What is it about?

Anaerobic biochemical reactors are biochemical reactors that use anaerobic biological bacteria to degrade organic pollutants in wastewater. It is a sustainable wastewater treatment technology with high treatment efficiency and low energy consumption. The paper analyses the sludge-water mixing process in two types of anaerobic reactors, i.e., the Upflow Anaerobic Sludge Bed (UASB) reactor and the Innternal Reflux Packed- Bed Anaerobic Reacto r(IRPAR). In these two types of anaerobic reactors, t he solid-phase anaerobic sludge does not move with the water flow and is confined or fixed in a localised area. In UASB reactors the anaerobic bacteria or anaerobic sludge involved in the decomposition reaction of pollutants in the water are confined to the sludge bed and do not move with the upward flow , whereas the anaerobic sludge of the IRPAR is attached to the surface of the fixed medium. These two types of anaerobic reactors mix sludge and water at low mixing intensities in water circulation or seepage-diffusion mixing modes, respectively, to achieve a completely mixed sludge-water reaction. Under low mixing intensity conditions, these two sludge-water mixing modes can effectively maintain the stability of the biota composition in the anaerobic sludge and control the morphology of the anaerobic sludge membranes or particles and their internal non-restricted mass transfer distances from 2.0 to 4.0 mm. In a similar anaerobic sludge morphology and its range of non-restricted internal solid-phase mass transfer scales, the working conditions required for efficient reaction were obtained by controlling the complete sludge-water mixing state of the anaerobic reactor. The mathematical models of sludge-water mixing were established according to the sludge-water mixing patterns of these two types of anaerobic reactors, and the working conditions required to achieve complete sludge-water mixing were also given in the study.

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Why is it important?

Whether aerobic or anaerobic biochemical reactor treatment to remove pollutants in wastewater, only in the reactor microbial community composition is stable, biological sludge and organic nutrients are uniformly distributed in the sludge - water mixing conditions, can be stable and efficient operation. The simple high-intensity mixing of sludge-water to achieve complete mixing conditions, high water shear stress may affect the stability of the microbial community. In addition, the biochemical degradation rate of organic pollutants in wastewater is low, the reaction time is long, and the reactor volume is large. Direct mixing of mixed sludge-water may require high energy consumption. Thus, in engineering, the biochemical reactor should be designed in accordance with the biological sludge stability conditions in the biochemical reaction and the biochemical degradation reaction time scale of organic matter, in order to construct a suitable sludge-water mixing structure with a sufficiently small mixing time scale, so that the biochemical reaction can be carried out in the completely mixed state of sludge-water. The sludgr-water mixing models of UASB reactor and IRPAR given in this thesis study can be used for the design and computational analysis of sludge-water mixing structures in similar anaerobic reactors, which is of great significance for improving the operational stability and efficiency of anaerobic reactors. The design and computational analysis of mixing models and mixing structures for aerobic biochemical reactors are fully described in study "Dynamic mixing models and analysis of the mixing processes for an Oxidation ditch and an aeration tank", (Yang, 2022).


The biochemical degradation of organic pollutants in wastewater is complex and has targeted requirements for mixing processes and conditions. Since anaerobic reactors began to be designed for the biochemical degradation of organic pollutants in wastewater treatment , sludge-water mixing in the reactor has been a key research issue in order to improve the efficiency of the biochemical reaction. In addition to the time scale of biochemical reaction completion and the mixing time scale of the biochemical reactor directly affects the mixing state of the biochemical reactor, unlike the aerobic biochemical reactor, anaerobic biochemical reactor also needs to take into account the morphology of the anaerobic sludge, the size of the sludge and its internal solid phase mass transfer distance. Thus, to improve the efficiency of anaerobic biochemical reactors, the focus is on the fixation or attachment of the anaerobic sludge, as well as the optimisation of the material and geometry and morphology of the attached packing material. Simple and reliable sludge-water mixing structures should be investigated to maintain the integrity and stability of the microbial community in the anaerobic sludge while mixing sludge-water.

Min Yang

Read the Original

This page is a summary of: A Sludge-Water Mixing Model of Anaerobic Reactors with Confined Sludge, Journal of Environmental Engineering, November 2022, American Society of Civil Engineers (ASCE), DOI: 10.1061/(asce)ee.1943-7870.0002049.
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