Modern sewage treatment methods are mainly divided into four categories: physical treatment,chemical treatment,physicochemical treatment and biological treatment.

Chemical precipitation method.

The chemical precipitation method refers to the addition of certain chemical agents to the wastewater to cause five-reaction reactions with the dissolved pollutants in the wastewater to form hard-to-water salts (precipitates) that precipitate out of the water,thereby reducing or removing Pollutants in water.The chemical precipitation method is mostly used to remove calcium ions,mirror ions,and heavy metal ions in wastewater,such as water,pots,lead,and bowls.According to different precipitants used,the precipitation method can be divided into lime method (also known as hydroxide precipitation method),sulfide method and silver salt method.

The sum of Ca 2+ and Mg2 + content in water is called total hardness,which can be divided into carbonate hardness and non-carbonate hardness.For carbonate hardness,lime can be added to reduce Ca 2+ and Mg2 + in the water to form CaC03 and Mg (OH) 2 precipitation.To remove non-carbonate hardness at the same time,lime-soda softening method can be used to make Ca 2+ It forms CaC03 with Mg2 + and llMg (OH) 2 is precipitated and removed.Therefore,when the hardness or alkalinity of the raw water is high,the chemical precipitation method can be used as the pretreatment for ion exchange softening first,so as to save the operating cost of ion exchange.