Virus Emissions from Toilet Flushing: Comparing Urine-Diverting to Mix Flush Toilets

High levels of viruses can be found in human excrement from infected individuals, a fraction of which can be emitted from toilet flushing. Unlike the common mix flush toilet (MFT), the urine-diverting toilet (UDT) separates urine from the toilet water. Specific focus on urine-associated viruses is needed because the UDT can emit different levels of urine-associated and fecal-borne viruses and urine has different properties compared to feces that can affect emission levels (e.g., protein content). In this work, we quantified emission levels of surrogate bacteriophages for urine-associated and fecal-borne viruses, MS2 and T3, from flushing a UDT and an MFT, with and without protein in the water. Emission levels of viruses in the water of the UDT were lower than that of the MFT by up to 1.2-log₁₀ and 1.3-log₁₀ for T3 and MS2, respectively. If urine is completely diverted in the UDT, virus emissions can be reduced by up to 4-log₁₀. Based on these results and typical levels in urine and feces, we estimate that up to 10⁷ and 10⁸ gene copies of human viruses per flush can be released from the UDT and MFT, respectively. Lower emissions observed with the UDT suggest reduced exposure to viruses from flushing the UDT.