MECHANISMS AND KINETICS OF PATHOGEN INACTIVATION IN BIOSOLIDS-AMENDED AGRICULTURAL SOIL
Stephen R. Smith, James Cass, Felipe Perez-Viana and Michael Rogers
Centre for Environmental Control and Waste Management, Department of Civil and
Environmental Engineering, Imperial College London
(free)
Protecting human health from infectious enteric disease is central to the controls and
management practices adopted when sewage sludge is used as a fertiliser and soil
conditioner on agricultural land. Taking advantage of the natural inactivation of enteric
pathogens in soil is one of the multi-barriers in place to prevent enteric microorganisms
causing infections in humans. Ecological processes may have a critical role in reducing
enteric bacteria, due to microbial predation, and this could be important for the destruction of
enteric organisms when biosolids are applied to agricultural soil.
To test this hypothesis, and to provide long-term decay information on a variety of enteric
pathogens, a series of field experiments have been established on two soils of contrasting
organic matter and fertility status, amended with different sludge types. Inoculation treatments
with a non-VTEC strain of E. coli O157, Salmonella enterica, Campylobacter jejuni, and
Listeria monocytogenes were also monitored. E. coli were indigenous to both soil types and
numbers fluctuated dynamically between 1-6 log10 100 g-1 dry soil (ds). Following application
of conventionally treated biosolids, E. coli numbers increased to approximately 5-7 log10 100
g-1 ds. E. coli numbers subsequently decreased by 2 log10 100 g-1 ds within 30 days. E. coli
content of enhanced treated biosolids was < 1 log10 100 g-1 ds. However, E. coli numbers in
soil treated with enhanced biosolids increased by 0.5-2 log10 100 g-1 ds compared to the
unamended controls, and it is suggested this is due to the influence of the increased
availability of substrate resources on the growth of the indigenous E. coli population in soil. E.
coli, E. coli O157, Salmonella and Listeria numbers generally declined to background or nondetectable concentrations within 60 – 160 days of sludge incorporation. Campylobacter
numbers were low throughout the monitoring period and were geneally equivalent to the
unamended control. Protozoa populations were measured in the same field experiments
using a novel bacteria-prey, fluoresence technique.
The unamended controls contained approximately 3.5 log10 g-1 ds of protozoa and sludge
application consistently increased the size of the protozoa population overall by 0.5-1.0 log10
g-1 ds. The results suggest that sludge application may potentially increase bacterial grazing
by protozoa, thus reducing the survival of enteric bacteria applied in sludge. Consequently,
the survival of enteric organisms may be a self-limiting process, due to the stimulation of soil
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 and 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.
KEYWORDS
Agriculture, biosolids, enteric bacteria, sewage sludge, soil, survival, pathogens
