Wastewater Treatment Terminology: Anaerobic Baffled Reactor (ABR)
The anaerobic baffled reactor (Anaerobic Baffled Reactor, abbreviated ABR) is a highly efficient, novel anaerobic biological wastewater treatment device developed around 1982 by Bachmann, McCarty and colleagues at Stanford University [13-14]. Its principle is to install a series of vertical baffles inside the reactor, dividing it into several series-connected compartments, each of which is a relatively independent upflow sludge-bed system [14] [16]. The water flow is guided by the baffles to move up and down in a baffled path, passing sequentially through the sludge beds of each compartment, so that the organic substrates in the influent come into full contact with the microorganisms and are degraded and removed [14] [26]. The ABR combines the advantages of various second-generation anaerobic reactors and realizes the core idea of the staged multi-phase anaerobic process (SMPA), separating the acidogenic and methanogenic phases along the flow path within a single reactor; in overall performance it is equivalent to a two-phase anaerobic treatment system and is considered to possess the characteristics of a third-generation anaerobic biological reactor [13] [19] [26].
The anaerobic baffled reactor (ABR) was developed and researched around 1982 by Bachman and McCarty and colleagues at Stanford University [5] [13]. In 1983, McCarty modified the ABR—whose up- and down-flow chambers were originally equal in width—into a new structure with a wider up-flow chamber and a narrower down-flow chamber, and added a deflecting angle at the end of the baffle [15]. The original structure was obtained by K. F. Fannin and others by adding vertical baffles to a plug-flow reactor. Later researchers proposed various improved versions, such as the horizontal baffled anaerobic reactor (HBAR) proposed by P. Y. Yang et al. A. Tilehe et al. made substantial modifications to the ABR, adding a settling chamber at the reactor end to recycle sludge and attaching packing at the top of each compartment to immobilize sludge and collect the gas produced in each compartment separately. In 1991, R. Boopathy et al. improved the two-compartment ABR structure by designing the volume ratio of the first and second compartments as 2:1. In 1998, I. V. Skiadas et al. designed a periodic baffled anaerobic reactor (PABR). In 2002, S. Uyanik et al. proposed a split-feed anaerobic baffled reactor (SFABR) [16].
The anaerobic baffled reactor combines the advantages of various second-generation anaerobic reactors; it belongs to the staged multi-phase anaerobic biological treatment process technology and is considered to possess the characteristics of a third-generation anaerobic biological treatment reactor [13].
The structural design of the ABR has a decisive influence on its hydraulic conditions and treatment efficiency, and related research aims to optimize its geometric parameters. The number of compartments is determined according to the wastewater concentration: 3-4 compartments are suitable for low-concentration wastewater, while 6-8 are appropriate for high-concentration wastewater. The width ratio of the up-flow to down-flow chambers is generally controlled at 5:1 to 3:1, and the baffle deflection angle is usually taken as 45°-60°. Influent modes include feeding at the top, middle, or bottom of the compartment, and gas collection modes include separate collection per compartment and centralized collection. Installing packing in the upper space of the reactor to form a hybrid ABR (HABR) can increase biomass and accelerate sludge-water separation [14] [16]. To address the problems of high load on the first compartment and sludge washout in the last compartment, studies have also proposed improvements such as increasing the volume of the first compartment and adopting zigzag baffles [14].
Its disadvantages include the tendency of the first compartment to over-acidify, a long start-up time, the existence of hydraulic and biological dead zones, and the need to optimize influent distribution [7] [16].
Key operating parameters include temperature, pH, hydraulic retention time, upflow velocity, volumetric load, and alkalinity. The ABR has two main start-up modes: fixing the influent concentration while shortening the hydraulic retention time, and fixing the hydraulic retention time while increasing the influent concentration; studies show the former start-up mode is superior to the latter. Installing packing in the upper part of the reaction compartments can form a hybrid anaerobic baffled reactor to improve performance [14].
The anaerobic baffled reactor has a wide range of applications and is suitable for the treatment of high-concentration organic wastewater from hotels, schools, military units, office buildings, various residential communities, livestock and poultry farming, food processing, pharmaceuticals, and chemical industries, among others [13].
Specific application cases include the treatment of municipal domestic sewage, such as coupled application with an anaerobic membrane bioreactor [12]; the treatment of pharmaceutical wastewater, such as acetylspiramycin production wastewater [15]; the treatment of high-salinity organic wastewater, such as pickled mustard-tuber wastewater [19]; the treatment of landfill leachate [21]; the treatment of livestock and poultry farming wastewater [20]; the treatment of printing and dyeing wastewater [18] [26]; and the treatment of waste-paper papermaking wastewater, etc. [22].
The anaerobic baffled reactor is often used as a pretreatment unit or core unit in combination with other processes to form integrated processes, such as ABR-modified SBR, ABR-constructed wetland, ABR-biological contact oxidation, ABR-MBR (membrane bioreactor), and ABR-Fenton [17-18] [20].
Microbial Community and Phase-Separation Research
One of the most significant features of the ABR is that its multi-compartment structure enables the separation of the acidogenic and methanogenic phases: the front compartments are dominated by hydrolytic and acidogenic bacteria, while the rear compartments are dominated by methanogens [16]. When treating printing and dyeing wastewater, the distribution of culturable dominant microbial populations (such as Bacillus and Pseudomonas) differs among the ABR compartments; decolorizing bacteria are mostly concentrated in the front compartments, while aniline-degrading bacteria are enriched in the rear compartments [26]. During gradient acclimation with phenol, the dominant bacterial families shift from Streptococcaceae and Enterobacteriaceae to Syntrophaceae, while the dominant archaeal families shift from Methanobacteriaceae to Woesearchaeales [25].
Reactor Structure Optimization and Design Research
The ABR has the advantages of simple structure, no moving parts, strong resistance to shock loads, strong adaptability to toxic substances, good solid-liquid separation, stable operation, flexible operation, and strong biological solids retention capacity [9-10] [13].
Operating Process Parameters and Efficiency Research
Hydraulic retention time (HRT) is the main parameter controlling ABR operation and should be selected reasonably according to the wastewater concentration. Temperature, pH, and alkalinity are important environmental factors for maintaining microbial activity. Controlling the accumulation of volatile fatty acids (VFA) inside the reactor is essential to prevent system acidification. There are different views on the effect of effluent recirculation: appropriate recirculation can dilute toxic substances and adjust the front-end pH, but inappropriate recirculation can destroy phase separation, increase dead-zone volume, and lead to sludge washout. Split feeding (SFABR), as an optimized feeding method, has been proven effective in alleviating the load shock on the first compartment [14] [16].
Coupling Research with Other Treatment Technologies
To overcome the limitation that ABR effluent quality may not meet standards or to further improve treatment efficiency, research often couples it with other technologies [16]. The coupling of ABR with a membrane bioreactor (MBR) forms the ABR-MBR (or CAMBR) process, which can effectively improve nutrient-salt removal rates [12] [18]. The coupling of ABR with a microbial electrolysis cell (MEC) can be used to degrade high-concentration organic wastewater [16]. In addition, the ABR is often used as a pretreatment unit in combination with aerobic processes, such as ABR-constructed wetland, ABR-SBR, and ABR-biological contact oxidation, to form a complete anaerobic-aerobic treatment system [17] [20].
Due to the complexity of wastewater composition and microbial action, ABR design relies heavily on experimental data. To reduce experimental workload and improve design reliability, using computer simulation methods to study the hydraulic characteristics of the ABR for structural optimization, as well as establishing relevant mathematical models in combination with microbial reaction and mass-transfer mechanisms, has become an important theoretical research direction [14] [24].
Overall, the ABR continues to develop in engineering practice, but fundamental theoretical research lags relatively behind practice [14]. Future research can focus on: further improving reactor structure; exploring more efficient start-up methods to shorten the start-up time; studying the adaptability of the ABR to the treatment of various complex-composition wastewaters and its deep coupling with other processes; and, in combination with simulation and modeling, deepening the understanding of the mechanisms of microbial metabolism and mass-transfer processes inside the reactor [16] [24].
Anaerobic Baffled Reactor Diagram
