Chemical reactions during pretreatment produce additional SS in industrial wastewater. The most common precipitating agent is lime slurry (10% calcium hydroxide plus H2O). It precipitates a variety of calcium compounds, both organic and inorganic in water. Dolomitic lime (Ca[OH]2 • Mg[OH] 2) is a superior precipitating agent for some waste material. These two forms of lime represent the best combination of availability, economy, and versatility.
Compounds precipitated by lime usually settle well and can be filtered at a rate of 2 to 6 lb/hr/ft2 on an RDVF (see Figure 7.47.5). Lime adds little to the dissolved solids content, and most calcium compounds have low solubility. Regardless of the alkali used, environmental engineers must perform filtration testing of the precipitates for design.
Frequently, the submicron industrial sludge formed by the reaction with lime should not be preconcentrated be-
cause filtration proceeds better on a dilute slurry (see Figure 7.47.6).
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