Am.  Jour.  Pharm.  ) 
May,  1910.  j 
Chemical  and  Sand  Filtration. 
231 
10  cubic  feet  in  twenty-four  hours  which  is  the  slow  sand  filtering- 
rate  for  each  square  foot  of  surface.  The  result  was  a  water  entirely 
palatable  and  faultless  for  laundry  work. 
Each  of  the  preceding  examples  described  applications  of  prin- 
ciples with  the  effluent  as  the  objective.  The  following  case  is  one 
where  the  filtered  water  is  run  to  waste  in  the  sewer  and  the  con- 
tamination saved,  just  reversing  the  conditions. 
At  Mt.  Vernon,  New  York,  there  is  a  large  silver  manufacturing- 
establishment.  For  various  reasons  considerable  silver  finds  its  way 
into  the  wash  water  used  by  the  silversmiths.  This  waste  is  allowed 
to  run  into  a  sedimentation  basin  where  it  is  treated  with  sodium 
chloride,  and  allowed  to  subside.  The  water  after  standing  is 
pumped  into  a  filter  where  any  suspended  matter  is  caught,  and  after 
this  process  has  been  carried  far  enough  the  filter  is  washed  and 
the  washings  returned  to  the  sedimentation  basin,  and  the  clear 
water  is  again  directed  into  the  sewer.  The  saving  to  the  manufac- 
turers is  $5000  per  annum,  the  cost  of  which  is  Si 50  for  one 
year's  operating  expenses. 
One  other  and  very  important  adaptation  of  rapid  filtration  is 
that  in  connection  with  so-called  water  softening.  This  process  con- 
sists in  removing  from  the  water  dissolved  salts  of  calcium  or  mag- 
nesium that  may  be  present.  The  handling  of  this  matter  requires 
precise  manipulation  as  will  be  understood  from  the  character  of 
the  reactions  involved — because  excess  of  the  reagents  would  also 
produce  hardness ;  hardness  it  will  be  remembered  is  commonly  due 
to  the  presence  of  bicarbonate  and  sulphate  of  calcium.  The  first 
step  is  to  add  a  slight  excess  of  calcium  hydroxide  to  the  water. 
The  result  of  this  is  to  neutralize  the  half  bound  carbonic  acid  of 
the  bicarbonate  of  calcium  and  precipitate  it  as  neutral  calcium 
carbonate.  After  an  interval  sodium  carbonate  solution  is  added, 
when  any  excess  of  calcium  hydroxide  is  changed  to  neutral  car- 
bonate of  calcium  and  the  sulphate  of  calcium  is  precipitated  also  as 
carbonate — the  resulting  sodium  sulphate  remains  in  solution. 
After  this  treatment  the  water  is  conducted  to  a  filter  where  the 
precipitates  are  removed  and  the  water  is  passed  on  practically 
free  of  hardness.  This  treatment  is  capable  of  reducing  water  of 
extreme  hardness  at  a  very  rapid  rate,  to  about  2  degrees  of  hard- 
ness. This  is  the  limit  because  neutral  calcium  carbonate  is  soluble 
in  water  to  this  slight  extent :  refinement  in  apparatus  is  absolutely 
essential  but  it  is  quite  practicable  in  competent  hands. 
