Completely Mixed Anaerobic Digestion

Anaerobic digestion involves the biological conversion of organic and inorganic matter in the absence of molecular oxygen to a variety of end-products including methane and carbon dioxide. The consortium of anaerobic organisms works both to degrade the organic sludges and wastes in three steps, consisting of hydrolysis of high-molecular-mass compounds, acidogenesis and methanogenesis.

The process takes place in an airtight reactor. Sludge is introduced continuously or intermittently and retained in the reactor for varying periods of time. After withdrawal from the reactor, while intermittent or continuous, the stabilized sludge is reduced in organic and pathogen content and is non-putrescible. The two most widely used types of anaerobic digesters are standard-rate and high-rate. Within the standard-rate digestion process, the contents of the digester are usually unheated and unmixed, and are retained for a period ranging from 30 to 60 days.

In the high-rate digestion procedure, the contents of the digester are heated and mixed fully, and are retained, classically, for a period of 15 days or less. A combination of these two primary procedures is known as the two-stage process, and is used to separate the digested solids from the supernatant liquor. Therefore, additional digestion and gas production might occur. Anaerobic digesters are generally used for the treatment of sludge and wastewaters along with high organic content. The disadvantages and advantages of a system of this type, as compared to aerobic treatment, stem straight from the slow growth rate of methanogenic bacteria. A slow growth rate needs a relatively long retention time in the digester for adequate waste stabilization to occur; however, that same slow growth means that only a small portion of the degradable organic matter is synthesized into new cells. Another advantage of this type of system is the production of methane gas, which can be used as a fuel source, if produced in sufficient quantities. Furthermore, the system produces a well-stabilized sludge, which can be safely disposed off in a sanitary landfill after drying or dewatering. On the other hand, the fact that high temperatures are required for adequate treatment is a major drawback.