Volume based crediting for street sweeping
The Minnesota Pollution Control Agency 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 metods described in this manual can be used. The preference for volume-based approaches is due to easier measurement of volume compared to mass. Volume of sweeping hoppers is know and there is no need to determine the water content of the sweeping material.
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 applied 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 |