The Washington Suburban Sanitary Commission recently approved water and sewer rate increases of 23 percent and 9 percent respectively. The new rates, which took effect July 1, are expected to raise the bill of an average family of four in Montgomery and Prince George's counties by $32 annually.

The new rates were enacted to support the WSSC's $352 million budget for fiscal 1979, which represents an increase of 22 percent over the previous budget, according to a WSSC spokesman.

The accompanying chart shows the newly adopted water and sewer rates. The first column, "Gallons," represents average daily consumption by a customer during the billing period. The second and third columns represent the cost of water and of sewage per 1,000 gallons.

For example, if a customer uses 18,000 gallons of water over a 90-day period, the total bill will be $33.66.

This figure is arrived at as follows: 18,000 gallons over 90 days is an average daily consumption of 200 gallons (first column). The water rate (second column) for 200 gallons is 88 cents per 1,000 gallons. Eighty-eight cents multiplied by 18 (the number of 1,000 gallon-units consumed) equals $15.84.

The sewage rate (third column) for 200 gallons is 99 cents per 1,000 gallons. Ninety-nine cents multiplied by 18 equals $17.82. The combined rates, $15.84 for water and $17.82 for sewage, equal $33.66.