Wastewater

Wastewater treatment plant effluent is the largest local human source of inorganic nitrogen loading to Puget Sound. Municipal wastewater treatment plants and other permitted industrial sources discharge wastewater effluent directly into Puget Sound waters via marine outfalls. Many wastewater treatment plants discharge to rivers further upstream.

Human wastewater contains nitrogen, and facilities achieve varying levels of nitrogen removal depending on their treatment processes. However, none of the wastewater treatment facilities in the Puget Sound region currently have a regulatory requirement to remove nitrogen except for the LOTT facility in Olympia, which discharges to Budd Inlet. 

Wastewater treatment plant nitrogen loading

This map shows dissolved inorganic nitrogen (DIN) loads to Puget Sound and the Straits from all permitted wastewater sources, including those in Canada. Values are based on an annual average over the years 1999 through 2008.

The total DIN load into Puget Sound (south of Edmonds) is about 32,200 kilograms per day (kg/day). Regional facilities that serve Puget Sound’s largest population centers (e.g. Seattle, Everett, Tacoma) have the largest nitrogen loads.

Wastewater treatment plant load contributions vary by regions within Puget Sound, with 70% of the DIN load entering the main basin of Puget Sound. These loads do not vary much seasonally since they treat about the same amount of water from month to month (based on the populations they serve).

Wastewater concentrations

Concentrations of nitrogen in effluent vary from plant to plant because each plant is designed and operated differently. The box plot to the right shows DIN concentrations in wastewater treatment plant effluent discharging to South and Central Puget Sound. These data were collected as part of a 2006-2007 monitoring effort in South and Central Puget Sound, as well as data reported in Discharge Monitoring Reports (DMRs).

Based on these data, the plants had a median concentration of 22 milligrams per liter (mg/L) of DIN in treated effluent, which is 10 to 100 times more concentrated than levels in rivers. The LOTT facility in Olympia has some of the lowest concentrations – it is also the only wastewater treatment plant in the Puget Sound region that has a permit limit for nitrogen concentrations and loads. (NOTE: the plot does not include treatment plants that discharge into Puget Sound north of Edmonds)

National Pollutant Discharge Elimination System, or NPDES permits, are required for facilities discharging to state waters, and these permits often mandate some level of monitoring and reporting of each plant’s effluent through DMRs. Currently, in Washington state, most plants are not required to monitor all the different forms of nitrogen in their effluent.

Future wastewater loads

This chart shows estimates of future DIN loads into Puget Sound from wastewater facilities. The top chart shows that wastewater DIN loads to Puget Sound will almost double in the future from a current (2006) load of 32,200 kg/day to almost 55,000-65,000 kg/day by 2070 due to projected population growth. This is simply because more people means more wastewater.

The bottom chart shows that if wastewater facilities upgrade their treatment technologies to include a higher level of nitrogen removal, we can more than offset the projected increases in DIN load due to population growth, and reduce the total DIN load from wastewater to below current levels by reducing the concentrations of nitrogen in wastewater effluent. We will be using the Salish Sea Model to see how water quality in Puget Sound might respond to such a reduction in loading from wastewater facilities.

 These estimates were developed using a range of population forecast estimates (high, medium, low) generated by the Office of Financial Management. The lower image assumes that DIN concentrations in wastewater effluent are no greater than 8 mg/L throughout the year.