Cas Js, Rogers M and Smith S R.
Centre for Environmental Control and Waste Management, Department of Civil and Environmental
Engineering, Imperial College London,


The decay of microorganisms in soil following land application of biosolids is a well-established principle in the multi-barrier approach to controlling the dispersal of infectious disease from the agricultural use of sewage sludge. However, the mechanisms responsible for the inactivation of enteric bacteria in sewage sludge amended soil have yet to be adequately quantified. To further this understanding, a field trial was established in April 2005 on two sites with differing soil characteristics, including a silty clay soil (Northside Lands) and the other a sandy silt loam (Brices Field), based in the south east of England. Escherichia coli numbers were recorded in unamended control soil and soil amended with biosolids and livestock manures. Numbers recorded for the control soils were generally in the range 4 – 6 log10 100 g-1 ds initially and declined to 2 log10 100 g-1 ds in the summer period, increasing again to 3 – 4 log10 100 g-1 ds by mid-September.

Following a rapid decay of organisms added in conventionally treated dewatered digested sludge (DMAD), equivalent to approximately 99 % of the initial population, a very small community of bacteria, corresponding to 100 cells g-1 ds, survived for a longer period compared to the smallest control values measured during the summer period, but were within the range of the control results overall. E. coli also decayed to the same extent in soil receiving raw sludge (DRAW), but the pattern was less consistent compared to DMAD and large fluctuations were observed in the population of indicator organisms in DRAW amended soil. Lime-treated cake (LC) and thermally dried (TDMAD) biosolids did not contribute E. coli to the soil population, but there was evidence during the latter monitoring period that these sludge types influenced the behaviour of the indigenous community of E. coli as numbers were consistently increased by approximately 1.0 log10 100 g-1 ds in some cases compared to the background population in unamended soil.

By contrast, farmyard manure (FYM), which contained more E. coli than either of these enhanced-treated products, but less than the soil when it was applied, had no apparent effect on background numbers. The results are interpreted in context of soil ecological processes and interactions that may have an important bearing on the dynamics of enteric bacteria survival in agricultural soil and in relation to the management of biosolids and livestock manures as agricultural soil amendments.

KEY WORDS Agriculture, Biosolids, Decay, Escherichia coli, Pathogen, Recycling, Sewage sludge, Soil.

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