Key points for water quality testing operations in sewage treatment plants part thirteen

70. How are the microscopic examination results recorded?

After microscopic examination of the activated sludge or biofilm biophase, the results can be recorded in Table 1.

Table 1 Recording form of biophotographic microscopy results
Floc size	Big, medium, small
Floc morphology	Round, irregular shape
Floc structure	Open, closed
Floc tightness	Tight, loose
Number of filamentous bacteria	0，±，+，++，+++
Free bacteria	Almost none, less, more
Micro animals	Dominant species (type, quantity and form)
	Other species (type, quantity and form)

 

 

71. What are the differences between biofilm biophase and activated sludge?

The biological phase characteristics of the biofilm treatment system are different from those of the activated sludge process, mainly in the types and distribution of microorganisms. Table 9-2 lists the comparison of the types, species and quantities of microorganisms appearing in biofilms and activated sludge.

Table 2 Comparison of microorganisms in biofilm and activated sludge
Microbial species	activated sludge process	biofilm method	Microbial species	activated sludge process	biofilm method
bacteria	++++	++++	Other ciliates	++	+++
Fungus	++	+++	Rotifers	+	+++
algae	-	++	Nematodes	+	++
Giardia	++	+++	Oligochaeta	-	++
Sarcopods	++	+++	other metazoans	-	+
ciliate caterpillar	++++	++++	Insects	-	++
ciliate strawworm	+	+

 

 

 

Generally speaking, due to the gradual change trend of Water Quality and the improvement of microbial growth environmental conditions, the biofilm system has more types and quantities of microorganisms than the activated sludge process, and the food chain is long and complex, especially filamentous bacteria, native The types of animals and metazoa have increased greatly, and there are also a certain proportion of anaerobic bacteria and facultative bacteria. Algae can appear in areas exposed to sunlight, and insect-like organisms such as filter flies can also appear. The distribution characteristic is that the types and quantities of microorganisms show great differences along the biofilm thickness (from the surface to the inside) or the inlet water direction (different from the inlet water contact time). In the first stage of multi-stage treatment or the upper part of the downflow packing layer, the biofilm is often dominated by jelly bacteria, and the film thickness is also larger (2~3mm); as the number of stages increases or the downflow packing In the lower part of the layer, because the water it comes into contact with has been partially treated, more filamentous bacteria, protozoa, and metazoa will gradually appear in the biofilm; the types of microorganisms continue to increase, but the thickness of the biofilm continues to decrease. (1～2mm). The microorganisms on the surface of the biofilm are all aerobic, and as the thickness increases, the microorganisms gradually become facultative or even anaerobic.

The biofilm is fixed on the filter material or filler, and the biosolids residence time SRT (mud age) is long, so it can grow microorganisms with long generation time and low proliferation rate, such as nitrifying bacteria, etc. A large number of filamentous bacteria may also appear on the biofilm, but sludge bulking will not occur. Compared with the activated sludge method, the proportion of animal nutrients among the organisms on the biofilm is larger, and the survival rate of micro-animals is also higher. They can inhabit high-trophic level organisms, such as predatory ciliates, rotifers, and nematodes. Oligochaetes and insects also inhabit it. Therefore, the food chain on the biofilm is longer than that in activated sludge, which is why the biofilm method produces less sludge than the activated sludge method.

Depending on the quality of the wastewater, the characteristic microorganisms on each stage or layer of filler will also be different. That is, changes in Water Quality will cause changes in the types and quantities of microorganisms in the biofilm. When the concentration of the incoming water increases, it can be observed that the characteristic microorganisms of the original layer move downward, that is, the microorganisms originally on the front-stage or upper-layer filler can appear on the back-stage or lower-layer filler. Therefore, such similar changes are found through biophase observations to infer changes in wastewater concentration or sludge load.

72. What is the meaning of the total number of bacteria in water?

The total number of bacteria refers to the number of colonies grown in 1mL of water sample in nutrient agar medium after incubation at 37oC for 24 hours. The unit of measurement is generally the total number of bacteria per mL of water. The total number of bacteria in water is often related to the degree of organic matter pollution of the water body. It is one of the important indicators to evaluate the degree of water pollution and the possible harm to the human body.

The analysis method of the total number of bacteria uses the standard plate method to count the bacteria in the water sample, which is a method to determine the density of aerobic and facultative anaerobic heterotrophic bacteria in the water. However, since no nutrient medium or any environmental condition can meet the physiological requirements of all bacteria in a water sample, and bacteria in water can exist in the form of individual individuals, pairs, chains, clusters or groups, so the measurement The bacterial colony count obtained is actually lower than the actual number of viable bacteria in the tested water sample.

73. What are the precautions for measuring the total number of bacteria?

Use sterile operation to absorb 1 mL water sample or 2 to 3 diluted water samples with appropriate dilution ratios, pour them into sterilized plates, then pour 15 mL nutrient agar culture medium and mix thoroughly with the water samples. Make two samples for each water sample. Parallel samples should be taken. In addition, a blank control in which only nutrient agar medium is poured should be done for each test.

Plate colony counts should be performed immediately after incubation. If counting must be postponed, the plates can be stored at 5 to 10°C, but not for more than 24 hours, and this practice cannot be regarded as a routine operation.

When counting the colonies on the plate, you can observe it with the naked eye. To prevent omissions, use a magnifying glass to check if necessary. Colonies that appear similar and are close but not touching should be counted separately as long as the distance is less than the diameter of the smallest colony. Colonies that are in close contact but differ in appearance (morphology or color) should also be counted separately.

When calculating the average number of colonies at the same dilution, if one of the plates has a large flake colony growing, it is not appropriate to use it. Instead, the plate without flake colonies should be used as the number of colonies at that dilution. If the flake-shaped colonies take up less than half of the plate and the rest of the colonies are evenly distributed, you can count the evenly grown colonies on 1/2 of the plate and multiply by 2 to represent the number of colonies on the whole plate.

The determination result of the total number of bacteria is the total number of colonies per plate or the average number of colonies on parallel experimental plates of the same dilution multiplied by the dilution factor. When the final result is within 100, record the result as the actual number of colonies; when it is greater than 100, use two significant figures and use an index of 10 to express the result. If the number of colonies cannot be counted, the dilution factor must be indicated when reporting the result.

74. How to calculate the total number of bacteria in water samples based on the colony count results?

When calculating the test results of the total number of bacteria, it is necessary to compare and calculate based on the average number of colonies at different dilutions. The method is as follows:

⑴ First, select the case where the average number of colonies is between 30 and 300 for calculation. When the average number of colonies using only one dilution meets this range, multiply the average number of colonies by its dilution factor as the result of testing the total number of bacteria in the water sample. .

⑵ If the average number of colonies in two dilutions is between 30 and 300, the calculation method should be determined based on the ratio of the two. If the ratio is less than 2, the average number of colonies multiplied by the dilution factor will be used as the result of testing the total number of bacteria in the water sample; if the ratio is greater than 2, the smaller of the average number of colonies multiplied by the dilution factor will be used. It is used as the result of testing the total number of bacteria in the water sample.

⑶ If the average number of colonies at all dilutions is greater than 300, the result of testing the total number of bacteria in the water sample should be multiplied by the average number of colonies with the largest dilution factor.

⑷If the average number of colonies at all dilutions is less than 30, the average number of colonies with the smallest dilution ratio should be multiplied by its dilution ratio as the result of testing the total number of bacteria in the water sample.

⑸ If the average number of colonies at all dilutions is not between 30 and 300, the average number of colonies closest to 30 or 300 should be multiplied by its dilution factor as the result of testing the total number of bacteria in the water sample.

75.What is the meaning of coliform count (value)?

Coliform bacteria refer to a type of aerobic or facultative anaerobic, lactose-fermenting, Gram-negative, spore-free bacilli, so they are sometimes also called fecal coliforms or Escherichia coli. Coliform bacteria are cultured on lactose. After being cultured at 37°C for 24 hours, the base can produce acid and gas. The coliform count (value) is generally measured in terms of the number of coliforms contained in 1L or 100mL of water.

If the water source is contaminated by feces, it may be contaminated by intestinal pathogens and cause intestinal infectious diseases. Since intestinal pathogenic bacteria account for a relatively small proportion of the total number of microorganisms, it is often very difficult to isolate pathogenic bacteria from water, especially tap water. Coliform bacteria are the most common and abundant type of intestinal aerobic bacteria, so they are often used as indicators of fecal pollution. That is, based on the number of coliforms in the water, it is judged whether the water source is contaminated by feces, and the possibility of the water source being contaminated by intestinal pathogens is detected.

76.What are the methods for measuring the number of coliform bacteria?

There are two commonly used determination methods for total coliforms: multi-tube fermentation method and filter membrane method.

The multi-tube fermentation method is based on the relevant characteristics of coliform bacteria such as lactose fermentation, Gram staining negative, no spores, and rod shape. It is tested through three steps to determine the total number of coliforms in the water sample. The multi-tube fermentation method uses the Most Probable Number to express the experimental results, also referred to as MPN. It is actually a method to estimate the density of E. coli and the sanitary quality of water based on statistical theory. This estimate tends to be greater than the actual number. . The estimate of coliform content is determined by the dilutions that show both positive and negative results. The actual number of replicates required for water sample testing depends on the accuracy of the data required.

The membrane filtration method uses a special sterilized microporous membrane to filter water samples. After the bacteria are trapped on the membrane, the membrane is attached to the magenta sodium sulfite medium for culture. Because coliform bacteria can ferment lactose, purple-red colonies with metallic luster will appear after culture on the filter membrane. By counting the number of colonies with this characteristic that appear on the filter membrane, you can calculate the amount of lactose contained in each L of water sample. Coliform count. The filter membrane method can measure a larger volume of water samples and can obtain results faster than the multi-tube fermentation method. However, the effect is poor when measuring high turbidity and high density of non-E. coli bacteria.

77.What is residual chlorine?

Residual chlorine is the chlorine remaining in the water after chlorination and disinfection for a certain period of time. Its function is to maintain continuous sterilization ability. From the time the water enters the pipe network to the point of use, the effect of disinfectants in the water must be maintained to prevent possible damage and repopulation of pathogens. This requires that the amount of disinfectant added to the water can not only meet the needs of killing pathogens in the water, but also retain a certain residual amount to prevent the re-proliferation of pathogens during the water transportation process. If chlorine disinfection is used , then the part of the disinfectant that exceeds the disinfection needs at that time is residual chlorine.

Residual chlorine has two forms: free residual chlorine (Cl2, HOCl and OCl-) and combined residual chlorine (NH2Cl, NHCl2 and NCl3). These two forms can exist in the same water sample at the same time. The sum of the two is called Total residual chlorine. Free residual chlorine has a strong bactericidal ability, but it is easy to decompose. Combined residual chlorine has a weak bactericidal ability, but it lasts longer in the water. Generally, when there is no ammonia or ammonium in the water, the residual chlorine is free residual chlorine. When the water contains ammonia or ammonium, the residual chlorine usually only contains combined residual chlorine. Sometimes, residual chlorine and combined residual chlorine coexist. The amount of residual chlorine must be appropriate. If it is too low, it will not be able to prevent and control pathogens. If it is too high, it will not only increase the cost of disinfection, but may also cause harm to the human body when it comes into contact with the human body.

Conceptually, residual chlorine refers to chlorine and chlorine series disinfectants. When using other non-chlorine disinfectants such as chlorine dioxide, residual chlorine should be understood as the residual disinfection remaining in the water after a certain period of contact. agent.

78.What are the methods for measuring residual chlorine? What is the scope of application of each?

Residual chlorine can be determined by iodometric titration, o-toluidine visual colorimetry, N,N-diethyl-p-phenylenediamine (DPD) ferrous titration (GB 11897-89), N,N- Diethyl p-phenylenediamine spectrophotometry (GB 11898-89), etc. The iodometric titration method can only measure the total residual chlorine in water samples; the o-toluidine visual colorimetry method can separately measure the total residual chlorine and free residual chlorine by changing the operating procedures; N,N-diethyl p-benzene Diamine titration or spectrophotometry can measure free chlorine or total chlorine in a concentration range of 0.03 to 5 mg/L. By changing the operating procedures, monochloramine, dichloramine and some combined chlorine components can also be measured respectively.

Iodometric titration is suitable for water samples with a total residual chlorine content greater than 1 mg/L, and is a commonly used method to determine the amount of chlorine added. The o-toluidine visual colorimetry method is simple to operate and is a common method for measuring residual chlorine in drinking water. The measurement range is 0.01 ~ 10mg/L. The N,N-diethyl-p-phenylenediamine titration method or spectrophotometry method is highly sensitive and can measure water samples with low residual chlorine content. It is suitable for determining the total available chlorine in sewage containing organic matter. The measurement range of the two methods They are 0.05～1.5mg/L and 0.03～5mg/L respectively.

79. What are the precautions for measuring residual chlorine?

Chlorine is very unstable in aqueous solutions, especially at low concentrations, and the content will decrease rapidly. The reduction of chlorine is accelerated by exposure to sunlight and other strong light or by agitation. Therefore, the sample cannot be stored after sampling, and the measurement of chlorine must be started immediately while avoiding light exposure and stirring of the water sample.

All operations during the measurement process should avoid direct sunlight. It is best to conduct it at the lowest possible temperature and soft light, and all colorimetric methods need to use color and turbidity blanks to compensate for the color and color of the original water. Blank values must be measured when the turbidity and color are high.

When using the o-toluidine visual colorimetry method to measure residual chlorine, if the water sample is mixed evenly with the standard o-toluidine solution and the color is measured immediately, the measured result is free residual chlorine. If it is placed in a dark place for 10 minutes, the highest level of residual chlorine will be produced. After measuring the color, perform color comparison, and the result obtained is the total residual chlorine. The total residual chlorine minus the free residual chlorine is the combined residual chlorine.

When using the o-toluidine visual colorimetry method, if the residual chlorine is large, orange will be produced; if the alkalinity of the water sample is too high and the residual chlorine is small, light green or light blue will be produced. At this time, you can add 1 mL more o-toluidine standard solution to produce a normal light yellow color.