Anaerobic granular sludge bed technology refers to a special reactor concept for the anaerobic treatment of wastewater with high throughput. The concept was introduced with the UASB reactor (UASB = upward-flow anaerobic sludge blanket). A schematic of a UASB reactor is shown in the figure.

From a hardware perspective, at first glance, a UASB reactor is nothing more than an empty tank (i.e. an extremely simple and inexpensive design).

The wastewater is fed into the tank via appropriately arranged inlets. The wastewater flows upward through an anaerobic sludge bed where the microorganisms in the sludge come into contact with the wastewater substrates. The sludge bed consists of microorganisms that naturally form granules (pellets) with a diameter of 0.5 to 2 mm, which have a high sedimentation rate and are therefore not washed out of the system even under high hydraulic loads. The resulting anaerobic degradation process is usually responsible for the production of gas (e.g. biogas containing CH4 and CO2). The upward movement of the released gas bubbles causes hydraulic turbulence, which ensures mixing of the reactor without mechanical parts. At the top of the reactor, the water phase is separated from the sludge solids and gas in a three-phase separator (also called a gas-liquid solids separator). The three-phase separator is usually a gas cap with a settler above it. Baffles are used below the gas cap opening to direct the gas to the gas cap opening.

Brief history of UASB

The UASB procedure was developed by Dr. Gatze Lettinga and colleagues developed it in the late 1970s at Wageningen University (Netherlands). Inspired by publications by Dr. Perry McCarty (Stanford, USA), Lettinga's team experimented with an anaerobic filter concept. The Anaerobic Filter (AF) is a high-speed anaerobic reactor in which biomass is immobilized on an inert porous support material. During experiments with the AF, Lettinga observed that, in addition to the biomass fixed to the carrier material, a large part of the biomass developed into free granular aggregates. The UASB concept crystallized during Gatze Lettinga's trip to South Africa, where he observed the sludge developing into compact granules in an anaerobic wine vinasse treatment plant. The reactor design of the plant visited was a "Clarigestor", which can be considered a precursor to the UASB. The upper part of the "Clarigestor" reactor has a clarifier but no gas cap.

The birth of the UASB

The UASB concept emerged from the realization that an inert support material for biomass attachment is not necessary to maintain a high proportion of active sludge in the reactor. Instead, the UASB concept is based on a high degree of biomass retention through the formation of sludge granules. When developing the UASB concept, Lettinga took into account the need to promote the accumulation of granular sludge and prevent the accumulation of disperse sludge in the reactor. The most important features for the development of granular sludge are, firstly, maintaining an upward flow in the reactor that selects microorganisms to aggregate, and secondly, ensuring adequate separation of solids, liquid and gas to prevent leaching of the sludge grains.

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

Grahik: By Tilley, E., Ulrich, L., Lüthi, C., Reymond, Ph., Zurbrügg, C. - Compendium of Sanitation Systems and Technologies - (2nd Revised Edition). Swiss Federal Institute of Aquatic Science and Technology (Eawag), Duebendorf, Switzerland. ISBN 978-3-906484-57-0., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=42267210