The threat of new sewer moratoria understandably has disturbed real estate developers in the Washington metropolitian area.

The future for construction here is clouded by continuing intergovernmental disputes about the use of limited sewage capacity at the Blue Plains treatment plant and about disposal of sludge.

Two other adverse circumstances raise questions about future building in this area:

The Enviromental Protection Agency has announced that federal funds for financing 75 percent of the cost of sewer lines, and treatment plants no longer will be approved for new residential areas. EPA's objective is to clean up water pollution in existing communities rather than to stimulate new growth. (In line with this policy, EPA refused to approve the proposed $400 million treatment plant at Dickerson in upper Montgomery County).

A future water shortage looms as a threat to new construction. Limited rainfall from last August to December led to water rationing in one nearby community, Round Hill, Va.However, recent heavy rains and snows have led to the opposite problem, leakage of ground water into aging sewer lines with a consequent overload at sewage treatment plants.

The Army Corps of Engineers has predicted future water shortages in three metropolitan areas of the well-watered northeastern states: Boston-Rhode Island, New York City and Washington. Thus, the long-term future of water-flushed sewage systems appears dim.

In the absence of EPA funds, developers are faced with the prospect of underwriting the entire cost of sewer systems and treatment plants unless local governments pick up part of the burden. And with costs for such central systems running up to $20,000 a house, the prospect is discouraging enough without worrying about future water shortages.

But developers can take heart from a recent conference in Ann Arbor, Mich., conducted by the National Sanitation Foundation. The meetings focused on the use of on-site disposal systems in lieu of centralized sewer lines and treatment plants.

A group of 300 public officials and sanitary engineers agreed that septic tanks are fully satisfactory when soil conditions allow proper percolation of the effluent and when sludge is pumped out periodically. But where the soil will not percolate, a number of innovations are being developed to solve the problem.

In some cases, mounds, or evapotranspiration beds, are being used to evaporate the effluent. In others, the effluent from several septic tanks is pumped to a central lagoon, or to a patch soil at some distance that does provide good percolation.

And a new tool is now available to sanitation authorities: an aerial survey in which infrared photos identify failing septic tanks.

Much interest was expressed in the new aerobic tanks now coming on the market. These systems use underground tanks and tile drain fields like a septic (anaerobic) system, but an electric pump churns air into the tank to break up solids and stimulate the growth of air-loving organisms that feed on anaerobic bacteria.

Aerobic systems use as much water as septic systems, but one type of aerobic tank filters the effluent so it can be recycled back to the flush toilets safely, reducing water use by about 40 percent. Similar, water savings are possible with composting or incinerating toilets, but then the "gray water" from basins, baths, sinks and laundry still must be disposed of.

Many authorities require a full-sized septic tank and drain field to handle such gray water, but others now accept a smaller tank and field. Some manufacturers are developing home filters to cleanse gray water so it can be used for watering lawns and gardens.

In addition to swerless systems that serve single-family houses, some new devices can serve clusters of eight or 10 houses or an apartment building without the need for a sewer connection. One such device is installed in a public comfort station at Olney Manor Park. They system, which is made by Saxton Air Systems, Willow Grove, Pa., serves 11 toilets, four urinals and five wash basins. The effluent is cleansed for recycling back to the toilets and urinals, and surplus water is evaporated.

A problem for makers of these new devices and for developers who want to purchase them is the atitude of sanitation officials in some states, counties and towns. They are reluctant to approve the installation of unfamiliar devices that may fail in use. Three recent developments may alleviate the situation:

The National Sanitation Foundation is testing some of the new devices, and several systems already have been approved.

The EPA has been authorized to establish a national clearinghouse of information on such alternative sanitation systems. When in full operation, this new center will assist public officials, sanitary engineers and developers in assessing the merits of on-site sanitary systems.

The Appalachian Regional Commission has pioneered a new approach to maintenance of on-site systems at its experimental site in Boyd County, Ky., where each on-site system is included in a public sanitary district that undertakes to inspect and maintain the new devices. Several state governments are revising their laws to encourage formation of such districts.

Real estate developers now have a number of new alternatives for disposal of wastewater. Where central sewage collection systems are too expensive for the developer to finance (in the absence of public funds), the on-site approach may prove to be a practical solution.