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.