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The Details Hidden Behind the Sewage Plant Tests are Ignored by 90% of People!


Time:

2022-10-11

In order to obtain reliable monitoring results, the inlet and outlet water quality of the sewage plant must be tested and monitored every day. Let's understand the details hidden behind the sewage test monitoring!

Common Analytical Instruments for Wastewater Treatment Plant Test

In general, the laboratory of sewage treatment plant has electronic balance, spectrophotometer, acidimeter and other commonly used analytical instruments. When using these instruments, special attention should be paid to some details.


For example, the electronic balance must be preheated to a specified time when the power is turned on. Also, never allow the weighing of the balance to exceed its maximum weighing limit to avoid damaging the balance.
For spectrophotometer, a voltage regulator shall be used to provide stable power supply voltage to ensure the stability of light source output; It should be placed in a relatively dry place.


Acidimeter shall be preheated for at least 15-20min before use. When measuring the water sample, rinse the sample cup and electrode with clean water, then rinse the measured water sample for more than 2 times, and then immerse the electrode for pH measurement. After the pH meter reading is stable, the measured value is the pH value of the solution. When replacing each sample to be tested, the electrode shall be fully rinsed with water, then the water drops shall be sucked off with filter paper and wiped dry, and finally the solution to be tested shall be used for rinsing to eliminate mutual influence.

 

Determination of Basic Items of Sewage Test

Determination of chemical oxygen demand
The principle of potassium dichromate method for determining chemical oxygen demand is: in strong acidic solution, a certain amount of potassium dichromate oxidation water sample contains reducing substances, excessive potassium dichromate is used as indicator, and ammonium ferrous sulfate solution is used for back dropping. Calculate the amount of oxygen consumed by reducing substances in the water sample according to the amount used.
For waste water samples with high chemical oxygen demand, take appropriate waste water samples and reagents, put them in 15x150mm hard glass test tubes, shake them well, and observe whether they turn green after heating.
If the solution is green, reduce the amount of waste water sampling appropriately until the solution is not green, so as to determine the volume to be taken for waste water sample analysis. (Note that the amount of waste water sample taken shall not be less than 5mL during dilution. If the chemical oxygen demand is very high, the waste water sample shall be diluted for many times.)
Some laboratories use a digestion instrument to digest the water sample. When the temperature reaches the set value, if there is enough time, it is better to let the temperature of the digestion furnace automatically balance for about 15min, so that the temperature uniformity of the digestion furnace is better, and the digestion temperature of each tube is more uniform and accurate. After sample digestion, for safety reasons, the digestion tube should be taken out after the power is turned off and cooled.

 

Determination of dissolved oxygen
Dissolved oxygen in water is determined by iodometry. In the "Fixation of Dissolved Oxygen" step, when the brown sediment drops to half of the bottle, mix it upside down again. When the sediment drops to the bottom of the bottle, proceed to the next step of "iodine precipitation". In the step of "iodine precipitation", after all the precipitates are dissolved, the dissolved oxygen bottle should be placed in a dark place for 5min. After the reaction is complete, Na2S2O3 should be used immediately for titration.


Determination of five-day biochemical oxygen demand
The classic determination method of BOD is dilution inoculation method.
For some surface water and most industrial wastewater, because they contain more organic substances, they need to be diluted and then cultured for determination to reduce their concentration and ensure sufficient dissolved oxygen. In the process of culture, attention must be paid to adding the sealing water of dissolved oxygen bottle, so that the mouth of dissolved oxygen bottle is always in a water sealed state.
After five days and nights from the start of putting into the incubator, discard the sealing water and measure the remaining dissolved oxygen.
Sometimes, after adding manganese sulfate and alkaline potassium iodide solution, white precipitate instead of brown precipitate will be generated, which indicates that the dilution ratio is too small, and there is not enough dissolved oxygen in the bottle to oxidize the white Mn (OH) 2 precipitate into brown MnO (OH) 2 or MnO2 · H2O after culture. The dilution ratio should be increased.
When the dilution ratio of water sample exceeds 100 times, it shall be preliminarily diluted with water in a volumetric flask in advance, and then appropriate amount shall be taken for final dilution and culture.


Determination of nitrogen
For ammonia nitrogen determination, the water sample shall be properly pretreated during analysis.
The water sample shall be pretreated with flocculation sedimentation method. During filtration, medium speed filter paper fully washed with ammonia free water shall be used for filtration, and 20mL of primary filtrate shall be discarded.
Pretreatment of water sample by distillation includes pretreatment of distillation unit and pretreatment of water sample. Attention shall be paid to the connection of distillation unit to prevent air leakage and backdraft. Avoid boiling during distillation, otherwise the temperature of distillate will rise and ammonia absorption will be incomplete.
Ammonia nitrogen shall be measured by titration method. Soft latex tube shall not be used for reflux condenser tube, otherwise it is easy to age, deform and the cooling water is not smooth. There should be no temperature sense when touching the cooling water, otherwise the measurement result is low.
Ammonia nitrogen is measured by Nessler reagent photometric method. The ratio of mercuric iodide and iodide bell in Nessler reagent has a great influence on the sensitivity of color reaction. The sediment generated after standing shall be removed.
There are many methods for the determination of nitrate nitrogen in water, and the Descartes alloy reduction method is most suitable for heavily polluted and dark water samples.
When this method is used in the experiment, the water sample is pretreated, and then the Dyslet alloy is added. Before distillation, it should be placed for a few minutes and heated carefully to gradually increase the temperature. When it is near the boiling point or fierce bubbles are generated, the temperature must be properly reduced to avoid the rapid escape of a large amount of gas affecting absorption.


Determination of phosphorus
Pre treat the water sample. Take mixed water samples (including suspended solids), and use potassium persulfate and other strong oxidants for oxidation and decomposition to measure the total phosphorus content in water.
When determined by molybdenum antimony anti spectrophotometry, the prepared 10% (m/v) ascorbic acid solution is stored in a brown glass bottle and can be stable for several weeks in a cold place. The prepared molybdate solution shall be stored in a brown glass bottle in a cold place and be stable for at least 2 months. In order to prevent reagent deterioration, it is not necessary to prepare too much at one time, but the required volume can be prepared according to the proportion in the book.


Preparation of activated sludge sample
The activated sludge in the activated sludge aeration tank is the main body of wastewater treatment. They are composed of bacteria, molds, yeasts, actinomycetes, protozoa and other microorganisms, as well as metazoans such as rotifers, nematodes and solid substances in wastewater. The preparation of activated sludge samples should be emphasized in the biological phase observation during the test.
There are two main points for the preparation of activated sludge samples:
First, take a small drop of mixed liquid from the activated sludge process aeration tank and place it in the center of the clean glass slide;
The other is to cover it carefully with a clean cover slip.
In the first article, it should be noted that if there is less sludge in the mixed solution, a small drop of the precipitated sludge can be added to the slide after it is precipitated; If there is a lot of sludge in the mixed solution, it should be observed after dilution. In Article 2, it should be noted that when the glass slide is covered, it should not be put down until its center has contacted the water drop, otherwise bubbles will form in the slide and affect the observation. 

 

Operating Procedures for Sewage Test

1) Washing glassware
The cleanliness of glassware directly affects the accuracy and precision of water quality inspection results. It is a necessary factor to ensure the cleanness of glassware used in the experiment by using different lotion according to different needs in the use process.


2) Collect water sample
The quality control of the sewage sampling process can ensure the representativeness and integrity of the samples obtained, and can fully reflect the quality of the incoming and outgoing water of the sewage treatment plant and the degree of reaching the standard of sewage treatment.
The collection of water samples with wide sewage can be roughly divided into three methods: fixed time interval sampling, cumulative flow sampling and instantaneous sampling. Different sampling methods can be adopted according to the actual situation and specific requirements. The preservation of samples shall also meet the preservation requirements of sewage water samples, such as adding oxidation inhibitors or maintaining a certain temperature.


3) Determine the blank value of the whole procedure
The size and dispersion of the whole procedure blank test value have a great impact on the precision and accuracy of the analysis results, reflecting the level of water quality analysis laboratories and analysts to a certain extent.
In routine analysis, two parallel samples of full procedure blank experiment are determined each time, and the relative deviation is generally<50%. The average value is taken as the blank correction value of the measurement results of the same batch of samples.
The blank value of the whole procedure can reflect the influence of many factors such as the quality of water used in the laboratory, the quality of reagents used, the environmental quality of the laboratory and the technical level of the analysts on the determination results.


4) Drawing and inspection of standard curve
For the standard series, after the solution is measured with pure solvent as reference, blank correction shall be made first, and then the standard curve shall be drawn.
Generally, the standard solution can be directly determined, but when the pretreatment of water sample is complex and the pollution or loss cannot be ignored, it should be determined after the same treatment as the water sample. For example, in the detection of total nitrogen, the standard solution should be digested first and then determined just like the water sample.
The standard curve often changes with the change of environmental temperature, reagent batch number and other conditions. It is important to correct the standard curve when conditions change.
The test of standard curve includes linearity test, intercept test and slope test. Linear test is to verify the precision of standard curve; Intercept test is to test the accuracy of standard curve; The slope test generally tests the sensitivity of analytical methods.


5) Determination of parallel samples
When parallel samples are measured, the same sample is required to be analyzed synchronously under identical conditions. The number of parallel samples can be arranged according to the complexity of the sample, the precision of the method and instrument used, and other factors. If conditions permit, parallel double sample analysis shall be conducted for all samples, otherwise parallel sample determination shall be conducted at least 20% of the number of samples in one month every year.
When the qualification rate of parallel double sample measurement is less than 95%, in addition to the re parallel double sample measurement for unqualified samples, 10% of the parallel double samples shall be added for measurement. When taking water samples for parallel determination, the analyst takes two samples of the same water sample for determination at the same time. The relative standard deviation of the test results of parallel water samples should not be greater than 2.83 times the relative standard deviation listed in the standard method.


6) Use of reference materials
The standard solution with known concentration can be prepared and determined by the same method as the sample detection. If the measured result is close to the standard, it indicates that the method is correct within the error range.
The spiking recovery is also an important means to effectively test the test results. When spiking, the concentration of the standard solution should be large and the volume should be small (the volume can be ignored), and the amount added is generally 0.5~2 times of the sample content. The total amount of the tested object after adding the standard shall not exceed the upper limit of detection.
The spiking recovery is one of the most commonly used methods to evaluate the accuracy of results, but it has some limitations. That is to say, when the spiking recovery rate is satisfactory, it is not sure that the measurement accuracy is OK, but when it exceeds the required range, the determination result must be wrong.
The recovery rate of all instrumental analysis items is measured twice a year. The measured spiked recovery rate shall not exceed the recovery rate range listed in the standard method or unified method, and the recovery rate range not listed is generally controlled at 90%~110%.
The spiking amount shall be controlled within the measurement precision range of the substance to be measured in the sample. When the content of the substance to be measured in the sample is close to the detection limit of the method, the spiking amount shall be controlled within the low concentration range of the standard curve.


7) Repeated determination of sample
There are many methods for repeated determination of samples, including the following:
First, the same analyst shall retest the reserved samples of different batches. The purpose is to investigate the measurement accuracy between two batches of samples.
Second, the same sample was determined by different analysts with the same method. This is to investigate the measurement error between different laboratory personnel.
Third, use different methods to determine the same sample. After the sample is determined by the specified method, it can be rechecked by other methods to find the possible systematic error of the specified method.


8) Correlation of test results of analytical samples
Different indicators of the same water sample often have a certain correlation, some are qualitative, some are quantitative. The correlation between different indicators of the same sample can sometimes directly reflect the accuracy of the test results.

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