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    Topic review

    Contaminants of Emerging Concern

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    Contributors: DRISS DHIBA , Souad El Hajjaji
    Submitted by: DRISS DHIBA
    (This entry belongs to Entry Collection "Wastewater Treatment ")


    Worldwide, the pollution of water bodies by contaminants of emerging concern (CECs) such as pharmaceuticals, endocrine disrupting compounds, flame retardants including brominated flame retardants (BFRs) and perfluorochemicals (PFCs), micro plastics, nanomaterials, and algal toxins, to name just a few, is creating a new set of challenges to the conventional wastewater treatment facilities, which demonstrate inefficiency in removing/degrading many CECs. As a consequence, environmentalists started to detect the presence of some of those contaminants at alarming levels in certain countries, with possible negative effects on aquatic species and often increased potential for human health risks through the exposure to the contaminated waters, or the reuse of treated wastewater in agriculture and household use. Such issues are more accentuated in the African continent due to various socio-economic problems giving rise to poor sanitation systems and serious shortages in wastewater treatment plants in many regions, making it difficult to tackle the problem of conventional pollutants, let alone to deal with the more challenging CECs. Thus, in order to effectively deal with this emerging environmental threat, African researchers are working to develop and optimize sound sampling and analytical procedures, risk assessment models, and efficient remediation technologies.

    1. Introduction

    Contaminants of emerging concern (CECs) are a cluster of chemical compounds newly identified by scientists as harmful substances to the environment and possibly to humans (through an inherent toxicity or a recalcitrant and persistent nature), but so far with no established regulatory guidelines for the majority of them [1][2].

    In the last decades, CECs such as human and animal pharmaceuticals, endocrine disrupting compounds (EDCs), personal care products (PPCPs), microplastics, nanomaterials, and so on, are increasingly and globally causing environmental and public health concern. Consequently, numerous scientific studies have started, and actively continue, to be conducted all over the world, with proven reports on the pollution of aquatic ecosystems by CECs, and most notably pharmaceutical drugs and their derivates [3][4][5].

    After assessing the occurrence of CECs in water bodies (lakes, rivers, etc.), most researchers traced back the origins of such pollution and proved that conventional and outdated wastewater treatment plants (WWTPs) were one of the main sources of pollution with CECs [6][7]. Indeed, many studies reported the inefficacy of the activated sludge-based WWTPs to treat such recalcitrant trace chemicals based on the monitored occurrence of selected CECs in WWTPs effluents and nearby water bodies [8][9]. Other studies also proved that even more advanced wastewater treatment technologies such the membrane bioreactor process were not able to remove all the CECs present in urban wastewaters [10][11].

    Then, research studies begun to be carried out to analyze the fate and ecotoxicology of many CECs in wastewater and aquatic environments [12][13]. In this regard, endocrine disrupting compounds (EDCs) are among the most investigated CECs. Indeed, their presence in the environment, raw sewage, treated sewage effluents, receiving rivers, and surface waters [14] constitute a serious menace due to the possible interference with normal function of the endocrine systems of wildlife and humans, directly through exposure or indirectly via the food chain [15].

    Many CECs are being investigated including human and veterinary pharmaceuticals, disinfection by-products (DBPs), personal care products (PCPs), new pesticides, flame retardants, microplastics, algal toxins, and engineered nanomaterials. In this context, it has to be noticed that the majority of the related research studies are concentrated in North American, European and selected Asian countries, and that this topic is still absent in the majority of African countries and relatively new in a few of them, with most research studies still in the early stages of assessing the occurrence and, to a lesser extent, the fate of CECs in WWTPs and receiving water bodies [16].

    After analyzing the threat of water pollution by CECs (i.e., occurrence, fate and impact), researchers in the water treatment field started to experiment with processes and technologies to tackle this issue. In this context, however, the high reactivity and mobility, and the very low concentrations, of most CECs posed a serious challenge to this R&D effort, especially when coupled with the large volumes of the to-be-treated wastewater streams and water bodies.

    2. Risk Assessment and Impacts of Waterborne CECs on the Environment and Human Health

    For many decades, CECs have been entering aquatic environments regularly through discharged WWTP effluents, industrial wastewaters, agricultural runoff, and municipal landfill leachates. Consequently, aquatic species (fauna and flora) are being continuously exposed to these contaminants of emerging concern, especially organisms that can bio-concentrate and bio-accumulate significant amounts of these CECs to concentrations several times higher than in the water body they live in [17][18].

    Globally, several assessment studies conducted in Europe, North America and China emphasized the adverse impacts of a wide range of CECs on the environment, especially aquatic ecosystems. EDCs and many pharmaceuticals, which are environmentally persistent and bioaccumulating compounds, have great potential environment risks when reaching drinking water supplies or through the food chain [19][20][21].

    For instance, it was revealed that the exposure to diclofenac, a NSAID drug, led to the near-extinction of vultures in the Indian subcontinent. The cause was that the birds were feeding on the carcasses of cattle treated with this drug [22]. In aquatic ecosystems, diclofenac, which is widely detected in African wastewater effluents [23][24], is suspected to cause damage to the inner organs in many fish species [25][26] and sharp declines in the populations of certain fish species due to the feminization of male fish [27]. The same adverse phenomenon, along with reduced fecundity, was also reported for fish species exposed to metformin, an anti-diabetes drug [28]. Such proven adverse eco-toxicological effects on local fish populations provide strong reasons for African researchers to examine such CECs-related issues, especially in African regions with poor sanitation systems or in densely populated areas.

    Regarding the assessment of CECs’ impacts on the aquatic environment in Africa, interesting studies, although few in number compared to the ones conducted in other continents, presented valuable results. In South Africa, a research team from the University of the Western Cape, Cape Town, investigated the impact of this issue by attempting to accurately analyze the concentrations of fifteen CECs (pharmaceuticals, personal care product, perfluoroalkyl and endocrine disrupting compounds) in different fish samples [29]. The contaminants were selected according to their high annual consumption, their stability and poor elimination during wastewater treatment, along with concerns about adverse effects on human and aquatic organisms. The results revealed that perfluorodecanoic acid (20.1–179.2 ng/g), perfluorononanoic acid (21.2–114.0 ng/g), and perfluoroheptanoic acid (40.1–138.3 ng/g) were the most predominant among the perfluorinated compounds. Moreover, diclofenac was reported to have the highest concentration in the investigated edible fish species out of all the pharmaceuticals detected (551.8–1812 ng/g). As for the risk assessment, the authors reported values above 0.5 and 1.0, respectively, for acute and chronic risk, thus highlighting high health risk to the pelagic fish, aquatic organisms and to humans via the food chain.

    In Egypt, a nationwide survey of several CECs in water sources, including bisphenol A (BPA), methylparaben, ethylparaben, propylparaben, butylparaben, and o-phenylphenol, was conducted, along with an assessment of the impacts of the measured concentrations on the aquatic organisms and human health [30]. The results showed that, on the one hand, BPA, methylparaben, propylparaben, and butylparaben were frequently detected in the investigated source and drinking waters. To illustrate the alarming level of the registered data, the authors reported that the highest concentrations of BPA and methylparaben in source water, and those of BPA and methylparaben, propylparaben, and butylparaben in drinking water, were the highest in the world. Regarding the environmental risk assessment, it was showed that among the studied EDCs, BPA presented the highest risk to the aquatic organisms, while human health risks of the exposure to BPA via the consumption of drinking water were at safe levels.

    In Nigeria, a research study published in 2020 investigated the distribution and bioaccumulation issues EDCs in Lagos Lagoon as the final sink accumulating and transporting numerous chemicals such as octylphenol (OP), nonylphenol (NP) and BPA [31]. In this study, water and sediment samples were collected at nine locations on the lagoon, along with five different species of fishes. It was observed that BPA was not detected in all the collected water samples. However, the concentrations of NP and OP reached up to 102 ng/L and 127 ng/L, respectively. In the sediment samples, the EDCs were substantially detected at concentrations reaching up to 5.1 μg/g for BPA, 1.9 μg/g for NP, and 2.5 μg/g for OP. In the fish samples, the highest concentrations were reported at around 1139 ng/g for BPA, 476 ng/g for NP, and 643 ng/g for OP. Such results confirm the bioaccumulation character of the studied EDCs in the Lagos lagoon biota, which can have detrimental effects on other organisms across the food chain.

    Globally, many researchers paid special attention to emerging risks associated with natural and synthetic steroid hormones related to their endocrine-disrupting activities observed in receiving surface waters [32][33]. In a related joint South African and Canadian study, the estrogenicity and anti-estrogenicity of EDCs in WWTPs’ influents and effluents were assessed using recombinant yeast estrogen receptor binding assays [34]. Thus, seasonal and daily variations were monitored at ten South African WWTPs, as well as the river streams located upstream and downstream of the WWTWs. This comparative analysis was carried out with the objective of assessing potential adverse health risks associated with the measured estrogenic concentrations.

    Furthermore, the influence of compounds that may antagonize estrogen-mediated receptor binding (anti-estrogens) in the bioassay was also investigated to further elucidate potential endocrine disrupting risks that WWTP effluents might pose on receiving surface waters. The main results revealed that despite the fact that WWTPs managed to substantially reduce this EDCs menace (Cf. Figure 1), the abatement efficiencies tended to vary from one season to another and from a set of locations to another.

    Figure 1. Mass load (g/day) of estrogen equivalent concentrations measured using the yeast estrogen screen for the various WWTPs during sampling campaigns in summer 2015/16 and winter 2016 [34]. Reproduction license secured from Elsevier.

    The reported reasons for such variations were higher weekday/weekend variations for WWTPs receiving wastewater from domestic sources and the amount of estrogenic EDC loads being directly influenced by the ongoing connection of more households to the sewage systems. In terms of season, the variations are linked to the differences in the influx of wastewater at the WWTPs, variable rainfall, and large runoff surges. The study also showed that concentrations in some treated effluent and river water samples were above effect-based trigger values, thus posing an endocrine-disrupting risk.

    3. Removal Technologies Targeting CECs

    The efficiency of contaminants of emerging concern removal depends on the treatment methods and the chemical characteristics of each contaminant, as well as the characteristics of the wastewater treatment plants. Several pharmaceuticals are not removed from wastewater during the treatment processes. This inefficiency depends on the wastewater treatment plant’s design, and can also be associated with the desorption phenomenon of CECs from the particulate matter during the treatment process. Other processes may take place during treatment, such as biotransformation/biodegradation, and abiotic elimination by adsorption to the sludge.

    For instance, for the case of pharmaceuticals, it was reported that several factors including physico-chemical properties of targeted compounds, and operating conditions of the processes in the WWTPs, did influence the removal efficiency of pharmaceuticals in wastewater [35].

    In addition to the application of adsorbents for removal of pharmaceutical contaminants in African waters [36], different technologies for water purification have been explored for various CECs including the use of membrane technology [37], and photo-electrocatalytic water treatment systems [38].

    Thus, as shown in Table 1, for some methods, there is a wide variability of removal efficiency for different compounds across various WWTPs in Africa.

    Table 1. Selected water treatment technologies targeting emerging pollutants in Africa.

    Methods Average Removal Efficiency of CECs Limitations CECs Nature References
    Physic-chemical treatment: Adsorption technique
    Activated carbon treatment techniques:
    Adsorption by entrapped activated carbon in alginate
    <79% -Not available Pharmaceuticals [36]
    Biosorption in a baffled duckweed pond system <90.6% -Not available Analgesic (acetaminophen), antibiotic (amoxicillin, ampicillin) and anti-inflammatory (diclofenac) [39]
    Adsorption via porous Sugarcane Bagasse activated carbon (SCB-AC) <92.4% (after 5 cycles) -Not available Anti-inflammatory (diclofenac sodium) [40]
    Biological treatment
    Biological treatment:
    microbial communities in the WSPs and biofilms.
    40–100% within 20–116 h -Not available Anti-inflammatory (paracetamol) [37]
    Biodegradation under aerobic conditions <100% -Not available Female sex hormone (17-β-Estradiol) [41]
    Activated sludge process 30–95% Negative removals being reported for some anti-inflammatory example diclofenac Analgesic/anti-inflammatory drugs and steroids [37]
    Advanced oxidation process (AOPs)
    Photo-electrocatalytic oxidation at photoanode (FTO (*)/BiVO4/BiOI) under visible 68% and 62% -Not available Analgesic (acetaminophen) and antibiotic (ciprofloxacin) [38]
    Treatment via photolysis
    Direct photolysis 48% and 18% -Not available Removal of anti-infective (sulfamethoxazole) and antibiotic (trimethoprim) in the effluent [37]
    Indirect photolysis 16–36% and 20–62% -Not available Removal of sulfamethoxazole and trimethoprim, in the effluent [37]
    Physical treatment
    Filtration and ultrafiltration treatment techniques: ceramic fine ultrafiltration membrane At least 70% Are not commonly accessible by poor populations in developing countries Anti-inflammatory (diclofenac), calming (diazepam), antibiotic (erythromycin), and antibacterial and antifungal (triclosan) [37]
    Coupled treatment
    Fenton-biological <100% -Not available Antibiotic (chloramphenicol) and anti-inflammatory (paracetamol, and diclofenac sodium and associated by-products) [42]

    (*) Fluorine-doped tin oxide (FTO) coated glass.

    Results depicted in Table 1 show the wide variability of removal efficiencies for different compounds across various WWTPs. A reduction of over 95% was recorded in all WWTPs, which shows that both of the biological processes were efficient in the removal. With respect to wastewater treatment, CECs are partially removed by wastewater stabilization ponds (WSPs) and trickling filter technologies to a similar extent as compared with other techniques (e.g., conventional activated sludge and constructed wetlands), but long hydraulic residence times and thus large surface areas are required. In the African context, numerous adsorbents have been investigated. Various adsorption mechanisms have been reported in different studies for the uptake of pharmaceuticals from water using various adsorbents. In these views, the mode of adsorption was observed to be exclusive for a specific adsorbent directed to a particular pharmaceutical or group of analytes.

    The entry is from 10.3390/su13031125


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