Kalciumklorid som hjälpkoagulant

Sammanfattning: As populations continue to grow, the demand for fresh drinking water is increasing. This puts a lot of pressure on drinking water producers to strive for more efficient solutions and techniques. Many producers worldwide use surface water as a raw water source, which they often treat through coagulation and flocculation techniques. This is done by adding coagulant (e.g. metal coagulants), creating instability in the suspension, causing flocculation. In this work, PIX-311 (a FeCl3 coagulant produced by Kemira) and Al2(SO4)3 (Kemwater ALG) were used as primary coagulants and CaCl2 (produced by TETRA Chemicals) as an adjunct in a coagulation and flocculation study. The goal of this work was to study the effects of CaCl2 additions to Fe(III) and Al(III) flocculation. The experiments were conducted at Ringsjöverket (a water treatment facility), using jar tests to simulate the treatment process on a laboratory scale. The raw water samples used in this study, were taken from Bolmen, a lake in southern Sweden. A spectrophotometer was used to monitor the efficiency of flocculation by UV-VIS absorption. In the first experiments, various CaCl2 additions were added to Fe(III) flocculation, with FeCl3 as a primary coagulant. To see if FeCl3 could be substituted with CaCl2, the amount of primary coagulant was reduced to about 80% of the optimal dosage (the dose used daily by the water treatment plant). In the next series of experiments, various amounts of CaCl2 were added with an optimal dose of FeCl3. After that, the effects of CaCl2 additions to Al(III) flocculation were conducted, using Al2(SO4)3 as a primary coagulant. The experiments followed the previous scheme used in Fe(III) flocculation. The UV-VIS results showed that no CaCl2 additions were effective enough to replace the primary coagulant. However, reduced amount of primary coagulant benefited slightly from small CaCl2 additions. A likely explanation for this is the ability of Ca2+ to aid in charge neutralization and reduce the repulsive forces between particles in suspension, aiding coagulation. Furthermore, CaCl2 additions, with reasonable certainty, did not increase the efficiency of Fe(III) flocculation. When the CaCl2 dose was increased (from about 13µl to 820 µl per liter raw water), the effect became negative. In other words, high doses of CaCl2 inhibited flocculation by, most likely, occupying adsorption sites for the primary coagulant. This was observed for an optimal FeCl3 dose, reduced FeCl3 dose and an optimal dose of Al2(SO4)3. In the more brief study on Al(III) flocculation, low CaCl2 doses did not appear to have any effect on flocculation at first.  However, when a reduced amount of Al2(SO4)3 was used, the samples with CaCl2 gave better UV-VIS results once the pH was increased from 6 to about 6.15. One explanation for this could be that the shift in flocculation mechanism at higher pH, causes CaCl2 to have an increased positive effect.