Nitrogen: A Problem for Puget Sound
Sewage typically contains high levels of nitrogen. Even with new technologies, on-site sewage (septic) treatment systems generally have limited ability for removing nitrogen in wastewater. This can result in increased nitrate concentrations, or "nitrogen loading" in ground and surface water. And high nitrate levels in drinking water can affect human health.
Nitrogen loading is an environmental concern in the Lower Hood Canal and other regions of the Puget Sound. Excess nitrogen fuels the growth of algae. As algae dies and decays, it consumes oxygen. This process contributes to depleted dissolved oxygen conditions in Puget Sound and can harm aquatic life.
For more information see our fact sheet How Nitrogen from Septic Systems Can Harm Water Quality (PDF).
Nitrogen Removal Verification
An objective of the denitrification project is to expand reliable and affordable nitrogen removal options for septic treatment systems in Washington State. Our Wastewater Management Section partnered with the University of Washington's Department of Civil and Environmental Engineering to evaluate various septic treatment systems for their nitrogen removal abilities. See our project overview (PDF).
Three public domain, cost-effective septic treatment technologies were selected for testing. These technologies have been successful in other parts of the United States in removing high amounts of nitrogen from sewage. Because denitrification processes are temperature dependent, we didn't know if they would be effective in Washington's climate.
The three systems were built, tested, and evaluated over a one-year period using the EPA Environmental Technology Verification (ETV) protocol for nutrient reduction. A stakeholder advisory committee was formed to help in the evaluation process. We compiled ETV test results for each system and wrote final reports based on those results. The systems evaluated were:
Vegetated recirculating gravel filter (VGRF) that is comparable to a recirculating vertical flow constructed wetland.
Passive two stage nitrification and denitrification system that includes a recirculating gravel filter followed by a vegetated denitrifying woodchip bed (RGFW).
Enhanced recirculating gravel filter (ERGF) that is also designed to maximize nitrogen removal efficiencies.
Technology Standards Development
The environmental technology verification results show both the vegetated recirculating gravel filter and the recirculating gravel filter with the woodchip bed systems are reliable and effective in removing nitrogen from wastewater. We are developing standards and guidance for the use of these two treatment technologies as supported by their ETV results.