Am' Mayr'i9ioarm' }        Chemical  and  Sand  Filtration.  227 
RAPID  CHEMICAL  FILTRATION  COMPARED  TO  SLOW 
SAND  FILTRATION. 
By  William  G.  Toplis. 
On  three  previous  occasions  1  it  has  been  my  privilege  to  address 
this  body  on  the  subject  of  water  purification — each  time  some  phase 
of  slow  sand  nitration  was  considered. 
To-day  the  subject  is  again  water  purification,  but  this  time 
we  consider  a  different  side,  which  is  known  as  rapid  filtration.  In 
order  that  we  may  have  a  better  understanding  of  the  difference 
between  the  two  methods,  it  seems  proper  to  restate,  briefly,  -the 
basic  principles  of  slow  sand  filtration.  This  method  is  favored 
for  the  purification  of  municipal  water  supplies  and  the  objects 
sought  to  be  accomplished  are :  first,  the  removal  of  suspended 
matter,  and,  second,  the  decomposition  of  organic  matter  in  solu- 
tion. The  first  is  simple,  the  mud  being  strained  out  as  the  water 
percolates  through  the  sand.  The  second  is  much  more  complex, 
and  is  brought  about  through  the  growth  of  living  vegetable  organ- 
isms, known  as  bacteria,  that  thrive  within  the  sand  bed,  and  are 
attached  to  the  grains  of  sand,  in  such  quantity  that  they  actually 
form  a  coating  over  the  surface  of  each  grain  of  sand.  The  sand 
bed  is  about  four  feet  deep.  The  water  is  controlled  in  its  passage 
through  the  sand  and  is  allowed  to  proceed  at  a  rate  of  from  three 
to  six  million  gallons  per  acre  per  twenty-four  hours.  This  means 
that  each  square  foot  of  area  filters  about  ten  cubic  feet  of  water 
in  twenty-four  hours  at  the  three  million  rate.  Now  as  the  strained 
water  passes  through  the  fine  spaces  between  the  grains  of  sand  in 
the  filter  it  carries  the  dissolved  organic  matter  together  with  some 
atmospheric  oxygen,  also  in  solution,  to  the  bacteria  growing  within 
the  sand  bed.  Here  the  organic  matter  is  absorbed  by  these  growths, 
and  it  furnishes  pabulum  for  their  existence  and  propagation.  The 
extreme  limit  of  organic  contamination  in  drinking  water  is  perhaps 
represented  in  urea,  and  as  the  greater  includes  the  less,  the  treat- 
ment of  urea  will  cover  all  kinds  of  organic  contamination.  Urea 
is  composed  of  four  elements :  carbon,  hydrogen,  oxygen,  and  nitro- 
gen.   As  the  contaminated  water  passes  slowly  through  the  sand  the 
1  Amer.  Jour.  Pharm.,  74,  67,  Feb.,  1902;  76,  March,  1904;  81,  220, 
May,  1909. 
