Modulation of sewage sludge microbiome for enhancing the transition from wastewater treatment plants into biorefineries in the circular economy era

Abstract

The sewage sludge (SS) microbiota operating in wastewater treatment plants (WWTPs) is determinant in pollutant degradation to obtain clean water. According to the WIDER-UPTAKE project [1], based on the circular economy principle of WWTP conversion into biorefineries, SS microorganisms can produce valuable materials, like polyhydroxyalkanoates (PHAs), using pollutants as nutrients [2]. Indeed, PHAs, produced as microbial intracellular storage compounds, are promising candidates as alternatives to petroleum-based plastics. PHAs can be efficiently produced by SS microorganisms through a process that involves the supplementation of volatile fatty acids (VFA) in specific PHA-producing reactors. Furthermore, VFAs can also be produced in specific WWTP fermenters by SS microorganisms through acidogenic fermentations. Therefore, studies on the relationships between operating conditions and SS microbiome can maximize VFA and, consequently, PHA production in a biorefinery system. In this study, different conditions were assayed at laboratory scale in batch-cultivation reactors and at pilot scale to improve VFA production by acidogenic fermentations, revealing microbiota structure changes by metataxonomics. In particular, in batch-cultivation reactors, initial pH 10 determined methanogenic bacteria decrease and VFA producer increase [3], while the addition of KMnO4 caused microbial diversity decrease and the acidogenic fermenting strain survival, thus increasing VFA yields. In pilot plant scale, the fermenter headspace reduction promoted VFA production, and comparative studies on SS microbiota structure are currently in progress. Thus, metataxonomics has been proven as a useful approach to highlight relationships between operating conditions, microbiomes, and the achievement of specific outputs.