Assistance, Quality, and Affordability Act of 2021 This bill provides support for drinking water infrastructure and requires the Environmental Protection Agency (EPA) to address the safety and affordability of drinking water. Specifically, the bill reauthorizes and establishes several programs for drinking water infrastructure. For example, it reauthorizes through FY2031 the drinking water state revolving fund. In addition, the bill revises requirements concerning the safety of drinking water, including by requiring the EPA to promulgate national primary drinking water regulations for perfluoroalkyl and polyfluoroalkyl substances (commonly known as PFAS), microcystin toxin, and 1,4–dioxane. PFAS are man-made and may have adverse human health effects. A variety of products contain the compounds, such as nonstick cookware or weatherproof clothing. Microcystin is a toxin that is produced by certain algae. The solvent known as 1,4–dioxane may also have adverse human health effects. It is used in variety of products, such as adhesives, sealants, and printing inks. The EPA must also implement a residential emergency relief program that assists customers of public water systems with paying drinking water bill. Under the program, the EPA must provide payments to public water systems to reimburse them for forgiving their customers' debts or fees for services provided since March 1, 2020, in order to help the customers retain, reconnect, or restore water services. Public water systems that receive payments may not disconnect or interrupt their customer's service because of such debt or fees. Finally, the bill directs the Government Accountability Office to report on issues relating to the potential for consolidation of certain distressed and small public water systems.