Treating Verdigris River Water - Disinfection Strategy for Minimal Disinfection By-Products Formation
Abstract:Water demand projections indicate that the City of Broken Arrow (COBA) will need additional water supply from the Verdigris River (VR) to supplement water purchased from the Oklahoma Ordinance Works Authority (OOWA). COBA's original treatment facility is over 40 years old and many of the treatment units are currently failing. Hence, the plant is not capable of consistently producing water in compliance with the latest Oklahoma Department of Environmental Quality (ODEQ) and U.S. Environmental Protection Agency (EPA) regulatory mandates. VR water is characterized by high levels of organic matter and turbidity, with TOC concentrations ranging from 4.4 to 9.5 mg/L in the past two years and an average of 6.0 mg/L (at plant intake). There are several water utilities that draw water from this source as well. Historical data illustrate that the water plants treating VR water have consistently exceeded disinfection-by-product (DBP) maximum contaminant levels (MCLs).
Since the VR water has been a challenge to treat, a comprehensive treatability study was performed to determine appropriate treatment strategies for upgrading the existing COBA's treatment plant with a 15 MGD facility. Specifically, jar tests and bench-scale analyses were conducted to identify possible pretreatment, clarification, disinfection, and post treatment processes. Following the different pretreatment scenarios, samples were chlorinated or chloraminated for 72 hrs to simulate disinfection within a full-scale distribution system. At 72 hours, the samples were quenched and preserved for DBP analyses. Evaluation indicated that ferric chloride was the most effective coagulant for organics removal. However, DBP analyses indicated that the use of chlorine as primary and secondary disinfectant was not feasible due to the high formation potential of DBPs. Although enhanced coagulation could achieve removal levels for TOC well above the Stage 1 D/DBPR requirements, the remaining organic matter in the water produced levels of DBP above MCLs. Therefore, the use of chloramines as the secondary disinfectant was investigated and demonstrated to significantly lower the DBP formation potential of the finished water.
This paper reviews the protocol and discusses the comprehensive treatability study performed with the VR water to determine the best process alternative to consistently produce treated water in compliance with the latest ODEQ and EPA regulatory mandates. The results of this paper benefits utilities that withdraw water from similar sources as the VR and struggle to meet DBP MCLs while ensuring adequate inactivation of pathogens. Furthermore, this paper presents a comprehensive methodology to assess treatability studies which utilities can apply to optimize their treatment schemes. This methodology emphasizes a holistic evaluation of the treatment efficacy, where pretreatment, clarification, and disinfection options are analyzed all together rather than distinct and separate processes.
Document Type: Research Article
Publication date: 2009-01-01
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