NatureofwaterpollutionWater,oneofman'smostpreciousresources,isgenerallytakenforgranteduntilitsuseisthreatenedbyreducedavailability（可用性）orquality.Waterpollutionisproducedprimarilybytheactivitiesofman,specificallyhismismanagementofwaterresources.Thepollutantsareanychemical,physical,orbiologicalsubstancesthataffectthenaturalconditionofwateroritsintendeduse.Becausewaterpollutionthreatenstheavailability,qualityandusefulnessofwater,itisofworldwidecriticalconcern.Theincreaseinthenumberandvarietyofusesforwaterthroughouttheworldhasproducedawiderangeofstandardsofwaterqualitythatmustbesatisfied.Thesedemandsinclude:①preservationofriversintheirnaturalstate;②potability（飲用）ofthewatersupply;?preservationandenhancementoffishandwildlife;④safetyforagriculturaluse;⑤safetyforrecreational（娛樂的）useincludingswimming;?accommodation（適應；供應）toagreatvarietyofindustrialpurpose;?freedomfromnuisance（有害的東西）;?generationofpowerforpublicutilities（公用事業(yè)）;⑨dilution（稀釋）andtransportofwastes.Besidesthespecificchemical,biological,andphysicalrequirementsforthemultitudeofusesnotedabove，thereareconstraints（約束、限制）reflectingpublichealthrequirements，aesthetics（美學），economics，andshortandlong-termecologicalimpacts.Consequently,thereisnorigid（嚴格的）orspecificdefinitionofwaterpollution，sincetheintendeduseorusesofthewatermustbetakenintoconsiderationinanydefinitionofwhatconstitutespollutedwater.Onemethodofclassifyingthegaseous，liquidandsolidconstituentsofwaterthatconstitutepollutiondependsontheintendeduseofthewater.Thepollutantsarethengroupedasnotpermissible，asundesirableandobjectionable（不適合的），aspermissiblebutnotnecessarilydesirable，orasdesirable.Forexample，ifwateristobeusedimmediatelyforanimalconsumption，toxic（有毒的）compoundsarenotdesirable,whereas（然而）acertainamountofoxygenisnotobjectionable.Ontheotherhand,ifthewateristobeusedinapowerplantforsteamgeneration,toxicmaterialsmightbeallowableorevenperhapsdesirable,whereasoxygenthatcouldpossiblycorrode（侵蝕、損害）equipmentwouldbeobjectionable.Anothermethodofclassifyingpollutantsthatenterwaterasaresultofman'sdomestic,industrialorotheractivitiesistodistinguishbetweenconservativeandnon-conservativepollutants.Conservativepollutantsarethosethatarenotalteredbythebiologicalprocessesoccurringinnaturalwaters.Thesepollutantsareforthemostpartinorganicchemicals,whicharedilutedinreceivingwaterbutarenotappreciablychangedintotalquantity.Industrialwastescontainnumeroussuchpollutants,includingmetallicsaltsandothertoxic,corrosive,colored,andtaste-producingmaterials.Domesticpollutionandreturnflowfromirrigationmaycontainnumeroussuchpollutants,includingchlorides（氯化物）andnitrates（硝酸鹽）.Non-conservativepollutants,ontheotherhand,arechangedinformofreducedinquantitybychemicalandphysicalprocessesinvolvedinbiologicalphenomenaoccurringinwater.Themostcommonsourceofnon-conservativepollutantsisdomesticsewage－h(huán)ighlyputrescibleorganicwastethatcanbeconvertedintoinorganicmaterialssuchasbicarbonates（重碳酸鹽），，sulfates（硫酸鹽）,andphosphates（磷酸鹽）bythebacteriaandothermicroorganisminthewater.Ifthewaterisnottooheavilyladenwithwastes,itwillundergo“self-purification”.Thisprocessinvolvestheactionofaerobicbacteria,thatis,bacteriathatrequirefreeoxygentobreakdownwastes,anditproducesnooffensiveodors.If,however,thewaterisladenwithwastesbeyondacertainamount,theprocessofbiologicaldegradationbecomesanaerobic.Thatis,itproceedsbytheactionofbacteriathatdonotrequirefreeoxygen.Intheprocess,noxioushydrogensulphidegas（硫化氫），methane（甲烷）,andothergasesareproduced.Theaer