near theMississippiRiver, forexample,you wouldcertainlynotwanttodrink
its water in thespringtime.Thenit is fullofsmalldirtparticlesandis muddy.
Ina laboratory,solid particlescan beseparatedfrom a liquid by using filter
paper.Ina large town or city, however, sandandgravel beds are used to filter
outsolids from the water.Graveland sand are the same exceptthatgravel con-
sistsofmuchlarger,coarserparticles.
Coagulation. Inthis process, potassium aluminumsulfate isadded to the
water beforeit is filtered. This makes many tinyparticles groupthemselves
togetherso they are largeenoughto be filteredoutin hugetankscalled filter
beds or settling tanks.Coagulation is used in areas where the solid particle
contaminantsofclay in thewaterare very small.Itis also used in large cities to
remove theremainsofgerms which havealreadybeen killed byothermeans,
but which are too small to be filteredoutby meansofsandor gravel alone.
Chlorination. Chlorinekillsdisease-causinggerms inwaterchemically,but
you would have to go to abiologicallaboratorytoperformexperimentsproving
this. You would have to growcultures(colonies grown in a test tube)ofharmful
bacteriaandthen kill them withchlorine.Both these things would be muchtoo
dangerousand too difficult to do at home. Furthermore,chlorine,in sufficiently
largeamounts,is apoisonousgasandverydangerousto work with. Byadding
a chlorinesolutiontowater,many cities and towns insurethe safetyoftheir
citizens fromdisease-causinggerms in the water. Duringthe war, servicemen
became familiarwith little pills called halazonetablets. These tablets,which
containeda chlorinecompound,halazone, very effectively killed allharmful
bacteriain theunpurifiedwaterthe men often had todrink.You areprobably
familiarwith the smellandtasteofchlorine,becauseit is often addedto the
water inswimmingpools.
Aeration.Ina watersystemthatuses thismethod,thereare huge nozzlesnear
the reservoirsthatspraythe water intothe air as if they were tremendous
fountains.As the nozzlesbreakthewaterintosmalldroplets,they also expose