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1. BOD and COD indicators in water environment monitoring
Chemical oxygen demand COD and biological oxygen demand BOD are the most important indicators in water environment monitoring, mainly characterizing the amount of reducing pollutants in water Due to the wide variety of organic compounds, it is impossible to measure their respective contents, so BOD and COD are commonly used as comprehensive indicators. The decomposition of organic matter in water is carried out in two stages. The first stage is the carbon oxidation stage, and the second stage is the nitrification stage. The amount of oxidation consumed in the carbon oxidation stage becomes the carbonation biochemical oxygen demand (CBOD). Hydrocarbons, proteins, oils, lignin, and other organic pollutants that exist in suspended or dissolved states in domestic sewage and industrial wastewater such as sugar, food, paper, and fiber are all organic pollutants that can be decomposed by aerobic bacteria through biochemical reactions. Due to the consumption of oxygen during the decomposition process, they are also known as aerobic pollutants. If too much of these pollutants are discharged into water bodies, it will cause a lack of dissolved oxygen in the water. At the same time, organic matter will also cause decay through the decomposition of anaerobic bacteria in the water, producing odorous gases such as methane, hydrogen sulfide, thiols, and ammonia, causing the water to deteriorate and become foul smelling. Organic matter is basically a reducing substance that can be oxidized by chemical oxidants. The more organic matter there is, the more oxidants are consumed, so the amount of oxidants consumed (converted to mg/L of O2) can indirectly reflect the content of organic matter. But not all organic compounds can be oxidized, such as lower fatty acids mainly composed of acetic acid, which can hardly be oxidized. In addition, oxidized pollutants also include reducing inorganic substances such as Fe2+and NO2-. The organic matter in wastewater mainly includes 40-60% of single discharge, 25-50% of carbohydrates, and 8-12% of oils and fats. Urine is also a major organic component in wastewater. Because urine decomposes relatively quickly, when removing fresh (fresh? ）It is difficult to find urea in other types of wastewater besides wastewater. Along with the proteins, carbohydrates, fats and oils, and urea, derived from food and human wastes, wastewater typically contains small quantities of a very large number of different synthetic organic molecules, with structures ranging from simple to extremely complex. Sources of synthetic organic molecules include unused medicine, personal care products, and household cleaning and maintenance products. COD( Chemical Oxygen Demand (COD): refers to the amount of oxidant consumed when treating water samples with a certain strong oxidant under certain conditions. It reflects the degree of material pollution in water, and the higher the chemical oxygen demand, the more severe the pollution of organic matter in water. COD is expressed in mg/L. According to the COD values detected by water quality monitoring instruments, water quality can be divided into five categories. Among them, Class I and Class II COD ≤ 15mg/L can basically meet the drinking water standards. Water with values greater than Class II cannot be used as drinking water. Class III COD ≤ 20mg/L, Class IV COD ≤ 30mg/L, and Class V COD ≤ 40mg/L belong to polluted water quality. T