Basic principles of B0D5:
Definition; Biochemical Oxygen Demand (BOD5) is defined as the amount of dissolved oxygen consumed by aerobic microorganisms in water. When the sample is placed in an incubator and the cultivation temperature is 20 ° C for five days, determine the amount of dissolved oxygen consumed to determine the water sample. BOD5 value
Biochemical Oxygen Demand (English abbreviation BOD) refers to the amount of dissolved oxygen that is consumed or required to decompose organic matter in water through the propagation and respiration of aerobic microorganisms in water under specific conditions. The BOD5 value in water is usually expressed as the amount of dissolved oxygen (mg / L number) consumed by the sample at 20 ° C for 5 days, and is recorded as “BOD5â€.
2. Measuring principle:
Pour the water sample with pre-selected range and measure the volume according to the range into the culture bottle, and stir continuously on the host mixer. Put the host and culture bottle in the incubator. Adjust the temperature in the incubator to 20 ℃ ± 1 ℃, wait for the sample to be incubated for five days. The water sample in the culture flask ensures sufficient dissolved oxygen for the biochemical reaction of microorganisms under continuous stirring. The organic matter in the water sample undergoes biological oxidation to be converted into nitrogen, carbon and sulfur oxides. In this process, the only gaseous carbon dioxide that overflows from the water sample is absorbed by sodium hydroxide (or potassium hydroxide). At this time, the amount of air pressure reduction in the culture bottle is equivalent to the dissolved oxygen content consumed by the microorganisms. That is, the BOD5 value of the water sample is proportional to the degree of air pressure reduction in the bottle. Increasing or decreasing the amount of sample taken can increase or decrease the pressure reduction value. This allows the operator to accurately measure a wide range of BOD5 values. The change of the air pressure in the culture bottle is detected by the change of the height of mercury in the pressure gauge, and the BOD5 value of the water sample can be read out through a simple calculation.
Factors affecting the determination of BOD5
—Dissolved oxygen:
The samples taken in winter are supersaturated with dissolved oxygen due to the test temperature being set at 20 ° C. The samples taken in summer may be undersaturated with dissolved oxygen at 20 ° C. Before BOD5 measurement, these samples should be stirred and aerated to adjust the dissolved oxygen to around 20 ℃ saturation point.
—PH:
The pH of the test water sample should be adjusted to 6.7 ~ 7.5 (the optimal point is pH 7.2). The drop in the BOD5 reading may be due to the pH value of the sample being tested exceeds this range. The BOD5 reading of the sample containing a large amount of acid or alkali may be lower than Actual content.
Acidic or alkaline water samples should be neutralized with appropriate concentrations of Na0H or H2S04.
If the industrial wastewater contains acidic or salt oxides, or high dilution is required, for the standard five-day BOD5 determination, use phosphate buffer pH 7.2 as the diluent.
-temperature:
During the experiment, the instrument amplifier was placed in an incubator at 20 ℃ ± 1 ℃. The measurement of BOD can also exceed this temperature range. For example, at 35 ℃ or 37 ℃, the time required to measure BOD can be shortened to 34 hours, and the measurement result is similar to the BOD5 measurement at 20 ℃ on five days. The BOD value obtained by increasing the temperature has a good correlation with the measurement result under the condition of 20 ° C on five days.
If the temperature of the sample exceeds 20 ° C during sampling, or the temperature of the sample is less than 20 ° C, the sample can be placed in a constant temperature in an incubator. The sample is required to be processed before the sample is measured.
-dilution:
The direct reading BOD5 measuring instrument has a measuring range of 0 to 1000 mg / L. If the estimated BOD5 value of the test sample is higher than 1000mg / L, the sample can be diluted with dilution water. The added dilution water should also be maintained at 20 ° C and aerated to saturate oxygen. If several identical samples are required for testing, a sufficient amount of raw water sample should be diluted and the volume corresponding to the range should be taken from the diluted sample . For example, when the original water sample is diluted twice, the dilution factor is 2; when the original water sample is diluted 1:10, it should be 10, and the measured result should be multiplied by the dilution factor.
—Inoculation:
The BOD5 experiment requires that the water sample contains aerobic bacteria for biological oxidation and an appropriate amount of oxidized organic matter, as well as other microorganisms that consume organic matter and promote the growth of aerobic bacteria. If there is no or almost no such microorganisms in the water sample, it is necessary to add a certain proportion of such microorganism solution to the water sample. This process is different from domestic sewage. Industrial wastewater may not contain enough bacteria for the complete biochemical decomposition of the organic matter contained in the sample. If the measured sample is such wastewater, the inoculation solution should be added.
A inoculation method
Preparation of inoculation solution
Inoculation solution
The solution used for inoculation is the supernatant of untreated fresh domestic sewage placed at 20 ° C for 24 to 36 hours. In the next experiment, if you want to use the previously measured water sample as the inoculum, it should be filtered through filter paper. This filtrate is stored under refrigeration (about 20 ℃) ​​and protected from light. It is generally effective within one month. Put the sample to be measured into a culture bottle, and use a pipette to add a certain amount of inoculum to the sample (according to the sample amount). Measure BOD5 in steps, and the reading obtained five days later can be used as the true BOD5 value of the test sample.
Note: When using the above method for inoculation. It is not necessary to correct the BOD5 reading of the inoculation. Because the amount of inoculum added is too small for the amount of water sample, so that it does not affect the reading of BOD5. If the above method fails to cause the biological decomposition of organic matter in the water sample, the amount of inoculum added can be increased. If the addition of inoculum affects the reading of BOD5, the inoculum and the sample can be tested in parallel for simultaneous sample determination.
Inoculation volume is usually taken as 5%, 10% two kinds
-data processing:
For samples that are neither diluted nor pretreated by inoculation, simply read the mercury manometer reading on the instrument and multiply the scale factor of the selected range to obtain the sample BOD5 value.
For the diluted and inoculated samples, the mercury manometer reading can be read out. The actual BOD5 of the sample is calculated according to the following formula:
Sample BOD5 reading × scale factor-inoculation solution BOD5 reading × scale factor × inoculation solution%
BOD5 (mg / L) = × dilution factor
Sample liquid%
Where: BOD5 reading of the sample—the mercury manometer reading of the corresponding sample, in centimeters;
Inoculation fluid BOD5 reading-corresponding to the inoculation fluid mercury manometer reading, cm;
Scale factor——Acquired from the water sampling scale 1 according to the selected range;
Inoculum%-volume percentage,%;
Sample liquid% —— volume percentage,%.
Example 1: There is a sewage sample whose BOD5 value is estimated to exceed 1000 mg / L. Therefore, a dilution pretreatment is performed. The dilution factor is 2 times (ie one sample plus one dilution water). Select a range of 0 to 600 mg / L and cultivate five. Days later, the mercury manometer reads 8.8 cm Hg.
8.8 × 60ï¼0
BOD5 value (mg / L) = × 2 = 1056 (mg / L)
Example 2: An industrial wastewater sample is estimated to have a BOD5 value of approximately 80 mg / L. Existing suitable domestic wastewater is used for inoculation and culture determination. The inoculation volume is 10% (ie wastewater sample: domestic wastewater = 9: 1)
The BOD5 value of domestic sewage is about 30mg / L, and the range of 0-50mg / L is selected, and the range of industrial wastewater is 0-100mg / L (the sampling volume is according to the sampling volume of each range provided in Table 1 of the manual. Of) parallel sample test. Five days later, the mercury pressure readings of the waste water sample and the domestic sewage sample were 7.8 cm Hg and 4.0 cm Hg, respectively. The BOD5 value of this wastewater is:
7.8 × 10ï¼4.0 × 5 × 10 ï¼…
BOD5 value (mg / L) = = 84.4 (mg / L)
90%
Example 3: There is a kind of industrial wastewater that is toxic. It is estimated that the BOD5 value is about 700mg / L. The sample was diluted 10-fold before the measurement and inoculated with domestic sewage. The inoculation liquid BOD5 value was about 60mg / L and the inoculation amount was 10%. Refer to Table 1. The treated industrial wastewater and inoculum were sampled in the range of 0-100 mg / L. After five days of cultivation, the mercury manometer readings were 5.2 cm Hg and 6.1 cm Hg, respectively. This industrial wastewater BOD5 is:
5.2 × 10ï¼6.1 × 10 × 10 ï¼…
BOD5 value (mg / L) = × 10 = 510 (mg / L)
90%
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