Flow drag force contributes high bio-treatment efficiency in a circulating fluidized bed reactor: Mechanism of selective separation of functional decayed sludge

[Display omitted] •Functional separation of activity sludge could be achieved by drag force.•All sludge were hypothesized to be composed by three component sludge.•Sludge with different size showed distinction of function and decay.•Computational fluid dynamics was used to simulate the distribution of sludge.•The bio-treatment efficiency could be improved by selectively discharge of sludge. The diversity and expression of microbial function of activated sludge are influenced by combined effect of environmental factors and selective separation. The selective separation of sludge greatly depends on the design of reactor. The discharge of decayed microorganisms and the retain of active microorganisms cannot be realized through sedimentation and reflux of the activated sludge in the conventional secondary sedimentation tank. Thus, this study proposed a strategy for in-situ selective separation of activated sludge by means of the circulating flow field. The fluid field in the fluidized bed reactor was found to influence the distribution of activated sludge with different physical properties, biological properties, and functions. The sludge with various particle sizes showed different bio-characteristics. Then, based on the cumulative distribution of mean floc size, three kinds of component sludge with different floc sizes (sludge-a: 10.56–81.42 μm, 28.3 %; sludge-b: 81.42–104.63 μm, 26.1 %; and sludge-c: 104.63–257.94 μm, 45.6 %) were hypothesized to constitute all the sludge in the fluidized bed reactor for simplification of heterogeneous sludge. Finally, computational fluid dynamical and simulation calculations were used to reveal the impact of selective sludge discharge on bio-treatment efficiency. The results indicated that the bio-treatment efficiency could be improved by more than 5.0 % based on the selective discharge of decayed sludge with the same input of energy. This work demonstrated a principle that a fluid field could be used to identify functionally decayed sludge in a fluidized bed reactor for wastewater treatment. The principle could play an important role in identification and separation of microbial function decayed sludge for improving the bio-reaction efficiency, energy utilization rate and stability of reactor.