03 August 2011

Water Treatment - Softening, Odor Removal, Sediment Removal & Disinfection




PAUL HAY Capital Projects
  

Water Treatment

Author:          Paul Hay
e-mail:            paul.hay@phcjam.com
profile:           www.linkedin.com/in/phcjam


1.0       INTRODUCTION TO WATER TREATMENT            


1.1            Precipitation typically has few impurities, almost no bacterial content, and minimal amounts of minerals and gases;

1.2      Precipitation may be contaminated with physical pollutants from the catching surfaces;

1.3      Ground water is susceptible to chemical alteration in percolation;

1.4     Potable water may be obtained from aquifers, surface run-off or through desalination (which is generally expensive); 
           1.4.1       Potable water is generally obtained from public water supply systems.
        1.4.2    Where available, public water supply systems are adequately monitored to provide a dependable safe source of water.
           1.4.3       Water taken from other sources should be carefully analyzed and treated before use.


1.5      90% of pathogens die naturally after storage over a 5 - 7 day period; and

1.6       Treatment processes range from sedimentation to distillation.



2.0       PUBLIC WATER SUPPLY MAY NEED SOFTENING     


2.1      Water treatment of the public water supply may only be needed in the event of unacceptable hardness:
            2.1.1   Hard water has over 58.6 ppm of Calcium and magnesium salts;
         2.1.2   Water softeners are the most common means of removing these salts from domestic water supply.

2.2       Water softeners are not always cost effective:
            2.2.1   They require additional expenditure on equipment, chemicals, and labour;
            2.2.2   Water is also more corrosive after softening.   

2.3       Operating costs are lower and require less equipment when used primarily for treatment of the hot water supply.
                                                                                               


3.0       GROUND WATER MAY NEED ODOR REMOVAL


3.1       Ground water is usually clean but softening and odor removal is important;

3.2       Aeration can (a) remove odors due to hydrogen sulfide and algae, (b) improve taste and colour of water, and (c) oxidizes iron and manganese, so that they may be removed by filtration:
            3.2.1   Aeration is achieved in tanks by passing water through a series of perforated plates in streams or droplets;
         3.2.2   Otherwise, aeration is achieved by maximizing the exposure of water to air (e.g. waterfalls, fountains, etc.).

3.3       Activated carbon filters remove tastes and odors by extracting dissolved gases, soluble organic material and fine solids.


4.0        SURFACE WATER MAY NEED SEDIMENT REMOVAL & DISINFECTION


4.1       Surface water is likely to be contaminated and should be treated by (a) sedimentation, (b) coagulation, (c) disinfection and (d) filtration;

4.2       Sedimentation is the process by which heavy  suspended particles settle in inactive water conditions:
           4.2.1   A minimum of two tanks should be used each with 24-hours storage capacity to allow de-silting operations without affecting distribution of water; and,
            4.2.2   baffles can be used to slow the flow of water.

4.3       Coagulation (Flocculation) removes colouration and suspended matter:
            4.3.1   Particles combine with alum (hydrated aluminium sulfate) in slow moving water to form floc;
            4.3.2   Heavy particles settle in a process similar to sedimentation, and some adjustment of pH may be required in the process.

4.4       Chlorine has become the standard means of disinfection:
            4.4.1   This process should be applied first where iron and manganese concentration is a problem, as these adversely affects plumbing and other methods of water treatment;
            4.4.2   Hypo-chlorinators automatically add chlorine solution to water;
            4.4.3   Effective disinfection requires (a) a minimum concentration of 4mg/L of chlorine in water for at
                       least 30 minutes, (b) lower pH; and (c) high concentrations should be used for faster
                       disinfection.


4.5       Filtration can remove (a) suspended particles, (b) some bacteria, (c) colour and (d) taste:
          4.5.1   Chlorination/fine filtration oxidizes iron and/or manganese to form precipitate that is removed by the fine filter; and
            4.5.2   Chlorination/fine filtration kills iron bacteria and disinfects.

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FURTHER READING

Mechanical and Electrical Equipment for Buildings, Benjamin Stein & John S. Reynolds, John Wiley & Sons Inc., U.S.A.
Construction Materials & Processes, Don G. Watson, McGraw-Hill Book Co., USA.

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