First of all, when talking about organic matter indicators in water, everyone should think of them being divisible into two categories: one category consists of indicators expressed by oxygen demand (O2) equivalent to the amount of organic matter in the water, and the other category consists of indicators expressed by carbon (C). The oxygen demand indicators are BOD/COD/TOD, and the carbon indicator is TOC. Below, I will introduce the above indicators of organic matter in water in detail.

The order of the above indicators according to their numerical value from largest to smallest is: TOD > COD > BOD > TOC. The specific analysis is as follows:

[Total Oxygen Demand TOD]

Total Oxygen Demand (TOD) refers to the amount of oxygen required when reducing substances in water are burned at high temperatures and turned into stable oxides, and the result is calculated in mg/L. The TOD value can reflect the oxygen consumption required when almost all organic substances in water (including components such as carbon C, hydrogen H, oxygen O, nitrogen N, phosphorus P, sulfur S, etc.) are burned and turned into CO2, H2O, NOx, SO2, etc.

[Total Organic Carbon TOC]

Total Organic Carbon (TOC) is a comprehensive indicator that indirectly represents the organic matter content in water. The data displayed is the total carbon content of organic matter in sewage, expressed in mg/L of carbon (C). Generally, the TOC of urban sewage can reach 200 mg/L. The TOC range of industrial sewage is wide, with the highest reaching tens of thousands of mg/L. The TOC of sewage after secondary biological treatment is generally < 50 mg/L.

[Biochemical Oxygen Demand BOD]

Biochemical Oxygen Demand, abbreviated as BOD, represents the amount of dissolved oxygen consumed during the biochemical oxidation process of aerobic microorganisms decomposing organic matter in water under conditions of 20°C and oxygen. That is, the amount of oxygen required for the stabilization of biodegradable organic matter in water, in mg/L. BOD includes not only the oxygen consumed by the growth, reproduction, or respiration of aerobic microorganisms in water, but also the oxygen consumed by reducing inorganic substances such as sulfides and ferrous iron, although this part usually accounts for a very small proportion.

Under natural conditions at 20°C, the time required for organic matter to oxidize to the nitrification stage—that is, to achieve complete decomposition and stabilization—is more than 100 days. However, in practice, the 20-day biochemical oxygen demand (BOD20) at 20°C is commonly used to approximately represent biochemical oxygen demand. In production applications, 20 days is still considered too long, and the 5-day biochemical oxygen demand (BOD5) at 20°C is generally used as an indicator to measure the organic matter content in sewage.

[Chemical Oxygen Demand COD]

Chemical Oxygen Demand (COD) refers to the amount of oxidant consumed by the action of organic matter in water with a strong oxidant under certain conditions, converted into oxygen, and calculated in mg/L of oxygen. When potassium dichromate is used as the oxidant, almost all (90%-95%) organic matter in the water can be oxidized. At this time, the amount of oxidant consumed converted into oxygen is what is commonly called chemical oxygen demand, often abbreviated as CODcr. The CODcr value of sewage not only includes the oxygen consumption of almost all organic matter in the water being oxidized, but also includes the oxygen consumption of reducing inorganic substances such as nitrites, ferrous salts, and sulfides in the water being oxidized.

YexSensor Online Monitoring Selection Matrix for Organic Matter

Frequently Asked Questions (FAQ)

Q1: Why is UV spectrometry COD monitoring generally recommended over the potassium dichromate method for online integration?
   A: The potassium dichromate method involves high maintenance costs, strong acids, and heavy metal reagents, plus hazardous waste disposal. UV spectrometry (YEX-COD-206) is reagent-free, provides real-time data, and is far more suitable for automation and early warning systems.

Q2: How is the relationship between BOD5 and COD applied in engineering?
   A: By determining the B/C ratio (BOD5/COD), integrators can assess biodegradability. If B/C < 0.2, the wastewater is generally unsuitable for direct biological treatment, and advanced oxidation pre-treatment units should be integrated.

Q3: Under what circumstances is the TOD indicator mandatory?
   A: TOD is typically required for specific industrial wastewaters with very high loads or as a baseline for research into the total theoretical oxidation potential of a water body. For most municipal and industrial outfall monitoring, COD and TOC are sufficient.

Q4: How is the accuracy of converting TOC to COD guaranteed?
   A: Accuracy depends on the stability of the water composition. In production lines with fixed components, TOC and COD have high linear correlation (R² > 0.9). YexSensor allows inputting compensation coefficients at the controller for high-precision simulation.

Q5: Will UV254 sensors fail in high-chroma (highly colored) wastewater, such as dye wastewater?
   A: Strong color affects light absorption. YexSensor uses dual-wavelength compensation (adding a 365nm or 546nm reference path) to cancel out measurement errors caused by turbidity and some color interference.

Q6: How are these sensors integrated into an IoT platform?
   A: Each YexSensor has a unique Modbus ID. Integrators simply hang them on an RS485 bus connected to a DTU or PLC and read floating-point values according to the provided register map.

Q7: What is the difference between BOD20 and BOD5?
   A: BOD20 represents the total oxygen demand for complete biochemical oxidation over 20 days; however, because the period is too long for practical use, the 5-day indicator (BOD5) is universally adopted as the measurement standard.

Q8: What is the typical maintenance cycle for these sensors?
   A: When equipped with automatic cleaning, the cycle for routine inspection and calibration of YexSensor digital sensors is generally 3-6 months, depending on the severity of scaling and fouling in the water.

Conclusion: Data-Driven Future of Water Environment Integration

The monitoring of organic matter in water has shifted from intermittent laboratory analysis to all-weather online perception. For system integrators, understanding the internal logic of TOD, COD, BOD, and TOC and selecting sensors with high-performance communication and self-cleaning capabilities is key to enhancing project competitiveness.

YexSensor will continue to provide underlying technical support for industrial partners, helping to build smarter, more efficient, and greener water treatment integrated systems through precise digital indicators.

Technical Support & Integration:
       To obtain detailed Modbus communication manuals, equipment selection tables, or integration advice for specific industries, please contact the YexSensor Engineering Center.