Removal of conventional and novel taste and odour compounds from drinking water using UV photolysis and UV/hydrogen peroxide advanced oxidation.

摘要

Taste and odour (T&O) has always been of concern in the drinking water industry because consumers that perceive T&O may not drink the water for aesthetic reasons or they may associate the T&O with the water not being safe to consume. The two terpenoids, geosmin and 2-methylisoborneol (MIB), are the most common compounds linked to odour outbreaks of the earthy, musty, and moldy category, while 2-isopropyl-3-methoxypyrazine (IPMP) and 2-isobutyl-3-methoxypyrazine (IBMP) are identified less often. As a result, the majority of research using UV photolysis and the advanced oxidation process (AOP) of UV in combination with hydrogen peroxide (UV/H2O 2) have been conducted on the conventional T&O compounds geosmin and MIB. All these compounds can be detected by humans at exceptionally low concentrations (i.e. low parts-per-trillion) and have been frequently reported during the late summer/early fall in several potable waters including the Great Lakes.; Some more novel T&O compounds include unsaturated aldehydes such as E2-heptenal, E2,E4-nonadienal, E2,E4-decadienal, and E2,E4-heptadienal (i.e. aldehydic compounds). These compounds have been measured in various source waters and identified as odourous compounds in the fruity, fishy, and swampy categories They have odour threshold concentrations (OTCs) that range from the parts-per-trillion to parts-per-billion range. Todate, there has been limited research conducted on the ability for UV and UV/H2O 2 to destroy aldehydic compounds. Furthermore, conventional treatment processes have difficulty removing these traditional and novel T&O compounds to below their respective OTCs.; The focus of this research was to explore the potential for UV and UV/H 2O2 to remove conventional and novel T&O compounds from potable waters. The bench-scale experiments demonstrated significant removal of all the compounds of interest even when implementing a hydrogen peroxide dose as low as 1.5 mg/L and an UV fluence greater than 500 mJ/cm2 for a given water quality. Furthermore, UV/H2O2 was shown to have the ability to remove geosmin, MIB, IPMP, and IBMP up to 2 times greater than with only direct UV photolysis. Moderate removal with direct UV photolysis was achieved for the aldehydic compounds. Conventional treatment prior to UV or UV/H2O2 did not change the water quality enough to effect T&O removal in this investigation (i.e. raw versus treated water results were similar), but this would be expected to be source-specific. Geosmin, MIB, and the pyrazines were most resistant to treatment in the bench-scale studies, regardless of whether or not hydrogen peroxide was present.; It was identified that UV photolysis may be capable of mitigating the aldehydic compounds thereby eliminating the need for a hydroxyl-radical driven process and, thus, the need to add and then quench hydrogen peroxide before secondary disinfection at a WTP. Residual hydrogen peroxide from the UV/H 2O2 AOP would exert a chlorine demand for secondary disinfection. Therefore, implementing only UV photolysis for the control of odourous aldehydes would provide an economic benefit to a WTP because chemical costs would be minimized as opposed to using the AOP of UV/H2O2. In agreement with previous experiments, it was demonstrated that in order to efficiently mitigate geosmin, MIB, and the pyrazines an AOP would likely be needed.; Experimental results from varying the hydrogen peroxide dose while implementing UV fluences that are typically used for disinfection of micro-organisms (40 to 200 mJ/cm2), demonstrated that high concentrations of hydrogen peroxide would be required to mitigate the conventional and novel T&O compounds. A log reduction close to or exceeding 1 was achieved for only the aldehydic compounds E2,E4-nonadienal, E2,E4-heptadienal, and E2,E4-decadienal at the highest hydrogen peroxide dose of 26 mg/L. The hydrogen peroxide experiments verified that the hydroxyl radical-driven reactions contribute only partially to the reduction