Relative risk for human illness of biogas effluent use in horticulture at small-scale pig farms in northern Vietnam

Treatment of animal manure in small-scale biogas systems are spreading rapidly in developing countries like Vietnam. The anaerobic fermentation breaks down solid matter and transforms it into methane which can be used for cooking and generation of light. Other benefits include a high-quality fertilizer effluent, reduction of problems with mal odour and a potential also to treat human waste products. Often the hygiene and health aspects of handling and digesting these organic wastes are unknown and the promotion of biogas technologies does rarely consider hygienic aspects. The aim of the current study was therefore to establish simple hygiene models for Vietnamese small-scale farmers that could describe the relative health risks associated with management of manure and consumption of the fertilized crop when using; i) fresh manure, ii) stored manure or iii) manure processed in the biogas plants. The hygiene models were developed based on information collected during interviews and observations of Vietnamese farmers operating biogas digesters as well as from the literature. Rather than calculating the specific risk for one person to become infected when handling a specific type of manure, we established hygiene models to calculate the relative risks of infection with the two model pathogens, Salmonella Typhimurium and Ascaris, allowing a comparison of risks for the different manure handling systems. Results showed that there was ten times higher risk of a human S. Typhimurium infection when handling fresh manure or composted manure as compared to handling of manure treated in a biogas system. In contrast, the risk for infection with the more resistant Ascaris was equivalent for all three manure handling systems. There is an urgent need to document the hygiene aspects of biogas systems developed and promoted to farmers in developing counties. Thus, further studies are needed on human exposure when handling animal manure and human excreta and pathogen survival in biogas systems as such information is essential to further refine the hygiene models developed and to formulate hygiene guidelines for biogas systems.