The biogas experimental data was fitted with the modified Gompertz and the first-order kinetic models with R2 higher than 0.994 and 0.949, respectively. The ratio of the experimental biogas production to the theoretical biogas production (ɛ) for control sample was 0.53 while for other samples containing additive were higher than 0.78.

BIO-METHANE POTENTIAL TEST FOR ANAEROBIC CO-DIGESTION OF FAECAL SLUDGE AND SEWAGE SLUDGE

Faecal sludge (FS) samples taken in Hanoi urban area had COD values ranging from 2.83 g/L to 83.83 g/L,TN values from 0.18 g/L to 3.95 g/L, and VS/TS ratio from 47.5 % to 87.7 %. Sewage sludge (SS) samples taken from sewage treatment plants in Hanoi had COD values ranging from 2.22 g/L to 24.97 g/L, TN from 0.16 g/L to 1.24 g/L, VS/TS ratio from 53.5 % to 69.5 %. Bio-methane potential (BMP) tests at mesophilic anaerobic condition (35 oC) provided results of methane production from FS, primary sewage sludge (PSS), waste activated sludge (WAS1 and WAS2) and mixture of primary and secondary sewage sludge at sludge thickener (WAS) as much as 242.3 NmL/gVS, 310.5 NmL/gVS, 294 NmL/gVS, 228.2 NmL/gVS and 282.3 NmL/gVS, respectively. Co-digestion of FS and WAS at different mixing ratios provided from 269.3 NmL to 294.8 NmL CH4 per gram of fed substrate VS. The values of methane yield have shown promising sludge-to-energy option with FS-SS co-digestion. FS-SS co-digestion in anaerobic digesters built at municipal sewage treatment plants which are often working under capacity provides chance to combat with FS and SS challenges, utilizing infrastructure facilities efficiently.

Overall evaluation of microwave-assisted alkali pretreatment for enhancement of biomethane production from brewers’ spent grain

In this study, microwave-assisted alkali pretreatment for brewers’ spent grain (BSG) waste was comprehensively investigated using Box-Behnken design under varying operating parameters, i.e. microwave power, pretreatment time and alkali/biomass (A/B) ratio. Both quantitative and qualitative characterization analysis such as crystallinity index (CrI), specific surface area (SSA), morphology and compositional analyses were conducted on pretreated samples. Standard biomethane potential (BMP) tests were performed for each pretreated sample to observe the biomethane enhancement after pretreatment. Under optimized conditions, it was observed that the pretreatment technique has facilitated removal of 46% of lignin and 38% of hemicellulose and improved the SSA of samples from 9.55 to 161.98 m2/g, contributing to 52% increase in BMP value. Meanwhile, mathematical models with correlation coefficients of 97.9% and 98.2% respectively were built to predict both the soluble chemical oxygen demand (COD) yield and BMP after pretreatment. Furthermore, a simplified concept of overall evaluation mechanism for pretreatment has been introduced, taking into account waste-removal, energetic and economic indices. Under the proposed evaluation mechanism, the caustic soda price was found to play a more important role than the prices of industry electricity and wholesale natural gas.

Correlation between hydrolysis rate constant and chemical composition of energy crops

Besides biogas yield, the kinetic of biogas production in a biomethane potential (BMP) test also provides important information for feedstock characterization. In this study, fodder analysis and BMP tests with high temporal resolution were performed in order to identify statistical correlations between the hydrolysis rate constant (kh) and the chemical composition of various energy crops. Different species and cultivars of energy crops were analyzed in order to develop a broadly applicable regression model for the prediction of kh. Two independent datasets (222 samples in total) were used, one for the calibration of the model and one for its validation. The results indicated that the analytical parameters non-fiber carbohydrates and crude protein were statistically suitable for a multiple linear regression model for the prediction of kh. Furthermore, a first-order kinetic model and the proposed regression models can be utilized for the prediction of the biogas production in a BMP test. The proposed approach offers a fast and reliable prediction of the biogas production rate and allows a feedstock assessment according to their biogas potential.

Faecal sludge treatment and utilization by hydrothermal carbonization

Hydrothermal carbonization (HTC) is a thermal conversion process that can be applied to convert faecal sludge into carbonaceous solids, called hydrochar. In this study, the technical feasibility of hydrochar production by HTC of faecal sludge was investigated. Experimental results showed energy contents of the produced hydrochar to be about 19–20 MJ/kg, comparable to natural coals and therefore usable as a solid fuel. The produced hydrochar contained a carbon content of approximately 40%wt, which could be processed further to make it suitable as an anode in batteries. The produced hydrochar also had adsorption characteristics for removing heavy metals and micropollutants in wastewater. Liquid by-products obtained from the HTC process were found to contain high concentrations of organic matter, while the amount of gas produced was 10 L-gas/kg-FS with CO2 is the main component. The bio-methane potential tests of this liquid product suggested the methane production of about 2.0 L-CH4 per kg-faecal sludge could be obtained.

Evaluation and comparison of product yields and bio-methane potential in sewage digestate following hydrothermal treatment

In recent years, sewage sludge management has been considered one of the biggest concerns in the wastewater industry for the environmental impacts linked to its high content of pollutants. Hydrothermal Treatments are a good option for converting wet biomass such as sewage sludge into high-value products. The digestate following anaerobic treatment of sewage sludge has high organic matter content despite initial conversion into biogas and is normally spread on land or composted; however, this does not fully harness its full potential. In fact, the digestate is a potential feedstock for hydrothermal processing and this route may produce higher value products. In this study, the potential of hydrothermal processing as a novel alternative to treat the digestate has been be evaluated. The effect of temperatures is evaluated with respect to product yields, biomethane potential and solubilisation of organic carbon. Three different temperatures were evaluated: 160, 220 and 250°C at 30min reaction time. The hydrochar yields obtained were 73.42% at 220°C, 68.79% at 250°C and 56.75% at 160°C treatment. The solubilisation of carbon was increased from 4.62% in the raw feedstock to 31.68%, 32.56% and 30.48% after thermal treatments at 160, 220 and 250°C, respectively. The thermal treatment enhanced the potential methane production in all products up to 58% for both, the whole fraction (hydrochar+processed water) and processed waters. The Boyle’s and Buswell’s equation were used to calculate theoretical methane yields for all hydrothermal products. Theoretical methane yields were compare with experimental data from biomethane potential (BMP) tests and it was found that the Boyle’s equation had closer agreement to BMP values.

Mesophilic co-digestion of municipal solid waste and sewage sludge: Effect of mixing ratio, total solids, and alkaline pretreatment

A large number of biogas plants feeding with municipal solid waste (MSW) are installed throughout the world. However, researches are widely conducted with the aim of maximizing the process yield, in order to make it more comparable with fossil fuels. In the present paper, biomethane potential test (BMP) assays (37 °C) were conducted to maximize the biomethane production from anaerobic digestion (AD) of municipal solid waste (MSW) under different mixing ratios with sewage sludge (SS) (50:50, 60:40, 80:20, and 90:10), the total solid (TS) concentrations (5%, 10%, 15%, and 20%), and pretreatment with sodium hydroxide. Increase in SS ratio up to 40% led to a substantial increase in biomethane yield. After 18 days of retention time, 90% of the methane (280 NmL/g VS) was achieved. The results showed that the methane yield in liquid anaerobic digestion (L-AD) (5%-10% TS) was greater than high-solids anaerobic digestion (HS-AD) (15%-20% TS). Biomethane yield was increased up to 18% with sodium hydroxide pretreatment. The findings shows that such optimization framework can generally increase the biomethane yield up to 57%.

Hydrothermal post-treatment of digestate to maximize the methane yield from the anaerobic digestion of microalgae

As an alternative to applying the hydrothermal treatment to the raw algal feedstock before the anaerobic digestion (i.e. pre-treatment), one considered a post-treatment scenario where anaerobic digestion is directly used as the primary treatment while the hydrothermal treatment is thereafter applied to the digestate. Hydrothermal treatments such as wet oxidation (WetOx) and hydrothermal carbonization (HTC) were compared at a temperature of 200°C, for initial pressure of 0.1 and 0.82MPa, and no holding time after the process had reached the temperature setpoint. Both WetOx and HTC resulted in a substantial solids conversion (47–62% with HTC, 64–83% with WetOx, both at 0.82MPa) into soluble products, while some total chemical oxygen demand–based carbon loss from the solid-liquid phases was observed (20–39%). This generated high soluble products concentrations (from 6.2 to 10.9g soluble chemical oxygen demand/L). Biomethane potential tests showed that these hydrothermal treatments allowed for a 4-fold improvement of the digestate anaerobic biodegradability. The hydrothermal treatments increased the methane yield to about 200 LSTP CH4/kg volatile solids, when related to the untreated digestate, compared to 66 LSTP CH4/kg volatile solids, without treatment.

Methane Production of Full-Scale Anaerobic Digestion Plants Calculated from Substrate’s Biomethane Potentials Compares Well with the One Measured On-Site

Biomethane potential (BMP) tests are used to determine the amount of methane that can be produced from organic materials in order to design different components of full-scale anaerobic digestion plants such as size of the digesters and units exploiting the produced biogas. However, little is known on how well BMPs compare with biogas production from the same organic materials in full-scale installations. In this study, two anaerobic digestion plants were chosen to carry out such comparisons, a dry anaerobic digestion (AD) plant treating green waste from urban areas and food waste from restaurants and supermarkets, and a liquid AD plant treating waste sludge from wastewater treatment and seven additional organic wastes. The BMPs of multiple samples of the individual organic materials collected during a period of seven to nine months were determined. Separate tests of mixtures of organic materials confirmed that the BMP of the mixtures can be calculated by adding the BMPs of the individual materials. The weekly methane production during the investigated periods was calculated from the full-scale installation data on the feeding of the digesters and the BMP’s of each substrate fed into the digesters and compared with the weekly methane production measured on-site. The latter was calculated from the most accurately measured entity, either the electricity or the volume of purified biomethane injected into the grid. The weekly methane production rates calculated from BMPs and the one measured on-site were very similar and followed the same pattern. Some exceptions could be explained by e.g. an overload of the full-scale installation. The measured weekly methane production accounted for 94.0±6.8% and 89.3±5.7% of the calculated weekly methane production for the wet and dry AD plant, respectively. For 26 out of 29 weeks, the calculated weekly methane production overestimated the measured one in the case of the wet AD plant and for 37 out of 39 weeks for the dry AD plant. Based on these results, it is proposed using an extrapolation coefficient of 0.8 to 0.9 to estimate the methane production of full-scale AD plants from BMPs of the substrates to be digested and their specific organic loads.

Characterization of selected municipal solid waste components to estimate their biodegradability

Biological treatments of Residual Municipal Solid Waste (RMSW) allow to divert biodegradable materials from landfilling and recover valuable alternative resources. The biodegradability of the waste components needs however to be assessed in order to design the bioprocesses properly. The present study investigated complementary approaches to aerobic and anaerobic biotests for a more rapid evaluation. A representative sample of residual MSW was collected from a Mechanical Biological Treatment (MBT) plant and sorted out into 13 fractions according to the French standard procedure MODECOM™. The different fractions were analyzed for organic matter content, leaching behavior, contents in biochemical constituents (determined by Van Soest's acid detergent fiber method), Biochemical Oxygen Demand (BOD) and Bio-Methane Potential (BMP). Experimental data were statistically treated by Principal Components Analysis (PCA). Cumulative oxygen consumption from BOD tests and cumulative methane production from BMP tests were found to be positively correlated in all waste fractions. No correlation was observed between the results from BOD or BMP bioassays and the contents in cellulose-like, hemicelluloses-like or labile organic compounds. No correlation was observed either with the results from leaching tests (Soluble COD). The contents in lignin-like compounds, evaluated as the non-extracted RES fraction in Van Soest's method, was found however to impact negatively the biodegradability assessed by BOD or BMP tests. Since cellulose, hemicelluloses and lignin are the polymers responsible for the structuration of lignocellulosic complexes, it was concluded that the structural organization of the organic matter in the different waste fractions was more determinant on biodegradability than the respective contents in individual biopolymers.

Comparison of approaches for organic matter determination in relation to expression of bio-methane potentials

Bio-Methane Potential (BMP) tests are used to evaluate the suitability of a biomass for anaerobic digestion. BMP data are usually presented as the amount of methane produced from a kilogram of volatile solids (VS) or chemical oxygen demand (COD) of the substrate. However, the most used methods for determination of VS and COD are not always accurate. Oven drying may underestimate VS content due to loss of volatile organic compounds, and incomplete chemical oxidation may lead to underestimation of COD content. Bomb calorimetry is an attractive alternative to COD measurements, because the physical state of the biomass sample does not influence the measurement, and because sample preparation is limited. In this study, 11 biomass samples, wet and dry, were analyzed with different methods for organic content determination. COD (determined by bomb calorimetry and by wet chemistry) and VS (by Karl Fischer titration and loss on drying; LOD) were compared, and used for determination of BMP. In general, the BMP estimated on a VS basis were higher than those estimated on COD basis. For certain biomass samples the method for VS determination also greatly influenced the results; for fishery waste the BMP was estimated as 928 L kg−1 based on LOD-VS compared to 394 L kg−1 based on KF-LOD. Thus, this study shows that determination of organic content is not trivial and the method of choice strongly influences the estimation of bio-methane potentials. Bomb calorimetry offers a possibility to measure energy content directly, independent of biomass composition and physical state.

Risks of using EDTA as an agent for trace metals dosing in anaerobic digestion of olive mill solid waste

ABSTRACTLow concentrations of trace elements in many organic wastes recommend their supplementation in order to avoid potential limitations. Different chelating agents have been used to ensure an adequate trace metal pool in the soluble fraction, by forming dissolved complexes. Ethylenediaminetetraacetic acid (EDTA) is probably the most common, although several negative effects could be associated with its usage. Biomethane potential tests were performed using Olive Mill Solid Waste as the substrate, supplementing different combinations of Fe, Co, Ni, Ba, always under the presence of EDTA. Results show that Ni and Co slightly recovered biodegradability. However, Ba supplementation resulted in worsening the methane yield coefficient in all cases. High concentration of EDTA led to decrease in the activity of anaerobic digestion. High availability of EDTA induces the capture of trace metals like Co or Ni, key trace metals for anaerobic biomass activity. While supplementing trace metals, the addition of Ba and/or EDTA must be carefully considered.