WWEMA Window: Reducing Emerging Contaminants In The Water Cycle By Advanced Oxidation Processes (AOP)

By Ron Port, Xylem Inc.

Due to the development of analytical methods and instruments in the past few years, pollutants which had not been detectable in our drinking water sources have now become visible. They are anthropogenic and persistent substances in the water cycle, which conventional treatment methods are not able to remove. Their potential impact on aquatic life and human health have made these emerging contaminants — such as pharmaceuticals, hormones, personal care products, and industrial chemicals — a focus of investigations and regulations. Efficient barriers, particularly for shortened recycle loops for potable water production, are needed.

Ozone treatment and ultraviolet (UV) irradiation are well-known single treatment steps for oxidation or disinfection purposes. The combination of ozone or UV-radiation with hydrogen peroxide results in a more powerful process referred to as Advanced Oxidation Process (AOP). These combined AOP processes are able to reduce many of the trace pollutants efficiently and in accordance with regulations. The application of ozone and UV-based AOPs for full-scale drinking water treatment has been proven by studies and through full-scale references to be an option for drinking water utilities to face the challenge of emerging contaminants in their water sources.

Both AOPs have their advantages and disadvantages, and it is difficult to predict which AOP is the best for an operator without knowing the characteristic of the existing treatment process and the chemistry of the water. A combination of both AOPs can also be the right solution, especially when a wide range of micro-pollutants with a diversity of chemical characteristics have to be removed from the water.

Applying AOPs for the treatment of drinking water consumes additional energy and can increase the overall treatment cost. Testing different technologies and their combination is vital for finding the best solution regarding treatment effectiveness and operational efficiency. In one specific case, testing revealed that the combination Ozone/Hydrogen peroxide and UV was the most economical process. The main criteria to evaluate the right solution are operational cost for energy, investment cost for equipment, flexibility in operation and formation of by-products.

The following “thought process” can be used:

1. Check the water matrix and compounds to be treated. The important figures to take into account for the water matrix are: DOC and nitrate concentrations, UV-transmission, amount of radical scavenger, ozone consumption, and bromide level. The compounds to be treated need to be characterized in different groups: ozone reactive, radical reactive, or UV-sensitive.


2. Check the expected concentrations of the target compounds and define the desired reduction rates. Depending on water matrix and needed reduction rates for specific compounds, different AOPs can be favorable.
   
   For the defined process conditions (points 1 and 2), a first calculation can be done on the basis of existing data. With this kind of principle check, the most interesting AOP combinations for specific cases can be pre-evaluated.


3. Run pilot trials to investigate the pre-selected AOP combination in more detail to get the design parameters for a full-scale plant.


 

In the end, the selected process will be able to achieve the required reduction rates for a wide range of trace pollutants with less energy, less capital cost, and more flexibility in operation than the standard processes.