Thanks to the interest of Sen. Jennings Randolph (D-W.Va.), the recent Clean Water Act amendments gave the Environmental Protection Agency a mandate to explore the use of on-site sanitation systems (such as septic tanks and compositng toilets) in lieu of centralized sewerage systems.

The senator's experience with expensive central systems in small towns in this home state convinced him of the need to find lower-cost alternatives. And the EPA is carrying out its mandate by holding conferences throughout the country to familiarize sanitary engineers and public officials with innovative and alternative methods. In addition, the EPA, soon will issue a manual on such systems.

More than 300 persons recently attended a three-day conference on alternative sanitation. The primary message was clear: The EPA's experience in its municipal grants program shows that gravity sewer sytems leading to sewage treatment plants are not appropriate for small towns with populations of 3,500 or less. Speakers described some horrible examples:

Construction costs of a central system running up to $16,000 a house. The EPA pays 75 percent of the costs but the remaining 25 percent still can be a heavy burden for a small town. (In contrast, a septic system can be installed for about $2,000 a house.)

Heavy annual operating charges for a sophisticated sewage treatment plant, plus the problem of recruiting and retaining a competent staff. (Annual user charges in some communities run up to $500 a house.)

An engineering study that greatly overestimated growth by designing a treatment plant for a village of 1,500 to serve people.

Because of these problems, the EPA began to consider alternatives such as upgrading existing septic systems, land application of treated effluent, wetland discharge, vacuum or pressure sewers, holding tanks and flow-reduction devices such as low-flush or composting toilets -- while discouraging use of kitchen garbage grinders.

A special problem became apparent in the EPA's Chicago region, where a number of grant applications were received from lakefront communities proposing sewer lines along th shores. (These did not involve the Great Lakes, but smaller lakes where summer cottages lined the shores.) Because of the high cost per dwelling, the regional office contracted with Wapora, Inc., a firm of environmental consultants in the Washington, D.C., area, to study lower-cost alternatives. An interview with Gerald O. Peters Jr., project manager for Wapora, provided the following hghlights:

The study, underway for 2 1/2 years, involves 7 areas having a total of 35 lakes and is called the Seven Lakes study.

Most houses along the lake shore have relied on septic tanks and subsurface drain fields for sanitation but many of these were not constructed or maintained properly. Most are functioning well or can be rehabilitated at a far lower cost than a central sewerage system.

Where septic systems cannot be rehabilitated, a low-pressure sewer line can carry the effluent from a cluster of houses to an area where a satisfactory drain field can be built.

Waste flows can be reduced by using low-flush toilets, composting toilets or other water-saving devices.

Organization of small waste-flow districts would help to keep the on-site systems functioning on a continuous basis by an inspection and maintenance program.

If centralized sewerage systems were adopted in the Seven Lakes communities, their high costs would force relocation of many residents who could not pay the costs, would encourage denser development and would result in converting many seasonal cottages to year-round use.

Final returns are not yet in for the Seven Lakes study, but if these communities chose the lower-cost alternatives, total savings of tens of millions of dollars could result.

Closer to home, the EPA conference paid special tribute to Fairfax County for its willingness to try innovative methods where sewer connections are not possible. As a case study, the following features of the county's program for better use of septic systems were cited:

Careful evaluation of building sites to insure that septic systems will function properly.

Inspection of each unit several times during installation for adherence to standards.

Splitting the drain field into two equal parts and using them alternatively for one year each so that each part can have an inactive period in which to recover absorptive capacity.

Issuance of a brochure for owners of septic systems giving instructions for proper use.

Setting up an inspection program so buyers of houses can be sure that septic systems are working properly.

Another recent conference, conducted by the National Sanitation Foundation in Ann Arbor, Mich., featured an additional example of Fairfax County's innovative approach: approval of several installations of the Cycle-Let recycling system made by the Thetford Corp. of Ann Arbor.

This unit receives wastewater from toilets and purifies it so it can be returned for re-use, with a great saving in water consumption. The first installation was made in 1976 at the Braddock Community Center. A sewer connection was not possible, and the soil was not suitable for a septic system.

According to John Clayton, environmental health director fo the county, "The system has the capacity to treat up to 45 flushes per day on a regular basis. So far, we are very pleased with its performance."

The next Cycle-Let installed in the county was on a far larger scale -- at the new Great Falls Village Center, a shopping mall modeled after the early American village of Old Sturbridge, Mass. Some 70 toilets and urinals are connected to a large Cycle-Let unit in the basement of one of the buildings. This will reduce water consumption and will cut down the size of the drain field that an equivalent septic system would need.

Cycle-Let units come in several sizes but the smallest is still too large and expensive for a single-family home. (But several adjacent houses could be hooked into one unit.) Thetford sees the potential market and is working on a household unit.

Some criticism has been voiced that sewerless systems like Cycle-Let will allow indiscriminate building on undeveloped land because local authorities often have used approval or disapproval of proposed sewer lines as a zoning instrument. Proponents of sewerless units counter by asserting that these systems will channel future building construction to hilly, rocky or otherwise unsuitable land that cannot be served economically by septic tanks or sewer systems, and thus will help to preserve good farmland.

They also claim these units will void construction of "strip cities" along sewer lines. Optimists say that the new sanitation devices now coming on the market such as recycling systems, composting toilets and aerobic units repesents a major step toward a sewerless future which would help to solve the world's critical water-supply and pollution problems.