Risk Assessment of E. coli Survival Up to the Grazing Exclusion Period After Dairy Slurry, Cattle Dung, and Biosolids Application to Grassland

S M Ashekuzzaman,Bryan Griffiths,Karl Ritz,Emma O'Leary,Karl Richards,Stephanie Ellis,Sean Tyrrel,Owen Fenton

doi:10.3389/fsufs.2018.00034

Abstract

Grassland application of dairy slurry, cattle dung and biosolids offers an opportunity to recycle valuable nutrients (N, P and K), which may all introduce pathogens to the soil environment. Herein, a temporal risk assessment of the survival of Escherichia coli (E. coli) up to 40 days in line with the legislated grazing exclusion time points after application was examined across six scenarios: (1) soil and biosolids mixture, (2) biosolids amended soil, (3) dairy slurry application, (4) cattle dung on pasture, (5) comparison of scenario 2, 3 and 4, and (6) maximum legal vs. excess rate of application for scenario 2 and 3. The risk model input parameters were taken or derived from regressions within the literature and an uncertainty analysis (n=1000 trials for each scenario) was conducted. Scenario 1 results showed that E. coli survival was higher in the soil/biosolids mixture for higher biosolids portion, resulting in the highest 20 day value of residual E. coli concentration (i.e. C20, log10CFU g−1dw) of 1.0 in 100% biosolids or inoculated soil and the lowest C20 of 0.098 in 75/25 soil/biosolids ratio, respectively, in comparison to an average initial value of ~6.4 log10CFU g−1dw. The E. coli survival across scenario 2, 3 and 4 showed that the C20 value of biosolids (0.57 log10CFU g−1dw) and dairy slurry (0.74 log10CFU ml−1) was 2.9-3.7 times smaller than that of cattle dung (2.12 log10CFU g−1dw). The C20 values of biosolids and dairy slurry associated with legal and excess application rates ranged from 1.14 to 1.71 log10CFU ha−1, which is a significant reduction from the initial concentration range (12.99 to 14.83 log10CFU ha−1). The E. coli survival in un-amended soil was linear with a very low decay rate resulting in a higher C20 value than that of biosolids or dairy slurry. The risk assessment and uncertainly analysis showed that the residual concentrations in biosolids/dairy slurry applied soil after 20 days would be 45−57% lower than that of the background soil E. coli concentration. This means the current practice of grazing exclusion times is safe to reduce the risk of E. coli transmission into the soil environment.

Highlights

Provision of a circular economy safeguards against volatile fertilizer prices, global diminishing resources and an increased demand for food (Heffer and Prud’homme, 2013)
The main objective of this study was to assess the risk of E. coli survival as an indication of the risk associated with land spreading biosolids to agricultural soils within the context of legislated grazing exclusion times
12 relevant papers were utilized to generate the data under five categories— (1) un-amended soil, (2) E. coli spiked soil, (3) biosolids, (4) dairy slurry, and (5) cattle dung

Summary

Introduction

Provision of a circular economy safeguards against volatile fertilizer prices, global diminishing resources (e.g. synthetic fertilizers, fossil fuel) and an increased demand for food (Heffer and Prud’homme, 2013). In the European Union (EU), the Landfill Directive (EC, 1999) promoted a circular economy by targeting an 85% reduction in the disposal of sewage sludge to landfill by 2014 from 1995 levels. Such an ambitious target was aided by the Sewage Sludge Directive (EEC, 1986), which directed a major proportion of sewage sludge to land (Lucid et al, 2013; Fijalkowski et al, 2017). The negatives can be heavy metal bioaccumulation, runoff losses of nutrient, metal, enteric pathogens and emerging contaminants, and bio-transfer of persistent organic pollutants to the food chain (Healy et al, 2016a,b, 2017; Clarke et al, 2017, 2018; Fijalkowski et al, 2017)

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Results