Nanoparticles augmentation on biogas yield from microalgal biomass anaerobic digestion

This study evaluates the influence of metal and metal-oxide nanoparticles (NPs) on biogas production from green microalgae Enteromorpha. The concentration of metallic NPs (Ni, Co) was 1 mg/L and oxides NPs (Fe₃O₄, MgO) was 10 mg/L. An anaerobic digestion was carried out batch-wise with working volume, operating temperature, mixing rate and hydraulic retention time as 500 ml, 37 °C, 150 rpm and 170 h, respectively. The measurements of chemical oxygen demand (COD), volatile fatty acids (VFAs), reducing sugar and biogas production were observed to monitor effectivity of nanoparticles. The results showed that NPs has moderate positive influence in biogas production until 60 h of retention time but significantly improve afterward. The maximum total biogas yield of 624 ml was achieved by Fe₃O₄ NPs whereas highest biohydrogen, 51.42% (v/v) was achieved by Ni NPs. The cumulative increase in biogas production for Fe₃O₄, Ni, Co and MgO NPs was 28%, 26%, 9% and 8%, respectively. A modified Gompertz and Logistic function model were used to determine kinetic constants of the reaction. The logistic model has the better predicting ability for microalgae anaerobic digestion.     

Modeling & optimization of renewable hydrogen production from biomass via anaerobic digestion & dry reformation

There is a growing interest in the usage of hydrogen as an environmentally cleaner form of energy for end users. However, hydrogen does not occur naturally and needs to be produced through energy intensive processes, such as steam reformation. In order to be truly renewable, hydrogen must be produced through processes that do not lead to direct or indirect carbon dioxide emissions. Dry reformation of methane is a route that consumes carbon dioxide to produce hydrogen. This work describes the production of hydrogen from biomass via anaerobic digestion of waste biomass and dry reformation of biogas. This process consumes carbon dioxide instead of releasing it and uses only renewable feed materials for hydrogen production. An end-to-end simulation of this process is developed primarily using Aspen HYSYS® and consists of steady state models for anaerobic digestion of biomass, dry reformation of biogas in a fixed-bed catalytic reactor containing Ni–Co/Al₂O₃ catalyst, and a custom-model for hydrogen separation using a hollow fibre membrane separator. A mixture-process variable design is used to simultaneously optimize feed composition and process conditions for the process. It is identified that if biogas containing 52 mol% methane, 38 mol% carbon dioxide, and 10 mol% water (or steam) is used for hydrogen production by dry reformation at a temperature of 837.5 °C and a pressure of 101.3 kPa; optimal values of 89.9% methane conversion, 99.99% carbon dioxide conversion and hydrogen selectivity 1.21 can be obtained.     

Two-stage anaerobic dry digestion of blue mussel and reed

Blue mussels and reeds were explored as a new biomass type in the Kalmar County of Sweden to improve renewable transport fuel production in the form of biogas. Anaerobic digestion of blue mussels and reeds was performed at a laboratory-scale to evaluate biogas production in a two-stage dry digestion system. The two-stage system consisted of a leach bed reactor and an upflow anaerobic sludge blanket (UASB) reactor. The two-stage system was efficient for the digestion of blue mussels, including shells, and a methane yield of 0.33 m³/kg volatile solids (VS) was obtained. The meat fraction of blue mussels was easily solubilised in the leach bed reactor and the soluble organic materials were rapidly converted in the UASB reactor from which 68% of the methane was produced. However, the digestion of mussels including shells gave low production capacity, which may result in a less economically viable biogas process. A low methane potential, 0.22 m³/kg VS, was obtained in the anaerobic two-stage digestion of reeds after 107 days; however, it was comparable to similar types of biomass, such as straw. About 80% of the methane was produced in the leach bed reactor. Hence, only a leach bed reactor (dry digestion) may be needed to digest reed. The two-stage anaerobic digestion of blue mussels and reeds resulted in an energy potential of 16.6 and 10.7 GWh/year, respectively, from the estimated harvest amounts. Two-stage anaerobic digestion of new organic materials such as blue mussels and reeds can be a promising biomass resource as land-based biomass start to be limited and conflict with food resources can be avoided.     

Biogas generation potential by anaerobic digestion for sustainable energy development in India

The potential of biogas generation from anaerobic digestion of different waste biomass in India has been studied. Renewable energy from biomass is one of the most efficient and effective options among the various other alternative sources of energy currently available. The anaerobic digestion of biomass requires less capital investment and per unit production cost as compared to other renewable energy sources such as hydro, solar and wind. Further, renewable energy from biomass is available as a domestic resource in the rural areas, which is not subject to world price fluctuations or the supply uncertainties as of imported and conventional fuels. In India, energy demand from various sectors is increased substantially and the energy supply is not in pace with the demand which resulted in a deficit of 11,436 MW which is equivalent to 12.6% of peak demand in 2006. The total installed capacity of bioenergy generation till 2007 from solid biomass and waste to energy is about 1227 MW against a potential of 25,700 MW. The bioenergy potential from municipal solid waste, crop residue and agricultural waste, wastewater sludge, animal manure, industrial waste which includes distilleries, dairy plants, pulp and paper, poultry, slaughter houses, sugar industries is estimated. The total potential of biogas from all the above sources excluding wastewater has been estimated to be 40,734 Mm³/year.     

A comprehensive review on pretreatment of microalgae for biogas production

Use of microalgal biomass for renewable energy production has gained considerable attention in the world due to increasing global energy demand and negative environmental impacts of nonrenewable fossil fuels. Anaerobic digestion is one of the renewable technologies that microalgal biomass is converted into biogas by anaerobic archea. One of the main drawbacks of using microalgal biomass for biogas production is that certain types of microalgae has rigid cell wall characteristics, which limits accessibility of anaerobic archea to microalgal intracellular organic matter during hydrolysis phase. This limitation lowers efficiency of biogas production from microalgal biomass. However, introducing pretreatment methods prior to anaerobic digestion provides disruption of rigid microalgal cell wall and improve biogas yields from microalgal biomass. The objective of this paper was to review current knowledge related to pretreatment methods applied prior to anaerobic digestion of microalgal biomass. Efficiency and applicability of pretreatment methods mainly depend on type of microalgae, cell wall characteristics, and cost and energy requirements during pretreatment process. In this review, various type of pretreatment methods applied to microalgal biomass was discussed in detail with background knowledge and literature studies in their potential on maximization of biogas yields and their cost effectiveness, which is important for large‐scale applications. In the view of current knowledge, it was concluded that each pretreatment method has a relative contribution to improvement in biogas production depending on the type of microalgae. However, energy and cost requirements are the main limitations for pretreatment. So, further studies should focus on reduction of cost and energy demand by introducing combined methods, novel chemicals, and on‐site or immobilized enzymes in pretreatment to increase feasibility of pretreatment prior to anaerobic digestion in industrial scale.     

Evaluation of biogas production from different biomass wastes with/without hydrothermal pretreatment

Municipal biomass waste is regarded as new available energy source, although it could cause serious environmental pollution. Generally, biogas recovery by anaerobic digestion was seen as an ideal way to treat biomass waste. Different types of biomass waste have different biogas production potential. In this paper, cow manure, pig manure, municipal sewage sludge, fruit/vegetable waste, and food waste were chosen as typical municipal biomass waste. In addition, hydrothermal pretreatment was used to accelerate digestion and increase biogas production. Biochemical methane potential (BMP) test was used to evaluate biogas production for raw biomass and hydrothermal treated waste. Raw materials of fruit/vegetable and food waste show higher methane production than that of cow manure, pig manure, and municipal sewage sludge. After hydrothermal pretreatment at typical condition (170 °C at 1 h), the biogas production of pig manure, cow manure, fruit/vegetable waste, and municipal sewage sludge increased by 7.8, 13.3, 18.5, and 67.8% respectively. While, for treated food waste, the biogas decrease by 3.4%. The methane yield of pig manure, fruit/vegetable waste, and municipal sewage sludge increased by 14.6, 16.1, and 65.8%, respectively. While, for treated cow manure and food waste, the methane decrease by 6.9% and 7.5%.     

Pretreatment of energy crops with sodium hydroxide and cellulolytic enzymes to increase biogas production

This work presents the influence of alkali pretreatment on the enzymatic hydrolysis and efficiency of anaerobic digestion of lignocellulosic biomass pretreated both in a one- (chemical or enzymatic) and two-step (chemical and enzymatic) process. In this study two species of energy crops were used Miscanthus giganteus and Sida hermaphrodita. The aim of this work was to compare biogas production and methane yield during fermentation of pretreated and untreated energy crops. The results show that alkali pretreatment is necessary for the effective biogas generation from plant material due to high delignification level and significant hemicellulose degradation. The two-step hydrolysis process consisting on the alkali and enzymatic step leads to the release of high concentrations of glucose (about 20 g L⁻¹). The best results were achieved for M. giganteus with biogas production yield of 421.5 Ndm³ kg TS⁻¹ and with methane production yield of 257 Ndm³ kg TS⁻¹.     

Optimization of mechanical pre-treatment of Laminariaceae spp. biomass-derived biogas

Macroalgae have not met their full potential to date as biomass for the production of energy. One reason is the high cost associated with the pretreatment which breaks the biomass's crystalline structure and better exposes the fermentable sugars to anaerobes. In the attempt to overcome this technological barrier, the performance of a Hollander beater mechanical pretreatment is assessed in this paper. This pretreatment has been applied to a batch of Laminariaceae biomass and inoculated with sludge from a wastewater treatment plant. The derived biogas and methane yields were used as the responses of a complex system in order to identify the optimal system input variables by using the response surface methodology (RSM). The system's inputs considered are the mechanical pretreatment time (5–15 min range), the machine's chopping gap (76–836 μm) and the mesophilic to thermophilic range of temperatures (30–50 °C). The mechanical pretreatment was carried out with the purpose of enhancing the biodegradability of the macroalgal feedstock by increasing the specific surface area available during the anaerobic co-digestion. The pretreatment effects on the two considered responses are estimated, discussed and optimized using the tools provided by the statistical software Design-Expert v.8. The best biogas yield of treated macroalgae was found at 50 °C after 10 min of treatment, providing 52% extra biogas and 53% extra methane yield when compared to untreated samples at the same temperature conditions. The highest biogas rate achieved by treating the biomass was 685 cc gTS⁻¹, which is 430 cc gTS⁻¹ in terms of CH₄ yield.     

Pretreatment of lignocellulosic biomass for enhanced biogas production

Lignocellulosic biomass is an abundant organic material that can be used for sustainable production of bioenergy and biofuels such as biogas (about 50–75% CH₄ and 25–50% CO₂). Out of all bioconversion technologies for biofuel and bioenergy production, anaerobic digestion (AD) is a most cost-effective bioconversion technology that has been implemented worldwide for commercial production of electricity, heat, and compressed natural gas (CNG) from organic materials. However, the utilization of lignocellulosic biomass for biogas production via anaerobic digestion has not been widely adopted because the complicated structure of the plant cell wall makes it resistant to microbial attack. Pretreatment of recalcitrant lignocellulosic biomass is essential to achieve high biogas yield in the AD process. A number of different pretreatment techniques involving physical, chemical, and biological approaches have been investigated over the past few decades, but there is no report that systematically compares the performance of these pretreatment methods for application on lignocellulosic biomass for biogas production. This paper reviews the methods that have been studied for pretreatment of lignocellulosic biomass for conversion to biogas. It describes the AD process, structural and compositional properties of lignocellulosic biomass, and various pretreatment techniques, including the pretreatment process, parameters, performance, and advantages vs. drawbacks. This paper concludes with the current status and future research perspectives of pretreatment.     

Structural optimization of biohydrogen production: Impact of pretreatments on volatile fatty acids and biogas parameters

The present study aims to describe an innovative approach that enables the system to achieve high yielding for biohydrogen (bio-H₂) production using xylose as a by-product of lignocellulosic biomass processing. A hybrid optimization technique, structural modelling, desirability analysis, and genetic algorithm could determine the optimum input factors to maximize useful biogas parameters, especially bio-H₂ and CH₄. As found, the input factors (pretreatment, digestion time and biogas relative pressure) and volatile fatty acids (acetic acid, propionic acid and butyric acid) had significantly impacted the biogas parameters and desirability score. The pretreatment factor had the most directly effect on bio-H₂ and CH₄ production among the factors, and the digestion time had the most indirectly effect. The optimization method showed that the best pretreatment was acidic pretreatment, digestion time > 20 h, biogas relative pressure in a range of 300–800 mbar, acetic acid in a range of 90–200 mg/L, propionic acid in a range of 20–150 mg/L, and butyric acid in a range of 250–420 mg/L. These values caused to produce H₂ > 10.2 mmol/L, CH₄ > 3.9 mmol/L, N₂ < 15.3 mmol/L, CO₂ < 19.5 mmol/L, total biogas > 0.31 L, produced biogas > 0.10 L, and accumulated biogas > 0.41 L.     

Energy recovery from grass of urban roadside verges by anaerobic digestion and combustion after pre-processing

Grass from urban roadside verges is a potential, though widely unused, resource for bioenergy recovery. Two possible bioenergy recovery techniques were tested, i.e. i) direct anaerobic digestion of the whole parent material and ii) the “integrated generation of solid fuel and biogas from biomass” (IFBB) procedure, which divides biomass into a press fluid and a press cake by mashing and mechanical dewatering. Biomass yield, chemical composition and canopy height of biomass, contribution of functional groups, fermentation characteristics of silage and press fluids, as well as characteristics of the produced solid fuel was investigated, applying a 4-cut management for anaerobic digestion, a 2-cut management for IFBB and an 8 times mulching as a reference. Mean annual biomass yield (2013 and 2014) was 3.24, 3.33 and 5.68 t dry matter ha⁻¹ for the mulching, 4-cut management and 2-cut management, respectively. Yields were higher in 2014 due to more favourable weather conditions. Fibre concentration was higher in material of the 2-cut management than in the 4-cut management, however, methane yield of the corresponding silages was the same. Highest methane yield was gained from press fluids with 292 l_N kg⁻¹ volatile solids. The press cake had a lower heating value of 16 MJ kg⁻¹ dry matter and a K₂O/CaO index of 0.51–0.88. Gross energy output was 26.4 GJ ha⁻¹ for anaerobic digestion and 84.4 GJ ha⁻¹ for IFBB. Thus, an altered roadside verge management with reduced cutting frequency might allow a significant energy recovery and improved ecosystem services, i.e. increased biodiversity.     

Community dynamics and significance of anaerobic protozoa during biomethanation of lignocellulosic waste

The diversity and community dynamics of anaerobic protozoa and their functional role during anaerobic digestion of a typical lignocellulose biomass in a lab scale leach bed coupled UASB reactor is reported in this study. The functional role played by different protozoa during various stages of methanogenesis was analyzed through linear regression analysis of individual protozoon counts with major hydrolytic enzyme activities, volatile fatty acid levels and biogas production. The protozoa community in the digester was represented by ciliates (Metopus, Cyclidium and Colpoda) and flagellates (Rhyncomonas, Menoidium and Bodo). Regression analysis revealed the relationship between total protozoa counts with the activity of cellulase (R² = 0.71) pectinase (R² = 0.50) amylase (R² = 0.53) and xylanase (R² = 0.34), total volatile fatty acid levels (R² = 0.86) and biogas production (R² = 0.78) in the digester. Moreover, it was found that both volatile fatty acid and biogas production is correlated with ciliate and flagellate populations. This study underlines the importance of both ciliates and flagellates in the anaerobic digestion process and, more specifically, the contribution by individual protozoa on hydrolysis, which is the rate limiting stage in anaerobic digestion.     

Electrical energy production from the integrated aerobic-anaerobic treatment of organic waste by ORC

The energetic performance of an ORC system fueled by the heat generated from the integrated aerobic/anaerobic treatment of organic waste was analyzed. The temperature and heat content of the exhaust air arising from the aerobic treatment were increased by the combustion of the biogas produced by the anaerobic digestion of a fraction of the same waste. On the basis of the amount of excess air exploited in the process, for each tonne of organic waste treated, it was possible to produce from 30 to 90 kg of exhaust air per day with a mean temperature ranging from 330 to 340 K. By processing from 0.5% to 16% of the whole organic waste in an anaerobic digestion section instead of the aerobic one, it was possible to increase the exhaust air temperature from 340 to 510 K, leading to an increase in the ORC size from about 0.05 to about 1 W/tonne/year. The best energetic utilization of the biogas was achieved for ORC compression ratios from 1.5 to 2 and for maximum air temperatures from 335 to 340 K. In these conditions, by using a micro-ORC system (i.e. <15 kW), it was possible to convert about 20% of the energy content of the biogas into electrical energy.     

Optimization of biogas production by anaerobic digestion for sustainable energy development in Zimbabwe

There is increasing international interest in developing low carbon renewable energy technologies. Biomass is increasingly being utilized as an energy source throughout the world. Several modern technologies have been developed that convert biomass to bioenergy. Anaerobic digestion is a mature energy technology for converting biomass to biogas, which is a renewable primary energy source. Biogas is a robust fuel that can be used to supply heat, electricity, process steam and methanol. There are vast biomass resources in Zimbabwe that have good potential for biogas production by anaerobic digestion. However, anaerobic digestion is not being optimally used as a biomass conversion technology in the country. This paper presents an overview of biogas production in Zimbabwe and outlines technical options that can be utilized to optimize biogas production by anaerobic digestion in the country.     

Characterization of Spartina alterniflora as feedstock for anaerobic digestion

Smooth cordgrass (Spartina alterniflora), a saltmarsh plant with high production, was characterized for its potential for use as feedstock for anaerobic digestion processes. The anaerobic digestibility and biogas yield of S. alterniflora were evaluated by anaerobic batch digestion experiments performed at 35 ± 1 °C at initial volatile solids (VS) of 6%. The nutrient content analysis indicated that S. alterniflora contained the required nutrition for anaerobic microorganisms, but its high C/N of 58.8, high K and Na contents of 8.1, 22.7 g kg^?1, respectively, may be disadvantageous to its anaerobic digestion. The cumulative biogas yield was determined to be 358 L kg^?1 VS and the biodegradation efficiency was 45% after 60 days of digestion. The methane content of biogas increased from 53% on day 3 to around 62% after 13 days of digestion. The changes of volatile fatty acids (VFAs) indicated that the acidification of S. alterniflora was propionate-type fermentation with proportion of acetate and propionate ranging from 54.8% to 98.4%, and the hydrolysis of lignocellulose was the rate-limiting step for its anaerobic digestion. The analysis of cations suggested that K⁺ and Mg²⁺, with the maximum concentration of 1.35 and 0.43 g L^?1 in fermentation liquor, respectively, could be inhibitory to the anaerobic digestion of S. alterniflora. It is concluded that S. alterniflora can be transformed into clean energy by anaerobic digestion and the high contents of K, Na, Ca and Mg may be the inhibitory factors when S. alterniflora is digested by continuous or semi-continuous anaerobic process.     

An integrated approach to energy production and nutrient recovery through anaerobic digestion of Vetiveria zizanoides

This paper reports on experimental results used to verify the applicability of Vetiveria zizanoides (VZ) as a virtuous energetic crop. VZ produces biogas through its anaerobic digestion, and its nutrient content can be recovered through reuse, after digestion, as an agricultural amendment. Biomethanation tests were conducted with fresh and pretreated VZ, and the results of these tests were compared with those from the anaerobic degradation of common garden grass. Specific methane production was found to be around 650 Nm³ per ton of total organic carbon (TOC) for Vetiveria zizanoides, and around 510 Nm³ per ton of TOC for common grass, with no significant improvement after thermal pretreatment. Germination tests conducted with the digested VZ showed that the produced digestate fulfills the requirements of a fertilizer.     

System optimization for effective hydrogen production via anaerobic digestion and biogas steam reforming

To construct a system for the effective hydrogen production from food waste, the conditions of anaerobic digestion and biogas reforming have been investigated and optimized. The type of agitator and reactor shape affect the performance of anaerobic digestion reactors. Reactors with a cubical shape and hydrofoil agitator exhibit high performance due to the enhanced axial flow and turbulence as confirmed by simulation of computational fluid dynamics. The stability of an optimized anaerobic digestion reactor has been tested for 60 days. As a result, 84 L of biogas is produced from 1 kg of food waste. Reaction conditions, such as reaction temperature and steam/methane ratio, affect the biogas steam reforming reaction. The reactant conversions, product yields, and hydrogen production are influenced by reaction conditions. The optimized reaction conditions include a reaction temperature of 700 °C and H₂O/CH₄ ratio of 1.0. Under these conditions, hydrogen can be produced via steam reforming of biogas generated from a two-stage anaerobic digestion reactor for 25 h without significant deactivation and fluctuation.     

Two-stage anaerobic process for bio-hydrogen and bio-methane combined production from biodegradable solid wastes

A two-stage anaerobic digestion process intended for biohydrogen and bio-methane combined production from organic fraction of municipal solid wastes was investigated. In thermophilic conditions blocking of methanogenesis at the first stage of the anaerobic fermentation was achieved at pH 9.0. Cumulative hydrogen production made 82.5 l/kg volatile solids. Pretreatment of organic fraction of municipal solid wastes and exploitation of mixed cultures of anaerobic thermophilic cellulolytic and saccharolytic bacteria of Clostridia sp resulted in the increase of hydrogen cumulative production up to 104 l/kg volatile solids. Content of methane in biohydrogen didn’t exceed 0.1%. Cumulative bio-methane production made 520 l/kg volatile solids. Methane percentage in produced biogas was 78.6%. Comparison of energy data for two-stage anaerobic digestion with those for solely methane production shows the increase in energy recovery from biodegradable fraction of municipal solid wastes. Results obtained make a foolproof basis for the development of cost-effective technological process providing hydrogen and methane combined production from solid organic wastes. Technology can be implemented at large scale biogas plants improving economical and ecological characteristics of the overall process.     

Evaluation of marine algae as a source of biogas in a two-stage anaerobic reactor system

The marine algae are considered an important biomass source; however, their utilization as energy source is still low around the world. The technical feasibility of marine algae utilization as a source of renewable energy was studied to laboratory scale. The anaerobic digestion of Macrocystis pyrifera, Durvillea antarctica and their blend 1:1 (w/w) was evaluated in a two-phase anaerobic digestion system, which consisted of an anaerobic sequencing batch reactor (ASBR) and an upflow anaerobic filter (UAF). The results show that 70% of the total biogas produced in the system was generated in the UAF, and both algae species have similar biogas productions of 180.4(±1.5) mL g⁻¹ dry algae d⁻¹, with a methane concentration around 65%. The same methane content was observed in biogas yield of algae blend; however, a lower biogas yield was obtained. In conclusion, either algae species or their blend can be utilized to produce methane gas in a two-phase digestion system.     

Approbation of an innovative method of pretreatment of dark fermentation feedstocks

According to the International Energy Agency, only a small part of the full potential of biomass energy is currently used in the world. The annual amount of agricultural waste in the Russian Federation is estimated at about 152 million tons, and the energy potential of animal waste is 201 PJ/year. Anaerobic digestion is an efficient method of converting organic waste into renewable energy sources. Previously, the positive effect of pretreatment of various organic feedstocks in vortex layer apparatus (VLA) on the characteristics of anaerobic digestion and energy efficiency was shown. Currently, there is a significant interest in the world in obtaining biohydrogen from organic waste using the dark fermentation (DF) process. During pretreatment in the VLA, the iron working bodies are abraded and iron particles are introduced into the feedstock of the DF reactor. This may have a positive effect on the production rate and yield of hydrogen, which has not been previously studied. This work is aimed at evaluating the possibility of using the VLA as a method for pretreatment of a dark fermentation feedstock for the intensification of biohydrogen production. To achieve this goal, an experimental setup was constructed. It consisted of a 45 L DF reactor, a VLA and a process control system to collect data on the DF process parameters every 5 min. At a hydraulic retention time in the DF reactor of 24 h and in the VLA of 30 s, the hydrogen content in the biogas increased from 51.1% to 52.2%. At the same time, the pH increased from 3.85 to 4.8–4.9, and the hydrogen production rate increased by 16% to 1.941 L/(L day). The hydrogen yield was 80.9 ml/g VS. Thus, pretreatment of the feedstock in VLA can be an effective way to intensify the DF process; however, further study of the VLA operating modes is required in order to optimize the concentrations of iron particles introduced into the feedstock for the most efficient continuous production of dark fermentative biohydrogen.