Diatomaceous earths may resemble the forms of the charcoals. The earths are primarily filter aids, precoats or adsorbents, the function of the filter medium being secondary. Fuller's earth and clays are used for decoloring applications; diatomaceous earths are used for clarification. The adsorbtivity of diatomaceous earth works in the same fashion as activated carbon, but isotherms (affinity) for many chemical species like the hydrocarbons is weaker. For this reason, activated charcoal or carbon is much preferred in wastewater treatment applications expecially when taste and odor issues are priorities.
Carbon materials are activated through a series of processes which includes:
• Removal of all water (dehydration)
• Conversion of the organic matter to elemental carbon, driving off the noncarbon portion (carbonization)
• Burning off tars and pore enlargement (activation).
Initially, the material to be converted is heated to 170° C to effect water removal Temperatures are than raised above 170° C driving off CO* CO, and acetic acid vapors. At temperatures of about 275" C, the decomposition of the material results and tar, methanol, and other by-products are formed. Nearly 80 percent elemental carbon is then effected by prolonged exposure to 400- 600° C. Activation of this product follows with the use of steam or carbon dioxide as an activating agent. The superheated steam, 750-950" C, passes through the carbon burning out by-product blockages, and expanding and extending the pore network.
142 WATER AND WASTEWATER TREATMENT TECHNOLOGIES PRECIPITATES AND SALTS
Precipitates or salts are used when corrosive liquor must be filtered, and where there is no available medium of sufficient fineness that is corrosion-resistant and will not contaminate the cake. In these cases, precipitates or salts are used on porous supports. In the filtration of caustic liquors, ordinary salt (sodium chloride) is used as the filter medium in the form of a precoat over metallic cloth. This procedure has the advantage that the salt medium will not be detrimental to either the cake or the filtrate if inadvertently mixed with it.
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