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Trihalomethane Removal In Municipal Drinking Water Storage Tanks

Mix Only Vs Mix-Ventilation Vs Spray Aeration Strategies
Thursday, May 25, 2017

There are many, many considerations to take into account when researching and implementing a trihalomethane (THM, TTHM) removal strategy for your drinking water system.  It can all get very complex in a hurry.  

For the sake of this blog article however, we are going to focus only on THM volatilization removal strategies through the prism of water surface area.
 

THM Removal Via Volatilization

THMs are removed by volatilization (evaporation) into the atmosphere.  For this to happen, the THM (also know as a volatile organic compound or VOC) must come into extremely near contact with the atmosphere.  

How close?  

Approximately 300 angstroms (30 nanometers) close or in laymen's terms, pretty much touching the atmosphere.  Within the 300 angstrom “skin" of the surface, there is a very good probability for THM volatilization. Any further away and the THM removal probability decreases significantly.


The Tank

A tank has a given water surface area and no matter the rise or fall, ins or outs, this surface area remains relatively constant.  For instance, a 100 ft diameter tank will always have an area of 7850 square feet.  Never more. Never less.   Very little THM removal can be achieved in this condition so we’ll give it a starting volatilization score of 1.00.

 

The Mix Only Strategy

Mixing a tank does not increase the surface area of the water but it does provide constant renewal which subsequently exposes more THMs to possible volatization.  Increased THM removal will take place; however, even with Medora’s mixers (by far the most powerful in the marketplace), any Mix Only strategy would have a volatilization score of only about 1.03.

*Note: Mixing does have other advantages that help lower THMs (such as as reducing or eliminating the need to boost chlorine which in turn reduces THM formation); however, this article is solely about THM volatilization as it relates to total water surface area.

 

The Mix / Ventilation Strategy

You can bolster the Mix Only Strategy by introducing headspace ventilation to avoid THMs from re-condensing (with no place to go, any removed THMs may re-enter the water).  Headspace ventilation does nothing to increase the surface area but it does assist removal just a teeny tiny bit more by clearing the headspace of any already volatilized THMs.  Mix / Ventilation is better than a Mix Only strategy but only by a very thin margin.   Our experience indicates a volatilization score of 1.05 at most.

 

If all that is needed is negligible THM removal (and for some tanks, this is the case), the Mix Only or Mix / Ventilation strategies may be worth considering.

 

The GridBee® Spray Aeration Trihalomethane Solution

Again, the more water surface area exposed to the atmosphere, the better probability for THM volatilization.  Spray aeration does this extremely well.  Many of these details are proprietary in nature but generally speaking, one spray aeration unit can easily triple the amount of surface area of a 100 foot diameter tank in just a 15 foot diameter spray footprint.  Huge amounts of new surface area are continually created every second and this increases the THM removal probability substantially.  With greatly improved THM volatilization, effective headspace ventilation becomes a much more important consideration to remove THMs from  the headspace (more THMs volatilized, the more need to effectively evacuate THMs out the tank).

The GridBee® Spray Aeration strategy scores well ahead of the pack at a minimum volatilization score of 4+ in the system mentioned above.
 

Call Us To Discuss THM Removal Solutions For Your System

As mentioned before, the overall trihalomethane removal process is a little more complex and a thorough evaluation is needed to ensure the best possible THM mitigation & removal strategy for your system.  GridBee® SolarBee® potable water equipment can provide an all of the above solution including Mix Only, Mix / Ventilation, and Spray Aeration.  

However you want to implement your THM removal plan of action, Medora Corporation can help.

 

Give us a buzz at 866-437-8076 or
Contact Us button for AP500 lift station wet well mixing equipment
through our website!

 

Want to learn more about the GridBee® SN Series THM Removal System?

Check out all the how's and why's in this white board video!