Chemistry: molecular biology and microbiology – Process of utilizing an enzyme or micro-organism to destroy...
Reexamination Certificate
1999-10-08
2001-09-18
Redding, David A. (Department: 1744)
Chemistry: molecular biology and microbiology
Process of utilizing an enzyme or micro-organism to destroy...
C435S289100, C095S139000, C095S229000, C095S235000, C095S236000, C096S181000, C096S203000, C210S605000
Reexamination Certificate
active
06291232
ABSTRACT:
This invention pertains generally to anaerobic digesters and, more particularly, to a system and process for improving the efficiency and stability of an anaerobic digester by extracting pure, dry methane gas from the raw gas generated by the digester and using that methane gas in the digester.
Methane gas, a major component of the raw gas by-product of the anaerobic digestion of wastewater sludge, has long been recognized as an important potential energy source. With mesophilic digestion, there is approximately one cubic foot of raw digester gas per day from every man, woman and child contributing to the waste stream. Recently, treatment plants have utilized this raw gas to fire boilers and/or power gas-engine driven electrical generators.
Raw gas generated by digesters will vary a few percentage points in its constituent amounts. A normal sample might typically consist of 61% methane (CH
4
), 33% carbon dioxide (CO
2
), 5% water vapor (H
2
O), and 1% hydrogen sulfide (H
2
S) Pure methane has a heat value of 1000 BTU/ft
3
of gas at standard conditions of temperature and pressure, and raw gas has a heating value of about 600 BTU/ft
3
.
Anaerobic digesters usually operate in the mesophilic temperature range of 30° C. to 38° C., with a set point of 35° C. There are also a few digesters that operate in the psychrophilic temperature range of 10° C. to 20° C. and the thermophilic temperature range of 49° C. to 57° C. The system and process described herein are applicable to all three ranges, although specific reference is made only to mesophilic digester operation at 35° C. and thermophilic digester operation at 55° C.
A digester operated in the mesophilic or thermophilic range must not only hold its temperature close to 35° C. or 55° C., but must also hold its pH in the range of 6.6 to 7.6. Pressure is usually held in the range of 4 to 10 inches above atmospheric pressure for gas flow control purposes, and the elevated temperature is responsible for a substantial part of the operational cost in a digester system. Hot water boilers, often fueled by raw gas from the digester, are commonly used to maintain this temperature control. In colder climates, special insulation is required, and burning raw gas for digester heat is inefficient and corrosive to boiler tubes. Using raw gas to power gas or gas/diesel engines for pumping or generating electrical power is undesirable from the standpoints of corrosiveness and volumetric inefficiency. The system and process of the invention not only remove all the unwanted components of the raw gas, but in addition heat the return gas used in mixing, thus significantly decreasing the requirement for additional digester heat.
Typically, the raw gas generated in wastewater, solid waste and/or landfill processes is recycled in order to provide mixing of the liquor in the digestion process. The use of raw gas in mixing is an aid to bacterial growths that break down the bio-solids in the anaerobic digestion process. Raw gas is recycled in the digestion process by various methods. The invention applies to all gas mixing methods.
Anaerobic digestion is a two-stage process which may take place in a single vessel, in which case it is sometimes referred to as single stage digestion. It may also take place in two separate vessels, in which case it is commonly referred to as two stage digestion. It is the combined action of two forms of bacteria that live together in the same environment and are commonly referred to as the “acid formers” and “methane fermenters” It is necessary to keep a balance between these two bacteria. Acid formers are abundant in raw sewage. Methane fermenters are not nearly so prevalent and require a pH of 6.6 to 7.6 to produce. A digester is sensitive to too much food, it may easily become too acidic, or “go sour”, and fail to produce the desired innocuous dewaterable sludge and valuable methane (CH
4
). The system and process of the invention remove the acidic component from the mixing gas, increase the volumetric gas flow, and stabilize the system over a much broader range of feed conditions by removing the acids and acid forming products from the mixing gas.
The carbon dioxide in the raw gas forms carbonic acid (H
2
CO
3
) which, when returned to the digester as a component of the mixing gas, moves the balance in the direction of the already predominant acid formers and destroys the volumetric efficiency of any gas-to-energy processes to which it may be applied. The water vapor condenses in lines, equipment, and instrumentation used to monitor and control gas flow. However, the methane component is a valuable component, both as a gas that may be drawn from the system for energy and as a mixing gas. It is also believed that the presence of methane is a further aid to the health of methane fermenters. Natural gas, which contains a high percentage of methane, is sometimes used to restart sour digesters.
Several methods of separating (scrubbing) methane from the other raw gas constituents have been developed. U.S. Pat. No. 3,981,800, for example, describes a system in which the gas in a digester is pressurized to a level of 2 to 5 atmospheres (approximately 30 to 75 psig) so that the digesting organic waste preferentially absorbs carbon dioxide. The application of these processes to wastewater treatment plants has not proven practical and/or economically feasible. The present invention provides clean methane gas that is dry, cold and dense at atmospheric pressure, and it also overcomes the limitations and disadvantages of other processes and enhances the operational efficiency of the plant.
The thermophilic digestion process has never been considered to be an economically viable solution to the treatment of sludge in a full sized digester with a capacity of some 300 to 500 thousand gallons. Prior to this invention, there has never been an economical heat source capable of maintaining the additional 20° C. required for thermophilic operation. Thermophilic digestion is about three times as fast as mesophilic digestion. For example, a thermophilic digester can reduce the same amount of volatile solids in 10 days that a mesophilic digester will reduce in 32 days. Consequently, a thermophilic digester produces about three times as much methane gas in a given time period. By providing the heat required to raise and maintain the operating temperature in the thermophilic range, the basic digester operating efficiency has increased by a factor of three.
Utilization of the methane energy component of this raw gas has been hampered by the presence of the other by-product components. The most harmful by-product with respect to repair, maintenance and replacement of equipment in the mixing and/or gas to energy systems (iLe., pumps, blowers, compressors, boiler tubes, cylinders, etc.) is the condensing water vapor and the dilute sulfuric acid (H
2
SO
4
) produced by the hydrogen sulfide and water in the raw gas. In addition, the volumetric inefficiency of using a gas that is 40% inert in gas-engines used for pumping or generating systems requires much larger and more expensive engines than the service requirement would otherwise dictate, accompanied by a corresponding reduction in operating efficiency. Additionally, passing carbon dioxide through the combustion process increases the “greenhouse” effect upon the atmosphere.
Because of the unreliability, high maintenance costs, and the low time between failures, many plants have abandoned the use of raw gas altogether in favor of natural gas (domestic or pipeline), opting to flare-off the raw digester gas and its harmful components to the atmosphere.
Secondary sludge (sludge from the secondary sedimentation basins and the aerobic treatment processes) tends to be thinner than primary sludge sludge from the primary sedimentation basins). In order to handle secondary sludge effectively, whether it is used in direct land application or cycled through the digester, it is customary to thicken this sludge. This requires rather elaborate and expensive equipment with chemicals such as polymers to aid the proces
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