Biochemical oxygen demand five-day culture method

Biochemical oxygen demand refers to the amount of dissolved oxygen consumed by aerobic microorganisms in the biochemical oxidation process of decomposing organic matter in water under dissolved oxygen conditions. It also includes the amount of oxygen consumed by the oxidation of reducing inorganic substances such as sulfides and ferrous irons, but this portion usually accounts for a small proportion.

The aerobic decomposition of organic matter under the action of microorganisms is roughly divided into two stages. The first stage is called the oxidation stage of carbonaceous materials, mainly the oxidation of carbon-containing organic matter to carbon dioxide and water; the second stage is called the nitrification stage, mainly the decomposition of nitrogen-containing organic compounds into nitrites and nitrates by the action of nitrifying bacteria. . However, these two phases are not completely separated, but each has its own priorities. For industrial wastewater with domestic sewage and its properties close to it, the nitrification stage is not carried out significantly after 5-7 days or even 10 days. Therefore, the BOD value of the 20 °C five-day culture method (BOD5 method) widely used at home and abroad is generally not Including the nitrification stage.

BOD is a comprehensive indicator reflecting the degree of contamination of water by organic matter. It is also an important parameter for studying the biodegradability and biochemical treatment of wastewater, as well as the process design and kinetics of biochemical wastewater treatment.

Five-day culture method (20 ° C)

Also known as the standard dilution method. The measurement principle is that after the water sample is diluted, it is cultured at 20±1°C for 5 days, and the dissolved oxygen content in the water sample before and after the culture is determined, and the difference between the two is BOD5. If the biochemical oxygen demand of the water sample does not exceed 7 mg/L on the 5th, it is not necessary to dilute it, and it can be directly measured. Many cleaner rivers fall into this category.

For industrial wastewater containing no or little micro-organisms, such as acidic wastewater, alkaline wastewater, high-temperature wastewater or chlorinated wastewater, BOD5 should be inoculated to introduce microorganisms capable of degrading organic matter in wastewater. When there is an organic matter or a highly toxic substance in the wastewater which is difficult to be degraded by the microorganisms in the general domestic sewage at a normal speed, the domesticated microorganism should be introduced into the water sample for inoculation.

Dilution water

For contaminated surface water and most industrial wastewater, because it contains more organic matter, it needs to be diluted and cultured to ensure sufficient dissolved oxygen during the cultivation process. The degree of dilution should be such that the dissolved oxygen consumed in the culture is greater than 2 mg/L, and the remaining dissolved oxygen is above 1 mg/L.

The dilution water is generally prepared by using distilled water, first introduced into the air treated by activated carbon adsorption and water washing, and aerated for 2-8 hours, so that the dissolved oxygen in the water is nearly saturated, and then placed at 20 ° C for several hours. Add a small amount of nutrient salt solution such as calcium chloride, ferric chloride, magnesium sulfate and phosphate buffer solution before use, and mix for use. The pH of the dilution water should be 7.2 and the BOD5 should be less than 0.2 mg/L.

If there is no microorganism in the water sample, the microorganism should be inoculated in the diluted water, that is, 1-10 mL of the supernatant of the domestic sewage or 20-30 mL of the surface soil leaching solution, or 10-100 mL of the river water and the lake water, per liter of the diluted water. This water is called inoculated dilution water. In order to check the quality of the dilution water and inoculum, and the level of operation of the laboratory personnel, the standard solution containing 150 mg of glucose and glutamic acid per liter is diluted at a dilution ratio of 1:50, and the BOD5 is measured in synchronization with the water sample. Should be between 180-230mg / L, otherwise, the reasons should be checked and corrected.

2. Water sample dilution factor

The dilution factor of the water sample should be estimated based on practical experience. This table lists the methods for estimating the dilution of ground water. The dilution factor of industrial wastewater is obtained by multiplying the CODcr value by a factor of 0.075, 0.15, and 0.25, respectively. Usually three water dilutions are made at the same time.

3. Calculation result calculation

Water samples directly cultured without dilution:

BOD5(mg/L)=C1-C2

Where: C1—the concentration of dissolved oxygen (mg/L) before the culture of the water sample; C2—the residual dissolved oxygen concentration (mg/L) after the water sample was cultured for 5 days.

Water sample after dilution:

Where: B1—the concentration of dissolved oxygen (mg/L) before dilution in a dilution water (or inoculated dilution water); B2—the concentration of dissolved oxygen in the dilution water (or inoculated dilution water) after culture (mg/ L); f1 - the proportion of dilution water (or inoculated dilution water) in the culture solution; f2 - the proportion of water sample in the culture solution.

When the water sample contains toxic substances such as copper, lead, zinc, cadmium, chromium, arsenic or cyanogen, the microbial activity is inhibited. The dilution water inoculated with the domesticated microorganism may be used, or the dilution factor may be increased to reduce the influence of the poison. If a small amount of chlorine is contained, it is usually left for 1-2 hours to disappear by itself; the water sample which can not be dissipated for a short time can be removed by adding sodium sulfite. The amount of addition is determined by experiment.

The method is suitable for determining water samples with a BOD5 greater than or equal to 2 mg/L and a maximum of no more than 6 000 mg/l; greater than 6 000 mg/L, a greater error due to dilution.