Xavier Lefebvre

Combined thermophilic aerobic process and conventional anaerobic digestion: Effect on sludge biodegradation and methane production

The efficiency of hyper-thermophilic (65 degrees Celsius) aerobic process coupled with a mesophilic (35 degrees Celsius) digester was evaluated for the activated sludge degradation and was compared to a conventional mesophilic digester. For two Sludge Retention Time (SRT), 21 and 42 days, the Chemical Oxygen Demand (COD) solubilisation and biodegradation processes, the methanisation yield and the aerobic oxidation were investigated during 180 days. The best results were obtained at SRT of 44 days; the COD removal yield was 30% higher with the Mesophilic Anaerobic Digestion/Thermophilic Aerobic Reactor (MAD-TAR) co-treatment. An increase of the sludge intrinsic biodegradability is also observed (20-40%), showing that the unbiodegradable COD in mesophilic conditions becomes bioavailable. However, the methanisation yield was quite similar for both processes at a same SRT. Finally, such a process enables to divide by two the volume of digester with an equivalent efficiency.

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.

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.

Effect of the Grease Solubilization and the Optimal Process Monitoring on the Grease Aerobic Digestion

Placement of lipid residues in sanitary landfills are now prohibited by French legislation. The lipid residues used in this work were collected from a wastewater plant and restaurant traps. Aerobic degradation of these mixtures by a mixed population originated from an activated sludge was studied. First, the lipid matter underwent a saponification reaction, which solubilized partially and emulsified the substrate. An activated sludge was acclimated to saponified grease carbon source in a sequencing batch culture mode. Microbial activity was evaluated by the on line measurement of the CO2 production rate in the exit gas. At a feed concentration of 2 gHEM/1, the substrate degradation rate was measured as high as 1.6 gHEM/1 /h, the COD and HEM removals were 75% and 90%, respectively. Finally the average biomass yield was 0.3 gMLSS/gHEM.