Wastewater Infrastructure Improvement Act of 2021 This bill sets forth requirements and establishes incentives to control water pollution. Specifically, the bill reauthorizes and revises several existing grant programs that address water pollution, including grants concerning implementing state water quality improvement programs, increasing the resilience of public wastewater utilities to man-made or natural disasters, recycling wastewater or stormwater, constructing sewer overflow and stormwater management projects, and supporting the clean water state revolving fund. In addition, the bill revises requirements concerning the National Pollutant Discharge Elimination System (NPDES) permit program of the Environmental Protection Agency (EPA). The bill allows the EPA to issue certain NPDES permits to states or municipalities for up to 10 years (currently, 5 years).