The most important index of PAC is basicity?
What is the most important index of a PAC? the concentration of alum or the basicity?
There are now a number of different grades of PAC with aluminium concentrations between 9.5% and 18%. The concentration of aluminium is not the only consideration when selecting a grade of PAC to use. (If it was, PWTAG would be recommending the much cheaper alum – aluminium sulphate.)
Continuous coagulation with polyaluminium chloride has long been recommended by PWTAG. Like all coagulants PAC works by extracting and then clumping together dissolved, colloidal and suspended matter. The resulting floc is then trapped on the filter.
Filtration through a porous medium such as a sand bed is an efficient and cost effective means of reducing water turbidity. But sand filters are very limited in their ability to remove impurities without coagulation.
Without coagulation removal rates in a single pass of a medium-rate filter were about 50%. This was dramatically improved, to over 90%, by dosing polyaluminium chloride.
Commercially available PAC products are characterised by their degree of basification – i.e. the concentration of hydroxyl groups (OH) relative to aluminium ions.
PAC products can be in the range of 5%-65% basicity. As a general rule, the higher the basicity, the higher the polymer content and thus cationic charge density and efficiency. Higher basicity products have a lower aluminium content but are more efficient in clarification of water (i.e. turbidity/suspended solids removal).
Because there is less aluminium content there is less aluminium in the backwash water. Furthermore, aluminium residuals are minimised. The lower aluminium content products have a higher basicity, so the best advice that PWTAG can give is to use the same dosing rate for all grades of PAC, that is 0.1ml/m3 of the total flow rate.
PAC should be dosed as far upstream of the filters as possible, but after the chemical controller’s sample point.
PAC should be dosed continuously using a peristaltic pump.
All grades of PAC should be dosed at a rate of 0.1ml/m3 of the total flow rate.
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