Pollutograph Mass Conversion Factor

Regarding the Mass Units Conversion Factor, below is the definition from the EPA-SWMM documentation. We will seek to include this in our Wiki knowledgebase if you find it helpful:

Units Factor

A numerical factor used to convert the units of pollutant mass flow rate into concentration mass units per second. For example, if the inflow data were in lbs/day and the pollutant concentration was chosen as mg/L, then the conversion factor value would be (453,590 mg/lb) / (86,400 sec/day) = 5.25 (mg/sec) per (lb/day). This field does not appear for FLOW inflow, and for concentration-type inflows any value entered will be overriden to 1.0.

More than one constituent can be edited while the dialog is active by simply selecting another choice for the Constituent property. However, if the Cancel button is clicked then any changes made to all constituents will be ignored.

• If a pollutant is assigned a direct inflow in terms of concentration, then one must also assign a time series inflow to flow, otherwise no pollutant inflow will occur. An exception is at submerged outfalls where pollutant intrusion can occur during periods of reverse flow. If pollutant inflow is defined in terms of mass, then a corresponding flow inflow is not required.

Regarding the Mass Units Conversion Factor, below is the definition from the EPA-SWMM documentation. We will seek to include this in our Wiki knowledgebase if you find it helpful:

Units Factor

A numerical factor used to convert the units of pollutant mass flow rate into concentration mass units per second. For example, if the inflow data were in lbs/day and the pollutant concentration was chosen as mg/L, then the conversion factor value would be (453,590 mg/lb) / (86,400 sec/day) = 5.25 (mg/sec) per (lb/day). This field does not appear for FLOW inflow, and for concentration-type inflows any value entered will be overriden to 1.0.

More than one constituent can be edited while the dialog is active by simply selecting another choice for the Constituent property. However, if the Cancel button is clicked then any changes made to all constituents will be ignored.

• If a pollutant is assigned a direct inflow in terms of concentration, then one must also assign a time series inflow to flow, otherwise no pollutant inflow will occur. An exception is at submerged outfalls where pollutant intrusion can occur during periods of reverse flow. If pollutant inflow is defined in terms of mass, then a corresponding flow inflow is not required.

Hi Jesse, thank you for the information, it is very helpful.

I would like to confirm my understanding of how SewerGems interprets the units factor. 

My inflow data is in kg/day and my chosen concentration is mg/L. The conversion factor would be (1000000 mg/kg) / (86400 sec/day) = 11.574 (mg/sec) per (kg/day)?

Thanks,

Maria Blake

What you are doing sounds as if it meets the requirements of the explicit SWMM solver.

The example given in the SWMM manual is for English units so it's difficult to tell which units mass flow are supposed to be. The original SWMM manual is not clear on this.

The underlying equation is

M = Q x C

Where M = mass flow rate, Q = flow and C = concentration.

A good check is to do a hand calculation where you know Q and C and calculate M. Then run it through SWMM and see if the results agree. 

Note that I have added some information based on this discussion to the following related Wiki article: Prediction of quality characteristics of sewers

Thanks to you both, Tom and Jesse!

After having the above discussion, I completed some testing of the mass conversion factor. My model and pollutant data are originally in SI units, but I also tested out US units since the SWMM documentation only gives an example for US units.

Below are pollutant results with the model/pollutant data in US units (lb/day, CFS, ft, etc..) and using conversion factor of 5.25 (mg/sec) per (lb/day) as explained in the SWMM documentation. I considered these results to be correct.

Below are results with my model in SI units (kg/day, LPS, meters, etc..) and using a factor of 11.574 (mg/sec) per (kg/day) that I calculated above:

The results are different which led me to believe that I calculated the wrong conversion factor when using SI units.

Oddly enough, when using SI units, I could only achieve the same results when using the SWMM recommend conversion factor of 5.25 (mg/sec) per (lb/day):

I read in Units in SWMM output reports - OpenFlows | Water Infrastructure Wiki - OpenFlows | Water Infrastructure - Bentley Communities that "when the SWMM solver is used, the program always computes in US Customary Units". 

From the testing above, my theory is that the mass unit conversion factor computes for US units even if the entire model and pollutograph data are in SI units, which isn't very intuitive.

Perhaps this mass unit conversion factor for pollutographs can be further tested to determine how it should be used for US/SI units and more information can be provided for SewerGems users? The information in the EPA SWMM documentation is not detailed, and I am not confident that my proposed theory is correct.

Thanks,

Maria Blake