Anaerobic Sludge Digester

  • Introduction
  •         This unit operation employs microorganisms to convert industrial waste water into readily disposable digested sludge. Anaerobic digestion is a bacterial process that breaks down organic materials within waste in the absence of oxygen. Itis generally run in closed tanks (1). Generally, biomass consisting of sewage or processing wastes is mixed with water and fed into the digester without air. The waste stream generally contains fats, oils, greases. These waste streams are produced in processes involving the the manufacture of detergents and soaps, as well as within the petrochemical industry. Municipal sewage treatment facilities also use sludge digesters. The generalized equation for anaerobic sludge digestion is:

    Organic Matter + Combined Oxygens --------->Anaerobic Microbes + New Cells

    + Energy for life processes + CH4 + CO2 + Other gases

    The combined oxygens consist of CO3-2, SO4-2, NO3-1, and PO4-3.

    A example of an anerobic digester process used in industry:

    The following link provides additional information on Waste Digester Design

  • Operational Information
  •         Biogas is produced when organic matter is degraded in the absence of oxygen. The biogas from anaerobic digester contains about 60% methane and 40% carbon dioxide, with trace amounts of nitrogen, hydrogen, and hydrogen sulfide. In addition, anaerobic digestion also produces methane, a valuable fuel, making it a more attractive as a waste water alternative. (1) Separation of metals from incoming wastes prior to digestion can also be a source of income. It is also important to note that the anaerobic digestion process is not instantaneous, thus requiring larger vessels because it takes time for the bacteria population to grow and stabilize.
            Generally, any recyclable or unwanted materials are separated from incoming wastes. If the waste is already very wet, the addition of water is not necessary. The average ratio of solid to liquid for a typical digester is 15-25%, but certain technologies can handle solid concentration as high as 30% (2).
            The digester operates around 35-27 degrees C for varying periods of 10-20 days (2). The time period depends on external temperature fluctuations, and waste composition.
            After the biological degradation is complete, the solid residue is removed and usually screened to remove oversized and unwanted items like glass or plastic.

  • Case Study
  •         In order to understand the usefulness of the anaerobic sludge digester, consider the following example of a municipal facility involving the remediation of a grease-cap. In this 100 million-gal/day municipal facility, only 1.6 million gallons were being utilized to the presence of a 10-ft layer of scum and grease at the top of the unit, consisting of approximately 20% total solids. Physical removal of this layer was estimated at 1 million dollars. (Huban-Plowman, 77). Due to this high cost, other alternatives were sought such as microbial degradation ofthe scum.
            Anaerobic digestion was used to remove this grease cap, and after 45 days, the sludge composition was reduced from 20% solids to less than 1.5%. The cost was less than $50,000 with no physical removal required. (Huban-Plowman, 78). The digester therefore reaps enormous economic and safety benefits due to the introduction of microorganisms.
            Anaerobic digestion has many advantages to it. It can make landfills easier to manage by removing potentially hazardous organic wastes. It avoids the generation of biogas in landfills and can recover this biogas for further use. (2) It contributes to recycling goals set by industries and government agencies, and the end product can often be used as soil conditioner.
            Some typical problems to be considered in the use of the digester focus on the microbial population stability. The stability of the digester depends on the type of strain selected for a given process. Certain strains of bacteria will produce better yields of digested sludge than others. (Huban-Plowman, 79). Bioaugmentation procedures are currently being researched and refined to aid in this process.
            One of the biggest costs incurred in the anaerobic digester involves selecting the appropriate strain of bacteria and conducting tests to determine degradation efficiency. The heat input to the digester will also have certain costs per year associated with it. The measurement apparatus costs would be minimal, consisting of thermocouples used to measure temperature, and flowmeters on the influent and effluent lines.
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