Assessing the probability of infection by Salmonella due to sewage sludge use in agriculture under several exposure scenarios for crops and soil ingestion

Abstract

A deeper understanding about the risks involved in sewage sludge practice in agriculture is required. The aims of the present study were to determine the annual risk of infection of consuming lettuce, carrots and tomatoes cultivated in soil amended with sewage sludge. The risk to agricultural workers of accidental ingestion of sludge or amended soil was also investigated.A Quantitative Microbial Risk Assessment was conducted based on Salmonella concentrations from five WWTPs were used to estimate the probability of annual infection associated with crops and soil ingestion. The risk of infection was estimated for nine exposure scenarios considering concentration of the pathogen, sewage sludge dilution in soil, variation of Salmonella concentration in soil, soil attachment to crops, seasonal average temperatures, hours of post-harvesting exposure, Salmonella regrowth in lettuce and tomatoes, Salmonella inhibition factor in carrots, crop ingestion and frequency of exposure, sludge/soil ingestion by agricultural workers and frequency of exposure. Annual risks values varied across the scenarios evaluated. Highest values of annual risk were found for scenarios in which the variation in the concentration of Salmonella spp. in both soil and crops (scenario 1) and without variation in the concentration of Salmonella spp. in soil and variation in crops (scenario 3) ranging from 10−3 to 10−2 for all groups considered. For agricultural workers, the highest annual risks of infection were found when workers applied sewage sludge to agricultural soils (2.26×10−2). Sensitivity analysis suggests that the main drivers for the estimated risks are Salmonella concentration and ingestion rate. These risk values resulted from conservative scenarios since some assumptions were derived from local or general studies. Although these scenarios can be considered conservative, the sensitivity analysis yielded the drivers of the risks, which can be useful for managing risks from the fresh products chain with stakeholders' involvement.