Showing papers on "Biogas published in 1995"

Pilot-scale high solids thermophilic anaerobic digestion of municipal solid waste with an emphasis on nutrient requirements

Abstract: A pilot study was conducted to assess the biodegradable organic fraction of municipal solid waste (BOF/MSW) as a substrate in a high-solids anaerobic digestion process. Results obtained indicate that a typical BOF/MSW in the United States is deficient in most macro and micro-nutrients required for robust and stable digestion. The BOF/MSW was supplemented with nutrient-rich organic wastes such as wastewater treatment plant sludges, dairy manure, and synthetic chemical solutions to correct nutritional deficiencies. The combined addition of wastewater treatment plant sludge and dairy manure to a typical BOF/MSW significantly elevated the gas production rate and enhanced the process stability. Microbial nutrient requirements are identified and nutrient concentrations for stable operation are quantified.

Methanogenesis in thermophilic biogas reactors

Abstract: Methanogenesis in thermophilic biogas reactors fed with different wastes is examined. The specific methanogenic activity with acetate or hydrogen as substrate reflected the organic loading of the specific reactor examined. Increasing the loading of thermophilic reactors stabilized the process as indicated by a lower concentration of volatile fatty acids in the effluent from the reactors. The specific methanogenic activity in a thermophilic pilot-plant biogas reactor fed with a mixture of cow and pig manure reflected the stability of the reactor. The numbers of methanogens counted by the most probable number (MPN) technique with acetate or hydrogen as substrate were further found to vary depending on the loading rate and the stability of the reactor. The numbers of methanogens counted with antibody probes in one of the reactor samples was 10 times lower for the hydrogen-utilizing methanogens compared to the counts using the MPN technique, indicating that other non-reacting methanogens were present. Methanogens that reacted with the probe againstMethanobacterium thermoautotrophicum were the most numerous in this reactor. For the acetate-utilizing methanogens, the numbers counted with the antibody probes were more than a factor of 10 higher than the numbers found by MPN. The majority of acetate utilizing methanogens in the reactor wereMethanosarcina spp. single cells, which is a difficult form of the organism to cultivatein vitro. No reactions were observed with antibody probes raised againstMethanothrix soehngenii orMethanothrix CALS-1 in any of the thermophilic biogas reactors examined. Studies using 2-14C-labeled acetate showed that at high concentrations (more than approx. 1 mM) acetate was metabolized via the aceticlastic pathway, transforming the methyl-group of acetate into methane. When the concentration of acetate was less than approx. 1 mM, most of the acetate was oxidized via a two-step mechanism (syntrophic acetate oxidation) involving one organism oxidizing acetate into hydrogen and carbon dioxide and a hydrogen-utilizing methanogen forming the products of the first microorganism into methane. In thermophilic biogas reactors, acetate oxidizing cultures occupied the niche ofMethanothrix species, aceticlastic methanogens which dominate at low acetate concentrations in mesophilic systems. Normally, thermophilic biogas reactors are operated at temperatures from 52 to 56° C. Experiments using biogas reactors fed with cow manure showed that the same biogas yield found at 55° C could be obtained at 61° C after a long adaptation period. However, propionate degradation was inhibited by increasing the temperature.

Viable energy production and waste recycling from anaerobic digestion of manure and other biomass materials

Abstract: Energy prices and energy sales possibilities seem to be the most important preconditions for the viability of agricultural biogas plants. Some countries provide opportunities for electricity sale at prices of approximately US$ 0.10-0.15 per kWh. This price level provides sufficient incentive for farmers and other investors to become interested. In other countries, where much lower prices are offered, agricultural biogas plants do not expand. General preconditions for farming probably comprise the second most important factor. Reduced odours and improved fertiliser characteristics are important benefits of the digestion process. The process can also play a key role for co-operatives dealing with organic fertiliser declaration and redistribution. A relatively wide range of investment and operating costs are experienced for similar plants. Most likely this reflects that market forces are relatively weak. Considerable reductions in costs may be achieved from efforts within the whole range of technical issues from slurry handling and onwards, with the issue of electricity production being one of particular importance. Anaerobic digestion may imply environmental benefits with regard to waste treatment, pollution reduction, energy production, and improvements in agricultural practices. Agricultural and environmental government agencies are in need of further information on this. Questions concerning environmental benefits of various anaerobic digestion solutions need to be answered in an increasingly more accurate manner.

Thermophilic anaerobic wastewater treatment : temperature aspects and process stability

Abstract: The main objective of this thesis was to assess the thermostability of thermophilic anaerobic wastewater treatment processes and the possibility to optimize the performance of thermophilic high-rate systems. Experiments were conducted to study the suitability of two types of seed material to start a thermophilic anaerobic process. Both mesophilic granular sludge and digested organic fraction of municipal solid waste were used as inoculum. The fate of mesophilic granular sludge under thermophilic conditions was studied in detail. Due to the temperature increase the mesophilic methanogens are replaced by thermophiles. In fact, the mesophilic granules appeared to serve mainly as carrier material for the thermophilic bacteria during the start-up. Since the thermophilic organisms attach quite well, the thermophilic specific methanogenic activity increased very rapidly in this period. Treatment of completely acidified wastewater leads to a deterioration of the 'mesophilic-thermophilic' granules. It therefore appeared extremely difficult to develop thermophilic granular sludge on this type of wastewater for both types of inocula. However, the thermophilic granulation process proceeded easily when sucrose was added to the influent. The temperature sensitivity of the various types of thermophilic anaerobic sludge depends strongly on the process conditions applied, such as temperature and reactor type. Thermophilic sludge cultivated in high-rate reactors with high solids retention shows a high thermostability. Therefore, thermophilic anaerobic treatment in high-rate reactors can be applied in a wide temperature range, even under mesophilic conditions. In contrast, sludge cultivated in batch reactors is very sensitive to relatively small temperature variations. Regarding the thermostability of the process, application of high-rate reactors is preferred over batch reactors or completely mixed reactors. The presence of granular sludge enhances the stability towards temperature fluctuations quite substantially. The maximum specific activity of the cultivated granules appeared to be limited by the mass transfer rate. Consequently, a 'biomass buffer' is created which can be drawn on if the specific activity drops as a result of a temperature decrease. A high process stability and high removal efficiencies were obtained in upflow staged sludge bed (USSB) reactors under extreme loading conditions. This USSB reactor consisted of 5 compartments along the reactor height. From each separate compartment of this reactor the produced biogas is withdrawn. The major effect of staging the thermophilic process is a very low concentration of intermediate products, such as hydrogen and acetate, in the last compartments of the system. A low concentration of these products enhances the anaerobic thermophilic degradation of all fatty acids. The properties of the sludge grown in the various compartments of the staged reactor depend on the environmental conditions prevailing in each compartment. Therefore, withdrawal of the produced excess sludge should be performed from each compartment or from the first compartment when an upflow reactor is used. Otherwise, a stable operation on the long term cannot be guaranteed because the voluminous acidifying sludge will eventually force out the extremely active acetogenic and methanogenic consortia in the subsequent compartments.

Liquid and gaseous fuels from biotechnology: challenge and opportunities

Abstract: This paper presents challenging opportunities for production of liquid and gaseous fuels by biotechnology. From the liquid fuels, ethyl alcohol production has been widely researched and implemented. The major obstacle for large scale production of ethanol for fuel is the cost, whereby the substrate represents one of the major cost components. Various scenarios will be presented for a critical assessment of cost distribution for production of ethanol from various substrates by conventional and high rate processes. The paper also focuses on recent advances in the research and application of biotechnological processes and methods for the production of liquid transportation fuels other than ethanol (other oxygenates; diesel fuel extenders and substitutes), as well as gaseous fuels (biogas, methane, reformed syngas). Potential uses of these biofuels are described, along with environmental concerns which accompany them. Emphasis is also put on microalgal lipids as diesel substitute and biogas/methane as a renewable alternative to natural gas. The capturing and use of landfill gases is also mentioned, as well as microbial coal liquefaction. Described is also the construction and performance of microbial fuel cells for the direct high-efficiency conversion of chemical fuel energy to electricity. Bacterial carbon dioxide recovery is briefly dealt with as an environmental issue associated with the use of fossil energy.

Chemo-autotrophic biogas purification for methane enrichment: mechanism and kinetics

Abstract: Off-gas from anaerobic digestion and landfills has significant potential as an alternative energy source. Current technologies to purify off-gas and increase its caloric value have been primarily limited to physicochemical methods. An alternative biological method has been proposed that increases the methane content. Through the use of a chemo-autotrophic methanogen (Methanobacterium thermoautotrophicum), uncoupled methanogenesis techniques and hollow fiber membranes, carbon dioxide is converted to methane and hydrogen sulfide is effectively removed from biological off-gases. This gas stream treatment process improves the quality and caloric value of the biogas. A continuous culture bench-scale system that utilizes hollow fiber membranes was employed to study the process. The gas-phase methane concentrations were found to increase from 60% to 96%.

Anaerobic digestion of municipal solid waste: Utility of process residues as a soil amendment

Abstract: Tuna processing wastes (sludges high in fat, oil, and grease [FOG]) and municipal solid waste (MSW) generated on Tutuila Island, American Samoa, represent an ongoing disposal challenge. The biological conversion of the organic fraction of these wastes to useful products, including methane and fertilizer-grade residue, through anaerobic high-solids digestion is currently in scale-up development. The suitability of the anaerobic digestion residues as a soil amendment was evaluated through extensive chemical analysis and greenhouse studies using corn as an indicator crop. Additionally, native Samoan soil was used to evaluate the specific application rates for the compost. Experiments established that anaerobic residues increase crop yields in direct proportion to increases in the application rate. Additionally, nutrient saturation was not demonstrated within the range of application rates evaluated for the Samoan soil. Beyond nutrient supplementation, organic residue amendment to Samoan soil imparts enhanced water- and nutrient-binding capacities.

Anaerobic Treatment of Eucalyptus Fiberboard Manufacturing Wastewater by a Hybrid USBF Lab-Scale Reactor

Abstract: A hybrid upflow sludge bed-filter anaerobic reactor (USBF) at lab-scale was used for the treatment of eucalyptus fiberboard manufacturing wastewater at 37oC. A physical-chemical pre - treatment adding a neutral polyelectrolyte was applied to remove inert solids present in the wastewater that may accumulate in the reactor. Chemical Oxygen Demand (COD) removals around 94% were obtained at an organic loading rate (OLR) of 14.8 kg COD m−3 d−1, corresponding to a hydraulic retention time (HRT) of 3 days. About 70% of COD was converted to methane, giving a volumetric yield of 2.6 m3 CH4 m−3 reactor d−1 and about 7.8 m3 CH4 m−3 wastewater. The biogas methane content was approximately 53%.