|M.Sc Student||Zarchi Idit|
|Subject||Polyaluminum Chloride as an Alternative to Alum for Direct|
Filtration of Drinking Water
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor Eran Friedler|
|Professor Emeritus Menahem Rebhun|
|Full Thesis text|
The efficiency of various Polyaluminum Chloride coagulants (PACls) was compared to the efficiency of Aluminum Sulphate (Alum) in the coagulation-flocculation process preceding the direct filtration of drinking water. The comparative study between the PACls and Alum was divided into four stages: In the first stage, filtration-jar-tests (FJTs) were conducted at the Technion laboratory in order to screen 15 PACls of differing basicity (from 40 - 85%, r = 1.2 - 2.6) from various suppliers. The eight PACls that performed best during this stage were selected for further study. The following two stages were comprised of short (5 - 7 hours), and long (24 hours) filter runs conducted at a pilot filtration plant equipped with full-scale sized filters. Partially treated surface water from the Sea of Galilee with very low turbidity (~ 1 NTU) was used. In the final stage the speciation of aluminum in situ was investigated utilizing the ferron assay method. Results from the FJTs and the pilot indicate that some PACls were as or more efficient a coagulant as Alum for the direct filtration of surface water without requiring acid addition for pH adjustment and consequent base addition for re-stabilizing the water. Consequently, cost analysis of the chemicals needed for the process showed that treatment with PACl would be significantly less costly than treatment with Alum. Contrary to commonly expressed views that high basicity PACls are more efficient for filtration purposes than those with low basicity, no specific preference for basicity, in terms of turbidity and particle reduction, was found in any of the screening stages. Ferron assay experiments revealed that the performance of the coagulant is more influenced by the species present during the coagulation process, irrespective of those present in the original reagents. A PACl of low basicity displayed a large monomeric fraction at the product’s original pH (pH < 4) but when added to water (pH 7 or 8) contained a predominantly large polymeric fraction typically correlated with the highly efficient Al13 polymer. A PACl of high basicity displayed a constant speciation at all pH values, with nearly equal fractions of polymers and large polymers/colloids. Alum displayed a large polymeric fraction at pH 7, however at pH 8 a large quantity of less effective Al hydroxide precipitate was noted. These results support results from the filtration pilot and explain PACls ability, in contrast to Alum, to act as an efficient coagulant within a wider pH range.