The Washington Suburban Sanitary Commission adopted yesterday a new "conservation-oriented" water and sewer rate for both homes and businesses that will lower costs for those who conserve and increase them for those who use more.

The new rate structure will take effect Jan. 1 in Montgomery and Prince George's counties.

The change is a reversal of the standard practice of rewarding larger users with discounts - a practice that still prevails both for water and power consumers in most parts of the country.

Critics, particularly members of the Prince George's County Council, have argued that the new system will hurt large families and employers using alot of water.

Advocates, including WSSC general manager Robert McGarry, have said that the new structure will simply encourage conservation at a crucial period of water shortage in the region.

For the typical consumer, the average cost - according to WSSC figures - will increase by about 5 per cent. For the conservationist who really cuts back, savings could range as high as 40 per cent, while the excessive water users could pay up to 26 per cent more.

The new rate structure charges - depending upon the amount of consumption - between 36 cents and up to $1.05 per 1,000 gallons of water used. For sewer usage, the costs range between 45 cents up to $1.31 per 1,000 used.

According to a survey by the WSSC staff, large water consuming companies such as J & M Dry Cleaning, Marlow Heights Shopping Center and Pepsi Cola Bottling Company, will pay increases ranging between $300 and $3,000 when the new rate goes into efect.

"In some respect these large water-dependent companies are responsible for the big facilities we have to build to cope with peak period water consumption . . . why shouldn't they pay more?" said Vera Berkman, WSSC chairwoman.

Several spokesmen for smaller, water-dependent companies such as car washes, restaurants and dry cleaners expressed concern about the rate change, but added that the increases in their water and sewer bills will be passed on to their customers.

"We will probably put in a water reclaim (recycling) system like the one on our Fairfax County car wash," said a spokesman for Beltsville Car Wash, who added that up to this point it was not "necessary" to install that type of water conserving system.

For cost conscious water users WSSC has recommended a number of steps that can help reduce the average water bill.

WSSC suggests that the consumer "flush the toilet sparingly," water outdoor plants by hand and and reduce the number of baths and showers taken.

WSCC also reminds consumers to check for water leaks and to turn off the water tap when brushing teeth.

The decision comes in the context of recurring water shortages throughout the metropolitan area. The WSSC last summer implemented special conservation measures following the breakdown of one of its main pumping stations.

The bicounty agency also made an experimental and successful effort at intensive conservation called "camel week" to prepare customers for future shortages.

Many Northern Virginians were also placed on a conservation footing during the fall following a long, dry spell that drastically reduced the level of the Occoquan reservoir.