The Clean Water Affordability Act amends the Federal Water Pollution Control Act to modify provisions related to additional subsidization within the Water Pollution Control Revolving Loan Funds (CWSRF). This legislation aims to enhance financial assistance for clean water infrastructure projects and ensure affordability for ratepayers. Specifically, the bill revises the amount states can use for additional subsidization, allowing them to allocate the greater of 50 percent of their annual capitalization grants or the 10-year average of excess state deposits. It also mandates that states use a minimum of 20 percent of their capitalization grants for additional subsidization, provided there are sufficient eligible applications. Furthermore, the act clarifies that assistance is intended to help ratepayers maintain access to wastewater and stormwater treatment services, and specifies that loans with an interest rate of 0 percent or more are not considered additional subsidization.