Culturing the uncultured microbial majority in activated sludge: A critical review

Activated sludge is a widely applied wastewater treatment process that mainly uses suspended microbial flocs to remove pollutants in wastewater. With the characteristics of high biomass content and high microbial diversity, activated sludge plays an important role in pollutant removal and contains various functional microorganisms as a valuable pool of various useful microbial resources. However, the majority of microorganisms in activated sludge have not been isolated, which substantially limits the improvement of treatment efficiency and the innovation of process technology in wastewater engineering. As the basic biological methodology which can extremely expand the downstream studies for microorganisms, the cultivation of new species in activated sludge is urgently needed to fill the gaps between the cultured and uncultured microbial communities. The growing emphasis on cultivation in recent years has spawned the creation of many innovative and high-throughput cultivation techniques. In this review, we summarized the microorganism “wanted list” in activated sludge, reviewed the potential cultivation methods that could extend our understanding of activated sludge microbiota, and discussed the significance and perspectives for activated sludge microbiota cultivation.