Humans have been disinfecting our water forever. Data from as early as 2000 BC shows that civilizations were boiling water, using sunlight (UV), and filtering drinking water through charcoal to remove impurities.
In industrial and commercial wastewater processing, water is primarily purified using chemical oxidization. Oxidants like ozone, chlorine, and hydroxyl radicals are used to remove impurities and make our water safe to drink.
Except processing doesn’t remove everything that it needs to. Micropollutants are bioactive, persistent contaminants that aren’t fully eliminated through traditional wastewater treatment. In fact, micropollutant residue has been found in almost every major body of water in the world.
Micropollutants often take the form of pharmaceuticals. Prescription drugs are improperly disposed of, and coupled with an increase in hormone and antibiotic use in animal farming over the last few decades, they’ve made their way into our drinking water supply. Because of micropollutants’ accumulative properties, they pose a threat to the health of wildlife and the environment.
Chlorine, chlorine dioxide, ozone, and hydroxyl radicals are widely applied methods of disinfection of drinking water and wastewater. Chlorine is the most commonly-used disinfectant for drinking water around the world, but in municipal and industrial settings, ozone is the gold standard.
While studies have shown the effectiveness of chlorine and ozone for decades, research has only recently begun to evaluate their effectiveness in transforming micropollutants.
The efficiency of the micropollutant transformation relies on two things: how reactive the oxidant is, and how the dissolved components (and their relative concentrations) in the water affect how stable the oxidants are.
Comparing relative effectiveness of oxidants
Yunho Lee and Urs von Gunten, through the Swiss Federal Institute of Aquatic Science and Technology, and the Institute of Biogeochemistry and Pollutant Dynamics (Sweden) compared the relative efficiency and effectiveness of wastewater micropollutant oxidants.
The study, titled Oxidative transformation of micropollutants during municipal wastewater treatment: Comparison of kinetic aspects of selective and non-selective oxidants, compares chlorine, chlorine dioxide, ferrateVI, and hydroxyl radicals, for their relative effectiveness.
The micropollutants reviewed were all pharmaceuticals; atenolol (a beta-blocker), carbamazepine (a seizure medication), 17a-ethinylestradiol (EE2, a steroidal estrogen), ibuprofen (a common pain reliever), and sulfamethoxazole (an antibiotic).
While no studies to date have found pollutant pharmaceuticals to have a significant effect on human health, pharmaceutical micropollutants are affecting the ecosystem. A 2011 study found that fish located downstream from wastewater treatment plants are more likely to be female or intersex, as a result of hormonal contamination like EE2. Concentrated amounts of other medications have been found in the brain tissue of nearby fish as well.
With a sizable portion of drugs ending up flushed down the toilet and in our water supply, there’s a need for effective micropollutant transformation and removal in our municipal wastewater sector.
How did they do?
Most commercially-used oxidants are selective; they react preferentially with electron-rich organic moieties (ERMs), such as activated aromatic compounds (i.e. phenol, aniline, and polycyclic aromatics), organosulfur compounds, and deprotonated amines.
Hydroxyl radicals, on the other hand, are less selective, and react easily with many organic compounds.
This high reactivity has both an upside and a downside as a micropollutant transformer; hydroxyl radicals were found to be more broadly effective at reducing contaminants in general, as it reacted more frequently and faster. Because of this high reactivity, though, it was less efficient at targeting specific micropollutants and instead was non-selective with regards to which compounds it reacted with. It's also important to note that this high reactivity leads it to be less effective in open air decontamination.
The study found that matrix components present in the water, such as dissolved organic matter (DOM), determined how stable the oxidants were. While both the selective and non-selective oxidants were affected by DOM concentrations and other substances present, hydroxyl radicals are more affected and required higher concentrations in order to be effective for micropollutant transformation.
Still, the presence of specific compounds decreased the effectiveness of the selective oxidants, too. Ammonia, for example, reacts easily with chlorine, and so high levels of ammonia in wastewater leads to a reduced transformation of micropollutants using chlorine. EE2, in particular, saw a transformation rate of 10% with both ozone and chlorine when high levels of ammonia were present.
Application in wastewater treatment
Sweden has funded the majority of research pilots looking to eliminate or reduce micropollutants in the water supply. Currently, the use of powdered activated carbon and ozone are the most widely-adopted and recommended for micropollutant transformation in wastewater management, but other methods have been found to be both cost-effective and transformation-effective.
Still, new studies and funding are examining how hydroxyl radicals can be used with improved efficiency when targeting specific unwanted compounds in wastewater.