During anaerobic treatment of organic waste most of the carbon is converted to biogas while the nutrients are retained, making the sludge an excellent fertiliser. However, the value of the sludge can be reduced by the presence of biological or chemical contaminants. Organic wastes are likely to contain many different types of bacteria, viruses, fungi and even parasites (Déportes et al. 1995). Before the anaerobic treatment process, the waste is heated in order to kill pathogenic micro-organisms. However, full control of the heating process is not possible for the entire mass of waste, hence the waste material not always is completely disinfected by this method. Furthermore, some micro-organisms (especially spore forming) might survive even a successful heating procedure and finally end up in the sludge. Presence of biological contaminants can expose people handling the waste or sludge to health hazards or can cause other problems related to the production of food and feed from soil amended with the sludge.
During this investigation we studied the presence and survival of spore forming microorganisms, six different fungi and Clostridium tyrobuturicum, during anaerobic treatment of organic household waste. The fungi included in the study were either commonly found in different types of compost processes, extremely thermotolerant, 'microaerophilic', known to produce mycotoxins or to cause allergic reactions. The selected species were; Aspergillus flavus, Aspergillus fumigatus, Penicillium roqueforti, Rizomuccor pusillus, Thermoascus crustaceus and Thermomyces lanuginosus. To date, most concern about fungi and waste has centred on the genus Aspergillus during composting processes. Various members of the genus Aspergillus, including A. fumigatus and A. flavus, have been isolated from different composting systems and high levels of fungal spores have been measured in the air close to compost systems (Millner et al 1980, Fischer et al 1998). Furthermore, several investigations suggest that workers involved in the compost process had fungi infections and allergic responses caused by aerosols of fungi spores to a higher degree than other workers (Clark et al 1984, Fischer et al 1998).
Clostridium tyrobutyricum is a gram-positive, spore forming anaerobic bacterium commonly found in soil, manure, sewage, sediments, decaying vegetation etc. It ferments different carbohydrates and organic acids to butyric acid, carbon dioxide and hydrogen. C. tyrobutyricum is causing deterioration of silage by consuming lactic acid, thereby increasing pH and allowing spoilage organisms to grow. Studies have shown that the number of this bacte-rium were higher in silage produced from soil subjected to repeated application of liquid manure than from unfertilized soil (Rammer and Lingvall, 1997). Furthermore, C. tyrobutyricum is causing problems not only for the farmers but also for the milk industry. It is considered to be the principal organism responsible for the late spoilage of brinesalted semi-hard and hard cheeses (Klinj et al. 1995). After the cow has been feed with silage the bacterium passes through the digestive tract without germination. However, during the digestion the spores are concentrated about 10 times (Stadhouders et al. 1985). At milking, dung or silage contaminates the milk and at the dairy the spores survive pasteurisation, germinate and multiply in the cheese producing butyric acid and gas. The butyric acid causes bad taste of the cheese and the gas causes the so called 'late-blowing effect'. C. tyrobutyricum is causing great economical looses for the diary industry.
The aim of the present investigation was to identify and enumerate Clostridium tyrobutyricum in household waste and the sludge produced after anaerobic treatment of the waste as well as to study the survival of six different fungi during the anaerobic degradation of the waste. The results will allow further evaluation of the potential risks with the anaerobic treatment of waste and the use of the produced sludge as a fertiliser.