Rapid measurement method for biochemical oxygen demand (BOD)
1.1 Theme content
This standard specifies a rapid method for the measurement of biochemical oxygen demand (BOD) in water and sewage. The biochemical oxygen demand specified in this standard refers to the amount of dissolved oxygen consumed by soluble biochemically degradable organic matter in water and sewage under the action of microorganisms.
1.2 Scope of application
This standard applies to the determination of BOD in surface water, domestic sewage and industrial waste water that does not have a significant toxic effect on microorganisms.
1.3 Interference and elimination
The maximum allowable amount of the following substances in water without significant interference to the determination of this method is: Co2 + 5mg / l; Mn2 + 5mg / l; Zn2 + 4mg / l; Fe2 + 5mg / l; Cu2 + 2mg / l; Hg2 + 2mg / l; Pb2 + 5mg / l; Cd2 + 5mg / l; Cr6 + 0.5mg / l; CN-0.05mg / l; 250mg / l suspension. For samples containing free or bound chlorine, 1.575g / l sodium sulfite solution can be added to make the free or bound chlorine in the sample invalid. Avoid adding excessive amounts of high-concentration bactericides and pesticides that have a toxic effect on the bacteria in the microbial membrane. Wastewater is not suitable for this method.
2.1 Biochemical oxygen demand
Under certain conditions, microorganisms decompose certain oxidizable substances present in water, especially organic matter, which consumes the amount of dissolved oxygen in the biochemical process.
2.2 Microbial membrane
The filamentous yeast is sealed in a membrane while maintaining its physiological function, and is called a microbial membrane or an immobilized microbial membrane.
2.3 Microbial sensors
The microbial sensor is composed of an oxygen electrode and an immobilized microbial membrane. It can detect changes in oxygen concentration caused by microorganisms when degrading organic matter.
The water sample or cleaning solution is continuously fed into the measuring cell by a certain amount ratio within a unit time under the action of the peristaltic pump.
2.5 Discontinuous (joint)
Add the buffer solution to the measurement tank, keep the microbial sensor (microbial membrane) in contact with the buffer solution, and then add a quantitative water sample to measure the BOD value of the water sample.
2.6 Constant temperature control device
The reaction performance of the microbial electrode depends on certain temperature conditions, so a stable temperature field is required during the test. This device is called a constant temperature control device in the instrument.
2.7 Cleaning solution (buffer solution)
The cleaning solution is composed of potassium dihydrogen phosphate and disodium hydrogen phosphate. Its main function is to adjust the pH value of the sample as a buffer solution, to clean and maintain the microbial sensor to make it work normally, and to settle heavy metal ions.
The microbial sensor for measuring BOD in water is composed of an oxygen electrode and a microbial membrane. The principle is that when a sample containing saturated dissolved oxygen enters the flow cell and comes into contact with the microbial sensor, the soluble biodegradable organic matter in the sample is exposed to bacteria in the microbial membrane This kind of effect, while consuming a certain amount of oxygen, reduces the mass of oxygen on the surface of the diffused oxygen electrode. When the diffusion rate (mass) of biodegradable organic matter in the sample to the bacterial membrane reaches a constant level, the mass of oxygen diffused to the surface of the oxygen electrode also reaches a constant level, so a constant current is generated. Because there is a quantitative relationship between the difference between the constant current and the amount of oxygen reduction, the biochemical oxygen demand in the sample can be calculated based on this.
Analyze pure reagents and distilled water. The distilled water should be boiled for about 2-5 minutes before use and left at room temperature before use.
4.1 phosphate buffer solution: 0.5mol / l
68g of potassium dihydrogen phosphate (KH2PO4) and 134g of disodium hydrogen phosphate (Na2HPO4 · 7H2O) were dissolved in distilled water, diluted to 1000ml, and used. The pH of this solution is about 7.
4.2 Phosphate buffer solution (cleaning solution): 0.005mol / l
4.3 Hydrochloric acid (HCl) solution: 0.5mol / l
4.4 Sodium hydroxide (NaOH) solution: 20g / l
4.5 Sodium sulfite (Na2SO3) solution: 1.575g / l, this solution is not stable, it should be configured immediately before use.
4.6 Glucose-glutamic acid standard solution
Weigh 1.705 g each of anhydrous glucose (C6H12O6) and glutamic acid (HOOC-CH2-CH2-CHNH2-COOH), which were dried at 103oC for 1 h and cooled to room temperature, and dissolved in a 4.2 phosphate buffer solution. The solution was diluted to 1000ml and mixed to obtain a 2500mg / l BOD standard solution.
4.7 glucose-glutamic acid standard use solution (configured before use)
Take 10.00ml of the standard solution in 4.6 and place it in a 250ml volumetric flask. Use a 0.005mol / l phosphate buffer solution to make up to the mark and shake well. The concentration of this solution is 100mg / l. .
The glass instruments and plastic containers used must be carefully cleaned, and no poisonous or biodegradable compounds can be stored on the container walls. Contamination should be prevented during operation.
5.1 Microbial sensor BOD rapid tester
5.2 Microbial film: The bacteria in the microbial film should be uniform, and the film should be as consistent as possible. The storage method can be stored wet or dried at room temperature. The continuous life of the microbial film should be greater than 30 days.
5.3 Activation of microbial membranes: Soak the microbial membranes in 0.005mol / l phosphate buffer solution for more than 48 hours, and then install them on the microbial sensor.
5.410L polyethylene plastic bucket
6 Storage of samples
When samples cannot be analyzed within 2 hours after collection, they should be stored at 0-4oC and analyzed within 6 hours. When analysis cannot be performed within 6 hours, the storage time and temperature should be reported together with the analysis results. It must never be stored for more than 24 hours at any time.
7. Operation steps
7.1 Sample pretreatment
7.1.1 Neutralization of samples
If the pH of the sample is not between 4-10, use a hydrochloric acid solution (4.3) or a sodium hydroxide solution (4.4) to neutralize the sample to a pH of about 7.
7.1.2 Preparation of test samples
(1) Place the sample at room temperature.
(2) Surface water samples can be directly measured in accordance with 7.3.3 without dilution (without special circumstances).
(3) The dilution factor of domestic sewage and industrial wastewater can be determined based on experience or expected BOD value, so that the BOD value is controlled below 50mg / l as the sample to be tested.
7.2 Measurement steps
7.2.1 The instrument should be turned on before measurement, and the microbial sensor should be washed with phosphate buffer solution (4.2) until the potential Eo (or current Io) is stable.
7.2.2 Drawing of working curve
(1) Take 5 50ml stopper colorimetric tubes and add glucose-glutamic acid standard use solution (4.7) 1.50ml; 3.50ml; 7.50ml; 12.50ml; 25.00ml, use 0.005mol / l phosphate buffer Dilute the solution (4.2) to the mark and shake well.
(2) Measure the difference between the potential Eo (or the current Io) by injection (the difference is proportional to the concentration of BOD).
(3) Use the concentration of 5 different standard solutions to plot the potential difference △ E (or current difference △ I) to draw a working curve.
7.2.3 Determination of samples
Take 50 ml of the pretreated sample and add 0.5 ml of 0.5 mol / l phosphate buffer solution (4.1).
8 result representation
Directly read the instrument to display the measured concentration value; or check the concentration of BOD in the water sample (mg / l) from the working curve.
Precision and accuracy
Four laboratories analyzed uniform distribution standard solutions with BOD content of 25.3mg / l and 10.3mg / l. The results are as follows:
The relative standard deviations in the laboratory were 3.0% and 2.6%.
The relative standard deviations between laboratories were 3.5% and 2.7%.
Four laboratories determined 50.6 mg / l of a uniformly known BOD sample with a relative error of -2.0-2.8.
Note: 1. The injection volume can be adjusted, but the injection volume of a single sample should not be less than 10ml in any case.
2. In order to shorten the measurement period, it is best to dilute the BOD value in the water sample to 25mg / l.
3. The storage conditions for measuring BOD water samples are the same as GB7488-87.