Grass for biogas production: The impact of silage fermentation characteristics on methane yield in two contrasting biomethane potential test systems

Grassland biomass is likely to be harvested and stored as silage to ensure a predictable quality and a constant supply of feedstock to an anaerobic digestion facility. Grass (Phleum pratense L. var. Erecta) was ensiled following the application of one of six contrasting additive treatments or a 6 h wilt treatment to investigate the effects of contrasting silage fermentation characteristics on CH₄ yield. In general, silage fermentation characteristics had relatively little effect on specific CH₄ yield (from 344 to 383 Nl CH₄ kg⁻¹ volatile solids). However, the high concentrations of fermentation products such as ethanol and butyric acid following clostridial and heterofermentative lactic acid bacterial fermentations resulted in a numerically higher specific CH₄ yield. While the latter fermentation products of undesirable microbial activity have the potential to enhance specific CH₄ yield, the numerically higher specific CH₄ yield may not compensate for the associated total solids and energy losses during ensiling.     

Associated effects of storage and mechanical pre-treatments of microalgae biomass on biomethane yields in anaerobic digestion

The pre-treatment of microalgae cell walls is known to be a key factor to enhance methane (CH₄) yields during anaerobic digestion. This study investigated the combined effects of two different biomass storage methods and physical pre-treatments on the anaerobic digestion for three different microalgae species. Acutodesmus obliquus, Chlorella vulgaris and Chlorella emersonii were cultivated in 80 L sleevebag photobioreactors (batch mode), and then subjected to different storage (cooling and freezing) and pre-treatment methods prior to anaerobic digestion using the biochemical methane potential (BMP) test. A. obliquus was selected to evaluate pre-treatment methods for further experimentation. Significantly higher CH₄ yields of cooled (4 °C) A. obliquus biomass were achieved through ultrasonication (+53% CH₄) and wet-milling (+51% CH₄). These methods were then applied in follow-up experiments to cooled (4 °C) biomass of C. emersonii and A. obliquus. Ultrasonication again led to significantly higher CH₄ yields for A. obliquus biomass (323 dm³ kg⁻¹ CH₄ yield calculated at standard gas conditions of 273 K, and 101.5 kPa per unit volatile solids, +41% CH₄), and C. emersonii biomass (308 dm³ kg⁻¹; +35% CH₄). In a third experiment series, frozen A. obliquus and C. vulgaris biomass were thawed prior to pre-treatment and BMP-testing. Among all BMP tests, the highest CH₄ yields were achieved with untreated, freeze-thawed C. vulgaris biomass (406 dm³ kg⁻¹); pre-treatment did not enhance CH₄ yields for C. vulgaris, but for A. obliquus (ultrasonication +20%). Pre-treatment was more effective for cooled than freeze-thawed microalgal biomass and combined effects acted strain dependently.     

One-stage H2 and CH4 and two-stage H2 + CH4 production from grass silage and from solid and liquid fractions of NaOH pre-treated grass silage

In the present study, mesophilic CH₄ production from grass silage in a one-stage process was compared with the combined thermophilic H₂ and mesophilic CH₄ production in a two-stage process. In addition, solid and liquid fractions separated from NaOH pre-treated grass silage were also used as substrates. Results showed that higher CH₄ yield was obtained from grass silage in a two-stage process (467 ml g⁻¹ volatile solids (VS)_original) compared with a one-stage process (431 ml g⁻¹ VS_original). Similarly, CH₄ yield from solid fraction increased from 252 to 413 ml g⁻¹ VS_original whereas CH₄ yield from liquid fraction decreased from 82 to 60 ml g⁻¹ VS_original in a two-stage compared to a one-stage process. NaOH pre-treatment increased combined H₂ yield by 15% (from 5.54 to 6.46 ml g⁻¹ VS_original). In contrast, NaOH pre-treatment decreased the combined CH₄ yield by 23%. Compared to the energy value of CH₄ yield obtained, the energy value of H₂ yield remained low. According to this study, highest CH₄ yield (495 ml g⁻¹ VS_original) could be obtained, if grass silage was first pre-treated with NaOH, and the separated solid fraction was digested in a two-stage (thermophilic H₂ and mesophilic CH₄) process while the liquid fraction could be treated directly in a one-stage CH₄ process.     

Co-digestion of poultry manure and residues from enzymatic saccharification and dewatering of sugar beet pulp

This study investigates the co-digestion of poultry manure (PM) with sugar beet pulp residues (SBPR) obtained from saccharification and dewatering of sugar beet pulp. The laboratory-scale experiments were conducted under batch and semi-continuous conditions at mesophilic temperatures (35 °C). Batch tests gave specific biogas and methane yields of 590 dm³/kgVS_fed and 423 dm³CH₄/kgVS_fed, respectively for SBPR, whereas the corresponding values for PM were 434 dm³/kgVS_fed and 300 dm³CH₄/kgVS_fed. The co-digestion of PM with SBPR was found to increase biogas and methane yields compared to the manure alone. In semi-continuous reactor experiments, the highest methane yield of 346 dm³ CH₄/kgVS_fed was achieved for the mixture containing poultry manure with 50% SBPR (weight basis) and a solids retention time (SRT) of 20 days. However, when poultry manure was digested as a sole feedstock, the biogas production was inhibited by ammonia, whereas the co-digestion of PM with 25% SBPR was slightly affected by volatile fatty acids, which concentrations exceeded 4000 g/m³.     

Mitigation of greenhouse gas emissions by adopting anaerobic digestion technology on dairy,sow and pig farms in Finland

The impact of anaerobic digestion (AD) technology on mitigating greenhouse gas (GHG) emissions from manure management on typical dairy, sow and pig farms in Finland was compared. Firstly, the total annual GHG emissions from the farms were calculated using IPCC guidelines for a similar slurry type manure management system. Secondly, laboratory-scale experiments were conducted to estimate methane (CH₄) potentials and process parameters for semi-continuous digestion of manures. Finally, the obtained experimental data were used to evaluate the potential renewable energy production and subsequently, the possible GHG emissions that could be avoided through adoption of AD technology on the studied farms. Results showed that enteric fermentation (CH₄) and manure management (CH₄ and N₂O) accounted for 231.3, 32.3 and 18.3 Mg of CO₂ eq. yr^?1 on dairy, sow and pig farms, respectively. With the existing farm data and experimental methane yields, an estimated renewable energy of 115.2, 36.3 and 79.5 MWh of heat yr^?1 and 62.8, 21.8 and 47.7 MWh of electricity yr^?1 could be generated in a CHP plant on these farms respectively. The total GHG emissions that could be offset on the studied dairy cow, sow and pig farms were 177, 87.7 and 125.6 Mg of CO₂ eq. yr^?1, respectively. The impact of AD technology on mitigating GHG emissions was mainly through replaced fossil fuel consumption followed by reduced emissions due to reduced fertilizer use and production, and from manure management.     

Effect of particle size on biogas yield from sisal fibre waste

The degradation and biogas production potential of sisal fibre waste could be significantly increased by pre-treatment for reduction of particle size. Batch-wise anaerobic digestion of sisal fibre waste was carried out in 1-l digesters with fibre sizes ranging from 2 to 100 mm, at an ambient temperature of 33 °C. Sediment from a stabilisation pond at a sisal production plant was used as starter seed. Total fibre degradation increased from 31% to 70% for the 2 mm fibres, compared to untreated sisal fibres. Furthermore, the results confirmed that methane yield was inversely proportional to particle size. Methane yield increased by 23% when the fibres were cut to 2 mm size and was 0.22 m³ CH₄/kg volatile solids, compared to 0.18 m³ CH₄/kg volatile solids for untreated fibres. By anaerobic digestion and biogas production, the 148,000 tonne of waste sisal fibres generated annually in Tanzania could yield 22 million m³ of methane, and an additional 5 million m³ of methane if pre-treatment by size reduction to 2 mm was applied.     

Automotive storage of hydrogen in alane

Although alane (AlH₃) has many interesting properties as a hydrogen storage material, it cannot be regenerated on-board a vehicle. One way of overcoming this limitation is to formulate an alane slurry that can be easily loaded into a fuel tank and removed for off-board regeneration. In this paper, we analyze the performance of an on-board hydrogen storage system that uses alane slurry as the hydrogen carrier. A model for the on-board storage system was developed to analyze the AlH₃ decomposition kinetics, heat transfer requirements, stability, startup energy and time, H₂ buffer requirements, storage efficiency, and hydrogen storage capacities. The results from the model indicate that reactor temperatures higher than 200 °C are needed to decompose alane at reasonable liquid hourly space velocities, i.e., > 60 h⁻¹. At the system level, a gravimetric capacity of 4.2 wt% usable hydrogen and a volumetric capacity of 50 g H₂/L may be achievable with a 70% solids slurry. Under optimum conditions, 80% of the H₂ stored in the slurry may be available for the fuel cell engine. The model indicates that H₂ loss is limited by the decomposition kinetics rather than by the rate of heat transfer from the ambient to the slurry tank.     

Predicting the increase of methane yield using alkali pretreatment for weeds prior to co-digestion

The increase was predicted of the methane (CH₄) yield between untreated and pretreated weeds prior to co-digestion with cow dung. The mixed substrate slurry of weed was prepared using a ratio of weed to cow dung to water of 10:10:80, respectively. The total volatile solids (TVS) and total solids (TS) of the mixed substrate slurry of the untreated weeds were the two parameters input in the developed equation. Four types of weeds were used in the test: French weed, para grass, water lettuce, and sedge. The results showed that the % increase in CH₄ yield depended on the ratio of TVS/TS of the mixed substrate slurry of the untreated weeds. The proposed developed equation was Y = 221.05X – 152.46, where X is the initial ratio of TVS/TS of mixed substrate slurry of untreated weed and cow dung, and Y is the % increase in CH₄ yield obtained when the weed is pretreated using 1% sodium hydroxide for 24 hr prior to co-digestion. Water hyacinth was used as the substrate to verify the equation, and the results indicated that the equation performed well. Verification confirmed that the proposed equation successfully provided an accurate (within 10%) prediction of the % increase in CH₄ yield. The equation could be useful in developing pretreatment options for weed co-digestion and environmental management.     

Enhancement of biogas production from sunnhemp using alkaline pretreatment

This study investigates enhancing the biogas production of sunnhemp by pretreatment, before the anaerobic digestion and co-digestion processes, to address the complex and recalcitrant structure of the plant. Fresh sunnhemp harvested at a cutting interval of 50 days is used in the study. Five systems (each with a 5 litre useable volume) are operated semi-continuously with five different ratios of the feedstock by feeding separate feedstocks every five days with a hydraulic retention time (HRT) of 40 days. The system operates at room temperature (30 °C). The study uses sunnhemp as 20% of the feedstock and also considers sunnhemp mixed with cow manure at different ratios, with the weighed sunnhemp being pretreated with dilute sodium hydroxide. Pretreatment of sunnhemp before digestion produces a methane (CH₄) yield 89% greater than that of the untreated sunnhemp. It requires 3.597 kg of dry sunnhemp to produce 1 m³ of CH₄ and the annual CH₄ yield per hectare is 19,015 m³. In the pretreatment of sunnhemp before co-digestion, the increased CH₄ yield depends on the amount of pretreated sunnhemp in the feedstocks. However, the %CH₄, the CH₄ production level and the system stability depend on the optimal ratio of the sunnhemp to cow manure. The initially prepared sunnhemp to cow manure ratio is recommended at 10 g:10 g in 80 mL of water. At this ratio, the %CH₄ and the CH₄ yield are 53.84% and 313 kg chemical oxygen demand (COD) removed, respectively, and the COD removal efficiency is 56.4%. Sunnhemp has high potential and it is worth pretreating before producing biogas. Using sunnhemp to produce biogas is recommended to decrease greenhouse gas emissions and mitigate global warming.     

Biomethane Production from co-fermentation of agricultural wastes

Biomethane (CH₄) was recovered from co-digestion process of waste glycerol and banana wastes. The wastes used contain waste glycerol with varying concentrations from 7.5 to 90 g L⁻¹ and banana peel in the range 2.5–10 %w·v⁻¹. The co-substrate mixture ratio was implemented in 0.5 L batch reactor operated at 37 °C and pH 7 for 120 h. The composition of biogas gas and liquid samples (COD, VFA, pH, alkalinity) were analyzed every 12 and 24 h, respectively. The optimum condition to produce CH₄ was found at 7.5 g L⁻¹ waste glycerol mixed with 7.5% banana peel. The highest CH₄ yield and CH₄ production potential were 0.281 m³ kg⁻¹ COD and 652 mL, respectively.     

Methane and hydrogen sulfide production during co-digestion of forage radish and dairy manure

Forage radish, a winter cover crop, was investigated as a co-substrate to increase biogas production from dairy manure-based anaerobic digestion. Batch digesters (300 cm³) were operated under mesophilic conditions during two experiments (BMP1; BMP2). In BMP1, the effect of co-digesting radish and manure on CH₄ and H₂S production was determined by increasing the mass fraction of fresh above-ground radish in the manure-based co-digestion mixture from 0 to 100%. Results showed that forage radish had 1.5-fold higher CH₄ potential than dairy manure on a volatile solids basis. While no synergistic effect on CH₄ production resulted from co-digestion, increasing the radish fraction in the co-digestion mixture significantly increased CH₄ production. Initial H₂S production increased as the radish fraction increased, but the sulfur-containing compounds were rapidly utilized, resulting in all treatments having similar H₂S concentrations (0.10–0.14%) and higher CH₄ content (48–70%) in the biogas over time. The 100% radish digester had the highest specific CH₄ yield (372 ± 12 L kg⁻¹ VS). The co-digestion mixture containing 40% radish had a lower specific CH₄ yield (345 ± 2 L kg⁻¹ VS) but also showed significantly less H₂S production at start-up and high quality biogas (58% CH₄). Results from BMP2 showed that the radish harvest date (October versus December) did not significantly influence radish C:N mass ratios or CH₄ production during co-digestion with dairy manure. These results suggest that dairy farmers could utilize forage radish, a readily available substrate that does not compete with food supply, to increase CH₄ production of manure digesters in the fall/winter.     

Investigation of anaerobic digestion of Chlorella sp. and Micractinium sp. grown in high-nitrogen wastewater and their co-digestion with waste activated sludge

This study investigated two wildtype green algae, Micractinium sp. and Chlorella sp., for their growth in high nitrogen wastewater (mixture of sludge centrate and primary effluent wastewater) and subsequent anaerobic digestion under mesophilic conditions. Extraction and analysis of extracellular polymeric substances (EPS) in both algal species during cultivation showed that Micractinium generated larger quantity of EPS-proteins than Chlorella. Anaerobic digestion of harvested algae showed the opposite trend that Chlorella allowed a higher CH₄ yield on the volatile solids fed the digester (VS_fed) of 230 dm³ kg⁻¹ than Micractinium (209 dm³ kg⁻¹). These results suggested that different growth patterns of two types of algae, with different quantity of EPS expressed, affected anaerobic digestibility and biogas yield. Co-digestion of algae with waste activated sludge (WAS) improved the volatile solids reduction, hydrolysis efficiency as well as the biogas yields of algae.     

Giant cane (Arundo donax L.) for biogas production: The effect of two ensilage methods on biomass characteristics and biogas potential

Arundo donax L. is a perennial plant that can substitute for traditional energy crops to produce biogas, reducing costs because of its high biogas yield per Ha cultivated and low agronomic and energetic inputs. Nevertheless, Arundo donax biomass needs to be ensiled to be preserved and used. Because no full-scale data exist about A. donax ensilage and the effect of this process on potential biogas production, in this work two different ensiling techniques, i.e. trench and silo-bag ensiling, were performed at full scale, and the processes studied for 200 days. Results obtained indicated that A. donax could be successful ensiled by using the two approaches. Ensilage proceeded by fermentation of organic acids already present in the biomass, i.e. malic and oxalic acids that were degraded, giving volatile fatty acid accumulation. This was different from corn ensiling characterized by starch fermentation to lactic acids. Biological processes determined a loss of the potential biomethane production, namely −20.1% and −7.6% for trench and silo-bag, respectively. Taking into consideration biomethane yield per Ha and ensilage losses, potential biomethane losses of 5000 Nm³ CH₄ Ha⁻¹ for trench silage and of 2000 Nm³ CH₄ Ha⁻¹ for silo bag, were estimated, respectively. Nevertheless, taking into consideration the higher biomass and biomethane yields Ha⁻¹ in comparison with the other energy crops, A. donax still remained more efficient and cheaper than traditional energy crops in producing biogas.     

From piggery wastewater nutrients to biogas: Microalgae biomass revalorization through anaerobic digestion

Microalgae grown in swine wastewater were used as a promising strategy to produce renewable energy by coupling wastewater bioremediation and biomass revalorization. The efficiency of a microalgae consortium treating swine slurry at different temperature (15 and 23 °C) and illumination periods (11 and 14 h) was assessed for biomass growth and nutrient removal at two NH₄⁺ initial concentrations (80 and 250 mg L⁻¹ NH₄⁺). Favourable culture conditions (23 °C and 14 h of illumination) and high ammonium loads resulted in higher biomass production and greater nutrients removal rates. The initial N–NH₄⁺ load determined the removal mechanism, thus ammonia stripping and nitrogen uptake accounted similarly in the case of high NH₄⁺ load, while nitrogen uptake prevailed at low NH₄⁺ load. Under favourable conditions, nitrogen availability in the media determined the composition of the biomass. In this context, carbohydrate-rich biomass was obtained in batch mode while semi-continuous operation resulted in protein-rich biomass. The revalorization of the resultant biomass was evaluated for biogas production. Methane yields in the range of 106–146 and 171 ml CH₄ g COD⁻¹ were obtained for the biomasses grown in batch and semi-continuous mode, respectively. Biomass grown under favourable conditions resulted in higher methane yields and closer to the theoretically achievable.     

Biohydrogen production from pig slurry in a CSTR reactor system with mixed cultures under hyper-thermophilic temperature (70 °C)

A continuous stirred tank reactor (CSTR) (750 cm³ working volume) was operated with pig slurry under hyper-thermophilic (70 °C) temperature for hydrogen production. The hydraulic retention time (HRT) was 24 h and the organic loading rate was 24.9 g d⁻¹ of volatile solid (VS). The inoculum used in the hyper-thermophilic reactor was sludge obtained from a mesophilic methanogenic reactor. The continuous feeding with active biomass (inoculum) from the mesophilic methanogenic reactor was necessary in order to achieve hydrogen production. The hyper-thermophilic reactor started to produce hydrogen after a short adapted period of 4 days. During the steady state period the mean hydrogen yield was 3.65 cm³ g⁻¹ of volatile solid added. The high operation temperature of the reactor enhanced the hydrolytic activity in pig slurry and increased the volatile fatty acids (VFA) production. The short HRT (24 h) and the hyper-thermophilic temperature applied in the reactor were enough to prevent methanogenesis. No pre-treatment methods or other control methods for preventing methanogenesis were necessary. Hyper-thermophilic hydrogen production was demonstrated for the first time in a CSTR system, fed with pig slurry, using mixed culture. The results indicate that this system is a promising one for biohydrogen production from pig slurry.     

Can slurry biogas systems be cost effective without subsidy in Mexico?

Biogas from pig slurry in Mexico has potential to produce 21 PJ per year, equivalent to 3.5% of natural gas consumption in 2013. In this paper, three different scenarios are analysed: mono-digestion of pig slurry in a finisher farm (scenario 1); co-digestion of pig slurry and elephant grass in a finisher farm in situ (scenario 2) and co-digestion of pig slurry and elephant grass in centralised biogas plants (scenario 3). The digesters proposed are anaerobic high density polyurethane (HDPE) covered lagoons. HDPE centralised plants can have capital costs 5 times cheaper than European biogas plants. The economics of utilisation of biogas for electricity generation and as biomethane (a natural gas substitute) were investigated. Economic evaluations for on-site slurry digestion (Scenario 1) and on-site co-digestion of elephant grass and pig slurry (Scenario 2) showed potential for profitability with tariffs less than $US 0.12/kWh_e. For centralised systems (Scenario 3) tariffs of $US 0.161/kWh_e to $US 0.195/kWh_e are required. Slurry transportation, energy use and harvest and ensiling account for 65% of the operational costs in centralised plants (Scenario 3). Biomethane production could compete with natural gas if a subsidy of 4.5 c/L diesel (1 m³ of biomethane) equivalent was available.     

Using sugarcane leaves and tops for exploiting higher methane yields: An assessment study

Sugarcane leaves and tops are lignocellulosic agricultural by-products which are considered a significant input for biogas production. Their potential pretreatment by sodium hydroxide prior to co-digestion with cow manure helps increase the methane (CH₄) content, biodegradation efficiency of the lignocellulosic materials and CH₄ yield. The untreated and pretreated sugarcane leaves and tops to cow manure and water of different ratios were digested and co-digested in the 5 L reactors by semi-continuous operation with hydraulic retention time of 40 days. The pretreated sugarcane leaves and tops to cow manure at the initial prepared ratio of 100 g:100 g, in 800 mL water which was corresponding to the organic loading rate (OLR) of 0.61 kg COD/m³·day was recommended. At this ratio, the chemical oxygen demand and total volatile solids degradation efficiency was 68.80 and 72.52%, respectively, the CH₄ content was 44.52% and the CH₄ yield was 331 L/kg COD degraded. According to the results, there is an average of 3.7% deviation between the practical model based on the thermodynamic balance equations carried out using the Aspen Plus and the experimental study. The highest exergy destruction rate is found at 21 kW where the sugarcane leaves and tops, and cow manure ratio is 100 g:100 g, in 800 mL water. The highest energy and exergy efficiencies of the overall system are calculated as 45.53% and 46.02%, respectively.     

Hydrogen and methane yields of untreated, water-extracted and acid (HCl) treated maize in one- and two-stage batch assays

In the present study, two-stage H₂ and CH₄ production was compared with one-stage CH₄ production from maize subjected to water extraction and acid (HCl) treatment. In addition, the effect of duration (2 and 14 days) of the first-stage H₂ process on the H₂ yields and subsequent CH₄ yields from the second-stage was also investigated. Results showed that the average H₂ yields from untreated maize were 5.6 and 9.9 ml/g volatile solids added (VS_added) after 2 and 14 days, respectively. On the other hand, H₂ yields from water-extracted and HCl-treated maize were 18.0 and 20.5 ml/gVS_added (14 d), respectively. On comparison to one-stage CH₄ assays, the average increase in CH₄ yields from two-stage assays with 2 d H₂ stage were 7, 9 and 27% for untreated, water-extracted and HCl-treated maize, respectively.     

Methane yield of oat husks

The biogas yield of solid manure from dairy cattle depends on its quality and the proportion of excreta and organic litter material contained within. The biogas yield of both faeces and straw is available in literature. Straw is a common litter material of mixed farms. However, straw is scarcely available on dairy farms. Oat husks are appropriate to replace or supplement straw for use as litter material. In this study, the actual methane yield and the total methane potential of oat husks were determined. Based on an optimized test with ground oat husks, the total methane potential resulted from regression and extrapolation of the experimental data. The total methane potential was determined with 242 L_N CH₄ kg⁻¹ VS added. Additionally, the actual methane yield over retention time at a digestion temperature of 37 °C was determined, using untreated oat husks. For 42 days of retention, the methane yield was 202 L_N CH₄ kg⁻¹ VS added at 52% CH₄ content. Results indicate that the methane yield of oat husks reaches the same level as that of straw. The total methane potential is not higher, but digestion of oat husks may proceed faster. Verification of the laboratory results on-farm revealed that the contribution of oat husks to overall methane production of a prototype biogas plant for solid manure might reach up to 80%.     

Hydrogen and methane potential based on the nature of food waste materials in a two-stage thermophilic fermentation process

The purpose of this study is to investigate the biological H₂ and CH₄ potential based on the nature of organic waste materials in a two-stage thermophilic fermentation process. Three varieties of actual waste specifically potato, kitchen garbage and bean curd manufacturing waste (okara) were selected. The production rates for H₂ and CH₄ were as follows: potato, 2.1 and 1.2 l/l/d; garbage, 1.7 and 1.5 l/l/d; okara, 0.4 and 1.4 l/l/d in the continuous processes. The H₂ and CH₄ yields were 20–85 ml H₂/g VS_added and 329–364 ml CH₄/g VS_added, respectively. The H₂ yield increased and the CH₄ yield decreased in the order of potato, kitchen garbage and okara. The H₂ yield was shown to be not only dependent on the proportion of carbohydrate but also on the hydrolysis pH of the organic waste, which was influenced by the nature of the organic waste materials. Higher yields of H₂ or CH₄ were obtained when the hydrolysis pH of the organic waste was close to the optimum pH range of H₂-producing bacteria or methanogenic archaea in the two-stage fermentation processes.