Polycyclic aromatic hydrocarbons (PAHs) in Greater Cairo water supply systems

Population growth is accompanied by the consumption of resources and the degradation of the environment through the activities associated with urbanization. Water is life, and no one ignores its contribution to agriculture and industry (Buettner 2020). Each type of water use requires a certain quality. Contaminated drinking water will affect public health (WHO 2021). The production of drinking water is challenging, especially in the face of increasing pollution as a result of the discharge of polycyclic aromatic hydrocarbons (PAHs).

PAHs are a class of xenobiotic compounds composed of carbon and hydrogen. They belong to a class of pollutants that have high melting and boiling temperatures, low vapor pressure, and low water solubility. PAHs are various organic compounds with two or more fused aromatic rings that are released into the atmosphere through the combustion of fossil fuels. The combination and proportions of each PAH are a footprint of their sources. PAHs are slowly biodegradable under aerobic conditions and in the environment, making them persistent organic pollutants (POPs). The most detected PAHs in drinking water are fluoranthene, pyrene, benz[a]anthracene, benzo[b]fluoranthene, benzo[j]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, dibenz[a,h]anthracene, benzo[ghi]perylene, and indeno[1,2,3-cd] pyrene (WHO 2021). The origin of PAH is diverse, with two main categories: natural and anthropogenic activities. Recently, significant concentrations of PAHs have been detected in the aquatic environment (Mojiri et al. 2019). Natural sources include volcanic eruptions, forest fires, decaying organic matter, plant synthesis, rare minerals, and natural petroleum. Anthropogenic sources are classified into three categories: (1) domestic sources (burning of oil, gas, garbage, wood, and cigarettes), (2) industrial sources (sewage sludge and organic wastes from various industries), and (3) agricultural sources (pesticides and burns).

PAHs are found in the air, sediments, water sources, sewage, fish, birds, and crustaceans, indicating their penetration into the environment in its various dimensions. They have also been found in street dust, rainwater, and urban runoff. PAHs can enter water bodies through a variety of routes, including dry and wet deposition, road runoff, industrial effluent, creosote-impregnated wood leaching, petroleum spills, and fossil fuel combustion (Karyab et al. 2013). The solubility of PAHs in water is also a determining factor for their presence in water or sediments. However, PAHs are not hydrophilic compounds in nature but are partially soluble in water. The higher the molecular weight of the PAH compound, the less soluble it is in water and vice versa (Liu et al. 2021).

The production of potable water is a strategic goal for policymakers. Surface freshwater is a common way for safe water supply which is the best-known technique used for this purpose in Egypt (Hussein et al. 2021). However, freshwater faces many challenges including pollution sources. Non-point sources are sometimes called diffuse pollution such as runoff from agricultural fields where substances like fertilizers, pesticides, and soil conditioners are swept into surface water resulting in contamination. On the other hand, point sources of pollution include factories wastewater and localized pollution spots, which could influence the increase in several parameters such as ammonia, nitrite, nitrates, aluminum, total organic carbon (TOC), pesticides, PAHs, and phthalate esters (Bodzek & Dudziak 2006).

PAHs in urban areas originate from petrogenic and pyrogenic sources. Petrogenic substances include crude oil and its products such as gasoline, fuel oil, asphalt, and coal. Pyrogenic substances are complex hydrocarbons formed from organic matter exposed to high temperatures where there is insufficient oxygen for complete combustion. Fires and furnaces produce pyrogenic PAHs. They are also formed when coke or gas is produced from coal or oil. Coal tar-based products such as roofing, road sealants, caulking, pesticides, and some shampoos contain pyrogenic PAHs (Abdel-Shafy & Mansour 2016). Natural incomplete combustion, on the other hand, can release them into the environment.

Organic pollution of surface waters in Egypt is caused by point and non-point sources. There are many causes for freshwater pollution in Egypt including but not limited to (1) untreated domestic sewage, (2) industrial and irrigation wastewater, (3) increased reuse cycles, and (4) decreased freshwater flows especially in winter (Abdel-Satar et al. 2017).

As POPs, significant concentrations of various PAHs have been found in Egypt in air (Hassanien & Abdel-Latif 2008), water (Nasr et al. 2010), and soil (Barakat et al. 2011a). PAHs have increased in Egypt and have transferred to living organisms, leading to their accumulation and, in some cases, toxicity (Abdel-Shafy & Mansour 2016). PAH cancer risk assessment for humans revealed significant levels, which motivated further research to evaluate PAH concentrations and treatment pathways (Tongo et al. 2017). They are teratogenic, carcinogenic, and mutagenic, in general, and can cause lung, bladder, and skin cancer. Furthermore, high amounts of PAHs have been proven to have immunosuppressive effects as well as the ability to cause oxidative stress during their metabolism (Ali et al. 2021).

The previous studies on PAHs in Egypt focused on the concentration in the sediments of the Egyptian Mediterranean coast, including the northern lakes (El Nemr & Abd-Allah 2003; Mostafa et al. 2003; Nasr et al. 2010; Barakat et al. 2011a, 2011b, 2013), PAH concentrations in air and road dust (Hassanien & Abdel-Latif 2008; Hassan & Khoder 2012; Haiba & Hassan 2018). However, few studies have been conducted on the presence and treatment of PAHs in surface freshwater (Badawy & Embaby 2010; Nassar et al. 2015; Haiba 2019). Few works have been conducted to investigate the presence of PAHs in the surface and drinking water of Greater Cairo. Therefore, this study aims to investigate the occurrence and treatment efficiency of 17 PAH compounds in the water supply system of Greater Cairo, which are priority pollutants according to the U.S. EPA list (U.S. EPA 1979). The study also investigates the correlation between PAHs and their sources in Greater Cairo.

Seventeen PAHs were investigated in the canals of Nile, Ismailia and Sharkawia, the main water source in Greater Cairo for drinking water supply. Eight water treatment plants (WTPs) were selected to represent different water supply systems in Greater Cairo (Figure 1). These WTPs provide drinking water to more than 18.9% of the Egyptian population according to the Egyptian statistics in June 2020 (CAPMAS 2020).

Seventeen PAHs were analyzed using the U.S. EPA method SW-846 (U.S. EPA 1997), such as naphthalene, 2-chloronaphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo (a) anthracene, chrysene, benzo (b) fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene, dibenze(ah)anthracene, and benzo(ghi)perylene, and standards were purchased from Merck (Germany).

All other chemicals and solvents used were of pesticide residue grade or equivalent from Merck (Germany). One hundred and ninety-two samples were collected in the period from January to December 2018, including 96 samples of raw water from the intake and 96 samples of treated water after pumping from reservoirs. Raw and treated water samples were collected according to Baird et al. (2017). Samples were transferred to an icebox; unless extraction started within 4–6 h, samples were preserved with 50 ml of CH₂Cl₂ and stored in a dark place for a maximum of 7 days. An appropriate volume of the water samples (2 l) was acidified to a pH value of 2 with 1.0 M sulfuric acid and extracted twice with redistilled CH₂Cl₂. The combined extracts were evaporated, dried with anhydrous Na₂SO₄, and then concentrated to 1 ml using a stream of pure nitrogen. Analysis was performed according to the U.S. EPA method SW-846 (U.S. EPA 1997).

PAH identification was performed using a Varian 3400 GC coupled to a Finnigan Mat SSQ 700 (manufactured by Agilent, USA) and equipped with a capillary column (30 m×0.25 mm and a 0.25 μm thick silica gel column SE54). Injections were performed in a split-less injector mode with a delay time of 3 min and were maintained at 280 °C. The GC oven was held at 80 °C for 2 min and then programmed to rise to 280 °C at a rate of 8 °C min⁻¹ for 30 min. Helium was used as the carrier gas at a flow rate of 1 ml min⁻¹. The transfer line was marinated at 300 °C, and the mass spectrometer was scanned from 40 to 500 a.m.u. every second. Calibration standards with at least three concentration levels were prepared (Badawy & Embaby 2010).

Blank samples were run with each batch of samples in the laboratory, during travel, and in the field. PAH concentrations in all blank samples were detected in all analyses. The quantification of PAHs was performed using external standards whose coefficient for the calibration curves was greater than 0.925. The calibration program was verified at each patch by measuring mixture standards. One sample was run in triplicate for every 20 samples. Each standard and sample was measured in duplicate, and the sample was reanalyzed if the relative standard deviation of the two measurements exceeded 5%. A control chart was prepared, and the action was taken as necessary if the action and warning limits were exceeded at ±2 and 3 standard deviations (SDs), respectively. Extraction efficiency was determined by adding a known concentration of 4,4′-difluorobiphenyl as a surrogate standard. Recovery rates for water samples ranged from 81 to 98%. In addition, all 20 samples were spiked with the external standard to ensure recovery and detection of the target PAHs. In the present study, all laboratory instruments used in sample collection, analysis, and storage were soaked in 10% HNO₃ for 2 days and then thoroughly rinsed with distilled and deionized water before use (Baird et al. 2017; Lesser et al. 2018). The study included statistical analyses such as minimum, maximum, average, standard deviation, and correlation matrix (Pearson).

Only naphthalene, anthracene, phenanthrene, fluorene, and pyrene were detected in the study. Naphthalene and phenanthrene appeared in all raw and treated waters tested. Due to heavy traffic and aircraft in the study area, they are emitted into the air and then precipitate in surface water (Mojiri et al. 2019). However, fluorene was found in only 50% of the sites, such as Embaba, Rod El-Farag, Shamal Helwan, and Shoubra El-Kheima, due to the activities at these sites, especially in the industrial areas of Shoubra El-Kheima, and Helwan (Shaltout et al. 2014, 2018). Pyrene was detected in significant concentrations in all samples except in the wastewater treatment plants of El-Fostat, El-Rawda, and Embaba. The conditions in the study area and the environment confirm the previous results, as the sampling sites are surrounded by different PAH-emitting activities. Numerous articles confirm our work in this area (Jahin et al. 2009; Badawy & Embaby 2010; Moawad et al. 2017).

As shown in Table 1, the detected concentrations of naphthalene and phenanthrene were confirmed in all the raw waters tested. Fluorene was present in only 50% of the sites studied, with the highest value recorded in Rod El-Farag (117 ± 130 ng/l) and the lowest in Shoubra El-Kheima (67 ± 87 ng/l). Anthracene was also found in 62.5% of the samples studied, with the maximum value in El-Tebeen (496 ± 124 ng/l) and the minimum in El-Fostat (220 ± 126 ng/l). Pyrene was detected in 75% of the studied sites, with the highest value in Shoubra El-Kheima (369 ± 128 ng/l) and the lowest in Mostorod (58 ± 77 ng/l). The sum of all PAHs detected was highest in Shamal Helwan (1,325 ± 631 ng/l) and lowest in Mostorod (468 ± 329 ng/l). Although Shamal Helwan is located upstream of Cairo, it is still surrounded by many industrial activities and combustion of petroleum products that produce large amounts of PAHs that dissolve and precipitate in surface water (Moawad et al. 2017). Finally, the absence of certain PAHs in certain locations is due to natural removal and reduction by evaporation through solar heat, ultraviolet radiation, and biological activity (Sanches et al. 2011; Ukiwe et al. 2013).

As can be seen from Figure 2, the concentration of total PAHs is highest in winter, which is due to the Egyptian water management plan applied. This is because there is little freshwater available in winter and more recycled untreated wastewater is added to meet the national demand. On the other hand, total PAH concentrations decreased in summer at all locations because a lot of freshwater flows into the Nile Basin during this season. This dilutes and minimizes the effects of polluted untreated wastewater mixed with it (Hussein et al. 2021). Shamal Helwan has the highest total PAH content because it is in the middle of the Helwan industrial area, which emits significant amounts of PAHs into the air and then precipitates into water bodies (Mojiri et al. 2019). Concentrations were affected significantly with freshwater flow (Hussein et al. 2021).

The detected concentration range in the raw water of the eight sites studied along the Nile was higher than that previously recorded in Egypt, except for those at the end of the Rosetta Arm in Alexandria. The accumulation effect of these POPs may be the reason for this (Table 2), which could be due to the industrial, combustion, and petroleum activities in the study area (Moawad et al. 2017). The levels of PAH in the rivers of China have higher concentrations than those in this study, indicating that many emissions of PAH and pollution may be common in China and Egypt.

The difference in detected PAHs in the Rosetta branch of Jahin et al. (2009) and Haiba (2019) is due to different time periods and increased emission and spill activities, industrialization, and urbanization.

PAHs enter water sources through dry and wet deposition, road runoff, leaching from creosote-impregnated wood, industrial effluents, petroleum spills, and fossil fuel combustion (Mojiri et al. 2019). Petroleum-derived PAHs contain excessive amounts of low-molecular-weight PAHs (LPAHs) containing less than four aromatic rings, such as naphthalene, phenanthrene, acenaphthylene, and fluorene (Helfrich & Armstrong 1986; Badawy et al. 1993). Combustion-derived PAHs from pyrolytic synthesis at elevated temperatures with limited oxygen tend to contain high-molecular-weight PAHs (HPAHs) such as fluoranthene, pyrene, and benzo (a)pyrene.

In the present study, the concentration of naphthalene in all the raw waters tested ranged from 164 ± 136 to 219 ± 119 ng/l for Rod El-Farag and Mostorod, respectively. Similarly, concentrations of phenanthrene were found between 163 ± 141 and 315 ± 132 ng/l in Shoubra El-Kheima and Shamal Helwan, respectively. These results indicate that the recent pollution of surface water in the studied areas originated from a petrogenic source. The LPAHs such as naphthalene and phenanthrene are more hydrophilic than HPAHs. This supports their spread and persistence from place to place without precipitation, which explains their presence in all raw waters with significant concentrations. Moreover, aeration, adsorption, absorption, and precipitation are critical factors for the persistence of PAHs in the aquatic environment (Rojo-Nieto et al. 2013; Mojiri et al. 2019).

(Anthracene/anthracene + phenanthrene) and (LPAHs/HPAHs) were used to clarify the PAH composition and determine the possible sources (Pies et al. 2008; Zhang et al. 2008). The (anthracene/anthracene + phenanthrene) ratio was >0.1 at all sites except Embaba, Mostorod, and Rod El-Farag (Table 3). On the other hand, the ratio (LPAHs/HPAHs) was >1 in all studied sites, except El-Fostat, El-Rawda, and Embaba. These results and ratios confirm that the main sources of the detected PAHs are the combustion of petroleum materials, such as traffic emissions, electric power plants, generators, and the combustion of fuels by industrial activities.

The ratio between LPAHs and HPAHs in raw water (Table 3) confirmed the previous observation and reported that PAHs originate from the combustion of petroleum products and other sources such as runoff, atmospheric deposition, wastewater, and industrial effluents (Gigliotti et al. 2005; Xu et al. 2007; Hassanien & Abdel-Latif 2008). Petrochemical sources, such as fuel, oil, or lightly refined petroleum products, were dominated by LPAHs and had LPAHs/HPAHs >1 (Soclo et al. 2000; Durand et al. 2004; Rocher et al. 2004; Zhang et al. 2008). This indicates that surface waters have taken up anthropogenic PAHs from various sources. However, certain sources are known to be responsible for the presence of PAHs in surface waters, including spills, precipitation, exhaust, smoke, fires, and industrial activities (Huntley et al. 1993; Wang et al. 2006).

An attempt to introduce the correlation matrix between the identified PAHs in each WTP and the WTPs studied is presented in Table 4 (in the Supplementary Material). From this table, all the detected PAHs in the same WTP were closely related to each other over the year and showed high correlation compared to all the WTPs in the study area, except fluorene, which has a low relationship with anthracene. The strong correlation between the detected PAHs indicates that the PAHs have similar sources in all areas (petrogenic sources).

Figure 3 shows that the naphthalene removal efficiency was 21% minimum in Embaba and 26% maximum in Mostorod. Moreover, phenanthrene removal was 26% minimum in El-Fostat and 30% maximum in El-Rawda. Fluorene was detected only in four raw waters and showed the best removal efficiency in Shoubra El-Kheima (29%) and the worst in Rod El-Farag (25%). Anthracene was also detected in only four sites and showed high removal in Shamal Helwan (38%) and the least removal in El-Tebeen (31%). Pyrene was least removed in Shamal Helwan (30%) and most removed in Shoubra El-Kheima (38%). The total PAH removal ranged from 25 to 31% in Embaba and Shamal Helwan, with an average value of 29% for the eight WTPs studied. The higher value of removal efficiency in Shamal Helwan was due to optimized operating conditions, including residence time, disinfectant and coagulant dosing, and filtration rate.

In Egypt, the conventional treatment includes coagulation, precipitation, filtration, and disinfection. LPAHs such as naphthalene are minimally removed by the conventional treatment, but HPAHs are better removed as shown in Figure 3(d). On the other hand, pyrene achieved the best removal at 35%. Naphthalene, on the other hand, was the least removed with 24%, which is confirmed by Badawy & Embaby (2010). The removal of PAHs by the conventional treatment depends on numerous parameters including retention time, adsorption on flocs and efficient alum, and chlorine dosing, which was optimized in Shamal Helwan WTP but not in Embaba WTP. It can be concluded that the conventional treatment is not remarkably effective against PAHs in freshwater (Rubio-Clemente et al. 2014).

The study included eight WTPs representing three different designs: round, pulsator, and return clarifiers. The pulsator clarifiers perform better than the other two clarifiers in PAH removal under optimal operating conditions. In a pulsator clarifier, raw water is passed through a pre-formed and regenerated sludge blanket (Kawakami et al. 2016), which acts as a filtration and absorption medium to increase PAH removal efficiency. The return sludge clarifier had the lowest PAH removal efficiency. Due to the clarification mechanism, the slurry circulates, which may increase the accumulation of PAHs (Kawakami et al. 2016). This is confirmed by the data in Figure 3(a)–3(c), which shows that the same treatment designs have similar removal efficiencies compared to each other.

Drinking water produced by the eight WTPs studied was analyzed through 2018. The average PAH concentrations were plotted in Figure 4 to allow comparison between the studied areas. The map was drawn, assuming that the treated water in the WTPs is of the same quality in the supplied localities. Shamal Helwan and Shoubra El-Kheima communities were exposed to the highest PAH concentrations due to industrial activities surrounding the surface water and lower PAH removal efficiency (Moawad et al. 2017). On the other hand, Mostorod and Embaba had lower PAH concentrations due to green areas that reduced the transport of PAHs and reduced industrial and PAH-emitting activities (Moawad et al. 2017). However, the detected PAH levels in the treated water were lower than the Egyptian standards for drinking water (EWQS 2007) and the levels determined by WHO (2021).

Seventeen priority PAHs were analyzed in raw and treated water from eight WTPs in the greater Cairo area. The source of the detected PAHs was petrogenic activities. All identified PAH species correlate with each other, indicating the similarity of their origin. Pulsator clarifiers showed significant PAH removal compared to circular and recirculated slurry clarifiers due to the treatment mechanisms in each plant. The communities in Shamal Helwan and Shoubra El-Kheima areas may be exposed to drinking water with elevated PAH levels, while the levels in Embaba and Mostorod were least. However, PAH concentrations in treated water are still below the Egyptian standards and the WHO guidelines. Since the efficiency of PAH removal was low in the studied WTPs, it is recommended that PAH emissions to the environment should be eliminated or reduced by source control. Otherwise, further research efforts should be made to reduce PAH concentrations in water.

Recommendations for future studies: the health risk assessment and long-term impact of PAHs in drinking water on communities in the study area is urgently needed due to the increasing PAH-emitting activities and the accumulating property of PAHs as POPs.

The authors would like to thank the reference laboratories of water and wastewater and the general department of quality and environmental affairs in the holding company of water and wastewater, Egypt, for their grateful support and efforts so that our research could reach its final form. Also, we appreciate the help and support received through peer reviewers and journal editor.

All relevant data are included in the paper or its Supplementary Information.