Nitrogen from Onsite Sewage Systems

Onsite sewage systems or onsite septic systems contribute nitrogen to both freshwater and marine water. However, nitrogen loads from these systems are difficult to quantify because of challenges in accurately measuring the nutrient transformations that occur as the septic effluent leaves the drainfield and interacts with soil layers and groundwater before it enters nearby waterbodies.

Conventional septic systems are not designed to reduce nitrogen levels in human wastewater. Even well maintained, properly functioning septic systems release a significant portion of otherwise-treated wastewater into the groundwater environment.

Once this nitrogen is discharged from septic system drainfield to sub-surface soil layers, there are a number of pathways which can lead to a loss of nitrogen (e.g. denitrification) before it enters a nearby surface waterbody. The amount of nitrogen removal that occurs is difficult to quantify, since it is highly variable due to differences in sub-surface soil characteristics and groundwater flow pathways. The proximity of septic systems to the shoreline is another factor that effects how much nitrogen enters Puget Sound since effluent from septic systems located closer to the shoreline will spend less time in the soil before it enters nearby surface waters, and therefore have higher nitrogen concentrations than effluent from septic systems located further inland.

Several studies have tried to estimate nitrogen loading from onsite sewage systems to different sub-regions of Puget Sound – the results of these studies are summarized in the image below, and discussed in more detail in the following paragraphs.

Steinberg et al. (2010) estimated the fraction of total watershed inputs attributed to human population that could include septic systems or other residential activities. These constituted 14% of the watershed inputs to Hood Canal overall but 31% of the inputs to Lower Hood Canal or Lynch Cover.

Click to scroll to info on septic system studies.

Nitrogen from Septic Systems Entering Lynch Cove

Cope and Roberts (2013) estimated shoreline onsite sewage system contributions to Lynch Cove (the area east of Sister’s Point in Hood Canal) as 0.3 to 1.9 MT/month, or 10 to 63 kg/d based on a comprehensive review of a number of other studies and available monitoring data. Shoreline seepage monitoring data collected by Mason County showed median and mean concentrations DIN concentrations of 0.23 mg/L and 0.65 mg/L, which were used to establish a low and high bound estimate of DIN loading when multiplied by shoreline groundwater flow estimates.

Nitrogen from Septic Systems Entering Quartermaster Harbor

Nitrogen loads from shoreline septic systems of 2 – 19 kg/d to Quartermaster Harbor were estimated by (King County, 2010), using an estimate of the number people living along the shoreline, per capita effluent volume, and typical septic drainfield nitrogen concentrations of 10 – 78 mg/L. This study assumed no transformation or reduction of nitrogen concentrations since the focus was on nearshore systems with limited potential for attenuation.

Nitrogen from Septic Systems Entering South and Central Puget Sound

Whiley (2010, in Appendix C of Mohamedali et al., 2011b) estimated that DIN loading from septic systems to South and Central Puget Sound (south of Edmonds) ranges from about 80 – 1,400 kg/d. The actual loading varies within this range depending on what assumptions are made when performing the calculation, and whether the loading estimate includes only septic systems located closer to the shoreline (< 150 m) or upland systems (> 150 m).

This estimate was based on existing literature and past studies, using a mean effluent nitrogen concentration of 31 mg/L in septic system effluent. The analysis then involved the application of a spatial GIS approach, combined with a Monte Carlo-type analysis by estimating the number of people in Puget Sound who are not serviced by municipal wastewater treatment plants, average wastewater flows per person, and a nitrogen loss ranging from 10 – 70% due to attenuation.

Study Evaluating Nitrogen Removal Treatment Processes

Since most septic systems are not optimized to treat wastewater for nitrogen removal, there is a need to evaluate treatment processes that are cost effective, reliable, and require low maintenance, and could be applied to single or multiple residences. A 2013 study by the University of Washington (UW) and the Department of Health (DOH) evaluated the effectiveness of three public domain nitrogen removal treatment processes, with the goal to produce effluent total nitrogen concentrations below 20 mg/L.  For more details about study, go to the DOH’s Denitrification Verification Project webpage.

The results from this study are summarized in the table below – all three treatment processes evaluated produced effluent with total nitrogen concentrations well below the goal of 20 mg/L.

Treatment Process Average Influent Total Nitrogen Concentration Average Effluent Nitrogen Concentrations Percent Removal of Total Nitrogen
NH3-N NOx-N Organic-N
Recirculating Gravel Filter and Vegetated Denitrifying Woodchip Bed 48.6 mg/L 0.5 mg/L 2.4 mg/L 1.1 mg/L 92%
Enhanced Recirculating Gravel Filter 6.8mg/L 0.6 mg/L 1.3 mg/L 82%
Vegetated Recirculating Gravel Filter 4.1 mg/L 9.5 mg/L 1.5 mg/L 69%

Source: University of Washington and WA State Department of Health, 2013 (Denitrification Verification report)