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Uplow Anaerobic Sludge Blanket Systems - UASBs

An OnLine Primer

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Anaerobic fixed-film reactors were developed in 1968 and have grown to represent an advanced technology that has been used effectively for treating a variety of industrial wastes.  A number of variations have been developed in the intervening years including the fully packed upflow anaerobic filter (AF) , the fully packed downflow anaerobic filter and the upflow  hybrid anaerobic filter (HAF).

Fixed-film reactors are basically contact processes in which wastes pass over or through a mass of biological solids contained within the reactor, which is attached to the surfaces of a media matrix as a thin biofilm, entrapped within the media matrix, or held as granulated or flocculated biomass within the reactor by the action of the media or a gas-solids separation device.  Soluble organic compounds passing in close proximity to this biomass diffuse into the surfaces of the attached or granulated solids, where they are converted to intermediates and to end products, specifically methane and carbon dioxide.

Upflow Anaerobic Sludge Blanket (UASB) reactors

UASB drawingUpflow Anaerobic Sludge Blanket (UASB) reactors retain biological solids in the form of granular pellets or large agglomerates of flocculated cells that are sufficiently large and dense to settle against the upflow velocity. No particular media is used to weight the granules, but particulate materials naturally present in the influent waste may be entrapped within the biomass, thereby aiding the flocculation and granulation process.
UASB reactors have seen widespread use in Europe, Canada, the United States and South America, with over 200 full-scale systems in operation. The reactor configurations may be cylindrical or rectangular, with full-scale reactors volumes varying from 30 to 5500 m3. The design of most gas/solids separators is proprietary with each manufacturer having its own specific configuration.
Recent developments include modifications that have high height-to-diameter ratios. These systems in effect become fluidized-bed reactors without a weighting agent. Their success requires the occurrence of granulation of the biomass and a highly efficient solids-liquid-gas separator.

UASB reactors typically operate at food-to-microorganism (F/M) ratios between 0.5 and 1 g COD/g VSS/day. Typical organic loading rates for UASBs can range from about 8 to 12 kgCOD/m3/d.

Expanded Granular Sludge Bed (EGSB) reactors essentially comprise stacks of two or more UASBs. Each vendor will tweak roughly similar configurations and add proprietary designs for the various components and/or possible arrangements/layouts.



The hybrid reactor design combines an lower section functionally identical to an UASB and an upflow AF on top, the idea being to combine the strengths of each approach in a single tank.  Thus, the lowermost 30 to 50 percent UASB-like portion of the reactor volume is responsible for flocculant and/or granular sludge formation.  The upper 50 to 70 percent of the reactor is filled with crossflow plastic media and behaves as an anaerobic filter.  


The media in fixed-film anaerobic reactors helps to provide uniform flow through the reactor, improves contact between the waste constituents and the biomass contained within the reactor, and causes accumulation of the large amounts of biomass needed to produce the long solids retention times that are required to treat complex industrial wastes.  Commercial media available for use in anaerobic filters include Pall rings (or similarly designed loose-fill media) and modular, block-type media formed from corrugated plastic sheets.  The channels in the media modules may be tubular so that no lateral flow occurs through the height of the block, or counter-stacked so that a crossflow redistribution effect occurs at the contact points within the unit's height.

The amount of media to use in upflow hybrid anaerobic filters is quite subjective.  Since the growth on the media surfaces provides substantial COD removal, and since the media aids in flocculating the biological solids, there is a limit to how little media can be used.  The media-to-height ratio seems to be the critical factor, and reactors having 70 percent or less of the volume occupied by media generally have experienced increased biomass loss and reduced efficiency.

Media used in full-scale upflow anaerobic filters averages 30 sq.ft./cu.ft. (100 m2/m3) specific surface area regardless of the type of media.  Research indicates that less than 5 percent improvement in COD removal efficiency is gained by more than doubling said specific surface area.  Therefore, it seems likely that the additional cost of high-density media cannot be justified routinely.  While pilot- and full-scale data shows lower efficiencies for loose-fill and tubular media than for crossflow media, site-specific considerations, economics, and operating factors should ultimately be the determining factors.

Clogging of media has been a concern of a number of designers and potential users of upflow anaerobic filters.  While clogging was a problem with early fully packed designs using rock and loose-fill media methods to scour and flush excess solids from the media periodically have been included in recent designs with loose-fill media.  No instances of plugging have been reported for crossflow or tubular modular-media having specific surface areas of about 30 sq.ft./cu.ft. (100 m2/m3)

Since waste conversion in downflow reactors is associated almost exclusively with attached biomass, these reactors must be essentially filled with media to realize maximum use of the contained volume.  While a number of types, configurations, and sizes of media have been evaluated in laboratory and pilot-scale downflow anaerobic filters, only tubular media having 30 sq.ft./cu.ft.specific surface area have been used in full-scale downflow reactors.


A major factor in the design of full-scale fixed-film reactors is the proper distribution of the influent wastewater and the associated effluent recycle.  Placement of the distribution orifices should take into consideration the hydraulic limits of piping and orifice sizing.  Generally, distribution orifices for upflow anaerobic filters are placed no farther apart than 1 m.  This spacing requires orifice diameters of about 18 mm (3/4") to maintain an outlet velocity of 1 m/s.  Smaller orifices would be subject to increased plugging potential, and larger orifices would require greater spacing, which could result in poor distribution.


Occurrence of high pH in conjunction with high VFAs indicates strongly a toxic impact, possibly due to high ammonia concentrations or chemicals used in the industrial processing plant.


It is not good to add sulfuric acid to anaerobic treatment processes. The associated production of hydrogen sulfide can lead to odors and toxic conditions. Hydrogen sulfide can react with trace minerals, specifically iron, to produce black finely dispersed solids.




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