Wastewater quality indicators

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　　Wastewater contains a wide variety of pollutants, and analytical and testing methods can be used to qualitatively and quantitatively determine these pollutants in order to reflect the water quality of the wastewater. **There are many standards for the analysis and testing of water quality, and its indicators can be divided into three categories: physical, chemical, and biological. I. Physical indicators 　　(1) Temperature 　　Many industrial wastewater streams have high temperatures; when these wastewaters are discharged into water bodies, they raise the temperature of that water, causing thermal pollution. Rising water temperatures affect the survival of aquatic organisms and their utilization of water resources. The solubility of oxygen in water decreases as the water temperature rises; as a result, on one hand, the amount of dissolved oxygen in the water decreases, and on the other hand, the rising water temperature accelerates the oxygen-consuming reactions, ultimately leading to oxygen deficiency in the water or deterioration of its quality. 　　(2) Chroma Chroma is a sensory parameter. Generally, pure natural water is clear and transparent, that is, colorless. However, wastewater containing colored pollutants such as metal compounds or organic compounds comes in various colors. The colored wastewater is diluted with distilled water and compared with the reference water sample, until the color difference between the two samples is the same; at this point, the dilution factor of the wastewater represents its chroma. 　　(3) Smell and taste  Smell and taste, just like color intensity, are sensory indicators that can qualitatively reflect the amount of a certain pollutant. Natural water is odorless and tasteless. When water bodies are polluted, they develop an unusual odor. The unpleasant odor of water stems from pollutants such as reducing sulfur and nitrogen compounds, volatile organic compounds, and chlorine. Different salts can impart different odors to water. For example, sodium chloride has a salty taste, magnesium sulfate has a bitter taste, and calcium sulfate has a slight sweet taste. 　　(3) Solid substances The total of all residues in water is called total solids (TS), which includes dissolved substances (DS) and suspended solid substances (SS). After the water sample is filtered, the solid obtained by evaporating the filtrate is the dissolved solids (DS), while the residue after dehydration and drying is the suspended solids (SS). Solid residues can be classified into volatile solids (VS) and non-volatile solids (FS) based on their volatility. The solid is calcined at 600°C; the amount that volatilizes constitutes the volatile solids (VS), while the residue after calcination is the fixed solids (FS). Soluble solids indicate the amount of salts, suspended solids represent the quantity of solid substances that are not soluble in water, while volatile solids reflect the amount of organic components in the solids. 　　High salinity in water bodies affects the osmotic pressure of biological cells and their normal growth. Suspended solids may cause waterways to become clogged. Volatile solids are an important source of organic pollution in water bodies. II. Chemical indicators (1) Organic matter Organic compounds such as carbohydrates, proteins, and fats contained in domestic wastewater and certain industrial wastewaters are ultimately broken down into simple inorganic substances, carbon dioxide, and water under the action of microorganisms. These organic substances require a large amount of oxygen during decomposition, and therefore are considered oxygen-consuming pollutants. Oxygen-consuming organic pollutants are one of the main causes of dark and foul odors in water bodies. 　　The composition of organic pollutants in wastewater is complex, and it is difficult with existing technologies to determine the concentration of each type of organic compound separately; usually, there is no need to do so either. In terms of organic pollutants in water bodies, their main hazard is the consumption of dissolved oxygen in the water. In practical work, various indicators such as biochemical oxygen demand (BOD), chemical oxygen demand (COD, OC), total organic carbon (TOC), and total oxygen demand (TOD) are used to reflect the amount of oxygen-demanding organic matter in water. The determination of TOC and TOD both involves chemical oxidation reactions through combustion; the results for the former are expressed in terms of carbon, while those for the latter are expressed in terms of oxygen. The oxygen consumption processes of TOC and TOD differ fundamentally from that of BOD, and due to the varying compositions of organic substances in different water samples, the biochemical processes also vary significantly. There is no fixed correlation between TOC and TOD and BOD among different water qualities. Under basically identical water quality conditions, there is a certain correlation between BOD and TOC or TOD. 　　(2) Inorganic indicators 　① Plant nutrients N and P in wastewater are plant nutrients. From the perspective of crop growth, these nutrients are valuable substances; however, an excessive amount of N and P entering natural water bodies can lead to eutrophication. The levels of nitrogen and phosphorus in water bodies are closely related to the degree of eutrophication in those waters. When it comes to the effect of wastewater on eutrophication, phosphorus plays a much greater role than nitrogen. 　　② pH value mainly indicates the acidity or alkalinity of a water sample. 　　③Heavy metals mainly refer to elements with significant biological toxicity such as mercury, cadmium, lead, chromium, nickel, and the metalloid arsenic; they also include common heavy metals that possess certain toxic properties, such as zinc, copper, cobalt, and tin. III. Biological indicators 　　(1) Total bacterial count 　　The total bacterial count in water reflects the degree of bacterial contamination of that water body. The total bacterial count does not indicate the source of contamination; it is necessary to consider the E. coli count as well in order to determine the source of water pollution and its level of safety. 　　(2) E. coli Water is an important vector for the transmission of intestinal diseases, and E. coli is considered the most fundamental indicator strain for fecal contamination. The value of the E. coli population can indicate the degree to which a water sample is contaminated by feces, and indirectly suggest the possibility of the presence of intestinal pathogens (typhoid, dysentery, cholera, etc.).