Enhancement of particle removal in water treatment by pre-ozonation

Abstract

Particle removal is an essential part of water treatment, used to minimise the passage of disinfection-resistant pathogens into the public water supply. Ozone is known to give rise to an enhancement in particle removal, with coagulation and filtration, under certain conditions. However, the conditions required for this are not fully understood. A novel experimental batch water treatment system, innovative means of quantifying floe morphology and new experimental methodology were developed in order to analyse the ozone-enhanced particle removal phenomenon in depth. These systems were used with both natural and model waters. By operating in batch mode, it was possible to continuously monitor each water quality parameter throughout every stage of water treatment. Preliminary results from the morphological analysis of floes suggest that ozone-enhanced particle removal occurs when ozone leads to the creation of larger, hence more settlable and filterable, floes. Ozone gave rise to an increase in particle removal in natural waters with high natural or spiked bicarbonate concentration and synthetic model waters containing either bicarbonate or tertiary-butanol. Ozone gave rise to a decrease in particle removal in waters with low bicarbonate concentration and synthetic model waters containing no bicarbonate or tertiary-butanol. From this it is observed, for the first time, that a hydroxyl radical scavenger (bicarbonate or tertiary-butanol) is required in order for ozone to have a positive effect on particle removal. It is hypothesised that ozone operates in a beneficial manner, with respect to particle removal, by direct molecular action and in a detrimental manner by radical pathways. The practical outcome of this is that, in order to optimise particle removal, ozone should be used at concentrations in which the local bicarbonate concentration is able to maximise the direct molecular action of ozone.