Browsing by Author "O'Leary, Emma"

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    Proportion of sewage sludge to soil influences the survival of Salmonella Dublin, and Escherichia coli
    (2018-01-27) Ellis, Stephanie; Tyrrel, Sean; O'Leary, Emma; Richards, Karl; Griffiths, Bryan; Ritz, Karl
    The survival of enteric pathogens in sewage sludge could lead to their transferral into the soil environment and subsequent contamination of crops and water courses. This, in turn, could increase the potential spread of gastrointestinal disease. This work aimed to determine the persistence of several microorganisms, co-introduced with sewage sludge, when exposed to varying proportions of sewage sludge to soil. Three microcosm-based studies were established, inoculated with Salmonella Dublin or an environmentally-persistent strain of Escherichia coli (quantified periodically over a period of 42 days), or indigenous sewage sludge E. coli (quantified over a period of 56 days). Treatments consisted of a mixture containing: 0, 15, 25, 50, 75 and 100% soil or sludge, depending upon the experiment. Each introduced microorganism declined significantly over time, with greater quantities of soil generally instigating greater die-off particularly in the cases of environmentally-persistent E. coli and S .Dublin. However, this relationship was not proportionally related as sludge/soil mixtures showed greater declines than pure soil treatments. In contrast, indigenous sewage sludge E. coli had a more consistent decline across all treatments. This may indicate that indigenous strains are more resilient and may be indicative of natural behaviour. Moreover, the effects of soil-borne factors on pathogen attenuation were context dependent and non-linear, possibly arising from the relative spatial distribution of introduced sludge and attendant microbes in soil.
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    Risk assessment of E. coli survival up to the grazing exclusion period after dairy slurry, cattle dung, and biosolids application to grassland
    (Frontiers, 2018-07-10) Ashekuzzaman, S. M.; Richards, Karl; Ellis, Stephanie; Tyrrel, Sean; O'Leary, Emma; Griffiths, Bryan; Ritz, Karl; Fenton, Owen
    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.