Improved Methane Yield Research Articles

English

One of the best options for African countries to meet rural energy needs is to grow care-free crassulacean acid metabolism plants on a massive scale in waste lands. This can enable bioenergy production without disrupting food supplies and hence sustainable energy supply for the future. Opuntia ficus indica is an ideal plant for arid regimes but has barely been studied as a potential bioenergy source. This study investigated the effect of aerobic pretreatment on methane yield of O. ficus indica biomass. This effect was investigated in batch bioreactors which were exposed to aerobic conditions by varying time from 3 to 72 h before the start of anaerobic digestion. Reducing sugar content and dissolved oxygen levels after pretreatment period was analyzed. Reducing sugar content in bioreactors increased with increase in pretreatment time from 12.22 ±0.69 to 59.08 ± 5.35 g/L in the untreated and 72 h pretreated batches, respectively. Methane yields after pretreatment were observed to range from 0.286 to 0.702 L/kg volatile solids at 9 and 72 h of pre-treatment, respectively. A 9 h pre-treatment of feedstock prior to anaerobic digestion yielded 123% higher methane yield when compared to that without pre-treatment. The findings that there was an  increase in reducing sugar production and methane yield at 9 h of aerobic pre-treatment suggests that there was increased hydrolysis with pretreatment. Hence, short pre-treatment period could be an option to increasing solubilization of cladodes and promoting methane productivity. Therefore, pre-aeration of O. ficus indica, was shown to be an effective method for enhancing both its digestibility and improved methane yield during anaerobic digestion.   Key words: Anaerobic digestion, biogas, methane, Opuntia, pretreatment, spineless cacti.

Production of lipid-containing algal-bacterial polyculture in wastewater and biomethanation of lipid extracted residues: Enhancing methane yield through hydrothermal pretreatment and relieving solvent toxicity through co-digestion

The feasibility of generating a lipid-containing algal-bacterial polyculture biomass in municipal primary wastewater and enhancing biomethanation of lipid-extracted algal residues (LEA) through hydrothermal pretreatment and co-digestion with sewage sludge (SS) was investigated. In high-rate algal ponds, the polyculture of native algal and bacteria species demonstrated a monthly average net and gross biomass productivity of 30 ± 3 and 36 ± 3 gAFDW m−2 day−1 (summer season). The algal community was dominated by Micractinium sp. followed by Scenedesmus sp., Chlorella sp., pennate diatoms and Chlamydomonas sp. The polyculture metabolic activities resulted in average reductions of wastewater volatile suspended solids (VSS), carbonaceous soluble biochemical oxygen demand (csBOD5) and total nitrogen (Ntotal) of 63 ± 18%, 98 ± 1% and 76 ± 21%, respectively. Harvested biomass contained nearly 23% lipid content and an extracted blend of fatty acid methyl esters satisfied the ASTM D6751 standard for biodiesel. Anaerobic digestion of lipid extracted algal residues (LEA) demonstrated long lag-phase in methane production of 17 days and ultimate methane yield of 296 ± 2 mL/gVS (or ~50% of theoretical), likely because to its limited biodegradability and toxicity due to presence of the residual solvent (hexane). Hydrothermal pretreatment increased the ultimate methane yield and production rate by 15–30% but did not mitigate solvent toxicity effects completely leading to less substantial improvement in energy output of 5–20% and diminished Net Energy Ratio (NER < 1). In contrast, co-digestion of LEA with sewage sludge (10% to 90% ratio) was found to minimize solvent toxicity and improve methane yield enhancing the energy output ~4-fold, compared to using LEA as a single substrate, and advancing NER to 4.2.

Hydrothermal catalytic processing of waste cooking oil for hydrogen-rich syngas production

Substantial amounts of waste cooking oil are obtained worldwide from household and catering enterprises because of deep-frying and other cooking activities. Supercritical water gasification is considered as an aqueous phase reforming process to produce hydrogen enriched syngas from biomass and other organic wastes. In this study, waste cooking oil was gasified at variable temperatures (375–675 °C), feed concentration (25–40 wt%) and reaction time (15–60 min) to investigate their effects on syngas yield and composition. Maximum yields of hydrogen (5.16 mol/kg) and total gases (10.5 mol/kg) were obtained at optimal temperature, feed concentration and reaction time of 675 °C, 25 wt% and 60 min, respectively. At 5 wt% loading, Ru/Al2O3 enhanced hydrogen yield (10.16 mol/kg) through water-gas shift reaction, whereas Ni/Si-Al2O3 improved methane yield (8.15 mol/kg) via methanation reaction. The trend of hydrogen production from catalytic supercritical water gasification of waste cooking oil at 675 °C, 25 wt% and 60 min decreased as Ru/Al2O3 > Ni/Si-Al2O3 > K2CO3 > Na2CO3. The results indicate the recycling potential of waste cooking oil for hydrogen production through hydrothermal gasification.

Evaluation of pressurised carbon dioxide pre-treatment aimed at improving the sanitisation and anaerobic digestibility of co-settled sewage sludge

This work reports on the use of pressurized CO2 pretreatment to improve methane yield and pathogen indicator organism die-off in co-settled sewage sludge (SS). Four semi-continuous mesophilic anaerobic digesters were fed on co-settled SS to establish a baseline for performance and stability. One pair of digesters was then fed with co-settled SS pretreated by P CO2 at 2800 kPa for 23 h. The trial continued for 70 days during which specific biogas and methane production, volatile solids destruction and loss of viability of Escherichia coli was monitored in test and control digesters. The pretreatment had no positive influence on any of these parameters, which was further confirmed using batch biochemical methane potential tests and direct measurement of die-off of E. coli and Salmonella enterica in samples of different sizes treated in pressure vessels of different sizes and in matrices of nutrient medium and co-settled SS. Pressurised CO2 pretreatment was effective at killing fecal indicator bacteria in nutrient medium but ineffectual in SS, strongly suggesting that the nature of suspending matrix was a principle determining factor. Paper concludes that pressurized CO2 pretreatment is not a satisfactory approach to improve either biogas production or pathogen destruction in anaerobic digestion.

Comparing the Biomass Yield and Biogas Potential of Phragmites australis with Miscanthus x giganteus and Panicum virgatum Grown in Canada

The production of bioenergy from plant biomass has the potential to reduce fossil fuel use. The number of biogas facilities around the world has risen dramatically, increasing demand for feedstocks. In this study the invasive perennial grass Phragmites australis was evaluated as a biogas feedstock in comparison with Miscanthus x giganteus and Panicum virgatum. Results from three field sites for each species demonstrated that biomass yields for P. australis averaged approximately 1.82 ± 0.9 kg dry matter (DM) m−2, comparable to that of M. x giganteus. Yield of P. australis was greater than P. virgatum, which ranged from 0.49 ± 0.06 to 0.69 ± 0.07 kg DM m−2 in July and October, respectively. In mesophilic bench-top digester experiments, methane yields were greater for July-harvested material than for October, ranging from 172.4 ± 15.3 to 229.8 ± 15.2 L CH4 kg−1 volatile solids (VS) for all perennial grasses. Methane yields per hectare were highest for October-harvested M. x giganteus, followed by July-harvested M. x giganteus and P. australis, whereas methane yield from P. virgatum at both harvest times was lower than the other two species. These results suggest that P. australis is not an economically viable biogas feedstock without pre-treatment to improve methane yield.

Using calcium peroxide (CaO2) as a mediator to accelerate tetracycline removal and improve methane production during co-digestion of corn straw and chicken manure

Antibiotics have been widely detected in livestock waste. The removal of antibiotics from livestock waste before they are exposed to the environment is crucial. Anaerobic digestion (AD) is an effective technique to deal with livestock waste. However, tetracycline was found to inhibit the AD process. In this research, effects of CaO2 addition on the tetracycline removal and methane production during the co-digestion of corn straw and chicken manure were investigated. Results showed the presence of tetracycline in AD system reduced methane yield by 8.8%. Addition of CaO2 at the dose of 0.032 g/g TVS (total volatile solid of substrate and inoculum) improved methane yield by 4.8% and shortened lag-phase by more than 2 days. Using CaO2 also accelerated the removal of tetracycline, therefore mitigated its inhibition effect. The microbial communities of the co-digestion system were also affected by tetracycline/CaO2 addition. The addition of tetracycline especially CaO2 led to increased relative abundance of microorganisms associated with complex substrate degradation. This study indicated that using CaO2 is a promising way to accelerate tetracycline removal and improve methane production during AD of antibiotic-containing organic wastes.

Removal of nitrogen from chicken manure anaerobic digestion for enhanced biomethanization

The effects of side-stream ammonia stripping as a pre- and post-treatment on the anaerobic mono-digestion of chicken manure were investigated in laboratory-scale continuous stirred tank reactors. Ammonia stripping columns were operated at pH 10 but different temperatures (35, 55, 70 °C). Results showed that digestate stripping at 70 °C and pH 10 got the highest ammonia removing rate (18.2–45.2%), and under these conditions, the methane yield at OLR of 9 g VS L−1 d−1 reached 0.199 L g−1 VSadded which was significantly higher than that of other digesters (0.01–0.03 L g−1 VSadded), although no improvement of methane yield was observed at OLR 3 and 6 g VS L−1 d−1. The microbial community analysis revealed that hydrolytic and fermentative bacteria dominated all bacterial communities. Methanogenic pathway of feedstock stripping digester was predominant by hydrogenotrophic Methanimicrococcus, while hydrogenotrophic/aceticlastic Methanoscarcina was dominant in the archaeal structures of the rest digesters. However, a shift from Methanosarcina to Methanoculleus was observed in R3 (digestate stripping at 70 °C/pH 10) when dominant species were inhibited and decreased in other digesters, indicating that the high methanogenic activity in R3 was maintained by shifting to methanogens with high tolerance to inhibitors.

Improved methane production of corn straw by the stimulation of calcium peroxide

This research focused on the effect of calcium peroxide (CaO2) addition on anaerobic digestion (AD) of corn straw. It was found that the CaO2 dose of 0.032–0.256 g·g−1 TVS (total VS of substrate and inoculum) had positive effect on the AD of corn straw by improving methane yield and content in biogas. Maximum methane yield was achieved at the CaO2 dose of 0.032 g·g−1 TVS, which was 241.6 mL·g−1 VSS (VS of substrate) and 11% higher than that of the control. Methane content in biogas also increased according to the in situ immobilization of CO2 by Ca(OH)2 that produced via the reaction of CaO2 with water (XRD analysis results). Microbial community structure was significantly affected by CaO2 addition. The variation of bacterial community structure led to higher hydrolysis rate, and the variation of archaea community structure ensured the proceeding of methanogenesis process. The use of CaO2 could be a cost-effective method to improve methane production of corn straw because it avoids harsh condition and high cost (e.g., chemical and energy inputs) associated with conventional chemical treatment techniques. The digestate from the AD with the addition of CaO2 can contain high CaCO3 content, making it a potential soil amendment for acidic soil.

Improved methane yield from wastewater grown algal biomass.

Methane production from the algal biomass cultivated in a laboratory scale continuous photobioreactor (PBR) using sewage was evaluated in the present work. During the preliminary experiments, algal biomass reached up to 1.69 ± 0.35 g L-1 in 12 days' growth period. Besides, 65 to 100% removal in concentrations of total dissolved phosphorus (TDP), nitrate nitrogen (NO3-N), total ammoniacal nitrogen (TAN) and soluble chemical oxygen demand (sCOD) was also recorded. The sCOD removal in the reactor was 100%, whereas removal of TDP, NO3-N and TAN were up to 75, 40 and 92%, respectively. Upon anaerobic digestion, the fresh algal biomass showed methane yield of 180 mL g-1 VSfed. Further, algal biomass was stored under natural conditions in open containers (aerobic conditions) in darkness at room temperature (27-30 °C) for 72 h. Interestingly, >48% COD solubilization from algal biomass was observed during storage. Pretreatment through natural storage was further confirmed with qualitative observations including scanning electron and fluorescence microscopic analysis. Moreover, higher methane yield (284.38 mL g-1 VSfed) was observed from the samples stored for 60 h. Thus, natural storage for a designated period may be recommended as a prerequisite stage in the process of methane production from wastewater-grown algal biomass.

Improving methane yield and quality via co-digestion of cow dung mixed with food waste

Methane (CH4) production and quality were enhanced by the co-digestion of cow dung and food waste (FW) mixed with organic fraction of municipal solid waste (OFMSW) under optimized conditions in bench and semi continuous-scale mode for a period of 30 days. A bacterium capable of high yield of CH4 was enriched and isolated by employing activated sewage sludge as the inoculums. The thirteen bacterial isolates were identified through morphological and biochemical tests. Gas chromatography was used to analyze the chemical compositions of the generated biogas. CH4 yields were significantly higher during co-digestion of Run II (7.59 L) than Run I (3.7 L). Therefore, the co-digestion of FW with OFMSW and Run II was observed to be a competent method for biogas conversion from organic waste resources.

Improving methane production from anaerobic digestion of Pennisetum Hybrid by alkaline pretreatment

Alkaline pretreatment with NaOH was used to improve methane yield from Pennisetum Hybrid. The pretreatments were carried out with different NaOH solutions (2–8% w/w) at three temperatures (35, 55 and 121 °C) for different periods of time (24, 24 and 1 h). All treated and untreated Pennisetum Hybrid were digested under mesophilic conditions (37 °C) to biogas, significant effects of the pretreatments on the yield of methane were observed. Results showed the modified Gompertz equation was reliable (determination coefficients (R2) greater than 0.96) to describe the kinetic behavior of anaerobic digestion of Pennisetum Hybrid. The best result, obtained by the treatment at 35 °C 2% NaOH for 24 h, resulted in the methane yield of 301.7 mL/g VS, corresponding to 21.0% improvement in the methane yield. Compositional, SEM, XRD and FTIR analysis confirmed that lignin removal, structural modification and cellulose crystalline variation were responsible for the improvement.

Evaluating the effects of activated carbon on methane generation and the fate of antibiotic resistant genes and class I integrons during anaerobic digestion of solid organic wastes

The effects of activated carbon (AC) on methane production and the fate of antibiotic resistance genes (ARGs) were evaluated through comparing the anaerobic digestion performance and transformation of ARGs among anaerobic mono-digestion of food waste, co-digestion of food waste and chicken manure, and co-digestion of food waste and waste activated sludge. Results showed that adding AC in anaerobic digesters improved methane yield by at least double through the enrichment of bacteria and archaea. Conventional digestion process showed ability in removing certain types of ARGs, such as tetA, tetX, sul1, sul2, cmlA, floR, and intl1. Supplementing AC in anaerobic digester enhanced the removal of most of the ARGs in mono-digestion of food waste. The effects tended to be minimal in co-digestion of co-substrates such as chicken manure and waste activated sludge, both of which contain a certain amount of antibiotics.

Combined low thermal alkali addition and mechanical pre-treatment to improve biogas yield from wheat straw

One of the main obstacles for a more widespread use of wheat straw for renewable energy production is the absence of a low-cost technology which promotes its biodegradation. The aim of this study was to determine whether methane yield from wheat straw could be improved by using alkali at low concentration and ambient temperature if the fiber fraction had previously undergone mechanical pretreatment. After combining mechanical and alkali pretreatment, a linear correlation between alkali concentration and hydrolysis of the fiber content, mainly hemicelluloses, was found. A positive effect by combining briquetting of wheat straw with sodium hydroxide addition was found with the hemicellulose content decreasing by up to 66%. In general, alkali improved the ultimate methane yield and the methane yield obtained after 30 days. Briquetting reduced the optimal alkali concentration needed to reach the maximal improvement in methane yield compared to merely shredding. The results proved that it is not necessary to work with the high alkali concentrations traditionally used to improve methane production if the straw is mechanically pretreated prior to alkali addition. Methane yield obtained after 30 days as well as net energy yield rose by around 40% when briquetting was combined with 2% sodium hydroxide. Therefore, a low concentration of alkali, especially sodium hydroxide, applied at ambient temperature can improve the energy and the economic balance of pretreatment.

Improving methane yield from organic fraction of municipal solid waste (OFMSW) with magnetic rice-straw biochar

Magnetic biochar is a potential economical anaerobic digestion (AD) additive. To better understand the possible role of magnetic biochar for the improvement of biomethanization performance and the retention of methanogens, magnetic biochar fabricated under different precursor concentrations were introduced into organic fraction of municipal solid waste (OFMSW) slurry AD system. Results showed that methane production in AD treatment with magnetic biochar fabricated under 3.2g FeCl3:100g rice-straw ratio increased by 11.69% compared with control treatment without biochar addition, due to selective enrichment of microorganisms participating in anaerobic digestion on magnetic biochar. AD treatment with magnetic biochar fabricated under 32g FeCl3:100g rice-straw ratio resulted in 38.34% decreasement of methane production because of the competition of iron oxide for electron. Furthermore, 25% of total methanogens were absorbed on magnetic biochar and can be harvested with magnet, which can offer a potential solution for preventing the methanogens loss in the anaerobic digesters.

Effect of Minimizing d-Limonene Compound on Anaerobic Co-digestion Feeding Mixtures to Improve Methane Yield

The purpose of this work was to assess the enhancement of the anaerobic digestion process of sewage sludge by the addition of orange peel as co-substrate. In this experimental study, an evaluation of the co-digestion process in a semi-continuous stirred tank reactor was carried out under mesophilic conditions (37 ± 0.2 °C) during an operation time up to 315 days. All trials were performed at a hydraulic retention time of 15 days and the AD reactor was fed daily with a mixture of sewage sludge and orange peel pulp, previously prepared. The organic loading rate was kept at 1.80 ± 0.31 g VS L−1 day−1 in all trials. A reference scenario (T0), mono-digestion, was performed using sewage sludge as substrate (primary sludge and activated sludge, 40:60, v/v). Thereafter, two different anaerobic co-digestion trials were carried out, one with pre-treated orange peel (T1) and the other with reduced d-limonene content (T2). Trials with pre-treated orange peel (T1 and T2) led to two- and threefold improvements in biogas and methane yields, compared with the reference scenario (T0). Furthermore, the reduction of d-Limonene content (T2) increased the methane yield by around 70% compared with T1. Therefore, it was proved that the sieving pre-treatment is effective to avoid the inhibitory effect of d-limonene.

Effect of microwave pretreatment on semi-continuous anaerobic digestion of sewage sludge

Sewage sludge is a waste produced worldwide in wastewater treatment plants (WWTP) that constitutes a major environmental and health problem. Although anaerobic digestion has become a widely used method for the management and treatment of sewage sludge, the biomethanization of this waste leads to low biodegradability and low methane production. Microwave pretreatment under fixed conditions of 20 × 106 J/kg TS (Total Solids) and 700 J/s has been described as an interesting alternative in order to improve the feasibility of the anaerobic digestion of sewage sludge. In this study at laboratory scale, the semi-continuous anaerobic digestion of raw sewage sludge and pretreated sewage sludge were compared in terms of organic loading rate (OLR), biodegradability and methane production. The main results were found to be OLR up to 2.80 × 10−5 kg TVS (Total Volatile Solids)/m3·s, 70% biodegradability and a 20% improvement in methane yield compared to sewage sludge without pretreatment. Higher loads led to free ammonia accumulation, increase of pH values and system destabilization.

Evaluation of chemical, thermobaric and thermochemical pre-treatment on anaerobic digestion of high-fat cattle slaughterhouse waste

This work aimed to enhance the anaerobic digestion of fat-rich dissolved air flotation (DAF) sludge through chemical, thermobaric, and thermochemical pre-treatment methods. Soluble chemical oxygen demand was enhanced from 16.3% in the control to 20.84% (thermobaric), 40.82% (chemical), and 50.7% (thermochemical). Pre-treatment altered volatile fatty acid concentration by −64% (thermobaric), 127% (chemical) and 228% (thermochemical). Early inhibition was reduced by 20% in the thermochemical group, and 100% in the thermobaric group. Specific methane production was enhanced by 3.28% (chemical), 8.32% (thermobaric), and 8.49% (thermochemical) as a result of pre-treatment. Under batch digestion, thermobaric pre-treatment demonstrated the greatest improvement in methane yield with respect to degree of pre-treatment applied. Thermobaric pre-treatment was also the most viable for implementation at slaughterhouses, with potential for heat-exchange to reduce pre-treatment cost. Further investigation into long-term impact of pre-treatments in semi-continuous digestion experiments will provide additional evaluation of appropriate pre-treatment options for high-fat slaughterhouse wastewater.

Integrated Process for Ethanol, Biogas, and Edible Filamentous Fungi-Based Animal Feed Production from Dilute Phosphoric Acid-Pretreated Wheat Straw

Integration of wheat straw for a biorefinery-based energy generation process by producing ethanol and biogas together with the production of high-protein fungal biomass (suitable for feed application) was the main focus of the present study. An edible ascomycete fungal strain Neurospora intermedia was used for the ethanol fermentation and subsequent biomass production from dilute phosphoric acid (0.7 to 1.2% w/v) pretreated wheat straw. At optimum pretreatment conditions, an ethanol yield of 84 to 90% of the theoretical maximum, based on glucan content of substrate straw, was observed from fungal fermentation post the enzymatic hydrolysis process. The biogas production from the pretreated straw slurry showed an improved methane yield potential up to 162% increase, as compared to that of the untreated straw. Additional biogas production, using the syrup, a waste stream obtained post the ethanol fermentation, resulted in a combined total energy output of 15.8 MJ/kg wheat straw. Moreover, using thin stillage (a waste stream from the first-generation wheat-based ethanol process) as a co-substrate to the biogas process resulted in an additional increase by about 14 to 27% in the total energy output as compared to using only wheat straw-based substrates.Graphical

Improvement of wheat straw anaerobic digestion through alkali pre-treatment: Carbohydrates bioavailability evaluation and economic feasibility

Lignocellulosic biomasses such as wheat straw are widely used as a feedstock for biogas production. However, these biomasses are mainly composed of a compact fibre structure and therefore, it is recommended to treat them prior to its usage for biogas production in order to improve their bioavailability. The aim of this work is to evaluate, in terms of performance stability, methane yield and economic feasibility, two different scenarios: a mesophilic codigestion of wheat straw and animal manure with or without a low-energy demand alkaline pre-treatment (0.08gKOHgTS−1of wheat straw, for 24h and at 25°C). Besides this, said pre-treatment was also analysed based on the improvement of the bioavailable carbohydrate content in the untreated versus the pre-treated wheat straw. The results pointed out that pre-treated wheat straw prompted a more stable performance (in terms of pH and alkalinity) and an improved methane yield (128% increment) of the mesophilic codigestion process, in comparison to the “untreated” scenario. The pre-treatment increased the content of cellulose, hemicellulose and other compounds (waxes, pectin, oil, etc.) in the liquid fraction, from 5% to 60%, from 11.5% to 39.1% TS and from 57% to 79% of the TS in the liquid fraction for the untreated and pre-treated wheat straws, respectively. Finally, the pre-treated scenario gained an energy surplus of a factor 13.5 and achieved a positive net benefit of 90.4€tVS-WS−1d−1, being a favourable case for an eventual scale-up of the combined process.

Thermal pretreatment and bioaugmentation improve methane yield of microalgal mix produced in thermophilic anaerobic digestate

Liquid fraction produced in anaerobic digestion (AD) of biodegradable waste can be treated on-site with microalgae, which can be recycled back as substrate to the biogas plant. For this research, a pilot high rate algal pond (HRAP) was set with connections to a full scale biogas plant that enabled the use of waste heat and CO2 from a combined heat and power gen-set (CHP). The microalgal mix produced in the thermophilic anaerobic digestate supernatant was tested as a substrate for biogas production in the thermophilic AD (i.e. untreated, bioaugmented with anaerobic bacteria Clostridium thermocellum, and thermally pretreated, respectively). The methane potential of the untreated microalgal mix was low (157.5 ± 18.7 mL CH4/g VS). However, after the thermal pretreatment of the microalgae, methane production increased by 62%, while in the bioaugmentation with C. thermocellum under thermophilic conditions (T = 55 °C) it was elevated by 12%. The outcome of our pilot trial suggests that microalgae produced in the thermophilic biogas digestate represent a prospective alternative AD feedstock. At the same time, microalgae reduce the digestate nitrogen and COD to the level sufficient for the outflow to meet the quality required by the sewage system (ammonia-nitrogen max 200 mg/L, nitrite max 10 mg/L).