Grantee Research Project Results
Final Report: Fluoride, DBP Precursors, and Particles: Simultaneous Removal with Aluminum Salts a Solution for Small Drinking Water Systems
EPA Grant Number: R835176Title: Fluoride, DBP Precursors, and Particles: Simultaneous Removal with Aluminum Salts a Solution for Small Drinking Water Systems
Investigators: Lawler, Desmond , Katz, Lynn
Institution: The University of Texas at Austin
EPA Project Officer: Page, Angela
Project Period: December 1, 2011 through November 30, 2014 (Extended to June 19, 2016)
Project Amount: $499,357
RFA: Research and Demonstration of Innovative Drinking Water Treatment Technologies in Small Systems (2011) RFA Text | Recipients Lists
Research Category: Drinking Water , Water
Objective:
The primary goal of this research was to better understand the removal of fluoride and its interactions with natural organic matter (NOM) and particles during the alum coagulation process in a small drinking water system. The objectives of this research shaped the project into three natural phases moving from experimentation to development of a treatment model. The first phase involved developing a mechanistic understanding of the interactions among fluoride, organic ligands, and aluminum during enhanced coagulation processes. The second phase applies this mechanistic understanding to waters containing NOM for a range of parameters including alum concentrations, NOM concentrations, and NOM sources. Results from this phase were used to verify the mechanistic interpretation developed within the first phase and serve as preliminary guidelines for implementation of enhanced coagulation processes for simultaneous fluoride and NOM removal in a small drinking water system. The third and final phase of the project consisted of conducting pilot tests at two different sites using the fluoride/NOM removal guidelines developed during the previous phase. These onsite studies provided verification concerning the efficacy of the guidelines and for the application of enhanced coagulation for fluoride and NOM removal.
Conclusions:
In this research, the use of alum coagulation to not only remove particles and NOM (as is commonly done) but also to remove fluoride was investigated. The research resulted in a far deeper understanding of the interactions of fluoride, NOM, particles, and alum in the context of water treatment. Fluoride at elevated concentrations (taken here to mean greater than 1.5 mg/L) is not a common problem in natural surface waters in the United States, and so, fortunately, few water utilities of any size exhibit the problem that this research addressed. Groundwaters that are used as a drinking water source and contain elevated concentrations of fluoride are more common, but they rarely contain appreciable concentrations of either particles or NOM; as a result, they would be better served to remove fluoride through an adsorption process (typically, using activated alumina as the adsorbent) than to adopt the process studied in this research. Nevertheless, some water utilities, especially small water utilities that might have limitations for capital expenditures and/or limited technical expertise to operate more complex systems, are likely to find the results of this research to be particularly applicable. In addition, some water systems treating a surface water might only need a small reduction in the fluoride concentration to meet the standard, and the process developed in this research would be of particular value to them.
The primary outcome is a treatment process that would be effective at reducing the fluoride concentrations to below the existing Second Maximum Contaminant Level (sMCL) of 2 mg/L as F or the World Health Organization target of 1.5 mg/L. We anticipate that the current EPA review of the fluoride MCL of 4 mg/L will result in a lowering of the limit to one of these other target values. The process developed in this research will have limited applicability because it is likely to require relatively high doses of alum; however, this process could be adopted at many plants with no additional capital costs. The decision to adopt, therefore, would be a tradeoff between capital and operating costs. While alum doses as high as 500 mg/L were investigated in this research, more reasonable doses in the range of 30 to 100 mg/L of alum were found to effectively remove fluoride, depending on the NOM content, initial fluoride levels, and other characteristics of the raw water.
A consistent finding of this research is that the optimal pH to remove fluoride with alum is between pH 6.5 and 6.8. This finding was confirmed in both laboratory and pilot studies. In this pH range, fluoride is incorporated into the Al(OH)3 precipitates that are characteristic of alum coagulation at doses above approximately 20 mg/L. Maximum removals are achieved when alum and fluoride are co-precipitated in this manner. The flocs that are formed in this process were found to have a slightly smaller average diameter (on a volume-weighted basis) but they would still settle well in a conventional continuous flow sedimentation process and, therefore, be able to be removed well in most water treatment plants.
One of the other significant impacts resulting from the presence of fluoride in waters undergoing alum coagulation is the impact on precipitation. Aqueous aluminum fluoride complexes formed upon addition of alum increase the solubility of aluminum, and concomitantly increase Al residuals in the treated water. This reduced solubility can impact both removal of turbidity and natural organic matter. In contrast, moderate levels of natural organic matter have minimal effect on fluoride removal because NOM removal typically occurs through an adsorption mechanism and fluoride removal can occur through a combination of co-precipitation and adsorption.
In accordance with EPA’s Disinfection/Disinfection By-Products (D/DBP) Rule, most water treatment plants are required to achieve some level of NOM removal (measured as total organic carbon, or TOC), and many plants employ so-called “enhanced coagulation” to achieve this removal. The specific requirements for the percent of dissolved organic carbon (DOC) to be removed depend on the alkalinity and raw water TOC value and range from 15 to 45%. In enhanced coagulation with alum, higher concentrations of alum than needed for particle removal are added to accomplish this TOC (or NOM) removal. However, utilities are allowed to achieve less DOC removal if they demonstrate that they can only achieve the mandated TOC removal at doses of alum higher than the “point of diminishing returns.” This point is reached when, in a series of alum coagulation jar tests at increasing doses, an additional dose of 10 mg/L of alum achieves less than an additional removal of 0.3 mg/L of TOC. In this research, it was found that fluoride interferes with the removal of NOM, so that a plant with an elevated fluoride concentration in its raw water is likely to reach the point of diminishing returns at a lower alum dose than a plant with lower fluoride concentration and otherwise similar water characteristics. The plant with the higher fluoride concentration could achieve a higher removal of TOC and also achieve a higher removal of fluoride if it operated at an alum dose higher than the minimum required to meet the D/DBP Rule.
Journal Articles:
No journal articles submitted with this report: View all 14 publications for this projectProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.
Project Research Results
- 2015 Progress Report
- 2014 Progress Report
- 2013 Progress Report
- 2012 Progress Report
- Original Abstract