Houston is facing a federal mandate to upgrade its embattled sanitary sewer system, stirring concerns among advocates and civic leaders that the estimated $2 billion bill — and the higher rates required to pay it — could overburden low-income families.

The average city sewer bill already exceeds what the Environmental Protection Agency considers affordable for more than 113,500 Houston families, Houston Public Works and Census Bureau data show. That could rise to more than a quarter of all Houston households if sewer costs increase by 19 percent.

Such a hike is unlikely to happen overnight, but the average city water bill has risen 17 percent in the last six years via annual increases for inflation alone.