Does human excreta fertilization increase pathogen contamination in sandy soils?

The use of human excreta in the context of the circular economy has the potential to recycle nutrients, save water and energy, and sequester carbon. However, the risk of pathogen contamination of soil and crops, particularly from land application of solid human excreta (feces), is a realistic scenario. In our study we investigated a double thermophilically treated and quality assured compost (Hygienized Human Feces Compost). We used indicator organisms for fecal contamination to monitor the risk of pathogen contamination of the soil after application of human feces compost. Indicator organisms were Escherichia coli, Salmonella spp., Enterococcus spp. and Clostridium perfringens. One pot experiment under semi-controlled conditions and three on-farm experiments were conducted on loamy sand and sand soils. The experimental period ranged from 58 to 1123 days. The levels of fecal indicator organisms in soil show a consistent pattern across all experiments. E. coli, Salmonella spp. and Enterococcus spp. were not detected in any of the soil samples. C. perfringens was found in concentrations ranging from <10 to 10³ CFU g^-1 FM. However, there was no evidence of a systematic increase in the soil concentrations of C. perfringens following human feces compost application. In different years and under different experimental conditions, we were not able to detect any increase in the concentration of indicator organisms for fecal contamination in our experiments. We therefore assume that sandy soils might be generally suitable for application of quality-assured fertilizers from human excreta.