The removal of micro pollutants from drinking water and water used in industrial processing is becoming increasingly important. In drinking water the discovery of pharmaceutical and personal care products in lakes, rivers and reservoirs has created uncertainty in the safety of these water sources because conventional water treatment plants can not remove these compounds. In industrial applications, especially semiconductor manufacture and pharmaceutical production increasing quality standards are requiring greater purity levels in the water used in these processes.
Advanced oxidation processes can address the need to remove low level contaminants in water without concentrating or absorbing these materials. As a result, they are permanently and safely removed, i.e. converted to CO2 and salts. Advanced oxidation process work by producing the hydroxyl radical. This short lived chemical species with a high oxidation potential can be produced in a number of processes, all referred to as advanced oxidation. These include exposing hydrogen peroxide or ozone to UV radiation, reacting ozone with hydrogen peroxide, and reacting hydrogen peroxide with certain Fe salts.
The hydroxy radical will react with virtually all organic compounds except certain heavily chlorinated compounds. The reaction time is exceedingly fast and indiscriminate, i.e. all organic compounds present will be attack.
In this article, we will focus on the ozone UV process for making the hydroxyl radical. When ozone is exposed to UV radiation in the 254 nm range, the ozone is converted in the presence of water to the hydroxyl radical via the following reactions:
O3 + hν > O2 + O
O + H2O > 2OH•
This reaction is fairly efficient, but depends on the transmission of UV through the fluid, known as UV transmittance or UVT. This can be hindered by suspended solids, or dissolved materials that absorb UV radiation in the 254 nm range. Even clear solutions can have low UVT values since many organic compounds absorb UV.
In relatively pure water systems with low levels of organic contamination, UVT levels can be high, i.e. greater than 95%. This makes the use of ozone UV as an advanced oxidation process practical.
The advantage of the ozone UV process is that no chemicals need to be purchased, stored or handled. The hydroxyl radical is made on site from air and electricity. Since the process is efficient with respect to ozone, there are no measurable byproducts, other than potential break down compounds of the organic materials in solution. Thus it is an extremely safe and sustainable process.
Because equipment is substituted for purchased chemicals, capital expense for the process tends to be higher than other advanced oxidation processes. These systems tend to have a small foot print and are highly automated eliminating the need for constant operator attention.
Thus ozone UV tends to be a better choice for applications with low flows or very low contaminant levels. Some applications where ozone UV has been applied are laboratory water reuse systems, groundwater remediation and semiconductor water reuse.
Since all advanced oxidation processes produce the same chemical agent, the selection of a particular process depends on how that process fits with the end use application in terms of capital expenditure, operating expenses, tolerance for chemical handling, byproduct formation, water quality, etc.