Storm Water Best Management Practices for the Removal of Suspended Solids
As storm water discharge regulations continue to be adopted and updated by state and federal agencies, the management of storm water pollutants including oil and grease, bacteria, suspended solids, and nutrients becomes paramount. State and federal permits are including discharge requirements for these pollutants, which can present a sizable challenge for industrial facilities. Such facilities are encouraged to install and maintain best management practices (BMPs) to manage and reduce pollution to receiving streams and to avoid financial and legal penalties.
The U.S. Environmental Protection Agency (USEPA) defines BMPs as schedules of activities, prohibitions of certain practices, maintenance procedures, or physical treatment structures that help reduce pollutants in storm water runoff [1]. Effective maintenance procedures may improve water quality by removing pollutant sources, but total suspended solids (TSS, a measurement of the total solids in a water sample that are retained by filtration) usually results from storm water runoff across exposed surfaces. TSS is therefore best removed by physical treatment technologies prior to discharge to receiving streams. Common physical treatment technologies include, but are not limited to, wet retention ponds, vegetated swales, catch basin inserts, and sand filters. The benefits and proper use of each of these technologies are discussed below.
Wet Retention Ponds
Wet retention ponds, or storm water ponds, are the most common BMP employed for the removal of suspended solids. The ponds retain a permanent pool of water year-round. These are different from dry detention ponds, which are typically empty basins that fill with storm water and either discharge to a receiving stream or allow infiltration to groundwater. In the wet retention ponds, storm water runoff is routed through the pond, slowing the velocity of the discharge and allowing for ample retention time for particles in the water to settle to the bottom of the pond. Wet retention ponds can offer up to 87 percent TSS removal when designed and installed properly [2]. Facilities with existing systems can install wet retention ponds by converting dry detention areas or ditches. New inlet and outlet structures would need to be installed based on specific site conditions. Wet retention ponds are simple to install, can be designed to take any shape, and create a park-like landscape on site.
Wet retention ponds have relatively low maintenance requirements when compared to other available BMPs. Maintenance tasks include vegetation removal to prevent overgrowth once or twice per year and digging out accumulated sediment, particularly in the entrance to the pond, every 5 to 10 years depending on the solids concentration of the runoff.
Vegetated Swales
Vegetated swales are another simple physical treatment technology that can be employed to remove solids from storm water runoff. Vegetated swales are storm water ditches or drainage areas that are planted with vegetation to help slow storm water flows, thereby filtering and settling solids. They can be retrofitted into existing storm water systems, but it takes time for the vegetation to grow and fully establish. Vegetated swales are effective in treating small- to medium-sized drainage areas. They can be supplemented with rock or dam structures to slow down the flow rate and increase retention time, thereby increasing TSS removal through settling. Swales are effective as either wet or dry BMPs. Maintenance requirements are similar to those of a wet retention pond, requiring periodic overgrowth removal and solids removal.
Catch Basin Inserts
Catch basin inserts are filters that are placed within catch basins to physically remove solids as storm water flows into the catch basin. They can remove TSS as well as larger trash pollution and debris. Catch basin inserts can be easily installed in existing basins as a non-intrusive BMP. Despite their simple installation, catch basin inserts require a significant amount of maintenance. Filters need to be regularly inspected to ensure they have not been torn or broken, reducing or removing the efficacy of the filter. The filters will also need to be frequently cleaned to remove trash and debris and ensure the drain is not being clogged. Catch basin inserts clogged by heavy amounts of debris, trash, or sediment will create ponding on paved surfaces. New storm water systems should be designed to avoid using catch basin inserts to remove TSS, but many employ them for the removal of other pollutants such as trash and oil and grease.
Sand Filters
Sand filters are constructed as multi-chamber structures that are most often installed underground. Typically, sand filters have two or three chambers; the first chamber being dedicated to settling out large solids, the second chamber serving as a filter to remove the remaining TSS, and when employed, the third chamber as the clean effluent discharge chamber. They are extremely effective at removing TSS when designed for site-specific conditions. Sand filters are easily incorporated into new system designs and can be quickly retrofitted into existing systems due to their small footprint and subsurface placement. They work best when treating runoff from smaller areas. Typical percent removal of TSS can be around 83 percent.
Maintenance requirements may be high for these filters depending on the trash and heavy solids contributed by the drainage area. Accumulated solids may need to be removed from the chambers using a vacuum truck. In other cases, filter media may need to be replaced every 3 to 5 years [3]. The maintenance requirements are offset by the high amount of TSS removal that is offered per unit area.
BMP Selection Factors
The type of BMP employed to remove TSS from storm water runoff depends on a variety of factors. These include site conditions, desired removal efficiencies, and maintenance concerns. There is no one-size-fits-all approach to removing TSS from storm water. Each BMP discussed has its own benefits and drawbacks, which must be thoroughly reviewed to determine what is the most effective system for each facility. PSARA Technologies offers permitting and design services for BMPs to maximize TSS removal and stay cost effective.
If you have any questions regarding storm water permitting, meeting effluent concentrations, or the design of BMPs, please contact PSARA Technologies at (513) 791-4418.
[1] “Frequent Questions,” Water Industry Effluent Guidelines, U.S. Environmental Protection Agency, 17 August 2012.
https://www.epa.gov/eg/learn-about-effluent-guidelines#BMPs
[2] “International Stormwater Best Management Practices (BMP) Database Pollutant Category Statistical Summary Report – Solids, Bacteria, Nutrients, and Metals,” International Stormwater BMP Database, December 2014.
[3] “Storm Water Technology Fact Sheet – Sand Filters,” U.S. Environmental Protection Agency, Office of Water, EPA 832-F-99-007, September 1999.
https://nepis.epa.gov/Exe/ZyPDF.cgi/200044AG.PDF?Dockey=200044AG.PDF