For economic, agricultural, and environmental reasons, composting is frequently used for organic waste recycling. One approach to limiting the potential risk from bacterial food-borne illnesses is to ensure that soil amendments and organic fertilizers are disinfected. However, more knowledge concerning the microbiological safety of composted substrates other than sludge and manure is necessary. Experimental in-vessel biowaste composts were used to study the survival of seeded Listeria monocytogenes, Salmonella enterica subsp. enterica serotype Enteritidis, and Escherichia coli. Four organic waste mixtures, containing various proportions of paper and cardboard, fruits and vegetables, and green waste, were composted in laboratory reactors with forced aeration. The physicochemical and microbiological parameters were monitored for 12 weeks during composting. The survival of bacteria over a 3-month period at 25°C was assessed with samples collected after different experimental composting times. Strain survival was also monitored in mature sterilized composts. Nonsterile composts did not support pathogen growth, but survival of seeded pathogens was observed. Salmonella serovar Enteritidis survived in all composts, and longer survival (3 months) was observed in mature composts (8 and 12 weeks of composting). Mature biowaste composts may support long-term survival of Salmonella serovar Enteritidis during storage at room temperature. E. coli and L. monocytogenes survival was observed only in 4-week-old composts and never in older composts. Proper composting may prevent long-term survival of E. coli and L. monocytogenes. These results suggest that like composted sewage sludge or manure, domestic waste composts may support pathogen survival. Survival was not related to the physicochemical characteristics of the composts.The presence of pathogens in food and outbreaks of foodassociated diseases can be linked to the poor microbial quality of raw agricultural products (5, 9). Furthermore, the transfer of microbial pathogens, such as Listeria monocytogenes, Escherichia coli O157:H7, and Salmonella enterica serovar Typhimurium, from amended soil to fresh agricultural products has been described (3,24,37). Composting is a time-honored biological treatment used for organic wastes; the resulting composts are useful in agriculture and horticulture for improving many soil attributes. However, microbial safety of organic amendments and fertilizers used in agriculture is required to prevent colonization by food-borne illness pathogens, like E. coli O157:H7 and Salmonella spp. (9). In order to reduce the likelihood of pathogen transfer from waste to fresh produce, several international standards for meeting pathogen limits through composting are available (14, 39). According to the U.S. Environmental Protection Agency (US-EPA) standard, class A composts must not exceed maximal limits for either Salmonella spp. (less than 3 most probable numbers [MPN]/4 g) or thermotolerant coliforms (less than 1,000 MPN/g) (39). A draft of a forthcomin...

Editor: Michael WagnerKeywords municipal solid waste; methanization; stable carbon isotopic signature; automated ribosomal intergenic spacer analysis; FISH. AbstractReconstituted municipal solid waste (MSW) with varying contents of putrescible and cellulosic waste was incubated anaerobically under mesophilic conditions. Standard physicochemical parameters were monitored, together with stable isotopic signatures of produced CH 4 and CO 2 . d13 C values for CH 4 indicated a change of methanogenic metabolism with time. CH 4 was predominantly produced from H 2 /CO 2 at the beginning of the incubations. This period was associated with important shifts in archaeal communities monitored by automated ribosomal intergenic spacer analysis (ARISA) and FISH of oligonucleotidic probes targeting specifically 16S rRNA gene of various methanogenic groups. The onset of the active methane generation phase was characterized by an increase of CH 4 d 13 C, indicating a progressive shift toward an aceticlastic metabolism. When the methane production levelled off, a decrease in the isotopic signature was observed toward values characteristics of hydrogenotrophic metabolism. ARISA profiles were, however, found to be stable from the beginning of the active methane generation phase until the end of the experiment. FISH observation indicated that members of the family Methanosarcinaceae were predominant in the archaeal community during this period, suggesting that these methanogens might exhibit a high metabolic versatility during methanization of waste.

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Taking into account isotope (13)C value a mathematical model was developed to describe the dynamics of methanogenic population during mesophilic anaerobic digestion of putrescible solid waste and waste imitating Chinese municipal solid waste. Three groups of methanogens were considered in the model including unified hydrogenotrophic methanogens and two aceticlastic methanogens Methanosaeta sp. and Methanosarcina sp. It was assumed that Methanosaeta sp. and Methanosarcina sp. are inhibited by high volatile fatty acids concentration. The total organic and inorganic carbon concentrations, methane production, methane and carbon dioxide partial pressures as well as the isotope (13)C incorporation in PSW and CMSW were used for the model calibration and validation. The model showed that in spite of the high initial biomass concentration of Methanosaeta sp. Methanosarcina sp. became the dominant aceticlastic methanogens in the system. This prediction was confirmed by FISH. It is concluded that Methanosarcina sp. forming multicellular aggregates may resist to inhibition by volatile fatty acids (VFAs) because a slow diffusion rate of the acids limits the VFA concentrations inside the Methanosarcina sp. aggregates.

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Similar evolution was obtained for the stable carbon isotope signatures delta (13)CH(4) and the model-predicted relative rate of aceticlastic methanogenesis during mesophilic methanization of municipal solid wastes. In batch incubations, the importance of aceticlastic and hydrogenotrophic methanogenesis changes in time. Initially, hydrogenotrophic methanogenesis dominated, but increasing population of Methanosarcina sp. enhances aceticlastic methanogenesis. Later, hydrogenotrophic methanogenesis intensified again. A mathematical model was developed to evaluate the relative contribution of hydrogenotrophic and aceticlastic pathways of methane generation during mesophilic batch anaerobic biodegradation of the French and the Chinese Municipal Solid Wastes (FMSW and CMSW). Taking into account molecular biology analysis reported earlier three groups of methanogens including strictly hydrogenotrophic methanogens, strictly aceticlastic methanogens (Methanosaeta sp.) and Methanosarcina sp., consuming both acetate and H(2)/H(2)CO(3) were considered in the model. The total organic and inorganic carbon concentrations, methane production volume, methane and carbon dioxide partial pressures values were used for the model calibration and validation. Methane isotopic composition (delta (13)CH(4)) evolution during the incubations was used to independently validate the model results. The model demonstrated that only the putrescible solid waste was totally converted to methane.