WO2010077767A1

WO2010077767A1 - Oxygen monitoring system and method for determining an oxygen density load of a fluid

Info

Publication number: WO2010077767A1
Application number: PCT/US2009/067525
Authority: WO
Inventors: Chad Bertram; Elmar Grabert
Original assignee: Hach Company
Priority date: 2009-01-02
Filing date: 2009-12-10
Publication date: 2010-07-08

Abstract

An oxygen monitoring system (200) for determining an oxygen density load of a fluid is provided. The oxygen monitoring system (200) includes an oxygen demand sensor (205) for measuring an oxygen demand of the fluid, a flow meter (210) for measuring a flow rate of the fluid, and a processing system (220) coupled to the chemical oxygen demand sensor (205) and to the flow meter (210). The processing system (220) is configured to receive the oxygen demand, receive the flow rate, and determine the oxygen density load of the fluid from the oxygen demand and the flow rate.

Description

OXYGEN MONITORING SYSTEM AND METHOD FOR DETERMINING

AN OXYGEN DENSITY LOAD OF A FLUID

Background of the Invention

1. Field of the Invention

The invention is related to the field of water treatment, and more specifically, to an oxygen monitoring system and method for determining an oxygen density load of a fluid.

2. Statement of the Problem

Water quality, whether surface, well, or aquifer water, or water that has been treated or processed, is an area of great importance. The water quality may be measured in order to determine the safety and drinkability of the water. Water quality testing can be performed to determine the condition of municipal water systems, including water treatment and/or water supply systems. The testing can be performed in order to ensure water quality and safety, water treatment efficacy, compliance with regulations and standards, etc. The water quality testing can be performed in order to measure and assess incoming water quality, outgoing water quality, and treatment effectiveness.

The water quality testing can test for organic molecules, chemical contaminants, suspended solids, etc. For example, the water quality may be measured in order to determine the presence and level of biological materials, including algae and bacteria, among others. The water quality may be measured in order to determine the presence and level of pollutants or contaminants. The water quality may be measured in order to detect water tampering or poisoning, such as acts of terrorism or sabotage.

The oxygen content is one water characteristic of interest. The oxygen content can be used to determine an organic content of the water and may be used to determine a microorganism load, for example. The oxygen content can also be used to detect pollutants and other undesired materials. Oxygen demand tests are therefore widely used.

The oxygen demand test has its widest application in measuring waste loadings of treatment plants and in evaluating the efficiency of treatment processes. Other applications include testing lake and stream water samples for organic pollution. For example, as microorganisms in the water consume organic material, oxygen is depleted. This can have an adverse effect of fish and plant life. In addition, the oxygen content measurement can be combined with other water quality measurements in order to obtain a more complete water quality assessment.

Oxygen demand testing generally does not determine the concentration of a specific substance. Instead, oxygen demand testing measures the effect of a combination of substances and conditions.

There are three widely used methods of measuring oxygen demand. The total organic carbon (TOC) test measures oxygen demand indirectly. Conversely, the biochemical (or biological) oxygen demand (BOD) and chemical oxygen demand (COD) tests measure oxygen demand directly. The total organic carbon (TOC) test was an early measurement process. The

TOC test uses heat, ultraviolet (UV) light, and a strong chemical oxidant (or a combination of these three factors) to oxidize organic material into CO₂ and H₂O. The carbon may be present in the form of foreign material, including biological materials. Oxygen demand is measured indirectly by determining the amount of CO₂ produced using infrared (IR) spectroscopy, conductivity, or coulometry (an electrochemical technique).

The TOC test has drawbacks. The TOC test can take a long time to complete. The TOC test produces less useful information than that obtained from BOD or COD analyses. Also, the TOC test does not differentiate between compounds with the same number of carbon atoms in different stages of oxidation and will thus produce different oxygen demand results.

Another oxygen content measurement is the biochemical (or biological) oxygen demand (BOD) test. In the BOD test, microorganisms consume organic compounds for food while consuming oxygen at the same time. The standard BOD test measures the amount of oxygen consumed in a sample over a five-day period, typically by measuring the dissolved oxygen in the water before and after the test.

The BOD test has drawbacks. Due to the length of time required to complete the test, the results provide historical data only. The BOD test cannot facilitate rapid water quality assessment or optimal process control. The BOD test is limited in some applications, such as industrial wastewaters, which often contain heavy metal ions, cyanides, and other substances toxic to microorganisms. Consequently, when the microorganisms become poisoned by toxic substances, they are unable to oxidize waste.

In such a scenario, the BOD test is ineffective for the measure of organic pollution. The COD test was developed more recently than the TOC or BOD tests. The basis of the COD test is that nearly all organic compounds can be oxidized when a strong oxidizing agent is used under properly acidic conditions. The COD test uses a strong oxidizing agent in an acid solution and with heat in order to oxidize organic carbon into CO₂ and H₂O. The oxidizing agent commonly comprises potassium permanganate or potassium dichromate, for example. Oxygen demand is determined by measuring the amount of oxidant consumed using titrimetric or photometric methods.

The COD result is commonly expressed in milligrams per liter (mg/L) and indicates the mass of oxygen consumed per volume of water. Advantageously, test data is typically available in one and one half to three hours, providing relatively fast water quality assessment and process control.

However, the traditional COD test has drawbacks. The oxidization of COD systems is not complete. Some organic compounds cannot be completely or even partially oxidized by using known COD oxidizing agents. High levels of inorganic materials in water may interfere with the determination of COD. Because of its high concentration in most wastewater, chloride is often the most serious source of interference. A number of other inorganic substances may also interfere, including nitrite, ferrous iron, and sulfides. Another drawback is that the COD process does not detect some organic materials. Typically, the COD process has difficulty in oxidizing (or fully oxidizing) long-chain hydrocarbons. The COD process may not be able to oxidize or subsequently detect these long-chain hydrocarbons, such as endocrine inhibitors from human or animal pharmaceuticals. Another drawback is that the oxidizing agents used in traditional COD tests are toxic. Disposal of a completed sample requires observation of applicable toxic material disposal procedures.

Another drawback is that the amount of needed oxidizing agent will vary with the organic material load. In order to ensure a complete an oxidization process as quickly as possible, an excess of oxidizing agent is typically added to the sample being tested. Otherwise, oxidization may not be complete. However, as discussed above, the oxidizing agent is not environmentally friendly and using an excess of the oxidizing agent presents both cost and disposal problems.

FIG. 1 shows a relatively new, but prior art, oxygen demand measurement device for quantifying organic load in water. The device is a photo-electro-chemical oxygen demand cell, such as disclosed in U.S. Patent Publications 2006/0240558 and 2006/0205083, assigned to Aqua Diagnostics Pty Ltd, South Melbourne, Australia. The photo-electro-chemical oxygen demand cell includes a container holding a water sample, a window that admits UV light from a UV light source, a working electrode including an oxidization portion, a reference electrode, and a counter electrode. The UV light source may illuminate the water sample in the region of the working electrode. An electrolyte is added to the water sample for improved conductivity.

The biggest advantage of photo-electro-chemical oxygen demand is that it generates a total oxygen demand, a value that includes oxygen demand not measured by the traditional COD process. The traditional COD process does not oxidize all carbon, for example. The working electrode includes an oxidization portion or coating. The oxidization portion or coating can comprise a suitable oxidization material, such as titanium dioxide (TiO₂), for example. The oxidization coating can comprise any semiconductive or nanoparticulate semiconductive material that is capable of photo- oxidation of organic compounds. Alternatively, other similar materials may be used, including tin dioxide, cadmium oxide, hafnium oxide, zirconium oxide, ferric oxide, zinc oxide, indium trioxide, tungsten trioxide, chromium trioxide, yttrium trioxide, niobium pentoxide, tantalum pentoxide, strontium titanate, calcium titanate, barium titanate, or potassium tantalate. This is not an exhaustive listing.

The oxidization coating serves as a catalyst for oxidation. The oxidization coating therefore replaces the traditional oxidizing reagents, such as potassium dichromate or potassium permanganate, for example. The oxidization coating promotes the oxidization action, in conjunction with the UV light. In addition, the oxidization coating enables measurement of an electrical current resulting from the oxidization process. In operation, the UV light source illuminates the water sample and the working electrode, including the oxidization coating. When the oxidization coating is exposed to UV light of a wavelength below about 385 nanometers (nm) and in the presence of water, two highly reactive substances are formed: hydroxyl radicals (OH) and a superoxide ion ( O₂ ¹ ). Hydroxyl radicals are very strong oxidizers and will attack all manner of organic materials, including those making up living cells. Consequently, when organic material lands on the oxidization coating, the hydroxyl radicals break down the cell wall and outer membrane, causing cell damage and death.

Advantageously, hydroxyl radicals are short-lived and do not present exposure, disposal, or contamination problems.

The oxidization of the organic materials in the water sample creates electron holes in the semiconductor oxidization coating. A voltage potential is placed across the electrodes in order to effect movement of the electron holes in the semiconductor oxidization surface. The voltage potential typically is only a few volts and causes the resulting electron holes to move in the electrode system, creating an electrical oxidization current in the electrodes. The oxidization current is related to the amount of oxidization occurring in the water sample. The oxidization current is therefore related to the organic load in the water sample. Consequently, the oxidization current can be quantified and used as a quantification of the total mass of oxygen used by the water sample, denoting or signifying the amount of organic material.

The oxidization current can be used as an instantaneous quantification of organic load. The instantaneous oxidization current can be related to a type and/or amount of organic material in some circumstances. For example, the instrument can be compared and/or fitted to an oxidization profile or curve, potentially identifying the organic material or materials in the water sample. Subsequently, the total organic load can be estimated or interpolated from the profile or curve.

The oxidization current can be integrated or otherwise accumulated over time in order to be used as an overall quantification of organic load. For example, the test can be allowed to occur until the oxidization current drops below a predetermined oxidization current threshold, signifying that almost all of the organic material in the water sample has been oxidized. The oxidization current during the entire test can then be integrated or otherwise processed in order to determine a total organic load. Aspects of the Invention

In one aspect of the invention, an oxygen monitoring system for determining an oxygen density load of a fluid comprises: an oxygen demand sensor for measuring an oxygen demand of the fluid; a flow meter for measuring a flow rate of the fluid; and a processing system coupled to the chemical oxygen demand sensor and to the flow meter, with the processing system being configured to receive the oxygen demand, receive the flow rate, and determine the oxygen density load of the fluid from the oxygen demand and the flow rate. Preferably, the oxygen demand sensor comprising a chemical oxygen demand sensor.

Preferably, the oxygen demand sensor comprising a photo-electro-chemical oxygen demand sensor.

Preferably, the oxygen monitoring system further comprises an oxygen injector coupled to the processing system, wherein the processing system is further configured to control the oxygen injector and therefore control addition of oxygen to the fluid. Preferably, the fluid comprises water. Preferably, the fluid comprises wastewater. Preferably, the fluid comprises drinking water.

Preferably, the oxygen density load comprises a predictor of oxygen needed for a fluid treatment process.

Preferably, the oxygen monitoring system further comprises the processing system controlling addition of oxygen to the fluid substantially according to the oxygen density load.

In one aspect of the invention, a method for determining an oxygen density load of a fluid comprises: measuring an oxygen demand of the fluid using a photo-electro-chemical oxygen demand sensor; measuring a flow rate of the fluid; and determining the oxygen density load of the fluid from the oxygen demand and the flow rate.

Preferably, the fluid comprises water. Preferably, the fluid comprises wastewater.

Preferably, the fluid comprises drinking water.

Preferably, measuring the oxygen demand comprises measuring the oxygen demand of the fluid using a chemical oxygen demand sensor.

Preferably, measuring the oxygen demand comprises measuring the oxygen demand of the fluid using a photo-electro-chemical oxygen demand sensor.

Preferably, the method further comprises controlling addition of oxygen to the fluid substantially according to the oxygen density load. In one aspect of the invention, a method for determining an oxygen density load of a fluid comprises: measuring an oxygen demand of the fluid using a photo-electro-chemical oxygen demand sensor; measuring a flow rate of the fluid; determining the oxygen density load from the oxygen demand and the flow rate; and controlling addition of oxygen to the fluid substantially according to the oxygen density load.

Preferably, the fluid comprises drinking water.

Description of the Drawings The same reference number represents the same element on all drawings. It should be understood that the drawings are not necessarily to scale. FIG. 1 shows a relatively new, but prior art, oxygen demand measurement device for quantifying organic load in water.

FIG. 2 shows an oxygen monitoring system for determining an oxygen density load of a fluid according to an embodiment of the invention.

FIG. 3 is a flowchart of a method for determining an oxygen density load of a fluid according to the invention.

Detailed Description of the Invention

FIGS. 2-3 and the following description depict specific examples to teach those skilled in the art how to make and use the best mode of the invention. For the purpose of teaching inventive principles, some conventional aspects have been simplified or omitted. Those skilled in the art will appreciate variations from these examples that fall within the scope of the invention. Those skilled in the art will appreciate that the features described below can be combined in various ways to form multiple variations of the invention. As a result, the invention is not limited to the specific examples described below, but only by the claims and their equivalents.

FIG. 2 shows an oxygen monitoring system 200 for determining an oxygen density load of a fluid according to an embodiment of the invention. The oxygen monitoring system 200 includes an oxygen demand sensor 205, a flow meter 210, and a processing system 220. The processing system 220 coordinates and performs an oxygen monitoring process in the fluid. The processing system 220 is coupled to the oxygen demand sensor 205 and the flow meter 210. In use, the oxygen demand sensor 205 measures an oxygen demand of the fluid and the flow meter 210 measures the flow rate of the fluid. The processing system 220 receives the oxygen demand from the oxygen demand sensor 205 and receives the flow rate from the flow meter 210. The processing system 220 processes the oxygen demand and the flow rate to generate the oxygen density load of the fluid from the oxygen demand and the flow rate.

The fluid can comprise any fluid where the oxygen demand is of interest and can be measured. The fluid in some embodiments comprises water. The fluid in some embodiments comprises wastewater. The fluid in some embodiments comprises drinking water or potable water. However, it should be understood that other fluids are contemplated and are within the scope of the description and claims, such as fluids that include at least some water.

The fluid can comprise an incoming fluid or an outgoing fluid. However, the method has special applicability for measuring the oxygen density load of an incoming fluid, wherein the oxygen density load can be used to alter or affect a treatment process. The oxygen density load comprises a measurement of oxygen per unit or quantity of fluid. The oxygen density load in some embodiments comprises a measurement of oxygen per volume of fluid. The oxygen density load in some embodiments comprises a measurement of oxygen per mass of fluid. The oxygen density load in some embodiments comprises a measurement of oxygen passing an observation point period of time in the flowing fluid.

The oxygen demand denotes the amount of oxygen needed in some embodiments to treat the fluid. The oxygen demand denotes the amount of oxygen needed in some embodiments to treat the fluid using oxidization. The oxygen demand denotes the amount of oxygen needed in some embodiments to treat the fluid using biological means. The oxygen demand denotes the amount of oxygen needed in some embodiments to treat the fluid using microorganisms. The oxygen demand therefore can indicate the presence of organic pollutants and/or microorganisms in the fluid, such as ammonia, phosphorus, enzymes, and bacteria, for example. Other organic pollutants and microorganisms are contemplated and are within the scope of the description and claims.

The oxygen demand denotes a need for agitation of the fluid. The agitation can be designed to incorporate the oxygen into the fluid and expose the microorganisms in the fluid to the oxygen. The agitation level can subsequently be controlled according to the amount of oxygen being added to the fluid. The oxygen density load is a predictor of the amount of oxygen needed by microorganisms in the fluid. The oxygen may be needed in order to oxidize organic material, including living and non-living organic materials and natural and man-made organic materials. Consequently, the oxygen density load can be used substantially in real time to determine the amount of oxygen to be added to the fluid in preparation for treatment. The oxygen can be added in any manner. The oxygen can be added as pure oxygen or in a mixture of gases, such as air, for example. Fluid treatment can include use of biological materials that are designed to consume materials in the fluid. Wastewater treatment, for example, can include treating wastewater by adding activated sludge. The activated sludge can comprise bacteria or other microorganisms that consume organic material in the wastewater. For example, the wastewater can include natural organic materials that need to be removed, including human and animal wastes, decomposing plant matter, etc. The microorganisms can be used to rid the wastewater of such materials by consuming the organic materials, typically referred to as carbonaceous pollution. The microorganisms in activated sludge, such as aerobic bacteria, typically need oxygen in order to live and consume waste material. As a result, oxygen (or air) may be added to wastewater in order to ensure the full and complete consumption of waste material by the microorganism. The oxygenation speeds up the consumption/oxidization of the carbonaceous foreign material.

However, the addition of oxygen or air incurs cost, as the additive air or oxygen must be pumped into the fluid. If concentrated oxygen is being used instead of atmospheric air, then more energy is required to concentrate/refme the oxygen before it is pumped into the fluid. More oxygen or air therefore comes with more cost. As a result, it is desirable to be able to add the right amount and not provide excessive and unnecessary oxygen or air.

In the prior art, oxygen or air is typically added in a fixed amount. The amount is selected in order to provide adequate oxygen for most microorganism loads. However, where excessive wastes are present in water, the microorganisms will reproduce more quickly and as a result may run out of oxygen, inhibiting the ability of the microorganisms to consume more waste materials when larger amounts of waste materials are present in the fluid. As a result, excessive microorganism loads may not be adequately treated. Conversely, at times the waste materials in the incoming material may be at a relatively low level. For example, an influx of rainwater may be included in wastewater, diluting the amount of waste material. In such cases, excessive oxygen or air will still be provided in the prior art, incurring unnecessary cost in waste treatment. The oxygen demand sensor 205 measures an oxygen demand in the fluid. The oxygen demand comprises the amount of oxygen needed to treat the fluid. The oxygen demand in some embodiments is measured indirectly, such as by determining the amount of carbon dioxide produced when foreign material in the fluid is oxidized.

The measurement of oxygen demand is desirable because the amount of incoming fluid may vary. For example, the volume of incoming fluid may vary where rainwater is present or where other fluids may add to the overall volume. Further, fluids of varying levels of foreign material may be present in the fluid. Consequently, the concentration of foreign material that affects oxygen demand may vary considerably over time.

In some embodiments, the oxygen demand sensor 205 comprises a chemical oxygen demand (COD) sensor 205, as is known in the art. The COD sensor 205 oxidizes the fluid using a combination of an oxidization agent, heat, and UV light to convert carbon in the fluid into carbon dioxide. The produced carbon dioxide can be subsequently quantified and used to determine the oxygen demand.

In some embodiments, the oxygen demand sensor 205 comprises a photo-electrochemical oxygen demand sensor 205, as is known in the art. The photo-electro- chemical oxygen demand sensor 205 oxidizes the fluid using a combination of a photo- catalyst oxidization portion, a voltage bias, and UV light to convert carbon in the fluid into carbon dioxide. The produced carbon dioxide can be subsequently quantified and used to determine the oxygen demand.

The photo-electro-chemical oxygen demand sensor 205 may perform a more complete oxidization that the COD sensor 205. As a result, the photo-electro-chemical oxygen demand sensor 205 may generate a more accurate and representative oxygen demand value than the COD sensor 205.

The flow meter 210 can comprise any suitable flow meter. The flow meter 210 can generate a volume flow rate. The flow meter 210 can generate a mass flow rate. The flow meter 210 can generate a flow speed. A variety of flow meter types are contemplated and are within the scope of the description and claims.

The processing system 220 can include a storage system 222. The storage system 222 can comprise any appropriate storage. The storage system 222 can include a control routine 225, an oxygen demand 226, a flow rate 227, and an oxygen density load 228. The control routine 225 performs the oxygen monitoring when executed by the processing system 220. The oxygen demand 226 comprises oxygen demand data received from the oxygen demand sensor 205. The flow rate 227 comprises flow rate data received from the flow meter 210. The oxygen density load 228 comprises a value or values that are determined from the oxygen demand 226 and the flow rate 227. It should be understood that additional data is contemplated and is within the scope of the description and claims. The oxygen monitoring system 200 in some embodiments can further include an oxygen injector 215 that is controlled by the processing system 220. The processing system 220 can control addition of oxygen to the fluid substantially according to the oxygen density load. Consequently, the oxygen demand measurement can be used to obtain a precise and regulated oxygen level in the fluid. The oxygen level can be varied as needed, depending on the organic load of the incoming fluid. The oxygen density load 228 therefore comprises information that can be used to operate the oxygen injector 215.

FIG. 3 is a flowchart 300 of a method for determining an oxygen density load of a fluid according to the invention. The fluid can comprise an incoming fluid or an outgoing fluid. However, the method has special applicability for measuring the oxygen density load of an incoming fluid, wherein the oxygen density load can be used to alter or affect a treatment process.

In step 301, the oxygen demand of the fluid is measured. The oxygen demand in some embodiments is measured using a chemical oxygen demand (COD) sensor. The COD sensor uses an oxidization agent and UV light to oxidize organic materials in the fluid, creating carbon dioxide. The carbon dioxide is quantified and serves as a measure of the oxygen demand in the fluid, as water in the fluid will supply oxygen to the carbon under the influence of the oxidization agent and the UV light.

The oxygen demand in some embodiments is measured using a photo-electro- chemical oxygen demand sensor. The photo-electro-chemical oxygen demand sensor does not use an oxidization agent, and instead uses UV light and a semiconductor oxidization portion that oxidizes the fluid when illuminated by the UV light. In addition, the semiconductor oxidization portion can enhance the oxidization process by the inclusion of a voltage bias on the semiconductor oxidization portion, enhancing electron mobility. The enhanced electron mobility in the semiconductor oxidization portion enhances the oxidization process and drives it to completion. The oxidization capability of the photo-electro-chemical oxygen demand sensor therefore is better than the oxidization capability of the COD sensor.

In step 302, the flow rate of the fluid is measured, as previously discussed. The flow rate can be measured in any desired manner by any suitable flow meter.

In step 303, the oxygen density load is determined from the oxygen demand of the fluid and the flow rate, as previously discussed.

In step 304, in some embodiments the oxygen demand is used to control addition of oxygen to the fluid substantially according to the oxygen density load. The oxygen can be added as air or as concentrated or purified oxygen, for example. Consequently, the oxygen added to the fluid can be controlled to substantially match oxygen requirements. As a result, the method can avoid adding too much or too little oxygen, guaranteeing that a treatment process will function optimally.

Claims

We claim:

1. An oxygen monitoring system (200) for determining an oxygen density load of a fluid, the oxygen monitoring system (200) comprising: an oxygen demand sensor (205) for measuring an oxygen demand of the fluid; a flow meter (210) for measuring a flow rate of the fluid; and a processing system (220) coupled to the chemical oxygen demand sensor (205) and to the flow meter (210), with the processing system (220) being configured to receive the oxygen demand, receive the flow rate, and determine the oxygen density load of the fluid from the oxygen demand and the flow rate.

2. The oxygen monitoring system (200) of claiml, with the oxygen demand sensor (205) comprising a chemical oxygen demand sensor (205).

3. The oxygen monitoring system (200) of claiml, with the oxygen demand sensor (205) comprising a photo-electro-chemical oxygen demand sensor (205).

4. The oxygen monitoring system (200) of claiml, further comprising an oxygen injector (215) coupled to the processing system (220), wherein the processing system (220) is further configured to control the oxygen injector (215) and therefore control addition of oxygen to the fluid.

5. The oxygen monitoring system (200) of claiml, where the fluid comprises water.

6. The oxygen monitoring system (200) of claiml, where the fluid comprises wastewater.

7. The oxygen monitoring system (200) of claiml , where the fluid comprises drinking water.

8. The oxygen monitoring system (200) of claiml , wherein the oxygen density load comprises a predictor of oxygen needed for a fluid treatment process.

9. The oxygen monitoring system (200) of claiml , further comprising the processing system (220) controlling addition of oxygen to the fluid substantially according to the oxygen density load.

10. A method for determining an oxygen density load of a fluid, the method comprising: measuring an oxygen demand of the fluid using a photo-electro-chemical oxygen demand sensor; measuring a flow rate of the fluid; and determining the oxygen density load of the fluid from the oxygen demand and the flow rate.

11. The method of claim 10, where the fluid comprises water.

12. The method of claim 10, where the fluid comprises wastewater.

13. The method of claim 10, where the fluid comprises drinking water.

14. The method of claim 10, with measuring the oxygen demand comprising measuring the oxygen demand of the fluid using a chemical oxygen demand sensor.

15. The method of claim 10, with measuring the oxygen demand comprising measuring the oxygen demand of the fluid using a photo-electro-chemical oxygen demand sensor.

16. The method of claim 10, wherein the oxygen density load comprises a predictor of oxygen needed for a fluid treatment process.

17. The method of claim 10, further comprising controlling addition of oxygen to the fluid substantially according to the oxygen density load.

18. A method for determining an oxygen density load of a fluid, the method comprising: measuring an oxygen demand of the fluid using a photo-electro-chemical oxygen demand sensor; measuring a flow rate of the fluid; determining the oxygen density load from the oxygen demand and the flow rate; and controlling addition of oxygen to the fluid substantially according to the oxygen density load.

19. The method of claim 18, where the fluid comprises water.

20. The method of claim 18, where the fluid comprises wastewater.

21. The method of claim 18, where the fluid comprises drinking water.

22. The method of claim 18, with measuring the oxygen demand comprising measuring the oxygen demand of the fluid using a chemical oxygen demand sensor.

23. The method of claim 18, with measuring the oxygen demand comprising measuring the oxygen demand of the fluid using a photo-electro-chemical oxygen demand sensor.

24. The method of claim 18, wherein the oxygen density load comprises a predictor of oxygen needed for a fluid treatment process.