For water treatment applications,Now a Days number of alternative aluminium-based coagulants have been developed instead of Alum.
ADVANTAGES OF POLYALUMINIUM COAGULANTS over ALUM
1. The Polyaluminium coagulants in general consume considerably less alkalinity than alum.
2. Compared to alum it works effective over a broader pH range,PACl works satisfactorily over a pH range of 5.0 to 8.0.
3. In water treatment processes is the reduced concentration of sulphate added to the treated water.This directly affects SO4 levels in domestic wastewater.
4. Low levels of residual aluminium in the treated water can be achieved, typically 0.01-0.05 mg/L
5. Less sludge is produced compared to alum at an equivalent dose,
6. Compare to Alum, PACl and ACH (aluminium chlorohydrate) work extremely well at low raw water temperatures. Flocs formed from alum at low temperatures settle very slowly, whereas flocs formed from polyaluminium coagulants tend to settle equally well at low and at normal water temperatures,
6. Lower doses are required to give equivalent results to alum.
7. The increase in chloride in the treated water is much lower than the sulphate increase from alum, resulting in lower overall increases in the TDS of the treated water.
a) .the cost of Poly Aluminium Chloride is twice/thrice of Alum ( roughly the cost of PAC-$0.4/ Kg)
b).One possible disadvantage in using Poly Aluminum Chloride (PAC) relates to the removal of dissolved organic carbon (DOC) from water. It is well documented that effective DOC removal is possible with alum,particularly when coagulating at lower pH values using so-called “enhanced coagulation”