Wastewater treatment applies processes that exploit naturally occurring microorganisms to remove nutrients, pathogens and chemical contaminants. While urban regions generally rely on centralized wastewater treatment facilities, in rural areas there is rising interest in decentralized systems that do not require complex and expensive infrastructure. We are investigating microbial processes and dynamics in various decentralized wastewater treatment modules in order to optimize their performance, and reduce release of contaminants to the environment.