What are the common indicators used in water quality testing and analysis?

1、Organic chemical indicators

Dissolved oxygen (DO for short)

Dissolved oxygen (DO) refers to the molecular oxygen (O2) dissolved in water. The content of dissolved oxygen in water is related to factors such as atmospheric pressure, water temperature and salt content. A decrease in atmospheric pressure, an increase in water temperature, and an increase in salinity can all lead to a decrease in DO content. Generally clean rivers, DO can be close to the saturation value of its temperature, when there is a large number of algal blooms, dissolved oxygen may be oversaturated; when the water body is contaminated by organic matter, inorganic reducing substances, it will make the dissolved oxygen content is reduced, or even tend to zero, when the anaerobic bacterial reproduction is active, water quality deterioration. When the dissolved oxygen in the water is lower than 3~4mg/L, many fish have difficulty in breathing and die of suffocation. Dissolved oxygen is one of the important indicators of water pollution.

Chemical oxygen demand (Chemical oxygen demand abbreviated COD)

Chemical oxygen demand refers to potassium dichromate (K2Cr2O7) or potassium permanganate (KMnO4) as an oxidizing agent, oxidation of reductive substances in the water to consume the amount of oxidizing agent, the results converted to the amount of oxygen (mg / L). Reducing substances in water include organic matter and nitrite, sulfide, ferrous salts and other inorganic substances. The COD reflects the extent to which the water is polluted by reducing substances. Based on the fact that it is common for water bodies to be polluted by organic matter, this indicator is also used as one of the comprehensive indicators of the relative content of organic matter, and it is adopted as a control item in various decrees related to water quality. Note: China’s promulgation of environmental ground water quality standards (1988), the COD value measured by acidic potassium dichromate method called chemical oxygen demand, (CODCr for short), and the COD value measured by potassium permanganate method called permanganate index, (CODMn for short).

Permanganate index, oxygen consumption (CODMn)

The permanganate index, also known as oxygen consumption, is a common indicator of organic and inorganic oxidizable material pollution in water bodies. It is defined as: under certain conditions, oxidizing certain organic and inorganic reducing substances in water samples with potassium permanganate, and calculating the equivalent amount of oxygen from the amount of potassium permanganate consumed. It reflects the amount of suspended and dissolved inorganic and organic matter in the water that can be oxidized by potassium permanganate. The permanganate index was also known as the potassium permanganate method of chemical oxygen demand (COD) in previous water quality monitoring analyses. However, because of this method in the specified conditions, organic matter in water can only be partially oxidized, and is not the theoretical oxygen demand, but also not reflect the total organic content of the water body in the scale, therefore, the term permanganate index as an indicator of water quality, in order to differ from the potassium dichromate method of chemical oxygen demand, more in line with the objective reality. CODcr is generally 2 to 5 times the CODMn, and the data we obtained in practice are basically the same. CODcr is generally 2 to 5 times of CODMn, and the data we get in the actual work are basically in this range.

Biochemical oxygen demand (Biochemical oxygen demand referred to as BOD)

Biochemical oxygen demand refers to the amount of dissolved oxygen in the conditions of dissolved oxygen, aerobic microorganisms in the decomposition of organic matter in the water in the process of biochemical oxidation of the amount of dissolved oxygen consumed. At the same time also includes such as sulfide, ferrous and other reducing inorganic substances such as oxidation of oxygen consumption, but this part usually accounts for a small proportion. Aerobic decomposition of organic matter under the action of microorganisms is generally divided into two stages. 1) carbonaceous material oxidation stage, mainly carbonaceous organic matter oxidized to carbon dioxide and water; 2) nitrification stage, mainly nitrogen-containing organic compounds in the nitrifying bacterial decomposition under the action of nitrite and nitrate. About 5-7 days after the significant. Therefore, the current commonly used 20 ℃ five-day incubation method (BOD5 method) to determine the BOD value generally does not include the nitrification stage.BOD is to reflect the degree of organic pollution of the water body is a comprehensive indicator, but also the study of wastewater degradation of biochemical and biochemical treatment effect, as well as biochemical treatment of wastewater process design and kinetic research in the important parameters.

Total Phosphorus (TP for short)

Total phosphorus is the main indicator for controlling eutrophication of water bodies. The total amount of various forms of phosphorus in water that can be oxidized into phosphate by strong oxidizing substances. Phosphorus is a nutrient element for plant growth and essential for life. If the phosphorus in the water exceeds the critical concentration, it will stimulate the growth of aquatic plants, so that “algal bloom” occurs, resulting in eutrophication of the water body. Phosphorus enters water bodies through a number of different pathways, such as the discharge of wastewater containing phosphorus compounds, surface runoff from agricultural fields, and livestock farms. In recent years, the use of phosphorus-containing detergents and other daily phosphorus-containing substances has also increased phosphorus emissions.

Ammonia nitrogen (NH3-N for short)

Ammonia nitrogen in water is nitrogen in the form of free ammonia NH3 (also called non-ionic ammonia) and ionic ammonia NH4+. For surface water, the determination of nonionic ammonia is often required. The ratio of the composition of the two is determined by the pH and temperature of the water; when the pH is high, the proportion of free ammonia is higher, and vice versa for ammonia salts. Ammonia nitrogen in water mainly comes from the decomposition products of nitrogenous organic matter subject to microbial action in domestic sewage, industrial wastewater such as coking and ammonia synthesis, as well as farmland drainage. When the content of ammonia nitrogen is high, it is toxic to fish and harmful to human beings in different degrees.

Total Nitrogen (TN for short)

The total amount of various forms of inorganic and organic nitrogen in water. Including NO3-, NO2- and NH4+ and other inorganic nitrogen and protein, amino acids and organic amines and other organic nitrogen, calculated in milligrams of nitrogen per liter of water. It is often used to indicate the degree to which a water body is polluted by nutrients. The total nitrogen content of water is one of the most important indicators of water quality. Its measurement helps to evaluate the polluted and self-purifying status of a water body. When nitrogen and phosphorus substances in surface water exceed the standard, microorganisms proliferate, plankton grows vigorously, and eutrophication occurs.

Total Organic Carbon (TOC)

It is a comprehensive indicator of the total amount of organic matter in a body of water expressed in terms of carbon content. As the measurement of TOC adopts the combustion method, it can oxidize all the organic matter, which can better reflect the total amount of organic matter than BOD5 or COD.

Total Oxygen Demand (TOD)

It refers to the amount of oxygen needed to oxidize the organic matter in water during combustion, and the result is expressed in mg/L of O2. TOD can only reflect the amount of oxygen needed to turn almost all organic matter into CO2, H2O, NO, SO2…after combustion. It is closer to the theoretical oxygen demand value than BOD and COD.

2、Inorganic chemical index

Hardness

Hardness initially expressed the size of the degree of soap foam in water, now, people in the chemical water Ca, Mg ion content, converted to its corresponding amount of CaCO3 to calculate the hardness value, with mg / L said. Hardness has total hardness, calcium hardness, magnesium hardness, carbonate hardness (temporary hardness), non-carbonate hardness (permanent hardness) and other expressions.

PH value

pH value indicates the strength of acidity and alkalinity in water, expressed as the negative logarithm of the activity of hydrogen ions in solution: pH = -lgαH+pH indicates the most basic property of water, which can control the dissociation degree of weak acids and weak bases in the water, reduce the toxicity of chlorides, ammonia, hydrogen sulfide and so on, and prevent the release of heavy metals from bottom mud. It has influence on the change of water quality, the growth and decline of biological reproduction, corrosiveness, water treatment effect, etc. It is an important parameter for evaluating water quality. The pH value of natural water is mostly in the range of 6-9; drinking water is in the range of 6.5-8.5; the pH value of certain industrial water must be maintained in the range of 7.0-8.5 to prevent metal equipment and pipelines from being corroded.

Conductivity (conductivity)

The conductivity of water with its inorganic acids, alkalis, salts have a certain relationship. This indicator is often used to estimate the total concentration of ions in water or salt content. Different types of water have different conductivity. Oxidation reduction potential (Oxidation reduction potential) redox potential is a variety of oxidizing substances in water and reducing substances in the redox reaction of the integrated results. Although this indicator can not be used as an indicator of the concentration of certain oxidizing substances and reducing substances, but can help us understand the electrochemical characteristics of the water body, analyze the nature of the water body, is a comprehensive indicator. The redox potential of the water body must be measured in the field.

3、Physical property index

Turbidity

Turbidity is the degree of obstruction that occurs when the suspended matter in the water is on the light transmission. Water containing sediment, clay, organic matter, inorganic matter, plankton and microorganisms and other suspended substances, can make the light scattering or absorption, turbidity. Turbidity size of water is not only related to the presence of particulate matter content in the water, but also with its particle size, shape, particle surface light scattering properties are closely related. Turbidity level generally can not directly explain the degree of water pollution, but the turbidity increases indicate that the water quality is bad.

Transparency

Is the degree of clarity of the water sample, clean water is transparent. Transparency and turbidity on the contrary, the more suspended solids and colloidal particles in the water, the lower its transparency. Determination of transparency of the method of lead, Seychelles disk method, cross method.

Suspended solids (Suspended solids abbreviated SS)

Solid pollutants in water mainly exist in the form of suspended state, colloidal state and dissolved state in the water body. Suspended state of solid pollutants are usually called suspended solids, refers to impurities, sediment inorganic material, plant and animal body corruption and organic matter and plankton. Solid suspended matter, it will cause deterioration in the appearance of the water body, increased turbidity, change the color of the water. Suspended matter deposited on the bottom of the river silt river channels, endangering the reproduction of aquatic organisms, affecting fisheries production; deposited in the irrigated farmland, it will block the soil capillaries, affecting the permeability, resulting in the soil 坂结, not conducive to the growth of crops.

4、Common metal indicators

Cadmium (Cadmium) (Cd)

Cadmium melting point of 320.9 ℃, boiling point of 765 ℃, is rich in ductility and soft metal, soluble in dilute nitric acid. Cadmium is very toxic, can be accumulated in the human liver, kidney and other tissues, causing damage to various organs and tissues, especially the most obvious damage to the kidneys, but also can lead to osteoporosis and softening, causing pain and pain disease. The vast majority of freshwater cadmium content is less than 1μg / L, cadmium in nature, mostly cadmium sulfide ore exists, and often coexist with zinc, lead, copper, manganese and other minerals. So in these metal refining process can be discharged a large amount of cadmium. In addition, electroplating, dyes, batteries and chemical industry and other wastewater discharged is also the main source of cadmium pollution.

Chromium (hexavalent) (Chromium)

(Cr6+) has a glossy silver-white solid metal, corrosion-resistant, heat-resistant, is one of the necessary trace elements of the human body, chromium compounds have common valence trivalent and hexavalent. Chromium toxicity and its existence of valence related to the metal chromium is harmless, hexavalent chromium has a strong toxicity, carcinogenic substances, and easy to be absorbed by the human body and accumulate in the body, usually considered that the toxicity of hexavalent chromium than the trivalent chromium 100 times greater. Trivalent chromium and hexavalent chromium compounds can be interconverted. Industrial sources of chromium pollution mainly come from wastewater from chromium ore processing, metal surface treatment, leather tanning, printing and dyeing, photographic materials and other industries. Chromium is an important indicator for water quality pollution control.

Copper (Copper) (element symbol: Cu)

Copper is ductile, easy to process, and a good conductor of heat and electricity. Copper is an essential trace element, the human body lack of copper will occur anemia, diarrhea and other diseases, but excessive intake of copper will also produce harm. Copper is harmful to aquatic organisms, oysters can accumulate high amount of copper in the water contaminated with copper ions, and cases of green oysters have occurred in Japan’s Yengang Bay and Taiwan’s Erren Creek. The toxicity of copper to aquatic organisms is related to its form, and the toxicity of free copper ions is much larger than that of complexed copper. The main sources of copper pollution are wastewater discharged from electroplating, smelting, hardware processing, mining, petrochemical and chemical industries.

Iron (Iron) (element symbol: Fe)

Iron is one of the trace elements found in natural waters, and its content in water depends on the geology of the area and on other chemical constituents in the water. Divalent and trivalent iron ions are the basic forms of iron in the aqueous environment. Divalent iron is found in water bodies lacking dissolved oxygen or in deep waters of stratifiable lakes with anaerobic substrates. When dissolved oxygen is increased in the water or when oxidizing substances are encountered, the divalent iron is rapidly oxidized to trivalent iron ions in the form of a form of ferric hydroxide or is deposited with other anions in the sediment at the bottom of the water, where the trivalent iron oxides are practically insoluble. If hydrogen sulfide is present in the substrate, ferrous sulfide is formed, respectively, and a black inorganic substance is produced. Iron is an indispensable trace element for plants and animals, and in some bodies of water it may be a constraint limiting the growth of algae and other plants, and in vertebrate and some invertebrate blood it is an extremely important factor in oxygen delivery. Iron has no toxicological effects on human health, only on the use of water. Iron significantly affects the taste of drinking water and is capable of staining laundry.

Zinc (Zinc)(Zn)

Zinc is a metal widely used in daily life, melting point 419.5 ℃, soluble in acid, concentrated alkali. It is often combined with sulfides of other metals, especially lead, copper, cadmium and iron sulfides. Zinc is an essential element for human body, but it has a greater impact on fish and other aquatic organisms, and the safe concentration of zinc for fish is about 0.1mg/L. In addition, zinc has a certain inhibitory effect on the self-purification process of the water body. Its main source of pollution is the drainage of electroplating, metallurgy, pigment and chemical industry.

Selenium (Selenium)(Se)

In water bodies, elemental selenium exists in the form of selenite or selenate, and the natural concentration of selenium in water bodies is directly proportional to the selenium content of soil. Selenium is an essential element for the human body, but toxicity can occur if too much is ingested. Metallic selenium has low toxicity and divalent selenium has very high toxicity, which is generally absorbed through the intestinal tract and accumulates in the liver and kidneys. The main source of pollution is the wastewater discharged from mining, metal smelting and selenium product factories.

Heavy metals

Chemically, according to the density of the metal is divided into heavy metals and light metals, often the density of the metal is greater than 4.5g/cm3 called heavy metals. Such as: gold, silver, copper, lead, zinc, nickel, cobalt, chromium, mercury, cadmium and so on about 45 kinds. Some of the heavy metal elements in water are macronutrients and trace elements necessary for human health, and some are harmful to human health, such as mercury, cadmium, chromium, lead, copper, zinc, nickel, barium, vanadium, arsenic and so on. The content of harmful metal compounds in polluted surface water and industrial wastewater tends to increase significantly. Harmful metal invasion of the human body, will make certain enzymes inactivate and varying degrees of poisoning symptoms. The size of its toxicity and metal type, physical and chemical properties, concentration and the presence of valence and form. For example, mercury, cadmium, chromium (hexavalent), lead, and its compounds are harmful metals that have a long-term impact on human health; mercury, lead, arsenic and other metals, organic compounds than the corresponding inorganic compounds are much stronger than the toxicity; soluble metals than the particulate state of the metal toxicity; hexavalent chromium than the trivalent chromium toxicity and so on.