Assessing the microbial risk of faecal sludge use in Ugandan agriculture by comparing field and theoretical model output

G Butte,C Niwagaba,A Nordin

doi:10.1016/j.watres.2021.117068

Abstract

Reuse of faecal sludge in agriculture has many potential benefits, but also poses risks to human health. To better understand the potential risks, Quantitative Microbial Risk Assessment (QMRA) was performed for three population groups in Kampala, Uganda: wastewater and faecal sludge treatment plant workers; farmers using faecal sludge; and consumers of faecal sludge-fertilised vegetables. Two models were applied for farmers and consumers, one based on pathogen concentrations from field sampling of sludge, soils and vegetables, and one based on theoretical pathogen contribution from the last sludge application, including decay and soil to crop transfer of pathogens. The risk was evaluated for two pathogens (enterohaemorrhagic E. coli (EHEC) and Ascaris lumbricoides). The field data on sludge, soil and vegetables indicated that the last application of faecal sludge was not the sole pathogen source. Correspondingly, the model using field data resulted in higher risks for farmers and consumers than the theoretical model assuming risk from sludge only, except when negligible for both. For farmers, the yearly risk of illness, based on measured concentrations, was 26% from EHEC and 70% from Ascaris, compared with 1.2% and 1.4%, respectively, considering the theoretically assumed contribution from the sludge. For consumers, the risk of illness based on field samples was higher from consumption of leafy vegetables (100% from EHEC, 99% from Ascaris) than from consumption of cabbages (negligible for EHEC, 26% from Ascaris). With the theoretical model, the risk of illness from EHEC was negligible for both crops, whereas the risk of illness from Ascaris was 64% and 16% for leafy vegetables and cabbage, respectively. For treatment plant workers, yearly risk of illness was 100% from EHEC and 99.4% from Ascaris. Mitigation practices evaluated could reduce the relative risk by 30-70%. These results can help guide treatment and use of faecal sludge in Kampala, to protect plant workers, farmers and consumers.

Highlights

Inadequate sanitation, estimated to cause 432,000 deaths from diarrhoea annually, is a major factor in several neglected tropical diseases and contributes to malnutrition (Murray et al, 2012; Prüss-Ustün et al, 2014)
The aim of the present study was to assess quantitatively the microbial risk related to treatment (WWFSTP workers) and reuse of treated faecal sludge from the Lubigi plant, based on pathogen concentrations measured along the faecal sludge management service chain
Drying of the sludge lowered the concentration of Ascaris eggs, with rather high variation in concentration (Fig. 2), which may reflect variations in raw sludge or indicate that the sludge had been subjected to different storage times

Summary

Introduction

Inadequate sanitation, estimated to cause 432,000 deaths from diarrhoea annually, is a major factor in several neglected tropical diseases and contributes to malnutrition (Murray et al, 2012; Prüss-Ustün et al, 2014). Provision of sanitation facilities does not necessarily mean that human excreta are safely managed. It is estimated that in SSA, up to 80% of the total population is served by sanitation facilities that do not safely manage excreta, which in some cases are dumped in the surrounding environment (WHO/UNICEF, 2019). An excreta flow diagram created for Kampala in 2016 (Schoebitz et al, 2016) indicated that 78% of excreta ends up as faecal sludge, of which 22% is contained and treated. This suggests that there is significant scope for improving faecal sludge management

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