A new self-contained human waste treatment system that has been test marketed in Virginia for the past three years is being marketed nationwide.

The Thetford Corp. of Ann Arbor, Mich, answered a call from state public health officials for a system that could be used for commercial, industrial and public buildings where sewer hook-ups were unavailable or too expensive, or where the soil will not perk or absorb waste.

Compared with the other alternatives-sceptic tanks and incineration methods-the Thetford system, called Cycle-Let, is more expensive but, according to users, more convenient and reliable. It uses about one-eighth the amount of water of a conventional toilet.

The 3-by-10-foot unit, hooked up to one or two toilets, processes waste with biological treatment and filtration, followed by ultra violet and ozone disinfecting. The water is colorless and odorless and it is not necessary to add water or chemicals, the manufacturer said.

The toilet can be flushed more rapidly than a conventional model because there is no need to wait for the tank to refill. Sink drains can also be hooked to the system.

The smallest unit, costing $17,000, can accomodate 50 people working eight or 10 hours a day or 18 people working 24 hours a day, the manufacturer said. The largest unit, priced at $33,000, can accomodate 280 daytime or 100 round-the-clock workers. Service is free the first year, but thereafter an annual labor charge of $500 to $1,000 is levied depending on the size of the unit.

Parts-primarily new ultraviolet lights-cost between $60 and $125 a year. The system's anticipated lifespan is 20 years, according to Thetford's manager of customer services, Dave Calhoun.

Wilson-Finley Co. of Gainesville, Va, which sells and rents heavy construction equipment, got its first prototype unit three years ago when it was discovered that the soil on the building site would not perk. The price of raw industrial land in that area runs around $2,000 an acre, compared with sewered land at $18,000 an acre. Branch manager Jerry Smith estimated that the savings of a Thetford unit on a 10 acre site would amount to between $70,000 and $160,000.

The Braddock Recreation Center in Fairfax County previously had a septic tank and paid $300 a month to have it emptied. Jim Bowman, of the county's division of environmental health, said the Thetford system was selected because no other system was available that would accomodate 65 or 70 people a day and meet other requirements.