On a watershed scale, the management of stormwater begins with Pollution Prevention (e.g. pet ordinances, buffer requirements, public education), followed by Source Controls (e.g. sweeping, Illicit Discharge Detection and Elimination), Onsite Stormwater BMPs (e.g. rain gardens, permeable pavement), and Regional BMPs (e.g. constructed pond or wetland, large underground infiltration system). On-site and regional practices treat stormwater runoff. The term Stormwater Treatment Train has loosely been used since the mid-1980s to represent a multi-BMP approach to managing the quantity and quality of stormwater runoff.
The concept of treatment trains has been commonly used in the field of wastewater treatment. Professionals in this field will often describe each component of the treatment train in terms of the structure within the plant, i.e. bar screens, grit chambers, primary settling tanks, secondary treatment tanks. For the purpose of stormwater management, the lexicon differs by focusing on the processes utilized within the BMP, rather than the structural feature of a BMP. For the purpose of the Minnesota Stormwater Manual and the information contained in this article, the definitions of practices vs. processes are based on the recommendations contained in Design of Urban Stormwater Controls, published jointly by the Water Environment Federation and American Society of Civil Engineers in 2012. The following approach uses different terminology than developed by WEF, which refers to Practices as Unit Operations. Practice has been selected for this article to parallel the common usage in Minnesota of Best Management Practices (BMPs) and Stormwater Management Practices (SMPs).
A closer look at on-site and regional BMPs shows that each BMP utilizes one or more components that work together to remove pollutants utilizing combinations of hydraulic, physical, biological, and chemical methods. A well-developed stormwater treatment train will combine these processes in a manner that ensures management of all pollutants that have been identified as affecting the receiving water.
Stormwater process describes the mechanism by which pollutants are removed. For example, infiltration and evaporation are different processes for managing stormwater volume. Stormwater practices are Best Management Practices. These are the stormwater controls in which the pollutant control process, or multiple processes, takes place. In the stormwater industry, many BMPs have evolved and have been given labels that describe the primary process utilized by that BMP/practice. A key example is the term infiltration, which is used interchangeably as both a process and as a practice.
Physically, each BMP/practice utilizes multiple processes. For example, the practice of a bioinfiltration BMP utilizes the following processes:
The following tables differentiate between stormwater process and practice. To view all the information in these tables in a single Excel file, link here.
A stormwater treatment train incorporates at least two processes to maximize the control of pollutants from the runoff. The BMP(s) selected may consist of one or multiple practices, depending on many considerations, including available space, physical conditions at a site, and regulatory requirements. See the tables above for a summary of processes and practices.
Hydraulic and physical processes remove larger solids and associated pollutants during storm events while biological and chemical processes that treat the finer solids and dissolved pollutants occur between storms (Scholes, et. al. 2007, Wong, et. al. 2001). Once complete, a treatment train can (adapted from Strecker, 2005)
Using this framework for pollutant removal, particulate size of the pollutant(s) of concern should be matched to the stormwater practice best suited to remove that pollutant from stormwater runoff (Wong, et. al. 2002, Strecker, 2005). The figure to the right suggests treatment processes for a range of particulate sizes commonly found in stormwater runoff. Chemical processes are rarely utilized, given the higher capital and operating costs of chemical feeds and controls.
The resulting stormwater treatment train may result in a single BMP, such as a stormwater wetland, that utilizes multiple treatment train processes and by definition can be considered a stand-alone stormwater treatment train. Or it could become a multi-BMP treatment train with BMPs operating in series or parallel to each other. Low Impact Development (LID) and Traditional development are two typical configurations of treatment trains currently utilized by site designers that meet the definition of stormwater treatment trains. The focus of LID is to keep the raindrop as close to it's source as possible utilizing techniques such as infiltration and capture/storage/reuse. Traditional development typically employs filtration and sedimentation practices such as swales and constructed ponds and wetlands. These practices may or may not treat rainwater close to its source but generally have minor impacts on stormwater volume. Where feasible, LID practices are favored from a stormwater management practice as they reduce both stormwater volume and pollutant loading. LID practices, however, are often constrained by site factors, such as shallow depth to bedrock, soil or groundwater contamination, and space limitations (e.g. ultra-urban settings).
Examples of these two types of treatment trains are provided below and illustrated in the schematic to the right. In the LID example, water falling on a rooftop is filtered through a green roof, which stores some water for eventual uptake by plants and routes the remaining water to a permeable pavement and then to an infiltration BMP. The traditional configuration routes water off-site through a swale, which provides some treatment, before the water is discharged to a regional system.
On-site treatment trains that have been found to be the most effective are those that maintain runoff onsite while allowing sufficient time for hydraulic, physical, biological, and chemical processes to take place (Rushton, 2004). This has been demonstrated to be true in multiple case studies, as well as in Minnesota case studies including the Empire Wastewater Treatment Plant project owned and operated by the Metropolitan Council.
On a larger catchment area or watershed scale the pollution reduction of treatment train BMPs is effective, but less so than on an on-site basis. Generally, the highest level of pollutant reduction is achieved in the first BMP, with each successive BMP becoming less effective. The theory of why this occurs is based on the concept of irreducible pollutants. For more information on irreducible concentration, link here. Essentially the second, third, etc. BMPs in the treatment train are receiving runoff that has considerably less concentration of pollutants and which at some point are below the theoretical irreducible concentration for the BMP. One recommended approach to adjust for the uncertainties of pollutant removals would be to create a prioritized list of BMPs that are listed or ranked according to the pollutant removal effectiveness for each pollutant. Modelers utilizing this technique are advised to adjust the pollutant concentrations and kinetic uptake factor (k) for each pollutant being assessed by the model (Scholes, et al, 2007).
A review of literature did not find any studies that assessed the cost effectiveness of stormwater treatment trains. It is assumed that the cost considerations for a stormwater treatment train would parallel these cost considerations for individual stormwater BMPs:
A stormwater treatment train begins at the point that the rainfall hits the ground and becomes runoff. Site owners, maintainers, designers, etc. that are establishing a stormwater treatment train should consider non-structural practices as well as structural BMPs. A comprehensive stormwater treatment train will include
The remainder of this section details the process of developing the structural practices of the stormwater treatment train.
Development of a stormwater treatment train is an iterative process that balances site constraints, project goals, and available budget. The following steps lay out the process for establishing a stormwater treatment train. The results of one of the steps may cause designers to reconsider earlier decisions on sizing, siting, etc., as the project progresses.
The site conditions, regulatory requirements, and project purpose will vary from site to site and from city to city. Information to assemble includes:
The purpose of this step is to create a list of BMPs that work together to remove the pollutants of concern identified in Step 1.
To make these determinations, use this table, which provides a summary of processes and practices.
Site constraints will affect the sizing, location, and performance of the BMPs identified in Step 2. The purpose of this step is to narrow down the BMP options based on such site constraints as:
Review each BMP identified in Step 3 to confirm that each pollutant removal process identified in Step 2 is present in the combination of BMPs selected in Step 3. If not, then Step 3 should be reviewed and alternative BMPs proposed.
Size the BMP and use the MIDS calculator or other technique detailed in the Credits section of the MN Stormwater Manual to assess the performance. Review results against goals set in Step 1. If goals are not fully achieved, then resize the BMPs or return to Step 3 to select alternative BMPs.
Designers should assess construction and operation considerations that need to be incorporated into the construction plans and/or the Operations and Maintenance Manual that are necessary to ensure the BMP operates as designed and is properly maintained.