Open access peer-reviewed chapter
Abstract
Contamination of aquatic environments by pollutants of emerging concern (PEC) creates new public health and environmental threats. Over the years, Africa has struggled to adequately treat wastewater before discharged into the environment. The situation is expected to be worsened by the more challenging to treat PEC like pharmaceuticals, endocrine disrupters, microplastics, surfactants, cyanotoxins, radioactive and flame retardants. Generally, the pollution of aquatic environments will have serious negative impacts on organisms that depend on the affected sources. Elsewhere, a number of research studies have reported the occurrence of these pollutants and in some cases exceeding the recommended levels. However, in Africa and other developing countries, a few studies have focused on PEC in aquatic resources. In this review, discussions are centered on the: (i) occurrence of PEC in African aquatic environments, (ii) potential risks to aquatic ecosystems and human health, and (iii) current chemical and biological monitoring techniques. There is need to include PEC in the research studies and routine environmental monitoring programmes particularly before the urban wastewater is discharged into the environment. Passive biomonitoring through using biomarkers like oxidative stress proteins and gonadal histopathology may be more informative and cheaper way of monitoring PEC than chemical analysis.
Keywords
- pollutants of emerging contaminants
- urban wastewater
- aquatic ecosystems
- biomonitoring
1. Introduction
Aquatic ecosystems are core to life as aquatic organisms, terrestrial animals and human beings depend on these fragile environments for water. However, aquatic ecosystems have been negatively affected by a number of anthropogenic factors like damming, overexploitation, agriculture and discharge of poorly treated wastewater. Among these factors, the discharge of poorly treated wastewater from urban settlements is one of the key challenges that continue to degrade the aquatic ecosystems in Africa [1, 2]. The urban wastewater-induced degradation of aquatic ecosystems may be attributed to the slackening development of infrastructure meant for effluent management. To date, most of the water quality studies on aquatic ecosystems in developing countries have largely focused on physicochemical parameters such as heavy metals, biological oxygen demand, dissolved oxygen, suspended solids, chemical oxygen demand etc., whilst negligible attention has been given to pollutants of emerging concern (PEC).
Globally, PEC is increasingly becoming an ecological and public health threat as most of wastewater treatment plants were not designed to extract and treat these. PEC include both synthetic and naturally occurring compounds that are not normally monitored within the aquatic environments but have been recognized as having adverse ecological and health effects. These pollutants have the ability to affect humans and aquatic animals’ resident in affected ecosystems even at low doses. The realization of the presence of these pollutants has raised research interests to address source pathways, their fate, transformations and impact on life. A study by [3] grouped the PEC into 11 major types: personal care products, pharmaceuticals, industrial chemicals, polycyclic aromatic hydrocarbons, volatile organic compounds, pesticides, mycotoxins, cyanotoxins, radioactives, microplastics and particulate organic matter.
In Africa, urbanization has been linked to the pollution of downstream water bodies, for example, the downstream pollution of Lake Chivero [1, 4] and Khami Dams [2, 5, 6] in Zimbabwe. This has been attributed to the discharge of poorly treated urban wastewater [7, 8, 9]. Therefore, in developing countries, water resources in urbanising catchments are likely to be the hotspots of PEC. Africa has been swiftly urbanizing over the last decades with its urban population having been predicted to triple over 40 years, from 395 million people in 2010 to 1.339 billion in 2050 [10]. However, it is important to note that swift urban expansion in Africa is coupled with slackened sewage infrastructural development resulting in the discharge of poorly treated sewage into the environment. Furthermore, the current sewage treatment technologies that are in use in Africa do not factor in the treatment of PEC. Owing to the pressure linked to the discharge of poorly treated urban wastewater into the environment and a general lack of awareness, the likelihood of pollution by PEC is great. Therefore, there is a need for research and monitoring programmes to focus on PEC, particularly on the urbanizing catchments.
2. Pollutants of emerging concern in developing countries
2.1 Pharmaceuticals
Pharmaceuticals are largely synthetic organic compounds used in alleviating pain or as antimicrobials, antivirals and contraceptives. The use of pharmaceuticals by humans is followed by excretion of the residual traces of these drugs through urine or faecal matter into the environment. Therefore, with the increasing urban population coupled with the dilapidating wastewater treatment infrastructure, it is expected that large quantities of these pharmaceuticals are discharged into the environment. Elsewhere, the availability of data on contamination of water resources by pharmaceuticals has been increasing [11, 12, 13, 14]. In Africa, some studies on pharmaceuticals have been conducted in South Africa [15, 16, 17], Kenya [18, 19, 20] and Nigeria [3, 21, 22]. This review will focus on antibiotics and contraceptive drugs due to their wider use in municipal areas of developing countries.
In Zimbabwe, the contraceptive method mix is dominated by the pill with more than two-thirds of women using this hormonal method [23]. Estrone was detected in the range of 0.90 to 4.43 ng/L in raw water samples of the Vaal river [24]. The Vaal River drains most of the wastewater from the metropolitan City of Johannesburg and has been termed one of Africa’s workhorse. Presence of these contraceptives in aquatic ecosystems has been linked with diverse negative effects that include intersex organisms [25, 26], abnormal secondary sex characteristics [27, 28, 29], reduced fecundity [30] and changes in population sex ratios [31]. Most of these effects have dire consequences on the populations of the affected organisms. Wastewater discharged from Bulawayo; Zimbabwe has been reported to be the major source of contraceptive contamination in affected water bodies [6]. In this city, the highest oestrogenic effects were reported in Umguza Dam [6], one of the most polluted dams in Bulawayo [2]. A study by Teta and others [6] reported feminization of male fish in Umguza Dam, a broader threat to aquatic life. On the other hand, Bacterial Antibiotic Resistance (BAMR) is now a global concern that is reversing the progress that had been made in containing bacterial infections [11, 32, 33]. Misuse and overuse of antimicrobials is the main contributing factor to the evolution of antimicrobial resistance (AMR). Elsewhere, antimicrobial-resistant bacteria have been found in the environment with drinking water reported to be the main transmission route of these pathogens to human beings [34]. BAMR is common in areas with extensive use of antibiotics [35] and thus urban wastewater that is discharged into the environment is expected to be rich in pharmaceutical residuals including antibiotics [36, 37, 38]. Although antibiotics have been vital in improving human and animal health, these drugs find their way into the aquatic environment largely because of their frequent and unregulated use plus lack of capacity by most urban wastewater treatment plants to remove the antibiotics during wastewater treatment [35, 36]. The antibiotic residues that find their way into the environment exert selective pressure on bacteria leading to the evolution of antibiotic resistance [35, 39].
In Zimbabwe, strains which caused outbreaks of both cholera and typhoid were reported to be drug-resistant with patients showing poor response to commonly used drugs [40, 41, 42]. The heavily polluted Lake Chivero, the portable water source for the capital city of Zimbabwe, Harare, and underground water contamination has been cited as some of the possible sources of pathogens causing the recurring outbreaks of diarrheal diseases including cholera and typhoid in the capital. Both chromatography and spectroscopy methods have been used to identify known pharmaceuticals in water [43, 44]. However, the drug sector is developing very fast thus it is always a challenge to come up with a list of chemical compounds to be included for analysis. This makes chemical analysis process much of a daunting task. Furthermore, the pharmaceuticals exist in water below the detection limits of most analytical equipment but at the same time have detrimental effects on resident organisms. Moreover, chemical analyses do not reveal biological effects of pharmaceuticals. Consequently, biomonitoring approaches have been preferred over chemical analyses to assess the effects of pharmaceuticals on organisms. Biomonitoring involves the systematic measurement of the effects of pollutants by focusing on structural, physiological and genetic changes in living organisms as a response to the exposure.
2.2 Microplastics
Collectively African countries are estimated to be the second largest contributors of plastic waste to rivers. The plastic waste eventually ends up in oceans [45]. Plastic industry remains core to modern economies and human development. In the year 2016, about 335 million tonnes of plastics were produced [46] and this figure continues to increase in proportion to the increasing human population. The level of microplastic pollution is predicted to be higher in developing countries due to lack of proper waste management facilities which may cause large quantities of plastics to end up in the environment [47]. Plastics can be classified into six types: polyethylene (PE), polypropylene (PP), polyamide (PA), polyvinyl chloride (PVC), polystyrene (PS), polyurethane (PUR), and polyethylene terephthalate (PET) [48, 49, 50]. Plastics can also be grouped by their physical structure into five groups: fragments, micro-pellets, fibres, films and foam [51]. Micro pellets originate from different sources like washing powders and paints, fibres derived from synthetic textiles, foam cushioning material and fragments a result of breakdown of items such as plastic bottles and packaging materials [52].
Most plastics degrade into smaller particles over time through mechanical forces, thermo-degradation, photolysis, thermo-oxidation and biodegradation processes [53]. Therefore, microplastics (< 5mm) might arise through degradation or directly from a range of products including washing powder, rinse-off cosmetics and personal care [54, 55]. It is these smaller microplastics (MPs) that are observed to be dominating in the aquatic food webs [56] but with biological effects not well understood. Generally, in Africa, the extent of MPs within the inland waters remains largely unreported. Some of the few studies in South Africa [57], Botswana [58], Nigeria [59], and Kenya [60] have observed the microplastics in gut contents of fish. Analytical techniques that have been used to assess microplastics include the basic light compound microscope, Fourier Transform Infrared Spectroscopy (FTIR), Rahman Spectroscopy, and pyrolysis followed by GC/MS.
2.3 Cyanotoxins
Cyanobacteria also known as blue-green algae are part of aquatic algae that are known for producing biotoxins called cyanotoxins. Nutrient enrichment of aquatic systems largely by the discharge of poorly treated urban wastewater [1, 5], agricultural and industrial runoff has increased the proliferation of these harmful algae. The cyanotoxins that are normally produced after cell lysis following the collapse of the algal blooms have hepatotoxic, neurotoxic, carcinogenic and teratotoxic, cytotoxic and dermatotoxic effects [61, 62, 63]. The cyanobacteria genera that have been observed to be responsible for the formation of toxic blooms in aquatic systems include
Serious chronic human and animal health problems, and in some cases mortalities have been linked to cyanotoxin poisoning [65, 66]. According to [67], 1.0 μg/L (0.001 mg/L) is the recommended level for microcystin in drinking water whereas 2 000
The cyanotoxins have been detected in nutrient-rich systems, particularly those systems that receive poorly treated urban wastewater. The microcystin concentrations ranged between 3.67 to 86.08 mg/L in hypertrophic Hartbeespoort Dam, South Africa and between 0.1 to 49.41 mg/L in Kruger National Park [76]. In a heavily polluted Lake Chivero in Zimbabwe, microcystin concentrations ranged between 18.02 to 22.48 μg/L [75]. High levels of microcystins, ranging from 0.58 to 2.65 μg L−1, were detected in Lake Tana, Ethiopia [77]. The assessment of microcystin in a major drinking water source, Legedadi Reservoir, of Addis Ababa, Ethiopia recorded levels ranging between 61.63 and 453.89 μg L−1 [78]. The few studies done in Maputo and Gaza provinces indicated the occurrence of microcystin-LR, ranging from 6.83 to 7.78 μg•L−1 [79]. Therefore, levels of cyanotoxins in eutrophic aquatic resources in Africa, such as those that receive poorly treated wastewater, may be above the WHO guideline value of 1.0 μg L−1 and thus pointing towards high risks to public and environmental health. We suggest that quantitative cyanotoxin measurements be included in the water quality monitoring programs as guiding precautionary actions to mitigate the risks to public health and biodiversity.
2.4 Surfactants
Globally, the market size of surfactants is about US$42.1 billion, and it is expected to rise to US$52.4 billion by 2025 [80]. Surfactants are utilised in the production of detergents, textiles, paints, polymers, pharmaceuticals, pesticides, paper, personal care products and in mining for the extraction of minerals through flotation. Surfactants are amphipathic molecules possessing both hydrophilic heads and hydrophobic tails [81]. Depending on the type of charge on the hydrophilic head, surfactants can be categorized into five groups: anionic, cationic, non-ionic, semi-polar and amphoteric. Cationic surfactants possess a positive hydrophilic group, while anionic surfactants contain negatively charged hydrophilic functional groups [82]. The non-ionic surfactants (TAS) possess a non-ionized hydrophilic group(s), while the charge on the hydrophilic sites of amphoteric surfactants changes as a function of pH [83, 84]. The anionic, cationic, and amphoteric surfactants constitute 65% of surfactants on the global market size.
Owing to the extensive use of surfactants in urban areas, these molecules are discharged with wastewater and end up contaminating the receiving water bodies. Some of these water bodies like Lake Chivero and Darwendale in Zimbabwe also serve as portable water sources for the municipalities thereby raising public and environmental health concerns. Anionic surfactants like the linear alkylbenzene sulfonates (LAS) have been associated with various ecotoxicological effects on aquatic/terrestrial ecosystems [82, 85] and humans [83]. Surfactants are also known to reduce the resistance of aquatic biota to environmental stresses affecting reproduction and growth processes [86, 87]. Surfactants also increase the solubility of most contaminants thus increasing their toxicity in aquatic environments [88, 89]. The enormous adverse effects of surfactants on health and the environment necessitates the need to include them in environmental monitoring programmes.
3. Monitoring of PEC
3.1 Chemical analysis and biomonitoring techniques
Monitoring of PEC is critical for conservation, guiding remediation efforts as well as biodiversity and human life protection from the adverse effects of these. This is highly challenging for several reasons: PEC are very diverse and together with their transformed products and their variations will increase as nations push the innovation agenda aimed at coming up with alternatives. Therefore, the available specialised equipment by far does not cover the full range of PEC. Some of these methods are either still being developed or are yet to be adapted to our systems. Furthermore, PEC affects life at low concentrations and there is a need for equipment to have low detection limits to allow for proper risk assessment. Currently, Africa and other developing countries are lagging behind with respect to the required state-of-the-art equipment to competitively monitor the PEC in aquatic bodies. The reasons being that of poor awareness and consequently low budget prioritisations.
In contrast to the challenging and expensive chemical analysis methods, biomonitoring technologies provide a relatively cheaper and more integrated method for monitoring the PEC. Biomonitoring in aquatic ecosystems is the detection of substances or their effects on organisms, compared to analyzing chemical pollutants in water samples. Biomonitoring follows two major approaches: the bioindicator and biomarker methods. The biological indicator method uses the presence or absence of organism(s) to indicate the level of certain critical factors including pollutants [90]. A biomarker is a naturally occurring biological molecule, gene, or structural characteristic expressed as a response to the exposure of an organism(s) or cell(s) to a particular pathological or physiological process, disease or in this case pollutants. Biomarkers are detectable biochemical and tissue-level changes in response to exposure to pollutants. Between these two approaches, it is the biomarker approach that is increasingly being supported for use in monitoring PEC in aquatic systems [91, 92, 93]. Biomonitoring through the use of biomarkers can detect the exposure, the effect, or reveal susceptibility. Biomarkers are the best approach to identifying an early response to contaminants [94] and are much more sensitive to identifying organism stress than the community responses. They are normally the key pillar of bioindication method [95]. Using fish and aquatic snails, a number of studies have been conducted in Zimbabwe to assess the biomarkers that are expressed in polluted aquatic ecosystems. These studies have shown the expression of antioxidant enzymes [96, 97, 98], histological pathology of gills and liver [99], reduced fecundity and feminization [6] in polluted aquatic ecosystems. These studies conducted in Zimbabwe are providing evidence that biomarkers might provide an alternative to the chemical analysis methods to monitor PEC in aquatic systems of developing countries such as Zimbabwe.
4. Conclusions and outlook
This review shows that in Africa, research and monitoring programmes on PEC are still very limited. The fact that PEC is still missing in most water quality and aquatic ecosystems monitoring and research programmes is an indication that awareness of these is still limited and thus they are less prioritised in various initiatives. The few published studies are pointing out that water bodies receiving urban wastewater are likely to be the hotspots of PEC, but the effects are still poorly understood. Chemical analyses, using very expensive equipment that might not be easily accessible in developing countries, are still the main means of generating information on PEC. The few studies that have been conducted in developing countries are showing the presence of PEC and in some cases exceeding the recommended limits. It was highlighted that the other limitation of using the chemical analysis methods stems from the equipment available on the market having poor sensitivity resulting in failure to detect PEC at low concentrations. Therefore, biomonitoring using biomarkers is suggested as an alternative. Although still in its infancy, biomonitoring using biomarkers will provide a cheaper and more integrated way of monitoring PEC in aquatic systems. Therefore, there is a need for more studies aimed at assessing the feasibility of using biomarkers for monitoring PEC in aquatic ecosystems, particularly those that receive urban wastewater. This paper further recommends the need to: (i) improve awareness of the existence of PEC, (ii) assess the effectiveness of the current wastewater treatment plants in the removal of PEC, (iii) include PEC in the routine monitoring programmes by municipalities, (iv) governments and other funding agencies should fund research on PEC to address knowledge gaps.
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