Recovery of energy from Taro (Colocasia esculenta) with solid-feed anaerobic digesters (SOFADs)

We present studies on solid-feed anaerobic digesters (SOFADs) in which chopped Colocasia esculenta was fed without any other pretreatment, in an attempt to develop an efficient means of utilizing the semi-aquatic weed that is otherwise an environmental nuisance. Two types of SOFADs were studied. The first type had a single vessel with two compartments. The lower portion of the digester, 25% of the total volume, was separated from the upper by a perforated PVC disk. The weed was charged from the top and inoculated with anaerobically digested cow dung-water slurry. The fermentation of the weed in the digester led to the formation of volatile fatty acids (VFAs) plus some biogas. The bioleachate, rich in the VFAs, passed through the perforated PVC disk and was collected in the lower compartment of the digester. The other type of digesters, referred to as anaerobic multi-phase high-solids digesters (AMHDs), had the same type of compartmentalized digester unit as the first type and an additional methaniser unit. Up-flow anaerobic filters (UAFs) were used as methaniser units, which converted the bioleachate into combustible biogas consisting of approximately 60% methane. All SOFADs developed a consistent performance in terms of biogas yield within 20 weeks from the start. Among the two types of digesters studied, the AMHDs were found to perform better with a twofold increase in biogas yield compared to the first type of digesters.     

Impact of different particle size distributions on anaerobic digestion of the organic fraction of municipal solid waste

Particle size may significantly affect the speed and stability of anaerobic digestion, and matching the choice of particle size reduction equipment to digester type can thus determine the success or failure of the process. In the current research the organic fraction of municipal solid waste was processed using a combination of a shear shredder, rotary cutter and wet macerator to produce streams with different particle size distributions. The pre-processed waste was used in trials in semi-continuous ‘wet’ and ‘dry’ digesters at organic loading rate (OLR) up to 6 kg volatile solids (VS) m^?3 day^?1. The results indicated that while difference in the particle size distribution did not change the specific biogas yield, the digester performance was affected. In the ‘dry’ digesters the finer particle size led to acidification and ultimately to process failure at the highest OLR. In ‘wet’ digestion a fine particle size led to severe foaming and the process could not be operated above 5 kg VS m^?3 day^?1. Although the trial was not designed as a direct comparison between ‘wet’ and ‘dry’ digestion, the specific biogas yield of the ‘dry’ digesters was 90% of that produced by ‘wet’ digesters fed on the same waste at the same OLR.     

Anaerobic co-digestion of aquatic flora and quinoa with manures from Bolivian Altiplano

Quinoa stalk (Chenopodium quinoa Willd.) from agricultural crop residue, totora (Schoenoplectus tatora) and o-macrophytes (aquatic flora) from Lake Titicaca (on the Bolivian Altiplano) were studied in a wet anaerobic co-digestion process together with manure from llama, cow and sheep. Anaerobic semi-continuous experiments were performed in (10) 2-l reactors at a temperature of 25 degrees C with 30 days of hydraulic retention time (HRT) and an organic loading rate (OLR) of 1.8kgVSm(-3)d(-1). Totora was found to be the best co-substrate. In mixture ratios of 1:1 (VS basis), it increased the biogas productivity by 130% for llama manure, 60% for cow manure, and 40% for sheep manure. It was possible to use up to 58% (VS basis) of totora in the substrate. Higher concentrations (including pure totora) could not be digested, as that caused acidification problems similar to those caused by other lignocellulosic materials. When quinoa and o-macrophytes were used as co-substrates, the increase in biogas productivity was slightly less. However, these co-substrates did not cause any operational problems. An additional advantage of quinoa and o-macrophytes was that they could be used in any proportion (even in pure form) without causing any destabilization problems in the anaerobic digestion process.     

Feasibility assessment of thermophilic anaerobic digestion process of food waste

In this study, a lab-scale thermophilic anaerobic digestion of food waste collected from G-district in Seoul was performed to assess its feasibility and applicability in field-scale biogas plants. Monitoring parameters included biogas production, methane composition, pH, alkalinity, and volatile fatty acid (VFA) concentrations. Accumulation of VFA caused successive depression in pH, which inhibited microbial activity of methane-forming microorganisms. Signals of biological instability and inhibition of methanogenesis suggest possible process failure, as indicated by reduction in methane production. Results revealed that modifications in certain conditions, such as decreased organic loading rate (OLR) or additional insertion of alkalinity, must be made for its application in industrial-scale biogas plants, and that thermophilic anaerobic digestion of food waste may not be feasible without any modification.     

Application of an early warning indicator and CaO to maximize the time–space-yield of an completely mixed waste digester using rape seed oil as co-substrate

In order to increase the organic loading rate (OLR) and hereby the performance of biogas plants an early warning indicator (EWI-VFA/Ca) was applied in a laboratory-scale biogas digester to control process stability and to steer additive dosing. As soon as the EWI-VFA/Ca indicated the change from stable to instable process conditions, calcium oxide was charged as a countermeasure to raise the pH and to bind long-chain fatty acids (LCFAs) by formation of aggregates. An interval of eight days between two increases of the OLR, which corresponded to 38% of the hydraulic residence time (HRT), was sufficient for process adaptation. An OLR increase by a factor of three within six weeks was successfully used for biogas production. The OLR was increased to 9.5 kg volatile solids (VS) m^?3 d^?1 with up to 87% of fat. The high loading rates affected neither the microbial community negatively nor the biogas production process. Despite the increase of the organic load to high rates, methane production yielded almost its optimum, amounting to 0.9 m³ (kg VS)^?1. Beneath several uncharacterized members of the phylum Firmicutes mostly belonging to the family Clostridiaceae, a Syntrophomonas-like organism was identified that is known to live in a syntrophic relationship to methanogenic archaea. Within the methanogenic group, microorganisms affiliated to Methanosarcina, Methanoculleus and Methanobacterium dominated the community.     

Impacts of microwave pretreatments on the semi-continuous anaerobic digestion of dairy waste activated sludge

Microwave (MW) irradiation is one of the new and possible methods used for pretreating the sludge. Following its use in different fields, this MW irradiation method has proved to be more appropriate in the field of environmental research. In this paper, we focused on the effects of MW irradiation at different intensities on solubilization, biodegradation and anaerobic digestion of sludge from the dairy sludge. The changes in the soluble fractions of the organic matter, the biogas yield, the methane content in the biogas were used as control parameters for evaluating the efficiency of the MW pretreatment. Additionally, the energetic efficiency was also examined. In terms of an energetic aspect, the most economical pretreatment of sludge was at 70% intensity for 12 min irradiation time. At this, COD solubilization, SS reduction and biogas production were found to be 18.6%, 14% and 35% higher than the control, respectively. Not only the increase in biogas production was investigated, excluding protein and carbohydrate hydrolysis was also performed successfully by this microwave pretreatment even at low irradiation energy input. Also, experiments were carried out in semi continuous anaerobic digesters, with 3.5 L working volume. Combining microwave pretreatment with anaerobic digestion led to 67%, 64% and 57% of SS reduction, VS reduction and biogas production higher than the control, respectively.     

Treatment of municipal landfill leachate using a combined anaerobic digester and activated sludge system

The main objective of this study was to assess the feasibility of treating sanitary landfill leachate using a combined anaerobic and activated sludge system. A high-strength leachate from Shiraz municipal landfill site was treated using this system. A two-stage laboratory-scale anaerobic digester under mesophilic conditions and an activated sludge unit were used. Landfill leachate composition and characteristics varied considerably during 8 months experiment (COD concentrations of 48,552–62,150 mg/L). It was found that the system could reduce the COD of the leachate by 94% at a loading rate of 2.25 g COD/L/d and 93% at loading rate of 3.37 g COD/L/d. The anaerobic digester treatment was quite effective in removing Fe, Cu, Mn, and Ni. However, in the case of Zn, removal efficiency was about 50%. For the rest of the HMs the removal efficiencies were in the range 88.8–99.9%. Ammonia reduction did not occur in anaerobic digesters. Anaerobic reactors increased alkalinity about 3.2–4.8% in the 1st digester and 1.8–7.9% in the 2nd digester. In activated sludge unit, alkalinity and ammonia removal efficiency were 49–60% and 48.6–64.7%, respectively. Methane production rate was in the range of 0.02–0.04, 0.04–0.07, and 0.02–0.04 L/g COD_rem for the 1st digester, the 2nd digester, and combination of both digesters, respectively; the methane content of the biogas varied between 60% and 63%.     

Differences in volatile methyl siloxane (VMS) profiles in biogas from landfills and anaerobic digesters and energetics of VMS transformations

The objectives of this study were to compare the types and levels of volatile methyl siloxanes (VMS) present in biogas generated in the anaerobic digesters and landfills, evaluate the energetics of siloxane transformations under anaerobic conditions, compare the conditions in anaerobic digesters and municipal solid waste (MSW) landfills which result in differences in siloxane compositions. Biogas samples were collected at the South District Wastewater Treatment Plant and South Dade Landfill in Miami, Florida. In the digester gas, D4 and D5 comprised the bulk of total siloxanes (62% and 27%, respectively) whereas in the landfill gas, the bulk of siloxanes were trimethylsilanol (TMSOH) (58%) followed by D4 (17%). Presence of high levels of TMSOH in the landfill gas indicates that methane utilization may be a possible reaction mechanism for TMSOH formation. The free energy change for transformation of D5 and D4 to TMSOH either by hydrogen or methane utilization are thermodynamically favorable. Either hydrogen or methane should be present at relatively high concentrations for TMSOH formation which explains the high levels present in the landfill gas. The high bond energy and bond distance of the Si–O bond, in view of the atomic sizes of Si and O atoms, indicate that Si atoms can provide a barrier, making it difficult to break the Si–O bonds especially for molecules with specific geometric configurations such as D4 and D5 where oxygen atoms are positioned inside the frame formed by the large Si atoms which are surrounded by the methyl groups.     

A pilot plant two-phase anaerobic digestion system for bioenergy recovery from swine wastes and garbage

A pilot plant bioenergy recovery system from swine waste and garbage was constructed. A series of experiments was performed using swine feces (SF); a mixture of swine feces and urine (MSFU); a mixture of swine feces, urine and garbage (MSFUG); garbage and a mixture of urine and garbage (AUG). The system performed well for treating the source materials at a high organic loading rate (OLR) and short hydraulic retention time (HRT). In particular, the biogas production for the MSFUG was the highest, accounting for approximately 865-930Lkg(-1)-VS added at the OLR of 5.0-5.3kg-VSm(-3)day(-1) and the HRT of 9 days. The removal of VS was 67-75%, and that of COD was 73-74%. Therefore, co-digestion is a promising method for the recovery of bioenergy from swine waste and garbage. Furthermore, the results obtained from this study provide fundamental information for scaling up a high-performance anaerobic system in the future.     

Development of on-line measurement techniques for siloxanes and other trace compounds in biogas

This paper focuses on the development of an on-line measurement method for siloxanes and other biogas trace compounds impeding the energy utilisation of biogas, as well as the main gas components, methane and carbon dioxide. The method is based on gas chromatography and FT-IR-analysis. The level of siloxane, hydrogen sulphide and halogens in biogas generated in a number of landfills and digesters in Finland is also presented and factors affecting the concentrations discussed. Generally, the level of biogas trace compounds hampering electricity production was lower than those measured at comparable sites in Central Europe and the US. Moreover, the paper discusses the significance of on-line monitoring of siloxane in connection to biogas-to-electricity applications and points out with activated carbon as an example the benefits of on-line siloxane measurement in the control of siloxane removal technology.     

Co-digestion of cattle manure with food waste and sludge to increase biogas production

Anaerobic co-digestion strategies are needed to enhance biogas production, especially when treating certain residues such as cattle/pig manure. This paper presents a study of co-digestion of cattle manure with food waste and sewage sludge. With the aim of maximising biogas yields, a series of experiments were carried out under mesophilic and thermophilic conditions using continuously stirred-tank reactors, operating at different hydraulic residence times. Pretreatment with ultrasound was also applied to compare the results with those obtained with non-pretreated waste. Specific methane production decreases when increasing the OLR and decreasing HRT. The maximum value obtained was 603 LCH(4)/kg VS(feed) for the co-digestion of a mixture of 70% manure, 20% food waste and 10% sewage sludge (total solid concentration around 4%) at 36°C, for an OLR of 1.2g VS/Lday. Increasing the OLR to 1.5g VS/Lday led to a decrease of around 20-28% in SMP. Lower methane yields were obtained when operating at 55°C. The increase in methane production when applying ultrasound to the feed mixtures does not compensate for the energy spent in this pretreatment.     

Pilot-scale anaerobic co-digestion of municipal biomass waste and waste activated sludge in China: Effect of organic loading rate

The effects of organic loading rate on the performance and stability of anaerobic co-digestion of municipal biomass waste (MBW) and waste activated sludge (WAS) were investigated on a pilot-scale reactor. The results showed that stable operation was achieved with organic loading rates (OLR) of 1.2–8.0 kg volatile solid (VS) (m³ d)^?1, with VS reduction rates of 61.7–69.9%, and volumetric biogas production of 0.89–5.28 m³ (m³ d)^?1. A maximum methane production rate of 2.94 m³ (m³ d)^?1 was achieved at OLR of 8.0 kg VS (m³ d)^?1 and hydraulic retention time of 15 days. With increasing OLRs, the anaerobic reactor showed a decrease in VS removal rate, average pH value and methane concentration, and a increase of volatile fatty acid concentration. By monitoring the biogas production rate (BPR), the anaerobic digestion system has a higher acidification risk under an OLR of 8.0 kg VS (m³ d)^?1. This result remarks the possibility of relating bioreactor performance with BPR in order to better understand and monitor anaerobic digestion process.     

Current EU-27 technical potential of organic waste streams for biogas and energy production

Anaerobic digestion of organic waste generated by households, businesses, agriculture, and industry is an important approach as method of waste treatment – especially with regard to its potential as an alternative energy source and its cost-effectiveness. Separate collection of biowaste from households or vegetal waste from public green spaces is already established in some EU-27 countries. The material recovery in composting plants is common for biowaste and vegetal waste. Brewery waste fractions generated by beer production are often used for animal feeding after a suitable preparation. Waste streams from paper industry generated by pulp and paper production such as black liquor or paper sludge are often highly contaminated with toxic substances. Recovery of chemicals and the use in thermal processes like incineration, pyrolysis, and gasification are typical utilization paths. The current utilization of organic waste from households and institutions (without agricultural waste) was investigated for EU-27 countries with Germany as an in-depth example. Besides of biowaste little is known about the suitability of waste streams from brewery and paper industry for anaerobic digestion. Therefore, an evaluation of the most important biogas process parameters for different substrates was carried out, in order to calculate the biogas utilization potential of these waste quantities. Furthermore, a calculation of biogas energy potentials was carried out for defined waste fractions which are most suitable for anaerobic digestion. Up to 1% of the primary energy demand can be covered by the calculated total biogas energy potential. By using a “best-practice-scenario” for separately collected biowaste, the coverage of primary energy demand may be increased above 2% for several countries. By using sector-specific waste streams, for example the German paper industry could cover up to 4.7% and the German brewery industry up to 71.2% of its total energy demand.     

Pilot project of biogas production from pig manure and urine mixture at ambient temperature in Ventanilla (Lima, Peru)

Parque Porcino de Ventanilla has an extension of 840ha with 2200 farmers dedicated to pig production. There is a lack of services in the area (i.e., water supply, electricity, or waste collection). Anaerobic treatment of pig manure would replace current dumping and incineration, reducing environmental pollution and hazards to public health, as well as providing an organic fertilizer and biogas. The objective of the present work was to study the viability of ambient temperature anaerobic digestion of pig manure diluted in urine, by means of on-site pilot scale reactors. The final goal was to establish design parameters for anaerobic digesters to be implemented; since it was part of a project to improve life conditions for the farmers through the incorporation of better management techniques. Experiments were carried out in a low-cost pilot plant, which consists of three anaerobic digesters (225L total volume), without heating or agitation, placed in a greenhouse. The start-up of the digestion process was performed with a mixture of temperature adapted pig manure-sludge and fresh rumen, and showed a good performance regardless of the dilution of pig manure with water or urine, which is a key parameter due to the scarcity of water in the area under study.     

Anaerobic biogasification of undiluted dairy manure in leaching bed reactors

Dry anaerobic digestion of high solids animal manure is of increasing importance since conventional slurry digestion is not an effective system for these manures. The investment costs for large-size reactors, costs for heating these reactors, handling, dewatering, and the disposal of the digested residue decrease the benefits of conventional slurry anaerobic digestion for high solids animal manure. Even though leaching bed reactors (LBR) constitute a promising option for dry anaerobic biogasification of animal manure, no study is cited in the literature for animal manure, excluding a single study on cattle waste which utilized a similar concept in a different experimental set-up, namely a packed bed digester. Therefore, this work was undertaken to investigate the anaerobic biogasification of undiluted dairy manure in LBRs. To this purpose anaerobic leaching bed reactors (ALBR) packed with a mixture of dairy manure, anaerobic seed and wood powder/chips were operated. The ALBRs were fed with water, and the leachate that was collected from their effluents was subjected to biochemical methane potential (BMP) experiments to determine the biogas production. The results revealed that LBRs can successfully be applied to anaerobic digestion of undiluted dairy manure with around 25% improvement in biogas production relative to conventional (slurry) anaerobic digesters.     

Balancing the energy demand by a resource circulation project in Daejeon,Korea

Demand for sustainable renewable energy is on an increase worldwide, whereas the supply is limited. This paper analyses the feasibility of generating electricity and supplying the surplus steam to Daeduk Industrial Complex, by incinerating the combustible municipal waste generated in Daejeon Metropolitan City. The economic feasibility of surplus biogas generated from the anaerobic digestion of food waste and food waste leachate has been analysed. This study estimated resource circulation facility to supply 23,200 m³/day of biogas generated to Daejeon Combined Heat and Power plant. By 2023, it is expected to supply 25.7 tons of steam per hour all year round. The additional steam demand in Daeduk Industrial Complex is estimated as 101,537 tons/year. Surplus biogas will be supplied through an additional 960-m new installation. The cost of biogas is estimated at 30% of the unit biogas production cost. Daejeon Combined Heat and Power plant expects to make 60% additional profit, and Daeduk Industrial Complex and the communities nearby expect to achieve 10% cost savings. It also reduces the dependence of energy on fossil fuels, contributes to national environmental energy policy in reduction in greenhouse gases, creates competitiveness in local business and reduces corporate tax and generates revenue.     

Anaerobic digestion and co-digestion of slaughterhouse waste (SHW): Influence of heat and pressure pre-treatment in biogas yield

Mesophilic anaerobic digestion (34 ± 1 °C) of pre-treated (for 20 min at 133 °C, >3 bar) slaughterhouse waste and its co-digestion with the organic fraction of municipal solid waste (OFMSW) have been assessed. Semi-continuously-fed digesters worked with a hydraulic retention time (HRT) of 36 d and organic loading rates (OLR) of 1.2 and 2.6 kg VS_feed/m³ d for digestion and co-digestion, respectively, with a previous acclimatization period in all cases. It was not possible to carry out an efficient treatment of hygienized waste, even less so when OFMSW was added as co-substrate. These digesters presented volatile fatty acids (VFA), long chain fatty acids (LCFA) and fats accumulation, leading to instability and inhibition of the degradation process. The aim of applying a heat and pressure pre-treatment to promote splitting of complex lipids and nitrogen-rich waste into simpler and more biodegradable constituents and to enhance biogas production was not successful. These results indicate that the temperature and the high pressure of the pre-treatment applied favoured the formation of compounds that are refractory to anaerobic digestion.The pre-treated slaughterhouse wastes and the final products of these systems were analyzed by FTIR and TGA. These tools verified the existence of complex nitrogen-containing polymers in the final effluents, confirming the formation of refractory compounds during pre-treatment.     

Anaerobic digestion potential of urban organic waste: a case study in Malm?.

A study of existing organic waste types in Malm?, Sweden was performed. The purpose was to gather information about organic waste types in the city to be able to estimate the potential for anaerobic treatment in existing digesters at the wastewater treatment plan (WWTP). The urban organic waste types that could have a significant potential for anaerobic digestion amount to about 50 000 tonnes year(-1) (sludge excluded). Some of the waste types were further evaluated by methane potential tests and continuous pilot-scale digestion. Single-substrate digestion and co-digestion of pre-treated, source-sorted organic fraction of municipal solid waste, wastewater sludge, sludge from grease traps and fruit and vegetable waste were carried out. The experiments showed that codigestion of grease sludge and WWTP sludge was a better way of making use of the methane potential in the grease trap sludge than single-substrate digestion. Another way of increasing the methane production in sludge digesters is to add source-sorted organic fraction of municipal solid waste (SSOFMSW). Adding SSOFMSW (20% of the total volatile solids) gave a 10-15% higher yield than could be expected by comparison with separate digestion of sludge respective SSOFMSW. Co-digestion of sludge and organic waste is beneficial not just for increasing gas production but also for stabilizing the digestion process. This was seen when co-digesting fruit and vegetable waste and sludge. When co-digested with sludge, this waste gave a better result than the separate digestion of fruit and vegetable waste. Considering single-substrate digestion, SSOFMSW is the only waste in the study which makes up a sufficient quantity to be suitable as the base substrate in a full-scale digester that is separated from the sludge digestion. The two types of SSOFMSW tested in the pilot-scale digestion were operated successfully at mesophilic temperature. By adding SSOFMSW, grease trap sludge and fruit and vegetables waste to sludge digesters at the wastewater treatment plant, the yearly energy production from methane could be expected to increase from 24 to 43 GWh.     

Effects of various fatty acid amendments on a microbial digester community in batch culture

Since biogas production is becoming increasingly important the understanding of anaerobic digestion processes is fundamental. However, large-scale digesters often lack online sensor equipment to monitor key parameters. Furthermore the possibility to selectively change fermenting parameter settings in order to investigate methane output or microbial changes is limited. In the present study we examined the possibility to investigate the microbial community of a large scale (750,000 L) digester within a laboratory small-scale approach. We studied the short-term response of the downscaled communities on various fatty acids and its effects on gas production and compared it with data from the original digester sludge. Even high loads of formic acid led to distinct methane formation, whereas high concentrations of other acids (acetic, butyric, propionic acid) caused a marked inhibition of methanogenesis coupled with an increase in hydrogen concentration. Molecular microbial techniques (DGGE/quantitative real-time-PCR) were used to monitor the microbial community changes which were related to data from GC and HPLC analysis. DGGE band patterns showed that the same microorganisms which were already dominant in the original digester re-established again in the lab-scale experiment. Very few microorganisms dominated the whole fermenting process and species diversity was not easily influenced by moderate varying fatty acid amendments - Methanoculleus thermophilus being the most abundant species throughout the variants. MCR-copy number determined via quantitative real-time-PCR - turned out to be a reliable parameter for quantification of methanogens, even in a very complex matrix like fermenter sludge. Generally the downscaled batch approach was shown to be appropriate to investigate microbial communities from large-scale digesters.     

Simultaneous removal of COD and ammonium from landfill leachate using an anaerobic-aerobic moving-bed biofilm reactor system

The performance of a moving-bed biofilm reactor (MBBR) system with an anaerobic-aerobic arrangement was investigated to treat landfill leachate for simultaneous removal of COD and ammonium. It was found that the anaerobic MBBR played a major role in COD removal due to methanogenesis, and the aerobic MBBR acted as COD-polishing and ammonium removal step. The contribution of the anaerobic MBBR to total COD removal efficiency reached 91% at an organic loading rate (OLR) of 4.08 kgCOD/(m3d), and gradually decreased to 86% when feed OLR was increased to 15.70 kgCOD/(m3d). Because of the complementary function of the aerobic reactor, the total COD removal efficiency of the system had a slight decrease from 94% to 92% even though the feed OLR was increased from 4.08 to 15.70 kgCOD/(m3d). Hydraulic retention time (HRT) had a significant effect on NH+4-N removal; more than 97% of the total NH+4-N removal efficiency could be achieved when the HRT of the aerobic MBBR was more than 1.25 days. The anaerobic-aerobic system had a strong tolerance to shock loading. A decrease in COD removal efficiency of only 7% was observed when the OLR was increased by four times and shock duration was 24 h, and the system could recover the original removal efficiency in 3 days. The average sludge yield of the anaerobic reactor was estimated to be 0.0538 gVSS/gCOD rem.