From Pollutants to Practices: Designing Effective Stormwater Treatment Trains

Introduction 

Impervious surfaces disrupt the natural water cycle by inhibiting the infiltration and retention of precipitation. In developed areas, excess runoff flowing from surfaces such as parking lots, roads and buildings can carry accumulated pollutants to receiving waters, which makes them a primary contributor to post-construction pollution in stormwater runoff.

Managing the wide range of pollutant types and forms found in runoff requires varied strategies and controls specifically designed to remove the pollutants of concern in a watershed. For some developments, this may require the use of multiple successive treatment strategies that combine different treatment unit processes, commonly called “treatment trains,” to achieve a comprehensive management solution. In addition to managing pollution, treatment trains can be implemented to improve the maintainability and long-term performance of individual practices or controls, thereby reducing lifetime maintenance costs.   

Common Pollutants and Their Forms 

Common Pollutants

Sediment is commonly used as a generic term for small solid particles that remain in suspension in water. These suspended solids are largely comprised of organic and inorganic particulate matter eroded from unstable or bare land, partially decomposed organic material, and weathered pieces of manmade infrastructure such as roadways or brake and tire wear. Excess sediment degrades water quality in receiving waters by increasing turbidity and streambed deposition, both of which are harmful to aquatic life. Sediment also regularly carries other attached pollutants that can be released into receiving waters.  

Nutrients of concern in stormwater are primarily excess nitrogen and phosphorus in runoff from over-fertilized agricultural fields and landscapes, pet and animal waste, natural weathering, illicit discharges and combined sewer overflows. Many watersheds are nutrient limited so when excess nutrients are introduced to a receiving waterbody, harmful algal blooms can develop that may produce toxins and deplete dissolved oxygen in the water in addition to diminishing recreational opportunities.  

Heavy metals common to stormwater include zinc, copper and lead; they’re often released from vehicle exhaust, tire wear and building materials—such as galvanized surfaces—and they can accumulate to toxic levels in aquatic ecosystems since they don’t break down.  

Hydrocarbons are primarily introduced to post-construction runoff because of leaking oil and grease from cars and other machinery, automotive exhaust, illegal dumping, and accidental spills. Hydrocarbons can be toxic to human and aquatic organisms. Large concentrations can coat the water surface, thereby blocking sunlight and inhibiting oxygen exchange, and may physically coat and smother wildlife. Hydrocarbons also bind to sediment where they may persist for decades, posing ongoing health risks.  

Pathogens and bacteria commonly come from illicit discharges as well as pet and wild animal waste. Elevated pathogen and bacteria concentrations increase human health risk and can make water unsafe for swimming, fishing and drinking.  

Debris and litter consist primarily of trash, yard waste and other gross solids that wash through the storm system. These pollutants can physically harm wildlife that ingest or become entangled in them, clog drainage infrastructure and become an eyesore in receiving waters. 

Emerging pollutants of concern include contaminants such as road deicing agents, household pesticides and insecticides, tire-wear particle compounds (such as 6-PPD-quinone that’s acutely toxic to certain fish species), and per- and polyfluoroalkyl substances (PFAS). Each of these contaminants poses a health risk to people, aquatic organisms or ecological systems. 

Pollutant Forms

Pollutants exist in both particulate and dissolved forms. This affects how the pollutants are transported in runoff and how they affect physical and aquatic ecosystems. Dissolved pollutants move more easily in water, making them highly bioavailable for plants and animals to absorb. Particulate pollutants are solid particles suspended in a medium. As such, their movement is dependent on physical force; during a storm, high runoff velocities transport larger and denser particles. 

Transport

Stormwater pollutant buildup, wash off and transport dynamics are complex and vary by pollutant type and storm characteristics. The most mobile pollutants can be disproportionately concentrated in runoff during the early portion of a storm, a pattern commonly referred to as the “first flush” effect. Capturing and treating such runoff can provide important water-quality benefits. Other pollutants, including coarse sediment, floatables and trash, may be more closely associated with higher-velocity flows during larger or more-intense storms. Many stormwater programs emphasize retaining or treating a large portion of the annual runoff volume, while allowing peak flows to bypass stormwater controls.  

Pollutant Removal Unit Processes 

Understanding which pollutants should be targeted, in what form they exist and how they are transported helps stormwater practitioners develop informed strategies to protect local waterways from development impacts. Water-quality improvement strategies can include many best management practices (BMPs). Stormwater BMPs include policies as well as structural and non-structural post-construction practices that target the management and improvement of stormwater runoff. The term stormwater control measure (SCM) is more targeted and generally refers to any structural engineered practice designed to control water quantity and/or improve water quality. Treatment SCMs utilize one or more types of pollutant removal unit processes to improve water quality; these can be categorized as the following: 

Physical

Physical processes capture particulate pollutant forms using mechanisms such as screening, gravity separation or filtration.  

• Pollutants can be screened from runoff using a barrier with defined openings to physically intercept pollutants of a target dimension and prevent them from moving further downstream in the stormwater system.  

• Gravity separation, also referred to as settling or sedimentation, is a process that targets solid pollutants denser than water such as sediment. Solids settle out of suspension through time because of gravity.  

• Gravity also can be used to separate and isolate floating pollutants such as hydrocarbons and trash/debris.  

• Physical filtration removes pollutants by passing stormwater through a porous medium that physically sieves or strains out particulate matter and promotes other physiochemical interactions within the filter medium.  

Chemical

Chemical processes affect or transform the chemical structure of pollutants to improve capture and removal. Where physical separation targets the particulate pollutant fraction, chemical processes target removal of the dissolved fraction and smaller suspended particles that don’t settle out easily by gravitational forces. This includes mechanisms such as the following: 

• Coagulation using additives to neutralize charges within microscopic sediment particles and increase precipitation of the pollutants. 

• Adsorption, whereby dissolved pollutants adhere on the surface of a reactive medium (e.g., activated carbon) that has a chemical affinity for the target pollutant, causing it to stick to the surface. 

• Absorption, whereby a reaction occurs with the targeted pollutants of concern, resulting in those pollutants being physically incorporated within the media matrix, like a sponge. 

Biological 

Biological processes are natural metabolic processes of vegetation, microbes and soil organisms that break down, absorb or transform pollutants. These unit processes rely on living organisms to neutralize and lock away pollutants using the following mechanisms: 

• Aerobic digestion, which occurs in oxygen-rich environments where microbes break down organic compounds and convert them into carbon dioxide and water. 

• Anaerobic digestion, which occurs in an environment without oxygen where microbes break down organics into biogas. 

• Denitrification, the process of converting dissolved nitrogen into nitrogen gas, driven by the metabolic activity of anaerobic bacteria. 

Mapping Processes to Practices 

Physical screening of pollutants is commonly found in pretreatment practices such as trash-capture devices, which are used to reduce the maintenance burden or prevent clogging of downstream treatment practices. 

 Gravity-separation SCMs rely on sufficient residence time to allow target pollutants in captured stormwater to settle or float before discharge. Settled materials are retained in sumps or sediment storage areas, and floating materials are retained behind baffles, hoods or similar controls.

Stokes’ Law describes the relationship between settling velocity and particle size and predicts that larger or denser solids will settle faster in relatively calm flow conditions. This principle is commonly used in the design of detention and retention ponds, hydrodynamic separators, and underground storage systems. System effectiveness generally increases with longer residence time, larger particle size and greater density. Therefore, gravity separation can be particularly effective for the removal of coarse solids and free-floating oils and debris, but effectiveness may be limited for fine-grained solids and dissolved pollutants. 

Physical filtration is one of the most effective physical unit processes for the removal of fine-grained solids and associated pollutants. Common SCMs that use physical filtration processes include surface-based sand or soil filters, proprietary media and membrane filters, and biofiltration systems. 

When enhanced treatment of stormwater is needed, filtration practices may be amended with materials that use chemical unit processes. This requires a reactive media or component within the system, or an inert media to be amended with reactive components such as iron, compost, activated carbon or activated alumina to name a few examples. Generally, the type of pollutant targeted by the treatment practice informs which amendment is used in the media, though some commercial options exist for pre-blended engineered media designed for the treatment of common post-construction stormwater pollutants. Common examples of such practices include iron-amended sand filters and proprietary high-rate media filters. 

Biological unit processes are fundamental to practices such as bioretention cells, constructed wetlands, vegetated swales and filter strips, and proprietary high-rate biofiltration. Vegetation can be used with engineered media practices to further promote biological activity that enhances pollutant transformation and removal as well as provides enhanced hydraulic conditions and long-term preservation of the stormwater practice. 

Sequencing Unit Processes in Treatment Trains 

When designing a stormwater treatment train, it’s of utmost importance to consider the unit processes likely to be most effective for removal of the targeted pollutants as well as the expected efficacy of the available SCMs targeting those pollutants. A treatment train functions as an integrated system: each upstream practice alters the stormwater quality, quantity or hydraulics entering the downstream SCM.

SCMs in a treatment train should be arranged so upstream practices manage hydraulic loading and remove pollutant fractions most likely to impair downstream practices, while subsequent practices provide progressively more targeted or refined treatment. Effective treatment train design recognizes that no individual SCM removes all pollutants equally well, and that a well-considered selection of practices can ease the burden of maintenance while meeting long-term performance goals. 

A fundamental principle of treatment train design is that upstream SCMs should protect downstream SCMs by removing the pollutant fractions most likely to impair their function. Pretreatment practices such as trash-capture devices, forebays and hydrodynamic separators are commonly placed at the beginning of a treatment train to intercept gross solids, debris, floatables and coarse sediments. Such pollutants are likely to impair the downstream flow controls of extended detention ponds or clog downstream filtration surfaces, thereby increasing the maintenance burden of such practices or preventing effective operation altogether. Not only does this approach protect the more-effective treatment solutions in the treatment train, but it also improves maintainability of the system by consolidating most of the captured pollutants in practices that are generally easier to access and maintain. 

Treatment train SCMs are generally organized according to the practice’s ability to target decreasing particle sizes, which tends to correspond with increasing treatment complexity. Coarse sediment and trash are first removed through screening and gravity separation processes, followed by filtration practices to target fine suspended solids and particulate-bound pollutants. Filtration practices may incorporate sorbing amendments or vegetation for enhanced dissolved pollutant removal if such treatment goals are needed.

This sequencing is important because dissolved-phase treatment mechanisms often are sensitive to particulate occlusion. For example, phosphorus-sorbing media can lose effectiveness when they become saturated or occluded, while biological transformation and uptake by vegetation may decrease if excessive sedimentation or inundation stresses the plant. Designing the treatment train around progressively refined treatment processes can improve the effectiveness of each successive removal mechanism while reducing the likelihood of premature failure. 

Another important consideration is the diminishing marginal benefit of successive SCMs. The first treatment practice in a train often achieves the largest absolute pollutant reduction because influent concentrations are highest at the system entrance. Downstream SCMs therefore receive progressively cleaner runoff, which can reduce the incremental pollutant load reduction observed at each subsequent stage. While cumulative treatment performance may remain high, designers must recognize that later-stage SCMs often function more as polishing systems than primary treatment units. As a result, the sizing and selection of downstream controls should reflect realistic influent loading conditions rather than assuming untreated runoff characteristics throughout the train. This concept is particularly important when estimating lifecycle performance or modeling pollutant removal credit stacking across multiple SCMs. 

Conclusion 

Effective stormwater treatment train design requires more than simply combining multiple practices; it requires understanding the pollutants being managed, the forms in which they occur and the removal mechanisms available to target them. By sequencing stormwater control measures according to progressively more complex treatment processes and recognizing the interactions between successive practices, designers can maximize pollutant removal, improve long-term reliability and reduce maintenance demands. A well-designed treatment train therefore provides a more comprehensive and sustainable approach to protecting receiving waters from the impacts of post-construction runoff. 

References 

Gulliver, J.S., A.J. Erickson, and P.T. Weiss, 2010, Stormwater Treatment: Assessment and Maintenance, University of Minnesota, St. Anthony Falls Laboratory, Minneapolis; https://stormwaterbook.safl.umn.edu/. 

Lee, J.H., K.W. Bang, L.H. Ketchum, J.S. Choe, and M.J. Yu, 2002, “First flush analysis of urban storm runoff,” Science of The Total Environment, 293 (1-3): 163-175; doi:https://doi.org/10.1016/S0048-9697(02)00006-2. 

Minnesota Pollution Control Agency, 2026, “Minnesota Stormwater Manual, Comprehensive approaches to stormwater management,” accessed May 2026; https://stormwater.pca.state.mn.us/comprehensive_approaches_to_stormwater_management. 

Minton, Gary R., 2005, Stormwater Treatment: Biological, Chemical, and Engineering Principles (2nd Edition), Sheridan Books Inc., Seattle. 

National Academies of Sciences, Engineering, and Medicine, 2024, “Developing a Guide for On-Bridge Storwmater Treatment Practices, The National Academies Press, Washington, D.C.; doi:https://doi.org/10.17226/27905. 

Stormwater Equipment Manufacturers Association, 2025, “Basics of Gravity Separation Devices,” Stormwater Equipment Manufacturers Association (SWEMA), accessed May 2026; https://www.stormwaterassociation.com/stormwater-white-papers.