Expanded Granular Sludge Bed Systems - EGSBs

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 EGSB reactors is the inventory of total and volatile suspended solids and the methane capacity of the anaerobic microorganisms in these solids. Satisfactory performance of EGSB 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 EGSB reactors should be granular.

What type of bacteria grow in an EXPANDED GRANULAR sludge bED 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 EGSB 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 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: 12 to 24 kg/d per m3

HRT (hydraulic retention time): 3 to 18 hours

Temperature: 25 to 35°C




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