Assessment of treated municipal wastewater irrigation for rice cultivation

ABSTRACT This study assessed the feasibility and safety of irrigating paddy rice with treated municipal wastewater under farm conditions in northeastern Thailand. Four irrigation regimes were evaluated across two cropping windows: reservoir water without agrochemicals, reservoir water with agrochemicals, pond-stored treated municipal wastewater (TMWW), and direct reuse of TMWW. Monitoring included source-water quality and volumes, soil characteristics, grain yield and composition, and consumer risk, using non-carcinogenic hazard assessment and a benchmark-based screening approach for lead. Outcomes were primarily influenced by seasonality and cultivar choice. Out-of-season crops achieved higher yields than in-season crops, while grain zinc content was higher in the in-season crop, and toxic-metal levels in grain remained uniformly low. Arsenic concerns occurred mainly during the longer, more reducing out-of-season period; combined non-carcinogenic hazard indices were below one only in selected in-season cases; and lead screening indicated no benchmark exceedance. Both direct and pond-mediated reuse performed comparably to reservoir water regarding grain toxic metals, while also providing nutrient co-benefits and buffering irrigation supply during dry periods. The findings support safe, fit-for-purpose wastewater reuse when multiple barriers, season-aware irrigation to maintain intermediate oxidation–reduction conditions, grain-metal verification at each harvest, and simple trigger-based adjustments are implemented.