Impurities in water and treatment methods
Impurities in water include coarse matter, suspended matter, colloid and dissolved matter. Coarse materials, such as aquatic plants floating in the river, garbage, large aquatic organisms, gravel in the waste water and large dirt, etc. In the water supply project, the coarse impurities are removed by the facilities of the water intake structure and are not included in the scope of water treatment.
In wastewater treatment, the removal of coarse impurities generally belongs to the pretreatment part of water. Suspended solids and colloids include sediment, algae, bacteria, viruses, as well as original and insoluble substances produced in water treatment. The dissolved substances include inorganic salts, organic compounds and gases. There are many methods to remove impurities in water, and the application scope of the main methods can be roughly divided according to the particle size of impurities (Fig. 1). Due to the great difference in the types and concentrations of impurities between the raw water and the product water, the water treatment process is also very different.
As far as domestic water (or urban public water supply) is concerned, raw water from high-quality water sources (well water or well protected special water supply reservoir) can be regarded as finished water only by disinfection; raw water from general rivers or lakes should first remove turbidity impurities such as sediment, and then be disinfected; raw water with serious pollution should also remove pollutants such as organic matter; raw water with iron and manganese (e.g In some wells, iron and manganese need to be removed. Domestic water can meet the water quality requirements of general industrial water, but industrial water sometimes needs further processing, such as softening, desalination and so on.
When the wastewater is discharged or reused with low water quality requirements, only screening and precipitation methods are needed to remove coarse impurities and suspended solids (often referred to as primary treatment); when organic matter is required to be removed, biological treatment method (often referred to as secondary treatment) and disinfection are generally used after primary treatment; for the wastewater after biological treatment, the treatment process is collectively referred to as tertiary treatment or advanced treatment For example, when the wastewater is discharged into the water body to prevent eutrophication, the nitrogen and phosphorus removal process belongs to three-stage treatment (see physical and chemical treatment of water). When the waste water is used as the water source, the water quality requirements of the finished water and the corresponding processing flow depend on its use. In theory, modern water treatment technology can produce any high quality product water from any inferior water.