Water Treatment > Quality Water Solutions > Operational Issues > Comparing Coagulants

The trick in comparing coagulants and their performance is to place them on an equal footing. This is best done by quantifying the active metal (e.g., iron or aluminum) that removes solids and is essential for controlling sludge volume, pH and other factors.

Plants that simply compare coagulants on a ppm 'as-is' basis do not address how the active metal varies in dry and liquid coagulants, in different liquid product concentrations, and in coagulants with different chemical substituents. In dry versus liquid alum, for example, the dry product has an aluminum content of 17.1% as Al2O3 and 9.1% as aluminum metal (Al), while the aluminum content of the liquid is 8.2% to 8.3% as Al2O3 and 4.1% as aluminum metal (Al). Similarly, the dry ferric salts can have more than two times the iron content when compared to the liquid form on a ppm as-is basis. In dry versus liquid ferric chloride or ferric sulfate, the dry products can have an iron content of 20% to 30% as Fe (iron metal), whereas the liquid products will have an iron content between 8% and 15% as iron.

The aluminum content in PACl varies from 6% to 23% as Al2O3 and 3.2% to 12.4% as aluminum metal (Al). However, the reactivity of the aluminum metal in PACl depends upon the basicity of the product. Basicity, or the amount of (OH-) present, is generally expressed as % basicity (defined as [OH]/3[Al] x 100). A higher % basicity product has a more highly aggregated or complex PACl species, which has aluminum species that are more highly charged. The metal in alum and iron salts has a valence of either +2 or +3. In PACl, aggregate valence can be as high as +6 or +7.

Since the metal in PACl is in a more active state, a lower dose is typically needed to remove turbidity compared to traditional coagulants. This ultimately reduces chemical sludge production, reduces aluminum residuals in the finished water and decreases the need for pH adjustment. PACl also consumes less of the water's natural alkalinity. This not only occurs due to a lower required coagulant dose, but because PACl itself contains some alkalinity (i.e., OH- ions). In a typical application, a ferric salt can lower source water pH from 7.5 to less than 6.5 while PACl lowers it to just 7.3. This can eliminate the need for pre-lime or pH adjustment with caustic soda.