An improved hydrodynamic model for percolation and drainage dynamics for household and agricultural waste beds.

This study focuses on the hydrodynamic modelling of percolation and drainage cycles in the context of solid-state anaerobic digestion and fermentation (VFA platform) of household solid wastes (HSW) in leach bed reactors. Attention was given to the characterization of the water distribution and hydrodynamic properties of the beds. The experimental procedure enabled the measurement of water content in waste beds at different states of compaction during injection and drainage, and this for two types of HSW and for two other type of wastes. A numerical model, set up with experimental data from water content measurements, highlighted that a capillary-free dual-porosity model was not able to correctly reproduce all the hydrodynamic features and particularly the drainage dynamics. The model was improved by adding a reservoir water fraction to macroporosity which allowed to correctly simulate dynamics. This model, validated with data obtained from agricultural wastes, enabled to explain more precisely the water behaviour during percolation processes and these results should be useful for driving either solid-state anaerobic digestion or fermentation reactors. Indeed, this implies that the recirculation regime will impact the renewal of the immobile water fraction in macroporosity, inducing different concentration levels of fermentation products in the leachate.

Towards a standardization of biomethane potential tests.

Production of biogas from different organic materials is a most interesting source of renewable energy. The biomethane potential (BMP) of these materials has to be determined to get insight in design parameters for anaerobic digesters. Although several norms and guidelines for BMP tests exist, inter-laboratory tests regularly show high variability of BMPs for the same substrate. A workshop was held in June 2015, in Leysin, Switzerland, with over 40 attendees from 30 laboratories around the world, to agree on common solutions to the conundrum of inconsistent BMP test results. This paper presents the consensus of the intense roundtable discussions and cross-comparison of methodologies used in respective laboratories. Compulsory elements for the validation of BMP results were defined. They include the minimal number of replicates, the request to carry out blank and positive control assays, a criterion for the test duration, details on BMP calculation, and last but not least criteria for rejection of the BMP tests. Finally, recommendations on items that strongly influence the outcome of BMP tests such as inoculum characteristics, substrate preparation, test setup, and data analysis are presented to increase the probability of obtaining validated and reproducible results.

Assessment of percolation through a solid leach bed in dry batch anaerobic digestion processes.

This work aimed at assessing water percolation through a solid cow manure leach bed in dry batch AD processes. A laboratory-scale percolation column and an experimental methodology were set up. Water behaviour was modelled by a double porosity medium approach. An experimental procedure was proposed to determine the main hydrodynamic parameters of the multiphase flow model: the porosity, the permeability and the term for water exchange from macro- to micro-porosity. Micro- and macro-porosity values ranged from 0.42 to 0.70 m(3) m(-3) and 0.18 to 0.50 m(3) m(-3). Intrinsic permeability values for solid cow manure ranged from 5.55·10(-11) to 4.75·10(-9) m(2). The term for water exchange was computed using a 2nd order model. The CFD tool developed was used to simulate successive percolation and drainage operations. These results will be used to design leachate recirculation strategies and predict biogas production in full-scale dry AD batch processes.

Analysis of the outcome of shredding pretreatment on the anaerobic biodegradability of paper and cardboard materials.

Paper and cardboard stand for the major biodegradable organic fraction of most of municipal solid waste (MSW). This article aims at discussing the possible positive impact of a thin shredding of this fraction on its biodegradability under mesophilic anaerobic conditions, either for landfilling or for digestion in industrial reactors. For that purpose, BMP tests were performed on two types of paper and cardboard mixtures: one sorted from a complex landfill French MSW income, one built from source separated papers and cardboards. For both of these substrates, comparison was made between assays on large pieces of waste and assays on tiny shredded waste (powder particles of less than 1mm diameter). For the second substrate, assays at two different inoculation levels were performed. All results are discussed both in terms of maximal methane conversion yields and in terms of kinetic rates. The main conclusion is that shredding does not improve methane potential of paper and cardboard, neither the biogas production rates. This leads the authors to put forward the hypothesis that shredding does not significantly either increase enzyme accessibility to cellulose nor favor the surface bacterial colonization, although it strongly affects the macrostructure of the waste.