Bromate Formation Differences between Ozone Dissolution Systems

摘要

Many surface water treatment plants utilize ozone to meet a variety of water quality andrntreatment objectives including disinfection and the oxidation of inorganic and organicrncontaminants. Disinfection guidelines for ozone have been defined by the USEPArnSurface Water Treatment Rule and Long Term 2 Enhanced Surface Water TreatmentrnRule largely based upon ozone dissolution using fine bubble diffuser (BD). Since therndevelopment of the USEPA guidance manuals, sidestream injection (SSI) has emergedrnas a viable alternative to BD for ozone dissolution. However, the disinfection zonesrnmay be different among these ozone dissolution methods resulting in differences inrndisinfection credit and bromate formation for similar applied ozone dosages. Theserndifferences were explored at full-scale and pilot-scale as part of this study.rnThe Regional Municipality of Halton (Halton) serves approximately 480,000 residentsrnwith treated surface water from Lake Ontario. All three Halton surface water treatmentrnplants have integrated ozone as part of its treatment strategy for primary disinfection.rnTwo Halton ozone systems use BD for ozone transfer, while the third facility uses SSIrnwith degasification. At the facilities using BD, bromate has been detected and has led tornoperational modifications (i.e. optimizing ozone residual) in order to control bromaternformation effectively below the regulatory level of 10 μg/L. However, the facilityrnusing SSI with degasification has experienced no significant bromate formation.rnOperating similarities include influent bromide concentration (30-60 μg/L) andrnoperating CT-value. For an equivalent ozone dose, the SSI facility achieves nearlyrntwice the CT as the BD facilities while producing similar concentrations of bromate.rnPilot-scale testing has been conducted at the Southern Nevada Water Authority.rnPreliminary findings have supported the full-scale observations at Halton regarding thernrelationships between ozone dose, ozone CT, and bromate formation. The effect ofrnvarying water flow conditions on BD systems was also investigated. Results showedrnthat operating a contactor below the design water flow rate can yield ozone CT that isrnnot included in the “compliance CT” calculation. This operating scenario can result inrnactual CT values greater than those calculated for regulatory compliance along withrnbromate formation prior to the disinfection zone. The full-scale and pilot-scalernobservations provide important information to ozone utilities and design engineers tornconsider when designing new ozone contactors or replacing BD with SSI ozonerndissolution systems in existing contactors.