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Nitrogen in Wastewater Treatment Plants and Other Permitted Sources

Many municipal wastewater treatment plants and other permitted industrial sources discharge wastewater effluent directly into Puget Sound waters via marine outfalls. Some discharge to rivers.

Wastewater treatment plants with marine outfalls are one of the largest local sources of nitrogen to Puget Sound.

Nitrogen contributions from municipal wastewater treatment plants are much greater than those from industrial facilities. This is because municipal wastewater has much higher nitrogen concentrations than industrial effluent and because there are many more wastewater treatment plants than there are industrial facilities with outfalls in Puget Sound.

The effluent from all these facilities is treated and regulated, but it still contains nitrogen. Effluent nitrogen concentrations vary between facilities depending on the type of treatment technology and other factors.

Approximately 78 U.S. municipal wastewater treatment plants, nine Canadian municipal wastewater treatment plants, five oil refineries, four active pulp/paper mills, and one aluminum facility discharge effluent directly into the marine waters of Puget Sound or the Straits. Sixty-one of these facilities discharge to Puget Sound marine waters south of Admiralty Inlet.

In addition, there are facilities which discharge upstream to freshwater bodies or groundwater within Puget Sound watersheds. Nitrogen loads for these upstream facilities have not been estimated separately. They are included indirectly when we measure nitrogen entering Puget Sound via rivers and streams.

Concentrations, Loads and Yields explains the differences in the ways of measuring and expressing nitrogen contributions.

Wastewater flows discharging to marine waters

The largest municipal wastewater plants, in terms of flows, serve the largest population centers.

The Puget Sound Dissolved Oxygen Study compiled flows for municipal wastewater treatment and industrial plants discharging to the marine waters of Puget Sound.  In Puget Sound, the two largest wastewater treatment plants include West Point and South King, which are both located in King County. West Point has an average annual flow of 107 million gallons per day (mgd), while South King has an average annual flow of 76 mgd. They both discharge into the main basin of Puget Sound.

The total flow from wastewater treatment plants and industrial facilities to Puget Sound is 390 million gallons per day, which is about 1.3% of all the freshwater inflows from the rivers and streams to Puget Sound, by volume.

Mean Annual Flow from Wastewater Plant Flows to U.S. and Canadian Waters

Region of Discharge into Marine Waters Flow from Wastewater Treatment Plants in Millions of Gallons Per Day (mgd) Flow from Industrial Facilities in mgd
Puget Sound - South of Admiralty Inlet 350 mgd 87 mgd
Straits of Juan de Fuca and Georgia – U.S 25 mgd 68 mgd
Straits of Juan de Fuca and Georgia – Canada 356 mgd 0 mgd

We do not have estimates of flows from facilities that discharge to rivers before reaching marine waters.

Nitrogen Concentrations in Wastewater Effluent

Concentrations of nitrogen vary from plant to plant, because each plant is designed and operated differently. National Pollutant Discharge Elimination System (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 Discharge Monitoring Reports (DMRs). However, most plants are not required to monitor all the different forms of nitrogen in their effluent. The LOTT facility in Olympia is the only wastewater treatment plant in the Puget Sound region which has a permit limit for nitrogen concentrations and loads.

Based on a 2006-2007 monitoring effort by Ecology and data from Discharge Monitoring Permits, Ecology estimated dissolved inorganic nitrogen (DIN) concentrations from all facilities discharging to Puget Sound, as illustrated in the map to the left.

Wastewater treatment plants have 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 highest concentrations are found in the effluent of Carlyon, Lakota, Central Kitsap, and South King wastewater treatment plants.

DIN concentrations in wastewater treatment plants. The heavy red line indicates the median, the box represents the 25th and 75th percentile concentrations, and the whiskers represent the minimum and maximum measured values (Source: Roberts et al., 2008).

Nitrogen Loads from Wastewater Treatment Plant Effluent

The total annual DIN load to Puget Sound from all permitted wastewater sources is 32,200 killograms per day (kg/d). The following five largest U.S. wastewater treatment plants in terms of flow contribute the largest nitrogen loads to Puget Sound even though some smaller wastewater treatment plants have higher concentrations.

West Point and South King are also the two largest regional wastewater treatment plants, serving Puget Sound’s largest population centers in King County. Nitrogen loads from the Annacis and Iona wastewater treatment plants, the two largest Canadian plants, are comparable in magnitude, contributing 12,650 kg/d and 8,360 kg/d of DIN, respectively, to the Strait of Georgia.

Five largest plants by DIN Load

Wastewater Treatment Plants DIN Loads in kg/d
West Point  10,450 kg/d 
South King 8,880 kg/d 
Chambers Creek  2,030 kg/d 
Everett/Snohomish 1,990 kg/d 
Tacoma Central  1,280 kg/d 

Wastewater treatment plant load contributions vary by regions within Puget Sound, with 70% of the DIN load entering the main basin of Puget Sound.

Annual WWTP DIN loads into different regions of Puget Sound (Source: Mohamedali et al., 2011a).

Load variation by season

The total nitrogen load from wastewater treatment plants to Puget Sound does not vary much seasonally, because the number of people served remains fairly constant. The average monthly load varied slightly from about 30,000 kilograms per day (kg/d) to 35,000 kg/d for the period 1999-2008.

In the winter, some treatment plants also receive stormwater from combined sewer systems. Even if plants are not designed to receive stormwater, stormwater and groundwater gets into wastewater pipes. These is called inflow and infiltration, and most municipal wastewater plants experience at least some winter increases in flow.

(Source: Mohamedali et al., 2011a)