【Daily Question No. 42】February 11, 2021

Error analysis: Chemical oxygen demand is an important indicator of the degree of water pollution. Water pollution indicators are numerical values that measure the degree of pollution in water bodies, and they also serve as an important basis for monitoring the operating condition of chemical wastewater treatment equipment. Among them, there are eight commonly used indicators for water pollution: 1. TOD – Total Oxygen Demand: the amount of oxygen consumed when organic matter is completely oxidized, with C, H, N, and S being oxidized respectively into carbon dioxide, water, nitric oxide, and sulfur dioxide. The unit is mg/L. 2.TOC: Total Organic Carbon – indicates the total carbon content of organic pollutants in water, expressed as a carbon percentage. The unit is mg/L. 3.SS suspended solids: The substances retained on the filter membrane or filter paper after the water sample is filtered. The unit is mg/L. 4. BOD: Biochemical Oxygen Demand – it indicates the amount of free oxygen consumed by aerobic microorganisms at 20 degrees Celsius over a certain number of days to degrade the organic matter in each liter of water, under conditions of saturated oxygen. The commonly used unit is mg/L; 5 days is typically the standard time for measuring BOD, which is denoted as BOD5. 5.COD Chemical Oxygen Demand: It represents the amount of oxygen equivalent required to oxidize organic substances into water molecules and carbon dioxide molecules using strong oxidizing agents. The commonly used oxidants are potassium dichromate or potassium permanganate, denoted as CODcr or simply COD; CODmn is also known as oxygen demand, abbreviated as OC, with the unit being mg/L. 6.PH: Indicates the acidity or alkalinity of wastewater. 7. Toxic substances: Indicates the level of substances present in water that are harmful to organisms. Such as cyanides, arsenides, mercury, cadmium, chromium, lead, etc., expressed in mg/L. 8. E. coli count: refers to the number of E. coli bacteria per liter of water. The unit is pieces/L.