What is a UASB?  Anaerobic granular sludge bed technology refers to a special kind of reactor concept for the "high rate" anaerobic treatment of wastewater. The concept was initiated with upward-flow anaerobic sludge blanket (UASB) reactor. A scheme of a UASB is shown in Figure 1 below. From a hardware perspective, a UASB reactor is at first appearance nothing more than an empty tank (thus an extremely simple and inexpensive design). Wastewater is distributed into the tank at appropriately spaced inlets. The wastewater passes upwards through an anaerobic  sludge bed where the microorganisms in the sludge come into contact with wastewater-substrates.  The sludge bed is composed of microorganisms that naturally form granules (pellets) of 0.5 to 2 mm diameter that have a high sedimentation velocity and thus resist wash-out from the system even at high hydraulic loads. The resulting anaerobic degradation process typically is responsible for the production of gas (e.g. biogas containing CH₄ and CO₂). The upward motion of released gas bubbles causes hydraulic turbulence that provides reactor mixing without any mechanical parts. At the top of the reactor, the water phase is separated from sludge solids and gas in a three-phase separator (also known the gas-liquid-solids separator). The three-phase-separator is commonly a gas cap with a settler situated above it. Below the opening of the gas cap, baffles are used to deflect gas to the gas-cap opening.

Brief History UASB. The UASB process was developed by Dr. Gatze Lettinga (Figure 2) and colleagues in the late 1970's at the Wageningen University (The Netherlands). Inspired by publications of Dr, Perry McCarty (from Stanford, USA), Lettinga's team was experimenting with an anaerobic filter concept. The anaerobic filter (AF) is a high rate anaerobic reactor in which biomass is immobilized on an inert porous support material. During experiments with the AF, Lettinga had observed that in addition to biomass attached on the support material, a large proportion of the biomass developed into free granular aggregates. The UASB concept crystallized during a trip Gatze Lettinga made to South Africa, where he observed at an anaerobic plant treating wine vinasse, that sludge was developing into compact granules. The reactor design of the plant he was visiting was a "clarigestor", which can be viewed as an ancestor to the UASB. The upper part of the "clarigestor" reactor design has a clarifier but no gas cap.

Birth of UASB. The UASB concept was born out of the recognition that inert support material for biomass attachment was not necessary to retain high levels of active sludge in the reactor. Instead the UASB concept relies on high levels of biomass retention through the formation of sludge granules. When the UASB concept was developed, Lettinga took into account the need to encourage the accumulation of granular sludge and discourage the accumulation of disperse sludge in the reactor. The main features for achieving granular sludge development are firstly to maintain an upward-flow regime in the reactor selecting for microorganisms that aggregate and secondly to provide for adequate separation of solids, liquid and gas, preventing washout of sludge granules.

First UASB. The UASB reactor concept was rapidly developed into technology, the first pilot plant was installed at a beet sugar refinery in The Netherlands (CSM suiker). Thereafter a large number of full-scale plants were installed throughout the Netherlands at sugar refineries, potato starch processing  plants, and other food industries as well as recycle paper plants. The first publications on the UASB design concept appeared in Dutch language technical journals in the late 1970's and the first international publication appeared in 1980 (Lettinga et al. 1980).

EGSB. An expanded granular sludge bed (EGSB) reactor  is a variant of the UASB concept (Kato et al. 1994). The distinguishing feature is that a faster rate of upward-flow velocity is designed for the wastewater passing through the sludge bed. The increased flux permits partial expansion (fluidization) of the granular sludge bed, improving wastewater-sludge contact as well as enhancing segregation of small inactive suspended particle from the sludge bed. The increased flow velocity is either accomplished by utilizing tall reactors, or by incorporating an effluent recycle (or both). A scheme depicting the EGSB design concept is shown in Figure 3. The EGSB design is appropriate for low strength soluble wastewaters (less than 1 to 2 g soluble COD/l) or for wastewaters that contain inert or poorly biodegradable suspended particles which should not be allowed to accumulate in the sludge bed.

Overview Reactor Performance. In a recent survey (Frankin, 2001), 1215 full-scale high rate anaerobic reactors have been carefully documented, which have been built for the treatment of industrial effluents since the 1970's throughout the world. An overwhelming majority (72% of all plants) of the existing full-scale plants are based on the UASB or EGSB design concept developed by Lettinga in The Netherlands. This statistic emphasizes that the anaerobic granular sludge bed design concept has been the most successful for scale-up and implementation. The average full-scale design loading of the UASB of 682 full-scale plants surveyed was 10 kg COD/m³.d. Note: COD stands for chemical oxygen demand and refers to the organic matter in the wastewater expressed as the weight of oxygen to combust it completely. The average full-scale design loading of the EGSB of 198 full-scale plants surveyed was 20 kg COD/m³.d.  COD removal efficiencies depend largely on wastewater type; however the removal efficiency with respect to the biodegradable COD is generally in excess of 85 or even 90%. The biodegradable COD is sometimes reflected in the parameter biological oxygen demand (BOD).

The four top applications of high rate anaerobic reactor systems are for:

1. Breweries and beverage industry
2. Distilleries and fermentation industry
3. Food Industry
4. Pulp and paper.

Together, these four industrial sectors account for 87% of the applications. However, the applications of the technology are rapidly expanding, including treatment of chemical and petrochemical industry effluents, textile industry wastewater, landfill leachates as well as applications directed at conversions in the sulfur cycle and removal of metals (see Other Applications). Furthermore in warm climates the UASB concept is also suitable for treatment of domestic wastewater.

Literature cited

Franklin, R. J. 2001. Full scale experience with anaerobic treatment of industrial wastewater. Wat. Sci. Technol. 44(8):1-6.

Kato, M., J. A. Field, P. Versteeg and G. Lettinga. 1994. Feasibility of the expanded granular sludge bed (EGSB) reactors for the anaerobic treatment of low strength soluble wastewaters. Biotechnol. Bioengineer. 44:469-479.

Lettinga, G., A. F. M. van Velsen, S. W. Hobma, W. De Zeeuw, A. Klapwijk 1980. Use of upflow sludge blanket reactor concept for biological waste water treatment, especially for anaerobic treatment. Biotechnol. Bioengineer. 22: 699-734.

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