Control of COD analysis conditions in sewage treatment
1. Key factor—representativeness of the sample
Since the water samples monitored in domestic sewage treatment are extremely uneven, the key to obtaining accurate COD monitoring results is that the sampling must be representative. To achieve this requirement, the following points need to be noted.
1.1 Shake the water sample thoroughly
For the measurement of raw water ① and treated water ②, the sample bottle should be tightly plugged and shaken thoroughly before sampling to disperse the particles and lumpy suspended solids in the water sample as much as possible so that a more uniform and representative sample can be obtained. Watery. For effluents ③ and ④ that have become clearer after treatment, the water samples should also be shaken well before taking samples for measurement. When measuring COD on a large number of domestic sewage water samples, it was found that after sufficient shaking, the measurement results of the water samples are not prone to large deviations. It shows that the sampling is more representative.
1.2 Take a sample immediately after shaking the water sample
Since the sewage contains a large amount of uneven suspended solids, if the sample is not taken quickly after shaking, the suspended solids will sink quickly. The water sample concentration, especially the composition of suspended solids, obtained by using the pipette tip for sampling at different positions at the top, middle and bottom of the sample bottle will be very different, which cannot represent the actual situation of the sewage, and the measured results are not representative. . Take a sample quickly after shaking evenly. Although bubbles are generated due to shaking (some bubbles will dissipate during the process of removing the water sample), the sampled volume will have a slight error in the absolute amount due to the presence of residual bubbles, but this is The analytical error caused by the reduction in absolute quantity is negligible compared with the error caused by the inconsistency of sample representativeness.
The control experiment of measuring water samples that were left for different times after shaking and rapid sampling and analysis immediately after shaking the samples found that the results measured by the former deviated greatly from the actual water quality conditions.
1.3 The sampling volume should not be too small
If the sampling amount is too small, certain particles that cause high oxygen consumption in the sewage, especially the raw water, may not be removed due to uneven distribution, so the measured COD results will be very different from the actual oxygen demand of the sewage. The same sample was tested under the same conditions using 2.00, 10.00, 20.00, and 50.00 mL sampling volumes. It was found that the COD results measured with 2.00 mL of raw water or final effluent were often inconsistent with the actual water quality, and the regularity of the statistical data was also very poor; 10.00 was used , the regularity of the results of the measurement of 20.00mL water sample has been greatly improved; the regularity of the COD results of the measurement of 50.00mL water sample is very good.
Therefore, for raw water with a large COD concentration, the method of reducing the sampling volume should not be used blindly to meet the requirements for the amount of potassium dichromate added and the concentration of the titrant in the measurement. Instead, it should be ensured that the sample has a sufficient sampling volume and is fully representative. The premise is to adjust the amount of potassium dichromate added and the concentration of the titrant to meet the special water quality requirements of the sample, so that the measured data will be accurate.
1.4 Modify the pipette and correct the scale mark
Since the particle size of suspended solids in water samples is generally larger than the diameter of the outlet pipe of the pipette, it is always difficult to remove the suspended solids in the water sample when using a standard pipette to transfer domestic sewage samples. What is measured in this way is only the COD value of sewage that has partially removed suspended solids. On the other hand, even if a part of the fine suspended solids is removed, because the pipette suction port is too small, it takes a long time to fill the scale, and the suspended solids that have been shaken evenly in the sewage gradually sink, and the removed material is extremely uneven. , water samples that do not represent actual water quality conditions, the results measured in this way are bound to have a large error. Therefore, using a pipette with a fine mouth to absorb domestic sewage samples to measure COD cannot give accurate results. Therefore, when pipetting domestic sewage water samples, especially water samples with a large number of suspended large particles, the pipette must be slightly modified to enlarge the diameter of the pores so that suspended solids can be quickly inhaled, and then the scale line must be corrected. , making the measurement more convenient.
2. Adjust the concentration and volume of reagents
In the standard COD analysis method, the concentration of potassium dichromate is generally 0.025mol/L, the amount added during sample measurement is 5.00mL, and the sewage sampling volume is 10.00mL. When the COD concentration of sewage is high, the method of taking less samples or diluting samples is generally used to meet the experimental limitations of the above conditions. However, Lian Huaneng provides COD reagents for samples of different concentrations. The concentrations of these reagents are converted, the concentration and volume of potassium dichromate are adjusted, and after a large number of experiments, they meet the COD detection requirements of all walks of life.
To sum up, when monitoring and analyzing water quality COD in domestic sewage, the most critical control factor is the representativeness of the sample. If this cannot be guaranteed, or any link that affects the representativeness of water quality is ignored, the measurement and analysis results will be inaccurate. errors leading to erroneous technical conclusions.
The rapid COD detection method developed by Lianhua in 1982 can detect COD results within 20 minutes. The operation is streamlined and the instrument has already established a curve, eliminating the need for titration and conversion, which greatly reduces errors caused by operations. This method has guided technological innovation in the field of water quality testing and made great contributions.
Post time: May-11-2024
