CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 61/388,585, fiied September 30, 2010, which is incorporated herein by reference in its entirety.

FIELD OF INVENTION

[0002] The present disclosure relates to methods for operating wet air scrubbers.

BACKGROUND

[0003] Rendering facilities can produce emissions in the form of volatile organic compounds (VOCs) as a result of their operations. Pollutants such as organic sulfides and disulfides (e.g., dimethyl disulfide), thiols (e.g., methane thiol), aldehydes (e.g., hexanal, 2-methy!butanal, and 3-methyibutanal), alcohols, ketones, amines, aliphatic hydrocarbons (e.g., octane), aromatic compounds (e.g., quino!ine and pyrazines) and organic acids are commonly produced. These VOCs are generally considered an odor nuisance when rendering facilities are in close proximity to residential areas, and so emission control measures are generally directed toward odor elimination. Air scrubbers may be employed at rendering facilities as one method of emission control and odor elimination.

SUMMARY

[0004] In an aspect the disclosure provides a method of operating a wet air scrubber, the method comprising: measuring at least one of water soil level in the wet air scrubber, amount of water discarded from the wet air scrubber, and contaminant level of an air stream entering and/or leaving the wet air scrubber; comparing at least one of the measured water soil level, the amount of water discarded, and the air stream contaminant level to at least one of a target water soil level, a target amount of water discarded, and a target air stream contaminant level; and adjusting the amount of makeup water added to the wet air scrubber when there is a difference between at least one of the measured water soil level, the measured amount of water discarded, and the measured air stream contaminant level and at least one of the target water soil level, the target amount of water discarded, and the target air stream contaminant level.

[0005] In another aspect the disclosure provides a method of reducing the contaminant levels in an air stream leaving a wet air scrubber comprising detecting at least one of the water soil ievel in the wet air scrubber and the contaminant Ievel of an air stream leaving the wet air scrubber; comparing the at least one of the water soil Ievel in the wet air scrubber and the contaminant level of the air stream leaving the wet air scrubber to at least one of a target water soil Ievel and a target air stream contaminant Ievel, the target level being lower than the detected level; and increasing the amount of makeup water added to the wet air scrubber to an amount effective to reduce the contaminant levels in the air stream.

[0006] In a further aspect the disclosure provides a method of improving the operating efficiency of air scrubber maintenance comprising: detecting at least one of the water soil Ievel in the wet air scrubber and the contaminant Ievel of an air stream leaving the wet air scrubber; comparing the at least one of the water soil level in the wet air scrubber and the contaminant Ievel of the air stream leaving the wet air scrubber to at least one of a target water soil level and a target air stream contaminant Ievel; and decreasing the amount of makeup water added to the wet air scrubber when at least one of the target water soil level and the target air stream contaminant Ievel is greater than the detected !evel of the at least one of the water soil Ievel in the wet air scrubber and the contaminant Ievel of an air stream leaving the wet air scrubber.

[0007] In some embodiments of the above aspects, the measuring comprises determining chemical oxygen demand of water in the wet air scrubber. In some

embodiments the measuring comprises determining the amount of water discarded using a water flow meter or water flow sensor. In some embodiments the measuring comprises detecting a volatile organic compound (VOC) in the air stream entering and/or leaving the wet air scrubber.

[0008] Other aspects and embodiments of the disclosure will become apparent in light of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 is a schematic diagram of a wet air scrubber,

[0010] FIG. 2 shows an embodiment of a rendering process connected to an air scrubber and sensor placement.

[0011] FIG. 3 is a graph of chemical oxygen demand (COD) as a function of conductivity of the water stream in a wet air scrubber.

[0012] FIG. 4 is a graph of COD as a function of total suspended solids (TSS) of the water stream in a wet air scrubber. [0013] FIG. 5 is a graph of COD as a function of oxidation-reduction potential (ORP) of the water stream in a wet air scrubber.

[0014] FIG. 6 is a graph of COD as a function of conductivity, TSS, and ORP of the water stream in a wet air scrubber, along with the accompanying function describing the relationship among the properties: COD(mg/L) = 0.0588 ^■ Cond( S/cm) + 1.070 ^■ TSS(ppm) + 0.3624 · ORP(mv) - 332.1.

DETAILED DESCRIPTION

[0015] The disclosure provides a method of operating a wet air scrubber in which the amount of makeup water added to the wet air scrubber is controlled in relation to at least one of 1) the soii level in the wet air scrubber; 2) the amount of water discarded from the wet air scrubber; and 3) the contaminant levels of air streams entering and/or leaving the wet air scrubber.

[0016] Before any aspects and embodiments are explained in detail, it is to be understood that the claims are not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The aspects and embodiments detailed herein can be practiced or carried out in various ways that will be apparent to one of ordinary skill in the art.

[0017] It also is specifically understood that any numerical value recited herein includes all values from the lower value to the upper value, i.e., all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application. For example, if a concentration range or a beneficial effect range is stated as 1% to 50%, it is intended that values such as 2% to 40%, 10% to 30%, or 1% to 3%, etc. are expressly enumerated in this specification. These are only examples of what is specifically intended.

[0018] In general, wet air scrubbers can remove pollutants or contaminants such as VOCs from a process air or gas stream by moving the air stream through an aqueous cleaning solution including one or more treatment chemicals. After passing through the aqueous cleaning solution, the air stream generally emerges without the pollutants (or at least with fewer or lesser amounts of the pollutants) that were present in the air stream before treatment in the air scrubber. Nevertheless methods that provide for more efficient operation of a wet air scrubber (e.g., VOC removal from an air stream) would be of benefit at least in that such methods would provide a cost savings and reduced environmental impact. [0019] A typical wet air scrubber is operated at a fixed level of makeup water, while the contaminant level of the air stream being scrubbed can vary widely. In order for VOGs to be efficiently removed from the air stream, it may be beneficial to control the amount of makeup water added to the wet air scrubber based on at Ieast one of the following measurements: 1) the soil level in the wet air scrubber; 2) the amount of water discarded from the wet air scrubber; and 3) the contaminant levels of air streams entering and/or leaving the wet air scrubber. By indexing makeup water addition to soil and/or contaminant levels, odor excursions from the wet air scrubber may be better controlled and water can be used more efficiently in operation of the wet air scrubber.

[0020] Accordingly, in a broad sense, the disclosure provides a method of operating a wet air scrubber comprising: measuring at Ieast one of water soil level in the wet air scrubber, amount of water discarded from the wet air scrubber, and contaminant level of an air stream entering and/or leaving the wet air scrubber; comparing at Ieast one of the measured water soil level, the amount of water discarded, and the air stream contaminant level to at Ieast one of a target water soil level, a target amount of water discarded, and a target air stream contaminant level; and adjusting the amount of makeup water added to the wet air scrubber when there is a difference between at Ieast one of the measured water soil level, the measured amount of water discarded, and the measured air stream contaminant level and at ieast one of the target water soil level, the target amount of water discarded, and the target air stream contaminant level.

Wet Air Scrubber

[0021] Referring now to the drawings, FIG. 1 is a schematic view of one embodiment of a high-intensity wet air scrubber suitable for use in removing VOCs from an air stream. FIG. 2 is a schematic view of one embodiment of a wet air scrubber 14. Referring to FIG. 2, the wet air scrubber 14 can include, for example, a packed tower scrubber, a spray tower scrubber, an orifice scrubber, a venturi scrubber, a fiber-bed scrubber, an impingement-plate scrubber, a spray nozzle scrubber, a fiuidized-bed scrubber, a packed-bed scrubber, multiple-stage scrubbers, baffle spray scrubber, a counter-flow scrubber, a crossflow scrubber, and combinations thereof. Wet air scrubbers can be custom designed by, for example, an independent designer or competent staff at a rendering facility. Wet air scrubbers are commercially available, for example, from Verantis, AC Corporation, Sep Control Inc., and Millpoint Industries inc. The wet air scrubber 14 receives a process air stream 12 contained in a first conduit 3 (i.e., pipe) from a rendering facility 10 according to the present application. The rendering facility 10 may be, for example, an integrated rendering plant (e.g., a plant that operates in conjunction with an animal slaughterhouse or poultry processing plant), or an independent rendering plant (e.g., a plant that collects raw materials from a variety of offsite sources). Raw materials can include, for example, at least one of whole animal carcasses, animal parts, blood, grease, feathers, offal, and

combinations thereof. Offsite sources can include sources such as, for example,

restaurants, butcher shops, slaughterhouses, farms, feedlots, animal shelters, and combinations thereof. The rendering facility 10 can operate edible and/or inedible rendering processes. The wet air scrubber 14 can include a pump 22 that circulates a water stream 23 through a second conduit 25 from a second outlet 24 to a second inlet 26 of the wet air scrubber 14.

Measurement of Soil Level in the Wet Air Scrubber

[0022] Soil level as used herein refers to the level of pollutants. As used herein, pollutants and contaminants may be used interchangeably. Pollutants and contaminants can include byproducts from any number of industrial processes such as, for example, VOCs and solid or aerosolized organic materials including byproducts from animal processing (e.g., fat, oil, grease, bone meal, and protein). Such processes can be used to convert inedible or edible animal parts into products suitable for consumption by humans, animals (e.g., livestock), or pets, or for industrial products such as, for example, lubricants, detergent, soap, cement, ink, lipstick, and pharmaceuticals.

[0023] The soil level in a wet air scrubber can be determined by any suitable method that can correlate at least one measurable property of the water or air stream when entering, residing within, or leaving the air scrubber to the relative amount of soil in the stream. Some non-limiting examples of measurable properties include, but are not limited to, chemical oxygen demand (COD), total suspended solids (TSS), pH, turbidity, oxidation-reduction potential (ORP), ammonia concentration, conductivity, inductivity, biological oxygen demand (BOD), hydrogen sulfide concentration, or other VOC concentration.

[0024] In some embodiments, measurement of the soil level in a wet air scrubber can include determination of COD of the water stream 23. COD is commonly expressed in milligrams of oxygen consumed per liter of solution (mg/L), and a COD test can be used to measure indirectly the amount of VOCs present in the water stream. The COD levels in the wet air scrubber may depend on the organic loading of the gases entering the wet air scrubber. The organic loading of the gases entering the wet air scrubber may fluctuate over time due to, for example, the amount and/or composition of raw materials in the cooker and the VOCs and aerosolized organic materials such as fat, oil, grease, bone meal, and protein emitted by the cooker during operation. [0025] The COD test is based on the premise that under acidic conditions and in the presence of a strong oxidizing agent, nearly a!! organic compounds can be fully oxidized to form carbon dioxide, water, and ammonia according to the following equation:

C _n H _a ObN _c + (n + a/4 - b/2 - 3c/4) 0 ₂ → nC0 ₂ + (a/2 - 3c/2) H ₂ 0 + cNH ₃ (1 )

Because equation (1) does not account for the oxygen demand due to nitrification, a second equation:

NH ₃ + 20 ₂ → N0 ₃ ^" + H ₃ 0 ⁺ {2} can be applied after equation (1) to account for oxidation due to nitrification. When the COD of the recirculating water of a wet air scrubber exceeds about 1 ,500 mg/L, it is likely that odor release from the wet air scrubber may exceed permitted levels. Accordingly, the amount of makeup water can be increased in order to reduce the COD to about ,500 mg/L or less, and thereby the soil level. Similarly, the amount of makeup water can be decreased once at least one of the measured water soil level, amount of water discarded from a wet air scrubber, or contaminant level of an air stream entering or leaving the wet air scrubber is at or below a target value for water soil level, amount of water discarded from a wet air scrubber, and/or contaminant level of an air stream entering or leaving the wet air scrubber.

[0026] The COD of the water stream 23 may be determined by on-site measurement or by sending a sample to an off-site laboratory for analysis. The COD test typically involves addition of oxidant solution to a sample followed by titrimetric or spectrophotometric deiermination of the amount of oxidant remaining in the sample after the oxidant has reacted with organic compounds in the sample. Oxidizing agents can include, without limitation, potassium dichromate, eerie sulfate, and potassium iodate.

[0027] In some embodiments, an approximate COD level may be determined using other measured values (e.g., TSS, ORP, pH, temperature, etc.) as a proxy for the COD value. Suitably, a mathematical relationship exists between easily measured values (TSS, ORP, pH, temperature, etc.) and COD, as shown in Example 1. Thus the approximate COD level can be known in real time, allowing a control system to respond quickly to changes in COD, and, in soil level. For example, if the soil level increased, the COD would necessarily increase, the sensors would send their signals to a controller-microprocessor, the proxy value of COD would be detected, and the flow of water in and/or out of the wet air scrubber could be adjusted, the chemical set point could be adjusted, and odors could be controlled in the rendering vicinity. [0028] In some embodiments, the water stream 23 may have at least one property, such as, for example, chemical composition, ORP, pH, turbidity, conductivity, and combinations thereof, that can be monitored with one or more sensors. For example, one or more ion- selective electrodes can be used to determine the chemical composition of the water stream 23. Ion-selective electrodes are commercially available from Advanced Sensor

Technologies, Inc., under the trade name IOTRON®. Other sensors that can be used in embodiments of the present application include, for example, ORP sensors, pH sensors, turbidity sensors, conductivity sensors, ultraviolet light detectors, and combinations thereof, some of which are described above. ORP sensors are commercially available from

Walchem Corporation under the trade name WEL pH/ORP Electrodes. pH sensors are commercially available from Walchem Corporation under the trade name WEL pH/ORP Electrodes. Turbidity sensors are commercially available from optek-Danulat, Inc.

Conductivity sensors are commercially available from Sensorex Corporation. Ultraviolet light detectors are commercially available from optek-Danulat, Inc. The at least one sensor may be positioned in the system, such that it can monitor and measure at any desired point in the water stream 23. For example, the at least one sensor may be positioned in the second conduit 25 through which the water stream 23 flows. The water stream 23 can pass through a sensor manifold 28. The sensor manifold can include at least one sensor, such as, for example, an ion-selective electrode, an ORP sensor, a pH sensor, a turbidity sensor, a conductivity sensor, and combinations thereof.

Measurement of the Amount of Water Discarded from the Wet Air Scrubber

[0029] Referring to FIG. 2, the wet air scrubber 14 typically includes makeup water 18 that is introduced to the interior of the wet air scrubber 14 at a first inlet 19. A drain 20 may collect fluid leaving the wet air scrubber 14 from a first outlet 21. The makeup water 18 can include water from at least one of a municipal water supply, a river, a pond, an industrial water supply, recovered from cooling tower blow-down, RO reject water, water from a cleaning process in an animal processing plant, storm runoff water, filter back-flush water, the drain 20 of another air scrubber, and combinations thereof.

[0030] Water in the wet air scrubber may be discarded to, for example, a retention pond or sewer. Measurement of the amount of water discarded can be made by using, without limitation, a water flow meter or water flow sensor. In some embodiments, one or more flow sensors may be used to measure the rate of makeup water 18 entering the wet air scrubber 1 and/or to measure the rate of water discarded from the wet air scrubber 14. In some embodiments, the flow of water may be measured in the conduit between the outlet 21 and the drain 20. Examples of flow sensor may include, without limitation, a mass flow sensor, a volumetric flow sensor, a velocity flow sensor, and combinations thereof. Flow sensors are commercially available from Ryan Herco under the trade name SIGNET® 2537.

Measurement of the Contaminant Levels of Air Streams Entering and/or Leaving the Wet Air Scrubber

[0031] Referring to FIG. 2, the air stream 12 can be collected from one or more sources in the rendering facility 10, such as, for example, from receiving bins, continuous cookers, drainers, screw presses, centrifuges, filters, processing rooms, and combinations thereof. The air stream 2 may have at least one property, such as, for example, chemical composition and temperature, which can be monitored with one or more sensors, for example, before the air stream 12 enters the wet air scrubber 14.

[0032] The one or more sensors may be positioned anywhere in the air stream 12. For example, the one or more sensors may be positioned in the first conduit 13 through which the air stream 12 flows.

[0033] Additionaliy, one or more sensors can be used to measure properties of the air stream 2 at or near its source in the rendering facility 0 {e.g., the continuous cooker, the screw press, etc.), or at a location distant from its source.

[0034] For example, one or more VOC sensors can be used to determine the chemical composition of the air stream 12. Examples of a VOC sensor include, without limitation, an organic sulfide sensor, an organic disulfide sensor, an aldehyde sensor, an alcohol sensor, a ketone sensor, an amine sensor, an aliphatic hydrocarbon sensor, an aromatic hydrocarbon sensor, an organic acid sensor, and an infrared sensor. These sensors are commercially available from Advanced Sensor Technologies, Inc., under the trade name IOTRON and from App-Tek Safety Pty Ltd under the trade name ODALOG®.

[0035] Examples of temperature sensors include, without limitation, a contact sensor, a non-contact sensor, a thermocouple-style sensor, and a resistance temperature detector. Temperature sensors can also be employed to measure, for example, the temperature of solid and/or liquid materials in a rendering process, such as, for example, the contents of continuous cookers and screw presses. These sensors are commercially available from Lesman Instrument Company under the trade names AMETEK®, PYRO AT!ON®, HONEYWELL®, and SIEMENS®.

[0036] In some embodiments, the process air stream may pass through a venturi scrubber 16, such as are known in the art, before entering the wet air scrubber 14. The venturi scrubber 16 can be, without limitation, an ejector venturi, an eductor venturi, a wetted-throat venturi, a non-wetted throat venturi, a rectangular-throat venturi, an adjustable- throat venturi with plunger, an adjustable-throat venturi with movable plate, and a venturi rod scrubber. Venturi scrubbers are commercially available from Anco-Eaglin, inc., Verantis, AC Corporation, Sep Control Inc., and Mii!point Industries Inc.

[0037] The magnitude of one or more of the measured properties of the solid and/or liquid materials in the rendering facility 10, the process air stream 12, and or the water stream 23 can be compared to a target value or magnitude of the property, and the amount of makeup water can be adjusted as needed or as desired in response to the difference between the values. Examples of some target levels include values of TSS, ORP, pH, temperature, turbidity, ammonia concentration, conductivity, inductivity, biological oxygen demand (BOD), hydrogen sulfide concentration, or other VOC concentration that correlate to a chemical oxygen demand (COD) of about 1 ,500 mg/L. Such correlations to COD can be made by any suitable method described herein or as otherwise known in the art. In some embodiments the target level for pH is about 7.5 (e.g., 7.0, 7.1 , 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9). In some embodiments the target level for temperature is 120°F or less (e.g., 120, 119, 118, 117, 116, 115, 114, 113, 112, 1 11 , 1 10, 105, 100, 95, or 90°F or less).

[0038] In some embodiments, the amount or flow rate of makeup water is increased, such as, for example, when the measured property exceeds the target value of the same property by at least about 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%, or at least about 25% or more. It will be appreciated that the foregoing numbers represent the lower range of the differences in measured and target values that may be determined and that, in some circumstances, the measured value can exceed the target value by very large amounts (e.g., 100% and above). The amount or flow rate of makeup water may be increased by an amount that is effective to decrease the total difference between the measured value of the property and the target value of the property. In some embodiments the amount or flow rate of makeup water is increased, at least about 20%, at least about 30%, at least about 40%, at least about 50%, at least about 60%, at least about 70%, at least about 80%, at least about 90%, at least about 00%, at least about 125%, at least about 150%, or at least about 200% or more.

[0039] In some embodiments, the amount or flow rate of makeup water is decreased, such as, for example, when the measured property is less than the target value of the same property by at least about 5%, at least about 0%, at least about 15%, or at least about 20%. As noted above, in some embodiments wherein the measured property is well below the target value (e.g., 100% or more) the amount of makeup water can be sharply decreased. In some embodiments, the amount or flow rate of makeup water can be decreased by, for example, at least about 20%, at least about 30%, at least about 40%, at least about 50%, at least about 60%, or at least about 70% or more.

Control of Makeup Water Addition to the Wet Air Scrubber

[0040] To determine how much makeup water to add to the air scrubber during operation, a variety of mechanisms known in the art may be used. For example, by measuring one or more of the soil level in the wet air scrubber, the amount of water discarded from the wet air scrubber, and the contaminant levels of air streams entering and/or leaving the wet air scrubber, a control circuit can be used to control the addition of makeup water to the air scrubber water to ensure that the concentration of contaminants in the water of the wet air scrubber does not exceed the level associated with odor release.

[0041] In one embodiment, a monitoring system and control loop may directly measure, for example, a chemical concentration in the recirculating water of the wet air scrubber, and provide a millivolt signal back to a dual channel controller. An ion-selective electrode (ISE) may be connected to a controller (for example, Rosemount model 1056), which may supply a 4-20 mA signal to a makeup water pump controller. The pump controller may manage the pump frequency and duration of makeup water fed into the wet air scrubber. This two channel controller may be configured to show, for example, a first ISE on one channel, and a second ISE on the other, or may be connected to two independent ISE sensors. As such, makeup water may be fed into the air scrubber based on feedback from an ISE monitoring and control feedback system. Thus, as described above, if the concentration of the measured chemical in the recirculating water is high relative to a target value, then more makeup water may be added to the air scrubber. Similarly, if the concentration of the measured chemical in the recirculating water is low relative to a target value, then less makeup water may be added to the wet air scrubber.

[0042] In another embodiment, a monitoring system and control loop may be directly measure, for example, a chemical concentration in the incoming and/or outgoing air stream of the wet air scrubber, and provide a millivolt signal back to a dual channel controller. A sensor may be connected to a controller (for example, Rosemount model 1056), which may supply a 4-20 mA signal to a makeup water pump controller. The pump controller may manage the pump frequency and duration of makeup water fed into the wet air scrubber. This two channel controller may be configured to show, for example, a first sensor on one channel, and a second sensor on the other, or may be connected to two independent sensors. As such, makeup water may be fed into the air scrubber based on feedback from a sensor monitoring and control feedback system, if the concentration of the measured chemical in the incoming and/or outgoing air stream of the wet air scrubber is high relative to a target value, then more makeup water may be added to the air scrubber. If the

concentration of the measured chemical in the incoming and/or outgoing air stream of the wet air scrubber is low relative to a target value, then less makeup water may be added to the wet air scrubber.

[0043] In another embodiment, a monitoring system and control loop may directly measure, for example, the flow rate of water out of the wet air scrubber, and provide a millivolt signal back to a dual channel controller. A flow sensor may be connected to a controller (for example, Rosemount model 1056), which may supply a 4-20 mA signal to a makeup water pump controller. The pump controller may manage the pump frequency and duration of makeup water fed into the wet air scrubber. This two channel controller may be configured to show, for example, a first flow sensor on one channel, and a second flow sensor on the other, or may be connected to two independent flow sensors. As such, makeup water may be fed into the air scrubber based on feedback from a flow sensor monitoring and control feedback system. Similar to the embodiments describing chemical concentrations above, if the flow out of the wet air scrubber is high, then more makeup water may be added to the wet air scrubber. If the flow out of the wet air scrubber is low, then less makeup water may be added to the air scrubber.

[0044] in some embodiments, the makeup water pump controller can respond to signals from one or more sensors detecting soil level in the wet air scrubber, the amount of water discarded from the wet air scrubber, the contaminant levels of air streams entering and/or leaving the wet air scrubber, and combinations thereof. In some embodiments, the water pump controller can respond to the signals by increasing the amount of makeup water flowing into the wet air scrubber. In some embodiments, the water pump controller can respond to the signals by decreasing the amount of makeup water flowing into the wet air scrubber.

[0045] In some embodiments, the makeup water pump controller can respond to signals from one or more sensors when the sensors indicate (or correlate to) a measured chemical oxygen demand that exceeds a target value for chemical oxygen demand of water in the wet air scrubber selected from at least about 750 mg/L, at least about 1,000 mg/L, at least about 1 ,500 mg/L, at least about 1 ,750 mg/L, or at least about 2,000 mg/L. EXAMPLES

Example 1. Relationships among Air Scrubber Properties

[0046] Prior to commercial start-up in rendering facilities, audit data were collected to determine the state of soil loading in the prospective customer's air scrubber. Referring to FIG. 2, the audit data that were collected included water stream 23 temperature, air stream 12 temperature, water stream 23 pH, water stream 23 conductivity, TSS of water stream 23, COD of water stream 23, and ORP of water stream 23.

[0047] The data recorded in the air scrubber start-up survey were typically measured in 5-6 measurements at each scrubber over a day. Measured properties included water in sump temperature (N=455), air temperature into scrubber (N=101), pH (N=481), conductivity in uS/cm (N=449), total suspended solids in ppm (N=414), COD in mg/!iter (N=461), and ORP in mV (N=53).

[0048] Historical audit data were analyzed using statistical techniques to discover trends and relationships among the aforementioned measured quantities. Specifically, a statistically significant relationship among the easily-measured, real-time quantities of temperature, pH, conductivity, TSS, and ORP, and the difficult-to-measure quantity COD, was the desired outcome.

[0049] Relationships were discovered using historical audit data. COD as a function of conductivity is shown in FIG. 3. COD as a function of TSS is shown in FIG. 4. COD as a function of ORP is shown in FIG. 5. COD as a function of conductivity, TSS, and ORP with the accompanying function is shown in FIG. 6.