Technical, economic and environmental analysis of energy production from municipal solid waste

Four technologies are investigated which produce energy from municipal solid waste (MSW): incineration, gasification, generation of biogas and utilisation in a combined heat and power (CHP) plant, generation of biogas and conversion to transport fuel.Typically the residual component of MSW (non-recyclable, non-organic) is incinerated producing electricity at an efficiency of about 20% and thermal product at an efficiency of about 55%. This is problematic in an Irish context where utilisation of thermal products is not the norm. Gasification produces electricity at an efficiency of about 34%; this would suggest that gasification of the residual component of MSW is more advantageous than incineration where a market for thermal product does not exist. Gasification produces more electricity than incineration, requires a smaller gate fee than incineration and when thermal product is not utilised generates less greenhouse gas per kWh than incineration. Gasification of MSW (a non-homogenous fuel) is, however, not proven at commercial scale.Biogas may be generated by digesting the organic fraction of MSW (OFMSW). The produced biogas may be utilised for CHP production or for transport fuel production as CH₄-enriched biogas. When used to produce transport fuel some of the biogas is used in a small CHP unit to meet electricity demand on site. This generates a surplus thermal product.Both biogas technologies require significantly less investment costs than the thermal conversion technologies (incineration and gasification) and have smaller gate fees. Of the four technologies investigated transport fuel production requires the least gate fee. A shortfall of the transport fuel production technology is that only 50% of biogas is available for scrubbing to CH₄-enriched biogas.     

Technical/economic/environmental analysis of biogas utilisation

Biogas may be utilised for Combined Heat and Power (CHP) production or for transport fuel production (CH₄-enriched biogas). When used to produce transport fuel either electricity is imported to power the plant or some of the biogas is used in a small CHP unit to meet electricity demand on site. The potential revenue from CH₄-enriched biogas when replacing petrol is higher than that for replacing diesel (Irish prices). Transport fuel production when replacing petrol requires the least gate fee. The production of greenhouse-gas is generated with cognisance of greenhouse-gas production with the scheme not in place; landfill of the Organic Fraction of Municipal Solid Waste (OFMSW) (20% of biomass) with and without combustion of landfill gas is investigated. The transport scenario with importation of brown electricity generates more greenhouse-gas than petrol or diesel, when the ‘do-nothing’ case involves combustion of landfill gas. The preferred solution involves transport fuel production with the production of CHP to meet electricity demand on site. A shortfall of this solution is that only 53% of biogas is available for export.     

Improved biogas yield from organic fraction of municipal solid waste as preliminary step for fuel cell technology and hydrogen generation

Biogas utilization in fuel cell technology and hydrogen generation is a modern and economically viable approach. A pretreatment step prior to anaerobic digestion (AD) is obligatory to increase the hydrolysis, solubilize the complex matter present in organic fraction of municipal solid waste (OFMSW) and to achieve higher yield of biogas. This study was intended to find out the effects of thermal, chemical and thermochemical pretreatments on the properties and structure of OFMSW and also on biogas production. There was an increase in chemical oxygen demand of 6.87, 1.61 and 11.60% for thermal, chemical and thermochemical pretreatments, respectively. Also, the content of volatile solids was reduced by 2.36% by thermochemical pretreatment. FTIR, XRD and SEM analysis revealed that these pretreatments also caused chemical and morphological changes on the substrate, as a result reduced its crystallinity and enhanced the rate of hydrolysis. A significant increase of 54% in biogas yield was achieved after thermochemical pretreatment in comparison to untreated OFMSW sample.     

Principles and potential of the anaerobic digestion of waste-activated sludge

When treating municipal wastewater, the disposal of sludge is a problem of growing importance, representing up to 50% of the current operating costs of a wastewater treatment plant. Although different disposal routes are possible, anaerobic digestion plays an important role for its abilities to further transform organic matter into biogas (60–70 vol% of methane, CH₄), as thereby it also reduces the amount of final sludge solids for disposal whilst destroying most of the pathogens present in the sludge and limiting odour problems associated with residual putrescible matter. Anaerobic digestion thus optimises WWTP costs, its environmental footprint and is considered a major and essential part of a modern WWTP. The potential of using the biogas as energy source has long been widely recognised and current techniques are being developed to upgrade quality and to enhance energy use. The present paper extensively reviews the principles of anaerobic digestion, the process parameters and their interaction, the design methods, the biogas utilisation, the possible problems and potential pro-active cures, and the recent developments to reduce the impact of the problems. After having reviewed the basic principles and techniques of the anaerobic digestion process, modelling concepts will be assessed to delineate the dominant parameters. Hydrolysis is recognised as rate-limiting step in the complex digestion process. The microbiology of anaerobic digestion is complex and delicate, involving several bacterial groups, each of them having their own optimum working conditions. As will be shown, these groups are sensitive to and possibly inhibited by several process parameters such as pH, alkalinity, concentration of free ammonia, hydrogen, sodium, potassium, heavy metals, volatile fatty acids and others. To accelerate the digestion and enhance the production of biogas, various pre-treatments can be used to improve the rate-limiting hydrolysis. These treatments include mechanical, thermal, chemical and biological interventions to the feedstock. All pre-treatments result in a lysis or disintegration of sludge cells, thus releasing and solubilising intracellular material into the water phase and transforming refractory organic material into biodegradable species. Possible techniques to upgrade the biogas formed by removing CO₂, H₂S and excess moisture will be summarised. Special attention will be paid to the problems associated with siloxanes (SX) possibly present in the sludge and biogas, together with the techniques to either reduce their concentration in sludge by preventive actions such as peroxidation, or eliminate the SX from the biogas by adsorption or other techniques. The reader will finally be guided to extensive publications concerning the operation, control, maintenance and troubleshooting of anaerobic digestion plants.     

Enhanced methane fermentation of municipal sewage sludge by microbial electrochemical systems integrated with anaerobic digestion

Sewage sludge from a municipal wastewater treatment plant was fed into a microbial electrochemical system, combined with an anaerobic digester (MES-AD), for enhanced methane production and sludge stabilization. The effect of thermally pretreating the sewage sludge on MES-AD performance was investigated. These results were compared to those obtained from control operations, in which the sludge was not pretreated or MES integration was absent. The soluble chemical oxygen demand (SCOD) in the raw sewage sludge after pretreatment was 31% higher than the SCOD in untreated sludge (5804.85 mg/L vs. 4441.46 mg/mL). The methane yield and proportion of methane in biogas generated by the MES-AD were higher than those of the control systems, regardless of the pretreatment process. The maximum methane yield (0.28 L CH₄/g COD) and methane production (1139 mL) were obtained with the MES inoculated with pretreated sewage sludge. Methane yield and production with this system using pretreated sewage were 47% and 56% higher, respectively, than those of the control (0.19 L CH₄/g COD, 730 mL). Additionally, the maximum SCOD removal (89%) and current generation were obtained with the MES inoculated with a pretreated substrate. These results suggested that sewage sludge could be efficiently stabilized with enhanced methane production by synergistic combination of MES-AD system with pretreatment process.     

Enhancing biomethanation of municipal waste sludge with grease trap waste as a co-substrate

Grease trap waste (GTW) presents a challenge to wastewater treatment processes due to its slow biodegradation kinetics, high oxygen demand, and risks of pipeline blockage. The objective of this work was to evaluate the feasibility of GTW as an organic-rich co-substrate to improve biomethane production in the anaerobic digestion of municipal waste sludge (MWS) from sewage treatment, one of the most abundant feed materials to municipal anaerobic digesters. Waste characterization confirmed the high organic content of GTW at 138 gVS/L, which was 626% higher than that of MWS (19 gVS/L). The methane potential of GTW approximated 145 L_Methane/L_GTW, which was more than 15 times higher than that of MWS (8.9 L_Methane/L_MWS). When GTW was added as a co-substrate in addition to MWS, the high methane potential and organic content of GTW resulted in significant improvement in methane production during the anaerobic co-digestion of MWS, e.g. a 65% increase at the GTW loading of 5.5 gVS/L, representing a less than 4% (vol/vol) addition of GTW. Thus, the operational feasibility of anaerobic co-digestion using GTW as the co-substrate is enhanced by the insignificant volumetric GTW loading required for significant improvements in methane production. Process inhibition and reduction in biogas production, however, occurred with higher GTW loadings, suggesting the importance of proper GTW loading rates for the implementation of anaerobic co-digestion processes effective in improving biomethanation of municipal waste sludge.     

Hydrogen production characteristics of the organic fraction of municipal solid wastes by anaerobic mixed culture fermentation

The hydrogen production from the organic fraction of municipal solid waste (OFMSW) by anaerobic mixed culture fermentation was investigated using batch experiments at 37 °C. Seven varieties of typical individual components of OFMSW including rice, potato, lettuce, lean meat, oil, fat and banyan leaves were selected to estimate the hydrogen production potential. Experimental results showed that the boiling treated anaerobic sludge was effective mixed inoculum for fermentative hydrogen production from OFMSW. Mechanism of fermentative hydrogen production indicates that, among the OFMSW, carbohydrates is the most optimal substrate for fermentative hydrogen production compared with proteins, lipids and lignocelluloses. This conclusion was also substantiated by experimental results of this study. The hydrogen production potentials of rice, potato and lettuce were 134 mL/g-VS, 106 mL/g-VS, and 50 mL/g-VS respectively. The hydrogen percentages of the total gas produced from rice, potato and lettuce were 57–70%, 41–55% and 37–67%.     

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.     

Economic and environmental assessment on the energetic valorization of organic material for a municipality in Quebec, Canada

Waste-to-energy provides a solution to two problems: waste management and energy generation. An integrated anaerobic waste valorization process is an interesting option, but because of investments cost and low energy value in the province of Quebec, it is hard for a municipality to commit to that solution. This paper investigated the economic possibilities to manage organic material, organic fraction of municipal solid waste, and municipal wastewater sludge by anaerobic digestion for a 150,000 inhabitant municipality, with consideration to energy generation and greenhouse gas emission reduction. Using the biogas to co-generation solution brings a payback time on investment (PBT) of 3.7 years with electricity price at 0.10 $Cdn/kW h. The addition of manure from surrounding farms increases the biogas production by 37%, but increases the PBT to 6.8 years unless the leftover digestate can be used for agronomic valorization; then it becomes economically advantageous. The natural gas purchasing cost is too low to promote the enrichment of biogas into renewable natural gas. However, this scenario has the lowest energetic payback time (3.3 years) and reduces the most greenhouse gas emissions (4261 tCO₂eq/a).     

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.     

Evaluation of methods for estimating energy performance of biogas production

This study focused on identifying various system boundaries and evaluating methods of estimating energy performance of biogas production. First, the output–input ratio method used for evaluating energy performance from the system boundaries was reviewed. Secondly, ways to assess the efficiency of biogas use and parasitic energy demand were investigated. Thirdly, an approach for comparing biogas production to other energy production methods was evaluated. Data from an existing biogas plant, located in Finland, was used for the evaluation of the methods. The results indicate that calculating and comparing the output–input ratios (R_pr1, R_pr2, R_ut, R_pl and R_sy) can be used in evaluating the performance of biogas production system. In addition, the parasitic energy demand calculations (w) and the efficiency of utilizing produced biogas (η) provide detailed information on energy performance of the biogas plant. Furthermore, R_f and energy output in relation to total solid mass of feedstock (F_O/TS) are useful in comparing biogas production with other energy recovery technologies. As a conclusion it is essential for the comparability of biogas plants that their energy performance would be calculated in a more consistent manner in the future.     

Durability of a Ni based monolithic catalyst in the autothermal reforming of biogas

A Ni based catalyst supported on a cordierite monolithic substrate was applied to the autothermal reforming (ATR) of biogas to produce hydrogen. When the feed rates of oxygen and steam were constant, the Steam/CH₄ (S/CH₄) and O₂/CH₄ ratios changed because of an increase or decrease in the methane concentration of the biogas. The concentration of methane in the biogas fluctuates roughly between 35% and 65% according to factors such as the properties or amount of the waste. Therefore, the effect of S/CH₄ and O₂/CH₄ ratios on catalyst durability was confirmed by using actual biogas, which was produced by anaerobic fermentation of biomass at the biogasification bench-scale plant in Kyoto. Reforming reactions were carried out at ratios of S/CH₄ = 0–4, O₂/CH₄ = 0.5 and at S/CH₄ = 2, O₂/CH₄ = 0.6. The S/CH₄ range of 0–2.0 and the O₂/CH₄ range of 0.5–0.6 had no effect on the catalyst durability and a S/CH₄ ratio of more than 3.0 led to decreased catalytic performance.     

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.     

Internal reforming SOFC running on biogas

Direct feeding of biogas to SOFC, which is derived from municipal organic wastes, has been investigated as a carbon-neutral renewable energy system. CH₄/CO₂ ratio in the actual biogas fluctuated between 1.4 and 1.9 indicating biogas composition is strongly affected by the kinds of organic wastes and the operational conditions of methane fermentation. Using anode-supported button cells, stable operation of biogas-fueled SOFC was achieved with the internal reforming mode at 800 °C. Cell voltage above 0.8 V was recorded over 800 h at 200 mA cm⁻². It has been revealed that air addition to actual biogas reduced the risk of carbon formation and led to more stable operation without compromising cell voltage due to the lowering of anodic overvoltage.     

Biogas Production and Utilization Potential of Wastewater Treatment Sludge

Abstract

Municipal wastewater treatment plants generate sludge as a by-product of the physical, chemical and biological processes used in the treatment of wastewater. Generally, this sludge must be subject to some form of treatment in order to alter its character. By using anaerobic digestion in the treatment of wastewater sludge, methane gas is produced and it is known as biogas. It must not only be seen as a renewable energy source, but even more as one of the promising solutions to the large environmental problem concerning waste handling, water pollution, CO₂ emission, etc. This article presents the biogas generation from wastewater treatment sludge, its energy potential and also its usage in some treatment plants operated in Turkey. Although the estimation of recoverable energy from municipal wastes and sewage is difficult to assess, total recoverable bioenergy potential is estimated as being 16,920 ktoe. Of this, 1,300 ktoe of municipal wastes and sewage whereas biogas production potential is 1.5–2 Mtoe in Turkey.     

Inline NH3 removal from biogas digesters

During biogas production from various types of substrates such as animal manure, fats and proteins, bacterial growth and biogas production can be inhibited by excessive ammonia (NH₃) concentrations. If NH₃ is removed from the biogas digester without damaging the digestion process, inhibition of the methane (CH₄) producing bacteria will diminish. This study shows that it is possible to remove a significant quantity of NH₃ from the biogas digester headspace and liquid phase by a simple gas circulation method where gas bubbles free of NH₃ is forced through the upper 30 cm of the liquid phase in the biogas digester, into the headspace and out of the digester. The suggested method improves conditions for anaerobic bacteria exposed to high concentrations of NH₃ by simply removing NH₃ from the digester.In full-scale biogas production the system presented in this study can be improved by circulating headspace gas through an ammonia absorber and returning the NH₃ depleted biogas into the biogas digester. This method can also replace the need for mixing in biogas digesters.     

Techniques for transformation of biogas to biomethane

Biogas from anaerobic digestion and landfills consists primarily of CH₄ and CO₂. Trace components that are often present in biogas are water vapor, hydrogen sulfide, siloxanes, hydrocarbons, ammonia, oxygen, carbon monoxide and nitrogen. In order to transfer biogas into biomethane, two major steps are performed: (1) a cleaning process to remove the trace components and (2) an upgrading process to adjust the calorific value. Upgrading is generally performed in order to meet the standards for use as vehicle fuel or for injection in the natural gas grid.Different methods for biogas cleaning and upgrading are used. They differ in functioning, the necessary quality conditions of the incoming gas, the efficiency and their operational bottlenecks. Condensation methods (demisters, cyclone separators or moisture traps) and drying methods (adsorption or absorption) are used to remove water in combination with foam and dust.A number of techniques have been developed to remove H₂S from biogas. Air dosing to the biogas and addition of iron chloride into the digester tank are two procedures that remove H₂S during digestion. Techniques such as adsorption on iron oxide pellets and absorption in liquids remove H₂S after digestion.Subsequently, trace components like siloxanes, hydrocarbons, ammonia, oxygen, carbon monoxide and nitrogen can require extra removal steps, if not sufficiently removed by other treatment steps.Finally, CH₄ must be separated from CO₂ using pressure swing adsorption, membrane separation, physical or chemical CO₂-absorption.     

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.     

Effects of anaerobic pre-treatment on the degradation of dewatered-sewage sludge

Effects of anaerobic pre-treatment were evaluated on the dewatered-sewage sludge from a municipal wastewater treatment plant in order to improve its biodegradability through anaerobic digestion. The pre-treatment was conducted in laboratory scale at 25, 50 and 70 °C for an incubation time of two days. As a reference, sludge sample was also autoclaved at 121 °C for 20 min to determine the thermal effect to the subsequent sludge digestion. Characteristics of dewatered-sludge such as viscosity, pH and soluble chemical oxygen demand (SCOD) were affected by the pre-treatment. A higher SCOD after the pre-treatment did not necessarily imply an increase in methane yield, although initial biodegradability rate was improved. In fact, a ‘great’ improvement in SCOD concentration (up to 27%) was translated in only 8% increase in the methane yield (298 ± 9 and 276 ± 6 Nml CH₄ gVS_added^?1 for pre-treated and untreated samples, respectively). Increasing the anaerobic pre-treatment time from 12 h to 2 days at 50 °C led to an 11% improvement in methane yield. Methane content in biogas increased from an average of 65–69% for the pre-treated and untreated substrates, respectively. Volatile solids (VS) reduction increased from 42% to 51%. The overall digestion time was not affected by the pre-treatment but 90% of methane was produced in the first 12 days of incubation for 50 °C pre-treated samples whereas it took 2–5 days more for 25, 70 °C pre-treated and untreated sludge samples. In this study, thermophilic digestion was also found to be a better option in terms of faster digestion and higher VS-reduction, but it showed lower methane yield as compared to mesophilic digestion, i.e. 9% and 11% increment in methane yields for thermophilic and mesophilic digestions, respectively.     

Grass as a high potential by-product: Buffalo grass to biogas and the increase of system performance and stability

Buffalo grass and alkaline-pretreated buffalo grass samples were co-digested with cow manure separately to generate biogas in anaerobic reactors. The study considered a solid content of 20% (10% buffalo grass and 10% cow manure). The methane (CH₄) content and CH₄ yield of the distinct experiments were compared. For the untreated buffalo grass, the weighed buffalo grass was mixed with cow manure and water. For the alkaline-pretreated buffalo grass, the weighed buffalo grass was soaked in 1% sodium hydroxide for 1 day prior to being mixed with cow manure and water. The untreated and pretreated buffalo grass-manure were fed semi-continuously at the rate of 125 mL/day for five days feeding in a 5 L reactor, with 40 days hydraulic retention time. The experiments were conducted for approximately 100 days. Results were reported when the systems were in steady-state conditions. The chemical oxygen demand (COD) conversion efficiency of co-digestion of the untreated and pretreated buffalo grass-manure were 46.21 and 62.76%, respectively, and for the total volatile solids (TVS) were 68.50 and 71.80%, respectively. The CH₄ contents generated from co-digestion of the untreated and pretreated buffalo grass-manure were 48.32% and 50.36%, respectively. The CH₄ yields generated from co-digestion of the untreated and pretreated buffalo grass-manure were 328 and 385 L/kgTVS conversion, respectively. It was observed from the experiments that pretreatment of the buffalo grass prior to co-digestion provided system stability during biogas production.