| 1. |
- Åkesson, Mattias
(författare)
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Biogas generation in landfills : equilibria, rates & yields
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Landfilling in "cells" has become more common in recent years. Different waste streams are guided to different cells, among which the biocell is a landfill designed for biogas production. In this thesis, the dependence of biogas generation on waste composition was investigated. Six 8,000 m3 test cells, with contents ranging from mainly commercial waste to pure domestic waste and equipped with gas extraction systems and bottom plastic liners, were monitored for seven years. Great emphasis was given to the characterization of conversion processes and governing mechanism in the topics of bioenergetics, kinetics and capacities. A thermodynamic model, in which the oxidations of volatile fatty acids (VFA) (2< C<7) and hydrogenotrophic methanogenesis were assumed to equilibrate at a certain lower limit for energy conservation (dGmin), explained the relative distribution of VFA's observed in situ. dGmin ranged between -11 to -15 kJ/reaction and decreased with increasing levels of undissociated acetic acid, indicating the reduction of proton gradients over cytoplasmic membranes. Comparisons of methane production rates and internal conditions observed during a two year period, demonstrated that high biogas rates corresponded with low VFA levels. Rates obtained in test cells with mainly commercial waste were 13 - 19 Nm3 CH4/dry tonne,yr, whereas VFA levels ranged between 10 and 24 gO2/l. Corresponding values in domestic waste and food-rich waste fractions were 10 - 14 Nm3 CH4/dry tonne,yr and 18 - 77 gO2/l, respectively. This demonstrates that substrate inhibition of acetotrophic methanogenesis is one of the most important factors governing the rate of biogas generation, a notion supported by the findings from the thermodynamic model, and that the shift from acidogenic to methanogenic condition is not a discrete one, but rather a continuous transition. To explain the discrepancies between theoretical methane potentials and quantified yields (in this study found to be 150-200 and 40-70 Nm3/dry tonne, respectively), the possible nutritional limitation was investigated. Pools and emissions of chemical oxygen demand, N, P and K were quantified. Biomass pools were estimated from methane yields, growth yield coefficients, and bacterial mineral contents. However, results from commercial waste test cells showed that the assimilation of P exceeded the refuse content, which suggests the turnover of microbial biomass and questions the notion of nutritional limitation. In sum, the results showed that the advantages of a reduced content of readily biodegradable material, achieved by guidance or pretreatment, encompass several aspects of the performance.
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| 2. |
- Åkesson, Mattias
(författare)
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Estimation of Landfill Properties through Simulation
- 1998
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Ingår i: Waste Management & Research. - 1096-3669. ; 16:1, s. 63-71
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Tidskriftsartikel (refereegranskat)abstract
- The total content of dry substance (DS) and readily biodegradable material (RBM) in waste streams directed to six landfill test cells were estimated. These estimates were necessary for the evaluation of the performance of the test cells and were used to both express methane production rates on a dry matter basis, and to obtain a measurement related to acidogenesis. A hierarchial description of the test cell content was used for the calculation of DS and RBM. Test cells were treated as mixtures of different waste streams, and these, in turn, were divided into composite materials, each of which was assigned values of the emphasized properties; DS and RBM. A variety of information sources (entrance registrations, sortings, visual inspections and estimates of waste stream fractions) were utilized and a method, which included both matrix calculations and simulation, was developed which included the standard deviation (SD) of each fraction. In this way, SD values for the total content of DS and RBM could be quantified. Exaggerations of the SD of estimated fractions affected the results to only a limited extent since the dispersion was leveled out by the multitude of compounds. The SD of the total content of DS and RBM were, therefore, sufficiently low to produce useful results. In comparison to the moisture contents measured after the final covering of the test cells, the results indicate that the material was dried during construction.
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| 3. |
- Åkesson, Mattias, et al.
(författare)
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Material Dependence of Methane Production Rates in Landfills
- 1998
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Ingår i: Waste Management & Research. - 1096-3669. ; 16:2, s. 108-118
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Tidskriftsartikel (refereegranskat)abstract
- An extensive sampling programme has been conducted in six test cells since 1989. The test cells are located at the Spillepeng landfill in the City of Malmo in southern Sweden. Each test cell received a specific waste composition. The main objective of the project was to evaluate the dependence of biogas production on landfill waste composition. Methane production rates in test cells with high amounts of commercial waste were found to be between 14.3 and 19.0 m(3) tonne(-1) dry weight yr(-1). In test cells with domestic waste or other putrescibles-rich waste fractions the corresponding values were 10.7 to 13.4 m(3) tonne(-1) dry weight yr(-1). In the evaluation, information regarding density, time of filling, moisture content, temperature, concentration of volatile fatty acids, pH, ammonium and gas extraction, were compared with methane production rates. The main conclusion was that a waste mixture with a high amount of readily biodegradable material gives rise to a high internal concentration of volatile fatty acids, which in turn inhibits the methane production rate.
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| 4. |
- Åkesson, Mattias, et al.
(författare)
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Seasonal changes of leachate production and quality from test cells
- 1997
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Ingår i: Journal of Environmental Engineering. - 0733-9372. ; 123:9, s. 892-900
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Tidskriftsartikel (refereegranskat)abstract
- Seasonal changes oi leachate production and water-flux-dependent leachate quality from two test cells containing 4,000 t of refuse each were investigated. Pronounced seasonal dependence of the leachate production appeared three years after the final covering of the test cells. With seasonal dependent production, a number of leachate parameters showed water-Aux-dependent trends. The dependence was quantified with results from a five-month-long sampling program using correlation coefficients. The magnitude of changes appeared to indicate alternation between methanogenic and acidogenic phases. Reasons for the la-ge variations observed were sought with the assumption that different biochemical conditions dominated in different locations in the test cells at the same time, Hypothetical configurations with various biochemical conditions In different locations were constructed and evaluated, Results from a gas extraction program, measurements of leachate production rates, and analyses of leachate and solid waste samples were used to evaluate the configurations, The evaluation identified two mechanisms as the origins of the observed phenomena: (1) biological filtration in the drainage layer; and (2) annual successive wetting of the waste material.
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