Liquid purification or separation – Processes – Including controlling process in response to a sensed condition
Reexamination Certificate
2000-05-03
2004-11-02
Barry, Chester T. (Department: 1724)
Liquid purification or separation
Processes
Including controlling process in response to a sensed condition
C210S740000, C210S143000, C073S061680
Reexamination Certificate
active
06811706
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
This invention relates to an apparatus and method for optimizing the performance of biological processes for wastewater treatment. In particular, the invention relates to optimization of the activated sludge process.
A variety of process combinations are used to treat wastewater before it is reused or discharged to the environment. Many of these combinations incorporate the activated sludge process. The activated sludge process was invented in the early 1900s by Ardern and Lockett (Ardem, E. and Lockett, W. T., “Experiments on the oxidation of sewage without the aid of filters,”
Journal of the Society of Chemical Industries,
33, 523, 1914). Today, it is the most commonly used process for treating industrial and municipal wastewaters. The activated sludge process has many variations, including conventional activated sludge, extended aeration, pure oxygen activated sludge, step-feed, sludge reaeration, contact stabilization, and the solids contact step in the trickling filter/solids contact process, etc.
In the activated sludge process, wastewater is introduced to a biological reactor, termed an “aeration tank” or “aeration basin,” that contains a suspended culture of microorganisms, often termed “biomass.” The culture of microorganisms is often termed “activated sludge” and the mixture of the microorganism culture and the wastewater is often called “mixed liquor.” After the wastewater is in contact with the activated sludge for a period of time sufficient for treatment to have occurred, the mixed liquor is discharged to a secondary clarifier in which flocculation and settling of the sludge occurs to form a sludge blanket. During activated sludge treatment, the biomass in the system increases as particulate and soluble organic materials in the wastewater are converted into biomass by the microorganisms. Most of the settled activated sludge, often termed “return activated sludge” or “RAS,” is returned to the aeration tank. A small portion, often termed “waste activated sludge” or “WAS,” is removed from the process.
The performance of the activated sludge process can be significantly affected by the design of its unit operations (e.g., aeration and clarification) and by the manipulation of one or men more of three primary control variables: aeration rate, return activated sludge flow rate and waste activated sludge flow rate. Operation of the activated sludge process is optimized when those independent variables are correctly manipulated within a range of values. Since its inception to approximately the 1960s, the design of the activated sludge process was based primarily on experience, rules of thumb, process loading factors, etc. In the 1970s, the understanding of bacterial growth kinetics greatly advanced a more scientific approach to the design of activated sludge systems. With this knowledge it became apparent—and is now universally recognized—that the growth rate of the microbial culture in the activated sludge process is the design and operational variable of most importance.
In essence, the microbial growth rate of activated sludge is the inverse of the mean cell residence time (known also as the sludge age, solids residence time, MCRT, or SRT). The MCRT is calculated by dividing the mass of microorganisms residing in an activated sludge system [typically taken as the mixed liquor total suspended solids (MLSS) or the mixed liquor volatile suspended solids (MLVSS)] by the mass of microorganisms removed—intentionally in the WAS stream and unintentionally in the secondary effluent steam—from the system per day.
Controlling the activated sludge process, like its design, has evolved since its inception. In the 1960s and 1970s, Alfred W. (Al) West of the U.S. Environmental Protection Agency (USEPA) developed control techniques that were initially based on sludge quality (West, A. W.,
Operational Control Procedures for the Activated Sludge Process, Part
1,
Observations
, Environmental Protection Agency Report No. EPA-330/9-74-001-a, April 1973). Three important elements of Al West's techniques were a settlometer test, a centrifuge spin test, and a measurement of the secondary effluent turbidity. The settlometer test is conducted by collecting a sample of mixed liquor (i.e., the aeration basin contents) as it exits the aeration basin, shaking it, pouring it into a settling container, and recording the solids/liquid interface height (or depth) with respect to time, typically 0, 5, 10, 15, 20, 25, 30, 40, 50, and 60 minutes. The two most commonly used settling containers used are a I-liter (L) graduated cylinder and a 2-L Mallory settlometer. Although any container can be used (any see-through container, e.g., a canning jar), the 2-L Mallory settlometer is the most widely used today and is the container recommended by Al West. An important step in this procedure is plotting the interface height as a function of settling time and noting the general shape of the settling curve to ascertain the sludge's settling characteristics.
The second key procedure to Al West's techniques is the centrifuge spin test. In this test, an aliquot of the mixed liquor sample used in the settlometer test is centrifuged for 15 minutes and the percent volume of the initial aliquot volume occupied by the solids is recorded. He referred to this percentage as the “aeration tank concentration” or “ATC.” The ATC was used as a surrogate for the mixed liquor suspended solids concentration, despite the fact that the ATC is a function of both the solids concentration and the structural integrity of the microbial cells and extracellular materials (e.g., the compressibility of the solids).
Finally, a sample of the secondary clarifier effluent is collected. The turbidity of this sample is measured because turbidity is much more quickly measured than the total suspended solids concentration.
There are at least two limitations of using Al West's procedures to guide operation of the activated sludge process. First, while the compaction characteristics of a sludge are important, nowhere in an activated sludge system are the solids subjected to the multiple gravitational forces occurring in the centrifuge test Activated sludge solids are separated in almost all cases by gravity, so how the sludge compacts by simple settling is more appropriate. Second, the turbidity of the secondary clarifier effluent is confounded by the performance of the secondary clarifier(s). For this reason, it is inappropriate to use secondary clarifier effluent turbidity to quantify the flocculation characteristics of the activated sludge solids. Secondary effluent turbidity is, however, useful for quantifying the performance of the secondary clarifier when compared to the supernatant turbidity after the entering mixed liquor is flocculated and settled.
As the microbial growth kinetic approach to the design of activated sludge processes gained momentum, Al West expanded his approach to process control to take into account biomass growth kinetics. His approach is essentially one of controlling the SRT of the process.
Today, most approaches to activated sludge process control are based on growth kinetics. Essentially, the growth rate is controlled by one of three methods: (1) maintaining a constant SRT (or MCRT or sludge age), (2) maintaining a constant food-to-microorganism (F:M) ratio, or (3) maintaining a constant mixed liquor suspended solids or mixed liquor volatile suspended solids concentration or mass. For a variety of reasons (e.g., operator lack of understanding, influent variability, etc.), this approach is not working well.
The USEPA and others have documented the poor overall performance of the Nation's activated sludge treatment plants. One EPA study (
Environmental Protection Agency Report No. EPA
-600/2-79-034) found that 50 to 90 percent of the plants investigated regularly violate treatment standards. That document also references earlier EPA studies that reported that a third to a half o
Barry Chester T.
Hunter Robert M.
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