Hood Canal Onsite Sewage System Nitrogen Reduction Project

Hood Canal has a history of low dissolved oxygen levels known to negatively impact marine life. High levels of nitrogen are known to cause lowered dissolved oxygen levels as a result of algal blooms, which thrive on available nitrogen. Dissolved oxygen is then consumed through the decomposition process of the algae.

While anthropogenic nitrogen inputs in the Hood Canal watershed remain largely unquantified, an opportunity to reduce any of these inputs should be taken. One of the known sources of nitrogen is onsite sewage systems (OSS) near the shores of Hood Canal. The Hood Canal Onsite Sewage System Nitrogen Reduction Project (HCOSSNR) is implementing and monitoring advanced treatment systems on existing OSS on lower Hood Canal near-shore properties. The project is a continuation of a study conducted by the Washington State Department of Health partnered with the University of Washington. For more information regarding this project and OSS technology visit the Denitrification Verification Project webpage.

The HCOSSNR project will be implementing two of the Recirculating Gravel Filter (RGF) + Vegetated Denitrifying Woodchip Bed (VDWB) systems. One will be placed at the Salmon Center in Belfair, and the other at a residence in Union. The two systems will be sampled and monitored monthly (minimum) for the first two years, followed by quarterly sampling for another six years, making 8 total years of system monitoring. The objective of the project is: A) Determine the performance of these systems under authentic residential loading trends, B) Observe the longevity of system performance, such as length of time before Woodchips in VDWB need to be replenished, and C) Determine system cost, and amount of service, repair and maintenance required to maintain a well performing system.

The systems’ effluents will be sampled and monitored for nitrogen levels (ammonia, nitrate & nitrite, total nitrogen), as well as Biochemical Oxygen Demand (BOD), Total Suspended Solids (TSS) and Fecal Coliform. The Salmon Center is going to analyze samples for TSS and Fecal Coliform in-house using our own state-accredited lab space, while all other parameters are analyzed by project partners at the University of Washington Civil and Environmental Engineering, as well as local business Centric Analytical Labs. A comprehensive final report will be completed after the first 2 year period outlining the findings of the study, as well as recommendations and considerations for future implementation.