AFebJr°uaryTS2m'}        Filtration  of  Drinking  Water.  69 
springs.  The  sand  filter,  therefore,  means  the  most  exacting, 
painstaking  care  to  establish  the  proper  conditions,  together 
with  the  wise  application  of  much  chemical,  bacteriological, 
and  engineering  knowledge.  The  container  is  commonly  built 
of  concrete,  though  masonry  and  puddled  clay  embankments 
are  not  infrequently  used.  The  bottom  of  the  container  is  care- 
fully graded  so  that  drainage  will  be  equal  from  every  part.  The 
underdrains  are  given  like  careful  attention  for  the  same  reason,  and 
are  built  of  broken  stone  or  large  gravel  measuring  2  to  3 
inches  on  3  diameters.  It  is  spread  in  a  layer  6  to  8  inches 
deep ;  on  top  of  this  is  spread  several  inches  of  smaller  gravel 
then  finer,  until  we  have  a  bed  12  to  16  inches  in  thickness.  Upon 
this  is  placed  4  feet  of  fine  sand,  exercising  care  to  pack  it  evenly, 
and  avoiding  holes  and  ways.  The  chemical  character  of  the  sand 
and  gravel  must  be  carefully  looked  into,  as  it  is  important  to  avoid 
carbonates  and  sulphates  of  the  second  group.  Much  carbonic 
acid  is  formed  during  the  operation  of  the  filter,  and  this  in  solution 
has  the  property  of  dissolving  carbonates  of  calcium  magnesium, 
barium,  strontium,  creating  increased  temporary  hardness,  while 
any  sulphate  of  calcium  would  materially  add  to  the  permanent 
hardness.  The  best  material  for  the  purpose  is  a  sharp  silicious 
sand.  Having  constructed  the  filter  it  is  filled  by  introducing  water 
at  the  bottom  to  avoid  disarranging  sand  by  escaping  air.  The 
filtration  is  at  once  begun,  and  bacteriological  and  chemical  samples 
of  water  are  regularly  collected  from  the  effluent  and  likewise  from 
the  applied  water.  At  first  there  is  but  little  difference  in  the 
character  of  the  water,  either  bacteriologically  or  chemically.  After  a 
few  days  a  comparison  of  the  bacteriological  counts  on  the  raw  water 
with  those  of  the  effluent  will  show  a  very  marked  increase  of 
bacteria  in  the  effluent  over  that  of  the  applied  water;  this  increase 
will  steadily  rise  until  after  a  variable  period  of  time,  usually  two 
weeks,  the  counts  rapidly  diminish  until  they  become  less  than 
one  per  cent,  of  the  number  in  the  applied  water.  At  the  same 
time  a  comparison  of  the  chemical  analyses  of  the  effluent  and 
applied  water  will  show  in  the  effluent  greatly  decreased  free  and 
albuminoid  ammonia,  practically  no  nitrites,  and  greatly  increased 
nitrates.  The  operation  during  this  period  of  time  is  known  as  the 
ripening  of  the  filter.  It  embraces  many  complex  changes  of  absorb- 
ing interest,  and  copies  with  scientific  exactness  the  example  which 
