Are the large central sewerage systems so popular today compatible with tomorrow's potential water shortages? The answer, at least theoretically, is no.

Why? Because the muddy stream of raw sewage that flows into a municipal treatment facility is roughly 99.5 per cent water - a huge percentage, but certainly no accident. Water, the road that carries the waste, must be present in vast quantities to keep the system operating properly.

Already ecologists are calling for serious, permanent water conservation techniques that could affect, among other things, the volume of water that runs through sewer pipes. In particular, the flushs toilet is viewed as an expendable villain - a beast that pollute about 13,000 gallons of fresh water per person annually to carry off 165 pounds of water. Other measures, if adopted, could also cut water consumption susbtantially.

The effect on sewage transport would be dramatic. Normally, sewage flows through the collector pipes at a fairly rapid clip, no less than two feet per second. When enough water is removed, the process becomes sluggish, eventually slowing to the point that solids begin to settle out in the pipes. The available oxygen is used up. Foul-smelling gases are produced, and sometimes acids. Depending on what the system is made of, the pipes can rot. Ultimately the transportation system can become bogged down entirely.

It is likely to happen? Sewer administrators tend to be skepitcal. So far, excess water (or overload) has been their problem rather than the opposite. Where water conservation measures have been employed, they have created welcome drops in sewage volume. Watching the slow, painful process which accompanies most water savings programs, administrators find it difficult to imagine the situation reversed.

"People haven't begun to accept water conservation yet as a given," says Paul White, head of the water supply program for the Metropolitan Washington Council of Governments. "Nothing is going to happen all of the sudden."

But with water rationing in California, and frighteningly low flows in the nation's largest rivers, such an attitude may become suddenly outdated. Innovative water conservation techniques are being tried in isolated areas around the country. Each could be adopted for use nationwide - and very well might be as the water situation begins to be regarded as critical.For example:

Water conservation is being written into some plumbing codes. Since 1973, the Washington Suburban Sanitary Commission has required watersaving toilets (3.5 gallons instead of the standard 5), low flow faucets, and reduced-flow shower heads in new dwellings. When the Washington Metropolitan COG adopts its own new plumbing code, also incorporating the water-saving features, conservation will be even more widespread.

Faced with overloaded sewage plants, some communities are encouraging commercial and industrial users to recycle. In Western Maryland, the city of Hagerstown recently rescinded a favorable sewage treatment rate it was extending to its largest single customer, the Mack Truck company. Hit with a staggering increase, Mack began to look at recycling. Within a year, the city expects the process to be in operation, cutting Mack's daily waste water load from 1 million gallons to 250,000.

Elsewhere, commercial recycling has been legislated. In Fairfax County, recycling is mandatory in carwashes and other operations with continuously-running equipment using more than five gallons of water per minute.

Gray water, that not be used in toilets, has been recycled successfully in individual dwellings. Under an Appalachian Regional Commission project, a number of homes in Boyd County, Kentucky, have been outfitted with both on-lot waste disposal plants and recycling equipment. The gray water from the houses is filtered, disinfected, and pumped back inside for sanitary purposes.

"A clear odorless water of excellent quality is produced," says ARC spokesman Larry Waldorf, who is in charge of the project. "Equally important has been the high degree of customer satisfaction with the day to day use of recycled water."

Under various EPA grants around the country, cities with older sewer systems are upgrading their collector pipes to cut down on infiltration of water from the streets. Presently, infiltration can contribute significantly to the water coming into a treatment facility, especially where the pipes are made of short concrete sections that leak avery four feet at the joints. Plastic liners and other devices used to upgrade the pipes make infiltration virtually impossible.

Indications, too, are that a changing public climate may make for extensive and rapid changes. One technological breakthrough today could have an impact unheard of five years ago - an inexpensive process, say, to recycle shower water for use in the toilet - because of rising water rates and expanded media interest in innovations. Commercial manufacturers, sensitive to the change, are quickly getting into the act. A water-saving washing machine is now advertised regularly on nationwide television. Other companies are promoting low-water dishwashers with natural drying cycles, conservative shower heads, and the like.

One problem with the sudden interest in water savings is, as Ted Graham of the Montgomery County Water Supply Task Force says, "that we don't have any good handle on what each of these measures is going to add up to in terms of gallons."

Ellyn and Terry Bache are in development in Hagerstown, Md.