Effect of Visible Light on Surface-Attached and Suspended Heterotrophic Bacteria in a Typical Household Rainwater Harvesting Tank

Abstract

Rainwater harvesting (RWH) systems can be used to mitigate global water crises; however, they have been poorly received by communities because of the sub-standard quality of harvested water. Heterotrophic bacteria present in the water can degrade the water’s microbiological quality and create health issues. Moreover, exposure to visible light can affect both suspended and surface-attached heterotrophic bacteria, a phenomenon that is poorly investigated. This study explored the effect of visible light on surface-attached heterotrophs (SAB) and suspended heterotrophs (SB) in an RWH tank for a period of three months. The SAB plate counts were observed to be significantly higher in the tank exposed to sun (TES) than in the tank not exposed to sun (TNES). Furthermore, the SB plate counts in the TNES reduced 10 folds faster than in the TES, especially at the top and middle levels. When exposed to visible light, the phototrophs present in the water sustained the heterotrophs by producing nutrients via photosynthesis. Based on the findings of this research, this paper recommends providing shade to the tanks that are exposed to sunlight. Additionally, it suggests not to disinfect the tank because it leads to a decrease in the self-purification effect of microbes.