In general terms, AGS systems are built on a cylindrical column with a variable height/diameter (H/D) ratio and are operated in sequential batch cycles. The cycles in batch mode firstly consist in an aeration stage, which is used for the degradation of organic matter, nutrients and pollutants. Then, the aeration is stopped, and the granular biomass is allowed to settle at the bottom of the reactor. This parameter is essential because biomass selection pressure depends on settle time. The wash-out of biomass is subsequently linked with the discarded effluent. Finally, the reactor is re-filled with raw water. However, this operational mode presents certain disadvantages, such as the relatively low volume of treated water compared with other technologies that operate continuously. Therefore, from a technical point of view, the development of AGS working in continuous flow may represent an advantage over sequential systems.
In recent years, different designs of potentially useful granular CFR systems have been tested, in each case in an attempt to attain an optimal system that would enable a reactor to keep the granules in a steady-state, preventing the proliferation of slow-growing microorganisms. These microorganisms are responsible for the instability of granular biomass as a consequence of the outgrowth of filamentous organisms. Despite the different approaches carried out, we can affirm that for the moment, none of the experiences reported at the laboratory scale have been able to be transferred to the real scale, sometimes due to the complexity of the design or the lack of requirements for implementation. In this context, a large number of CFRs are being tested at the laboratory scale using different strategies, for example, bubble columns, serial multiple chambers, clarifiers after or within the bioreactors, or submerged membranes. Some designs are even hybrid reactors, combining aspects of SBR and CFR. A brief description of these designs is presented below.
Bubble columns with baffles
Chen et al. [ 48 ] reported an upflow sludge bed reactor (USB) connected to an aeration column as a continuous flow granular system ( Figure 3 ). The effluent from the USB was flown to the aeration tank and water and granular biomass returned to the USB to ensure a high level of biomass retention. In order to avoid the loss of biomass, a solid–liquid–gas separator was set to the top of the USB reactor. A similar separator has been employed by other authors [ 50 51 ]. Thus, Zhou et al. [ 50 ] operated a continuous flow airlift fluidized bed reactor (CAFB) that consisted in two columns, one inside the other, with a bubble burst at the top ( Figure 3 ). The granules flew up through the internal tube and fell down by the annular tube that rounds the internal cylinder. To acclimate the sludge, the reactor operated in batch mode for 2 days and was then configured in continuous mode. In the same way, Wan et al. [ 51 ] designed a continuous-flow aerobic granular reactor (CFAGR) composed of a cylindrical reactor containing a three-phase separator and a two-decker stabilizer to avoid the loss of biomass. Kishida et al. [ 52 53 ] also designed a reactor with a similar baffle that operated in an aerobic upflow fluidized bed (AUFB) in continuous flow. This reactor consisted of a column with a gas–solid separator (located in the upper part of the reactor) and a lamella dividing the aeration zone from the effluent output zone ( Figure 3 ).
Serial multiple chambers
A different mechanism to avoid the wash-out of granular biomass was operated using serial multiple chambers to retain the granules, such as the bioreactor designed by Li et al. [ 54 ]. The length of the reactor was divided by baffles to create 10 chambers in order to increase the H/D of the full reactor. Each chamber was connected with the chambers placed next to it. There was an aerator in all the cells. The direction of the influent flow changed every 2 h. On the opposite sections of the extremes, there was an agitator, but only one of them was working at the same time: the agitator located in the chamber of the influent input. Following this idea, the aerator of the last section was disconnected to create a settling zone at the bottom, where the granules were accumulated to avoid their wash-out. This configuration improved the feast–famine mechanism ( Figure 4 ).
Use of clarifiers
Several studies have presented CFR designs connected with clarifiers, where granules could be retained. Long et al. [ 55 ] reported a CFR system with a double column cyclic aerobic granular reactor (DCCAGR), which consisted in two equal bioreactors composed by a column and a settling tank connected by an inclined tube ( Figure 5 ). Both reactors were able to separate the solid, liquid and gas phases. The settling tank had a removable baffle, allowing the selection pressure to be modified. Moreover, both reactors functioned alternatively as the first and the second reactor, using the effluent of the first as the influent of the second one. Another example of a CFR system combined with clarifiers was reported by Chen et al. [ 56 ]. In this case, the CFR system was built with a cylindrical column with an H/D ratio of 1, composed by a mixer and an aerator. Moreover, the system incorporated a clarifier tank, where granules were accumulated and returned to the aeration tank ( Figure 5 ). Continuous flow AGS has also been developed to remove P from wastewater. For instance, Li et al. [ 57 58 ] reported a CFR system composed of two chambers (anaerobic and aerobic) followed by one settling tank, named the ‘Tube Seller’, which recirculated 50% of the granules’ volume to the anaerobic zone ( Figure 5 ). According to these authors, this continuous flow granular technology allowed granular stability to be maintained and very high C, N and P removal yields to be obtained.
Recently, Sun et al. [ 59 60 ] described a very innovative CFR system incorporating a plug-flow aerobic granulation reactor, formed by multiple completely stirred tank reactors (CSTRs). The main objective of this design was to create different feast–famine conditions and evaluate the influence of this parameter on granular stability. In this sense, a value of 0.33 was determined to be the optimum feast–famine ratio. Thus, granules were very stable for a feast/famine ratio of 0.33, while for ratios of 0.5 and 1, their stability was greatly affected, and their breakdown was frequently observed.
Li et al. [ 61 ] reported an innovative system that conceptually integrated an SBR system and an advanced continuous flow reactor (ACFR). Basically, this technology consisted of an aerobic zone with a specific area for granular selection ( Figure 5 ). The granules with good settleability went back to the aeration zone, while poorly decanting fluffy granules were accumulated and discharged every 24 h for effluent clarification purposes. Another example of a CFR system that incorporates clarifiers was described by Xu et al. [ 62 ]. These authors reported a continuous-flow reactor with a two-zone sedimentation tank (CFR-TST). The reactor was composed by an airlift system followed by an aeration tank. After the aeration zone, a double sedimentation tank was implemented, where well-settling granules from the first clarifier were returned to the airlift system. At the same time, the floccular biomass from the second clarifier was discharged ( Figure 5 ).
CFR with submerged membranes
A truly innovative technological alternative is the design of granular systems coupled with submerged MBRs. In such cases, the application of continuous AGS-MBR systems enjoys the advantages of both granular and membrane technologies, enabling the production of treated water with tertiary quality. Moreover, AGS-MBR technology significantly reduces membrane-fouling processes, probably a very important problem in membrane systems [ 63 64 ].
When AGS-MBR systems are implemented in continuous mode, the main problem pertains to keeping the granular biomass stable for long periods of time, due to the lack of a starvation phase, which avoids the collapse of granules. However, this problem can be solved by setting a famine phase. In this sense, the design of reactors is constituted by two serial reaction chambers ( Figure 6 ), one of them working at a high OLR and the other at a low OLR. With this plant configuration, feast and famine conditions can be respectively promoted [ 63 65 ].
Liu et al. [ 66 ] reported a continuous-flow granular self-forming dynamic membrane bioreactor (CGSFDMBR) that consisted in a sequencing batch airlift reactor (SBAR), a settling tank, a dynamic membrane bioreactor (DMBR) and a sludge selection tank ( Figure 6 ). The granular biomass was produced in the SBAR tank, while the separation of granular and filamentous biomass was produced in the settling tank. Then, the granular sludge was recirculated to the SBAR tank, and the flocculent sludge was driven to the DMBR. Subsequently, the treated water went out as effluent, while the mixed liquor of DMBR passed to the sludge selection tank once a day, where the granules that did not cross a sieve were returned to the DMBR tank, and the floccules that passed the sieve were discharged as excess sludge.
Hybrid SBR-CFR system
Li et al. [ 67 ] developed a hybrid SBR-CFR system comprising four identical columns ( Figure 7 ). One of the reactors acted as an anaerobic column and was also responsible for feeding two aerobic reactors. These two reactors then fed a fourth reactor that acted as a clarifier. Therefore, the bioreactors worked like clarifiers or anaerobic or aerobic chambers, depending on the direction of the inflow ( Figure 7 ).
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