1. Introduction
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Up-flow Anaerobic Sludge Blanket (UASB) reactor is an anaerobic digester for wastewater treatment, and its operational concept can be described as a vertical up-flow pumping of liquid substrate, including wastewater or growth media, through a layer of anaerobic sludge [ 1 6 ]. Microbial consortia inside the sludge layer consume digestible components as substrate and decompose them into smaller chemical compounds [ 7 ]. Within the scope of a wastewater treatment, the goal of anaerobic digestion is a complete mineralization of organic compounds combined with the production of biogas for the purpose of energy recovery.
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A distinguishing feature of UASB reactors is the formation of microbial conglomerates, where the metabolic product of one microbial group is a consumable substrate for another microbial group [ 8 ]. Such microbial conglomerates grow into spherical or bean-shaped granules over time [ 9 13 ]. The sizes of granules may vary, but typically are reported in the range 0.5 to 6 mm, where longer operation leads to larger sizes [ 14 16 ]. Granulation of sludge is promoted by the presence of microorganisms that are able to produce and secrete Exocellular Polymeric Substances (EPS) [ 17 ]. The term &#;EPS&#; includes multiple types of compounds, which serve as a glue to agglomerate microorganisms together and to add some mechanical strength to a granule [ 18 ].
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A combination of developing trophic microbial connections and mechanical cementation with EPS, results in higher resilience of larger granules to sudden changes of operation conditions including a change in pH, temperature mode failure, substrate switch or inconsistency of a substrate strength and content, feeding rate fluctuations, etc. [ 19 20 ]. In some cases, granules can be disrupted due to hydrodynamic forces or inner gas pressure into several smaller fragments [ 12 22 ] which become cores for the formation of new granules [ 23 ].
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The traditional concept of a UASB reactor, as suggested by Lettinga [ 10 26 ], is represented in Figure 1 a. The substrate is pumped to a reactor through the distribution system into a bottom layer of anaerobic sludge. Equally distributed in normal cross-section of the reactor, the substrate is pushed through the sludge layer (called a &#;digestion zone&#;) creating a vertical up-flow. This process is concurrent with the decomposition of organic compounds of substrate and a formation of gaseous products. Besides feeding the reactor, the continuous vertical up-flow of substrate prevents the sludge layer from clogging, keeping it afloat. However, the up-flow does wash out the unattached biomass (microorganisms, that did not start to form flocs) and small flocs/granules. The liquid part above the sludge layer (called &#;settling zone&#;) serves as a vertical settler and/or coagulation column to initiate the biomass and solids retention process before the actual separation. The separation process occurs in the compartment called Gas&#;Liquid&#;Solids separator (GLSS, a.k.a. three-phase separator). GLSS is traditionally located on top of the reactor column and it starts with a baffle-shaped structure in its bottom part, which serves the purpose of collecting and re-directing the gas bubbles to the main gas collection part and preventing gas bubbles from escaping with effluent.
The construction concept of the GLSS is shown in Figure 1 a, where it&#;s implemented via narrowing the outlet of the reaction tube with baffles. Such baffles are typically referred to as &#;deflectors&#; or &#;collar&#;. The side effect of narrowing the reaction tube outlet is a creation of local velocity gradient (velocity shear), which slightly enhances the formation of granulated particles, their separation from liquid and settling back to the bottom of the reactor. Above the baffles, the GLSS contains the gas collecting structure, where the cross-section looks like a flipped upside-down funnel. In some studies, this funnel is replaced by a tubular structure with diameter larger than the distance between baffles [ 27 28 ]. The liquid is forced to flow through the space in between the lower edge of the gas collector and the baffles, to go around the funnel and leave the reactor at the effluent port.
Other existing modifications of GLSS in laboratory-scale reactors can improve the higher solids retention time, such as installing a high rate settler in headspace [ 29 ] or modification of three-phase separators [ 30 ].
In addition to the operational concept of the UASB reactor shown in Figure 1 a, the same authors [ 11 ] also describe UASB reactor with a modified gas collector, which is demonstrated in Figure 1 b. However, some studies [ 31 ] call such a modification of the Up-flow Anaerobic Sludge Baffled Reactor (UASBR). It may also contain the inner mechanical agitation device to prevent foam formation in the gas collecting area [ 32 ]. Recently, the Y-shaped variation of UASB reactor also became popular and is depictured in Figure 1 c. In the case of the Y-shaped reactor, the GLSS is split into two individual separators: one separator is used to separate gas from the liquid and collect it directly at the top of a main tube, whereas a second collector is a sidearm tube that serves as an inclined settler for separating solids from liquid (similar to a Lamella clarifier). Use of a funnel-shaped gas-collecting element becomes optional in such case, since it serves only the purpose of preventing gas flow to an effluent side-arm.
Considering the concepts described, the optimization goal of a laboratory scale UASB reactor operation is to achieve better performance, where optimization targets for UASB performance include the following:
Higher removal of contaminants;
Higher biogas production rate;
Shortening of adaptation period; and
Resilience (robustness) of sludge.
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To achieve some of those optimizations, the classical UASB concept can be combined with other types of reactors, resulting in a range of composite reactors. Some modifications are found in the literature and are presented in Table 1 . This table represents options, where another reactor type is incorporated into the UASB itself, but not a sequence of two consecutive reactors.
In a holistic view, the purpose of UASB reactor optimization is to keep the microorganisms in a stage of maximum substrate consumption and active growth. However, from an operational perspective, the optimization of UASB functioning is achievable via adjusting operational parameters, including, but not limited to:
Organic Loading Rate (OLR) and Hydraulic Retention Time (HRT);
Recycle ratio of effluent;
Regulation of pH;
Retention of biomass; and
Granulation enhancement.
Despite the long history since the invention and description of the UASB concept by Lettinga et al. [ 41 ] and increasing its application in industry, UASB laboratory scale reactors used for treatability studies are highly variable with regard to terminology, design, construction, and operation processes. This lack of uniformity leads to different results regarding water quality indicators, for example, Chemical Oxygen Demand (COD), as well as bioenergy production, for example for biomethane and biohydrogen. There is a lack of uniformity with regard to the guidelines for operation of laboratory scale conditions, which is highlighted in this manuscript and recommendation are provided for making UASB laboratory studies and results more uniform with results more transferrable among laboratories and more useful for scale up activities. These lack of uniformity with laboratory scale UASB reactors is addressed in this study and guidelines are provided for increasing the uniformity so that results are comparable across different laboratories and are also more meaningful for scale up applications of the UASB reactor process.
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