Purified terephthalic acid (PTA) and related aromatic compounds are produced from petroleum and widely used as raw materials for plastics, which are essential to modern society. Wastewater from PTA-manufacturing industry is treated using anaerobic biological wastewater treatment processes. Within the process, functionally and phylogenetically diverse microorganisms form a “sludge” that plays an important role in the bioconversion of PTA-related compounds. Over the past decade, microbiological studies employing multiphase approaches, including cultivation, microscopic observations, and ecogenomics, have been performed to uncover the metabolic functions and biological interactions of microbes involved in the degradation of PTA-related compounds. Cultivation and ecogenomic experiments of three purely isolated bacteria, Syntrophorhabdus aromaticivorans strain UI, Pelotomaculum isophthalicum strain JI, and P. terephthalicum strain JT, and metagenome-assembled genomes (MAGs) have revealed the degradation pathways for phthalate isomers. Along with these aromatic compound-degrading bacteria, it is becoming evident that functionally diverse microorganisms are involved in the treatment process, including bacteria that degrade detrital compounds and fermentation by-products, methanogenic archaea, and uncultivated microorganisms at the phylum level. Using microscopic observations, the “cross-domain symbiosis” between uncultivated “Candidatus Patescibacteria” and methanogenic archaea in a methanogenic bioreactor has recently been discovered. Understanding the biological functions of host methanogens, which are thought to anchor the mineralization of organic compounds in anaerobic ecosystems, and the “Ca. Patescibacteria” that interact with them may contribute to the stabilization of wastewater treatment processes. This chapter describes the current knowledge of microbial communities in anaerobic bioreactors treating PTA-related wastewater.