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.
