UV disinfection studies on chlorine tolerant bacteria recovered from treated sewage

Abstract

Chlorine tolerance is a major concern during disinfection of treated wastewater. Here, the UV disinfection efficacy was assessed for controlling two previously reported chlorine tolerant bacteria, Stenotrophomonas maltophilia and Klebsiella sp. along with a standard E.coli strain using a collimated beam apparatus. UV exposure was applied in two sets - short exposure (0–60 s) and long exposure (0–60 min). Bacterial counts immediately after, and 24 h post irradiation (regrowth) were analysed using media plating. Morphological characteristics and intracellular Mn/Fe ratio were examined by SEM and ICP-OES respectively, to understand the possible reasons for sensitivity/ resistance to irradiation. E.coli and Klebsiella sp. were effectively reduced (> 4 log) but S. maltophilia showed low Effective Reduction (ER < 4) at short exposures. S. maltophilia also showed ER < 4 at long UV exposure due to a high regrowth potential (1.25–3.95). Membrane damage was apparent in SEM analysis in Klebsiella sp. whereas cellular aggregation was evident in S. maltophilia upon UV exposure. Intracellular Mn/Fe ratio was found to be low in both sensitive (Klebsiella sp.) and resistant (S. maltophilia) strains, but the Mn concentration in UV irradiated S. maltophilia cells was not reduced as compared to the non-exposed cells indicating a possible accumulation of Mn released by the damaged cells in the surviving cells. The results indicate that chlorine tolerant S. maltophilia is also resistant to UV due to cellular aggregation and possible Mn accumulation. However, further studies are needed to prove this mechanism of UV resistance.