Mềm là thuật ngữ cho quá trình loại bỏ các ion can thiệp vào việc sử dụng xà phòng. Những ion này được gọi là ion độ cứng do sự hiện diện của các cation đa hóa trị, chủ yếu là canxi và magiê. Trong vùng nước tự nhiên, các ion khác có thể là 2 + hiện nay gây ra độ cứng nhưng không phải trong một lượng đáng kể | Part III Unit Processes of Water and Wastewater Treatment Part III covers the various unit processes employed in water and wastewater treatment including water softening water stabilization coagulation removal of iron and manganese by chemical precipitation removal of nitrogen by nitrification-denitrification removal of phosphorus by chemical precipitation ion exchange and disinfection. 2003 by A. P. Sincere and G. A. Sincere 10 Water Softening Softening is the term given to the process of removing ions that interfere with the use of soap. These ions are called hardness ions due to the presence of multivalent cations mostly calcium and magnesium. In natural waters other ions that may be present to cause hardness but not in significant amounts are iron Fe2 manganese Mn2 strontium Sr2 and aluminum Al3 . In the process of cleansing using soap lather is formed causing the surface tension of water to decrease. This decrease in surface tension makes water molecules partially lose their mutual attraction toward each other allowing them to wet foreign solids thereby suspending the solids in water. As the water is rinsed out the solids are removed from the soiled material. In the presence of hardness ions however soap does not form the lather immediately but reacts with the ions preventing the formation of lather and forming scum. Lather will only form when all the hardness ions are consumed. This means that hard waters are hard to lather. Hard waters are those waters that contain these hardness ions in excessive amounts. Softening using chemicals is discussed in this chapter. Other topics related to softening are discussed as necessary. HARD WATERS The following lists the general classification of hard waters Soft 50 mg L as CaCO3 Moderately hard 50-150 mg L as CaCO3 Hard 150-300 mg L as CaCO3 Very hard 300 mg L as CaCO3 A very soft water has a slimy feel. For example rainwater which is exceedingly soft is slimy when used with soap. For this reason hardness in water .
