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Sunday, 14 April 2019

WATER TREATMENT - IMPORTANCE OF REMOVAL OF IMPURITIES

          The major concern in industrial water treatment, where thw water used directly or indirectly in an industrial process, is to treat the water to be suitable for that particular application/process. The use of water in boiler for steam generation is an obvious industrial use. Depending on the process, varying degrees of purity of treated water are required. For example, a textile processing unit will require soft and clea water for process use; a chemical plant will require pure water for process not exceeding 1.0 mg/ltr of dissolved impurities or electronic components manufacturing unit require ultra pure water containing total dissolved impurities not exceeding 0.5 mg/ltr or less. So depending upon the requirement, various water treatment processes are adopted to ttreat the water, to make the water suitable for that particular application. The details of the process are as follows:

FILTRATION
          Filtration is the process of passing a liquid containing suspended matter through a suitable porous material (filtering medium) to efficiently remove the suspended matter in the liquid. This is basically a physical treatment of water.
          Filtration is employed in the treatment of industrial water in order to remove or reduce suspended solids and turbidity. This is of special importance in boiler feed water as otherwise there will be formation of sludge and slit deposits. These will restrict flow, causes overheating and consequent failure of water wall tubes. Furthur, in combination with hardness, these sludge and slit deposits will add to the volume and has the insulating effect of scale deposits.

          1. Filtration is employed as a pre-treatment to softening or demineralising plants to protect the resins in them. It is also employed for treating potable water.
          2. The weakly basic anion resins exchanges only the strong acids such as Hydrochloric acid, Sulphuric acid and Nitric acid.

          There are basically two types of Filters - Gravity type, and Pressure type. For industrial applications the latter are preferred; they can be linked into the mains as they work under pressure and used in conjunction with other water treating equipment like softners, demineralisers without recourse to re-pumping. Also, Presure Filters are easier to install and operate, require less space and minimal civil work.

          Filter media commonly employed are graded and washed sand of effective size 0.35 mm to 0.5 mm resting on supporting underbed of crushed gravel and pebble of four varying sizes, with the coarsest size at the bottom of the bed. The sand depth is 500 mm and the underbed depth also 500 mm.

          The major components of a pressure filter are a steel pressure vessel with dished ends (normally vertical and cylindrical, though larger flows horizontal and cylindrical vessels are employed); internals comprising raw water distributor and filtered water collector-cum backwash water distributor; external pipe work and valves; filter media and instruments like pressure gauges, flow indicator etc.,

          Backwashing of the filter bed has to be carried out periodically (normally once in 24 hours, more frequently if the pressure drop across the bed exceeds 0.7 kg/cm and which indicates accumulation of dirt in the bed) with filtered water at a minimum head of 10 MWC. If air agitation facilitates are provided then the backwash rate can be reduced. The normal backwash time is 5-6 minutes and the scour time 2-3 minutes.

          NormallyFilters should not be fed with water carrying suspended matter and turbidity content of more than 30 to 50 NTU. Above these limits the water should be settled and clarified before Filtration. Also to increase the efficiency of Filtration and to ensure that even fine Suspensions are removed, the ususal practice is to dose Coagulant Chemicals like alum, ferrous sulfates or sodium aluminate at the inlet to the filter by means of a effluent. Activated carbon is used as a filtering Medium when oil/chlorine removal, etc, are required. A layer of processes Manganese Dioxide is incorporated in sand filters for iron removal. A layer of anthracite once the sand bed enhances the filtering capasity by providing in-depth filtration. When anthracite is used for filter can handle turbidity of upto 100 NTU.   

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