Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
2002-10-22
2004-11-02
Prince, Fred G. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S615000, C210S196000, C435S262500, C071S010000, C071S021000
Reexamination Certificate
active
06811701
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an apparatus for lowering odors and recovering biogas from flushed livestock manure and, more specifically, to a closed system apparatus that provides for the fixed-film anaerobic digestion of flushed manure.
Anaerobic digestion of livestock manure has been implemented for many years. In anaerobic digestion, a mixed culture of bacteria mediates the degradation of the putrescible fraction of organic matter ultimately to methane, carbon dioxide and mineralized nutrients. Upon storage, livestock manure begins this process of degradation resulting in the production of intermediate compounds, which are volatile and often a source of odors. Since methanogenic microorganisms grow slowly and are present in limited numbers in fresh manure, these volatile intermediates accumulate in stored manure. In an effective anaerobic digester, the growth of methanogens is promoted such that the intermediate compounds are converted to biogas and nutrients, and the odor potential of the manure is greatly reduced.
The principal means for promoting methanogenic growth in anaerobic digestion of manure are controlling the operating temperature and/or controlling the residence time of the bacteria within the process. The types of anaerobic digester that have been implemented in the digestion of manure are rather limited due to the nature of manure as a substrate. The digester types have included variations of batch and semi-continuous processes, which include plug-flow digesters, complete-mix digesters, covered lagoons and a few demonstrations of mixed reactors with flexible-film support media (U.S. Pat. Nos. 5,096,579; 6,254,775). Except for covered lagoons, these manure digesters are usually operated at mesophilic temperatures (usually 35° C.), which requires energy input. Often a portion of the biogas is used to heat the manure slurry to the operating temperature. The requirement for heating dictates that the manure slurry fed to these reactors should have as high a total solids (TS) content as possible to minimize the water content which must be heated. In practice, the manure slurry added to these heated digesters should have a TS content of 4-12%. In temperate climates, often manure slurry with 1% TS or less will fail to provide enough biogas to heat the slurry to 35° C.
Without any support media for bacterial residence/attachment, plug-flow and complete-mix manure digesters rely on the hydraulic retention time (HRT) to control the solids residence time sufficiently to promote methanogenic growth. At mesophilic temperatures, effective treatment dictates that the HRT be maintained at greater than 10 days and, in practice, a 20-40 day HRT is common. The volume of the digester is directly proportional to the chosen HRT and the volumetric rate of manure production. Again, like temperature, the long HRT requirements of these manure digesters dictate that feed manure slurries must have as high a TS content as possible to minimize excess water, which takes up digester volume and results in a higher digester volume requirement to achieve the design HRT.
Currently, many livestock facilities use large volumes of water for barn flushing, resulting in excessive amounts of dilute wastewater (<1% TS). This effectively precludes these operations from using conventional plug-flow and complete-mix manure digesters due to both the uneconomical digester volume requirements and the excessive energy required to heat the dilute manure to mesophilic temperatures for effective digestion. Ideally, an anaerobic digestion apparatus for effective treatment of flushed manure should be able to operate at ambient temperatures, tolerate much shorter HRTs, and handle small amounts of fibrous solids.
Fixed-film anaerobic digesters use an internal support media to provide large surface areas for bacterial attachment. Thus, a greater concentration of bacteria is available to mediate the degradation of organic matter. This allows bacterial residence time to be maintained independently of the HRT of the liquid phase. Using much higher concentrations of attached bacteria allows fixed-film digesters to operate at much shorter HRTs and at much lower temperatures while achieving similar treatment efficiencies as conventional plug-flow and complete-mix systems. Currently, designs for high-rate anaerobic processing systems that use fixed-film digesters are available. However, none of the existing fixed-film designs are suitable for wastewaters with significant levels of suspended solids, such as those found in flushed manure. Suspended solids loading for existing fixed-film reactors are limited to less that 10% of the influent chemical oxygen demand (COD).
Livestock manure often includes materials used for bedding, such as hay, sawdust or sand. Often, such materials are poorly degraded or non-biodegradable. Where manure is in liquid form, the liquid is often conveyed into a “lagoon” after separation using solid-liquid separation equipment, with the resultant solids spread on land. Manure presents a complex substrate for anaerobic digestion because the volatile solids content is comprised of readily digestible soluble materials; fine particles that have a high surface-to-volume ratio and are readily accessible to bacterial enzymes; and larger fibrous particles that are relatively inaccessible to microbial attack. These larger fibrous particulates can contribute to clogging of packing material or media. The larger fibrous particulates can also hinder the attachment of bacteria to the media. Ultimately, these situations can lead to short-circuiting of the anaerobic system, which reduces the effectiveness of the biological treatment system. In addition, scum formation is a problem as well as blockage of pipes and other ancillary equipment caused by floating and suspended solids.
For example, certain anaerobic processing systems, such as those disclosed in U.S. Pat. No. 4,183,809, provide for anaerobic microorganisms suspended in a liquid medium to which wastewater is fed. Such processing systems, also known as upflow anaerobic sludge blanket (UASB) reactors, rely on the tendency of anaerobic microorganisms to form flocs or granules (sludge) which are retained within the system by an efficient gas/solids/liquid separation device. Unfortunately, with this system, the microorganisms may be washed out along with the effluent when high levels of particulates are contained in the wastewater. Because of this and the fact that it is difficult to obtain granular sludge with flushed manure, these systems have not been implemented for managing livestock manure.
In other anaerobic processing systems known as fluidized or expanded-bed reactors, the microorganisms are retained within the processing system by attachment to small inert particles (or “packing material”). Suitable particles include sand, anthracite, granular activated carbon, PVC particles, or diatomaceous earth. For example, U.S. Pat. Nos. 3,846,289, 3,956,129, 4,009,099, 4,284,508, and 5,232,585 disclose methods and apparatuses for denitrifying wastewater using solid particulate carriers where particle size generally ranges from about 0.2 to 3 millimeters. Such systems, however, suffer from washout of media and/or reduced media separation efficiency when wastes with suspended solids are treated.
Additional known anaerobic processing systems immobilize the microorganisms on a matrix within the reactor, called fixed-bed reactors. As disclosed in U.S. Pat. Nos. 4,366,059, 4,530,762, 4,561,168, and 4,599,168, the matrix is composed of an inert packing material, or “media,” to provide a surface for microorganism attachment and biofilm development. Unlike the fluidized system and the expanded-bed systems described above, the media includes sheet, ring, or spherical material configured in either a random-pack or an oriented arrangement.
Random-pack (or “loose-fill”) media include such materials as gravel, wood chips, or special plastic pieces designed with a high “surface to volume” ratio and are packed in loose-fill configuration in fixed-bed r
Prince Fred G.
Saliwanchik Lloyd & Saliwanchik
University of Florida Research Foundation Inc.
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