Tracer studies and mixmaster tanks
Starting with the publishing of Safe Drinking Water Program Policy 4 in 2010, the department division began an effort to reassess all surface water treatment plants to ensure they use proper disinfection of surface water to meet both 4-log removal and inactivation of viruses (i.e. 99.99% removal and kill) and 3-log removal and inactivation of Giardia Lamblia cysts (i.e. 99.9% removal and kill). In 2014, the reassessment effort was restructured to provide more guidance and assistance to public water systems, and was named the Disinfection Outreach and Verification Effort (DOVE). DOVE includes an assessment of disinfection and documents treatment at all active surface water treatment plants. There were approximately 360 treatment facilities active in 2014. To date, the division has completed roughly 150 of those assessments with approximately 210 more to go.
A major issue that has arisen over the years is whether a treatment plant is over-estimating its baffling factor that it uses in relation to a tank that achieves contact time to kill viruses or bacteria. The baffling factor is essentially a penalty applied to each tank or pipe segment that estimates short circuiting in that particular tank. Most unbaffled tanks have a high degree of short circuiting, so these tanks typically receive credit for about 10% of the operating volume. For example, a rectangular concrete clearwell that operates at 10,000 gallons may only use 1,000 gallons in the calculation for disinfection treatment. Therefore, a baffling factor of 0.1, which represents 10% of the volume, is approved for most tanks. However, if there are special flow configurations and baffle walls within a tank, a tracer test may be run at the facility to determine the volume of water acting towards disinfection. This empirical testing at a facility is the most accurate method for determining whether the proper assumptions are being used to perform disinfection calculations, and the division recommends that each public water system runs the empirical testing at their own facilities.
Prior to the 2014 DOVE initiative, the division worked with the Colorado State University to determine appropriate baffling factors for pressurized tanks in series. These results were published by the division and CSU together. Also prior to 2014, a commercially available tank called the Flexcon® Mixmaster BAF120 was being used at certain public water systems. The Flexcon® company claimed a baffling factor of >0.8. After review at the time, the division allowed 0.7. In 2015, the division received information that perhaps the 0.7 baffling factor was an overestimation. Therefore, since the division had allowed many of these tanks to be installed, the division performed a set of baffling factor tests on the BAF120 tank and determined that the 0.7 baffling factor is correct. The division published an alternative acceptance document summarizing our findings. It is important to remember that the BAF120 tanks must be plumbed correctly and inspected during installation to ensure proper hydraulics (see figure).
Recently, additional manufacturers have come to market with tank systems that are designed to receive baffling factors similar to the Flexcon® Mixmaster tank series. Most of these commercially available tank systems are meant to assist small systems that have limited footprints and budgets in achieving appropriate disinfection contact time. Unlike the previous effort with the BAF120 tanks, the division will not perform tracer testing on behalf of manufacturers. Rather, the division will accept third-party-generated tracer test data and analysis that meet our tracer test guidance and make a determination of the baffling factor. Once a determination is made, the division will publish the baffling factor acceptance on our alternative acceptance page. Prior to gaining acceptance, proposed tanks will be approved with a 0.1 baffling factor unless a site specific tracer study is performed.
➽ Tyson Ingels, lead drinking water engineer and Gordon Whittaker, drinking water coach
