Sewage Treatment Overflow Prevention through Community Sanitation Outreach Act of 2021 or the STOP CSO Act of 2021 This bill revises the National Pollutant Discharge Elimination System permit program to require certain publicly owned water treatment facilities to monitor, report on, and notify the public of sewer overflows. For example, the facilities must notify the public within a specified number of hours after (1) sewer overflows that have the potential to affect human health, and (2) sewer overflows that may imminently and substantially endanger human health.