On-site wastewater treatment systems (on-site systems) provide treatment of domestic wastewater and return it to the environment within the boundaries of the property of origin. There are many different types of on-site systems, and they are designed to treat household wastewater to varying levels before it is released back into the environment.
Septic tanks are a common example of a basic or 'primary' treatment system in New Zealand. Septic tanks have two components: a solids settling tank (which is, in fact, the septic tank) and an effluent disposal field, such as soakage trenches or subsurface drip irrigation. With primary treatment systems like septic tanks, the majority of the treatment of the wastewater actually occurs in the soil into which the wastewater is discharged, so it is important that the soils are not overloaded with wastewater.
Secondary and tertiary systems involve biological processes and further 'polishing' of the wastewater by using various techniques and equipment that help bacteria and other bugs to digest and break down the wastes in the wastewater before it is released into the environment. (For more detail, see Appendix 1: Definitions).
Figure 2: A primary treatment system, comprising a septic tank and a disposal field
Source: New Zealand Water and Wastes Association (NZWWA) and Ministry for the Environment, 2006.
This figure shows a house with a cut-away view of a septic tank, or on-site wastewater system. The figure illustrates that the sewage from a domestic household is piped to an underground tank where the first stage of treatment occurs. The effluent then travels to an underground disposal area comprised of a series of buried pipes with small holes in them to let the wastewater drain into the surrounding soil. The bugs in the soil have a very important role to play as they work to breakdown the nutrients and pathogens in the wastewater.