A. Applicants will frequently need to evaluate the potential pollutant removal effectiveness of stormwater practices when conducting a pollutant loading analysis. To do this, applicants can use the rated pollutant removal effectiveness as listed in the tables in §§ 8.38(D) and (E) of this Part as a basis of estimating pollutant removal. These values have been derived from a variety of sources based on actual monitoring data and modified, where appropriate, to reflect the specific design and sizing criteria contained in §§ 8.19 through 8.35 of this Part.B. In some cases, the pollutant removal rating values use median values from prior monitoring studies when the studies included a significant number of facilities of similar design criteria as those required in this Part. In other cases, the 75th quartile values (or high end) are reported where it is recognized that the prior monitoring was of insufficient sample size or was of practices with design criteria not as robust as those required in this Part. Lastly, in many cases, there is insufficient prior monitoring of practices for some or all of the reported pollutants, but primary pollutant removal mechanisms are similar to other practices; thus, a removal value is assigned, based on general literature values and/or as a policy decision. In addition, most of the design criteria for water quality BMPs incorporate pre-treatment requirements, such as the requirement for a forebay or grass channel prior to infiltration. In these cases, the rated removal efficiency of the practice is for the total system.C. In general, where pollutant loading assessments are requested, applicants may use the rated removal values as a basis for estimating pollutant load. However, other pollutant removal estimates may be acceptable, provided the applicant submits the source of these estimates and data used. All information supplied by the applicant will be reviewed by the approving agency to determine the relevance of the removal estimates to the situation.D. Pollutant Removal Efficiency Rating Values for Water Quality BMPs. Water Quality BMPs (Those Meeting Minimum Standard 3, § 8.9 of this Part) | Median Pollutant Removal Efficiency (%) |
TSS | TP | TN | Bacteria |
WVTS | Shallow WVTS | 85%2 | 48%3 | 30%2 | 60%2 |
Gravel WVTS | 86%3 | 53%1 | 55%3 | 85%2 |
Infiltration Practices | Infiltration Basin | 90%2 | 65%3 | 65%2 | 95%2 |
Infiltration Trench | 90%2 | 65%3 | 65%2 | 95%2 |
Subsurface Chambers | 90%2 | 55%2 | 40%2 | 90%2 |
| Dry Well | 90%2 | 55%2 | 40%2 | 90%2 |
Permeable Paving | 90%1 | 40%1 | 40%2 | 95%2 |
Filters | Sand Filter | 86%3 | 59%3 | 32%3 | 70%2 |
Organic Filter | 90%2 | 65%2 | 50%2 | 70%2 |
Bioretention | 90%1 | 30%2 | 55%2 | 70%2 |
Tree Filter | 90%1 | 30%2 | 55%2 | 70%2 |
Green Roofs | Green Roofs | 90%4 | 30%4 | 55%4 | 70%4 |
Open Channels | Dry Swale | 90%1 | 30%2 | 55%2 | 70%2,6 |
Wet Swale | 85%3 | 48%3 | 30%2 | 60%2 |
1 UNHSC, Roseen, R., T. Ballestero, and Houle, J. 2007b. UNH Stormwater Center 2007 Annual Report. University of New Hampshire, Cooperative Institute for Coastal and Estuarine Environmental Technology, Durham, NH.
2 Center for Watershed Protection. 2007. Urban Stormwater Retrofit Practices. Urban Subwatershed Restoration Manual Series - Manual 3. Ellicott City, Maryland.
3 Fraley-McNeal, T. Schueler, R. Winer., 2007. National Pollutant Removal Performance Database, v. 3. Center for Watershed Protection. Ellicott City, MD.
4 Prescribed value based on general literature values and/or policy decision.
5 50% of reported values of low end for extended detention basins.
6 Presumed equivalent to bioretention; will require diligent pollutant source control to manage pet wastes in residential areas.
E. BMP Pollutant Removal Rating Values for Other BMPs Other BMPs | Median Pollutant Removal Efficiency (%) |
TSS | TP | TN | Bacteria |
Pretreatmen t BMPs | Grass Channel | 70%1,2 | 24%3 | 40%2 | NT |
Sediment Forebay | 25%4 | 8%5 | 3%5 | 12%5 |
Filter Strip | 25%4 | ND | ND | ND |
Deep Sump Catch Basin | 25%4 | NT | NT | NT |
Hydrodynami c Device | 25%1 | NT | NT | NT |
Oil and Grit Separator | 25%4 | NT | NT | NT |
Storage BMPs | Dry Extended Detention Basin | 50%2 | 20%2 | 25%2 | 35%2 |
Wet Extended Detention Basin | 80%3 | 52%3 | 31%3 | 70%3 |
Underground Storage Vault | 20%2 | 15%2 | 5%2 | 25%2 |
"ND" means no data.
"NT" means no treatment.
1 UNHSC, Roseen, R., T. Ballestero, and Houle, J. 2007b. UNH Stormwater Center 2007 Annual Report. University of New Hampshire, Cooperative Institute for Coastal and Estuarine Environmental Technology, Durham, NH.
2 Center for Watershed Protection. 2007. Urban Stormwater Retrofit Practices. Urban Subwatershed Restoration Manual Series - Manual 3. Ellicott City, Maryland.
3 Fraley-McNeal, T. Schueler, R. Winer., 2007. National Pollutant Removal Performance Database, v. 3. Center for Watershed Protection. Ellicott City, MD.
4 Prescribed value based on general literature values and/or policy decision.
5 50% of reported values of low end for extended detention basins.
6 Presumed equivalent to bioretention; will require diligent pollutant source control to manage pet wastes in residential areas.
F. Using the rated efficiencies from the tables in §§ 8.38(D) and (E) of this Part, applicants can reduce post-development loadings to receiving waters when BMPs are designed, installed, and maintained in accordance with the provisions of this Part.G. Estimating Pollutant Removal of BMPs in Series 1. In some cases, applicants may have one or more BMPs installed in a series as a so-called "treatment train." In these cases, available research has shown that the pollutant removal efficiency of specific BMPs, for specific pollutants, is reduced for subsequent BMPs in the treatment train arrangement. As stormwater migrates through the treatment train, coarser-grained particles are preferentially removed by the prior BMP, leaving progressively finer particles for practices down the line. The result is that for pollutants associated with fine particulates, removal efficiency drops off significantly.2. To account for this phenomenon, a widely applied and generally accepted method has been to discount the rated removal efficiency of the second BMP by a factor of between 75% and 50%, with subsequent BMPs being reduced accordingly.3. The Georgia Manual Method applies BMP removals as below. This method does not apply to bacteria, where removal is more a function of die-off than settling/attenuation; thus, the full efficiency is applied to subsequent BMPs.a. 100% of the rated TSS removal efficiency to the first BMP (1) If the TSS removal efficiency > 80% for the first BMP; removal efficiency for the second BMP = 50% (regardless of the pollutant constituent).(2) If the TSS removal efficiency <80% for the first BMP; removal efficiency for the second BMP = 75% (regardless of the pollutant constituent).b. For succeeding BMPs, removal efficiency is applied at either 50% or 75% depending on the equivalent TSS removal efficiency for the preceding BMPs (regardless of the pollutant constituent).250 R.I. Code R. 250-RICR-150-10-8.38
Amended effective 11/13/2018