Stormwater Best Management Practices in an Ultra-Urban Setting: Selection and Monitoring
Fact Sheet - Organic Media Filters
There are two types of organic filter media typically used for stormwater management - peat/sand and compost. The use of organic media in surface or subsurface filter designs is intended to provide a higher level of stormwater treatment than a sand-only filter. Both of these organic media are typically installed in filters to depths between 460 to 600 mm (18 to 24 in), and are drained by piped underdrain systems. (Figures 15 and 16 illustrate typical filter cross sections.)
The organic media filters improve water quality through a combination of sedimentation, filtration, and adsorption processes. The sedimentation section located just upstream of the filter section serves as pretreatment, removing larger diameter suspended solids and capturing floating hydrocarbons. Partially treated stormwater then flows slowly into the filter section where fine-grain material is strained from stormwater as it passes through the filter media.
The subsurface or underground filter design is well adapted for applications with limited land area and provides turnkey performance that is independent of local soil conditions, groundwater levels, and other factors. The underground filter design typically consists of a multi-chamber vault that is completely below grade and is covered with a grating or structural concrete. It is most useful for multipurpose land uses, that is, where committed land area will also be used for automobile parking or for public parks. The surface filter design, sometimes called the Austin filter, also consists of a multichambered facility. While most of the filter is located at or slightly below grade the filter is not covered and so requires a commitment of land area (refer to the Fact Sheets on Underground Sand Filters and Surface Sand Filters for additional information).
As with other stormwater filters, the purpose of organic media filters is to manage the first flush, which typically contains the highest concentration of pollutants. If designed as an off-line facility, however, such filters can provide true capture and treatment of any water quality volume.
A number of design variations or proprietary systems featuring organic media are currently available (e.g., CSF® Stormwater Treatment System, now StormFilterTM). While these systems basically use the same treatment mechanisms, there are differences in the size of settling areas or chambers, loading rates, and media configuration.
Applicability
Organic media filters can be used in underground and surface filter designs. Of these, the underground sand filter is considered to be more applicable to the ultra-urban setting. It requires a small commitment of land area, provides dependable service, and is relatively effective in removing urban pollutants. Furthermore, its design is inherently flexible, and the size and shape of the unit can be set based on local requirements.
Surface filter designs can also utilize organic media and are typically less expensive to construct and maintain than underground filter designs. Unfortunately, surface designs typically prevent multipurpose land uses and therefore are limited in their application to ultra-urban settings. In roadside settings where there is sufficient space (typically two to three percent of the drainage area served), a surface filter design may be preferred.
If they are placed below the frost line, the performance of organic media filters is relatively independent of season. In addition, the level of treatment is generally independent of placement and in situ soil conditions do not affect performance. For most designs pretreatment is integrated into the filter facility in the form of a settling chamber. Additional pretreatment may be provided by streetsweeping to remove accumulated sand and trash, which can diminish the useful life of the filter.
Effectiveness
Organic media filters are highly efficient in removing fine-grain material (small particles in stormwater runoff between 6 and 41 microns). As an additional benefit, organic media are capable of removing a portion of dissolved material found in stormwater. For example, the peat medium has a cation exchange capacity (CEC) 500 times that of sand. This greatly increases its ability to adsorb or capture positively charged dissolved metals and hydrocarbons, increasing the removal performance.
Organic media filters have demonstrated good total suspended solids (TSS) removals, typically providing 90 to 95 percent removal (Claytor and Schueler, 1996; Stewart, 1992). Performance for nutrients is less significant; in fact, the organic media may be a source of soluble phosphorus and nitrate (NO3). Total phosphorus (TP) removals range up to 49 percent, while variable removal of metals is typically between 48 and 90 percent (Figure 14). Removal of oil and gasoline averages about 90 percent (Claytor and Schueler, 1996).
Siting and Design Considerations
Two broad categories of organic media designs exist: (1) variations on existing sand medium filter designs and (2) proprietary designs that are optimized for organic media. For the first design category, organic media are simply substituted for sand, affecting the size of the filter portion of the facility. Information on existing sand filter designs is provided in the Surface Sand Filters and Underground Sand Filters Fact Sheets. These sand medium designs should be varied to reflect the permeability of the substituted organic media. It has been recommended in a recent evaluation that combination peat/sand filters be designed based on a permeability of 0.8 m/day (2.75 ft/day), or a value approximately 79 percent of that recommended for sand-only filters (City of Austin, 1991). On the other hand, compost medium filters have a wide range of permeability values depending on their age and degree of clogging. Designers should be aware that initial permeability can be very high (in the range of 122 m/day [400 ft/day], a value much higher than that used to specify the filter area); Claytor and Schueler (1996) recommend a design permeability value of 2.7 m/day (8.7 ft/day). Several good sources are available for detailed design procedures and information on underground and surface filter designs, including Design of Stormwater Filtering Systems (Claytor and Schueler, 1996) and Evaluation and Management of Highway Runoff Water Quality (Young et al., 1996).
One proprietary underground design that features organic media is the CSF® Type II system, which uses cylindrical filter cartridges filled with a granular organic medium consisting of composted leaves. (Figure 16 illustrates a recent advancement in StormFilterTM technology, formerly the CSF® system.) The filter works by percolating stormwater through the cylindrical cartridges containing certified CSF® compost media. Because of the highly porous nature of the granular media, the flow through a newly installed cartridge is restricted by a valve to 57 L/min (15 gal/min). This allows more time for sediment to settle and ensures adequate contact time for pollutant removal. The CSF® system is equipped with scum baffles that trap floating debris and surface films; even during overflow conditions. A typical unit requires 0.67 m (2.2 ft) of drop from the inlet invert to the outlet invert. A portion of the sediment settles out in the area around the cylinders; more sediment, including particulate forms of nutrients and heavy metals, are trapped by the porous structure of the compost. Sizes range from 1.83 m X 2.44 m (6 ft X 8 ft) (treating about 284 L/min [75 gal/min] peak flow) to 2.44 m X 5.49 m (8 ft X 18 ft) vaults (which treat about 1360 L/min [360 gal/min], or 0.023 m3/s [0.8 ft3/s]). Housed in standard size precast or cast in place concrete vaults, the filter systems are installed in-line with storm drains.
Maintenance Considerations
Annual maintenance costs for organic filters vary as a function of the design used. Surface filter designs using a peat/sand medium require periodic mowing and removal of the grass cuttings to avoid unwanted plant growth. In addition, at least an annual inspection is required for this design and reseeding of the grass cover crop may be required.
Filter designs that feature horizontal compost bed filters will likely be replaced every three to four years to prevent heavy metal concentrations from reaching levels that exceed the "clean sludge" definition under 40 CFR Part 503 (USEPA, 1994). These designs also require removal of accumulated material and rototilling of the compost to reestablish the required permeability.
Maintenance for underground designs that use organic media can be inferred from information given for sand-only medium filters given in the Fact Sheets for Underground Sand Filters and Surface Sand Filters. A D.C. underground sand filter serving a 0.4 ha (1 ac) area was serviced by removal and replacement of a gravel ballast and filter cloth, for $1300 in 1994 (Bell, 1996). It is reasonable to assume organic media filters would require comparable service. It should be noted that repair of subsurface filters requires confined space entry, which dictates larger management crews and a higher cost to repair than surface filters.
The maintenance of proprietary organic media filters varies with the manufacturer; it is likely that maintenance will include removing accumulated material that has settled in the facility and periodic replacement of organic media cartridges on an annual or biennial basis. For example, manufacturers of the CSF® system indicate annual maintenance costs will range from $500 to $1200 (for 280 and 1360 L/min [75 and 360 gal/min] systems, respectively).
Cost Considerations
The cost of surface facilities using organic media filters is comparable to the cost of filtration facilities that use sand medium (with the exception of proprietary systems). For conceptual costing a price of $8,400 to $39,500 per impervious hectare served (or $3,400 to $16,000 per impervious acre served) can be used to estimate the construction cost of a proposed facility, excluding real estate, design, and contingency costs (Schueler, 1994).
Underground filters are generally considered to be a high-cost BMP option for water quality management. The construction cost per hectare served is typically around $34,600 and the cost per acre served is typically around $14,000, excluding real estate, design, and contingency costs (Schueler, 1994).
Drop-in CSF® vertical organic media units are typically precast vaults delivered to the site either partially or fully assembled. Typical cost variables include the need for ballast, type of lids and doors, customized casting of sections or holes, and depth of the vault. Systems treating peak flows of 280 and 1360 L/min (75 and 360 gal/min) have an estimated installed cost of $10,000 and $25,000, respectively (Stormwater Management, 1996).
References
Bell, W. 1996. BMP Technologies for Ultra-Urban Settings. In Proceedings of Effective Land Management for Reduced Environmental Impact, Tidewater's Land Management Conference on Water Quality, August 22, 1996.
City of Austin. 1991. Austin Environmental Design Criteria Manual. Environmental Resources Management Division, Environmental and Conservation Services Department, City of Austin, Austin, TX.
Claytor, R.A., and T.R. Schueler. 1996. Design of Stormwater Filtering Systems. The Center for Watershed Protection, Silver Spring, MD.
Schueler, T.R. 1994. Developments in Sand Filter Technology to Improve Stormwater Runoff Quality. Watershed Protection Techniques 1(2):47-54.
Stewart, W.S. 1992. Compost Storm Water Treatment System. W&H Pacific Consultants, Portland, OR. Final Report.
Stormwater Management. 1994. Three Year Performance Summary of Stormwater Pollutant and Treatment - 185th Avenue, Hillsboro, Oregon. Technical Memorandum. Stormwater Management, Portland, OR.
Stormwater Management. 1996. Product literature.
USEPA. 1994. Land Application of Sewage Sludge. EPA 831-B-93-002b. U.S. Environmental Protection Agency (USEPA), Office of Enforcement and Compliance Assurance.
Young, G.K., S. Stein, P. Cole, T. Kammer, F. Graziano, and F. Bank. 1996. Evaluation and Management of Highway Runoff Water Quality. FHWA-PD-96-032. Federal Highway Administration, Office of Environment and Planning.
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