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.

UASB granules (Courtesy of www.Envitreat.com)

UASB granules (Courtesy of www.Envitreat.com)

Influent flow in UASB systems is typically equalized, neutralized and partially acidified in a separate tank ahead of the reactor.  The influent flow is often mixed with effluent recycle and then distributed into the lower part of the reactor below the sludge bed.  The upper portion of the reactor typically has a gas-liquid-solids separator (GLSS) that removes biogas and clarifies the effluent.  UASBs typically require low influent TSS concentrations (less than ca. 15 percent of the influent COD concentration) and low O&G, i.e. less than 100 mg/L.

Influent Distribution System

A major factor in the design of full-scale UASB 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 UASBs 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.

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