The Washington Suburban Sanitary Commission, the agency that provides water and sewer service to Montgomery and Prince George's counties, has approved a 1983 budget that calls for an average rate increase of 15 percent.

The $439.8 million budget request, $19 million higher than the current budget, must be acted upon by both the Montgomery and Prince George's county councils.

WSSC budget director Albert Joy said most of the increase will be used for increased debt payments for projects well under way. Joy said the agency's costs also have been affected by high interest rates.

According to WSSC estimates, the average household bill would be increased by $33.77 a year, bringing the total to about $264.63. The annual bill of the most conservative user would be increased by $9.86, to $74.47.

Joy said the county councils increased last year's budget by about $3.2 million over what the WSSC had proposed, but that increase resulted from an accounting change. The approved water and sewer rates actually were lower than those the WSSC had requested, Joy said.

The county executives from both Montgomery and Prince George's have until March 15 to comment on the budget, then both county councils have an opportunity to reiview it. Both counties must agree on any changes by June 1.