Suitability of chlorine dioxide as a tertiary treatment for municipal wastewater and use of reclaimed water for overhead irrigation of baby lettuce

Reclaimed wastewater used for agricultural irrigation should meet specific microbiological standards in order to prevent microbial contamination of the irrigated produce. The objective of this study was to evaluate the suitability of chlorine dioxide (ClO₂) for the disinfection of secondary-treated municipal wastewater and its subsequent use for overhead irrigation in greenhouse production of baby lettuce. The impact of reclaimed water tertiary-treated with ClO₂ on E. coli concentration, the presence of pathogenic bacteria, and the occurrence of chlorates as disinfection by-products was evaluated in water and in baby lettuce. E. coli was quantified using both conventional plating methods and a quantitative real time PCR (qPCR) method with propidium monoazide (PMA) pre-treatment to differentiate between viable and non-viable bacteria. Population density of cultivable E. coli was significantly lower (p < 0.05) in reclaimed water, tertiary-treated using ClO₂ (ClO₂W), when compared with secondary-treated municipal wastewater (SW). However, no significant differences in viable but non-cultivable (VBNC) E. coli loads were observed between treatments when quantifying using the PMA-qPCR method. These results could indicate that ClO₂ treatment of water did not kill the bacteria but it induced bacteria to enter a VBNC state. The proportion of samples positive for the presence of pathogenic bacteria was lower in ClO₂W (1/8) compared with SW (7/8). Significantly lower E. coli counts (p < 0.05) were detected in plants irrigated with ClO₂W compared with those irrigated with SW. Relationship between higher E. coli counts and the presence of pathogens was observed when lettuce samples were analyzed by PMA-qPCR (Mann-Whitney U Test, p < 0.05). Baby lettuce irrigated with ClO₂W showed a significantly higher concentration of chlorates than lettuce irrigated with SW. The quantification of viable bacteria using molecular methods suggests that the efficacy of ClO₂ could be overestimated when conventional plating quantification methods are used. Additionally, the accumulation of chlorates in the tissue should be considered as it represents an adverse effect of this disinfection treatment.