Methods for measuring BOD and dissolved oxygen

Check the national standards for analysis; they contain methods for testing BOD and dissolved oxygen. If there are specialized testing instruments, follow the instructions.

This post was last edited by cdpulin on 2011-2-10 21:13. Biochemical oxygen demand is abbreviated as BOD. It is a comprehensive indicator indicating the content of oxygen-demanding pollutants such as organic matter in water. It indicates the total amount of dissolved oxygen consumed in water when organic substances in that water are oxidized and decomposed through microbial biochemical processes, thereby being converted into inorganic substances or gases; its unit is ppm (milligrams per liter). BOD generally refers to the amount of organic matter that can be degraded by microorganisms, that is, the amount of biodegradable organic matter in wastewater. The methods for determining BOD include: 1. Standard dilution method – this is the most classical and commonly used method. Simply put, it is to measure the difference in dissolved oxygen levels in the solution before and after five days of incubation at a temperature of 20±1°C. The calculated BOD value is referred to as “five-day biochemical oxygen demand (BOD5)”. 2. The principle of the biosensor method is to allow water samples and air to flow into a flow cell at a certain rate, where they come into contact with microbial sensors. When the diffusion rate of soluble, biodegradable organic substances in the water sample reaches a constant level, the amount of oxygen that diffuses to the surface of the oxygen electrode also becomes constant, resulting in a steady current. Since this current is directly proportional to the amount of biodegradable organic substances in the water sample and to the decrease in oxygen levels, it is possible to calculate the biochemical oxygen demand of the water sample based on this relationship. Typically, a BOD5 standard sample is used for comparison in order to calculate the BOD5 value of the water sample. 3. In the activated sludge aeration degradation method, the temperature is controlled at 30°C–35°C; the sample is degraded through forced aeration using activated sludge for 2 hours. After biodegradation, the sample is digested with potassium dichromate, and the chemical oxygen demand before and after biodegradation is measured, with the difference representing the BOD. Based on the results of comparative experiments with standard methods, it can be converted into a BOD5 value. 4. The pressure measurement method: In a sealed culture flask, the dissolved oxygen in the water sample is consumed by microorganisms. These microorganisms produce CO2 in amounts equivalent to those of oxygen consumed through respiration. When this CO2 is absorbed, the pressure in the sealed system decreases. The BOD value of the water sample can be determined based on the pressure drop measured.

There are national standards for the methods of determining BOD; you can find them by searching online. The method for measuring dissolved oxygen, or DO, generally requires accuracy and thus makes use of the iodometric method; however, in engineering applications where speed is important, DO meters are usually used. There are also corresponding **standards** for iodometric determination; checking them will solve the issue. There is nothing secret about these measurement methods; what’s important is to become proficient at them. With enough practice, there are no issues at all. Of course, carefulness and summarizing one’s experiences are essential.

I think instrument monitoring is more convenient and accurate