Selection of measurement methods (1) Direct culture method: This method is suitable for water samples with a BOD
5 value not exceeding 7 mg/L. (2) Dilution culture method: This method is generally used when the BOD5 of water samples is above 10mg/L. (3) Determination of instantaneous oxygen demand. For water samples containing reducing inorganic substances such as sulfides, sulfites, and ferrous substances, it is sometimes necessary to measure instantaneous oxygen demand (1DOD). In general, this step can be omitted. Factors affecting the purity of BOD dilution water The so-called BOD is the amount of oxygen consumed by the organic matter in the water sample during the biochemical decomposition process. It is expressed by the reduction of dissolved oxygen after the water sample is stored in an airtight container at a certain temperature (20°C) for a certain period of time (usually 5 days). When the oxygen consumption exceeds the dissolved oxygen in the water sample, it is necessary to dilute the water sample properly with the prepared saturated 'dilution water', and then measure the oxygen consumption. Therefore, the purity of the dilution water is an important factor affecting the determination results. According to the experimental research, the following factors cannot be ignored. (1) The blank value of BOD dilution water Under uncontrolled experimental conditions, the BODs value of 81 times of dilution water was measured according to the conventional method, and these data were processed systematically, the minimum value was 0.01mg/L, and the maximum value was 1.06mg /L, and taking the measured value as the abscissa, and the detection frequency % as the ordinate, plotting on the semi-logarithmic coordinate, only 34.6% of the measured data are below 0.2mg/L. The results of this experiment show that when BOD dilution water was prepared, the experimental conditions were not controlled, but it was polluted by organic solvents, air dust and bacteria in the laboratory, resulting in an increase in the value of B005. For this reason, the experimental conditions must be strictly controlled, so that the BOD5 value can reach the specified O.2mg/L requirement. (2) The influence of the source of BOD dilution water on the BOD5 value. Use double-distilled water and ordinary distilled water added with KMnO4, put them into culture bottles respectively, store them at 20-25°C, and put them in a 20±1°C incubator every day. , BOD5 was measured by conventional analysis method, and the results showed that the two dilution waters could reach below 0.2mg/L from the fourth day, and there was no significant difference. Foreign reports use distilled water, ion-exchanged distilled water, and distilled ion-exchanged water, which are stored at 20°C and measured daily. B005 value, the order of BOD5 value measured on the same day is as follows: Ion-exchanged water after distillation>Distilled water after ion exchange>Distilled water, the BOD5 value of the dilution water prepared only with distilled water can reach below 0.2mg/L. It can be seen that the selection of water source for BOD dilution water is very important. Ion-exchange water is easily polluted by the resin bed and should not be used. Generally, distilled water is used as the source of BOD dilution water, and its blank value can meet the specified requirements. (3) The effect of temperature on BOD dilution water. The prepared BOD dilution water is divided into BOD culture bottles and stored at four different temperatures of 15°C, 20°C, 22-25°C and 30-34°C. The method is to measure BOD5, and choose the best storage days. Stored at 15°C from the fifth day, at 20°C from the second day, at 20-25°C from the fourth day, and at 30-34°C from the fifth day, up to 0.2mg/L respectively. Experiments have shown that temperature undoubtedly has a significant impact on the BOD5 value of BOD dilution water. BOD dilution water is best stored at 20±1°C. If the laboratory is limited by conditions, the storage time can be determined according to different temperatures. Experiments have proved that the easiest way is to place the prepared dilution water at room temperature for 5-7 days, and the BOD can fully meet the specified requirements. (4) Effects on BOD5 value during the aeration process Use fresh distilled water to prepare BOD dilution water, one part is aerated with air filtered by activated carbon, and the other part is not filtered. Store in an incubator at 20±1°C, and measure the BOD5 value daily by conventional methods. The daily BOD5 of the diluted water filtered with activated carbon was 0.15 mg/L, and the daily BODs of the diluted water filtered without activated carbon was 0.25 mg/L. The results showed that the use of activated carbon to adsorb and treat pollutants in the air significantly improved the quality of the diluted water.
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