An alternative system is, in simple terms, any system that produces a higher quality effluent than just a conventional septic tank. These designs are much more complicated, with some requiring a design by a professional engineer. Read more about how waste water treatment works from the EPA.
New regulations were issued in 2000 that require a minimum separation distance of eighteen inches to seasonal water table or restrictions, with the minimum installation of 18 inches for trenches. So if you want to install a "conventional" drainfield, you need to have thirty-six inches of suitable soil available in the disposal area.
During the evaluation process, if it is determined that your lot does not meet the minimum requirements for a “conventional” system, the new regulations allow you to install the disposal field shallower than eighteen inches, or with less than 18 inches to water table or restriction. In either case, in order to install a system you must use "secondary" treated effluent.
Secondary treated effluent can be achieved by the use of "alternative" septic systems such as: