Volume based crediting for street sweeping

The Minnesota Pollution Control Agency (MPCA) has established mass-based credits for phosphorus reductions associated with street sweeping. These mass-based approaches are based on relationships established between mass of leaf or sweeping material collected and phosphorus concentrations in that material. For information on mass-based methods, including credits and methods for determining mass and phosphorus in sweepings, see this section in the stormwater manual.

Street sweeping entities indicate a preference for volume-based methods for determining phosphorus credits associated with street sweeping. With volume-based approaches a relationship must be established between the volume of material collected and the mass of that material. If volume can be converted to mass, then the phosphorus credit methods described in this manual can be used. The preference for volume-based approaches is due to easier measurement of volume compared to mass since the volume of sweeping hoppers is known.

Volume to mass conversion requires knowing the bulk density of the material collected. A literature review indicated several entities in the U.S. developed relationships between bulk density and material collected. However, the methods for determining these values and the variability in sweeping materials creates uncertainty in deriving a specific conversion value. Bulk densities show a wide range in values, and these are dependent on factors such as time of year and water content. The adjacent table summarizes the data found in the literature. To see the technical memo summarizing the literature review, link here.

Source	Overall bulk density (g/cm3)	Leaf drop bulk density (g/cm3)	Non-leaf drop bulk density (g/cm3)
San Diego		0.8	1.1
Hobbie et al.		0.2	0.8
Massachusetts DOT	1.8
Incline Village, NV	1.36

Screen shot of MPCA Street Sweeping Credit Calculator, using Option 2 to determine TP removed from initial input of wet volume converted to wet mass for a non-leaf collection event

The MPCA has not yet established and approved a specific credit based on volume measurements. However the following two approaches may be used to estimate phosphorus reductions until a specific credit is established.

• Assume a bulk density of 0.5 g/cm³ for sweepings collected during leaf drop and a value of 1.0 g/cm³ for non-leaf drop sweepings.
• Determine bulk density of sweepings. The procedure for this is described below, though it should be noted this procedure has not formally been approved by the MPCA.

Measured solids converted to wet mass using a bulk density value A community may choose to conduct their own bulk density analysis instead of using the default bulk densities provided by MPCA. After determining the volume of swept material, a representative subsample of sweeping material should be collected and sent to a laboratory for bulk density analysis. This lab-based bulk density replaces default values of 0.5 or 1.0 g/cm³.

Wet mass (g) = wet volume (in cubic yards (yd³) x 0.7646 (yd³/m³) x 1000000 (cm³/m³) x bulk density (g/cm³)

Wet mass (pounds) = wet mass (g) x 0.002205 (g/pound)

Example for volume measurement based using a determined bulk density A small town collected 5.5 yd³ of street sweeping debris during street sweeping efforts in June. The town wants to replace the default bulk density values with bulk density specific to their sweeping efforts. A subsample is collected from the sweeping material and sent to the lab for analysis. The result is a lab-calculated bulk density of 1.4 g/cm³. The conversion to wet mass is as follow.

Wet mass (g) = 5.5 yd³ x 0.7646 (yd³/m³) x 1000000 (cm³/m³) x 1.4 g/cm³ = 5,887,420 g

Wet mass (pounds) = 4,205,300 x 0.002205 (g/pound) = 12,981.8 pounds.

The wet mass can now be entered into Option 2 in the Credit Calculation Tool, and the resulting TP reduction is 4.20 pounds. Note: If the initial volume is measured in m³, the yd³/m³ conversion factor should be omitted from the above calculations.