chemistry technology

Most municipalities must use a source of water in which the probability of pollution is rather high. Certainly, all our natural rivers and lakes and even the water stored in most reservoirs may be subjected to pollution,  and  generally  cannot  be  considered  safe  for  drinking  purposes  without  some  forms  of treatment. The type and extent of treatment will vary from city to city, depending upon the conditions of
the  raw  water.  Treatment  may  comprise  various  processes  used  separately  or  in  combinations,  such  as storage,  aeration,  sedimentation,  coagulation,  rapid  or  slow  sand  filtration,  and  chlorination,  or  other accepted forms of disinfection.
When  surface  waters  serve  as  a  municipal  water  supply,  it  is  generally  necessary  to  remove suspended solid, which can be accomplished either by plain sedimentation or sedimentation following the addition of coagulating chemicals. In the water from most streams that are suitable as a source of supply,
the sediment is principally inorganic, consisting of particles of sand and clay and small amount of organic matter.  In  this  water  there  will  also  be  varying  numbers  of  bacteria,  depending  upon  the  amount  of
bacteria  nutrients,  coming  from  sewage  or  other  sources  of  organic  matter,  and  upon  the  prevailing
temperature. Many of the bacteria may have come from the soil and, as a result, during a season of high turbidity when there is a large amount of eroded soil in the water, the bacterial count from this source
may be relatively high. If the organisms are derived from sewage pollution, the number will be highest during periods of low flow when there is less dilution, and at this time the turbidity will, in general, be low.  The  amount  of  sediment  may  vary  a  great  deal  from  one  river  to  another,  depending  upon  the
geological character of the various parts of the drainage system. The size of the suspended particles can also vary greatly. In some waters the clay particles may be extremely fine, in fact, they may be smaller
than bacteria. The time required for satisfactory sedimentation differs for different waters, and generally must  be  established  by  actual  experiments.  Some  waters  can  be  clarified  satisfactorily  in  a  few  days,
while others may require weeks or months. As far as total weight of sediment is concerned, the bulk of it
is  probably  removed  in  a  few  days,  but  this  may  not  bring  about  a  corresponding  change  in  the appearance of the water, since the smaller particles may have greater influence than the large ones upon
the apparent color and turbidity. When plain sedimentation is used primarily as a preliminary treatment, a high degree of clarification is not needed and, as a result, shorter periods of settling are adequate.
After  flocculation  treatment,  water  is  passed  through  beds  of  sand  with  diatomaceous  earth  to accomplish  sand  filtration.  As  we  mentioned  previously,  some  protozoan  cysts,  such  as  those  of G.lamblia, appear to be removed from water only by such filtration treatment. The microorganisms are trapped mostly by surface adsorption in the sand beds. They do not penetrate the tortuous routing of the sand beds, even through the openings might be larger than the organisms that are filtered out. These sand filters are periodically backflushed to clear them of accumulations. Water systems of cities that have an exceptional  concern  for  toxic  chemicals  supplement  sand  filtration  with  filters  of  activated  charcoal (carbon).  Charcoal  has  the  advantage  of  removing  not  only  particulate  matter  but  also  some  dissolved organic chemical pollutants.
Before entering the municipal distribution system, the filtered water is chlorinated. Because organic matter neutralized chlorine, the plant operators must pay constant attention to maintaining effective levels
of chlorine. There has been some concern that chlorine itself might be a health hazard, that it might react
with organic contaminants of the water to form carcinogenic compounds. At present, this possibility is considered minor when compared with the proven usefulness of chlorination of water.
One substitute for chlorination is ozone treatment. Ozone (O3) is a highly reactive form of oxygen that  is  formed  by  electrical  spark  discharges  and  ultraviolet  light.  (The  fresh  odor  of  air  following  an electrical storm or around an ultraviolet light bulb is from ozone). Ozone for water treatment is generated electrically  at  the  site  of  treatment.  Use  of  ultraviolet  light  is  also  a  possible  alternative  to  chemical disinfection. Arrays of ultraviolet tube lamps are arranged in quartz tubes so that water flows close to the lamps. This is necessary because of the low penetrating power of ultraviolet radiation.