Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
2001-01-19
2003-10-14
Prince, Fred G. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S609000, C210S613000, C210S631000, C210S181000, C210S205000, C210S258000
Reexamination Certificate
active
06632362
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention pertains generally to anaerobic digesters and, more particularly, to a system and process that significantly extends the efficiency, control, and applicability of anaerobic digesters to all of the many and variously different liquefied bio-waste products over a wide variety of conditions and concentrations.
2. Introduction and Related Art
Designers of wastewater facilities have always been concerned with energy costs. Historically, however, rather large engineering “houses” have designed wastewater treatment plants (WWTPs) with hydraulic loads of millions of gallons per day.
Designing these plants involved large structures, the design responsibility for each was assigned to a person or group that specialized in that particular function. A process engineer who was forced by the dictates of Federal, State, and local agencies to aim the overall design performance almost entirely with respect to the effluent requirements of these entities determined the make up of the process itself. Although energy considerations have in recent years received some attention little if any serious effort has ever been directed to the energy savings possible by the integration of these processes. Evolution not invention predominated the picture.
As each group perceived a problem in its area of expertise, or if operational problems developed after construction, enterprising engineering, inventor, entrepreneur types were brought into the picture. This approach has resulted in the application of inventive genius to a lot of fixes, a lot of complexity, and a large increase in construction, maintenance, and energy costs in WWTPs.
Aerobic And Anaerobic Processes
Domestic wastewater, liquefied bio-waste, commercial and industrial liquid waste processes have historically used two distinct classes or systems of bacteria to reduce the biosolids contained therein to more biochemically safe water and solids that can be used for fertilizer and a variety of other products. These two bacterial systems are termed aerobic and anaerobic.
The Aerobic Process
Aerobic processes require the mixing of air or pure oxygen into the liquor being treated so that aerobies (aerobic bacteria) grow, attack, and bio-chemically reduce the solids. Aerobic processes are relatively easy to devise and there are many such systems in use worldwide.
The drive for higher and higher quality effluents has contributed to the expansion and proliferation of aerobic processes. However, there are a number of disadvantages to aerobic processes: they are in general open processes that have odor problems; they tend to require large tanks or ponds that require considerable space; and they consume large quantities of energy in the form of electrical power. Sixty to seventy percent of the energy required in modern domestic wastewater treatment plants is directly attributed to aerobic processes.
The Anaerobic Process
Conversely, anaerobic processes can be net energy producers. They operate in closed tanks or vessels devoid of oxygen, at an elevated temperature, are more difficult to control, produce a gas that contains approximately 64% methane (natural gas), 34% carbon dioxide, and 2% hydrogen sulfide. It is the methane component of this raw gas mixture that is valuable for its energy content (nominally 1000 Btu/ft
3
). Gas production rates are a function of the type and density of the bio-feedstock and general digester efficiency.
Water To Volatile Solids Ratio
The limiting factor that has prevented all wastewater feedstock from being treated anaerobicly is the high ratio of water to bio-solids (volatile solids or VS) contained in the feedstock. Domestic wastewater typically exhibits as little as 0.01% VS. And yet, it is normally difficult to maintain anaerobic action below a minimum threshold of about 3 to 5% VS. Therefore common practice limits anaerobic digestion to that relatively small part of the influent that either settles readily or floats to the top of large primary and secondary sedimentation tanks, thus delegating a very large portion of the influent to aerobic activated sludge processes. The invention described herein completely eliminates this minimum VS requirement so that all biosolid liquor mixtures may be anaerobicly reduced irrespective of their biosolid (VS) concentrations.
Previous Anaerobic Limitations
The energy produced by anaerobic systems in the form of methane gas is a direct function of the quantity of biomass reduced (VSR) in the process. Therefore, the net positive energy generated is normally severely limited by the water to VS ratio of the digester influent, irrespective of the several chemical-thermal-mechanical factors that determine digester efficiency. And, depending upon the feedstock there has normally been an operating point at which it becomes more efficient to delegate a portion of the treated influent to aerobic processing. This limitation can be overcome to some extent by the addition of external biosolids such as: food, animal, agricultural, grass clippings, tree trimming, cardboard, and other bio-waste products to the anaerobic influent. Therefore, the ability of this invention to control and maintain the desired water to VS ratio in the digester eliminates the necessity for, but not the usefulness of, such considerations.
Anaerobic Temperature
Anaerobic digesters have been operated in a number of temperature ranges. This invention is applicable to all anaerobic digesters regardless of temperature. Most common digesters operate in the mesophilic range of approximately 35° C. or the thermophilic range (55° C.). The preferred embodiment and the description of this invention refer to thermophilic operation.
Raw Gas Constituents
In an anaerobic reactor, retort, or vessel operated in the thermophilic bacterial temperature range of approximately 53 to 58° C. (nominally 55° C.) there is a certain space above the liquor (hereinafter referred to as the dome, however this reference does not necessarily limit the shape of the vessel) that collects the raw gas produced in the reactor by the anaerobic action. The constituents of this raw gas vary a few percentage points but generally may be expressed as being 60% methane (CH4), 31% carbon dioxide (CO2), 1% hydrogen sulfide (H2S), and 8% water vapor (H2O).
The Vacuum Retort
The partial pressures of these gases are a function of the temperature and pressure in the dome. The quantity of methane, carbon dioxide, and hydrogen sulfide available to this mixture is limited by the digester gas production rate. Only the water content of the liquor, the temperature of the vessel, and the pressure in the dome however, limit the quantity of the water vapor available. Since the liquor is generally more than 95% water the quantity of water vapor available may be considered infinite within the confines of this discussion. And, since the temperature of the vessel is set by the anaerobic requirement the surface of the liquor may be considered constant at 55° C. However, the total pressure and to some extent the temperature in the gaseous space of the dome above the liquor may be varied widely and in itself will have virtually no effect upon the temperature or operation of the digestion process. Decreasing the pressure will increase the partial pressure of water vapor increasing the ratio of water vapor to gas. It is a major action and purpose of this invention to decrease the absolute dome pressure (creating a vacuum); thus increasing the percentage of water vapor; drawing off this water vapor; to result in the lowering of the water to VS ratio in the vessel, at a rate and to an extent that maximizes methane production and VSR.
The Retort Process
At the temperature of 55° C. water boils under a vacuum of 12 psi relative to standard conditions. In the range between atmospheric pressure (14.7 psia) and 12 psi vacuum (3 psia) the water vapor available to be drawn off by the process increases in a linear fashion and heat is drawn from the process. At the boiling point however, the rapid boiling of the water impedes further redu
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