After two months of struggling with the Washington Suburban Sanitary Commission, Edna Lauer of Adelphi has manged to win a $400 reduction in her six-month water bill. But she's still not satisfied.

"If I have any more trouble with bigh bills, there's going to be a mess because I'm not paying it anymore," she said yesterday.

In October, Lauer received a sewer and water bill for $556 covering the period from March through June-even though her highest previous six-month bill at her home in Adelphi had been slightly over $40.

Lauer had her house checked for water leaks-there were none-and asked the WSSC to replace her meter. But even though her bill charged her for the use of 2,130 gallons of water a day, WSSC officials insisted their calculations were correct.

Last week, however, after Lauer's case had been publicized, she reached a compromise of sorts with the WSSC.

After installing a new meter at her home, WSSC officials agreed to lower the charge for the last three months of the billing period to $36,13, a reduction of more than $250.

In addition, the charge for the first three-month period between March and June was set at $128.25-below the Cotober billing but still above the $42.60 estimate covering that period she received-and paid-in June.

Lauer paid both bills, totaling $164.38, with the help of her five children. A widow, Lauer says she lives on her Social Security payments.

"I still say I could have never used as much water as I paid for," she said. "But at least I didn't have to pay $500."