Treatment Functions and BMP Effectiveness

[transeco]

I'm trying to model the effectiveness of curb inlet filter insert BMPs using SWMM 5.0.007.

One option, according to the on-line help, is to enter a treatment function such as the fractional removal of a pollutant via the "Treatment" property of a Node object. For example, if the filter insert removes approximately 75 percent of total suspended solids from stormwater, the treatment function would be R=TSS*0.75.

While this seems intuitive enough, the report status summary shows very little variation when I tweak the node treatment properties.

For instance, the Wet Weather Inflow (WWIn), for the model is a constant 155,000 kg. Without any treatment specified for the nodes, the External Outflow is 95,000 kg. External outflows for three filter insert removal efficiencies are shown below:


R=TSS*0.10
External Outflow is 28,750 kg

R=TSS*0.40
External Outflow is 28,598 kg

R=TSS*0.50
External Outflow is 28,573 kg

R=TSS*0.75
External Outflow is 28,573 kg

Does anyone have an idea as to: (1) why, at 10 percent removal efficiency, external outflows are reduced approx 70 percent and also; (2) why there is little variation in the above outflow values?

Thanks for your help.
Scott Smith