Upflow Ananerobic Sludge Blanket

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Fig 1: Conceptual diagram of UASB. Source: UASB

Upflow Ananerobic Sludge Blanket (UASB)

The UASBis a wastewater treatment system that uses microorganisms to biologically degrade pollutants. The advantages of this system include the ability to keep large amount of the biomass intended for degradation and the production of biogas.

Structure of the UASB Reactor

Fig 2: Schematic diagram of the laboratory UASB. Source: Brazilian Journal of Chemical Engineering

A typical UASB reactor consists of the following compartments: the sludge bed, the fluidized zone, the separator and the settling zone. Wastewater enters from an opening (influent) at the bottom of the reactor and leaves via another opening (effluent) at the top. The sludge bed contains granular sludge that is formed from diverse microorganisms. Within this granular sludge, organic compounds are degraded. The final products of UASB degradation processes include gases such as CO₂, CH₄ and H₂S; these gases are collectively known as biogas. The biogas is separated from the water at the separator and they exit via another separate opening (gas outlet) [1].

Physical Environment

The sludge is considered a methanogenic environment; there is no inorganic electron acceptor [4]. Sludge is an abundant source of organic nutrients. The physical conditions in the UASB and associated microorganisms and processes are dependent on the location within the sludge [2][3]. Some of the physical conditions can be manually customized to allow other bioremediation processes.

Key Processes

Fig.3 Degradation pathway of Organic matter to methane in the UASB granular sludge.

• Pre-UASB treatments • Initial granulation and development of granular sludge involves Methanosaeta spp. and Methanosarcina spp. [1][2][3]. They form filamentous network throughout the granules which allows other bacteria species to colonize [2][3], though, other theories have also been proposed [3]. • Formation of biogas by complete anaerobic degradation of organic compounds. The process requires the interaction between multiple types of bacteria.

1. Acidogenesis is carried out by diverse population of acidogens An example will be Propionibacterium spp.

2. Acetogenesis includes many syntrophic bacteria: -Pelobacter carbinolicus: degrades ethanol -Syntrophobactor wolinii: degrades propionate -Symtrophomonas wolfei: degrades butyrate Microcolonies are usually observed at this layer.

3. Methanogenesis: Mainly Methanosaeta spp. and Methanosarcina spp., but other genuses are also being observed. An example is Methanobrevibacter spp. • Separation of Biogas from the water • Post-UASB treatments

Layered Structure of the Sludge

The typical UASB granule has 3 distinct layers: The following is an example when sucrose substrate is used

The outer layer is a mixture of various types of bacteria and includes some scattered colonies of Methanosaeta spp. and Methanosarcina spp. Acidogens are concentrated on the outer layer. The middle layer is dominated by syntrophic microcolonies with some scattered Methanosaeta spp. and Methanosarcina spp. The interior layer, which is also known as the centre core, is densely packed with short rod-shaped Methanosaeta spp. and Methanosarcina spp [3].

Usually, the product of the external layer will be the substrate of the next inner layer. An example would be in the situation when biopolymers are degraded by the acidogens into short chained volatile fatty acids (SCFA) or alcohols. The fatty acids or alcohols then diffuse down a concentration gradient to the middle layer. The middle layer bacteria, known as acetogenic bacteria, use the SCFA and produce acetate. Acetate is the substrate for the methanogens, Methanosaeta spp. and Methanosarcina spp. [2][3].

One or more of the layers could be missing or undistinguishable when certain wastewaters are used. The layers could be limited to the degradation of carbohydrate substrates. For example, granules, which degrade glutamate, do not have any layers. The rationale behind these seems to be related to the rate of uptake of the substrates for acidogenesis. The initial uptake via the layers for acidogenesis will be low if glutamate was the substrate; thus, it is reasonable that no layers are observed [2].

Methanosaeta spp. and Methanosarcina spp. are found in all layers of the sludge. This structure supports that these bacteria are responsible for the initial granulation [1][2][3].

Biological Interactions

Substrates for acetogenesis, such as butyrate and propionate, can only be degraded at low partial pressure; these processes are thermodynamically unfavorable otherwise. Hence, hydrogen produced during other acetogenesis processes must be removed by species that metabolize hydrogen.

Syntrophic microcolonies are formed when two or more different species are in close contact with each other within the granular sludge. These colonies allow the process known as interspecies hydrogen transfer where hydrogen producing species rapidly transfer hydrogen to hydrogen consuming species.

Customization of the UASB to Degrade Other Compounds

1) Additional species can be inoculated into the granular sludge to perform other processes of xenobiotic compounds. For example:

-Desulfomonile tiedjei -Dechlorosporium hafniense

Inoculation of the above species results in the novel process of dechlorination of chlorophenol to phenol in the UASB reactor [1][5]. Sulphate-reduction and methanogenesis can take place simultaneously [6].

2) In manmade thermophilic (55°C) UASB reactors, the two processes can take place simultaneously at moderate pH (6.75-7.4). When pH>8, sulphate-reduction is the dominant process while methanogenesis is inhibited [6].

3) Pre-UASB treatments and post-UASB treatments are required for complex wastewater. Complex wastewaters include those with insoluble precipitates or are likely to foam [7][8].

4) Expanded granular sludge bed (EGSB)

EGSB is a custom version of the UASB that is designed to treat non-complex wastewater with high efficiency. The rate which the wastewater flows is significantly faster than the average UASB [9].

Limitations

Although UASB is an effective and environmentally beneficial system, it is not widely used. This is because the effectiveness of the system is determined by the sludge retention time (SRT). SRT, in turn, is dependent on temperature. Hence, the system is limited to regions with moderate and relatively constant temperature [10].

References

(1) Schmidt, Jens E., and Birgitte K. Ahring. "Granular sludge formation in upflow anaerobic sludge blanket (UASB) reactors." Biotechnology and Bioengineering 49.3 (2000): 229-246.

(2) MacLeod, F. A., S. R. Guiot, and J. W. Costerton. "Layered structure of bacterial aggregates produced in an upflow anaerobic sludge bed and filter reactor." Applied and environmental microbiology 56.6 (1990): 1598-1607.

(3) Wastewaters." Water Science and Technology 30.12 (1994): 87-96.

(4) Stams, Alfons JM. "Metabolic interactions between anaerobic bacteria in methanogenic environments." Antonie van Leeuwenhoek 66.1 (1994): 271-294

(5) Christiansen, Nina, et al. "Degradation of chlorinated aromatic compounds in UASB reactors." Water Science and Technology 31.1 (1995): 249-259.

(6) Visser, A., Y. Gao, and G. Lettinga. "Effects of pH on methanogenesis and sulphate reduction in thermophilic (55 C) UASB reactors." Bioresource technology 44.2 (1993): 113-121.

(7) Gonçalves, M. M. M., S. G. F. Leite, and G. L. Sant'Anna Jr. "The bioactivation procedure for increasing the sulphate-reducing bacteria in a UASB reactor." Brazilian Journal of Chemical Engineering 22.4 (2005): 565-571.

(8) Lettinga, G., and LW Hulshoff Pol. "UASB-process design for various types of wastewaters." Water Science & Technology 24.8 (1991): 87-107.

(9) Lettinga, Gatze, Salih Rebac, and Grietje Zeeman. "Challenge of psychrophilic anaerobic wastewater treatment." TRENDS in Biotechnology 19.9 (2001): 363-370.

(10)Seghezzo, Lucas, et al. "A review: the anaerobic treatment of sewage in UASB and EGSB reactors." Bioresource Technology 65.3 (1998): 175-190.