Common indicators in water quality analysis

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1. Organic chemistry parameters: Dissolved oxygen (abbreviated as DO) refers to molecular oxygen (O2) dissolved in water, abbreviated as DO. The level of dissolved oxygen in water is related to factors such as atmospheric pressure, water temperature, and salinity. A drop in atmospheric pressure, an increase in water temperature, and an increase in salinity can all lead to a decrease in dissolved oxygen levels. In rivers that are generally clean, the DO level can approach the saturation value corresponding to their temperature. When large amounts of algae grow, dissolved oxygen levels can become supersaturated. When the water is polluted by organic substances or inorganic reducing agents, the dissolved oxygen content decreases, sometimes even dropping to zero; in such conditions, anaerobic bacteria multiply rapidly, leading to a deterioration in water quality. When the dissolved oxygen in water is below 3–4 mg/L, many fish experience difficulty breathing and die from suffocation. Dissolved oxygen is one of the important indicators reflecting the state of water pollution. Chemical oxygen demand (abbreviated as COD) refers to the amount of oxidizing agent consumed in the oxidation of reducing substances in water, using potassium dichromate (K2Cr2O7) or potassium permanganate (KMnO4) as oxidants; the result is expressed as an amount of oxygen in mg/L. Reducing substances in water include organic substances as well as inorganic substances such as nitrites, sulfides, and ferrous salts. Chemical oxygen demand reflects the degree of contamination in water by reducing substances. Given that water bodies are frequently polluted by organic substances, this indicator is also used as one of the comprehensive measures for assessing the relative content of organic matter, and it is adopted as a control parameter in various regulations related to water quality. Note: According to the environmental groundwater quality standards issued in our country in 1988, the COD value determined using the acidic potassium dichromate method is referred to as chemical oxygen demand (abbreviated as CODCr), while the COD value obtained through the potassium permanganate method is called the permanganate index (abbreviated as CODMn). The permanganate index, or CODMn, is also known as oxygen demand; it is a commonly used indicator to reflect the pollution caused by organic and inorganic oxidizable substances in water bodies. It is defined as: under certain conditions, using potassium permanganate to oxidize certain organic substances and inorganic reducing agents in the water sample, and calculating the equivalent amount of oxygen based on the amount of potassium permanganate consumed. It reflects the amount of inorganic and organic substances suspended and dissolved in water that can be oxidized by potassium permanganate. In previous water quality monitoring and analysis, the permanganate index was also referred to as the potassium permanganate method for chemical oxygen demand. However, since under the specified conditions this method can only oxidize a portion of the organic matter in water, it does not represent the theoretical oxygen demand nor a measure of the total amount of organic matter in the water. Therefore, using the term permanganate index as an indicator of water quality, to distinguish it from the chemical oxygen demand determined by the potassium dichromate method, is more in line with objective reality. Generally, CODcr is 2 to 5 times that of CODMn; the data we obtain in actual work mostly fall within this range. Biochemical Oxygen Demand (BOD) refers to the amount of dissolved oxygen consumed by aerobic microorganisms during the biochemical oxidation process of decomposing organic matter in water, under conditions where there is sufficient dissolved oxygen. It also includes the amount of oxygen consumed in the oxidation of reductive inorganic substances such as sulfides and ferrous iron; however, this portion usually accounts for a very small proportion. The aerobic decomposition of organic matter under the action of microorganisms generally occurs in two stages. 1) The oxidation stage of carbon-containing substances, during which carbon-rich organic compounds are oxidized into carbon dioxide and water; 2) The nitrification stage, during which nitrogen-containing organic compounds are broken down into nitrite and nitrate under the action of nitrifying bacteria. It proceeds significantly only after about 5–7 days. Therefore, the commonly used 20°C five-day incubation method (BOD5 method) for determining BOD values generally does not include the nitrification stage. BOD is a comprehensive indicator that reflects the degree of organic pollution in water bodies. It is also an important parameter in studying the biodegradability of wastewater, the effectiveness of biological treatment, as well as in the design of biological wastewater treatment processes and related kinetic studies. Total Phosphorus (abbreviated as TP) is a key indicator for controlling eutrophication in water bodies. It is calculated as the total amount of all forms of phosphorus in water that can be oxidized by strong oxidizing agents to form phosphates. Phosphorus is a nutrient element for plant growth and is also essential for life. If the phosphorus concentration in water exceeds the critical level, it will stimulate the growth of aquatic plants, leading to the formation of algal blooms and causing eutrophication of the water body. Phosphorus enters water bodies through various pathways, such as wastewater containing phosphorus compounds, surface runoff from agricultural fields, and livestock farms. In recent years, the use of phosphorus-containing detergents and other phosphorus-containing household products has also increased phosphorus emissions. Ammonia nitrogen (abbreviated as NH3-N) refers to the nitrogen present in water in the form of free ammonia NH3 (also known as non-ionic ammonia) and ionized ammonia NH4+. For surface water, it is often required to measure non-ionic ammonia. The composition ratio of the two is determined by the pH value and temperature of water; when the pH value is high, the proportion of free ammonia is higher, whereas when the pH value is low, the proportion of ammonium salts is higher. Ammonia nitrogen in water mainly comes from the decomposition products of nitrogen-containing organic substances in domestic wastewater due to microbial action, industrial wastewater from industries such as coking and ammonia synthesis, as well as agricultural drainage. When the ammonia-nitrogen content is high, it is toxic to fish and can also cause varying degrees of harm to humans. Total Nitrogen (abbreviated as TN) is the total amount of inorganic and organic nitrogen in various forms present in water. It includes inorganic nitrogen such as NO3-, NO2-, and NH4+, as well as organic nitrogen such as proteins, amino acids, and organic amines, measured in milligrams of nitrogen per liter of water. It is often used to indicate the degree of nutrient pollution in water bodies. The total nitrogen content in water is one of the important indicators for assessing water quality. Its measurement helps to assess the degree of water pollution and its self-purification capacity. When the levels of nitrogen and phosphorus in surface water exceed acceptable limits, microorganisms multiply in large numbers, phytoplankton grow vigorously, and eutrophication occurs. Total Organic Carbon (TOC) is a comprehensive indicator that represents the total amount of organic matter in water, expressed in terms of carbon content. Since the determination of TOC uses a combustion method, all organic substances can be oxidized, allowing it to reflect the total amount of organic matter more accurately than BOD5 or COD. Total Oxygen Demand (TOD) refers to the amount of oxygen required for the oxidation of organic substances in water during combustion; it is expressed in mg/L of O2. TOD can only reflect the amount of oxygen required for almost all organic matter to be converted into CO2, H2O, NO, SO2, etc. through combustion. It is closer to the theoretical oxygen demand value than BOD and COD. 2. Inorganic chemical indicators – Hardness
Originally, hardness referred to the degree to which soap could lather in water. Nowadays, chemically speaking, the content of Ca and Mg ions in water is converted into the equivalent amount of CaCO3 to calculate the hardness value, which is expressed in mg/L. Hardness can be expressed in terms of total hardness, calcium hardness, magnesium hardness, carbonate hardness (temporary hardness), non-carbonate hardness (permanent hardness), etc. pH value: The pH value indicates the acidity or alkalinity of water, and it is expressed as the negative logarithm of the activity of hydrogen ions in the solution: pH = -lgαH+. pH represents a fundamental property of water; it can control the degree of dissociation of weak acids and bases in water, reduce the toxicity of substances such as chlorides, ammonia, and hydrogen sulfide, and prevent the release of heavy metals from sediment. It affects changes in water quality, the proliferation and decline of organisms, corrosiveness, and the effectiveness of water treatment; thus, it is an important parameter for evaluating water quality. The pH value of natural water is generally within the range of 6-9; drinking water falls between 6.5-8.5; for some industrial waters, the pH value must be maintained between 7.0-8.5 to prevent corrosion of metal equipment and pipes. Conductivity: The conductivity of water is related to the amount of inorganic acids, bases, and salts it contains. This parameter is often used to estimate the total concentration of ions or the salinity in water. Different types of water have different conductivities. Oxidation-reduction potential (ORP) is the overall result of redox reactions between various oxidizing and reducing substances in water. Although this indicator cannot be used as a measure of the concentrations of oxidizing and reducing substances, it helps us understand the electrochemical properties of water bodies and analyze their characteristics; it is a comprehensive indicator. The redox potential of the water body must be measured on-site. 3. Physical property indicators: Turbidity. Turbidity indicates the degree to which suspended particles in water interfere with the passage of light. When water contains suspended substances such as silt, clay, organic matter, inorganic matter, plankton, and microorganisms, it causes light to be scattered or absorbed, resulting in high turbidity. The turbidity of water is not only related to the amount of particulate matter present in it, but also closely associated with the size and shape of these particles, as well as the light-scattering properties of their surfaces. 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. Transparency refers to the clarity of a water sample; clean water is transparent. Transparency is the opposite of turbidity; the more suspended solids and colloidal particles there are in water, the lower its transparency. Methods for measuring transparency include the typeface method, Say’s disc method, the cross method, etc. Suspended solids (abbreviated as SS) are solid pollutants in water that exist in the water body primarily in suspended, colloidal, or dissolved forms. Solid pollutants in a suspended state are commonly referred to as suspended solids; they include impurities, inorganic substances such as sediment, organic matter derived from plants and animals, and plankton. Suspended solids, which can deteriorate the appearance of water bodies, increase turbidity, and change the color of water. Suspended solids settle on the riverbed, accumulating there and harming the reproduction of aquatic organisms living on the bottom, which in turn affects fishery production. When they settle in irrigated farmlands, they block the soil’s capillaries, reducing its permeability and causing soil compaction, which is unfavorable for crop growth. 4. Common metal indicators: Cadmium (Cd). The melting point of cadmium is 320.9°C, and its boiling point is 765°C. It is a highly ductile and soft metal that dissolves in dilute *ao acid. Cadmium is highly toxic and can accumulate in tissues such as the liver and kidneys in the human body, causing damage to various organs and tissues; the impact on the kidneys is particularly severe. It can also lead to osteoporosis and softening of bones, resulting in itai-itai disease. The cadmium content in the vast majority of freshwater sources is below 1 μg/L. In nature, cadmium exists primarily as cadmium sulfide and often occurs together with minerals such as zinc, lead, copper, and manganese. Therefore, large amounts of cadmium can be released during these metal refining processes. In addition, wastewater discharged from industries such as electroplating, dyes, batteries, and the chemical industry is also a major source of cadmium pollution. Chromium (hexavalent) (Cr6+) is a shiny, silver-white solid metal that is resistant to corrosion and heat. It is one of the essential trace elements for the human body. The common valence states of chromium compounds are trivalent and hexavalent. The toxicity of chromium is related to its valence state; metallic chromium is harmless, while hexavalent chromium is highly toxic, carcinogenic, and easily absorbed by the human body where it accumulates. It is generally believed that the toxicity of hexavalent chromium is 100 times greater than that of trivalent chromium. Trivalent chromium and hexavalent chromium compounds can be converted into each other. The main industrial sources of chromium pollution come from the wastewater generated by industries such as chromium ore processing, metal surface treatment, leather tanning, printing and dyeing, and photographic materials. Chromium is an important indicator for water quality pollution control. Copper (element symbol: Cu) is malleable and easy to work with; it is an excellent conductor of heat and electricity. Copper is an essential trace element for the human body. A copper deficiency can lead to conditions such as anemia and diarrhea; however, excessive intake of copper can also be harmful. Copper is highly harmful to aquatic organisms; oysters can accumulate large amounts of copper in water contaminated with copper ions. Cases of green oysters have occurred in Enogawa Bay in Japan and Erin Creek in Taiwan. The toxicity of copper to aquatic organisms is related to its form; free copper ions are much more toxic than copper in complexed forms. The main sources of copper pollution are wastewater discharged from industries such as electroplating, smelting, metal processing, mining, petrochemicals, and the chemical industry. Iron (element symbol: Fe) is one of the trace elements present in natural water bodies. Its concentration in water depends on the geological conditions of the area as well as on other chemical components present in the water. Divalent and trivalent iron ions are the basic forms of iron in aquatic environments. Ferrous iron is present in water bodies lacking dissolved oxygen or in the deep waters of stratified lakes with an anaerobic bottom. When the dissolved oxygen level in the water increases or when it comes into contact with oxidizing substances, ferrous iron is rapidly oxidized to ferric ions, which then settle to the bottom as hydroxide iron or together with other anions. Oxides of ferric iron are actually insoluble. If hydrogen sulfide is present in the sediment, ferrous sulfide is formed, resulting in black inorganic substances. Iron is an essential trace element for plants and animals. In some water bodies, iron can be a limiting factor that hinders the growth of algae and other plants. In the blood of vertebrates and certain invertebrates, iron plays a crucial role in oxygen transport. Iron has no toxic effects on human health; it only affects the use of water. Iron noticeably affects the taste of drinking water and can stain laundered clothes. Zinc (Zn) is a metal that is widely used in everyday life. Its melting point is 419.5°C, and it is soluble in acids and strong alkalis. It is often associated with sulfides of other metals, particularly sulfides of lead, copper, cadmium, and iron. Zinc is an essential element for the human body, but it has a significant impact on fish and other aquatic organisms; the safe concentration of zinc for fish is approximately 0.1 mg/L. Furthermore, zinc has a certain inhibitory effect on the self-purification process of water bodies. Its main sources of pollution are the wastewater from industries such as electroplating, metallurgy, pigments, and chemicals. In water bodies, elemental selenium exists in the form of selenite or selenate; the natural selenium concentration in water is proportional to the selenium content in the soil. Selenium is an element essential for the human body, but excessive intake can also lead to poisoning. Metallic selenium has low toxicity, while divalent selenium is highly toxic; it is generally absorbed through the intestines and accumulates in the liver and kidneys. Its main sources of pollution are wastewater discharged from mining operations, metal smelting plants, and selenium product factories. Heavy metals: Chemically, metals are classified as heavy metals or light metals based on their density; metals with a density of more than 4.5 g/cm3 are generally considered heavy metals. For example, about 45 types such as gold, silver, copper, lead, zinc, nickel, cobalt, chromium, mercury, cadmium, etc. Some heavy metal elements in water are essential macro- and trace elements for human health, while others are harmful to it, such as mercury, cadmium, chromium, lead, copper, zinc, nickel, barium, vanadium, arsenic, etc. The levels of harmful metal compounds in polluted surface water and industrial wastewater often increase significantly. When harmful metals enter the body, they can deactivate certain enzymes, leading to poisoning symptoms of varying degrees. Its toxicity depends on the type of metal, its physicochemical properties, concentration, as well as the valence state and form in which it exists. For example, mercury, cadmium, hexavalent chromium, lead, and their compounds are harmful metals that have long-term effects on human health; organic compounds of metals such as mercury, lead, and arsenic are much more toxic than their inorganic counterparts; soluble metals are more toxic than particulate metals; hexavalent chromium is more toxic than trivalent chromium, and so on.