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Examining the Phosphorus Problem: What the Data Tells Us About Reducing Phosphorus in Stormwater

Course Information

Introduction

In many parts of the United States, stormwater quality regulations are expanding to include treatment criteria for total phosphorus. Phosphorus is the second-most-regulated pollutant in stormwater runoff after total suspended solids. However, engineers and regulators are still learning how to remove phosphorus from stormwater to promote healthy waterways and meet agency requirements. This article will provide an overview of the current state of phosphorus treatment research and how this research can help guide decisions when specifying stormwater Best Management Practices (BMPs).

Why Phosphorus Is Important

Phosphorus is a naturally occurring element and a nutrient necessary for the health of all forms of biological life. In the aquatic environment, elemental phosphorus is rarely found by itself; it is typically present as a compound. It can be in inorganic and organic forms, and it can be found attached to particulate matter or in dissolved forms. Adding to the complexity, phosphorus can shift forms between being dissolved and attaching to particulate matter within the water stream as it reacts to other compounds and variables in stormwater runoff, such as dissolved oxygen.

Phosphorus also is often the limiting nutrient for organisms in freshwater and can be a primary cause of eutrophication and harmful algal blooms (HAB) (Figure 1). The U.S. Environmental Protection Agency identifies HABs as a major environmental problem in all 50 states. Eutrophication occurs when the aquatic environment becomes enriched with nutrients, increasing plant and algae growth. The excess algae and plant matter eventually decompose, reducing oxygen to levels that can kill fish and seagrass as well as reduce essential habitats, often resulting in mass die-off events. This growth also limits light penetration, which hinders the development of ecologically beneficial plants.

The chemical form of phosphorus (also known as speciation) significantly impacts how the presence of phosphorus affects the water body. For example, dissolved phosphorus is more readily available to algae than particulate-based phosphorus, making dissolved phosphorus concentrations more directly relevant to eutrophication and HABs.

Author

Jeremiah Lehman, P.E., and Craig Fairbaugh

Learning Objectives

By the conclusion of this article, the reader should be able to:

• Explain the importance of removing phosphorus from stormwater

• Explain how phosphorus is measured in stormwater

• Explain why phosphorus is difficult to remove from stormwater

• Evaluate various phosphorus removal mechanisms and performance data

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