Human activity releases nutrients, pharmaceuticals, personal care products, pesticides and other contaminants, and many reach rivers, lakes and seas. In built-up areas, wastewater treatment plants are the key defence, collecting wastewater and cutting the contaminants it carries before discharge to rivers, coasts or groundwater. Most use several steps: primary treatment removes solids; secondary treatment uses biological and chemical processes to remove most organic matter; tertiary treatment reduces nitrogen, phosphorus or synthetic organic compounds; and the water is disinfected before release.
A U.S. Geological Survey study examined how much of 114 organic wastewater contaminants three plants removed before discharging to fresh and marine waters in a fast-growing part of northwestern Washington: the Stillaguamish River Basin, about 700 square miles and the fifth-largest watershed draining to Puget Sound.
The three plants
| City of Arlington (2009) | City of Arlington (2011) | Angel of the Winds Casino (2011) | Warm Beach Camp (2009) | |
|---|---|---|---|---|
| Serves | 17,711 residents | 17,930 residents | 3,600 visitors a day | 800 seasonal visitors |
| Capacity, million gallons a day (connections) | 1.00 (4,347) | 2.65 (4,427) | 0.18 (3) | 0.035 (350) |
| Primary | 6 mm screen, grit removal | 3 mm screen, grit removal | 2 mm screen | 6 mm screen |
| Secondary | aeration basins | aeration basins | aeration basins | two aerated lagoons, then two constructed wetlands |
| Tertiary | none | membrane bioreactor; biological nutrient removal | membrane bioreactor | the same lagoons and wetlands |
| Disinfection | chlorination / dechlorination | ultraviolet | ultraviolet | chlorination |
| Discharges to | Stillaguamish River | Stillaguamish River | groundwater recharge | Port Susan |
Before the 2011 sampling, the Arlington plant was upgraded with a membrane bioreactor and biological nutrient removal to improve contaminant removal.
How it was measured
At each plant, operators or the Stillaguamish Tribe of Indians collected paired samples of incoming (influent) and outgoing (effluent) water every 6 hours and combined them over the same 24 hours. The USGS filtered and processed the samples and analysed them at its National Water-Quality Laboratory. Each contaminant's removal percentage came from comparing influent and effluent concentrations, following an EPA method. Where a compound was found going in but not coming out, the figure is a minimum estimate.
What the plants removed
- Of the 114 contaminants, 71 turned up in the influent of at least one plant.
- Removal could be calculated for 52 at one or more plants. Of those, 51 showed some removal, and 49 had removal above 50 percent at one or more plants.
- For 16 compounds removal could be calculated at all three plants (the chart above): steroids and metabolites such as cholesterol and skatole; personal care products such as menthol, camphor, triclosan and the musk HHCB; and consumer product additives such as phenol and several phosphate flame retardants and plasticisers.
- After Arlington's upgrade, eight compounds were removed significantly better, and none significantly worse.
The results show that all three plants cut the concentrations of many contaminants before the water re-enters the environment. Confidence is limited, though: each plant's influent and effluent were sampled only once in each year studied.
Sources
- Barbash, J.E., Moran, P.W., Wagner, R.J., and Wolanek, M., 2015, Contaminant removal by wastewater treatment plants in the Stillaguamish River Basin, Washington: U.S. Geological Survey Fact Sheet 2015–3023. https://pubs.usgs.gov/publication/fs20153023
- Figures: U.S. Geological Survey; the map is modified from Wagner and others (2014), on a base from The National Map. The sheet's photographs of the plants — from the City of Arlington, a Google Earth montage and an outside report — are left out.
- The sheet's table of the remaining contaminants, plant by plant, is in its PDF.
- Rewritten in hubnx's own words.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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