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Title: Soil microbial interactions affecting enteric pathogen survival in sewage sludge-amended agricultural soil
Author: Perez Viana, Felipe
ISNI:       0000 0004 2688 7498
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2010
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The natural inactivation of enteric pathogens in soil is a critical component of the multi-barrier approach to prevent infectious disease in humans by enteric microorganisms when biosolids are used as a fertiliser and soil conditioner on agricultural land. The addition of biosolids to agricultural soil modifies the soil microbial community and ecological interactions. Ecological processes, especially the activities of predatory protozoa, may have a critical role in reducing the survival of enteric pathogenic bacteria when biosolids are applied to agricultural soil. To test this hypothesis a series of field experiments were established on two soils of contrasting organic matter content and fertility status, amended with different sludge types, to examine the interactions between the soil microbial biomass, total protozoa numbers, environmental and soil factors and their effects on the decay of the enteric indicator bacteria, Escherichia coli, in biosolids-amended soil. Soil microbial biomass carbon (SMBC) concentrations were influenced by soil physico-chemical properties and, in particular, larger background biomass concentrations were measured in unamended control soil containing the largest amount of organic matter. The microbiological content and substrate availability of the supplied materials also influenced the extent of the increases in SMBC. Soil protozoa numbers consistently increased in both experimental field soils from background values of 3-3.5 log10 g-1 ds to 4-4.5 log10 g-1 ds after sludge application. The extent of the increase was consistent with the effect of the organic amendments on SMBC. Laboratory investigations indicated the direct involvement of bacteriophagous protozoa activity in the soil ecological processes responsible for E. coli inactivation in biosolidsamended agricultural soil. This was linked to the addition of an active protozoa population to the soil in sludge, as well as to the stimulation of protozoa indigenous to the soil due to inputs of substrates and microbial biomass in sludge. Consequently, the survival of enteric organisms is a self-limiting process, due to the stimulation of microbial predatory activity in amended soil. Overall, the results provide assurance that assumptions relating to soil decay during waiting periods stipulated for agricultural use of sludge are highly conservative. They also confirm that the cropping/harvesting restrictions prescribed in legislation and guidance controlling the application of biosolids on farmland allow the natural attenuation of pathogens to protect human health with a significant margin of safety.
Supervisor: Smith, Stephen Sponsor: EPSRC ; DEFRA ; NFU ; UKWIR ; Anglian Water ; Scottish Water ; Thames Water ; Yorkshire Water
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral