because  of  its  appearance  and  the  taste  and odors  sometimes  associated  with  it. Turbidity  is  a  muddy  or  unclear  condition of  water  caused  by  particles  of  sand,  clay,  or organic  matter  being  held  in  suspension.  Clay and  silt  remain  suspended  in  water  for  the longest period of time because of their particle size   and   specific   gravities.   The   removal   of turbidity   is   essential   to   the   production   of potable   water.   Removal   reduces   water contamination,  extends  the  time  between backwashing  of  filters,  decreases  chlorine demand,  improves  disinfection,  and  enhances user acceptability of the finished water. Proper water  treatment  requires  turbidity  removal because   suspended   particles   often   contain organisms  that  may  cause  diseases. Turbidity  is  removed  by  coagulation  and sedimentation.  Since  the  physical  characteristics of   raw   water   vary   widely   in   different locations,  dosages  of  coagulant  chemicals  must be  determined  at  each  water  point  to  ensure maximum  efficiency  with  minimal  waste  of chemicals.  After  coagulation  and  settling,  the water should not have more than 20 percent of the  original  turbidity.  Daily  jar  tests  will  help check the optimum chemical dosage required to meet  this  standard. COAGULATING   CHEMICALS The  type  of  chemicals  that  should  be  used for coagulating raw water can be determined by using  the  results  from  jar  tests,  plant  tests,  or by   using   the   data   shown   in   table   9-3(A). Theoretically  table  9-3(A)  is  correct;  however, these values can be misleading when applied to some types of raw water. The chemical content of  water  may  have  a  considerable  influence  on the   optimum   pH   range   for   the   various coagulants.   For   example,   coagulation   with ferrous  sulfate  is  usually  best  accomplished  at relatively high pH values in the alkaline zone. With   soil,   colored   waters,   ferric   coagulant may  sometimes  be  used  with  considerable success   at   pH   values   of   4.0   or   less. Because   of   this   wide   variation   in   the optimum  pH  range  of  coagulant  (caused by   individual   characteristics   of   the   raw water),   the   coagulant   dosage   and   the optimum zone for floc formation should be determined  by  jar  tests,  rather  than  just relying  on  table  9-3(A). Table   9-3(A).—Optimum   pH   Ranges for   Common   Coagulant COAGULANT pH Aluminum 5.0  to  7.0 sulfate Ferrous 9.5  and  above sulfate Chlorinated 4.0  to  6.5  and copperas above  9.5 Ferric 4.0  to  6.5  and chloride above  9.5 Ferric sulfate 4.0  to  10.0 JAR  TEST The   jar   test   is   the   most   common method  of  determining  proper  coagulant dosages.  When  there  is  a  question  as  to which   chemical   should   be   used   as   a coagulant,  it  is  often  necessary  to  run more   than   one   series   of   jar   tests. Different   coagulant   chemicals   and   pH ranges should be used to determine which one produces the most satisfactory results at   the   lowest   cost.   The   step-by-step procedures  for  ajar  test  are  as  follows: 1. Prepare a standard solution of each 9-15