8 Fate of Trace Organic constituents in Septic System Plumes
A number of trace organic constituents can be present in septic tank effluent at concentrations that are of environmental and health concern, and consequently, their treatment in septic systems is of interest. Effluent from the Cambridge site shown in Figure 2, for example, was found to contain more than 30 volatile organic compounds (VOCs), mostly substituted benzenes, related to oil and grease residues introduced during dish washing, clothes laundering, and other cleaning tasks. However, none of these compounds were detected (i.e., none were present at a concentration greater than 1 µg/L) in the underlying groundwater plume, indicating relatively complete VOC transformation in the 2-m thick, sandy, unsaturated zone at this site (Robertson, 1994). In a tracer injection test conducted in the same study, dichlorobenzene contained in a plumbing line cleaner, was dosed into the septic tank, and was also found to be almost entirely degraded in the unsaturated zone. The drainfield unsaturated zone at the Cambridge site, provides a high degree of DOC removal (3 mg/L in the plume versus 38 mg/L in the effluent, Table 2) and provides complete oxidation of the wastewater NH4+ (Robertson, 1994). Swartz and others (2006) measured a suite of pharmaceutical compounds and estrogenic substances in the plume from a residential septic system on Cape Cod, Massachusetts, USA and found that several pharmaceutical compounds, including caffeine and paraxanthine, were highly consumed in the 3-m thick, sandy, unsaturated zone at that site, but that several estrogenic substances, including 17β-estradiol and estrone, persisted in the groundwater plume at concentrations that were close to the septic tank effluent values.
Carrara and others (2007) measured a suite of 12 pharmaceutical and trace organic constituents in the plumes from three large septic systems in Ontario, Canada including the Long Point site shown in Figure 3, and found persistence of several compounds, including ibuprofen, gemfibrozil and naproxen. The widely prescribed anti-convulsive drug, carbamazepine, is also known to persist during sewage treatment, and in subsurface wastewater plumes (Clara et al., 2004; Heberer et al., 2004; Nakada et al., 2008), and consequently, has also been proposed as a potential wastewater tracer. However, this compound is not ubiquitously present in all domestic wastewater sources (Oppenheimer et al., 2011), which reduces its robustness as a tracer. Some other tracer candidates, such as the artificial sweeteners, acesulfame and sucralose, are more ubiquitously present.
Overall, properly functioning septic systems provide a high degree of removal of trace organic constituents, particularly in the drainfield unsaturated zones, although some recalcitrant compounds can persist. However, these same compounds also persist during conventional sewage treatment as well.