Reconsidering the Rationality of Ultrafiltration as the Ultimate Safeguard for Reclaimed Water Biosafety: Unexpected Risks Arise under Different Storage Conditions

Ultrafiltration, an advanced process for producing biologically safe reclaimed water, has confronted challenges from bacterial regrowth. We conducted a comprehensive investigation into the biosafety risks of ultrafiltration-treated reclaimed water under five distinct storage conditions, considering temperature variations, airborne bacterial invasion, and light. Although ultrafiltration removed over 97% of the bacterial load, bacterial regrowth occurred during storage. Elevated temperatures resulted in the highest heterotrophic plate count (HPC), while lower temperatures favored increased total cell count (TCC). Notably, lower temperatures facilitated a surge in the number of potential pathogenic bacteria. Light exposure also elevated the potential pathogenic bacteria. Surprisingly, high temperatures posed minimal biosafety risk. While airborne bacterial invasion insignificantly affected potential pathogenic bacterial count, it constituted 67 ± 4% of total regrowth bacteria, with membrane breakthrough bacteria comprising the remaining 33 ± 4%, as revealed by SourceTracker. The impact of storage conditions on the bacterial community structure varied, with temperature exerting the most significant effect. Correlation analysis affirmed the potential of UV₂₅₄ and total fluorescence intensity as alternative indicators for HPC, and polysaccharide content emerged as a suitable substitute indicator for potential pathogenic bacterial count. Future endeavors should prioritize the development of integrated ultrafiltration processes designed to inhibit pathogen regrowth in reclaimed water treatment.