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Uplow Anaerobic Sludge Blanket Systems - UASBs

An OnLine Primer

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Anaerobic treatment is a biological process that utilizes a mixed culture of bacteria in the absence of free oxygen to remove organic matter that is present in wastewater.  The overall process yields a useful byproduct in the form of biogas, primarily methane (CH4) and carbon dioxide (CO2)).  This unique feature means that much of the available energy in the wastewater is converted to a gaseous form, resulting in very little energy left over for new cell growth.  In a nutshell, three significant benefits are associated with this process, namely the production of biogas energy, much less biosolids waste and low energy requirement for the treatment process, in addition to these benefits:

As with any process, however, anaerobic  treatment does have certain drawbacks, including the following:

Several different anaerobic technologies are available in the marketplace.  The best technology in one case may not be the best in another.  Wastewater characteristics affect the success or failure of specific processes.  Fixed-media processes seem to be most suitable for treating low-yield waste constituents, while suspended-growth reactors are most suitable for treating high-yield wastes that readily produce granules.  However, with proper selection of organic loading rate and operating conditions, each system can treat almost any type of wastewater but possibly will not perform optimally.  Unless previous experience is available with treating a specific wastewater, treatability tests are recommended highly.


A factor of importance of UASB reactors is the inventory of total and volatile suspended solids and the methane capacity of the anaerobic microorganisms in these solids. Satisfactory performance of UASB systems typically relies on contact of the influent wastewater with a bed of biomass granules that are formed in the reactor as a product of biomass growth and settling. Typically, around 90% of the sludge mass in UASB reactors should be granular.

What type of bacteria grow in an upflow anaerobic sludge blanket reactor?

Mostly facultative fermentation microorganisms that carry out hydrolysis of complex organics and fermentation of short-chain organic to for volatile fatty acids along with methane formers that convert hydrogen and acetic acid to methane.


When benchmarking a given UASB installation the following framework can be used for performance evaluation. Theoretically, 395 mL CH4 / g COD methane would be produced for say 100% conversion of COD to methane. This figure can be compared with actual plant biomass performance obtained by sample lab tests.


The maximum specific methanogenic activity (SMA) represents the "activity" of the biomass. Typically, activities for UASB reactors range from 0.5 to 1.0 g COD / g VSS/day. Lower than range lab scores point to severely decreased or inhibited methane production activity.


Process modeling provides a way to determine whether an anaerobic reactor is performing up to expectations based on fundamental process relationships. If a process model provides accurate predictions of reactor performance, that reactor is performing as expected. Model predictions that are substantially different from expected values indicate the presence of [significant] issues.


COD loading rates: 8 to 12 kg/d per m3

HRT (hydraulic retention time): 6 to 48 hours

SRT (solids retention time): greater than 20 days

Temperature: 30 to 35°C

Volatility: 70%

COD/VSS (lb/lb/day): less than 0.5

Can the bacteria coming out of the UASB register as a fecal coliform bacteria?

Almost all facultative microorganisms can mimic the basic parameter for coliform bacteria -- that is, production of hydrogen and volatile fatty acids -- but it is highly unlikely that these organisms will pass the true coliform screening tests; and even more unlikely that this effect would pass through an aerobic treatment process.


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.



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