In-Line Diffused Aeration to Reduce THMs in Distribution Piping

This post from University of New Hampshire's M.R. Collins is a continuation of a project update originally shared in our Technology News newsletter. 

Several methods exist for controlling THM formation, including reducing natural organic matter (NOM) prior to chlorine disinfection, and using an alternate disinfectant such as ozone, chloramines, or UV. Using these disinfectants will prevent or reduce the formation of THMs, but could facilitate the production of other potentially harmful byproducts.  Also, using ozone or UV as a disinfectant will not provide a residual in the distribution system (USEPA, 1981 & USEPA, 1999).  While effective at reducing THM formation, changing or upgrading the water treatment plant to include these control techniques could be costly and negatively affect other plant processes.

Posttreatment aeration is another strategy to control THMs, and involves removing the THMs after formation.  Countercurrent packed towers, diffused aeration in open reactors, and spray aeration in storage tanks are all viable aeration methods to remove THMs (USEPA, 1981 and Brooke & Collins, 2011).  While the above methods are viable and have been applied in the field, all require depressurization of the water, and are limited in terms of placement in the water distribution system.  Placement in the distribution system is important since THMs continue to form in the system, and often exceed regulations when at the far end of the system.  This research explores both vertical in-line diffused aeration (VILDA) and horizontal in-line diffused aeration (HILDA) to reduce THMs, which has the potential to be cost effective and conveniently placed where needed in the distribution system.

A schematic of an in-line diffused aeration system is depicted in Figure 1. The basic components consist of an air compressor, air-water reactor, air injector, air release system and associated air and water flow meters. The basic difference between VILDA and HILDA systems is the configuration of the air-water reactor. The VILDA system will utilize a countercurrent arrangement where air is injected in the bottom of the vertical reactor while the water enters at the top of the reactor. The HILDA will have air and water flowing concurrently through the horizontal reactor.

Figure 1. Schematic of basic in-line diffused aeration system. 

The most efficient air-water reactor is one where equilibrium or saturation THM removals can be achieved. The work of Matter-Müller et al. (1981) provides a mass balance method which allows equilibrium or saturation THM removal values to be predicted as depicted in Equation 1:

Equation 1:                             

where Q_G = the gas flow in (m³/s), H_cc = the dimensionless Henry’s law constant of compound, and Q_L= the liquid flow rate (m³/s). Hence, the higher the air/water ratio, the greater the removals of THMs.

For Equation 1 to be applicable to distribution piping conditions, the Henry’s Law Constant will have to be adjusted for both temperature and pressure. The former has been well documented in the literature but the latter had to be determined during this research. Basically, a second order relationship as shown in Equation 2 was used to correct for pressure (Zwerneman, 2012).

Equation 2:                                

where H_cc  = the dimensionless Henry’s law constant at system pressure, H_cc,o   = the dimensionless Henry’s law constant at atmospheric pressure, P = the system pressure (psi), and k= the experimentally determined rate constant (psi^-1).

HILDA REACTOR DESIGN. The most problematic concern with a HILDA system was to configure an air-water reactor where equilibrium or saturation conditions will be achieved since injected air in a horizontal pipeline will have a strong tendency to raise to the top of the pipe and not be mixed with the water sufficiently to reach saturation. Through trial and error, Komax static mixers were determined to provide adequate air-water mixing to approach saturation removal conditions provided fluid turbulence or mixing intensity was high enough. A modified Reynolds Number (Re’) was developed to capture the magnitude of fluid momentum and mixing intensity and can be seen in Equation 3 (McCowan, 2015).

Equation 3:                                           

where, v= velocity of water (ft/s), d=pipe diameter (ft), L_m=length of reactor (ft), and v=kinematic viscosity (ft²/s). Figure 2 shows a graphical representation of % removal vs. Re’ at 10:1 and 20:1 A:W ratios.  Re’ appears to be a good indicator of when saturation values are achieved and could be used to design of the HILDA reactor.

Figure 2. HILDA model predictions of THM removals for various A:W ratios as a function of modified Reynolds Number.

VILDA REACTOR DESIGN. Equilibrium or saturation removal conditions in a VILDA system is easier to achieve than with a HILDA system although a HILDA system could be easier to install in the field. The countercurrent flows in a VILDA reactor encourages adequate contact time between the air bubbles and bulk water to achieve saturation conditions quickly as depicted in Figure 3. Basically, in-line diffused aeration is a fast treatment process.

Figure 3. Influence of A:W ratios on VILDA EBCTs required to reach saturation THM removals.

SUMMARY. Both HILDA and VILDA have shown potential to achieve significant reductions in THMs in distribution pipelines especially if the most dominant species is chloroform. Research on bench and pilot scale versions of these posttreatment aeration systems have resulted in prediction models that could be used to design HILDA and VILDA reactors in the field. Field assessment and treatment verification of this innovative technology are currently being explored and developed. Opportunities to participate in these assessment studies are still available. Please inquire within.

REFERENCES

Brooke, E. & Collins M.R., 2011. Posttreatment Aeration to Reduce THMs. Journal AWWA, 103:10.

Komax Systems, Inc. Triple Action Static Mixer. http://www.komax.com/triple-action-static-mixer/ (accessed 3/31/15).

Matter-Müller, C.; Gujer, G.; Giger, G., 1981. Transfer of Volatile Substances from Water to the Atmosphere. Water Research, 15:1271.

McCowan, M.L.,2015. Developing a Horizontal In-Line Diffused Aeration System for Removing Trihalomethanes from Water Distribution Mains. Master’s Thesis, University of New Hampshire, Durham, N.H.

USEPA (United States Environmental Protection Agency), 1999.Alternative Disinfectants and Oxidants Guidance Manual. EPA 815-R-99-014.

USEPA (United States Environmental Protection Agency), 1981. Treatment Techniques for Controlling Trihalomethanes in Drinking Water. EPA/600/2-81/156, Washington, DC.

Zwerneman, J., 2012. Investigating the Effect of System Pressure on Trihalomethane Post-Treatment Diffused Aeration. Master’s Thesis, University of New Hampshire, Durham, N.H.

Developing A Better Understanding of Drinking Water Technology Approval: WINSSS Center Project B1

When EPA in 2014 chose to fund the National Centers for Innovation in Small Drinking Water Systems, their vision for the Centers was much more than developing new drinking water technologies; they asked them to also consider facilitating acceptance of both new and existing technologies, improving relationships between stakeholders, fostering dialogue among regulators, and facilitating the development of uniform data collection approaches for new technologies. All of the non-treatment pieces of the vision have been incorporated into the WINSSS Center’s Project B1.

Project B1 has three objectives:

1. Conduct a survey of the states to determine the barriers and data needs for technology acceptance.
2. Develop a states workgroup and use the survey results as a starting point to discuss how to overcome those barriers and develop a set of uniform data needs.
3. Take the workgroup results and apply them to the New England states to work toward multi-state acceptance.

The first objective has been completed, and the workgroup called for in the second has been meeting every other month since December.

Recognizing the importance of state buy-in to the project, the PI’s proposed to include the Association of State Drinking Water Administrators (ASDWA) as a partner in the survey implementation at the proposal stage. They have been a great partner, and the success of this project is a reflection of their involvement. It was also clear early on  that both Centers had proposed work related to developing a better understanding of acceptance of technologies, so we joined forces. It proved instrumental in the development of the questions, and there were at least eight participants from WINSSS, ASDWA, and DeRISK, that had a hand in the question development.

The survey included 16 questions asking states about their approach to technology acceptance, their experiences with new technologies, barriers to getting these technologies to small systems, data needs for acceptance of any new technology, and their interest in participating in our effort. Forty  states responded, again thanks to ASDWA’s involvement, and the data were telling. We learned that many states don’t consider new technologies for small systems because of cost and risk and that states generally struggle with having the staff and technical expertise to understand and approve new technologies. The most common barriers were a lack of staff and staff time to approve technologies, adequate performance data from vendors, funding for testing/evaluation, and training for state staff.

We asked the states to tell us what questions they needed answered to approve a technology, and over half of the states listed performance data to support the technology, pilot data from multiple locations or water qualities, residuals produced, third party certification and understanding of where technology is appropriate, and understanding the operator skills needed to operate the technology. They also listed the data deficiencies they see most often. These included range of water qualities tested, length of pilot testing, scale of pilot testing, and operating costs, among others.

The good news is that 11 of 14 “emerging” technologies provided to the states in the survey have already been implemented in at least 10 states. This suggests that more technologies are in use than we initially believed and for some technologies, better sharing and communication mechanisms between states are the most immediate needs.

We also asked states how they used the data from EPA’s Environmental Technology Verification Program (ETV) and Arsenic Demonstration Program in accepting new technologies. Nineteen states said they rely on ETV certification or testing protocols as part of their process. Fifteen states said that the Arsenic Demo Program influenced their decisions related to the tested technologies. These programs no longer exist, but they provide valuable insight into how we might consider developing a new program to support the states for sharing data and communicating technology approval information.

The last part of the survey focused on technology acceptance and asked the states if they would be interested in sharing data, developing common standards with neighboring states, or partnering with nearby states to coordinate technology approval. Six states did not answer this question, but 33 of the 34 who did were at least somewhat interested in developing a data sharing network. Twenty-eight states were also interested in developing common standards with nearby states, and 23 were interested in developing partnerships with nearby states to approve technologies. These are very encouraging results.

The survey data were shared with the states, and a workgroup of Centers, ASDWA, and state staffs was formed. The first meeting was in December 2015, and much progress has been made since. The workgroup has developed a draft framework for an entity that would support a shared data repository. They are currently developing a plan/proposal to share with the Interstate Technology & Regulatory Council board (ITRC) to consider how this entity might work with or within the existing ITRC framework. No decisions on this have been made and the workgroup is evaluating options. An open call to all industry stakeholders is planned for late July or early August to share progress to date and to get feedback.

There are no illusions that this can all be accomplished in a short time; the issues and barriers related to technology acceptance have been discussed within the industry for more than 25 years. But this project has created buzz within the industry, as well as with the states and USEPA. It has momentum, and the idea of developing a consensus approach for sharing data and fostering cooperation among all stakeholders that both supports the states need to protect public health and makes it easier for technologies to be accepted by states is now being discussed among all of the relevant players.