Unit13Text：WaterTreatmentProcessesOneofthegreatachievementsofmoderntechnologyhasbeentodrasticallyreducetheincidenceofwaterborneofdiseasessuchascholeraandtyphoidfever.当代化技术所取得伟大成就之一就是极大降低了霍乱和伤寒等水源性疾病发生。Thesediseasesarenolongerthegreatriskstopubichealththattheyoncewere.这些疾病曾经对人类健康产生过巨大威胁，但现在不是了。Thekeytothisadvancewastherecognitionthatcontaminationofpubicware,suppliesbyhumanwasteswasthemainsourceofinfection，andthatitcouldbeeliminatedbymoreeffectivewatertreatmentandbetterwastedisposal.进步关键在于人们认识到被人为污染公共水供给是主要传染源，这能够经过更主动更先进水处理技术所消除。Today'swatertreatmentplantsaredesignedtoprovidewatercontinuouslythatmeetsdrinkingwaterstandardsatthetap.今天水处理厂被设计为能连续提供符合饮用水标准水。Therearefourmanconsiderationsinvolvedinaccomplishingthis：sourselection,protectionofwaterquality，treatmentmethodstobeused，andpreventionofrecontamination.有四个主要方面要考虑：水源选择、水质保护、所使用处理方法和怎样预防二次污染。Commonprecautionstopreventgroundwaterandsurfacewaterpollutionincludeprohibitingthedischargeofsanitaryandstormsewersclosetothewaterreservoir,installingfencestopreventpollutionfromrecreationalusesofwater,andrestrictionsontheapplicationoffertilizersandpesticidesinareasthatdraintothereservoir.普通预防地下水污染和地表水污染办法包含预防排泄物管道和雨水管道排放口靠近蓄水池，安装栅栏预防娱乐用水污染，限制在水库流域使用化肥和杀虫剂。Screening，coagulation/flocculation，sedimentation,filtration，anddisinfectionarethemainunitoperationsinvolvedinthetreatmentofsurfacewater.隔离、混凝/絮凝、沉淀、过滤是地表水处理四个主要步骤。Watertreatmentoperationsfulfilloneormoreofthreekeytasks:removalofparticulatesubstancessuch.assandandclay，organicmatter,bacteria，andalgae；removalofdissolvedsubstancessuchasthosesingcolorand.Hardness;andremovalordestructionofpathogenicbacteriaandviruses.水处理要实现一到三个目标：去除沙和尘土等颗粒物质、有机物质、微生物和藻类；去除引发色度和硬度溶解物质；去除或破坏致病细菌和病毒。Theactualselectionoftreatmentprocessesdependsonthetypeofwatersourceandthedesiredwaterquality.实际处理工艺选择有赖于水源类型和所需要水质。Occasionally，rawwaterwithlowturbiditycanbetreatedbyplainsedimentation(nochemicals)toremovelargerparticlesandthenfiltrationtoremovethefewparticlesthatfailedtosettleout.有时，不太混浊进水能够