AFebJrXyT?902m'}        Filtration  of  Drinking  Water.  71 
It  is  broken  down  into  ammonia — as  the  first  change  by  one  class 
of  organisms,  and  here  becomes  truly  inorganic.  The  second  step  is 
one  of  oxidation,  and  the  ammonia  becomes  nitrous  acid  through 
the  agency  of  another  entirely  separate  organism  quite  different 
from  the  first.  In  the  third  and  final  step,  the  oxidation  is  completed 
by  another  organism  entirely  distinct  from  the  other  two.  Here  the 
nitrous  acid  becomes  nitric  acid,  which  unites  with  any  base  at 
hand,  and  is  delivered  as  such  in  the  effluent.  This  is  why  our  filter^ 
working  under  favorable  conditions,  shows  neither  free  nor  albu- 
minoid ammonia,  but  does  return  the  equivalent  in  nitrates  that  an 
ammonia  determination  on  the  raw  water  would  call  for.  As  before 
stated,  the  proper  conditions  must  be  preserved,  and  one  of  these  is 
the  element  of  time ;  how  rapidly  may  we  pass  the  water  through 
the  sand  as  an  economical  proposition  ?  As  might  be  predicted, 
the  character  of  these  changes  would  require  a  slow  rate  of  flow; 
therefore,  filtration  must  be  restrained  or  controlled  and  maintained 
at  a  uniform  rate,  notwithstanding  a  constantly  diminishing  filtering; 
capacity  due  to  clogging.  This  is  accomplished  in  several  ways  by- 
automatic  devices.  It  is  not  safe  to  carry  the  filtering  rate  much 
beyond  3,000,000  gallons  per  acre  per  twenty-four  hours.  This  has 
been  found  by  actual  working  conditions  to  be  the  safe  limit,  so  far  as 
bacteriological  and  chemical  conditions  are  concerned.  A  3,000,000- 
gallon  rate  is  equivalent  to  filtering  10  vertical  feet  of  water  over 
the  entire  area  of  filter  in  .twenty-four  hours.  The  problems  met 
with  in  water  purification  seem  to  change  with  each  source  of  supply, 
and  so  variable  are  they  that  no  municipality  would  undertake 
the  erection  of  a  filtration  plant  without  exhaustive  study  of  the 
conditions  covering  practically  a  whole  year.  In  some  waters,  color 
is  the  objection  ;  in  others,  taste  is  complained  of,  while  turbidity 
and  sewage,  with  every  imaginable  combination  of  all  the  faults,  is 
commonly  found. 
The  principal  problem  encountered  by  the  city  of  Philadelphia  ins 
its  effort  to  purify  the  water  is  that  of  turbidity.  True,  we  have 
sewage  contamination,  odors,  tastes,  etc.,  but  they  readily  disappear 
under  treatment.  The  turbidity,  however,  gives  trouble,  particularly 
at  times  of  freshet,  when  the  suspended  matter  may  rise  to  200  or 
300  parts  per  million.  This  requires  frequent  scraping  of  the  filter, 
resulting  in  loss  of  water  and  cost  for  attention.  Under  ordinary 
working  conditions,  with  water  carrying  less  than  40  parts  per 
