228 
Chemical  and  Sand  Filtration. 
J  Am.  Jour.  Pharm. 
\       May,  1910. 
bacteria  absorb  the  dissolved  matter  and  digest  it  and  resolve  it 
into  the  simple  oxides  of  the  elements  composing  it,  viz.,  carbon 
becomes  carbon  dioxide  (C02),  hydrogen  and  oxygen  combine  to 
form  water  (H20),  nitrogen  is  finally  changed  into  compounds  of 
nitric  acid. 
Chemical  analyses  of  water  properly  treated  by  the  method  thus 
described  prove  the  statements  to  be  facts,  and  the  water  wholly 
acceptable  from  both  hygienic  and  aesthetic  considerations.  There 
is  no  better  method  known  for  the  purification  of  municipal  water 
supplies  when  properly  conducted.  It  will  be  noted,  however,  that 
the  conditions  are  rigid  and  inflexible,  and  that  the  method  is  suit- 
able for  but  one  purpose  and  limited  to  that  one,  especially  so  because 
of  the  large  areas  of  land  necessary  for  its  execution. 
The  purification  of  municipal  water  supplies  is  but  one  of  the 
many  water  purification  problems,  which  problems  seem  to  be  of 
unending  variety,  and  call  for  wide  knowledge  and  experience  for 
their  successful  handling.  The  variety  of  requirements  for  indus- 
trial purposes  calls  for  a  system  of  great  flexibility,  one  that  can  be 
quickly  adapted  to  changing  needs,  and  deliver  large  volumes  of 
treated  water  in  little  time  and  by  apparatus  that  must  be  confined  to 
restricted  space.  These  conditions  are  quite  the  reverse  of  those 
necessary  in  slow  sand  filtration. 
There  are  two  types  of  rapid  filtering  apparatus,  one  of  the 
open  variety  known  as  the  "  gravity  filter.''  and  one  which  is  closed 
and  constructed  in  a  metallic  case,  known  as  a  "  pressure  filter.'' 
The  closed  type  is  used  where  filtered  water  is  to  be  delivered  to  a 
level  higher  than  that  of  the  filter,  and  the  open  type  is  used  where 
the  water  may  flow  from  the  filter  by  gravity  to  a  point  of  delivery 
below  the  level  of  the  filter. 
The  same  principles  are  involved  in  the  operation  of  both  types : 
and  both  types  are  built  on  the  same  general  plan  though  the  pres- 
sure filter  must  be  provided  with  a  cover  bolted  upon  it.  The  most 
common  form  is  that  of  a  cylinder,  of  any  suitable  material.  It  may 
be  of  wood,  metal,  or  concrete,  provided  with  proper  inlets  and  out- 
lets. At  the  bottom,  inside  of  the  container,  a  special  line  of  piping 
is  arranged.  Upon  the  upper  surface  of  this  piping  a  number  of 
sand  valves  are  placed.  The  sand  valve  is  the  most  important  feat- 
ure connected  with  this  system  of  filtration.  Its  function  is  to  retain 
the  sand  in  the  filter  but  allow  the  water  to  pass  freely  out  after 
filtration,  and  in  addition  to  this  allow  the  freest  ingress  of  water 
