Insights into microbial diversity in wastewater treatment systems: How far have we come?

Biological wastewater treatment processes are based on the exploitation of the concerted activity of microorganisms. Knowledge on the microbial community structure and the links to the changing environmental conditions is therefore crucial for the development and optimization of biological systems by engineers. The advent of molecular techniques occurred in the last decades quickly showed the inadequacy of culture-dependent methodologies to unveil the great level of diversity present in sludge samples. Initially, culture-independent technologies and more recently the application of -omics in wastewater microbiology, have drawn a new view of microbial diversity and function of wastewater treatment systems. This article reviews the current knowledge on the topic placing emphasis on crucial microbial processes carried out in biological wastewater treatment systems driven by specific groups of microbes, such as nitrogen and phosphorus removal bacteria, filamentous and electrogenic microorganisms, as well as Archaea. Despite the recent -omics has offered substantial insights into the diversity and ecophysiology of these bacteria never envisioned before by providing millions of sequence reads at an unprecedented scale, studies based on high-throughput sequencing are still scarce. In order to obtain significant gains in the analysis of structure-function relationships, a greater sequencing investment is needed, particularly to uncover gene expression patterns of functionally relevant genes.