After years of promoting expensive, elaborate plants for treating sewage before discharging it into streams and rivers, the federal Environmental Protection Agency, in a major policy shift, plans to "press vigorously" for recycling sewerage on land.

Under the new policy, partially treated sewage would be absorbed slowly and enrich the soil. The change was detailed last week in a memo from EPA administrator Douglas M. Costle to officials at the agency's regional offices througout the country.

One immediate effect, according to an official at the regional office covering metropolitan Washington, is tha "it will stiffen our backs" in resisting Montgomery Country's efforts to build a 60-million-gallon regional sewage treatment plant at Dickerson that could cost over $400 million. THe plant would discharge treated wastes above water intake pupes on the Potomac River.

Because EPA dispenses the federal grants that pay for 75 per cent of the cost of constructing water clean-up projects, the new policy could have a major impact on the technology and politics of sewage treatment.

Instead of concentrating on choosing among types of expensive sewage-plant hardware, as many local jurisdictions id in the past, communities seeking federal money to help build sewage tretment facilities will have to show they gave thorough consideration to land treatment.

Such treatment involve extensive spraying of pretreated sewage onto agricultural land or the other open space. THe sewage is absorbed and purified by the soil instead of being discharged into rivers and streams, officials say. The spraying operation is similar to irrigation projects on farmland in the Western states in the sense that movable sprayers are used.

If land treatment becomes widespread, it could have a major impact on land use. A large sewage treatment operation would require thousands of acres that would be used over and over again. The sprayed fields could be farmed but not developed for housing or other similar uses.

Besides getting a big, new push from EPA, land treatment also is gaining more suppor in Congress, Conferees from the House and Senate public works committees are considering legislation that would increase the federal contribution from 75 to 87 1/2 per cent, to promote use of the widely known but not extensively used process.

In his memo, EPA administrator Costle said:

" . . . The utilization If land-treatment systems has the potential for saving billions of dollars. This will benefit not only the nationwide water pollution control program, but will also provide and additional mechanism for the recovery and recycling of waster water as a resource.

This is because substances that would be pollutants in the water become nutrients when sprayed on the soil.

Applicants for cleanup funds, to build treatment plants, he said, in an underlines pasage "should be required to provide complete justification for the rejection of land treatment."

In asking for federal funds to building a big regional plant at dickerson, Montgomery County officials said they had considered land treatment, but rejected the alternative because it was too costly and require to much land.

However, EPA, in rejecting the Montgomery application last year, said the county had not given land treatment enough attention and that, furthermore, the proposed water plant was itself too expensive as well as oversized.

One of the few major municipal land-treatment operations started in recent years is in Muskegon, Mich. For some time after it opened, in 1973, the facility limped along and was widely criticized for not living up to expectations.

But all that has changed, says its director, Dr. Ara demirjian, after irrigation equipment became fully operational, objectionable odors were eliminated through equipment changes, and corn yields on land sprayed with the treated sewage were increased from 30 to 81 bushels per acre.

Demirjian said Muskegon's land treatment, which covers 5,200 acres, costs 17 cents per thousand gallons, versus 50 to 60 per cents per thousand gallons in an advanced treatment plant.

Last year's corn crop, he said, brought in $1 million, and revenues from this year's should be higher. There have been no recent increases in sewage service rates, he said, even though the costs of energy - used in the pumping and spraying operation - has doubled.

In addition to influencing negotiations between EPA and MOntgomery County on the Dickerson proposal, the new policy thrust also could have an impact on rapidly growing Poolesville in northwestern Montgomery.

The town commissioned Kamber Engineering Inc. of Rockville to assess land treatment, but after the consulting firms's negative analysis was criticized, local officials hired another firm from Chicago to take another look.

We've got one helluva fight going on," Sam E Zattiero, president of the town council, said. Some landholders, he said, oppose land treatment because "let's face it the process takes a lot more acreage out of development than a sewage - treatment plant."