Development of a Nordic system for measuring the inactivation of pathogens during composting
Summary
This report presents the results of a study initiated to prepare a full-scale investigation for evaluating the inactivation of pathogens during composting of biodegradable waste by means of a direct process evaluation. In the direct process evaluation, the process is analysed by inoculating the raw material with selected pathogens or indicator organisms in bags. After a sanitary composting phase (typically 2-4 weeks), the bags are collected and analysed for the survival of the inoculated organisms.
The main preparations needed before the direct process evaluation is initiating, includes
 | Production or collection of the inoculum organisms Escherichia coli, Streptococcus faecalis, Plasmodiophora brassicae, Rhizoctonia solani, Fusarium oxysporum, and tobacco mosaic virus. |
| Evaluation of methods applied for identification of the organisms. |
| Evaluation of the survival of the organisms for 4 weeks under standard conditions. |
The indicator organisms for human and animal pathogens, Escherichia coli and Streptococcus faecalis, were propagated from commercially available strains to culture concentrations between 107 and 109 bacteria per mL. By inoculating the raw materials with 10-mL culture per 100-g waste, suitable concentrations of the inoculated organisms were obtained for the direct process evaluation. This was indicated by a germ concentration in the raw materials totally dominated by the inoculated organisms. Generally, good agreement was found between the germ concentration added to the raw materials and the measured concentrations when using the methods published by the Nordic Committee on Food Analysis (NMKL). Poor survival of the inoculated organisms was found when incubating the raw material in humid sand at 20° C for 4 weeks. The poor survival of the organisms may be a consequence of a relatively dry environment in the sand compared to the raw materials. It is therefore recommended to incubate in media with higher water-holding capacity when performing the full-scale control incubations.
The plant pathogens R. solani and F. oxysporum were propagated on wheat seeds, tobacco mosaic virus was propagated on tobacco plants and P. brassicae was collected in an infected red cabbage field. For all organisms biotests were developed for identification of the pathogens. P. brassicae was identified on mustard (Brassica juncea), R. solani on bean (Phaseolus vulgaris), F. oxysporum on tomato (Lycopersicon ecculentum), and tobacco mosaic virus on tobacco plants (Nicotiana tabacum). The concentrations of the pathogens were for all biotests quantified visually by various indexes. In addition to these indexes, commercially ELISA kits were evaluated for quantification of R. solani and tobacco mosaic virus. All the plant pathogens survived the 4-week incubation period under standard conditions in sand without problems.
On the basis of the presented preparation study, it is recommended to include the direct process evaluation in the Nordic full-scale investigation for evaluating the sanitary aspects of composting biodegradable waste.