1 What is a Septic System?
In rural and many suburban areas, household wastewater is treated on-site, in disposal systems such as septic systems. Although versions of the flush toilet with associated sewer systems, have been in operation in urban centers since Roman times and earlier, in rural areas, outdoor pit privies were the common method for waste disposal until relatively recently. A privy is a small building surrounding a bench with holes through which a person may urinate or defecate. The modern ‘septic system’ was invented by Jean-Louis Mouras in France, around 1860. It involved indoor plumbing, with wastewater piped to a collection tank in which fats floated and solids settled while the liquid flowed to an associated drainage pit. After 10 years of operation, he was surprised to discover that the tank contained mostly liquid and remained relatively free of solids. A patent was granted in 1881, and these systems began appearing in the U.S. shortly thereafter. Their use became widespread during the 1940s, and in the 1970s, regulations were introduced in many jurisdictions, that standardized their design and installation (Van Delden, 2015). In the USA, more than 22 million septic systems are in operation, servicing ~ 25 percent of the population and it is estimated that more than 500,000,000 on-site waste-water treatment systems are in use worldwide (McCray et al., 2005; Conn et al., 2006).
Septic systems typically consist of a septic tank, which collects wastewater and an associated drainfield (also referred to as a leach field), where the wastewater is discharged into the subsurface (Figure 1).
Figure 1 – Typical 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).
Wastewater treatment in septic systems can be considered analogous to that provided by conventional communal treatment systems. Solids settle out in the tank and are pumped out periodically, providing the function of primary filtration, whereas the drainfield allows the filtered liquid to percolate into the subsurface where oxidation reactions occur, analogous to the oxidation provided by mechanical aeration during conventional secondary sewage treatment.
Generally, septic tanks are sized to provide several days of wastewater retention, which is the average time that the wastewater spends in the tank, so the volume of the tank is large enough to hold several days of wastewater. Drainfield size is based on the wastewater loading rate (i.e., volume of waste water per day) and the permeability of the drainfield sediments. A ‘percolation test’ is a simple-to-perform, semi-quantitative estimate of permeability (OMAFRA, 2019). The percolation rate is the rate of decline of the water level in a shallow hole in the soil that will contain the drainfield. The drainfield must be large enough to allow infiltration of the daily wastewater volume each day. A household septic system located on relatively permeable sediments, typically has a septic tank volume of about 3000 liters (L) and a drainfield that is about 100 m2 in area.
Wastewater is discharged from the septic tank by gravity flow (or by pumping) to infiltration pipes that are typically trenched into the drainfield sediments to about 0.7 m depth. This allows the wastewater to migrate through the unsaturated zone before entering the groundwater flow system at the water table (Figure 1). Typically, a drainfield consists of multiple lengths of perforated pipe installed in trenches that are backfilled with permeable pea-sized gravel. However, there are multiple drainfield designs, including importing higher-permeability sediment (usually coarse sand), at sites where low permeability sediments are present. Documents that provide more information on drainfield designs are available on-line (e.g., Septic Smart, OMAFRA, 2019). State and provincial regulations usually mandate minimum separation distances between the infiltration pipes and the water table or underlying impermeable formations. In the province of Ontario, Canada, infiltration pipes must be positioned a minimum of 1.9 m above the water table or bedrock formations (OMAFRA, 2019). Additional regulations govern the placement of drainfields, and mandate minimum separation distances: e.g., 15 m from drilled wells; 30 m from dug wells; 15m from surface water courses; and 5 m from permanent structures in Ontario (OMAFRA, 2019).