What exactly is chemical precipitation?
Chemical precipitations (with the exception of those engaged in coagulation-flocculation) are mostly employed in water or wastewater treatment to remove one or more mineral compounds that negatively influence water utilization, such as hardness (Ca2+ and Mg2+), metals and certain anions such as SO42–, PO43– and F–.
For a given temperature and pH, residual solubility is determined by the precipitating compound's solubility product. As a result, we can change the residual value of the undesired ion by moving the balance with a reagent overdose (exceeding the stoichiometric amount).
How Sludge produced during chemical precipitation of wastewater?
All of t?he chemical precipitation processes create sludge that may be classified into two types:
• Granular sludge composed of crystals ranging in size from a few micrometres to a few tens of micrometres, e.g. CaCO3, CaSO4.2H2O, CaF2, Ca3(PO4)2.
• Metal hydroxide sludge, to which silica-containing precipitates can be added (amorphous).
When pure, the former will retain little water and hence provide significant static thickening possibilities (several hundred kg/m2/h). For example, carbonate removal sludge containing more than 95 percent CaCO3:
-will thicken to around 200-400 g/L;
-will dewater on a belt filter up to more than 50% dry solids content, but caution must be exercised owing to the danger of pipe obstructions if the system is shut off (e.g. intermittent extraction of this sludge and scraper mechanism shut down).
However, if this same granular sludge contains a certain proportion of hydroxides {Fe(OH)3, Al(OH)3, Mg. (OH)2}, to which any precipitated silicates can be added, its suitability for thickening and dewatering decreases sharply.
Pure hydroxide sludge performs just somewhat better {in the case of Fe(OH)3, Cu(OH)2} than excess activated sludge, but Al(OH)3, Zn(OH)2, and Mg(OH)2 perform almost equally.
Ratio=Crystallized sludge/Hydroxide sludge
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