Performance of Spray Aeration for Stage 2 DBP Compliance in Brominated Waters

Authors:

  • David S. Briley PE, Erik Rosenfeldt PhD, PE - Hazen and Sawyer
  • Ben Kearns - Cape Fear Public Utility Authority

Bromide levels have been increasing in some water supplies throughout the Carolinas as well as other parts of the country. Bromide sources range from coal-fired electrical generation plants to other industrial sources. Bromide is not a regulated parameter, so rarely are discharge limits established for these sources. Bromide presents several unique challenges for Stage 2 Disinfectants/Disinfection ByProducts Compliance. Bromide gets incorporated into total trihalomethanes (TTHMs) first by chlorine oxidizing bromide to hypobromous acid. Hypobromous acid is a stronger halogen substitution agent than hypochlorous acid. When significant levels of bromide are present in the raw water, TTHM speciation shifts more towards the brominated species. Brominated species have higher molecular weight, so the mass concentration of TTHMs can increase even if molar concentrations are similar. Also, TTHM formation occurs at a more rapid rate in the presence of hypobromous acid.

One unique approach to DBP compliance uses post-treatment aeration at the water treatment plant as well as tanks in the distribution system. A case study will be presented for a North Carolina water system facing fluctuating bromide levels in their source water and challenges with TTHM compliance. Spray aerators were installed in a 4 million gallon (MG) clearwell at the water treatment plant. The TTHM removal goal for the aeration systems at the WTP was 25 percent, which was expected to ensure DBP compliance for most of the distribution system. To reduce TTHM levels in areas with the highest water age, spray aeration systems were installed in two elevated tanks. The TTHM removal goal for the elevated tanks was 40 percent.

Performance data has shown that the aeration systems at the WTP have exceeded the 25 percent goal. There were challenges with meeting the TTHM removal goal in the elevated tanks, particularly during winter months.

The approach for assessing spray aeration included a review of historical DBP records to assess the required percent THM reductions at compliance sites. Next, a model of in-tank spray aeration systems was developed to assess the impact of installing aeration in a clearwell at the water treatment plant. The model was also used to assess variation in THM reduction as a function of flow, temperature, THM formation kinetics, and bromide concentration. Based on the model, upon aeration in the 4 MG clearwell, THMs are expected to be reduced by approximately 25 percent with 60 percent of flow recirculated throughout the clearwell aerators. After leaving the clearwell, however, TTHMs will continue to form in the distribution system. An assumption was made that THM formation in the distribution system would occur at the same rate. Theoretically, THM formation potential has been reduced but practically, this means that the magnitude of THM reduction in the tank will translate through the system. This analysis demonstrated that several monitoring locations would still have exceeded the TTHM goal. Therefore, aeration of distribution system tanks was assessed to confirm whether further THM reductions could be achieved.

Online THM analyzers were also used to develop an understanding of the interactions between bromide, natural organic matter, and operational strategies/practices on DBPs. Correlations were developed between finished water DOC and TTHM formation to develop a predictive tool for process optimization throughout the plant.

The case study on will focus on the following:
1. The evaluation approach utilized to ascertain potential benefits of aeration and select where to aerate within the water system.
2. Insights and lessons learned during construction, startup, and testing of spray aeration systems in a clearwell and in elevated tanks.
3. Performance data from operating aeration systems installed in clearwells at the WTP as well as aeration systems installed in elevated tanks.
4. Impact of consecutive spray aeration on TTHM levels in the distribution system.
5. Seasonal trends in TTHM removal with spray aeration.

For more information, please contact the author at dbriley@hazenandsawyer.com.

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