Summary
This field study compared two reclaimed wastewater (RW) treatment systems—conventional activated sludge with sand filtration and chlorination (CAS + SFC-RW) and membrane bioreactor (MBR-RW)—against tube well control water to assess microbial safety during lettuce irrigation. Initial RW samples contained 1–1.5 orders of magnitude more bacteria than control water, with distinct bacterial and ARG/MGE profiles dependent on treatment type; Class 1 integrons and Tn916/Tn1545 were significantly more abundant in CAS + SFC-RW. Storage and soil depth emerged as key factors: storage reduced microbial and resistance marker differences between RW types and control, though pathogenic taxa showed greater persistence, and RW influence on soil microbiota diminished with vertical distance from the irrigation point.
Regional applicability
The findings are relevant to United Kingdom policy on recycled water use in horticulture, as water scarcity and nutrient recovery are drivers for water reuse schemes. However, the study was conducted internationally; direct applicability would depend on UK climate conditions, soil types, and the regulatory framework for reclaimed water quality standards, which may differ from those implied in this study design.
Key measures
16S rRNA gene copies (ddPCR and sequencing), antibiotic resistance genes (ARGs), mobile genetic elements (MGEs), bacterial taxonomic composition (Actinomycetota, Bacteroidota, Pseudomonadota proportions), Class 1 integrons, Tn916/Tn1545 abundance, soil bacterial composition at varying depths
Outcomes reported
The study assessed how wastewater treatment type and storage duration influence microbial composition, antibiotic resistance genes (ARGs), and mobile genetic elements (MGEs) in reclaimed water and their transfer to irrigated soils during lettuce cultivation. Bacterial communities were profiled using 16S rRNA sequencing and digital PCR, with particular attention to pathogenic taxa persistence and soil depth effects.
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