**Subject:** Mass and Concentration Water Quality Loadings in SWMM 5

If you have a time series of flow and water quality at a node in SWMM 5 you have the option of using either a Mass loading or a concentration loading (Figure 1). If you have a concentration then the load to the node internally in SWMM 5 is the flow times the concentration. Alternatively, if you have Mass loading then the program will calculate the concentration from the flow and the load. The table below shows some combinations of flow in cfs and load in pounds per day to yield various BOD 5 concentrations it the network nodes and links (Figure 2). For example, at a flow of 10 cfs you can get a BOD5 concentration of 100 mg/l with a loading of about 5400 pounds of BOD5 per day (Figure 3).

**Figure 1.** If you use a time series load in SWMM 5 you need TWO time series, one for the flow and one for the mass load or concentration.

**Figure 2.** The Mass loading needed to generate a concentration at a particular flow rate.

Flow (CFS)

BOD5 (mg/l)

BOD 5 Pounds Per Day

1

1

5.39

1

10

53.89

1

50

269.45

1

100

538.89

1

200

1077.79

10

1

53.89

10

10

538.89

10

50

2694.47

10

100

5388.93

10

200

10777.87

100

1

538.89

100

10

5388.93

100

50

26944.66

100

100

53889.33

100

200

107778.66

**Figure 3**. The calculated BOD 5 concentration in the link from the Mass Loading.