Hardness decreased further during ozonation, with the decrease proportional to the amount
of ozone applied (e.g., 4.1% removal in Treatment 1 and 7.8% removal in Treatment 3, a
statistically significant difference). Ozone-induced hardness removal was probably because
increased carboxyl acid concentration due to ozonation led to greater magnesium and
calcium association, resulting in precipitation of metal-humate complexes (Grasso & Weber
1988). Higher hardness removal at higher ozone doses was attributable to more destabilized
carboxyl acids binding with a larger amount of hardness ion species. Noting that increasing
hardness to 150 mg/L CaCO3 in ozonated water improved removal of TSS (Rueter &
Johnson 1995), BRA effluent exhibited favorable conditions for TSS removal. After being
formed in the aqueous medium, TSS complexes likely were caught on bubble surfaces and
buoyed to the top as foam, explaining the high TSS removal efficiency during ozonation
(Sandu et al. 2008). Once in the foam, however, the association of hardness with TSS seems
to have been attacked by ozone, which broke and dissolved TSS particles.