Joseph C. McGrath, the Republican candidate for Montgomery County executive, yesterday outlined his plan for dealing with the county's garbage, proposing a system in use in 20 countries from Vietnam to Brazil in which garbage is mixed with wet sewage sludge.

The procedure, composting, is being demonstrated by the District of Columbia government at Blue Plains. If adopted in the county, McGrath said, composting would be substantially cheaper than a proposed incinerator in Gaithersburg and could extend the life of the county landfill at Laytonsville.

McGrath, opposing incumbent Executive Charles W. Gilchrist in next Tuesday's election, said up to 60 percent of the county's garbage could be composted.

He said the composted material could be shipped by rail to West Virginia, for use in filling strip mines. The process, he said, could reduce the county's garbage dumping fee charged to taxpayers and perhaps even eliminate the need for another landfill.

Composting accelerates the decomposition of waste. Garbage is first passed through a magnetic separator to extract ferrous metals, which are sold. The remainder is mixed with wet sludge and dumped into a rotating cylinder. The material is then stored to ferment and stabilize, and eventually is bagged for sale.

County officials said composting is an option, but they raised several questions about its cost and the county's capacity to handle such a procedure.

Jerome Leszkiewicz, director of the office of environmental construction, said it takes "anywhere from two weeks to a month" for the pile of garbage to decompose sufficiently. Alexander J. Greene, special assistant to Gilchrist, added that the county would need 14 composting drums to handle its 1,400-tons-per-day garbage load. He said each 100-ton drum costs up to $3 million.

Greene said the process has not been used in this country because Americans have more plastics in their garbage, which are harder to grind up. Also, Greene said the county would need to haul in 1,200 tons of wet sludge each day to mix with the garbage, which would add costs.

McGrath and the county officials disagreed over whether the process generates offensive odors. "You'll need 50 acres of land, heaping with that composted garbage," Leszkiewicz said. "It would smell worse than any garbage dump in the world."

"That's not true," McGrath said. "There's no smell to it. He knows dammed well there isn't." He also said plastics can easily be ground up and his plan would be cheaper than the proposed incinerator.