Comparison of the Ability of Three Coagulants to Enhance Filter Performance

In the operation of porous media filtration systems, poor initial performance is often observed, in which particle removal is less than desired. Alum (${\mathrm{Al}}_2({\mathrm{SO}}_4{)}_3•14{\mathrm{H}}_2\mathrm{O}$), ferric chloride (${\mathrm{FeCl}}_3$), and polyaluminum chloride (PACl), are often used in drinking-water treatment to modify the surface properties of the particles being filtered and enhance their removal by filters; however, modification of the filter medium is not a common practice. In this research, these three coagulants were used to artificially modify a sand medium in situ by adding metal hydroxide precipitates onto the top of the filter before challenging the filter with an otherwise untreated kaolin suspension. After modification of the filter medium, high particle-removal efficiencies were achieved using a short (7.5 cm) sand column with $\cong 1\mathrm{mm}$ diameter sand. The best observed particle removal (96%) using alum and PACl occurred at the surface loading of 550 mmol of $\mathrm{Al}/{\mathrm{m}}^2$. The filter performance deteriorated at higher surface loadings. Fractional colloid removal increased with increased ferric chloride surface loading over the entire range of dosages tested (up to 97.5% at 2,200 mmol $\mathrm{Fe}/{\mathrm{m}}^2$). The experimental results suggest that pretreatment of filter media by application of Fe or Al hydroxides can eliminate the initial poor performance.