Micropollutants in wastewater – low concentrations of great importance

Mikrozanieczyszczenia w ściekach – niskie stężenia o wielkiej wadze

Currently, there are more than 350,000. registered chemicals and their mixtures, which are used in industrial production and daily use by consumers [1]. Many of these will eventually end up in municipal and industrial wastewater, posing a growing challenge to treatment plants. Although modern analytical methods give us increasing capabilities to detect chemical compounds in complex environmental matrices, we still know relatively little about the future ecological and health consequences of continued contact, even with low doses of pollutants.

What are micropollutants?

Over the past 30 years, developments in analytical techniques have made it possible to detect even very low concentrations of chemical compounds in the aqueous environment, at the micro and nanogram per liter levels. Initially, substances with such low concentrations were considered harmless to the environment and living organisms. However, over time, the group of micropollutants has expanded to include other substances, and researchers have observed that organic compounds belonging to it, have disturbing properties that can affect living organisms.

Micropollutants are both substances of anthropogenic origin and natural chemical compounds. They are usually toxic (cytotoxic, mutagenic, carcinogenic), endocrine disruptors, have the ability to accumulate in living organisms and even pass from generation to generation through the placenta or breast milk [2]. In addition, they exhibit increased durability of chemical bonds, making their biodegradation and removal from the environment difficult [1].

The presence of micropollutants, or otherwise known as “microcontaminants. Emerging substances (a name introduced due to the ever-lengthening list of compounds) have been confirmed in all aquatic environments, including tap water, and there is widespread agreement that their main source is wastewater – both untreated and treated. Micropollutants come from pharmaceuticals (anti-inflammatory drugs, lipid regulators, anticonvulsants, antibiotics, beta-blockers, stimulants), personal care products (fragrances, disinfectants, UV filters, insect repellents) and steroid hormones (estrogens) [2].

More than 100,000 have already been classified within the European Union. micropollutants. In contrast, the list of chemicals continuously controlled in tap water and wastewater represents only a small fraction of currently available scientific knowledge. This is due not only to the lack of physical capacity of water and sewage companies to control a wide range of pollutants, but also to the still unclear and incomplete picture of their harmfulness. Micropollutants are often referred to as “pollutants of concern” due to the lack of information about the magnitude of the threat they pose to the environment and humans [2, 3].

Why investigate micropollution further?

In wastewater or surface water, micropollutants are more polar, acidic or basic than other chemicals, making them particularly dangerous. Many of them are hydrophobic, traveling up the food chain as they accumulate in lipid-rich tissues or can affect the endocrine system of animals and humans [4]. By reaching the body, they can mimic or block natural hormones and disrupt their function (estrogens, androgens, thyroid hormones).

For example, perfluorinated organic compounds, found in cosmetics, among others, have proven carcinogenic properties (liver cancer) and contribute to an increased risk of infertility [4]. In turn, triclosan, commonly found in antibacterial soaps or toothpastes, may serve as a potentiating factor in the development of antibiotic resistance in bacteria, which will pose a growing global challenge to health systems in the future [5].

In the aquatic environment, micropollutants can be transformed into less toxic products or into even more toxic substances. There is also an increased risk of adsorbing them in sediments and periodic leaching, so aquatic organisms may be exposed to elevated concentrations of toxic pollutants [4].

Since micropollutants occur in mixtures in the aquatic environment, the toxic effects of single substances can accumulate and generate synergistic or antagonistic interactions, leading to the so-called “toxicity”. cocktail effect, which increases the difficulty of analyzing potential risks [5].

Micropollutants and their removal efficiency in wastewater treatment plants

The purpose of wastewater treatment is primarily to remove organic pollutants, nutrients (nitrogen and phosphorus compounds), as well as sludge and particulate matter. This is what the treatment plants are currently focusing on. Conventional methods include mechanical, chemical, biological and physicochemical processes, conducted among others. On grids, screens, in sand traps, primary settling tanks and biological chambers.

We are then talking about mechanical-biological, two-stage treatment plants. Still few facilities choose to add a third step (such as artificial wetlands or advanced membrane processes), which could increase the amount of organic pollutants removed and improve water conservation. Often this is due not only to economic reasons, but also to the lack of available land around the treatment plant or the concerns of local communities.

Wastewater treatment processes are mainly based on the use of microorganisms that remove contaminants through biological processes. In practice, the amount of organic compounds is characterized by surrogate parameters in the form of Biological Oxygen Demand and Chemical Oxygen Demand, which do not give full information about the pollutants present in wastewater and hence the need to identify the concentrations of specific substances listed on priority or watch lists [6]. However, as mentioned earlier, the number of these compounds is severely limited.

Biological treatment processes are mainly based on the use of active microorganisms – bacteria, protozoa, fungi, rotifers.

Microbial removal of micropollutants can include biodegradation, which is the complete mineralization of substances into biomass and gases. But often the transformation processes of persistent organic pollutants only lead to biotransformation, i.e. incomplete removal of the parent compound and production of its metabolites [6]. Micropollutants at low concentrations, combined with a stable chemical structure and suboptimal technological conditions, are difficult to remove.

Although recent analyses report that activated sludge microorganisms have adapted to remove micropollutants, this is often true for compounds that have been used in industry for almost 100 years, so their long-term presence has allowed evolutionary pathways in bacteria. However, among the persistent organic pollutants are substances such as per- and polyfluoroalkyl compounds (PFASs), which constitute the so-called “per- and polyfluoroalkyl compounds”. perpetual contamination, despite more than fifty years on the market and a partial recall. Their persistence in the environment has been dated for another 100 or even 2,000 years [6].

Studies show that the pollutant removal efficiency of conventional wastewater treatment plants is low and even negative. The reason may be the transformation of conjugated forms into free ones or the already mentioned biotransformation [7]. The removal efficiency of micropollutants in treatment plants ranges from 13 to 100% and varies, among other things. depending on the physicochemical nature of the substance, bioavailability, the operating conditions of the biological treatment chambers or the structure of the microorganism community. Analyses report that micropollutants in conventional systems can be removed more efficiently, but this requires an increase in sludge retention time and aeration, which generates significant additional costs [9].

Wastewater treatment capacity will not be the same in different countries, and trends vary globally, often due to access to modern solutions and economic conditions. The selection of sustainable wastewater treatment technology is a very complex task, requiring consideration of economic, environmental and social criteria. Also of increasing importance is the demand from industry for the recovery of valuable raw materials such as water, biogas and fertilizer.

We know that micropollutants negatively affect our health, but we still lack the tools to combat the degradation of water resources. The coming decades will certainly focus on the modernization of available technologies and the gradual introduction of modern solutions in the form of advanced membrane or electrochemical processes. However, what may contribute most to the technological transformation are legal solutions.

In the article, I used, among others. From the works:

[1] Yunqiao Zhou et al, Which type of pollutants need to be controlled with priority in wastewater treatment plants: Traditional or emerging pollutants? Environment International 2019, 131, 104982 https://doi.org/10.1016/j.envint.2019.104982
[2] Anindita Gogoi et al, Occurrence and fate of emerging contaminants in water environment: A review, Groundwater for Sustainable Development 2017, 6, 169-180 https://doi.org/10.1016/j.gsd.2017.12.009
[3] Ionela Cătălina Vasilachi et al, Occurrence and Fate of Emerging Pollutants in Water Environment and Options for Their Removal, Water 2021, 13, 2, 181 https://doi.org/10.3390/w13020181

[4] Monika Dubey et al, Occurrence, fate, and persistence of emerging micropollutants in sewage sludge treatment, Environmental Pollution 2021, 273, 116515, https://doi.org/10.1016/j.envpol.2021.116515
[5] Ana B. Rios Miguel et al, The role of mobile genetic elements in organic micropollutant degradation during biological wastewater treatment, Water Research X 2020, 9, 100065 https://doi.org/10.1016/j.wroa.2020.100065
[6] Ana B. Rios-Miguel et al, Predicting and improving the microbial removal of organic micropollutants during wastewater treatment: A review, Chemosphere 2023, 333, 138908 https://doi.org/10.1016/j.chemosphere.2023.138908

[7] Jemal Fito & Stijn W.H. Van Hulle, Wastewater reclamation and reuse potentials in agriculture: towards environmental sustainability, Environment, Development and Sustainability 2021, 23, 2949-2972 https://doi.org/10.1007/s10668-020-00732-y
[8] Teddy Kabeya Kasonga et al, Endocrine-disruptive chemicals as contaminants of emerging concern in wastewater and surface water: A review, Journal of Environmental Management 2021, 277, 111485 https://doi.org/10.1016/j.jenvman.2020.111485
[9] Nadeem A. Khan et al, Recent trends in disposal and treatment technologies of emerging-pollutants- A critical review, TrAC Trends in Analytical Chemistry 2020, 122, 115744 https://doi.org/10.1016/j.trac.2019.115744

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