This unit operation can be used to simulate anaerobic digestion when the reaction kinetics are unknown or unimportant but the mass stoichiometry is known. If the reaction kinetics are important, please use the Kinetic Anaerobic Digestion operation instead.
Anaerobic digestion is one of the oldest processes used for the stabilization of sludges. It involves the decomposition of organic and inorganic matter in the absence of molecular oxygen. The major applications have been, and remain today, in the stabilization of concentrated sludges produced from the treatment of wastewater and in the treatment of some industrial wastes. More recently, it has been demonstrated that dilute organic wastes can also be treated anaerobically (Tchobanoglous and Burton, 1991).
Anaerobic digestion utilizes airtight tanks in which anaerobic microorganisms stabilize the organic matter producing methane, carbon dioxide and other end products:
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Roughly four groups of microorganisms sequentially degrade organic matter. Hydrolytic microorganisms degrade polymer-type material such as polysaccharides and proteins to monomers. The monomers are then converted into fatty acids with a small amount of H2. The principal acids are acetic, propionic, and butyric with small quantities of valeric. All acids higher than acetic acid are converted into acetate and H2, by acetogenic microorganisms. The acetic acid and H2 are converted to CH4 by methanogenic organisms (Eckenfelder, 1989).
There are two main types of anaerobic digesters, standard rate and high rate. In the standard-rate digestion process, the digester contents are usually unheated and unmixed. The digestion period may vary from 30-60 days. In a high-rate digestion process, the digester contents are heated and completely mixed. The required detention period is 10-20 days.
The digester gas contains approximately 60-70% methane, 25-30% carbon dioxide and small amounts of hydrogen, nitrogen, hydrogen sulfide, and other gases. The gas has a heating value of 21,000-25,000 kJ/m3.
The hot water or steam used to heat digesters is most commonly generated in a boiler fueled by digestion gas. Up to 80% of the gas heating value can be recovered in a boiler. Additional heating (if required) is provided by natural gas.
● Stoichiometric Anaerobic Digestion Procedure
See Anaerobic Digestion: Modeling Calculations.
The interface of this operation has the following tabs:
● Oper. Cond’s, see Anoxic Reaction Operations: Oper. Conds Tab
● Volumes, see Continuous Vessel Operations (Design Mode): Volumes Tab and Continuous Vessel Operations (Rating Mode): Volumes Tab
● Reactions, see Stoichiometric Reaction/Fermentation Operation: Reactions Tab
● Gas Release, see Anoxic Reaction Operations: Gas Release Tab
● Sorption, see Environmental Reaction Operations: Sorption Tab
● Labor, etc, see Operations Dialog: Labor etc. Tab
● Description, see Operations Dialog: Description Tab
● Batch Sheet, see Operations Dialog: Batch Sheet Tab
● Scheduling, see Operations Dialog: Scheduling Tab