Water rates would go up 19 percent and sewer rates would rise 9 percent for Montgomery and Prince George's residents this July, under the proposed fiscal 1982 budget of the Washington Suburban Sanity Commission (WSSC).

The increases would boost the average households's water-sewer bill by about $29, from $183 to $212 a year. Higher rates are necessary, a WSSC spokesman said, because of inflation, sludge-disposal costs and new water and sewer plants.

A public hearing on the proposed rate increases is set for 8 p.m. Feb. 11 at the agency's headquarters in Hyattsville. Both county councils will hold additional hearings before final rates are approved late this spring. w

The bi-county agency raised rates by an overall average of 22 percent last July, with the major portion of the increase coming from a sewer rate hike of 39 percent. Water rates went up 4 percent. WSSC customers were granted a 10 percent reduction in water rates in 1979.

The proposed increases in suburban Maryland, and in several Northern Virginia jurisdictions, are not related to the drought that is draining reservoirs and rivers along the East Coast this winter. Inflation and the costs of making federally required improvements in sewage-treatment plants are the main culprits, say area officials.

The WSSC's proposed budget calls for $230 million for operating expenses, a 22 percent or $32.5 million increase over the current, 1981 fiscal-year budget. A 3 percent, or $12.4 million increase in capital-construction costs is projected.

By far the biggests increase in operating expenses is the $12.8 million jump in debt service, the cost of repaying bonds sold to finance construction of sewer and water extensions and improvements. High interest rates on borrowed money are a major factor in this increase, according to WSSC budget documents. The other large budget increase, $6.9 million, is in the estimated cost of disposing of suburban Maryland's 50 percent share of sewage sludge from the Blue Plains Sewage Treatment Plant. The sludge will be trucked to composting sites at Dickerson and Western Branch.

Water and sewer rates vary widely in the Washington area, with District residents paying the lowest charges for both water and sewer service ($122 a year), followed next by Arlington residents, who pay an average of $163 a year. Both get their water from the Potomac, the area's cheapest and most dependable source of water.

Suburban Maryland and Fairfax County residents now pay the same average sewer rates, $112.80 a year, and virtually the same water rate, $69 a year in Fairfax and $68 a year in Montgomery and Prince George's counties. Fairfax is currently also considering a sewer rate increase.

The primary reason for the high prices in Northern Virginia is the hodge-podge of water and sewer services.

Alexandria and Dale City, for example, are served by the privately owned Virginia-American Water Co. who, along with residents of an Occoquan-Woodbridge sanitary district, buy their water from the Fairfax County Water Authority. Their high bills are primarily the result of a complicated, two-price system in which the amount of water bought above 1974 levels is three times as expensive.