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  5. Designing Effective Stormwater Treatment Systems Part 1: Recognizing Common Pollutant Forms
Designing Effective Treatment Systems Part 1: Recognizing Common Pollutant Forms

Designing Effective Treatment Systems Part 1: Recognizing Common Pollutant Forms

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The starting point for the selection and design of stormwater control measures (SCMs) on a project is usually to identify regulatory requirements for post-construction water quality management (e.g. 80% removal of total suspended solids for the 90th percentile storm). The next typical step starts with identifying the pollutants of concern and then selecting an SCM that has the appropriate removal effectiveness required for compliance. With just a few small adjustments to that approach, we can optimize the effectiveness of the SCMs we select. The details that make treatment systems most effective in the field depend on understanding how the selected SCM removes the pollutant. This is the first in a three-part series, building to an informed workflow that helps designers and engineers maximize treatment efficiency for different common stormwater pollutants. The first step in this workflow is to better understand which pollutants are present in post-construction stormwater runoff and the forms in which we commonly encounter them.

Identify Commonly Regulated Pollutants

This is perhaps the first thing you look at when considering stormwater treatment – if you don’t know what you’re targeting, how can you effectively design an SCM to remove it? It seems simple, but it’s the fundamental building block that impacts and informs all subsequent decisions. Sediment is perhaps the number one pollutant of interest in a broad sense. It is commonly used as a generic term for any small solid particles that are suspended in the runoff and is often considered to be comprised of organic and inorganic matter (possible sources include erosion of nearby land, weathered infrastructure, and wear from vehicles).

  • Nutrients are another primary pollutant of concern due to their impact on receiving waters. This pollutant group generally enters storm sewers as excess phosphorus and nitrogen from over-fertilization, pet and animal waste, and decomposing organic debris.
  • Heavy metals like zinc, copper, and lead commonly come from vehicle wear and building materials.
  • Debris and litter consist primarily of trash, yard waste (like leaves and grass clippings), and other large solids that end up in our curblines – quite a bit larger than sediment and sometimes buoyant in water.

Recognize Common Pollutant Forms

Sediment has been used as a broad surrogate for many post-construction pollutants. When the term is used in this way to meet regulatory requirements, there is often an implied assumption that any attempt to treat sediment will also capture other common pollutants. But that assumption can leave significant treatment potential unrealized since not all pollutants are effectively removed in the same manner. The form in which pollutants exist in post-construction runoff directly impacts the efficacy of a given treatment process. Stormwater treatment tends to focus on two major pollutant forms: particulate and dissolved.

  • Particulate pollutants appear as solid, discrete particles suspended in the stormwater. Transport of particulates is dependent on the size, shape, and density of the pollutant – often requiring the physical force of moving water to mobilize and transport particles downstream. High runoff velocities transport larger and denser particles (in addition to the smaller particles that are more easily mobilized), making high-intensity storms a key consideration for managing heavy particulate pollutants.
  • Dissolved pollutants are much smaller than particulate pollutants and move more easily in water. They are not affected by gravity and the forces of water in the same way as particulate pollutants and often require advanced treatment methods to remove them from suspension.
  • ·Many pollutants can occur in both forms. Nutrients and metals, for example, may be bound to larger particles, may enter the storm system in particulate form but transform into dissolved form over time, or may enter the storm system already in dissolved form.

How Form Impacts Treatment

The details of treatment unit processes will be covered in the second part of this series; however, it’s important to recognize at this stage of the workflow that the form of a pollutant present in the runoff directly informs how you design an SCM for effective treatment. Coarse particles like sand settle out relatively easily and can be removed with SCMs that employ sedimentation – a relatively simple treatment approach. More advanced treatment like filtration and bioretention will be necessary to target dissolved pollutants.

To design an effective stormwater treatment system, it is fundamental to understand not only the types of pollutants being targeted, but also the forms in which they are likely to appear. Once you have that, you’re ready to take on the next part of the workflow: understanding pollutant treatment processes to properly select the most effective control measure.