How Water Quality Is Measured And Assessed

Water is integral to life on Earth, It is essential to the survival of people, organisms, and economies.Water quality is not just an individual issue but also a matter of environmental justice. However issues surrounding water resources and management are not rooted in the question of whether there is enough water on our planet; rather, they are driven by the state of the water available to us. Is it clean or contaminated? Access to clean water is, therefore, a matter of life and death.

Common water quality standards can be divided into physical, chemical and biological indicators. For example, indicators such as color, temperature, and turbidity are physical indicators, and PH, COD, ammonia nitrogen, total nitrogen, total phosphorus, etc. are chemical indicators, while indicators involving microorganisms such as fecal coliforms are biological indicators.

Chemical indicators

1. Chemical oxygen demand (COD)

Chemical oxygen demand is the amount of oxidant consumed when a certain strong oxidant is used to treat water samples under certain conditions. It is an indicator of the amount of reducing substances in water. Reducing substances in water include various organic substances, nitrites, sulfides, ferrous salts, etc. But the main thing is organic matter. Therefore, chemical oxygen demand (COD) is often used as an indicator to measure the content of organic matter in water. The greater the chemical oxygen demand, the more serious the water body is polluted by organic matter. The measurement of COD is the main daily monitoring item of the sewage treatment plant. By measuring the COD of the inlet and outlet water of different structures, the operation of the structure can be accurately grasped. By analyzing the data for a period of time, the operation of the structure can be appropriately adjusted to ensure Wastewater treatment effect. In addition, for sewage plant effluent, COD is an item that must be monitored, and the effluent should meet the corresponding national standards. For the measurement of chemical oxygen demand (COD), the measured values vary depending on the reducing substances in the water sample and the measurement methods. The most commonly used methods at present are the acidic potassium permanganate oxidation method and the potassium dichromate oxidation method. Potassium permanganate (KmnO4) has a low oxidation rate, but is relatively simple and can be used when measuring the relative comparative value of organic matter content in water samples. The potassium dichromate (K2CrO7) method has high oxidation rate and good reproducibility, and is suitable for determining the total amount of organic matter in water samples.

2. Biochemical oxygen demand (BOD)

Biochemical oxygen demand is the amount of oxygen consumed when the organic matter that can be decomposed in the water is completely oxidized and decomposed due to the action of microorganisms under aerobic conditions. It is expressed by the amount of dissolved oxygen reduced (mg/L) after the water sample is stored in a sealed container at a certain temperature (such as 20°C) for a certain period of time. When the temperature is 20°C, it takes about 20 days for general organic matter to basically complete the oxidative decomposition process, and it takes 100 days to completely complete the decomposition process. But such a long time loses its practical value for actual production control. Therefore, it is currently stipulated that culturing for 5 days at 20°C is used as the standard for measuring biochemical oxygen demand. If the quantity and composition of organic matter in sewage are relatively stable, there may be a certain proportional relationship between the two, and they can be calculated from each other. The ratio of BOD to COD in domestic sewage is roughly 0.4 to 0.8. For certain sewage, generally speaking, COD>BOD20>BOD5.

BOD5 is also one of the important daily monitoring items in sewage treatment plants. The specific meaning of BOD5 monitoring is basically the same as COD.

However, due to the drainage system of rivers in our country, urban sewage plant sewage contains a certain amount of industrial wastewater. Compared with domestic sewage, the quality of industrial wastewater changes greatly and is difficult to degrade. By monitoring BOD and COD in the incoming water of sewage plants, , can roughly judge the biodegradability of sewage. The classic method for measuring biochemical oxygen demand is the dilution inoculation method.

3. Dissolved oxygen DO

Molecular oxygen dissolved in water is called dissolved oxygen. The dissolved oxygen content of natural water depends on the balance of oxygen in the water body and the atmosphere. The saturation content of dissolved alcohol is closely related to the partial pressure of oxygen in the air, atmospheric pressure, and water temperature. The solubility of clean surface water is generally close to saturation. Due to the growth of algae, dissolved oxygen may be supersaturated. Dissolved oxygen decreases when water bodies are polluted by organic and inorganic reducing substances. When there is no time to replenish the oxygen in the atmosphere, the dissolved oxygen in the water gradually decreases until it approaches zero. At this time, anaerobic bacteria flourish and the water quality deteriorates, leading to the death of fish and shrimp. During the entire operation of the sewage plant, great importance is attached to the measurement of dissolved oxygen in the water. The main method for urban sewage treatment at home and abroad is to consider biological secondary treatment systems, mostly aerobic methods. As the name suggests, it uses the metabolic process of aerobic microorganisms to decompose and remove organic matter in the water. It can also be seen from this that the control of DO oxygen is very important. First of all, it should be ensured that there is enough dissolved oxygen in the water so that aerobic microorganisms can work normally. This is a prerequisite for achieving better operating results. However, if there is too much oxygen, it will cause waste and increase operating costs. Therefore, DO in the aeration tank is generally controlled between 2 and 4 mg/L. Treatment system failures occur when there is insufficient dissolved oxygen due to equipment problems or other reasons. For example, insufficient DO in the aeration tank may result in the expansion of filamentous bacteria in activated sludge. The reason is that bacteria and filamentous bacteria compete for insufficient DO. However, under conditions of insufficient DO, the competitiveness of filamentous bacteria is much greater than that of bacteria. Therefore, bacteria will obtain less DO and their growth will be inhibited. Instead, the filamentous bacteria are given the opportunity to multiply, and the end result is filamentous fungus expansion. In A/O, A2/O and other processes with certain denitrification and phosphorus removal, the control of DO is also very important. In order to obtain the appropriate N and P removal rates, a suitable DO value must be ensured. It can be seen that in the daily operation monitoring of sewage plants, the monitoring of DO is very important. The commonly used methods include iodometric method and its correction method, membrane electrode method and on-site rapid dissolved oxygen meter method.

4. Oxygen demand (TOD)

Total oxygen demand Organic matter contains C, H, N, S and other elements. When all the organic matter is oxidized, these elements are oxidized into CO2, H20, NO2 and SO2 respectively. At this time The oxygen demand is called total oxygen demand (TOD). The principle and process of total oxygen demand measurement is to inject a certain amount of water sample into the oxygen content, send it into a combustion tube with platinum steel as a catalyst, and burn it at a high temperature of 900°C. The organic matter in the water sample is burned by Combustion consumes the oxygen in the carrier gas. The remaining oxygen is measured with an electrode and recorded with an automatic recorder. The remaining oxygen after the water sample is burned is subtracted from the original oxygen amount of the carrier gas, which is the total oxygen demand. The measurement of this indicator is faster and easier than the measurement of BOD and COD, and the result is closer to the theoretical oxygen demand than COD.

5. Total organic carbon (TOC)

Total organic carbon represents the total carbon content of all organic pollutants in water and is a comprehensive parameter for evaluating organic pollutants in water. It is a comprehensive measurement index that uses the combustion method to measure the total organic carbon element in water samples to reflect the total amount of organic matter in the water. The measurement results are expressed as C content in mg/L. Its measurement principle and process are: add acid to the water sample, blow out the inorganic carbonate in the water through compressed air to eliminate interference, and then quantitatively inject the water sample into a combustion tube with platinum steel as a catalyst, and in the presence of oxygen In an air flow with sufficient and constant content, it is burned at a high temperature of 900°C. Carbon dioxide is produced during the combustion process, which is measured by an infrared gas analyzer and recorded with an automatic recorder, and then folded.Calculate the amount of carbon in it. The measurement of TOC uses the combustion method, so it can oxidize all organic matter. It can express the total amount of organic matter more directly than BOD5 or COD, so it is often used to evaluate the degree of organic matter pollution in water bodies. In recent years, various types of TOC analyzers have been developed at home and abroad. According to different working principles, it can be divided into combustion oxidation-non-dispersive infrared absorption method, conductivity method, gas chromatography, wet method, non-dispersive infrared absorption method, etc. Among them, the combustion oxidation-non-dispersive infrared absorption method only needs to be used once Conversion, the process is simple, the reproducibility is good, and the sensitivity is high, so this TOC analyzer is widely used at home and abroad.

6. Nitrogen (organic nitrogen, ammonia nitrogen, total nitrogen)

Organic nitrogen is a water quality indicator that reflects the total amount of nitrogen-containing organic compounds such as protein, amino acids, and urea in the water. If organic nitrogen is biologically oxidized under aerobic conditions, it can be gradually decomposed into NH3, NH4+, N02-, NO3- and other forms. NH3 and NH4+ are called ammonia nitrogen, NO2- is called nitrite nitrogen, and NO3- is called nitric acid. Nitrogen, the content of these forms can be used as water quality indicators, representing the different stages of the conversion of organic nitrogen into inorganic matter. Total nitrogen is a water quality indicator that includes all contents from organic nitrogen to nitrate nitrogen. Ammonia nitrogen (NH3-N) is an important monitoring indicator of sewage plant effluent. The source of ammonia nitrogen in the water is mainly the decomposition products of nitrogen-containing organic matter in domestic sewage by microorganisms. Certain industrial wastewaters, such as coking wastewater and synthetic ammonia fertilizer plant wastewater, etc. and farmland drainage. In addition, in an oxygen-free environment, nitrite present in water can also be reduced to ammonia by microorganisms. In an aerobic environment, ammonia in water can also be converted into nitrite, and even continue to be converted into nitrate. Measuring various forms of nitrogen compounds in water helps to evaluate the pollution and "self-purification" status of water bodies. Fish are sensitive to ammonia nitrogen in the water. High levels of ammonia nitrogen will cause fish death. It exists in water in the form of free ammonia NH3) or ammonium salt (NH4-). The composition ratio of the two depends on the pH value and water temperature of the water. When the pH is high, the proportion of free ammonia is higher. On the contrary, the proportion of ammonium salt is high and the water temperature is the opposite. Therefore, sufficient attention should be paid to pH and water temperature when monitoring. The methods for measuring ammonia nitrogen usually include Nessler colorimetry, gas phase molecular absorption method, phenol-hypochlorite (or salicylic acid-hypochlorite) colorimetry and electrode method. N in water will cause eutrophication of water bodies. N in effluent from sewage plants should be treated in accordance with the corresponding requirements of the national and local governments before being discharged up to standards. Therefore, the monitoring of N in effluent is one of the important items for water quality monitoring in sewage plants. In addition, for urban sewage plants that widely use secondary treatment, in order to ensure the normal operation of the sewage plant, the nutritional needs of microorganisms in the biochemical pool must be ensured. The aerobic method is generally controlled at: BOD:N:P=100 :5:1. Therefore, the monitoring of N in the incoming water of sewage plants is conducive to the control of microbial nutrients. When the proportion of phosphorus in the sewage is small, artificial supplementation is needed to ensure the nutritional needs of microorganisms and thus ensure the sewage. Process the normal operation of the system.

7. Phosphorus (total phosphorus, soluble phosphate and total soluble phosphorus)

In natural water and wastewater, phosphorus almost always exists in the form of various phosphates, which are divided into orthophosphates, condensed phosphates (pyrophosphate, metaphosphate and polyphosphate) and organically combined phosphorus (such as phospholipids) etc.), they exist in solutions, in humus particles or in aquatic organisms. Generally, the phosphate content in natural water is not high. Industrial wastewater and domestic sewage collected from chemical fertilizer, smelting, synthetic detergent and other industries often contain large amounts of phosphorus. Phosphorus is one of the essential elements for biological growth. However, excessive phosphorus content in water bodies (such as more than 0.2 mg/L) can cause excessive reproduction of algae until the quantity reaches a harmful level (called eutrophication), resulting in reduced transparency of lakes and rivers and deterioration of water quality. Phosphorus is an important indicator for evaluating water quality. In order to further prevent P in water from causing eutrophication of water bodies, P in sewage treatment plant effluent should be treated in accordance with the corresponding requirements of the national and local governments before being discharged up to standards. Therefore, the monitoring of P in effluent is one of the important items for water quality monitoring in sewage plants. In addition, for urban sewage plants that widely use secondary treatment, in order to ensure the normal operation of the sewage plant, the nutritional needs of microorganisms in the biochemical pool must be ensured. The aerobic method is generally controlled at: BOD:N:P=100 :5:1. Therefore, the monitoring of P in the incoming water of sewage plants is conducive to the control of microbial nutrients. When the proportion of phosphorus in the sewage is small, artificial supplementation is needed to ensure the nutritional needs of microorganisms and thus ensure the sewage. Process the normal operation of the system.

8. pH value

The pH value is an important indicator of the acidity and alkalinity of water and is numerically equal to the negative logarithm of the hydrogen ion concentration. The measurement of pH value usually uses the glass electrode method based on the electrochemical principle, and the colorimetric method can also be used. If the pH value of sewage is too high or too low, it will affect biochemical treatment, because the pH value range suitable for biological survival is often very narrow and very sensitive. For example, in the aeration tank of the activated sludge process system, if the pH changes, such as from the normal 6.5 to 8.5 to 5.5, then the system is likely to experience expansion of filamentous bacteria in the activated sludge. This will directly affect the quality of the effluent and lead to deterioration of the effluent. The main reason is that bacteria should be dominant in activated sludge, and their preferred optimal pH range is 6.5 to 8.5. When the pH value is normal, bacteria dominate and the number of filamentous bacteria is limited. However, when the pH changes to 5.5, it is very suitable for the growth of filamentous bacteria and lacks inhibition of bacterial growth. This will cause filamentous bacteria to dominate the activated sludge, causing the sludge to expand. In addition, when sludge or high-concentration wastewater is subjected to anaerobic digestion, special attention should be paid to the control of pH value. Because, during the anaerobic digestion process, methanogenic bacteria and non-methanogenic bacteria mainly play a role. Among them, the methanogenic bacteria have very strict requirements on the pH value and need to be controlled between 6.5 and 7.5, preferably between 6.8 and 7.2. Otherwise, the methane gas production rate will decrease significantly and affect the digestion effect. Generally, the pH value of treated sewage is required to be 6 to 9. When the pH value is less than 5, ordinary fish will die.

9. Suspended solids (SS)

Suspended solids (SS) refers to solid matter that cannot pass through a filter (filter paper or membrane). Solid matter in sewage includes suspended solids and dissolved solids. Suspended solids refer to solid matter suspended in water. Suspended solids are also called suspended matter or suspended matter, usually represented by SS. Suspended solids have poor light transmittance, making the water turbid and affecting the growth of aquatic life. Large amounts of suspended solids can also cause river obstructions. In terms of corresponding national and local sewage discharge standards, SS is one of the important items for monitoring.

10. Toxic substances

Toxic substances refer to substances that, when reaching a certain concentration in sewage, can harm human health, harm aquatic life in water bodies, or affect the biological treatment of sewage. Due to the greater harm of such substances, the content of toxic substances is an important water quality indicator in sewage discharge, water body monitoring and sewage treatment. Toxic substances are a common concern for people. Toxic substances can be divided into inorganic poisons and organic poisons. The main representatives of inorganic substances are some heavy metal ions such as mercury, chromium, cadmium, etc. If these ions are not removed or treated poorly in water, they will enter natural water bodies or biological systems, and can eventually be transferred to the food chain through the food chain A large amount of waste is collected in the human body, eventually leading to the emergence of various harmful diseases. Such as Minamata disease, bone pain disease, etc. Typical representatives of organic poisons include cyanide, phenol, organic chlorides, etc. These substances can also cause serious injuries. Therefore, it is necessary to conduct careful, strict and scientific monitoring of toxic and harmful substances in the effluent and sludge of urban sewage treatment plants.

11. Hardness

Hardness originally represented the foaming degree of soap in water. Now, people chemically convert the Ca and Mg ion content in water into the corresponding amount of CaCO3 to calculate the hardness value, expressed in mg/L. Hardness can be expressed in total hardness, calcium hardness, magnesium hardness, carbonate hardness (temporary hardness), non-carbonate hardness (permanent hardness), etc.

Physical indicators

1. Temperature

It has a direct impact on the physical, chemical and biological properties of sewage and sludge. In the aeration tank of the activated sludge system, treatment mainly relies on a large number of active microorganisms. Their suitable temperature is generally around 20 to 30°C. Therefore, if you want to ensure better organic matter treatment effect, the temperature should be controlled as much as possible Around 20~30℃. Temperature monitoring is carried out on site. Commonly used methods include water thermometer method, deep water thermometer method, inversion thermometer method and thermal thermometer method.

2. Chroma

The sewage from urban sewage treatment plants is different from the sewage from industrial wastewater. Its color is not very obvious, but this does not mean that the monitoring of color is not important. In fact, by observing the color of the sewage entering the sewage treatment plant, the freshness of the sewage can be judged. Usually fresh urban sewage is gray, but if it undergoes anaerobic decay during pipeline transportation and has very little DO, the sewage will be black and smelly. In addition, in our country, since industrial wastewater and domestic sewage are usually discharged together in a drainage system, the color of urban sewage plants sometimes has a large difference, and the color gives people an unpleasant feeling. my country's sewage plant discharge standards There are emission requirements for chroma, so if the chroma of the incoming water is large, the chroma of the effluent monitoring indicators should be taken seriously.

3. Stink

The odor in water mainly comes from the decay of organic matter. It can also bring discomfort to people and even affect human physiology, such as difficulty breathing, vomiting, etc. Therefore, odor is an important physical indicator. However, currently, sewage treatment plants do not specifically monitor odor.

4. Turbidity

Turbidity refers to the degree of obstruction caused by suspended solids in water from transmitting light. When the water contains suspended matter such as sediment, clay, organic matter, inorganic matter, plankton and microorganisms, it can cause light scattering or absorption, resulting in high turbidity. The turbidity of water is not only related to the content of particulate matter in the water, but also closely related to its particle size, shape, and light scattering characteristics of the particle surface. The level of turbidity generally cannot directly indicate the degree of water pollution, but an increase in turbidity indicates that the water quality has deteriorated.

5. Transparency

Transparency refers to the clarity of water. Clean water is transparent. Transparency is the opposite of turbidity. The more suspended solids and colloidal particles in the water, the lower its transparency. Methods for measuring transparency include the lead type method, the Seth plate method, the cross method, etc.

6. Suspended solids (SS for short)

Solid pollutants in water mainly exist in the water body in the form of suspended state, colloidal state and dissolved state. Solid pollutants in suspended state are usually called suspended matter, which refers to impurities, inorganic matter such as sediment, organic matter and plankton produced by the decay of animals and plants. Suspended solids, which can cause deterioration in the appearance of water bodies, increase turbidity, and change the color of water. Suspended matter deposited on the bottom of rivers silts channels, harming the reproduction of underwater organisms and affecting fishery production; deposited on irrigated farmland, it blocks soil capillaries, affects permeability, and causes soil sloping, which is not conducive to the growth of crops.

biological indicators

1. Total number of bacteria

The total number of bacteria refers to the total number of various bacteria contained in 1mL of water. An indicator reflecting the degree of bacterial contamination of water. In water quality analysis, a certain amount of water is inoculated into an agar medium, and after incubation at 37°C for 24 hours, the number of growing bacterial colonies is counted, and then the number of bacteria contained in each milliliter of water is calculated. The total bacterial count is a method for measuring the density of aerobic bacteria, facultative anaerobic bacteria and anaerobic bacteria in water. Because bacteria can exist in the form of single individuals, pairs, chains, clusters, etc., and no single culture medium can meet the physiological requirements of all bacteria in a water sample. Therefore, the colonies obtained by this method may be lower than the total number of viable bacteria actually present.

2. Coliform count

The coliform count refers to the number of coliform bacteria contained in 1L of water. Although coliforms themselves are not pathogenic bacteria, because the survival conditions of coliforms in the external environment are similar to those of bacteria and parasite eggs of intestinal infectious diseases, and the number of coliforms is large, it is easier to detect, so the number of coliforms is used as biological indicators. The more common pathogenic microorganisms include typhoid fever, hepatitis virus, adenovirus, etc., and some parasites also exist. Among the testing methods for total coliforms, the multi-tube fermentation method can be applied to various water samples (including sediment), but the operation is complicated and takes a long time; the membrane filter method is mainly suitable for water samples with less impurities and is simple to operate. fast. If the filter membrane method is used, the total coliforms can be redefined as: It is possible to grow dark-colored grapes with metallic luster on the Yuanteng's medium containing lactose within 24 hours at 37°C. Aerobic and facultatively anaerobic Gram-negative bacilli. In addition, in addition to paying attention to the monitoring of microorganisms in the effluent, it is actually very necessary to pay attention to the monitoring of microorganisms during the operation process. For example, microscopic examination of sludge in sewage treatment plants is mainly to observe the shape and composition of biological phases. Through regular microscopic examination, we can judge whether the operating facilities are working normally or not, and even prevent some abnormal phenomena in advance, such as: Through inspection, it is found that there is a tendency of accelerated proliferation of filamentous bacteria in the sludge. Certain measures can be taken to nip the possible expansion of filamentous bacteria in the bud, effectively ensuring the operation of the sewage plant and ensuring that the effluent reaches the Require. To sum up, if you want to ensure normal operation, the fundamental guarantee comes from scientific and effective operation management. Among them, regular and accurate monitoring of the operation indicators of the sewage plant, and analysis and statistics of the obtained data to guide the operation of the sewage plant are the foundation of the work of the sewage plant.