Alum flocculation process, removal

The selective removal of very low levels of dioxin from large volumes of process wastewater has become a concern to a number of companies and municipalities. Very little has been reported on how dioxin behaves in conventional water treatment processes (2-3). The work of Thebault, Cases, and Fiessinger suggested that alum flocculation would be marginally effective for the removal of dioxin. Our need to remove up to several parts per trillion dioxin from water in the lagoon prompted us to evaluate flocculation as a faster and more cost effective treatment for dioxin removal. 

Inorganic chemicals may also be used. Beatonite may be used as a flocculant ia combination with polymer treatmeat. Alum, oace a common coagulant, is less used because its concentration can build up ia recycle water. Alum oftea biads ink to fibers and iacreases the difficulty of deinking. Removal of the very small flexographic ink particles ia process water is difficult. Ultrafiltratioa (qv) has beea proposed for removing these very small dispersed ink particles (53). 

Flocculation or clarification processes are solids-liquid separation techniques used to remove suspended solids and colloidal particles such as clays and organic debris from water, leaving it clear and bright. Certain chemicals used (such as alums) also exhibit partial dealkaliz-ing properties, which can be important given that the principal alkaline impurity removed is calcium bicarbonate—the major contributory cause of boiler and heat exchanger scales (present in scales as carbonate), although closely followed by phosphate. 

This process is enhanced by the addition of polymers, alum, or other flocculants. This chemical addition provides for coagulation and flocculation of colloidal oils and solids into larger particles that can be easily removed. 

As described earlier, flocculation is a physical process used to encourage small particles to aggregate into larger particles or floe. It is an essential component of most water treatment plants in which flocculation, sedimentation, and filtration processes are integrated to effectively remove suspended particles from water. Chemicals (such as alum, polyelectrolytes, etc.) are usually added to achieve agglomeration among small particles in water. 

The impurities in water may be either suspended or dissolved. I- iltration is necessary to remove matter held in suspension. I he addition of a precipitant to the water before it is filtered greatly improves the efficiency of the process. Aluminium hydroxide forms a flocculent precipitate which retains coloured matter and colloidally-suspended impurities. It is common practice to add aluminium sulphate, in the form of alum, to the water but, because this salt gives an acidic solution, no aluminium hydroxide will be precipitated until the pH has been adjusted to between pH5 and pH7. 

Clarification of water The treatment of drinking water provides multiple barriers to protect public health by removing microorganisms, and natural and man-made chemicals that may cause illness in consumers. Generally, a conventional treatment process employs coagulation/flocculation-sedimentation-filtration, and various modifications of the above configuration can be adopted based on specific needs. Aluminum- and iron-based chemicals, particularly alum (Al2( 504)3 I8H2O) and iron(III) chloride. 

For surface water, treatment is aimed primarily at removal of turbidity (fine suspended matter) and perhaps softening the water. The typical treatment processes for removal of turbidity involve the addition of chemicals such as alum or ferric chloride. The chemicals are rapidly mixed into the water so that they react with alkalinity in the water, then slowly mixed (flocculation) to form a settleable precipitate. After sedimentation, the water passes through a sand filter and finally is disinfected wdth chlorine. If the water is to be softened as part of the treatment, lime, Ca(OH)2, and soda ash, Na COj, are used in place of alum or ferric chloride, and the water hardness (calcium and magnesium ions) is removed along with its turbidity. 

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