The effect of UV/H₂O₂ treatment on biofilm formation potential

Greater Cincinnati Water Works (GCWW) evaluated the efficacy of ultraviolet light/hydrogen peroxide advanced oxidation (UV/H₂O₂) for reducing trace organic contaminants in natural water with varying water qualities. A year-long UV/H₂O₂ pilot study was conducted to examine a variety of seasonal and granular activated carbon (GAC) breakthrough conditions. The UV pilot-scale reactors were set to consistently achieve 80% atrazine degradation, allowing comparison of low pressure (LP) and medium pressure (MP) lamp technologies for by-product formation. Because hydroxyl radicals react non-selectively with organic compounds, unintended by-product formation occurred. Total assimilable organic carbon (AOC) concentration increased through the reactors from 14 to 33% on average, depending on water quality. Natural organic matter (NOM) contains the precursors for AOC production, so when post-GAC water (versus conventionally treated water) served as reactor influent, less AOC was produced. No appreciable difference in AOC concentration was observed between LP and MP UV reactors. The Spirillum strain NOX fraction of the AOC increased from 50 to 65% on average, depending on the quality of the water. The increase in this fraction of AOC occurred because oxidation of NOM yielded smaller more assimilable organic compounds such as organic acids that are necessary for NOX growth. The Pseudomonas fluorescens strain P17 AOC concentration increased only when conventionally treated plant water was used as pilot influent. This organism thrives in waters of differing organic energy sources, but does not thrive well in carboxylic acids alone. The CONV water had more overall TOC that could contribute to higher P17 AOC counts. Biofilm coupon studies indicated that biofilms with greater heterotrophic plate counts were observed in the granular activated carbon (GAC) effluent streams receiving UV/H₂O₂ pre-treatment. Biofilm coupon studies additionally indicated that the effluent stream of the GAC column proceeded by the MP reactor exhibited more viable biofilm than the other GAC effluent streams based on an ATP-bioluminescence method. The increased viability of the biofilm produced by the MP UV reactor is likely a result of the multiple UV wavelengths and higher energy input characteristic of this technology.