4.1 Treatment in the Septic Tank (Anaerobic)

Septic systems usually collect all household wastewater in a single tank that is partitioned into two sections (Figure 1), so that solids settle out in the first compartment. Domestic wastewater is a rich source of labile organic carbon and consequently, the effluent normally becomes anaerobic during storage in the septic tank. Anaerobic conditions can lead to a number of reactions in the tank, including organic nitrogen mineralization to NH4+ as shown in Equation 1 (Wilhelm et al., 1994).

CO(NH3+)2 (urea) + H2O → 2NH4+ + CO2­ (1)

Anaerobic conditions also lead to ammonium, NH4+, conversion to NH3 gas as shown in Equation 2 (Wilhelm et al., 1994).

NH4+ + OH → NH3 + H2O (2)

In addition, anaerobic conditions result in sulfate reduction as shown in Equation 3 and methane generation as shown in Equation 4 (Wilhelm et al., 1994).

SO42 + 2CH2O + 2H+ → H2S + 2CO2 + 2H2O (3)
CO2 + 4H2 → CH4 + 2H2O (4)

Gases generated as a result of these reactions (CO2, NH3, H2S, CH4) are usually removed using a ventilation stack as shown in Figure 1, which is repeated here for the reader’s convenience. These gases are assumed to rapidly oxidize in the atmosphere. In some cases, the drainfield is not vented, and gases are conveyed to the leach field, where they migrate upward from the perforated drainfield piping.


Repeat of Figure 1Typical household septic system showing wastewater treatment steps that occur in the septic tank, and in the unsaturated sediments underlying the drainfield (adapted from Wilhelm et al., 1994). This repeat of Figure 1, provides an opportunity to notice the gases that are released into the atmosphere from the vent pipe or through the soil above the drainfield.


Septic System Impacts on Groundwater Quality Copyright © 2021 by William Robertson. All Rights Reserved.