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

19. How many water sample dilution methods are there when measuring BOD5? What are the operating precautions?

When measuring BOD5, water sample dilution methods are divided into two types: general dilution method and direct dilution method. The general dilution method requires a larger amount of dilution water or inoculation dilution water.

The general dilution method is to add about 500mL of dilution water or inoculation dilution water to a 1L or 2L graduated cylinder, then add a calculated certain volume of water sample, add more dilution water or inoculation dilution water to the full scale, and use a rubber at the end to The round glass rod is slowly stirred up or down under the water surface. Finally, use a siphon to introduce the evenly mixed water sample solution into the culture bottle, fill it with a little overflow, carefully cap the bottle stopper, and seal it with water. Bottle mouth. For water samples with the second or third dilution ratio, the remaining mixed solution can be used. After calculation, a certain amount of dilution water or inoculated dilution water can be added, mixed and introduced into the culture bottle in the same way.

The direct dilution method is to first introduce about half the volume of dilution water or inoculation dilution water into a culture bottle of known volume by siphoning, and then inject the volume of water sample that should be added to each culture bottle calculated based on the dilution factor along the bottle wall. , then introduce dilution water or inoculate dilution water to the bottleneck, carefully close the bottle stopper, and seal the bottle mouth with water.

When using the direct dilution method, special attention should be paid to not introducing the dilution water or inoculating the dilution water too quickly at the end. At the same time, it is necessary to explore the operating rules for introducing the optimal volume to avoid errors caused by excessive overflow.

No matter which method is used, when introducing the water sample into the culture bottle, the action must be gentle to avoid bubbles, air dissolving into the water or oxygen escaping from the water. At the same time, be sure to be careful when capping the bottle tightly to avoid air bubbles remaining in the bottle, which may affect the measurement results. When the culture bottle is cultured in the incubator, the water seal should be checked every day and filled with water in time to prevent the sealing water from evaporating and allowing air to enter the bottle. In addition, the volumes of the two culture bottles used before and after 5 days must be the same to reduce errors.

20. What are the possible problems that may arise when measuring BOD5?

When the BOD5 is measured on the effluent of a sewage treatment system with nitrification, since it contains many nitrifying bacteria, the measurement results include the oxygen demand of nitrogen-containing substances such as ammonia nitrogen. When it is necessary to distinguish the oxygen demand of carbonaceous substances and the oxygen demand of nitrogenous substances in water samples, the method of adding nitrification inhibitors to the dilution water can be used to eliminate nitrification during the BOD5 determination process. For example, adding 10mg 2-chloro-6-(trichloromethyl)pyridine or 10mg propenyl thiourea, etc.

BOD5/CODCr is close to 1 or even greater than 1, which often indicates that there is an error in the testing process. Each link of the testing must be reviewed, and special attention must be paid to whether the water sample is taken evenly. It may be normal for BOD5/CODMn to be close to 1 or even greater than 1, because the degree of oxidation of organic components in water samples by potassium permanganate is much lower than that of potassium dichromate. The CODMn value of the same water sample is sometimes lower than the CODCr value. a lot of.

When there is a regular phenomenon that the greater the dilution factor and the higher the BOD5 value, the reason is usually that the water sample contains substances that inhibit the growth and reproduction of microorganisms. When the dilution factor is low, the proportion of inhibitory substances contained in the water sample is greater, making it impossible for bacteria to carry out effective biodegradation, resulting in low BOD5 measurement results. At this time, the specific components or causes of the antibacterial substances should be found, and effective pretreatment should be carried out to eliminate or mask them before measurement.

When BOD5/CODCr is low, such as below 0.2 or even below 0.1, if the measured water sample is industrial wastewater, it may be because the organic matter in the water sample has poor biodegradability. However, if the measured water sample is urban sewage or mixed with certain Industrial wastewater, which is a proportion of domestic sewage, is not only because the water sample contains chemical toxic substances or antibiotics, but the more common reasons are non-neutral pH value and the presence of residual chlorine fungicides. In order to avoid errors, during the BOD5 measurement process, the pH values of the water sample and dilution water must be adjusted to 7 and 7.2 respectively. Routine inspections must be conducted on water samples that may contain oxidants such as residual chlorine.

21. What are the indicators indicating plant nutrients in wastewater?

Plant nutrients include nitrogen, phosphorus and other substances that are required for plant growth and development. Moderate nutrients can promote the growth of organisms and microorganisms. Excessive plant nutrients entering the water body will cause algae to multiply in the water body, resulting in the so-called "eutrophication" phenomenon, which will further deteriorate Water Quality, affect fishery production and harm human health. Severe eutrophication of shallow lakes can lead to lake swamping and death.

At the same time, plant nutrients are essential components for the growth and reproduction of microorganisms in activated sludge, and are a key factor related to the normal operation of the biological treatment process. Therefore, plant nutrient indicators in water are used as an important control indicator in conventional sewage treatment operations.

Water Quality indicators indicating plant nutrients in sewage are mainly nitrogen compounds (such as organic nitrogen, ammonia nitrogen, nitrite and nitrate, etc.) and phosphorus compounds (such as total phosphorus, phosphate, etc.). In conventional sewage treatment operations, they are generally Monitor ammonia nitrogen and phosphate in incoming and outgoing water. On the one hand, it is to maintain the normal operation of biological treatment, and on the other hand, it is to detect whether the effluent meets the national discharge standards.

22.What are the water quality indicators of commonly used nitrogen compounds? How are they related?

Commonly used water quality indicators representing nitrogen compounds in water include total nitrogen, Kjeldahl nitrogen, ammonia nitrogen, nitrite and nitrate.

Ammonia nitrogen is nitrogen that exists in the form of NH3 and NH4+ in water. It is the first step product of the oxidative decomposition of organic nitrogen compounds and is a sign of water pollution. Ammonia nitrogen can be oxidized into nitrite (expressed as NO2-) under the action of nitrite bacteria, and nitrite can be oxidized into nitrate (expressed as NO3-) under the action of nitrate bacteria. Nitrate can also be reduced to nitrite under the action of microorganisms in an oxygen-free environment. When the nitrogen in the water is mainly in the form of nitrate, it can indicate that the content of nitrogen-containing organic matter in the water is very small and the water body has reached self-purification.

The sum of organic nitrogen and ammonia nitrogen can be measured using the Kjeldahl method (GB 11891--89). The nitrogen content of water samples measured by the Kjeldahl method is also called Kjeldahl nitrogen, so the commonly known Kjeldahl nitrogen is ammonia nitrogen. and organic nitrogen. After removing ammonia nitrogen from the water sample, it is then measured by the Kjeldahl method. The measured value is organic nitrogen. If Kjeldahl nitrogen and ammonia nitrogen are measured separately in water samples, the difference is also organic nitrogen. Kjeldahl nitrogen can be used as a control indicator for the nitrogen content of the incoming water of sewage treatment equipment, and can also be used as a reference indicator for controlling the eutrophication of natural water bodies such as rivers, lakes and seas.

Total nitrogen is the sum of organic nitrogen, ammonia nitrogen, nitrite nitrogen and nitrate nitrogen in the water, which is the sum of Kjeldahl nitrogen and total oxide nitrogen. Total nitrogen, nitrite nitrogen and nitrate nitrogen can all be measured using spectrophotometry. For the analysis method of nitrite nitrogen, see GB7493-87, for the analysis method of nitrate nitrogen, see GB7480-87, and for the total nitrogen analysis method, see GB 11894- -89. Total nitrogen represents the sum of nitrogen compounds in water. It is an important indicator of natural water pollution control and an important control parameter in the sewage treatment process.

23. What are the precautions for measuring ammonia nitrogen?

The commonly used methods for determination of ammonia nitrogen are colorimetric methods, namely Nessler's reagent colorimetric method (GB 7479--87) and salicylic acid-hypochlorite method (GB 7481--87). Water samples can be preserved by acidification with concentrated sulfuric acid. The specific method is to use concentrated sulfuric acid to adjust the pH value of the water sample to between 1.5 and 2, and store it in a 4oC environment. The minimum detection concentrations of the Nessler reagent colorimetric method and the salicylic acid-hypochlorite method are 0.05mg/L and 0.01mg/L (calculated in N) respectively. When measuring water samples with a concentration above 0.2mg/L When , the volumetric method (CJ/T75--1999) can be used. In order to obtain accurate results, no matter which analysis method is used, the water sample must be pre-distilled when measuring ammonia nitrogen.

The pH value of water samples has a great influence on the determination of ammonia. If the pH value is too high, some nitrogen-containing organic compounds will be converted into ammonia. If the pH value is too low, part of the ammonia will remain in the water during heating and distillation. In order to obtain accurate results, the water sample should be adjusted to neutral before analysis. If the water sample is too acidic or alkaline, the pH value can be adjusted to neutral with 1mol/L sodium hydroxide solution or 1mol/L sulfuric acid solution. Then add phosphate buffer solution to maintain the pH value at 7.4, and then perform distillation. After heating, ammonia evaporates from the water in a gaseous state. At this time, 0.01~0.02mol/L dilute sulfuric acid (phenol-hypochlorite method) or 2% dilute boric acid (Nessler's reagent method) is used to absorb it.

For some water samples with a large Ca2+ content, after adding phosphate buffer solution, Ca2+ and PO43- generate insoluble Ca3(PO43-)2 precipitate and release H+ in the phosphate, which lowers the pH value. Obviously, other Ions that can precipitate with phosphate can also affect the pH value of water samples during heated distillation. In other words, for such a water sample, even if the pH value is adjusted to neutral and a phosphate buffer solution is added, the pH value will still be far lower than the expected value. Therefore, for unknown water samples, measure the pH value again after distillation. If the pH value is not between 7.2 and 7.6, the amount of buffer solution should be increased. Generally, 10 mL of phosphate buffer solution should be added for every 250 mg of calcium.

24. What are the water quality indicators that reflect the content of phosphorus-containing compounds in water? How are they related?

Phosphorus is one of the elements necessary for the growth of aquatic organisms. Most of the phosphorus in water exists in various forms of phosphates, and a small amount exists in the form of organic phosphorus compounds. Phosphates in water can be divided into two categories: orthophosphate and condensed phosphate. Orthophosphate refers to phosphates that exist in the form of PO43-, HPO42-, H2PO4-, etc., while condensed phosphate includes pyrophosphate and metaphosphoric acid. Salts and polymeric phosphates, such as P2O74-, P3O105-, HP3O92-, (PO3)63-, etc. Organophosphorus compounds mainly include phosphates, phosphites, pyrophosphates, hypophosphites and amine phosphates. The sum of phosphates and organic phosphorus is called total phosphorus and is also an important water quality indicator.

The analysis method of total phosphorus (see GB 11893--89 for specific methods) consists of two basic steps. The first step is to use oxidants to convert different forms of phosphorus in the water sample into phosphates. The second step is to measure orthophosphate, and then reverse Calculate the total phosphorus content. During routine sewage treatment operations, the phosphate content of the sewage entering the biochemical treatment device and the effluent of the secondary sedimentation tank must be monitored and measured. If the phosphate content of the incoming water is insufficient, a certain amount of phosphate fertilizer must be added to supplement it; if the phosphate content of the secondary sedimentation tank effluent exceeds the national first-level discharge standard of 0.5mg/L, phosphorus removal measures must be considered.

25. What are the precautions for phosphate determination?

The method for measuring phosphate is that under acidic conditions, phosphate and ammonium molybdate generate phosphomolybdenum heteropoly acid, which is reduced to a blue complex (referred to as molybdenum blue) using the reducing agent stannous chloride or ascorbic acid. Method CJ/T78--1999), you can also use alkaline fuel to generate multi-component colored complexes for direct spectrophotometric measurement.

Water samples containing phosphorus are unstable and are best analyzed immediately after collection. If the analysis cannot be carried out immediately, add 40 mg mercury chloride or 1 mL concentrated sulfuric acid to each liter of water sample for preservation, and then store it in a brown glass bottle and place it in a 4oC refrigerator. If the water sample is only used for analysis of total phosphorus, no preservative treatment is required.

Since phosphate can be adsorbed on the walls of plastic bottles, plastic bottles cannot be used to store water samples. All glass bottles used must be rinsed with dilute hot hydrochloric acid or dilute nitric acid, and then rinsed several times with distilled water.

26. What are the various indicators that reflect the content of solid matter in water?

Solid matter in sewage includes floating matter on the water surface, suspended matter in the water, sedimentable matter sinking to the bottom and solid matter dissolved in the water. Floating objects are large pieces or large particles of impurities that float on the water surface and have a density less than water. Suspended matter is small particle impurities suspended in the water. Sedimentable matter is impurities that can settle at the bottom of the water body after a period of time. Almost all sewage contains sedimentable matter with complex composition. The sedimentable matter mainly composed of organic matter is called sludge, and the sedimentable matter mainly composed of inorganic matter is called residue. Floating objects are generally difficult to quantify, but several other solid substances can be measured using the following indicators.

The indicator that reflects the total solid content in water is total solids, or total solids. According to the solubility of solids in water, total solids can be divided into dissolved solids (Dissolved Solid, abbreviated as DS) and suspended solids (Suspend Solid, abbreviated as SS). According to the volatile properties of solids in water, total solids can be divided into volatile solids (VS) and fixed solids (FS, also called ash). Among them, dissolved solids (DS) and suspended solids (SS) can be further subdivided into volatile dissolved solids, non-volatile dissolved solids, volatile suspended solids, non-volatile suspended solids and other indicators.