Viscosity is a nonnegligible factor in the waste activated sludge fermentation: Taking hyaluronan as an example

The waste activated sludge (WAS) exhibits typical viscoelasticity due to the presence of viscous and gelling organics in extracellular polymeric substances (EPS). However, the positive role of reducing viscosity in WAS fermentation by degrading viscous polysaccharides has been historically overlooked. This work demonstrates the occurrence of viscous hyaluronan-like polysaccharides in the WAS for the first time. Approximately 6.8 % of bacteria, such as Zoogloea (1.0 %), were identified as the potential producers. The viscosity of hyaluronan could be significantly reduced by 99 % within 1 hour by the oriented hyaluronan-degrading consortium (HDC), and a reduction of 20 % was also observed for WAS after 24 h. This resulted in a 18 % improvement in methane production and a 35 % improvement in the maximum production rate in WAS fermentation. The conversion of viscous hyaluronan was mainly through the hyaluronan lyase (EC 4.2.2.1) dependent pathway. An unfamiliar genus of Paludibacter (9.6 %) was identified as a key bacterium, responsible for excreting five extracellular enzymes of EC 4.2.2.1, EC 3.2.1.35, EC 3.2.1.31, EC 3.2.1.52, and EC 3.2.1.180. Consequently, this study has elucidated reducing viscosity as a substantial factor in WAS fermentation by the oriented HDC, thus providing a novel paradigm to enhance methane production.