Study on Conventional Drinking Water Treatment for Removing Emerging Contaminants: A Literature Review

Emerging contaminants (ECs) are substances that can be synthetic or natural, or even microorganisms that are usually not monitored in the environment and could be harmful to the environment and human health. These chemicals can include pharmaceuticals, such as ibuprofen and acetaminophen, and industrial chemicals, e.g., perfluorooctanoic acid (PFOA). Common conventional drinking water treatment plants (CDWTP) are not designed to remove emerging contaminants, so these compounds can enter the water system and affect the drinking water treatment process. This study aims to focus on the performance of CDWTP in removing ibuprofen, acetaminophen, and PFOA parameters, as well as determine suitable water treatment units to eliminate these parameters from the system. An extensive literature review was conducted and further analysed using descriptive and qualitative analysis to understand the unit performance in removing emerging contaminants, followed by the simple simulation to determine the types of advanced drinking water treatment facilities that perform better in eliminating ECs. The results show that CDWTP could reduce ibuprofen concentration in water with 40%, 20%, and 36% efficiency through coagulation-flocculation, sand filtration, and disinfection, respectively. Acetaminophen removal is up to 67%, 51%, and 66.45% during coagulation-flocculation, sand filtration, and disinfection, respectively. However, PFOA removal is only up to 5%, 7%, and 2% during coagulation-flocculation, sand filtration, and disinfection, respectively. Membrane treatment technology with reverse osmosis could remove ibuprofen, acetaminophen, and PFOA compounds more effectively with removal efficiencies of 99.99%, 96%, and 100%, respectively.


Introduction
In recent times, emerging contaminants have escalated into a significant challenge for both the environment and human well-being.These substances can stem from synthetic or natural sources, including under-monitored microorganisms in the environment, which can yield detrimental consequences for both ecological systems and human health [1].
One subset of emerging contaminants arises from domestic waste, particularly originating from pharmaceutical products.Among these, nonsteroidal anti-inflammatory drugs, such as ibuprofen and acetaminophen, have emerged as noteworthy concerns.Notably, ibuprofen stands as the third most consumed drug worldwide, making its way into water systems through pharmaceutical usage.The presence of ibuprofen in aquatic environments has surged due to heightened consumption driven by 1307 (2024) 012013 IOP Publishing doi:10.1088/1755-1315/1307/1/012013 2 increasing human population pressures [2].Similarly, acetaminophen, a widely consumed pharmaceutical, exhibits characteristics of polymorphism in its crystal structure, allowing easy dissolution in liquids and dispersion, thereby contributing to environmental contamination [3], [4].The infiltration of inflammatory drugs like ibuprofen and acetaminophen into the environment poses a range of toxic effects, extending from gastrointestinal disturbances to potential kidney impairments [5].
The existence of ibuprofen and acetaminophen compounds bears the potential for inducing persistent drug resistance, which can have far-reaching consequences.It has been observed that ibuprofen compounds, at concentrations greater than 10 µg/L, exhibit toxic effects on marine life, specifically zebrafish.Instances of developmental delays, decreased hatching rates, and heart anomalies have been noted in zebrafish embryos exposed to these compounds [6].
Beyond their presence in domestic waste, emerging contaminants also stem from industrial sources, such as perfluorooctanoic acid (PFOA) compounds.PFOA compounds, commonly employed in the production of non-sticky coatings for frying pans or Teflon, illustrate remarkable stability and solubility in both water and fat.This characteristic enables their widespread distribution in the environment.Prolonged human exposure to PFOA compounds has been linked to the onset of certain diseases [7].These compounds have been detected in human blood, and their prevalence is expanding.Moreover, they have infiltrated drinking water sources due to their high persistence, easy mobility in water, and capacity for bioaccumulation [8].
The presence of these emerging contaminants has broader implications, stretching to the quality of water sources and subsequently affecting the drinking water treatment processes in installations.This challenge is exacerbated by the fact that conventional drinking water treatment systems are not tailored to counteract these emerging contaminants, which underscores the need for innovative solutions [9].
Previous research indicates that standard treatment methods struggle to completely eliminate ibuprofen and acetaminophen compounds.This means that even after treatment, the water might still have these compounds in it.Instances in Brazil and Spain reported concentrations of 37.3 ng/L and 10.7-22 ng/L, respectively [5], [10].Similarly, treated water in Spain retained PFOA compounds at levels of 49-44 ng/L [11].When these emerging contaminants accumulate in the environment, they can lead to hormonal imbalances causing reproductive issues, lower fertility, increased chances of breast cancer, testosterone elevation, and persistent antibiotic resistance in humans [12].
Hence, it becomes imperative to investigate how well conventional drinking water treatment methods can remove these emerging contaminants.The goal is to pinpoint the most effective treatment unit for removing emerging contaminants like ibuprofen, acetaminophen, and PFOA in drinking water treatment plants based on existing literature.

Materials and Methods
The study method applied was to analyse various literature sources, such as national and international research journals, textbooks, dissertations, and final project reports relevant to the research topic, namely water pollution by ibuprofen, acetaminophen, and PFOA compounds and the performance of drinking water treatment units to remove these compounds.The analysis of the literature review was conducted using both descriptive qualitative and quantitative methods.Furthermore, the analysis is completed with a simple simulation based on the outcome of the literature review to determine the appropriate technology in removing ibuprofen, acetaminophen, and PFOA compounds to be applied at the DWTP.

Analysis of the Occurrence of Emerging Contaminants in Conventional Water Treatment Plant
From a public health perspective, drinking water treatment facilities can be a preventive measure to ensure that emerging contaminants like ibuprofen, acetaminophen, and PFOA do not re-enter the human body [13].By referring to Tables 1 to 3, graphical representations were generated to illustrate the correlation between processing flow rates and the concentration levels of ibuprofen and acetaminophen compounds in both the influent and effluent at DWTP.The concentration of ibuprofen in the influent of conventional DWTPs sourced from surface water in various countries has different concentration levels with variations in the wastewater flowrate of each conventional DWTP.Variations in flowrates do not affect the size of the concentration value of ibuprofen compounds.The difference in concentration values of ibuprofen compounds can be caused by differences in consumption patterns of ibuprofen and waste disposal of ibuprofen compounds by hospitals.The concentration of ibuprofen compounds can come from the disposal of human and animal waste that ends up in surface water, goods produced at home, and industrial plants located around the water bodies [1], [26].

Figure 1. Ibuprofen Compound Concentration Based on DWTP in Different Countries
Based on the study by, it is known that the population of northeastern and southeastern Spain tends to consume and produce more ibuprofen compound waste, resulting in high concentrations compared to the other 4 (four) countries.The highest influent concentration of ibuprofen compounds was in the Southeast Spain DWTP at 307 (257-357) ng/L.The concentration value of ibuprofen compounds in DWTP effluent is influenced by differences in drinking water treatment units used and can be influenced by the size of the concentration value of ibuprofen compounds in the influent water used.The variation of flow rates did not affect the concentration value of acetaminophen compounds.The difference in the concentration value of acetaminophen compounds can be caused by differences in consumption patterns of acetaminophen and waste disposal of acetaminophen compounds by hospitals [1], [26].Based on the study by, it is known that the population of Spain (Northeast) tends to consume more and produce more waste.The highest acetaminophen compounds are in the influent of conventional DWTPs in northeastern Spain when compared to conventional DWTPs in other countries.Where the concentration of acetaminophen compounds in the influent of Spain's DWTP (Northeast) amounted to 211.5 ng/L.The concentration of PFOA chemical compounds in conventional DWTP influent from surface water in various countries also has different concentration levels with different treatment discharge variations.The variation of treatment discharge does not affect the size of the concentration value of PFOA compounds.The difference in the concentration value of PFOA compounds can be caused by differences in the results of wastewater containing PFOA compounds produced from industries around the waters [1], [26].Based on the study by, it is known that Spain (Llobregat) produces wastewater containing PFOA compounds more than other countries, such as the United States, China (Zhujiang River), China (Beijiang River), Japan, Korea (Nakdong River), and Thailand.Where the concentration of acetaminophen compounds in the influent of the Spanish Water Treatment Plant (Llobregat) was 25.7 ng/L.The concentration of PFOA compounds in WWTP effluent is influenced by types of drinking water treatment units used and can be influenced by the size of the concentration of PFOA compounds in the influent water used.

Removal Efficiency of Emerging Contaminants in Conventional Drinking Water Treatment Units
The effective removal of ibuprofen, acetaminophen, and PFOA compounds from water sources can be achieved through the application of conventional drinking water treatment methods, including coagulation-flocculation units, sand filtration, and disinfection.The efficiency of the removal process for ibuprofen, acetaminophen, and PFOA compounds during water treatment is significantly impacted by the specific parameters and characteristics of the employed treatment units.This underscores the importance of considering the specifications of these units in assessing the efficiency of compound removal.A comprehensive overview of the removal efficiency for these compounds using conventional drinking water treatment units across diverse countries is available in Table 4 through Table 6 below.The removal efficiency of ibuprofen compounds through the application of conventional Drinking Water Treatment Plants (DWTP) featuring coagulation-flocculation units often exhibits a lower rate.This can be attributed to the inherent hydrophobic nature of ibuprofen compounds.Their hydrophobic characteristics stem from their non-polar attributes, which result in relatively weaker Van der Waals bond interactions.Despite possessing a carboxylic group (-COOH), the presence of this functional group in ibuprofen does not ensure comprehensive binding with the coagulant.Consequently, achieving effective removal of ibuprofen compounds requires the integration of supplementary processes.These additional procedures serve to significantly enhance the removal efficiency of ibuprofen compounds within the treatment process [28].
In terms of removal efficiency, acetaminophen compounds exhibit a superior performance when contrasted with ibuprofen and PFOA compounds.This heightened effectiveness can be attributed to the significantly greater polarity inherent in acetaminophen, as compared to both ibuprofen and PFOA.Acetaminophen's molecular structure features a range of diverse polar groups, including hydroxyl groups and amide groups, which greatly enhance its affinity for binding with coagulants utilized within the coagulation-flocculation process [29].
Among the various compounds, PFOA compounds demonstrate the least effective removal efficiency.This can be attributed to their hydrophobic nature, rendering them insoluble in water.The fluorocarbon carbon group within PFOA exhibits a limited tendency to interact with water, resulting in a reduced capacity for coagulants to effectively bind with these compounds.Consequently, this reduced binding affinity contributes to challenges in successfully removing PFOA compounds through the coagulation process, leading to suboptimal removal outcomes [30].
In the filtration unit, the elimination process for ibuprofen, acetaminophen, and PFOA compounds involves their attachment to coagulants before being directed to filtration.The filtration procedure involves the use of sand media primarily to target and remove larger particles.
The removal of ibuprofen, acetaminophen, and PFOA compounds through disinfection units involves the utilization of disinfectants such as chlorine (Cl2), ozone, or other similar agents.This disinfection procedure initiates a chemical breakdown of the structures present in ibuprofen, acetaminophen, and PFOA compounds.By inducing this structural breakdown, these compounds are transformed into simpler substances during the disinfection process.This transformation renders the compounds environmentally benign, mitigating any potential harm [24].
Based on the data in Table 1, it can be plotted into a box plot graph to determine the median value of the removal efficiency of each unit against ibuprofen compounds from conventional water treatment plants in various countries.This is necessary because there is varying data on the removal efficiency of the compound.Box plot graphs can provide a visual description of the distribution of data, including the median, quartiles, range, and the presence of outlier data.The box plot graph in Figure 4 shows that the centre value or average removal efficiency obtained from the median removal efficiency of ibuprofen compounds by coagulation-flocculation, filtration, and disinfection units is 40%, 20%, and 35%, respectively.Based on Table 2, it can be plotted into a box plot graph to determine the median value of the removal efficiency of each unit against acetaminophen compounds from conventional water treatment plants in various countries.The box plot graph in Figure 5 shows that the central value of removal efficiency obtained from the median value of removal efficiency against acetaminophen compounds by coagulation-flocculation, filtration, and disinfection units is 67%, 51%, and 66.45%, respectively.Based on Table 3, it can be plotted into a box plot graph to determine the median value of the removal efficiency of each unit against PFOA compounds from conventional water treatment plants in various countries.

Figure 6. Box Plot Analysis of PFOA Removal Efficiency in Conventional Drinking Water Treatment Units
The box plot graph in Figure 6 shows that the center value or average removal efficiency obtained from the median removal efficiency of PFOA compounds by the coagulation-flocculation, filtration, and disinfection units is 5%, 7%, and 2%, respectively.Given the limited effectiveness of conventional Drinking Water Treatment Plants (DWTP) in removing emerging contaminants, the implementation of advanced treatment methods becomes imperative.To address this challenge, three distinct treatment units have been introduced to DWTP settings, specifically designed for the removal of emerging contaminants compounds.

Assessment of Advanced Treatment Units for the Elimination of Emerging Contaminants in
Drinking Water Treatment Plants 3.3.1.Ultrafiltration.One of the advanced treatment methodologies is ultrafiltration.However, the efficiency of ultrafiltration in removing ibuprofen, acetaminophen, and PFOA compounds remains limited.For instance, at a drinking water treatment plant in Spain, the initial concentration of ibuprofen was measured at 155.7 ng/L (with a range of 81.4-230 ng/L).The employed ultrafiltration unit at this Spanish facility exhibited a modest 10% efficiency in removing ibuprofen compounds, resulting in an ibuprofen concentration of 140.4 ng/L (with a range of 78.8-202 ng/L) post-treatment [10].
Similarly, for acetaminophen compounds, the influent concentration at the Spanish Water Treatment Plant was 211.5 ng/L (with a range of 163-260 ng/L).The utilization of the ultrafiltration unit within the same plant led to a reduction in acetaminophen compound concentration to 188.5 ng/L (with a range of 147-230 ng/L), reflecting an 11% removal efficiency [10].
In addition, it is known that water treatment plants in Spain using ultrafiltration units are not effective in removing PFOA compounds [10].These data prove that the binding process of emerging contaminants is in fact much more difficult because the bonds that occur are not too strong, thus reducing the reactivity of the bonds which has implications for the efficiency value.The low removal efficiency of the ultrafiltration unit in removing ibuprofen, acetaminophen, and PFOA compounds makes this unit unsuitable for application in advanced drinking water treatment.

Reverse Osmosis.
The implementation of a reverse osmosis unit for the elimination of ibuprofen, acetaminophen, and PFOA compounds at the Spanish Drinking Water Treatment Plant has demonstrated an exceptional level of removal efficiency.This is evident from the exceptionally high removal efficiency values achieved during the removal process of these compounds using 11,000 Dow Filmtec LE440i membranes.
In the case of ibuprofen, the reverse osmosis treatment unit exhibited an outstanding removal efficiency of 99.99%.For acetaminophen compounds, the reverse osmosis treatment unit successfully reduced their concentration at the Spanish Water Treatment Plant to a range of 1.6-12.8ng/L (with a mean of 7.2 ng/L).This impressive result was attained through the collaborative action of an ultrafiltration unit, which managed to lower the concentration of acetaminophen compounds from an initial range of 163-260 ng/L (with an average of 211.5 ng/L) to a narrower range of 147-230 ng/L (with an average of 188.5 ng/L) [10].
The employment of a reverse osmosis unit yields an impressive 96% removal efficiency for acetaminophen compounds.Similarly, the Spanish Drinking Water Treatment Plant in Llobregat demonstrates remarkable efficiency in eliminating PFOA compounds.This is underscored by the remarkable 100% removal efficiency achieved for PFOA compounds, resulting in a concentration below 4.2 ng/L [11].The efficacy of reverse osmosis units in combatting emerging contaminants can be attributed to the minute size (<1 nm) of the membrane.This attribute enables the membrane to exclusively allow the passage of water, excluding all other impurities [31].The notably high removal efficiency showcased by the reverse osmosis unit in addressing ibuprofen, acetaminophen, and PFOA compounds establishes its suitability for integration within advanced drinking water treatment methodologies.

Electrodialysis.
Electrodialysis treatment is considered an effective technology with high energy efficiency for desalting brackish water, making it suitable for treating surface water for drinking purposes at water treatment facilities.The electrodialysis process utilizes direct current as a driving force to transfer ions from the source water through cation and anion exchange membranes into the concentrate waste stream, resulting in a more diluted product water [27].
Alongside advanced membrane-based treatments, electrochemical approaches involving electrodialysis units have also emerged.The application of electrodialysis units for the removal of emerging contaminants compounds has been employed in drinking water treatment plants, notably demonstrated in Spain's Llobregat region.Notably, the Spanish water treatment plant in Llobregat achieved a 53% removal of ibuprofen compounds through the utilization of electrodialysis units.
While literature review data primarily provides removal efficiency values for ibuprofen compounds, it is worth noting that acetaminophen compounds are typically subjected to prior unit processes before undergoing treatment within the electrodialysis unit [27].Conversely, the research conducted did not yield information about the removal of PFOA compounds using an electrodialysis unit, making it challenging to ascertain the efficiency level of PFOA compound removal through this method.
The relatively lower efficiency of the electrodialysis unit can be attributed to the limited ion interaction exhibited by emerging contaminants.Their inherent stability results in a lower electron affinity, rendering the electrodialysis membrane's functioning more challenging.Consequently, the electrodialysis unit demonstrates a lower removal efficiency in addressing ibuprofen, acetaminophen, and PFOA compounds.This drawback renders the unit unsuitable for incorporation into advanced drinking water treatment protocols.

Simplified Simulation of an Advanced Treatment Unit Capable of Removing Emerging Contaminants at DWTP
In this study, we conducted a simplified simulation to assess the elimination of ibuprofen, acetaminophen, and PFOA compounds.The simulation commenced with conventional treatment units, followed by advanced treatment involving a reverse osmosis membrane.
As an illustration, we demonstrate the calculation procedure for the advanced treatment unit's efficacy in removing ibuprofen compounds.For this purpose, we assumed the lowest removal efficiency values found in the research data for each respective unit: coagulation-flocculation (2%), sand filtration (8%), disinfection (1%), and reverse osmosis (100%).
In line with this, the influent concentration was set at the highest recorded ibuprofen compound concentration in the influent of a drinking water treatment plant, which is 307 ng/L as presented in Table 1.Consequently, employing these assumptions, we obtained an exemplified calculation of the effluent concentration for the removal of ibuprofen compounds.The concentration calculation outcomes for ibuprofen compounds, obtained from the simplified simulations, are outlined in Table 7.The results of a straightforward simulation involving a conventional drinking water treatment unit, supplemented by a reverse osmosis unit, revealed an effluent concentration of ibuprofen compounds amounting to 0.03 ng/L.Notably, this concentration remains well below the acceptable limit for drinking water, which stands at 2.8 mg/L [5].
Switching focus to acetaminophen compounds, the calculations concerning their concentration in the DWTP effluent were conducted through simulations encompassing both conventional and advanced treatment units.These simulations assume that the removal efficiency values for acetaminophen compounds align with the lowest values derived from research data for each unit: coagulation-flocculation (12.06%), sand filtration (0%), disinfection (59%), and reverse osmosis (96%).The influent concentration for acetaminophen compounds was extrapolated from the highest recorded concentration in the DWTP influent, as indicated in research data, standing at 211.5 ng/L, and detailed in Table 2.As observed in the outcomes of our straightforward simulations, the effluent concentration of acetaminophen compounds was determined to be 3.05 ng/L.It's noteworthy that this calculated effluent concentration continues to conform to the prescribed quality standard for acetaminophen compound concentration in drinking water, which is set at 0.2 mg/L [15].
Turning attention to the results of our simple simulation calculations, we delve into the removal efficiency of PFOA compounds employing both conventional and advanced treatment units.In this assessment, we adopt the assumption that the lowest removal efficiency values for each unit coagulationflocculation (5%), sand filtration (7%), disinfection (2%), and reverse osmosis (100%) align with research-derived data.
Moreover, the influent concentration for PFOA compounds is established based on the highest recorded concentration within the WWTP influent, as illustrated by the research data provided, standing at 25.7 ng/L and outlined in Table 3.The outcomes of the straightforward simulations reveal an effluent concentration of PFOA compounds amounting to 0 ng/L, comfortably falling within the quality standard prescribed by the United States Environmental Protection Agency (USEPA), which is set at 70 ng/L [8].These simulation results underscore that PFOA compounds can be effectively and completely removed, ensuring their safety for human consumption.

Figure 2 .
Figure 2. Acetaminophen Compound Concentration Based on DWTP in Different Countries

Figure 4 .
Figure 4. Box Plot Analysis of Ibuprofen Removal Efficiency in Conventional Drinking Water Treatment Units

Figure 5 .
Figure 5. Box Plot Analysis of Acetaminophen Removal Efficiency in Conventional Drinking WaterTreatment Units

Table 1 .
Ibuprofen Compound Concentration Based on DWTP in Different Countries

Table 2 .
Acetaminophen Compound Concentration Based on DWTP in Different Countries

Table 3 .
PFOA Compound Concentration Based on DWTP in Different Countries

Table 4 .
Removal Efficiency of Ibuprofen in Conventional Drinking Water Treatment Units

Table 5 .
Removal Efficiency of Acetaminophen in Conventional Drinking Water Treatment Units C Negative removal efficiencies can be caused by time differences or desorption of pollutants accumulated on the filter layer

Table 6 .
Removal Efficiency of PFOA in Conventional Drinking Water Treatment Units

Table 7 .
Simple Simulation of Advanced Treatment Unit that Can Remove Ibuprofen in Drinking Water Treatment Plant

Table 8 .
Simple Simulation of Advanced Treatment Unit that Can Remove Acetaminophen in Drinking Water Treatment Plant

Table 9 .
Simple Simulation of Advanced Treatment Unit that Can Remove PFOA in Drinking Water Treatment Plant