Unlimited Concentration

Question

Hello 
 What does 'Concentration Limit' do if 'Unlimited Concentration' box is unchecked and a value set for it? I did it but nothing happened for 'Concentration (Calculated)'. On which it affects? 
 Sim

Mal Sharkey · Accepted Answer

Hi Sim, 
 
You set a concentration limit of 10 mg/L, then entered an initial concentration of 20 mg/L at the reservoir. This initial concentration is higher than your limit! How do you suggest the software handles this? Ignore your user input? 
 
I can call you to discuss if you like. Where are you located? Your IP address says you are from Iran, but I checked and we don't have any official users there. 
 
Regards, 
 
Mal

Scott Kampa · Answer

Hello Sim, 
 
Unlimited Concentration would mean there is no upper limit for concentration of a constituent. Constituent analyses are usually computed for a relatively long period, such as over a course of a number of days. For instance, with a model run time of 228 hours, you should see the concentration will be capped at the Concentration Limit you enter. You may also want to use a smaller hydraulic time step, as this can help with the water quality results. 
 
Keep in mind that your input should make sense based on the constituent you are modeling, as this will impact the results. Bad data into the model will mean bad results. Also the limit is meant to be limit on the concentration over time. So a reasonable run time and input are needed. The limit will obviously be some reasonable value larger than the initial input. If you add 1 mg/L of chlorine at a reservoir, a limit of 1 mg/L as well would not be reasonable. A limit of 10 mg/L might be more reasonable. 
 
Regards, 
Scott

Scott Kampa · Answer

Hello Sim, 
 
You have an initial concentration of 1.2 mg/L and a concentration limit of 1.0 mg/L. This by itself is not intuitive. The concentration limit is meant to stop an exponential growth of the constituent and should be set to a reasonable value. The following forum post has some information on this: communities.bentley.com/.../9019 . 
 
That said, the concentration at the downstream junction is less than 1.0 mg/L anyway, regardless of the limit. This is because of the other factors in the Constituent manager (bulk reaction rate and first order wall reaction rate). That might be one reason there is no change: it is already below the limit of 1.0 mg/L. But again, a smaller value for the concentration limit really doesn't make much sense in this case, since it would still be smaller than the initial concentration. 
 
Basically, the concentration limit should be set to a reasonable value to inhibit exponential growth. The value you will obviously be related to your system. 
 
Regards, 
Scott

Scott Kampa · Answer

Hello Sim, 
 
1) This is likely occurring because you have concentration limit set to a value less than initial concentration. This is not really a valid setting. A future release of WaterGEMS will have a user notification generate when something like this occurs. 
 
2) The results are highly dependent on only on the input (diffusivity, bulk reaction rate, first order wall reaction rate), but the setup of the model too. A larger model with high demands, tanks, etc., may see different results. As mentioned in the forum link I included with my earlier email, the user was see a large concentration in his model. The concentration limit is meant for cases like this. 
 
Regards, 
Scott

Jesse Dringoli · Answer

1) Imposing a concentration limit does not stop the calculations to stop. It will cap the concentration value in the calculations at the limit you enter. 
2) Try entering a positive value for the bulk/wall reaction rate. Example1 has a scenario and some constituents set up for you to experiment with. 
 
Are you asking these questions because of a particular study that you're doing? Some background information on that would help you get answers that are not academic but that actually relate to what you ultimately need to do.