Fundamentals

Anaerobic processes involve converting the organic matter in wastewaters to methane and carbon dioxide through a series of reactions involving a consortium of facultative and obligate anaerobic microorganisms.  Complex organic waste constituents - starches, fatty acids, proteins, alcohols, complex organic chemicals, and the like - are converted first through enzymatic hydrolysis to lower-molecular-weight soluble intermediates such as sugars, alcohol and amino acids.  These soluble substances are converted further through fermentation reactions to form organic acids.  Acetogenic microorganisms convert the higher-molecular-weight organic acids to acetic acid plus hydrogen in a free or bound form.  Hydrogen and acetic acid are the primary precursors to the mineralization end products: methane and carbon dioxide.  This methane gas has potential value for heating the digester for improved treatment efficiency or for production of electrical power for other in-plan processes.  A principal advantage of anaerobic treatment is the small amount of excess biomass produced during biodegradation.  Net yields as low as 2 kg VSS/kg COD removal are not uncommon.

Upflow Anaerobic Sludge Reactors: A Brief Overview of Available Systems

Most representative configurations include UASB (upflow anaerobic sludge blanket) and EGSB (expanded granular sludge bed) reactors.  Granular sludges exhibit high settling velocities and activity rates that reduce required reactor volumes and increase allowable organic loading rates.  Thus, these processes are considered to be high-rate systems.  The factors that create the formation of good granular sludge are complex and considerably researched both by investigators and vendors.  These factors are varied but principally relate to wastewater characteristics, system configuration and loading condition.  Typically, these systems retain granular sludge by employing specially designed often proprietary gas-liquid-solids (GLS for short) separation devices.

EGSB granules (Courtesy of www.Envitreat.com)

EGSB granules (Courtesy of www.Envitreat.com)

EGSB reactors utilize the same operating principles as UASBs but differ in terms of geometry, process parameters and usually, construction materials.  EGSBs have substantially smaller footprints with tank heights anywhere from 12 to 18 meters.  Construction materials can be glass-reinforced plastic or stainless steel.  Some types of EGSBs as simply vertically stretched out versions of one stage UASB-type reactors. 

Another type of EGSB design (see below) consists of two UASB-type reactors, one on top of the other and housed in the same vessel, giving rise to two stage EGSBs.

Detail of Three-Phase Separator Arrangement

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