Raw Water Total Oxygen Demand (TOD) Testing: Principles, Methods, and Applications

Raw water total oxygen demand (TOD) testing quantifies the total amount of oxygen required to completely oxidize organic and oxidizable inorganic matter in source water, delivering a rapid and comprehensive indicator of pollution load. This technical guide explains the measurement principle, applicable standards, sampling requirements, testing workflow, and typical industrial applications of TOD analysis for raw water. Whether you operate a municipal treatment plant, a desalination facility, or an industrial water system, understanding TOD testing supports better process design and water quality control decisions.

Technical Overview

Total Oxygen Demand (TOD) is a water quality parameter that measures the amount of oxygen consumed when all organic matter and oxidizable inorganic species in a water sample are completely combusted. It is expressed in milligrams of oxygen per litre (mg O2/L) and represents the theoretical oxygen requirement of the sample, making it one of the most complete indicators of the oxidizable pollution load in raw water.

For raw water — the untreated water drawn from rivers, lakes, reservoirs, wells, or the sea — TOD testing provides valuable insight into the concentration of biodegradable and non-biodegradable organics, as well as inorganic reducing substances such as sulfide, sulfite, nitrite, and ferrous iron. Because TOD analysis relies on high-temperature catalytic combustion rather than wet-chemical oxidation, it captures compounds that conventional dichromate COD methods may oxidize only partially.

The most widely recognized method basis for TOD determination is ASTM D6238, Standard Test Method for Total Oxygen Demand in Water. Compared with the five-day biochemical oxygen demand (BOD5) test, TOD delivers results in minutes, enabling rapid screening, online monitoring, and timely process control for water intake and pretreatment systems.

Testing Samples

Accredited third-party laboratories typically accept the following categories of raw water samples for TOD testing. Proper sample identification, container selection, and preservation are essential to ensure representative results.

Surface Water

River water, lake water, and reservoir water collected at intake points or environmental monitoring stations. These samples often contain suspended solids and variable organic loads, so homogenization before subsampling is recommended.

Groundwater

Well water and spring water used as municipal or industrial sources. Groundwater generally exhibits lower TOD values, but reducing species such as ferrous iron and sulfide can contribute measurably to the oxygen demand.

Municipal and Industrial Intake Water

Untreated feed water entering treatment plants, cooling systems, or process water trains. TOD data at the intake stage establish a baseline for evaluating downstream removal efficiency.

Seawater and Brackish Water

Source water for desalination facilities. High chloride concentrations require appropriate instrument settings, dilution, or matrix-matched calibration to obtain reliable TOD results.

Pretreated Process Water

Water sampled after coarse screening, settling, or pre-oxidation steps, used to verify the performance of pretreatment units ahead of membranes, ion exchange, or boiler systems.

Testing Items

TOD is frequently tested alongside related oxygen demand and organic carbon parameters to build a complete picture of raw water quality. Common testing items include:

• Total Oxygen Demand (TOD) — the primary parameter, determined by combustion-catalytic oxidation
• Chemical Oxygen Demand (COD) — dichromate or sealed-tube method per ISO 15705 or ASTM D1252
• Biochemical Oxygen Demand (BOD5) — per ISO 5815-1 for biodegradable organic load
• Total Organic Carbon (TOC) and Dissolved Organic Carbon (DOC) — per ISO 8245 or EN 1484
• Permanganate Index — per ISO 8467, widely used for relatively clean surface waters
• Dissolved Oxygen (DO) — per ISO 5814 or ASTM D888, to characterize background oxygen levels
• Oxidizable inorganic species — sulfide, sulfite, nitrite, and ferrous iron that contribute to TOD

Testing Methods

TOD determination is performed by the combustion-catalytic oxidation method. A measured aliquot of the homogenized raw water sample is injected into a combustion tube packed with a platinum-based catalyst and maintained at approximately 850 to 950 °C. A carrier gas stream containing a precisely known concentration of oxygen flows through the tube, and all oxidizable constituents are converted to stable end products such as carbon dioxide, water, and oxidized forms of nitrogen and sulfur. The depletion of oxygen in the carrier gas, detected by an oxygen-sensitive sensor such as a zirconia element, is directly proportional to the total oxygen demand of the sample.

Sample Collection and Preservation

Samples should be collected in clean glass containers with minimal headspace. Where holding is unavoidable, samples are typically acidified to below pH 2 and stored refrigerated at approximately 4 °C, then analyzed as soon as practicable in accordance with the requirements of the applied standard and the laboratory's validated procedure. Samples containing settleable solids must be homogenized before subsampling, since TOD reflects the total oxidizable content including particulate organics.

Instrumentation and Calibration

The TOD analyzer is calibrated using standard solutions of potassium hydrogen phthalate (KHP), which has a well-defined theoretical oxygen demand. A multi-point calibration curve is established across the expected concentration range, and quality control checks are performed at regular intervals using independent verification standards.

Measurement Procedure

• Homogenize the raw water sample and, if necessary, dilute it to bring the expected TOD within the calibrated range
• Inject a precise aliquot, typically in the microlitre to low-millilitre range, into the combustion tube
• Allow complete catalytic oxidation of all organic and oxidizable inorganic species at high temperature
• Measure the oxygen depletion in the carrier gas with the detector and convert the signal to concentration using the calibration curve
• Report the result as mg O2/L, including dilution factors and measurement uncertainty where applicable

Complementary Methods

To strengthen interpretation, TOD results are often correlated with COD per ISO 15705 or ASTM D1252, BOD5 per ISO 5815-1, TOC per ISO 8245 or EN 1484, and permanganate index per ISO 8467 or equivalent national standards such as GB 11914 and GB/T 11892. These complementary data help distinguish biodegradable fractions from refractory organics and reducing inorganic species.

Applications

Raw water TOD testing supports a broad range of engineering, operational, and environmental objectives:

• Baseline characterization of source water quality for the design of treatment plants and pretreatment systems
• Continuous or periodic monitoring of raw water pollution load at municipal and industrial intakes
• Rapid screening ahead of BOD5 testing when fast turnaround is required
• Performance verification of oxidation, coagulation, and filtration processes in water treatment trains
• Feed water quality control for desalination, boiler makeup, cooling, and ultrapure water systems
• Environmental impact assessments and pollution source tracing for rivers, lakes, and groundwater
• Correlation studies between TOD, COD, BOD, and TOC for process modeling and regulatory reporting support
• Research and development in water treatment chemistry and oxidation technology

FAQ

What is the difference between TOD and COD?

TOD measures the oxygen required for complete high-temperature combustion of all oxidizable substances, including compounds that dichromate COD oxidizes poorly, and it also captures reducing inorganic species. COD is a wet-chemical oxygen demand based on chemical oxidation. TOD results are therefore generally equal to or slightly higher than COD for the same sample, and the TOD test is faster and free of hazardous chromium and mercury reagents.

Why choose TOD testing over BOD for raw water?

BOD5 requires a five-day incubation and reflects only the biodegradable fraction of the organic load. TOD produces results within minutes, includes both biodegradable and refractory organics plus oxidizable inorganics, and is well suited to online monitoring and rapid decision-making at water intakes.

How should raw water samples be handled before TOD testing?

Collect samples in clean glass containers with minimal headspace, keep them refrigerated at approximately 4 °C, and deliver them to the laboratory promptly. Homogenize turbid or particulate-laden samples before analysis, since TOD represents the total oxidizable content of the water.

What is the typical detection range of TOD testing?

Modern combustion-catalytic TOD analyzers can quantify oxygen demand from a few milligrams per litre up to several thousand milligrams per litre, with appropriate dilution for highly polluted samples. The practical reporting limit depends on the instrument, injection volume, and calibration range validated by the laboratory.

Can TOD results be used for regulatory compliance?

Acceptance of TOD as a compliance parameter depends on the jurisdiction and the applicable regulation, which more commonly specify COD, BOD, or permanganate index. TOD is nevertheless widely used for process control, rapid screening, and as supporting data, and its strong correlation with COD and BOD makes it a valuable complement to standardized regulatory methods in accredited third-party laboratories.