Septic Tanks
A septic tank, trash tank, or a processing tank is an enclosed watertight container made of concrete, polyethylene, or fiberglass that receives sewage from the source and provides primary treatment by separating solids from wastewater (See below).
Because the liquid level in the tank remains relatively constant, the rate of flow through the tank is slowed, promoting solids separation. Settleable solids fall to the bottom of the tank as sludge, while floatable solids (fats, oils and grease or FOG) rise to the top as scum. The clarified liquid (effluent) in the clear zone moves out of the outlet pipe to the next system component. Effective solids separation depends upon amount of time effluent remains in the septic tank. Detention time is a measure of the time effluent remains in a tank and is calculated by dividing the tank volume by the daily flow. Inclusion and configuration of inlet deflection devices such as baffles and compartment walls result in a less direct flow path for the effluent to allow for further separation in the tank.
Inflow (extraneous water directly entering a component, such as via a sump pump, downspout, foundation drain, or condensate line) or infiltration (entry of surface water through a leaking pipe, pipe penetration or access riser/tank seam) increases flow beyond design capacity and reduces detention time. This results in solids carrying over that can clog the distribution pipes, and excess hydraulic loading that can saturate the soil treatment area and cause the system to malfunction.
Exfiltration, or effluent exiting the tank, occurs when structural integrity is compromised in the form of leaking pipe penetrations or cracks. Effluent leaking from the tank can contaminate groundwater. If effluent surfaces, it poses a risk to the surface water resources and public health. All tanks must be watertight to prevent water from entering or leaving the system through openings other than designated inlet and outlet piping. Septic tanks Used properly, the septic tank provides primary treatment of wastewater. The fundamental function of the septic tank is to remove solids from raw wastewater, which is called primary treatment. Primary treatment generally refers to allowing solids to settle to the bottom, and oils, grease, and other floatables to migrate to the surface. Primary treatment reduces two commonly used measures of contaminant concentration: five-day biochemical oxygen demand (BOD5) which is reduced by 31 to 54 percent or more, and total suspended solids (TSS) which is reduced by 60 to 80 percent. Oil and grease are typically reduced by 79 percent. Of the solids removed from the wastewater, some are digested, and some are stored in the tank. Up to 50 percent of the solids retained in the tank decompose; the rest accumulate as sludge and scum and must be removed periodically. Pumping the tank is performed by a professional, as residual solids (septage) must be handled and disposed of according to accepted practices and regulatory parameters. Pumping is typically done every 3-5 years depending on the size of tank and number of individuals in the home and water use.
The septic tank is a facultative or anaerobic treatment system. An anaerobic system does not have free oxygen and the microbes breaking down the waste must be able to live on the organic material in the septic tank. They are thus referred to as anaerobic microbes. facultative microbes can thrive with or without free oxygen and are also present in septic tanks. Anaerobic bacteria do not thrive in environments with free oxygen. Wastewater entering the septic tank has dissolved free oxygen, and this is quickly removed because of the oxygen requirements of the wastewater treatment processes. As the system matures, the anaerobic bacteria become more efficient, because the oxygen demand in the system rapidly removes free oxygen entering with the influent and maintains the anaerobic environment. Greater removal rates of BOD5 and TSS are achieved under this fully anaerobic environment.
An effluent screen should be placed in the outlet of the septic tank for additional filtration.
Effluent screens remove solids that could instead be carried out of the tank and potentially clog downstream treatment devices. Processing tanks usually have an effluent screen within the pump vaults. Screens must be cleaned periodically at least once per year depending on water use.
Source: NOWRA Operation and Maintenance Manual, 2025
