ARPA-H20 Act of 2020 This bill establishes the Advanced Research Projects Agency–Water within the Environmental Protection Agency to provide funding for projects concerning the treatment of drinking water or wastewater at water systems that serve the public. The water systems must be large decentralized systems that provide treatment for at least two households. The projects must be intended to enhance, through the deployment of advanced technologies, the treatment, monitoring, delivery, affordability, and safety of drinking water provided by a water system, wastewater or stormwater discharged by a water system, groundwater or well water used for drinking water, or activities carried out by a water system for the purposes of enhancing public health or environmental safety.