DECAY OF ESCHERICHIA COLI IN SOIL FOLLOWING THE APPLICATION OF BIOSOLIDS TO AGRICULTURAL LAND

Abstract

INTRODUCTIONThe natural decay of enteric micro-organisms in soil following the application of sewage sludge, or any other type of faecal manure including livestock wastes, provides a final environmental barrier to the potential transmission of infectious disease. Understanding the survival characteristics of pathogenic micro-organisms in soil is therefore important for managing the potential microbiological risk to human and animal health associated with the use of biosolids as soil amendments in agriculture.In general, enteric organisms are poorly adapted to survival in the environment, and pathogens which are introduced to soil (in sewage sludge) are influenced by climatic and agronomic variables. Soil and environmental factors (including moisture, temperature, sunlight, competitive organisms, nutrients and type of soil) and the method and timing of sludge application, influence the decay of faecal micro-organisms in biosolids treated soil. Crops which are ready to eat from the field are not produced in the UK on recently amended soil, and specified minimum harvesting periods apply to these and other types of crop to allow the natural decay of enteric organisms to control the potential risk of infection".2'.The harvesting restrictions for agricultural land receiving biosolids are matched to the degree of pathogen removal which is achieved by different sludge-treatment processes. Conventionally treated sludge is regarded as having undergone stabilisation by a range of defined treatment methods (e.g. mesophilic anaerobic digestion), to ensure at least a 2 login removal of E. coli, and should contain less than lo5 E. coli per g of dry solids (DS) l3) . (Note: l ogm is used throughout the paperJ The absence of epidemiological evidence linking disease outbreaks in the human populationI4l, or in farm live~tock'~' from the agricultural use of sewage sludge, strongly supports the role and effectiveness of these dual barriers in preventing the potential spread of infectious diseases from re-use on farmland.Enhanced treatment is regarded as virtually eliminating the pathogenic content of sludge, and less-stringent management restrictions are applied when this category of biosolids is spread on farmland, compared with conventionally treated material. However, it could be argued that cropping, planting or harvesting restrictions are unnecessary for this type of biosolids product, because the sludge-treatment process provides a critical control point for managing the microbiological risk from land application. Nevertheless, a minimum 10-month harvesting restriction is a precautionary requirement of the 'safe sludge matrix' for enhanced treated biosolids which are used in fruit, salad and vegetable cultivation and other horticultural applications, consistent with the waiting period which is stipulated in the DirectiveI6'. The time intervals which are required before certain crops can be grown after sludge application have been established on a precautionary basis. However, there are relatively few published data describing ...

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