Dm for monochloramine?

Question

While performing a chlorine constituent analysis I noticed the default molecular diffusivity (Dm) value for the WaterCAD is 1.3e-8, which is correct for free chlorine (HOCL-). Our system, like many others uses chloramines for secondary/distribution disinfection. Does anybody know the Dm for monochloramine?

Jesse Dringoli · Answer

Hello Wayne, 
 I asked around and received some feedback on this from a few of my colleagues. 
 Determining the diffusivity may be difficult. Historically, one source of values for things like this is the Perry's handbook of Chemistry or CRC Tables. 
 Depending on the relative size of diffusivity vs, advection, often the diffusivity is negligible. So, a sensitivity analysis may help; start with chlorine and see how sensitive results are to this parameter. 
 One thing to consider is that a regular constituent analysis may not work well for Chloramine. You may want to look into the Multi-species Extension (MSX) functionality, which is included in recent versions of WaterCAD and WaterGEMS. You can read more about MSX in the Help and here: 
 What's New with WaterGEMS/WaterCAD V8i SELECTSeries 5 
 Here is an article on our Wiki about challenging with mixing Chlorine and Chloramines: 
 Mixing Chlorine and Chloramines 
 If you need more help, you might try posting on other places such as the EPANET listserv from the University of Guelph: 
 LISTSERV.UOGUELPH.CA LISTSERV Server (16.0) LISTSERV@LISTSERV.UOGUELPH.CA 
 We'd be interested to hear about successes you have with modeling this using MSX.

Tom Walski · Answer

Modeling chloramine decay can be straightforward. Measure the bulk decay in a bottle decay test in the lab and adjust the wall coefficient based on field measurements to achieve calibration just as you would for free chlorine. 
 
Modeling what happens at a chloramine booster station is much more difficult. You could model it using the multi-species extension (MSX) option in WaterGEMS. However, you probably don't have all the coefficients needed to do this. 
 
I'm skeptical about the value of simply adding chlorine as you mention because of breakpoint chlorination issues. You must do some lab experiments to determine dosages for your water. I would think that you would need to add both chlorine and ammonia but I'm not sure if you can get by with only using stoichiometric feed rates because you can't be sure of the chloramine coming into the booster. 
 
This is gong to require a pretty sophisticated control setup. 
 
Once you figure out how to achieve a set chloramine concentration coming out of the booster, you can model the downstream system using a Setpoint Booster with a known outlet concentration. 
 
This is a downside of using chloramines. If you don't get it right, you can end up with no residual disinfectant and/or nitrification.