Introduction to sewage

Oxygen consuming pollutant

In domestic sewage, food processing and paper and other industrial wastewater, there are carbohydrates, proteins, fats, lignin and other organic substances. These substances exist in a suspended or dissolved state in sewage, and this pollutant can cause a decrease in dissolved oxygen in the water, affecting the growth of fish and other aquatic life. After the depletion of dissolved oxygen in water, organic matter is anaerobic decomposition, producing hydrogen sulfide, ammonia and mercaptan odor, which further worsens the water quality. The composition of organic matter in water is very complex, and the concentration of oxygen-consuming organic matter is commonly expressed by the oxygen consumed in the biochemical decomposition process of oxygen-consuming substances per unit volume of water, unit mg/L.

Generally expressed by Chemical Oxygen Demand, COD (Chemical Oxygen Demand), it is a chemical method to measure the amount of reducing substances in the water sample that need to be oxidized.

A portion of it can be broken down by the biochemical action of microorganisms called biochemical oxygen demand (BOD). Generally, at 20 ° C, the five-day biochemical oxygen demand (BOD5) is expressed.

Plant nutrients

Plant nutrients mainly refer to nitrogen, phosphorus and other substances that can stimulate the growth of algae and aquatic grasses, interfere with water purification, and increase BOD5. The "eutrophication" caused by excessive nutrients in water bodies has become a serious problem in water source protection for lakes and slow-moving waters.

Under the influence of human activities, eutrophication refers to the phenomenon that nitrogen, phosphorus and other nutrients required by organisms enter a large number of slow-flow water bodies such as lakes, estuaries and bays, resulting in the rapid reproduction of algae and other plankton, the decline of dissolved oxygen in water bodies, the deterioration of water quality, and the death of a large number of fish and other organisms. Under natural conditions, the lake will also transition from a nutrient-poor state to a eutrophic state, with increasing sediment, first becoming a swamp and then a land. This natural process is very slow, often taking thousands or even tens of thousands of years. The phenomenon of eutrophication caused by artificial discharge of industrial wastewater and domestic sewage containing nutrients can appear in a short period of time.

The sources of plant nutrients are wide and large, including domestic sewage (organic matter, detergent), agriculture (fertilizer, farm fertilizer), industrial wastewater, garbage, etc. Each person brings about 50g of nitrogen into sewage every day. The phosphorus in domestic sewage mainly comes from washing wastewater, and 50% to 80% of the fertilizer applied to farmland flows into rivers, lakes, seas and underground water bodies. The content of phosphorus and nitrogen (especially phosphorus) in natural water bodies is to some extent a controlling factor for plankton growth. When a large amount of nitrogen and phosphorus plant nutrients are discharged into the water, some organisms (such as algae) are rapidly multiplied and grown, and the growth cycle becomes shorter. After the death of algae and other plankton, they are decomposed by aerobic organisms, constantly consuming dissolved oxygen in the water, or are decomposed by anaerobic microorganisms, constantly producing gases such as hydrogen sulfide, which deteriorates the water quality, causing a large number of fish and other aquatic organisms to die. Algae and other plankton residues in the process of decay, and the biological needs of nitrogen, phosphorus and other nutrients released into the water, for the new generation of algae and other organisms to use. Therefore, after eutrophication, even if the source of external nutrients is cut off, it is difficult to self-purify and return to normal levels. When the water body eutrophication is serious, the lake can be silted up by some numerous plants and their remains, and become a swamp or even dry land. Some areas of the sea can become a "Dead Sea" or a "red tide" phenomenon.

Nitrogen and phosphorus content, productivity (O2) and chlorophyll-α are commonly used as indicators of water eutrophication degree. Table 3-7 shows the index of water eutrophication degree divided by total phosphorus and inorganic nitrogen. To prevent eutrophication, the content of nitrogen and phosphorus entering water must be controlled.

Toxic pollutant

Toxic pollutants refer to substances that accumulate to a certain amount after entering the organism and can cause biochemical and physiological function changes in body fluids and tissues, causing temporary or lasting pathological states, and even endangering life. Such as heavy metals and organic pollutants that are difficult to decompose. The toxicity of pollutants is closely related to the amount ingested into the body. The toxicity of the same pollutant is also closely related to its presence. The toxicity can vary greatly depending on the valence or morphology. For example, the toxicity of Cr (Ⅵ) is greater than that of Cr (Ⅲ); The toxicity of As (Ⅲ) was higher than that of As (Ⅴ). Methylmercury is much more toxic than inorganic mercury. In addition, the toxicity of pollutants is closely related to several comprehensive effects. From the perspective of traditional toxicology, there are three kinds of combined effects of toxic pollutants on organisms: