Innovative Approach for Evaluating Stage 2 DBP Rule Compliance Strategies
- David Briley - Hazen and Sawyer
The Rivanna Water and Sewer Authority (RWSA) is a wholesale utility that provides potable drinking water to the City of Charlottesville, Virginia and to portions of Albemarle County. Hazen and Sawyer assisted RWSA in evaluating alternatives to reduce Disinfection Byproducts (DBP) formation to ensure compliance with the US EPSA’s Stage 2 DBP Rule. RWSA operates 5 water treatment plants with water supply sources characterized by low to moderate levels of organics. Due to these low organic levels, RWSA has been able to achieve Stage 1 DBP Rule compliance while practicing free chlorine disinfection. However, RWSA’s Initial Distribution System Evaluation indicated challenges with State 2 DBPR compliance.
The initial step was to conduct a critical evaluation of distribution system DBP data (Stage 1 and IDSE) to reveal the level of DBP reductions needed to achieve Stage 2 DBP Rule compliance (see graph below). This analysis was also used to assess two potential operational goals for DBP reduction:
1. Quarterly DBP samples below MCL at all Stage 2 sites.
2. Locational running annual average (LRAA) at 80% of MCL at all Stage 2 sites.
Once the DBP reduction targets and DBP goals were determined, reduction strategies could be assessed and only those viable for meeting the RWSA’s reduction goals were selected for more in-depth analyses. Based on this information, a bench-scale and full-scale testing program was developed to evaluate WTP optimization strategies for TOC removal and DBP reduction. Results of the testing program indicated whether lower cost, optimization strategies could assist RWSA in meeting Stage 2 standards or whether more costly options, such as advanced treatment technologies were needed.
Advanced treatment technologies known to achieve the level of DBP reduction required were evaluated based on lifecycle cost as well as non-economic factors. A multivariate decision support system was used to compare results of the evaluation, which considered the following:
• Expected DBP reduction
• Impact on WTP operations and maintenance
• Impact on finished water quality and corrosion control
• Ease of construction
• Regulatory acceptance
• Implementation schedule with respect to Stage 2 DBP compliance schedule
• Consideration of future regulations for nitrosamines, emerging contaminants, etc.
• Lifecycle costs
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