CC WORKSESSION 12142015 - Laserfiche WebLink

Home Browse Search

2.2 WATER QUALITY PARAMETERS 2.2.1 UV Transmittance UV transmittance (UVT) is the ratio of UV light transmitted through the sample to that transmitted through a reference solution. UVT is measured using a UV spectrophotometer. Reagent grade water is typically used as the reference solution (i.e., UVT = 100%). UV absorbance (A λ )] measures the amount of light absorbed by a solution over a given path length (l) and at a given wavelength (λ). UVT and UV absorbance are related by the following equation: UVT=10-Aλx100 The typical cell pathlength is 1 cm and both transmittance and absorbance values are commonly reported per cm. A key reference wavelength, and one at which UVT is often reported, is 254 nm. This wavelength is used because it is the wavelength at which a low pressure mercury UV lamp emits light. Transmittance decreases in the presence of UV absorbing substances and particles that either absorb or scatter UV light. This results in a reduction of available UV energy for disinfection and oxidation. The UV transmittance is the most important water quality parameter used in the sizing of a UV system. A UV system designer may compensate for low transmittance by increasing the residence time or the amount of equipment. 2.2.2 Hydroxyl Radical Scavenging Demand While the desired reaction in UV oxidation systems is between photogenerated hydroxyl radicals and contaminant molecules the unselective nature of hydroxyl radical reactions result in reaction pathways that consume hydroxyl radicals by reaction with constituents of the background water matrix. Examples of these hydroxyl radical scavenging reactions are the oxidation reactions with the natural organic matter (NOM) present in natural waters or reactions with carbonate and/or bicarbonate ions. Hydrogen peroxide itself will react with hydroxyl radicals and, therefore, is considered a hydroxyl radical scavenger. All of these scavenging reactions have the effect of reducing the steady state concentration of hydroxyl radicals in the water. Since the rate of contaminant degradation is proportional to the steady state concentration of hydroxyl radicals, these hydroxyl radical scavenging reactions reduce the rate of contaminant degradation. The level of scavenging reactions associated with a water sample can be quantified and is referred to as the hydroxyl radical scavenging demand of the water. Trojan routinely determines the scavenging demand of water samples at its laboratory in London, Ontario. 2.3 THE ELECTRICAL ENERGY PER ORDER PARAMETER In sizing UV systems for Environmental Contaminant Treatment, a different metric is used than for UV systems for disinfection. This metric is called Electrical Energy per Order, or E EO (Bolton et al. 1996). E EO is the electrical energy (measured line power draw) required to reduce the contaminant concentration by one order of magnitude (one log, or 90%) in one cubic meter (m3) or 1000 gallons (kgal) of water (depending on the choice of flow units). Typical units are:       •orderkgal kWh or     •orderm kWh 3 . 3