The number of functional traits of a wastewater treatment plant (WWTP) microbial community (i.e. functional richness) is thought to be an important determinant of its overall functional performance, but the ecological factors that determine functional richness remain unclear. The number of taxa within a community (i.e. taxonomic richness) is one ecological factor that might be important. Communities that contain more taxa are more likely to have more functional traits, and a positive association is therefore expected between functional and taxonomic richness. Empirical tests for this positive association among WWTP communities, however, are lacking. We address this knowledge gap by measuring the functional and taxonomic richness of 10 independent WWTP communities. We demonstrate that functional and taxonomic richness are positively associated with each other. We further demonstrate that functional and taxonomic richness are negatively associated with the effluent NH4-N and BOD5 concentrations. This led us to hypothesize that correlated variation in functional and taxonomic richness is likely related to variation in ambient nitrogen and carbon availability. We finally demonstrate that this hypothesis is consistent with the functional and taxonomic attributes of the WWTP communities. Together, our results improve our basic understanding of the ecology and functioning of WWTP communities.