水的净化(water purification)
the proviion of afe water neceitate one of the rnajor expenditure of manpower and revenue in our modern citie. the purification of water i baically a two-tep or three-tep proce carried out under the trict uperviion of public health cientit and engineer. a the firt tep, natural water from the leat contaminated ource i allowed to tand in large reervoir, where mot of the mud, clay, and ilt ettle out; thi i called "edimentation". often in water with high mud content, lime and aluminum ulfate are added to the water in the ettling reervoir. thee chemical react in the water to form aluminum hydroxide, which ettle lowly and carrie much of the upended material, including mot of carrie much of the upended material, including mot of the bacteria, to the bottom of the reervoir. a the econd tep, the water i filtered through bed of and and grovel, which remove other impuritie and chemical in it. during or after filtration, chemical are ordinarily added to the water to kill any remaining harmful bacteria. chlorine i one of the mot common chemical ued for thi purpoe. a third tep taken by ome rmnicipalitie i adding to the water other beneficial chemical uch a fluoride to make tooth enamel hard, and oda ah to make the water itelf oft. the water purification proce, carried out with little variation from one large city to another, i perhap the bigget factor in the prevention of major outbreak of dieae in thi country.[由wWw.HaozuoWen.com整理]