120  Refined  Methods  in  Water  Purification.  {AmMi?S;F^"m- 
Philadelphia  project,  and  from  an  engineering  point  of  view,  that 
which  has  given  the  heads  of  departments  in  this  city  the  most  con- 
cern is  perhaps  turbidity.  The  wide  limitations  and  the  constantly 
varying  amounts  of  suspended  matter  carried  by,  particularly,  the 
Schuylkill  River,  served  to  make  a  very  complicated  problem. 
There  is  no  great  difficulty  in  filtering  water  carrying  suspended 
matter  up  to  40  parts  per  million,  but  above  that  figure  the  scrap- 
ings become  inconveniently  frequent,  and  the  effort  has  been  to  pre- 
pare the  water  by  sedimentation  and  other  means  before  passing  it 
to  the  filters. 
At  times  of  freshet  the  Schuylkill  River  carries  every  kind  of  sub- 
stance from  coal  dust  to  microscopic  clay  particles,  the  amount  run- 
ning well  up  into  the  hundreds  of  parts  per  million,  and  here  is 
where  the  great  problem  lay  to  supply  water  of  uniformly  low 
suspended  matter  to  the  filters.  Sedimentation  alone,  such  as  was 
possible,  was  inadequate,  and  to  build  for  this  purpose  not  econom- 
ical. But  a  chain  is  no  stronger  than  its  weakest  link.  Those 
freshets  were  a  stubborn  fact,  and  must  be  met.  Yet  how  ?  The 
answer  most  hopeful  was  sedimentation  with  preliminary  fil- 
tration. Then  came  the  struggle  for  a  proper  preliminary  filter. 
Its  great  office  to  remove  mud  and  do  it  regularly,  whether  the 
suspended  matter  be  500  parts  or  50  parts  per  million, — that  was  a 
task  to  stagger  the  most  optimistic.  Still  they  have  gone  quite  a 
long  way  on  the  road  toward  its  realization.  At  the  lower  Rox- 
borough  filter  plant  there  is  in  operation  a  preliminary  filter  doing 
very  satisfactory  work  in  a  practical  way,  and  at  the  testing  station 
there  has  been  one  of  the  same  type  at  work  for  a  long  time,  from 
which  experience  was  gained  as  to  its  durability  and  efficiency ;  it 
has  given  great  promise  of  good  and  permanency. 
It  presents  some  novel  features.  The  walls  of  the  container  are  of 
concrete  construction,  and  it  is  divided  into  about  ten  elements ; 
these  are  controlled  separately.  That  enables  the  cleaning  of  each 
without  interfering  with  the  others.  The  filtering  material  con- 
sists of  several  sizes  of  broken  slag  ;  the  larger  at  the  bottom 
and  the  smaller  sizes  toward  the  top.  On  the  surface  of  the  slag  is 
placed  a  layer  of  sponge  clippings  I  foot  in  thickness.  This  is  com- 
pressed to  about  6  inches  and  held  down  by  a  lattice  of  woodwork 
over  all.  This  sponge  or  elastic  layer,  as  it  is  called,  really  consti- 
tutes the  strainer,  while  the  slag  divides  the  water  into  innumerable 
