In the last few years, most California cities have begun regulating the permanent stormwater BMPs that are required to be implemented with new construction or redevelopment projects. There is an overarching theme to the various Post-Construction Design BMP Manuals that have been recently published, which is, the requirement to utilize Low-Impact Development (LID) or Green-Infrastructure BMPs where at all possible.
Biofiltration areas are one of the most commonly known and referenced options for post-construction LID BMPs, yet California has far too few examples of successful biofiltration options. That’s because traditional biofiltration systems take up too much space, get clogged easily, are difficult and cumbersome to maintain, and sometimes never worked in the first place. Convergent Water Technologies and their regional distributors have a great solution for the problems seen with traditional biofiltration. The solution is FocalPoint High Performance Modular Biofiltration System (HPMBS).
FocalPoint’s next generation biofiltration media is the key to its high performance. This advanced media technology provides high flow rates, pollutant removal rates consistent with traditional bioretention, and the option of increased removal characteristics for specific pollutant targets. Media flow-rate impacts system performance in all aspects, from scale and cost-effectiveness to factors of safety, longevity and maintenance minimization. Infiltration flow rates for FocalPoint standard media exceed 100” per hour.
At 100” per hour, FocalPoint is able to meet and even exceed the pollutant reduction requirements set forth by municipalities. City Stormwater BMP Manuals typically define options for documenting compliance as an Alternative Biofiltration Practice. Along with satisfying the requirements of LID and proprietary media for alternative biofiltration options, the FocalPoint system is an acceptable alternative due to its third-party pollutant treatment performance consistent with performance levels associated with Technology Acceptance Protocol-Ecology certifications.
Using the city of San Diego as an example, the following performance results and testing protocols illustrate how and why FocalPoint is an ideal and acceptable alternative biofiltration system. Appendix F of the 2016 City of San Diego and San Diego County Stormwater BMP Manuals define several options for documenting compliance as an Alternative Biofiltration Practice. Since FocalPoint has third-party pollutant treatment performance of the system consistent with performance levels associated with Technology Acceptance Protocol-Ecology certifications, it falls under approved City and County Alternative Biofiltration Practice Criteria.
Table 1. A listing of State or Localities where BMP is currently approved for use
City of San Diego Flow Thru Proprietary Practice Criteria
Appendix B.6.2.2 of the 2016 City of San Diego Stormwater BMP Manual provides criteria for selecting flow thru proprietary treatment BMPs. This includes meeting the performance standard in Table B.6-3 below as certified through third-party, field scale evaluation. It is important to note that there is no exclusion of other field scale verification of 80% TSS.
The Convergent-ACF Alliance commissioned a field study through a third-party consultant in October of 2014 and installed our system for field evaluation in July of 2015. The location of the field study was Campbell’s Run Road in Pittsburgh, PA, a cold climate with and mean annual rainfall of 38.9 inches per year. The area is characterized by the United States Natural Resource Conservation Service (NRCS) as a Type II rainfall distribution, which is indicative of intense, short-duration rainfall events that produce greater runoff volumes than a Type I or IA rainfall distribution, characteristic of Washington State and Southern California. Therefore, our system sizing would be considered conservative on the high side compared to these States.
Results of this third-party field study can be found in the Final Technical Evaluation Report (TER). A summary of our TSS pollutant removal performance and Appendix Table B-6.3 performance criteria is summarized in the Tables 2, 3 and 4 below.
Table 2. Summary of Removal Efficiencies from TER
Table 3. Summary of Individual Storm Removal TSS Efficiencies from TER
Table 4. Comparison of Field Study results to Performance Criteria
FocalPoint is a modular, high performance biofiltration system that often works in tandem with other integrated management practices (IMP). Contaminated stormwater runoff enters the biofiltration bed through a conveyance swale, planter box, or directly through a curb cut or false inlet. Energy is dissipated by a rock or vegetative dissipation device and is absorbed by a 3-inch layer of aged, double shredded hardwood mulch, with fines removed, on the surface of the biofiltration media.
As the water passes through the mulch layer, most of the larger sediment particles and heavy metals are removed through sedimentation and chemical reactions with the organic material in the mulch. Water passes through the biofiltration media where the finer particles are removed and numerous chemical reactions take place to immobilize and capture pollutants in the soil media. In every application, the FocalPoint HPMBS is designed and maintained in a manner consistent with its performance certification.
The FocalPoint should be sized and configured based on our field study to include the following.
- The FocalPoint system must be sized in accordance with the manufacturer’s latest field test results and manage the water quality event defined by the local jurisdiction using a combination of filter bed area, ponding depth and ponding volume.
- The surface area of the media within the FocalPoint must be a minimum of 174 square feet per 1 acre of impervious area treated (26 sq. ft. per 0.15 acres). The thickness of the media is to be at least 1.5 ft. (18 inches) and the recommend ponding volume above the FocalPoint will be 5 times the filter bed area (e.g., 5 CF of ponding volume to 1 SF of filter bed) or alternatively, 25% of the WQv.
- The FocalPoint system consists of five components that include: 1) an open cell underdrain; 2) a wide aperture separation mesh wrap around the underdrain; 3) a layer of clean washed, 3/8” diameter bridging stone; 4) advanced high flow rate engineered media with an infiltration rate of 100 inches per hour; and 5) double shredded hardwood mulch. These components are built from the bottom up to create a mostly permeable profile that measures 3 feet from bottom of underdrain to top of mulch.
- The ponding depth above the mulch surface is typically 6 to 12 inches and varies based on site conditions. An overflow outlet should be placed above the ponding depth.
- The FocalPoint system requires the establishment of vegetation that is tolerant of wet and dry conditions. Plants that are not performing as desired should be replaced as needed.
California Filtration Specialists, LLC., is the exclusive California Distributor of the next generation of biofiltration. Contact us today at 858-705-6483 for assistance with introducing FocalPoint as an alternative BMP to your local municipality. FocalPoint systems are designed and maintained in a manner consistent with its performance certification; which will get you approval in even the most stringent of municipalities. FocalPoint will save you space and money on your biofiltration areas, and give you back building pads or parking spaces. It is 20% more efficient than traditional biofiltration, it doesn’t clog, and has high-performing pollutant-reduction rates. FocalPoint takes the outrageous cost and space requirements out of Green-Infrastructure.