The septic dye test is sometimes REQUIRED as part of a Septic Certification by a lending institution before financing can be obtained from them for a home purchase.
Most septic systems that were installed following modern code guidelines are an anaerobic system. This means that waste broken down in the system without the introduction of an auxilliary source of air.
Waste is carried from the home to the septic tank by means of a pipe (usually 3" or larger) where bacteria break down the waste. Baffles in the tank prevent solid waste from exiting the tank. Solid waste is broken down by bacteria into a liquid form, which floats to the surface of the septic tank. The liquid exits the tank via the tank outlet, where it is directed to a distribution box (or boxes) and introduced into the leach lines.
Leach lines are lengths of piping, usually made of perforated PVC, which carry the liquid waste and deposit it into the absorption field. The absorption field is the area surrounding the leach lines, and it is designed to distribute the liquid waste over a wide area where it can safely diffuse into the ground. During installation, the area beneath the leach lines is excavated to a depth of 18" (generally) and the trenches are filled with porous gravel or aggregate, upon which the leach lines are laid. The gravel or aggregate helps to create a reservoir for the liquid waste and prevents the leach lines from becoming clogged with dirt and clay. Absorption fields are also designed to allow some of the liquids to migrate to the ground surface where evaporation can eliminate some of the moisture.
A septic dye test is considered to be part of a Routine Maintenance Inspection of the septic system. The procedures are non-invasive, require no excavation, and are generally limited to what can be viewed above the ground surface. It is intended as an observation of the parts of the typical waste-handling system - the septic tank, distribution boxes, leach field, and any related portions of the home's plumbing and water fixtures. This type of septic inspection is performed by a home inspector, and in most cases will satisfy any requirements set forth by lenders as related to financing.