Potential Hormetic Effects of Cimetidine on Aerobic Composting of Human Feces from Rural China

Aerobic composting is widely used worldwide as a natural process for handling human waste. Such waste often contains pharmaceutical residues from human consumption, yet their impact on composting has not been studied. The aim of this study is to investigate the impact of the antihistamine cimetidine (10 mg/kg, 100 mg/kg) on the aerobic composting of human feces. The key results show that 10 mg/kg of cimetidine accelerates temperature increase and moisture removal of the composting substrate. The organic matter in all the groups gradually decreased, and the pH values increased first and then declined with the composting time, with no significant differences between the groups. The NH 4 + -N concentrations and NH 3 emission reached the maximum at 1.5 days and then declined rapidly, while the NO 2 − -N concentrations increased and then decreased, and the NO 3 − -N contents tended to increase all the time during the composting. The 100 mg/kg cimetidine caused a higher maximal NH 4 + -N concentration of compost, and a lower maximal NH 3 emission at 1.5 days, while 10 mg/kg cimetidine led to more NO 2 − -N and NO 3 − -N contents. In addition, 10 mg/kg cimetidine enhanced the aromatization and humification of dissolved organic matter and promoted the degradation of aliphatic substances. Furthermore, 100 mg/kg cimetidine generated a larger influence on the microorganisms than 10 mg/kg cimetidine, especially for the microorganisms related to nitrogen transformation. The findings imply that cimetidine has a dose-dependent impact on the decomposition of organic matter and the conversion of nitrogen in human feces during composting. It deserves further investigation of the possible hormesis effect.