Fundamental and Practical Aspects of Cometabolite Addition for 1,4-Dioxane Removal in Biofiltration

Abstract

1,4-Dioxane is a probable human carcinogen and common contaminant in potable reuse water, drinking water, wastewater, groundwater, and landfill leachate. Historically used as a chlorinated solvent stabilizer, 1,4-dioxane can be found in legacy wastes, contaminated groundwater sites, and in trace amounts as a byproduct formed in modern manufacturing. The United States Environmental Protection Agency has set the one in a million cancer risk level for 1,4-dioxane at 0.35 µg/L. 1,4-Dioxane is resistant to many conventional water treatment technologies, such as air stripping and adsorption, and typically requires an advanced oxidation process to break it down. Historically, 1,4-dioxane was considered recalcitrant to biological degradation. However, recent work in landfill leachate as well as contaminated groundwater has shown that 1,4-dioxane is amenable to treatment via cometabolism, where the addition of a cometabolite provides a carbon and energy source to support microbial growth and production of enzymes that biodegrade 1,4-dioxane. The work herein demonstrates cometabolic 1,4-dioxane removal as a viable potable water reuse treatment process using biologically active carbon filtration (BAC), while also advancing synergistic research to advance this technology towards application. Propane gas was found to be an effective cometabolite for the removal of 1,4-dioxane in full-scale, open-air BAC filters, achieving sub microgram per liter concentrations of 1,4-dioxane. These concentrations are particularly relevant for potable reuse and drinking water scenarios, as well as for meeting the 0.35 µg/L cancer risk level. Propane addition was not found to affect total organic carbon removal or the removal of N-Nitrosodimethylamine (NDMA), a nitrogenous disinfection by product produced in upstream ozonation. BAC, with or without propane amendment, was found to be effective for the removal of low molecular weight organics produced in ozone, such as acetone, acetaldehyde, and formaldehyde. DNA sequencing analysis was carried out on the BAC medium and was suggestive that Mycobacterium species appeared to be responsible for the cometabolic 1,4-dioxane removal in the BAC. Further analysis of functional genes implicated that a soluble methane monooxygenase, part of a family of functional enzymes called soluble diiron monooxygenase (SDIMO), was the most important enzyme for cometabolic propane and 1,4-dioxane degradation. The enzyme's expression was correlated with propane addition. The DNA sequencing data also implicated a toluene monooxygenase and a phenol monooxygenase, both also SDIMOs, as potentially participating in the propane and 1,4-dioxane biodegradation. The functional genes proliferated downstream of BAC in the granular activated carbon (GAC) contactors, which are designed for removal via physical sorption, but in reality are also richly colonized by microbes. Putative functional genes involved in cometabolic 1,4-dioxane degrdation were not detected in the finished product water or the facility's wells post recharge of the finished water. In a follow up study of drinking water, cometabolic 1,4-dioxane degradation with propane gas using BACs was found to be unsuccessful, most likely due to a growth factor limitation in the ogliotrophic water. However, propane gas amendment decreased the startup time requirement for new BACs to achieve complete NDMA removal. Cometabolic 1,4-dioxane degradation with tetrahydrofuran (THF) was also found to be an effective treatment for 1,4-dioxane contaminated landfill leachate, but not practical for potable water treatment because of the toxicity of THF. The research carried out as part of this dissertation furthers the understanding of BAC as a powerful treatment technology for potable reuse, as well as furthering the knowledge of cometabolism as an effective treatment technology for 1,4-dioxane as a key CEC of concern. BAC was found to be a powerful, yet economically advantageous, treatment tool for the removal of disinfection byproducts and low molecular weight organic compounds formed during ozonation. Cometabolic removal of 1,4-dioxane in BAC with propane gas is an effective and low-cost treatment method to achieve very low concentrations of 1,4-dioxane, well below health advisory concerns, without impeding other treatment objectives.