The Washington Suburban Sanitary Commission is considering a new "conservation-oriented" water consumption and sewer usage rate that would slightly increase water and sewer rates of average residential users - 9 cents a month for water and 2 cents for sewers - while imposing heavier charges on nonresidential users.

The WSSC voted unanimously yestersay to send the new rate structure to informal public hearing next month in both Montgomery and Prince George's counties and to adopt the new rate structure by Jan. 1.

For the nonresidential user under the new rate structure, the average cost will jump from 70 cents to 98 cents in monthly water costs and from 76 cents to $1.01 in their sewage bills. The average residential home is considered one with four people.

WSSC officials say the increase in water and sewage rates for both the residential and nonresidential users should be offset by the proposed elimination of the summer water charge and a service charge based on meter size.

"This is not a hidden rate increase," said Robert Reynolds, who helped develop the new rate structure.

John M. Brusnighn, assistant general manager for WSSC, said the new rate structure is designed with a number of different levels that will mean higher or lower bills depending upon the average daily consumption of the consumer.

Touted as "the first of its kind ever advanced by an American water utility," the new rate reflects actual consumption rather than flat rates, said WSSC spokesman Arthur P. Brigham.

Several WSSC officials said the average family of four will save on water bills if they conserve water according to the guidelines set out by WSSC during the recent water crisis in both Montgomery and Prince George's counties.

"If they stick to 40 gallons per person daily they will save," said Brusnighn.

The commission, which currently has a flat rate that charges 70 cents per 1,000 gallons of water used an 98 cents for sewer usage, has decided to change its rate structure because "there has been a change in philosophy," WSSC officials say.

"There was a time when utilities tried to encourage people to use natural resources by charging them less for larger consumption," said Brusnighn.

WSSC officials now say that the consumer must pay for excessive use of resources.

One of the new rate structure's designers said the proposal does not encourage conservation for apartment dwellers. He said that will have to be worked out during the public hearings.

Another weakness in the plan, according to WSSC officials, is that it does not "fairly" deal with other nonresidential water and sewer users because of the difficulty large users have in controlling the use of water.

Brusnighn said there are two proposals for the nonresidential users that will be debated during the public hearings.