Am.  Jour,  rharni.  ) 
May,  1910.  / 
Chemical  and  Sand  Filtration. 
229 
at  the  bottom  of  the  fitter  for  washing  purposes ;  and,  further,  the 
sand  valve  must  be  so  constructed  that  it  will  automatically  open 
and  discharge  any  particles  of  sand  that  may  effect  lodgement  in 
its  parts. 
The  action  of  these  filters  is  wholly  chemical.  In  the  reaction 
between  alum  and  bicarbonate  of  calcium  is  found  the  agency  to 
which  this  method  owes  its  efficiency.  All  river  and  spring  waters 
have  hardness  to  a  greater  or  less  degree,  and  very  few,  if  any,  are 
devoid  of  it.  Hardness  is  commonly  due  to  the  presence  of  lime 
compounds  dissolved  in  the  water,  generally  in  the  form  of  bicar- 
bonate of  calcium.  If  a  solution  of  alum  be  added  to  such  water, 
being-  careful  to  keep  the  lime  compounds  in  excess,  an  interchange 
between  the  alum  and  lime  follows,  with  the  result  that  aluminum 
hydrate  is  precipitated,  carbon  dioxide  is  evolved,  and  sulphate  of 
calcium  produced,  the  latter  remaining  in  solution.  The  objective 
point  of  this  reaction  is  the  production  of  aluminum  hydrate.  This 
substance  when  freshly  precipitated  forms  a  very  voluminous,  gelat- 
inous, white  precipitate  that  embraces  all  suspended  matter  in  a 
sticky  envelope  and  holds  it  in  a  layer  upon  the  sand.  The  water 
passes  through  at  a  rate  greatly  in  excess  of  the  slow  sand  method, 
about  100  to  1.  So  thoroughly  is  the  filtering  performed  that  the 
water  is  quite  as  brilliant  as  the  most  carefully  prepared  distilled 
water. 
The  result  in  this  respect  is  very  often  better  than  the  slow  sand 
filter  attains.  This  is  not  surprising  when  it  is  remembered  that  the 
particles  of  clay  suspended  in  water  are  often  less  in  size  than  bac- 
teria, and  frequently  are  as  small  as  one  one  hundred  thousandth 
of  an  inch  in  diameter,  as  has  been  determined  by  the  Massachu- 
setts State  Board  of  Health. 
After  the  filter  has  been  operating  for  a  period  it  becomes  notice- 
ably clogged,  resulting  in  what  is  technically  known  as  loss  in  head — 
to  remedy  this  condition  the  filter  is  washed  by  passing  in  at  the 
bottom  filtered  water  through  the  sand  valves,  at  such  a  rate  that 
the  sand  is  turned  over  and  lumps  and  adhesions  are  broken  up, 
and  all  of  the  adhering  mud  and  separated  matter  are  washed  out 
of  the  filter  into  the  sewer.  This  is  accomplished  in  from  five  to 
ten  minutes  and  the  filter  again  performs  its  duty  when  the  flow 
of  water  is  directed  downward  through  the  sand. 
As  stated  before  no  oxidizing  action  takes  place  in  this  method 
of  filtration,  but  recently  successful  effort  has  been  made  to  bring 
