Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
2001-06-15
2003-05-27
Barry, Chester T. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S103000, C210S143000, C210S197000
Reexamination Certificate
active
06569335
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to wastewater treatment control methods and devices. Specifically, the present invention is a method and apparatus for controlling activated sludge wastewater treatment systems to optimize the treatment and minimize swings in the quality of the resulting treated effluent.
BACKGROUND OF THE INVENTION
The process using activated sludge to treat wastewater is well known in the art. Briefly stated, activated sludge is a sludge containing living micro-organisms, mainly bacteria. Most wastewater treatment plants, except for the very smallest plants, include a primary treatment stage, typically a settling process, followed by a secondary treatment stage, typically a biological process utilizing an activated sludge process. The primary stage, known in the art as “primary treatment,” usually consists of a tank, or a number of tanks operating in parallel, where flow velocity is slowed to permit settling and consequent separation of settleable solids.
In the second stage of the treatment plant, known in the art as “secondary treatment,” the wastewater is directed to an aeration tank where activated sludge is combined with the wastewater influent to form a suspension or a “mixed liquor.” The micro-organisms of the activated sludge consume and digest suspended and colloidal organic solids in an aerobic process. In this aerobic metabolic process, the micro-organisms break down complex organic molecules into simple waste products, which are, in turn, broken down by other micro-organisms until the micro-organisms can no longer break down the waste products. The micro-organisms in the aeration tank grow and multiply as allowed by the quantities of air and consumable organic solids available.
Continuing with the second stage of the process, the mixed liquor from the aeration tank is directed to a secondary clarifier or secondary sedimentation tank. The effluent is stored to allow solids and suspended activated sludge to settle. The treated wastewater is removed from the secondary clarifier.
Contemporaneously, a portion of the activated sludge, referred to as return activated sludge or RAS, is drawn from the secondary clarifier and returned to the aeration tank to maintain the population of micro-organisms in the aeration tank. That is, a portion of the activated sludge leaving the aeration tank as a suspension in the mixed liquor is returned to the aeration tank from the secondary clarifier to replace some of the activated sludge lost as a suspension in the mixed liquor as well as some of that lost due to death of the micro-organisms in the aeration tank. However, the quantity of RAS does not precisely equal the quantity of sludge required in the aeration tank because the reproduction of micro-organisms in the aeration tank must also be accounted for. Therefore, a certain amount of micro-organisms must be wasted from the system by wasting a portion of the sludge that has settled in the secondary clarifier. The portion of the activated sludge that is wasted is referred to as waste activated sludge or WAS. It is well known in the art that the precise amount of wasted micro-organisms can be difficult to calculate because of varying influent and effluent flow rates and concentration amounts during any particular time period.
As alluded to above, one challenge of controlling the activated sludge treatment process is controlling the flow of WAS to optimize the wastewater treatment process. One known method for controlling the flow of WAS and, therefore, the wastewater treatment process, includes manually grabbing a sample of the mixed liquor, manually analyzing the mixed liquor sample to determine the mixed liquor suspended solids concentration (“MLSS concentration”), and estimating or calculating the WAS flow rate to reach and maintain the desired MLSS concentration. The drawback to this method is that the manual collection and analysis is time consuming and labor intensive. Also, it is subject to errors in sampling and lab analyses. Moreover, if the treatment includes more than one parallel activated sludge process, the collection and analysis must be performed for each process. Also, the time and effort involved in calculating the WAS flow rate is time consuming, especially if it has to be frequently during the day as happens in many treatment plants.
However, even if manual collection and analysis is replaced with instrumentation to allow automatic collection of samples and analysis of data, controlling the treatment process based on the MLSS concentration does not adequately account for variables such as the flow rates and organic loading rates that could be changing at any particular time. For example, it is known that wastewater influent flow rates into the wastewater treatment system increase during morning hours and that organic concentrations also vary during the day. Also, these flows and concentrations change from day to day, week to week, and season to season. Such variables can be significant because swings in flow rate can cause swings in concentrations in the various treatment processes that result in possible swings in the effectiveness in the treatment process. In other words, during such swings, the effluent water leaving the treatment process may not have been treated adequately and may be hazardous and violate regulatory standards.
Therefore, it can be seen that there is a need in the art for a control method and apparatus that allows stable and continuous control of a wastewater treatment process.
SUMMARY OF THE INVENTION
The control method and apparatus of the present invention is directed for an activated sludge wastewater treatment process. More specifically, the present method and apparatus is directed for use with a wastewater treatment process in which wastewater is combined with activated sludge in an aeration tank to form a mixed liquor. The mixed liquor is treated in an aerobic metabolic process and the treated mixed liquor is directed to a clarifier where the suspended activated sludge is allowed to settle out of the mixed liquor. A portion of the activated sludge is returned to the aeration tank (“return activated sludge” or “RAS”) and a portion of the activated sludge is wasted off (“waste activated sludge” or “WAS”) and handled in another portion of the wastewater treatment plant.
According to the present invention, instrumentation for measuring suspended solids concentration is located in the aeration tank and in the WAS flow stream. These instruments measure mixed liquor suspended solids concentration (“MLSS”) and waste activated sludge suspended solids concentration (“WASSS”), respectively. Optionally, the instruments may be a probe and suspended solids analyzer known in the art. In an optional embodiment, the instrumentation takes measurements and transmits data every second.
The MLSS instrumentation and the WASSS instrumentation communicate with a data processor having at least one data structure. The data structure stores algorithms along with constant values including the volume of the aeration tank (V
AerationTank
) and unit conversion constants. An input device also communicates with the data processor.
In use, the user inputs a desired solids retention time (SRT
Desired
) to Optimize the treatment process. The data processor utilizes data gathered from the MLSS instrumentation and the WASSS instrumentation to calculate the desired flow rate of the WAS according to the following formula:
WAS
flow
rate
=
MLSS
×
V
AerationTank
SRT
Desired
×
WASSS
While the MLSS and WASSS measurements used could be instantaneous measurements, in an optional embodiment, average or mean MLSS and WASSS values calculated over a set predetermined period of time, such as twenty-four hours, may be used. Such a period of time is optimized to reflect the effects of control adjustments.
In an optional embodiment, the processor merely outputs the WAS flow rate calculated to achieve the desired SRT to a display. However, in an alternate embodiment, the data processor communicates with WAS
Anderson & Morishita
Barry Chester T.
Morishita Robert Ryan
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