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I. Overview Anaerobic biological treatment is an important method among wastewater biological treatment technologies. To improve the efficiency of anaerobic biological treatment, in addition to providing microorganisms with a favorable growth environment, maintaining a high sludge concentration in the reactor and ensuring good mass transfer are also key factors. The first generation of anaerobic reactors, represented by the anaerobic contact process, have roughly the same sludge retention time (SRT) and hydraulic retention time (HRT), with a low sludge concentration within the reactor. To achieve good treatment results, wastewater usually needs to remain in the reactor for several days to several dozen days. The second generation of anaerobic reactors, represented by the UASB process, rely on the formation of granular sludge and the function of a three-phase separator to retain the sludge within the reactor, thereby achieving SRT > HRT and increasing the sludge concentration inside the reactor; however, the mass transfer process in such reactors is not ideal. To improve mass transfer efficiency, the most effective method is to increase the surface hydraulic load and surface gas generation load. However, the intense stirring caused by high loadings causes the sludge in the reactor to become fully expanded, shifting the condition from SRT > HRT to SRT = HRT, resulting in excessive loss of sludge and a deterioration in treatment efficiency. Over the past decade or so, many UASB reactor production facilities for treating industrial wastewater have been built. Experts have stated that in order to prevent excessive upward flow velocities from causing significant loss of suspended solids, when treating wastewater with medium to low concentrations (1.5–2.0 kg COD/(m3•d)), the inlet volumetric load rate for UASB reactors is generally limited to 5–8 kg COD/(m3•d). At this load rate, the minimum HRT is 4–5 hours ； When treating high-concentration organic wastewater with a COD concentration of 5–9 g/L, the influent volumetric load rate of the reactor is generally limited to 10–20 kg COD/(m3•d), in order to avoid increased turbulence caused by a too high gas generation load rate, which could lead to the loss of suspended solids. To overcome the limitations of these conditions, the Netherlands developed an internal circulation (IC) reactor; when treating wastewater with moderate to low concentrations, the influent volume load rate of this reactor can be increased to 20–24 kgCOD/(m3•d) ； When treating high-concentration organic wastewater, the influent volume load rate can be increased to 35–50 kg/(m3•d). Compared with UASB reactors, under the condition of achieving the same treatment efficiency, IC reactors have higher influent volumetric load and sludge load rates; the average upward flow velocity in IC reactors can be about 20 times that of UASB reactors used to treat similar wastewater. When treating low-concentration wastewater, the HRT can be reduced to 2.0–2.5 hours, allowing the reactor volume to be made smaller. Therefore, it has even more advantages. An international environmental protection company based in China has applied IC reactors in industries such as beer, fermentation, papermaking, food, beverages, and chemicals. And it achieved good results. II. Advantages The advantages of IC reactors are mainly reflected in the following aspects: (1) It has a very high volumetric load rate. Due to the internal circulation in IC reactors, the first reaction chamber features a high upward flow velocity, resulting in excellent mass transfer and high sludge activity; as a result, its organic volume load rate is much higher than that of conventional UASB reactors, typically exceeding it by more than 3 times. When treating high-concentration organic wastewater, such as that from potato processing, when the COD level is between 10,000 and 15,000 mg/L, the influent volume load rate can reach 30–40 kg COD/(m3•day). For treating low-concentration organic wastewater, such as beer wastewater, when the COD level is between 2000 and 3000 mg/L, the influent volume load rate can reach 20–50 kg COD/(m3•day), the HRT is only 2–3 hours, and the COD removal rate can be around 80%. (2) Save on infrastructure investment and floor space. Since the volumetric load rate of the IC reactor is **higher than that of the UASB reactor, the effective volume of the IC reactor is only 1/4 to 1/3 of that of the UASB reactor, which allows for a significant reduction in the capital investment required for the reactor. Since IC reactors are not only small in size but also have a high height-to-diameter ratio, they require very little floor space, making them highly suitable for factories and enterprises with limited space. Small IC reactors can be prefabricated in the factory, while large ones can be constructed on-site. The construction period is short, installation is simple, and the amount of earthwork required for IC reactors is minimal, which helps to save on construction costs. (3) Achieve internal circulation through biogas lift. There is no need for external power; the fluidization in anaerobic fluidized bed and expanded granular sludge beds is achieved through forced circulation via pump-driven return of effluent, thus some power must be consumed. The IC reactor utilizes the biogas generated by itself to perform work through adiabatic expansion, thereby enabling internal circulation of the mixture; no separate pump is needed for forced internal circulation, which helps to save energy consumption. (4) Strong resistance to shock loads: Due to the internal circulation achieved in IC reactors, when treating low-concentration water (such as beer wastewater), the circulation flow rate can reach 2–3 times the inlet flow rate ； When treating high-concentration water (such as potato processing wastewater), the recirculation flow rate can reach 10 to 20 times the inlet flow rate. Since the circulating flow mixes thoroughly with the influent water in the first reaction chamber, the harmful substances in the raw wastewater are adequately diluted, reducing their toxicity. This also prevents localized acidification, thereby enhancing the reactor’s ability to withstand shock loads. (5) It has the ability to buffer pH. The internal circulation flow rate is equivalent to the return flow rate of the effluent from the first stage of anaeration. The alkalinity generated by the conversion of COD can be used to buffer the pH, thereby maintaining a stable pH level within the reactor. When treating wastewater with low alkalinity, the amount of alkali added to the feedwater can be reduced. (6) The stability of the effluent is better than that in the first and second reaction chambers of the IC reactor; it is equivalent to two UASB reactors connected in series. The first reaction chamber has a very high organic volume load rate, serving a role in ‘rough’ treatment, while the second reaction chamber has a lower organic volume load rate, serving a role in ‘fine’ treatment. The entire IC reactor is actually a two-stage anaerobic treatment process. Under normal circumstances, two-stage anaerobic treatment offers better stability than single-stage anaerobic treatment, and the effluent is also more stable. III. Disadvantages: Although ICs enable a significant increase in the COD volumetric load, and their high treatment capacity yields some excellent results. However, this has also brought about many new problems. Some scholars believe that the main problems of IC lie in the following areas. (1) Structurally, the internal design of an IC reactor is more complex than that of a conventional anaerobic reactor, requiring higher standards for design and construction. The high height-to-diameter ratio of the reactor increases the power consumption of the feed water pump, thereby raising operating costs ； On the other hand, it accelerates the upward flow of water, resulting in more fine particulates in the effluent than in UASB, thereby increasing the burden on subsequent treatment processes. Furthermore, the upward movement of the slurry in the internal circulation can easily lead to blockages, causing the internal circulation to fail and reducing the treatment efficiency. (2) Fermenting bacteria use extracellular enzymes to hydrolyze insoluble organic substances into soluble ones, and then convert these soluble macromolecular organic substances into fatty acids and alcohols, etc.; the hydrolysis process carried out by such bacteria is quite slow. The shorter hydraulic retention time in IC reactors inevitably affects the removal efficiency of insoluble organic matter. (3) In anaerobic reactions, there is a close relationship and balance among the organic load, gas production volume, and degree of treatment. Generally, a higher organic load results in a greater gas production volume, but the degree of treatment decreases. Therefore, the overall removal efficiency of IC reactors is lower compared to that of UASB reactors. (4) There is a lack of key technologies for cultivating granular sludge with good activity and sedimentation properties under the hydraulic conditions of IC reactors. Currently, all IC reactors introduced in China use granular sludge inoculated from the Netherlands, which increases the project cost. IV. Market Prospects Although the use of IC reactors in China is not yet fully developed, the market development phase has essentially been completed. Since last year, some customers have begun to express a demand for IC reactors. Although, in terms of current market share, IC reactors are clearly behind UASB in terms of application, the former is expected to surpass the latter in the future