What is treated wastewater that flows out from the septic tank to the drain field called?

The term used to describe treated wastewater that flows out from the septic tank to the drain field is effluent. In the context of a septic system, effluent refers specifically to the liquid waste that has undergone some level of treatment within the septic tank before being distributed to the drain field for further filtration and absorption into the soil.

Effluent is essential for the proper functioning of a septic system as it is the treated component that is allowed to flow into the drain field, where further natural treatment occurs through soil and microorganisms. This healthy functioning is crucial for protecting groundwater quality and ensuring the overall effectiveness of wastewater disposal.

Other terms in the provided choices have distinct meanings. Greywater relates to the relatively clean wastewater from sinks, showers, and washing machines, but it does not encompass the treated waste leaving a septic tank. Sewage typically refers to a mixture of wastewater containing various contaminants, including solid waste, and is not specifically about the treated discharge from a septic tank. Leachate is a term more commonly used in the context of landfills and refers to liquid that has percolated through waste materials, which is not the same as the effluent being discussed. Thus, effluent is the most accurate term in this scenario.