Fungi characteristics of biofilms from sewage and greywater in small diameter gravity sewers

Small diameter gravity sewers (SDGSs) have been applied globally and provide a sewage recycling method whereby collected greywater can be directly reused as reclaimed irrigation water, which can be particularly useful for low-income rural areas. However, the fungi characteristics of SDGS biofilms remain unclear, especially with respect to the abundance of pathogenic fungi. To investigate the fungi characteristics of SDGS biofilms, we use rural sewage and synthetic greywater in an integrated experimental set-up containing six sewage SDGSs and six greywater SDGSs, respectively. Results indicate that Ascomycota and Hypocreales were the most predominant fungi phylum and order in the SDGS biofilms, respectively. Wastewater type caused little difference in the total number of fungi (8.5 × 10⁴ colony-forming unit (CFU) cm^-2 in sewage, and 1.1 × 10⁵ CFU cm^-2 in synthetic greywater), whereas a significant distinction was observed in fungi communities. Mucor and Rhodotorula were significantly abundant in the sewage SDGS biofilms whereas Exophiala was abundant in the greywater SDGS biofilms. Fungi in greywater SDGS biofilms showed a more complex network and had more keystone species than fungi in sewage SDGS biofilms based on network analysis. Animal pathogen Exophiala and plant pathogen Gibberella in the greywater SDGS biofilms were more abundant when compared with sewage SDGS biofilms. We conclude that reuse of greywater collected by SDGSs for irrigation should be further evaluated due to the relatively abundant pathogenic fungi in our SDGS biofilms. Further study combined with the present study could provide useful information for supporting decisions made by governments.