Evaluating the feasibility of sequential tank aeration and re-chlorination for removal of disinfection byproducts under extreme water age in support of the Navajo Gallup Water Supply Project Design

Project ID: 23031
Principal Investigator: Neal Gallagher
Research Topic: Desalination and Water Treatment
Funded Fiscal Years: 2023 and 2024
Keywords: None

Research Question

This work will seek answers to the following research questions in support of the NGWSP SJL system design and feasibility:

1) How does successive tank aeration and re-chlorination impact the DBP species formed in the distribution system?

2) Do changes in the balance of DBP species formed after successive tank aeration and re-chlorination result in poor or unacceptable DBP removal performance and therefore require SJL design approach changes away from the use of tank aeration for this purpose?

3) What are the characteristics of the fraction of DOC which reacts with chlorine to form DBPs during initial chlorination and after successive tank aeration and re-chlorination steps, for example does the more readily oxidizable DOC which reacts with chlorine to form DBPs initially produce a different balance of DBP species when compared to less readily oxidizable DOC which reacts during successive re-chlorination events?

4) How do DBP and DBP CEC formation curves (formation rate, DBP concentration) change after successive tank aeration and rechlorination?

5) Does current modeling under predict DBP formation after successive tank aeration and re-chlorination?

Need and Benefit

Tank aeration has been identified as a potential technology to save substantial cost for SJL system operations and previous modeling results and literature have shown successful TTHM removal using tank aeration. Variability in DBP speciation after successive aeration and subsequent re-chlorination events has not been well characterized in prior studies. Furthermore, information the fate of DBP CECs is not well documented in the literature. Therefore, the resulting DBP formation and speciation in the complex SJL system, with extreme water ages, are unknown and require conservative assumptions to design without further testing. This challenge is not unique to the NGWSP, many municipalities face similar challenges during the design and retrofit of their water distribution systems as the systems expand and operating conditions and regulations change.

DBPs are a significant concern in water transmission and distribution networks associated with the NGWSP SJL system due to extremely high water age (residence time) and the use of chlorine as a disinfectant. Chlorine has been identified as the most effective chemical for disinfection in the SJLWTP and distribution system after careful evaluation of alternatives. The use of chloramines for disinfection is typically considered as a viable alternative to chlorine and can mitigate DBP formation concerns. However, many of the existing community water systems which the SJL system connects to were designed for disinfection using free chlorine. Switching to chloramines would require costly upgrades to the system infrastructure to replace incompatible materials such as pipe gasket material. There are also concerns with changes in oxidation-reduction state of the water causing issues with corrosivity/scale potential of the water and general end user perception. Chloramines also increase the concentration of nitrogen in the system and can promote N-DBP formation.

Reclamation has an urgent need to evaluate tank aeration systems as a method for DBP mitigation in distribution systems with long residence times, such as the NGWSP SJL system. Tank aeration is a cost effective and feasible treatment technique when compared to DBP precursor treatment technologies (membranes, ion exchange, GAC) due to its effectiveness at targeting volatile TTHMs, low capital and operating costs, ease of installation, and ease of O&M. Numerous studies exist on the effectiveness of tank aeration in reducing DBPs (Brooke & Collins, 2011, Ghosh et al., 2015, Duranceau & Smith, 2016), however, limited research has been published regarding the formation of DBPs (and DBP CECs) and the change in DBP speciation after tank aeration and re-chlorination.

The successful outcome of this project is to provide a complete understanding of DBP speciation changes after sequential aeration/re-chlorination events using a bench scale WTP and distribution system. This study will inform cost saving decisions during design of the NGWSP by improving understanding of DBP formation and treatment in complex distribution systems. The data collected in this proposal will provide critical input to optimize GAC sizing at the SJL WTP (to reduce DBP formation potential) and tank aeration/re-chlorination systems within the SJL distribution system (to remove DBPs and maintain chlorine residuals).

The capital cost to install a tank aeration and re-chlorination system is roughly $1 million to $1.25 million. It is expected that 10 to 20 tank aeration and re-chlorination systems may be required within the SJL distribution system, depending on treatment efficacy. The results of this study will refine the cost balance point between DBP mitigation at the WTP using GAC and removal of DBPs in the distribution system using tank aeration. The results of this work will provide more certainty in the number of aeration/re- chlorination systems required, reducing overall costs.

Contributing Partners

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Research Products

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Last Updated: 6/22/20