Water-Transmitted Fungi are Involved in Degradation of Concrete Drinking Water Storage Tanks

Abstract

Global warming, globalization, industrialization, and the rapidly growing population at present increasingly affect the production of safe drinking water. In combination with sustainable bio-based or recycled materials, used for water distribution systems, these factors promote emerging pathogens, including fungi. They can proliferate in oligotrophic water systems, affect the disinfection process, degrade building materials, and cause diseases in humans. In this study, we explored fungal-based degradation of modern concrete water storage tanks and the presence of fungi in chlorinated drinking water at the entrance and exit of the tanks. The degradation potential of isolated 52 fungal strains and their growth at different oligotrophic conditions was tested in vitro. Forty percent of strains grew at extremely oligotrophic conditions, and 50% classified as aerophilic. Two-thirds of tested strains produced acids, with Penicillium strains as the best producers. Only 29.7% of the strains were able to grow at 37 °C, and none of them was isolated from drinking water at consumers' taps. Although not yet part of the guidelines for building materials in contact with drinking water, fungi should be taken into consideration in case of visible degradation. Their number of consumers' endpoints should be checked to exclude possible health risks for consumers.

Keywords: chlorination; degradation; drinking water; environmental factors; fungal contaminants; growth of fungi; materials in contact with drinking water; water storage tanks.

Figure 1

Sampling spots throughout water distribution system and inside concrete water storage tanks. Water samples are indicated as Sample-1, to -3, and Sample-10 to -12. Thick blue arrows indicate water flow. Samples inside the two concrete tanks that were taken by swabbing the surface are indicated as Sample-4 to -9. Thin blue arrows indicate water level as fluctuating or permanent.

Figure 2

Presence and diversity of fungal genera throughout water distribution network. Drinking water (DW) was sampled at eight distant locations. Both, the diversity and the numbers (CFU/L) were highest at the point of entrance into the concrete water storage tanks and the lowest at consumers’ endpoints (taps), indicating sensitivity to chlorine under prolonged exposure time. The genus Cladosporium was the most frequently isolated (four out of eight sampling spots).

Figure 3

Presence of fungi inside concrete water storage tanks. Fungal growth with visible mycelia was observed as black and ochre colonies on concrete ceilings at the entrance (A1) and inside the cell (B1), on a dry wall (C1), humid wall (D1), and wet wall (E1). Samples were taken also from the bottom of the tank (F1) with visible peeling and change of the colour and from the metal pipe for water outflow (G1). (A2–G2): Microscopic observation of adhesive tape samples revealed diverse mycobiota producing brown conidia and brown to hyaline hyphae at all sampling sites. (A3–G3): Fungal colonies on MEA medium grown from swab samples taken from 1 cm² surface.