Stormwater Best Management Practices for the Removal of Suspended Solids

Friday, February 24, 2017

Storm water discharges are becoming a key issue with state and federal rule-makers.  You may have heard some common buzz words such as green infrastructure (GI) or low impact development (LID), which fall under the same umbrella as storm water best management practices (BMPs) with the main goal of reducing pollutants discharged to receiving streams.  Pollutants such as nutrients, oil and grease, bacteria, and suspended solids are major drivers in the permitting process, and removing these pollutants can present a sizeable challenge for municipalities and industrial facilities depending on the removal needs and ability to install BMPs.  By employing effective BMPs, permitted facilities can meet permit discharge requirements and avoid financial and legal penalties.  This article focuses on some of the most common BMPs available to remove total suspended solids (TSS) from storm water runoff.

A BMP, as defined by the U.S. Environmental Protection Agency (USEPA), can be anything from a schedule of activities, prohibition of certain practices, maintenance procedure, or physical treatment structure that helps reduce pollutants in storm water runoff.[1]  Effective maintenance procedures improve water quality at the source, but removal of TSS in storm water is best achieved through the use of physical treatment technologies.  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, often simply referred to as storm water ponds, are a popular BMP employed for the removal of suspended solids.  They are constructed so that they always retain a permanent pool of water, no matter the time of year.  These are different from dry detention ponds, which perform similarly but may be dry some or most of the time depending on their design.  Storm water runoff is routed through the pond so that ample retention time is provided for particles to settle to the bottom of the pond.  Many facilities with existing systems can add wet retention ponds into ditches and dry detention areas by constructing an outlet structure designed for specific site conditions.  Design of pond systems can be fairly simple once the key goals of the project are understood.  The park-like landscape allows for some creativity; for instance, a nature path can be installed for employees or the public to enjoy, native and visually appealing vegetation can be introduced to the pond, and it can be designed to take any shape.

Wet retention ponds have relatively low maintenance requirements when compared to other available BMPs and can offer up to 87 percent TSS removal when designed and installed properly.[2]  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 can be employed as a physical means of removing solids from storm water runoff, similar to a wet retention pond.  Vegetated swales are simply 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 but are not cost-effective at the watershed level.  They can be supplemented with rock or dam structures to slow down the flow rate and increases retention time, thereby increasing TSS removal.  Swales are effective as wet and dry BMPs.  Maintenance requirements are similar to those of a wet retention pond.

Catch Basin Inserts

Catch basin inserts are essentially filters that are placed within existing catch basins to physically remove solids as storm water flows into the catch basin.  They can remove TSS as well as trash and debris and can be easily added to an existing storm water system.  The inserts do add a significant amount of maintenance to a system because they have to be regularly inspected and cleaned to ensure that they are not clogged and are functioning effectively.  The inserts can be heavy when they are filled and, if they do become clogged, will create ponding on paved surfaces around catch basins.

Catch basins are not the most ideal way to remove TSS, but due to the ease of installation in existing systems, they offer facilities a non-intrusive way to supplement their storm water treatment.  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 in the Midwest.  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 are high for these filters, but that is offset by the high amount of TSS removal they can offer per unit area.  In some cases, filter media may need to be replaced every 3 to 5 years to prevent sediment from clogging the filter.[3] Also, depending on the trash and heavy solids contributed by the drainage area, accumulated solids may need to be removed from the first chamber using a vacuum truck on a regular basis.

As you can see, 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 here has its own benefits as well as drawbacks, which must be thoroughly reviewed to find the best fit for each facility.  That is why BMPs must be properly designed in order to maximize TSS removal while also being cost-effective for the facility.  If you have any further questions regarding storm water permitting, meeting effluent concentrations, or BMPs, please contact Jessica Callaway at (513) 791-4418.

[1] “Frequent Questions,” Water Industry Effluent Guidelines, U.S. Environmental Protection Agency, 17 August 2012.

[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.