水的净化(Water Purification)

水的净化(waterpurification)

theprovisionofsafewaternecessitatesoneofthernajorexpendituresofmanpowerandrevenueinourmoderncities.thepurificationofwaterisbasicallyatwo-steporthree-stepprocesscarriedoutunderthestrictsupervisionofpublichealthscientistsandengineers.asthefirststep,naturalwaterfromtheleastcontaminatedsourceisallowedtostandinlargereservoirs,wheremostofthemud,clay,andsiltsettleout;thisiscalled"sedimentation".ofteninwaterwithhighmudcontent,limeandaluminumsulfateareaddedtothewaterinthesettlingreservoirs.thesechemicalsreactinthewatertoformaluminumhydroxide,whichsettlesslowlyandcarriesmuchofthesuspendedmaterial,includingmostofcarriesmuchofthesuspendedmaterial,includingmostofthebacteria,tothebottomofthereservoirs.asthesecondstep,thewaterisfilteredthroughbedsofsandandgrovel,whichremoveotherimpuritiesandchemicalsinit.duringorafterfiltration,chemicalsareordinarilyaddedtothewatertokillanyremainingharmfulbacteria.chlorineisoneofthemostcommonchemicalsusedforthispurpose.athirdsteptakenbysomermnicipalitiesisaddingtothewaterotherbeneficialchemicalssuchasfluoridetomaketoothenamelhard,andsodaashtomakethewateritselfsoft.thewaterpurificationprocess,carriedoutwithlittlevariationfromonelargecitytoanother,isperhapsthebiggestfactorinthepreventionofmajoroutbreaksofdiseaseinthiscountry.