|Don’t Be Baffled: New Guidance for Baffling Factors for Small Systems
by Mark Henderson, Senior Review Engineer, Water Quality Division of the Colorado Department of Public Health & Environment
Baffling factor requirements for insuring that your system has adequate contact time for disinfection can be, frankly, baffling. Understanding baffling factors and how it plays a role in disinfection could mean the difference between your system spending a few hundred dollars or a few thousand. A recent study completed by Colorado State University for the Colorado Department of Public Health and Environment clears up some of the confusion and provides accurate guidelines for how baffling factors impact the required contact time for disinfection.
Disinfection requirements are typically expressed in terms of a “log inactivation.” These terms are used to communicate what percentage of waterborne pathogens may be killed (inactivated, during the disinfection process). A 1-log inactivation represents a 90% kill of pathogens while 4-log represents 99.99% kill. In order to calculate the level of inactivation the term “CT” is used. This is the product of the residual concentration “C” of a disinfectant added to the water and the contact time “T”. The contact time (also referred to as T10) is defined as the time required for the first ten percent of a pulse of disinfectant tracer to travel through the disinfection segment to the residual sampling point. The baffling factor (BF) is multiplied by the detention time at the peak flow through the disinfection segment to ultimately determine “T”. Based on a given set of water quality parameters and flow rate; the required size for a contact tank is ten times larger at a BF=0.1 versus the required size at BF=1.
Pressurized and non-pressurized tanks seem to be most often utilized as contactors for small systems. Below are some key points and hints for proper implementation.
- BF achieved can vary greatly dependent on type and number of tanks.
- Can achieve a BF up to 0.55 with multiple tanks.
- Multiple tank systems must have tanks installed in series and remain full.
- When installing multiple tanks, check that pressure drop across tanks is not too high.
- The inlet and outlet of a tank must not be on the same side. Typical bladder tanks receive zero credit based on shared inlet/outlet configurations.
- Typically achieve a BF between 0.1 and 0.2.
- Inlet and outlet to the tank should be separated as much as possible.
- Tank geometry is important. Rectangular, “doorway” style tanks perform better than round, cylindrical tanks.
- Inlet configurations can impact the BF of the tank with a tee inlet performing the best over a larger range of flow rates (10-30 gpm).
Inlet manifolds and packing material may provide a good option for systems that already have contact tanks in place but find it infeasible or too costly to install additional tanks or install additional chlorine contact systems.
The complete documentation and results of the study can be accessed at the CDPHE Water Quality Control Division webpage.
Photo right: Random Packing Material ( from the Baffling Factors Guidance Manual)
©2015, Rumbles, November 2014. All rights reserved. Excerpted with permission