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Developing Ozone-Biofiltration Treatment Processes for Potable Reuse Applications
Civil and Environmental Engineering
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Ozone-biological activated carbon filtration (ozone-BAC) treatment plays a major role in advanced water treatment (AWT) for potable reuse applications in locations where reverse osmosis (RO) treatment is not cost effective and/or feasible. However, regulators, designers, and operators are ambiguous about long-term ozone-BAC reliability and mechanisms involved in the removal of certain contaminants of emerging concern (CECs) such as N-Nitrosodimethylamine (NDMA) and perfluorooctanoic acid (PFOA), and bulk organics such as total organic carbon (TOC). For 16 months, two parallel BAC filters were operated at different loading rates and empty bed contact times (EBCTs) of 10 minutes and 20 minutes treating up to 60,000 and 30,000 bed volumes (BVs), respectively, of sand filtered effluent from a municipal water resource recovery facility. BAC 1 (EBCT = 10 min) and BAC 2 (EBCT = 20 min) used Filtrasorb 400 (F400) granular activated carbon (GAC) as filter media, with equal bed depths. NDMA was generated after ozonation. Longer acclimation time (140 days or 20,000 BVs) was required for non-carbon-based (e.g., biodegradation) removal of NDMA in BAC 1. Three distinct phases were observed for NDMA removal: an initial phase dominated by carbon-based mechanisms (e.g., adsorption), the second phase influenced by both carbon- and non-carbon-based mechanisms and the third phase dominated by non-carbon-based mechanisms. In BAC 1, desorption of CECs such as sulfamethoxazole, DEET, and sucralose was observed after 450 days of operation (or 60,000 BVs). In BAC 2, almost all CECs, excluding NDMA, were removed consistently throughout the study. BAC 1 effluent TOC plateaued at around 6 mg/L, whereas BAC 2 effluent plateaued at around 4 mg/L. Under plateau conditions, BAC 1 and BAC 2 appear to have a gross TOC removal potential of around 0.2 to 0.3 kg/d/m3 of carbon. A comparative analysis of findings from this study and results from a past ozone-BAC study shows that higher TOC removal capacity was observed in BAC with shorter EBCT and upstream sand filter compared to BAC with longer EBCT and upstream membrane filter. TOC-based specific UV absorbance (SUVA) was recommended for continuous online monitoring of ozone-BAC performance.