Quantitative exposure of root crops to indicator enterobacteria from composted spent broiler litter under sub-tropical environment

The study aimed to quantify and compare the incremental exposure of root crops, at point of harvest, to enteric pathogens from untreated vs. composted spent broiler litter (SBL)/bagasse mix in field-crop application. An exposure assessment based on the Source–Pathway–Receptor approach was developed for bacterial indicator species, total coliforms, faecal coliforms, Escherichia coli and faecal enterococci. Event trees were constructed to model the pathways leading to the partitioning of pathogens present in the SBL blend during composting and after land application. The main barriers are induction of composting, high-rate thermophilic phase, maturation phase, and, decay and dilution of the indicator pathogens in the soil. The computed exposures have been expressed in terms of the arithmetic mean. TC, FC, E. coli and FE levels on root crops were reduced to very remote fractions of 0.01826, 0.00046, 0.000132 and 0.000013 kg−1, respectively. The degree of by-pass (π) of the treatment at operational scale showed that less than 1-log reduction has been by-passed during each turning event, revealing the effectiveness of turning for process control. The predicted E. coli counts on root crops at point of harvest provided a basis for estimating the exposure potential by the β-Poisson model. Probability of exposure was 0.782 for raw SBL mix compared to 1.40 × 10−11 with composting. It can be concluded that there is a definite advantage in optimally composting SBL mix before land application. The exposure assessment may essentially require modification and fine tuning as and when further data become available.