Micropollutants are bioactive and persistent contaminants that cannot be fully eliminated with traditional wastewater treatment methods and thatare not completely biodegradable.
Micropollutants, when continuously released with wastewater effluents, can lead to long-term hazards because the contaminants bioaccumulate and can even form new mixtures in water. The exact effects of these contaminants are not yet fully known which is a cause of concern for many EU health authorities.
hyxdyl | Shutterstock
Sources of Micropollutants
Micropollutants are released from products that are used daily, such as industrial chemicals, pharmaceuticals and personal care products (PPCPs), pesticides, and hormones. Residue from such products appear in almost all water bodies.
Hazardous micropollutants, formed during domestic usage of electronics, hygiene and cosmetic products, textiles and pharmaceuticals, normally reach the wastewater treatment plants (WWTPs). Various other sources of these micropollutants are surface run-off from agricultural areas, industrial discharge and stormwater run-off from the cities.
Investigations have attributed 70% of the pharmaceutical residue found in the wastewater to household use; another 20% is attributed to livestock farming. Of the remaining 10%, 5% is attributed to hospital effluent, and the other 5% is due to runoff from non-specific sources. However, these figures could vary highly in different parts of various countries.
Whatever the source of these micropollutants, they reach the water resources as they are persistent and non-biodegradable, which means that when released into the nature, these contaminants pass through the soil and ultimately reach the groundwater. Even if they reach the WWTPs, a major portion of these micropollutants are released along with wastewater effluents and reach the surface water.
Micropollutants have several sources, one of which is pharmaceuticals. Kaesler Media | Shutterstock
Impacts of Micropollutants on the Environment
The current biological WWTPs are not explicitly designed to eliminate the micropollutants, discharges from these WWTPs are a major point source of these chemicals in the environment.
The prevalence and continuous input of these organic micropollutants into water resources, inclusive of groundwater, is an ever-expanding environmental problem. Many of the micropollutants have been determined to be highly hazardous threats to animals, aquatic species, as well as human beings because they are non-biodegradable and bioaccumulative.
The occurrence of micropollutants in the ecosystem can lead to toxic biological effects such as mutagenicity, estrogenicity, and genotoxicity. An evident example is the feminization of male fish, caused due to the fish population being exposed to endocrine disrupting compounds (EDCs). Even when released at low levels, the continuous release of EDCs into the environment may cause reproductive and developmental abnormalities on highly sensitive species.
Yet another alarming issue is the increase of antibiotic-resistant organisms in the environment, which proves to be an added risk to the microbial ecosystems. The vigorous usage of antibiotics to improve the health of animals and humans has resulted in antibiotic-resistant genes in various environmental matrices. As a result of population explosion and higher reliance of modern societies on pharmaceuticals, the release of micropollutants into the ecosystem is anticipated to increase in the future. In addition, exposure to complex mixtures is more serious than single compound because of their possible synergistic effects.
Micropollutants can contribute towards antibiotic resistance. Jarun Ontakrai | Shutterstock
Reducing Micropollutant Concentrations Requires a Long-Term Strategy
Currently in Europe there are around 100,000 commercially registered compounds, and the residue from most of these will ultimately reach the water cycle. The manufacture of these chemicals is expected to increase. Two different approaches can be applied to decrease the micropollutant concentrations in the future: source control and end of pipe removal.
Source control is a long-term procedure, incorporating the prohibition of toxic contaminants and the promotion of green chemistry. In the field of pharmaceuticals, prohibiting effective drugs will lead to an ethical dilemma as humans as well as animals rely upon these drugs for survival. End of pipe solutions, or wastewater treatment, will also be highly important in the future.
The European Union is Taking Action
The European Union (EU) has published a list of prioritized substances that pose a threat to ground and surface resources. To adhere to the environmental standards set by the EU, starting from September 2015, the member countries have to regularly monitor the occurrence of the listed substances (Directive 2000/60/EC, i.e. Water Framework Directive, and Directive 2013/39/EU, i.e. priority substances in the field of water policy). Switzerland has already resolved to reduce micropollutant content and toxicity in the wastewater released by upgrading 100 WWTPs (which treat close to 50% of the municipal wastewater) in the next 20 years.
The amount of micropollutants cannot be reduced by traditional mechanical and biological wastewater treatment techniques, in which only phosphorus, nitrogen, and degradable organic matter are eliminated. The issue of micropollutants can be suitably resolved by carrying out advanced water treatment methods.
In order to reduce micropollutants, an additional barrier of advanced water treatment methods should be added to conventional WWTPs. Various existing techniques are proven to be good for the same. Rik I. L. Eggen and colleagues indicate that certain research carried out prior to the Swiss resolution has proven that powdered activated carbon (PAC) and ozonation are the most technically feasible techniques. It was shown that both techniques reduced the load of broad range micropollutants by over 80%, and a polishing step, such as sand filtration, should be performed to remove any existing bioavailable oxidation products and particles.
In Switzerland the cost aspect, which is highly important, was found to be nearly the same for both techniques. However, research carried out in Sweden indicated that the ozone treatment costs less than the PAC treatment by 50%.
Various full-scale and pilot studies have shown that ozone treatment is the most efficient technique to remove micropollutants in wastewater. The exceptional oxidation and disinfection properties of ozone enable even the most persistent and non-biodegradable substances to be degraded.
How Wastewater Treatment with Ozone Works
Ozone can be used to remove micropollutants, which are resistant to degradation, from water. zffoto | Shutterstock
As ozone is a well-known powerful oxidizing and disinfectant agent, it has been used historically to disinfect drinking water and to remove color, taste, and odor. The application of ozonation to wastewater treatment to eliminate organic micropollutants resistant to biological degradation has been becoming increasingly more popular.
The amount of ozone required for wastewater treatment relies on the characteristics and nature of the wastewater matrix. High levels of dissolved organic remnants and the presence of other inorganic species even after biological treatment require ozone treatment. In contrast, secondary oxidants such as hydroxyl (OH) radicals generated by the reaction of ozone with dissolved organic matter may enable the removal of ozone-refractory micropollutants. According to the treatment objective, ozone can be added either immediately after an extensive biological treatment or as a polishing step at the end of the treatment to reduce the required ozone dose.
The pH of the wastewater is another important factor that impacts ozone reaction. A lower pH enables direct reaction with molecular ozone, while a higher pH causes an increase in ozone decomposition which leads to the production of OH radicals that are advantageous, because these radicals non-selectively react with trace organic compounds present in the wastewater.
The chemical structure of the organic micropollutants, especially the reactivity of a particular functional group or substituent, determines the transformation of these chemicals. Ozone directly reacts with electron-rich sites, such as carbon-to-carbon double bonds, tertiary amines, and phenols. The micropollutants that exhibit low ozone reactivity are efficiently eliminated by hydroxyl radical mechanism. Even though complete mineralization is out of question, ozone treatment enhances biodegradability by breaking down organic micropollutants into simpler, smaller molecules, which can be eliminated with subsequent biological treatment.
This information has been sourced, reviewed and adapted from materials provided by Primozone.
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