Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
2001-09-06
2002-12-03
Simmons, David A. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S626000, C210S629000, C210S630000, C210S631000, C210S195300, C210S202000, C210S206000, C210S221200, C210S259000
Reexamination Certificate
active
06488854
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates generally to the field of treatment of wastewater, and more particularly to an improved system and method for treating wastewater containing contaminants which system and method offer a broad array of advantages over conventional activated sludge wastewater treatment systems, including smaller size, higher rates of operation, high oxygen transfer efficiency, lower operating costs, and a decreased level of excess sludge production.
With an increased awareness of problems with water quality, particularly those caused by the discharge of wastewater from industrial sources, has come a demand for improved equipment and methods to treat wastewater prior to discharging it into a sewer, as surface water, or to other destination for effluent discharge. While such treatment systems and methods are generally not required to produce potable water, they are increasingly required by law to enhance the quality of wastewater prior to discharging it as effluent. For industrial waste, this treatment process must typically remove certain type of pollutants such as organic contaminants, nitrogen and phosphorus, metals, and suspended solids.
The first wastewater treatment systems were of simple design, with a single container or tank being used for both treatment of the wastewater and the removal of solids from the wastewater, typically by allowing them to settle out. These early wastewater treatment systems were not aerated, and typically generated foul odors as a byproduct of the process utilized by these systems. Over time, these early wastewater treatment systems evolved into systems which use a popular type of wastewater treatment process referred to as the activated sludge wastewater treatment method.
The activated sludge wastewater treatment system and method use an aeration tank which is followed by a solid/liquid separator which acts as a secondary clarifier to remove separated solids from the liquid, which is discharged by the system. As its name suggests, the contents of the aeration tank are aerated and mixed to facilitate an aerobic reaction (a reaction taking place in the presence of oxygen) which is facilitated by the presence of activated sludge. This activated sludge, which is an accumulation of microorganism-rich residue contained in the solids which are separated from the liquid in the solid/liquid separator, is seeded into the incoming wastewater in the aeration tank. In conventional activated sludge wastewater treatment systems, the concentration of activated sludge solids is typically 2,000 to 5,000 milligrams per liter in the aeration tank.
The aerobic reaction which takes place in the aeration tank includes three types of phenomena—absorption, adsorption, and biological digestion. Absorption takes place when a contaminant is absorbed into the cell wall of the bacteria contained in the activated sludge. Adsorption, on the other hand, is a surface phenomenon which takes place when there is an interaction between a contaminant and the surface of the activated sludge whereby the contaminant adheres to the surface of molecules of the bacteria. Any one of these three phenomena will result in contaminants reacting with the bacteria contained in the activated sludge. Biological digestion takes place when the bacteria contained in the activated sludge consume waste constituents contained in the wastewater. Biological digestion can occur after the material has been absorbed or adsorbed.
As mentioned above, the reaction is an aerobic reaction occurring in the presence of oxygen, which decreases both the amount of time required for the reaction to occur and the level of foul odors produced by the reaction. Typically, the aeration and mixing may be produced by injecting compressed air or oxygen into the mixture, typically through diffuser devices located near the bottom of the aerator tank. As the air bubbles to the surface of the mixture, the diffused air provides both oxygen to the mixture and a vigorous mixing action. The amount of material contained in the wastewater may be characterized by the “chemical oxygen demand” or COD of the material. A chemical oxygen demand of one pound indicates that the material contained in the wastewater requires one pound of oxygen to degrade.
Air may also be added by the churning action of mechanical mixers located near the surface of the mixture contained in the aeration tank. In still another variation, mixing of the contents of the aeration tank may be caused by hydraulic pumping in which liquid is pumped out of the tank and back in through nozzles causing highly efficient mixing of the contents of the aeration tank. In a still further variation, air nozzles may be arranged around the liquid nozzles to further stimulate the mixing and simultaneously provide oxygen to the mixture. Still further variations include processes known as extended aeration and contact stabilization, both of which omit the primary settling step, and high-purity oxygen aeration, which can substantially reduce both the aeration time and the size of the aeration tank.
The conditions which are thus provided in the aeration tank promote the growth of the microorganisms introduced in the activated sludge with the resultant reaction removing contaminants from the wastewater. In conventional activated sludge technology, a predetermined period of time related to the strength of the wastewater and treatment objectives is required for the mixture to react in the aeration tank in a batch flow process. This time is required to allow the bacteria in the aeration tank to react with the contaminants contained in the wastewater, with much of the material being oxidized by the microorganisms. Generally, in conventional activated sludge processes, the contaminants are completely digested in the aeration tank.
The mixture is then allowed to flow from the aeration tank into the solid/liquid separator, which can be any of a number of different mechanical devices, all of which are well known in the art. The solid/liquid separator may be as simple as a secondary clarifier, which allows activated sludge to settle out by gravity. The clean liquid then overflows from the clarifier and it is discharged as secondary effluent, while the activated sludge may be separated out in a settling tank. The bacteria will tend to clump together and settle to the bottom of the settling tank, from which the activated sludge may be pumped out.
Some of the activated sludge will be recirculated back into the aeration tank, with this sludge being referred to as “return activated sludge” or RAS. The microorganisms contained in the return activated sludge are thus well acclimated to the environment in the aeration tank. The remaining activated sludge is treated and disposed of in a conventional solids processing technique which is well known to those skilled in the art. This sludge is referred to as “waste activated sludge” or WAS. In conventional activated sludge technology, the waste activated sludge may amount to as much as seventy percent of the sludge recovered in the solid/liquid separator.
The amount of excess activated sludge which is generated by an activated sludge waste treatment system may be controlled by a term referred to as “solid retention time” or SRT, which is the amount of time an average particle of solid material remains in the waste processing system. The solid retention time is inversely proportional to the relative volume of excess activated sludge which must be disposed of. Conventional extended activated sludge waste processing systems (designed for surface water discharge of effluent) have a solid retention time of approximately twenty days.
The excess solids produced may be determined by the yield of the activated sludge process multiplied by the mass of the contaminants removed. The yield may be measured in units of pounds of “chemical oxygen demand” or COD, which is a term commonly used to measure the amount of contaminants which are removed. Conventional extended activated sludge waste treatment systems produ
O'Leary Kevin P.
Probst Henry J.
Probst Thomas H.
Wang Jenchie
Wilson David A.
Prince Fred
Procorp, Inc.
Reinhart Boerner Van Deuren S.C.
Simmons David A.
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