High salinity in sewage sludge can affect not only the operation of wastewater treatment plants (WWTPs) but also the quality of treated water generated, thereby limiting its downstream reuse. Using data on geochemical parameters, both for the central WWTP in Abu Dhabi, UAE, and literature values for potential regional saline water sources (e.g., shallow groundwater and regional Arabian Gulf seawater), a variety of chemical fingerprinting diagnostic ratios were calculated and plotted in order to determine the source of salinity in the municipal sewage. Data were compared with data from a regional WWTP that was not impacted by salinity. Monitoring data demonstrated persistently elevated levels of salinity in the municipal wastewater arriving at the central WWTP from the city. Dilution/concentration analysis using a conductivity vs. chloride plot showed both potential sources, i.e. Arabian Gulf seawater and coastal hypersaline groundwater, as feasible sources of wastewater salinization. Further diagnostic analysis using a Panno Plot indicated that coastal groundwater was the only likely source of salinization of municipal sewage. Additional confirmation of the identity of the source and the extent of mixing using different lines of evidence like stable isotope ratios is recommended for future study.

Salinity impacts in the environment can occur from a variety of sources, including road salting for deicing in colder climates, landfill leachate discharge, and basin brines during oil and gas exploration and production (Richter & Kreitler 1993). In the coastal environment, salinity can also be introduced via seawater intrusion from over-pumping groundwater, and through atmospheric deposition of marine aerosols (Kelly et al. 2010). Besides damaging groundwater resources, salinity can impact urban built infrastructure such as building foundations (Ababneh et al. 2003; Hossain et al. 2009) and subterranean utility lines (Kaushal 2016).

Corrosion from inside sewage lines is a widely studied problem that occurs as a result of biodeterioration via mechanisms such as acid-producing activity of sulfate-reducing bacteria (Mori et al. 1991), sulfate-oxidizing bacteria (Davis et al. 1998), and salt precipitation by specific fungi (Gu et al. 1998). The less frequently reported phenomenon of corrosion and deterioration of sewage lines from outside the pipeline has been gaining more attention and is believed to occur due to salinity in the marine environment (Hyman 2005) or contamination in groundwater (Oualit et al. 2012). Sewage lines, which are laid generally within 3 m from ground surface to the invert of the sewage pipe, and below potable water lines, can often be submerged below the water table in coastal locations or in areas where the water table elevation fluctuates. In coastal regions of the Abu Dhabi Emirate, which extend more than 5 km inland, shallow groundwater occurs within 5 m of the ground surface elevation, making the possibility of sewage lines coming into contact with saline water quite high. Sewage lines might also be impacted by leaching of water through evaporitic soils, like the gypsiferous and halite-rich salts common to the coastal regions of the Abu Dhabi Emirate (Abdelfattah et al. 2009; Environment Agency – Abu Dhabi (EAD) 2009a).

High salinity in municipal sewage can deleteriously affect the operation of not only conventional wastewater treatment technologies like activated sludge (Wu et al. 2008), but also more modern ones like membrane bioreactors (MBRs), where high salinity induces increased production of extracellular polymeric substances, resulting in a rapid loss in membrane permeability (Reid et al. 2006). Long-term wastewater treatment studies in places like Hong Kong, where seawater is used for public toilet flushing in coastal areas, have shown the formation of brominated trihalomethane (THM) (Xue et al. 2008) and halo-acetic acid (HAA) (Sun et al. 2009) disinfection byproducts (DBPs) in treated wastewater (TWW), post-disinfection. Brominated DBPs tend to be far more carcinogenic than chlorinated ones. This phenomenon of brominated THM formation during chlorine disinfection has previously been demonstrated in Abu Dhabi (Aina & Ahmad 2013), in samples taken from the same wastewater treatment plant (WWTP) as this study.

Salinity impacts on environmental resources, such as surface and groundwater resources, have been studied extensively in the past using geochemical parameters (Richter & Kreitler 1993; Panno et al. 2005). The toolbox generally utilized to assess such impacts consists of a variety of chemical fingerprinting diagnostic ratios that rely on the measurement of inorganic ions and elemental stable isotopes. Similar approaches have been applied to track sewage through its chloride content (Kelly et al. 2010). In this study, we attempt to utilize a variety of geochemical inorganic ion ratios for identifying potential sources of water salinization impacting municipal sewage arriving at the central WWTP in the city of Abu Dhabi, UAE.

Wastewater and TWW sample collection and analysis

TWW samples were collected immediately after the chlorine contact units from two WWTPs in Abu Dhabi on a quarterly basis, over the course of a year. Al Mafraq or Mafraq WWTP plant is the centralized activated sludge treatment (AST) plant with a design capacity of 320,000 m3/day, receiving municipal wastewater from the city of Abu Dhabi, UAE. The other WWTP was the Masdar sequential MBR plant that receives municipal wastewater from the suburb of Masdar City and has a design capacity of 1,500 m3/day. Samples were analyzed for total dissolved solids (TDS) using a specific conductance correlation, and anions, such as chloride and bromide, were analyzed on a Dionex ICS3000 reagent-free ion chromatography system. In addition to these data, specific conductance and chloride ion monitoring data collected over a period of 7 months (frequency of approximately once every other day) were acquired for the influent municipal wastewater from the Mafraq AST management. These data were logged using a conductivity probe and a membrane selective electrode for specific conductance measurement and chloride ion concentration, respectively.

Hydrogeochemical data

Several literature sources were utilized to acquire data on the region's shallow groundwater resources. Non-isotopic data consisting of TDS, conductivity, and anion concentrations were acquired for costal groundwater (Masdar Corporation 2010), the Liwa area (Al-Katheeri et al. 2009), Sweihan region (Al-Alawi & Ali 2014), Al Ain city (Mohamed & Hassane 2016) and its adjacent Jabal Hafeet region (Murad et al. 2012a), and the border Al Hayer (Gadalla & Hosny 2015) and Al Wagan (Ali Khalifa 2014) regions with Oman. Summary statistics were performed on each data-source-specific parameter. The locations of these sites are depicted on a UAE map presented in Supplementary Information (SI) Figure S1 (available with the online version of this paper).

Isotopic data for δ 2H and δ 18O for SI Figure S3 were acquired from an Abu Dhabi-wide study (Imes & Wood 2007) spanning the shallow alluvial fan aquifer from Jabal Hafeet to the coastal area near Abu Dhabi city. In addition to this study, other sources of literature were utilized for the Jabal Hafeet region (Murad et al. 2012b), Sweihan region (Al-Alawi & Ali 2014), and the Liwa region (Al-Katheeri et al. 2009).

Seawater and desalination data

Seawater data for the Arabian/Persian Gulf were obtained from literature sources (e.g., Adham et al. 2013) as well as the DesalData database (www.desaldata.com) as feedwater for different coastal seawater desalination plants located in the Arabian Gulf region. For desalinated data, process-specific literature sources (Ludwig 2010) were used, focusing mainly on thermal desalination processes like multi-stage flash (MSF) and multiple effect distillation (MED), which constitute much of the desalination capacity in the region.

Salinity levels in the central WWTP

The city's oldest centralized municipal WWTP, Al Mafraq, operational since 1982, is located approximately 40 km inland from downtown Abu Dhabi and has an approximate design capacity of 320,000 m3/d (Kumaraswamy et al. 2014; Statistics Centre Abu Dhabi (SCAD) 2015). The central WWTP employs conventional AST and shares the municipal sewage flow from the city, which is conveyed by a sewerage system in place since the early 1980s. For this study, the wastewater arriving at Al Mafraq was monitored daily for chloride content and electrical conductivity (i.e. specific conductance in μS·cm−1) as surrogates for salinity for a period of 7 months (Figure 1). Both of these salinity surrogates demonstrate good correlation with each other (correlation coefficient = 0.929) and persistent elevated levels (electrical conductivity = 5,085.4 ± 682.8 μS/cm, chloride concentration = 1,443.8 ± 207.4 mg/L). Hence, one can conclude that the salinity problem in wastewater is a persistent one and occurs upstream of the centralized WWTP, possibly in the sewage conveyance infrastructure.
Figure 1

Electrical conductivity (specific conductance in μS·cm−1) and chloride concentrations in influent wastewater arriving at Al Mafraq WWTP from the city of Abu Dhabi over a 7 month period. Consistently elevated conductivity and chloride levels are indicative of a persistent salinity intrusion problem into the municipal sewage system.

Figure 1

Electrical conductivity (specific conductance in μS·cm−1) and chloride concentrations in influent wastewater arriving at Al Mafraq WWTP from the city of Abu Dhabi over a 7 month period. Consistently elevated conductivity and chloride levels are indicative of a persistent salinity intrusion problem into the municipal sewage system.

Close modal
With desalinated water and groundwater making up 79 percent and 12 percent, respectively, of the water demand of the city, it stands to reason that municipal wastewater should not carry elevated levels of salinity provided there are no sources of salinity introduced during the use of that water. In fact, when comparing salinity in the Mafraq treated effluent (TDS = 2,766.2 ± 305.4 mg/L), which retains much of its incoming salinity, with that of desalinated water, a two orders-of-magnitude difference can be observed in the TDS salinity content (Figure 2). A comparison with treated effluent from a small decentralized WWTP that is unimpacted by salinity, such as the MBR WWTP in the neighborhood of Masdar City (capacity of 1,500 m3/d), shows the centralized WWTP effluent to be one order of magnitude higher (Figure 2). Interestingly, the Mafraq treated effluent falls within the range of salinities observed in shallow groundwater located farther inland of the coastal regions, specifically within the range of Al Hayer and Hafeet waters at the border with Oman (Figure 2 and Figure S1). Conversely, the Arabian Gulf seawater and the coastal shallow groundwater show salinity levels that are one and two orders of magnitude higher, respectively, than the Mafraq effluent, pointing to these types of waters as potential sources of the high salinity in the city's wastewater.
Figure 2

Total dissolved solids (TDS) values for waters in the Emirate of Abu Dhabi. Figure conveys the relative order-of-magnitude salinity levels in different shallow groundwaters in Abu Dhabi (for locations see Figure S1 in the SI, available with the online version of this paper) and compares them with the salinity treated wastewater (TWW) coming out of Al Mafraq WWTP. Dashed lines display reference salinity levels for desalinated water, TWW from the Masdar City MBR WWTP, which is unimpacted by salinity, and Arabian Gulf seawater.

Figure 2

Total dissolved solids (TDS) values for waters in the Emirate of Abu Dhabi. Figure conveys the relative order-of-magnitude salinity levels in different shallow groundwaters in Abu Dhabi (for locations see Figure S1 in the SI, available with the online version of this paper) and compares them with the salinity treated wastewater (TWW) coming out of Al Mafraq WWTP. Dashed lines display reference salinity levels for desalinated water, TWW from the Masdar City MBR WWTP, which is unimpacted by salinity, and Arabian Gulf seawater.

Close modal

Salinity fingerprinting

An electrical conductivity versus chloride plot (Figure 3) is typically utilized to predict mixing effects, such as dilution and concentration, resulting from the mixing of different types of water. The first piece of information that Figure 3 yields is that there is no fundamental difference in this relationship between the influent wastewater to Mafraq and the TWW generated by that plant, indicating limited changes to these parameters through the wastewater treatment process. Also, the linear regression between the two types of TWW samples, unimpacted from Masdar MBR and salinity-impacted from Mafraq, when extrapolated, extends to samples from both of the two potential salinity sources, namely, Arabian Gulf seawater and the coastal groundwater. This phenomenon suggests a common marine origin of all types of waters on this plot, including TWW, which has its origins in desalinated marine water. Hence, Figure 3 does not help isolate a single salinization source of the TWW out of the two potential sources, due to the similar characteristics of these sources on this plot.
Figure 3

Electrical conductivity (specific conductance in μS·cm−1) as a function of chloride concentration (mg·L−1) for the 2 types of TWW (salinity impacted from Mafraq and unimpacted from Masdar WWTPs), impacted wastewater, and two potential sources of salinity, Arabian Gulf seawater and coastal groundwater. The graph is commonly used to convey dilution and concentration effects from mixing of different types of waters. The line indicates an extrapolated least squares fit of the TWW data. Note that the number in parentheses in the key indicates the number of samples in each dataset.

Figure 3

Electrical conductivity (specific conductance in μS·cm−1) as a function of chloride concentration (mg·L−1) for the 2 types of TWW (salinity impacted from Mafraq and unimpacted from Masdar WWTPs), impacted wastewater, and two potential sources of salinity, Arabian Gulf seawater and coastal groundwater. The graph is commonly used to convey dilution and concentration effects from mixing of different types of waters. The line indicates an extrapolated least squares fit of the TWW data. Note that the number in parentheses in the key indicates the number of samples in each dataset.

Close modal
Figure 4 presents the chloride-to-bromide ratio versus the chloride concentration, plotted in a ‘Panno plot’. The different diagnostic areas within this plot were delineated based on an extensive database from the Illinois Geological Survey and other data sources in the United States (Panno et al. 2005; Kelly et al. 2010). The first piece of information evident from the plot is that the unimpacted TWW from the Masdar MBR WWTP is quite different in its chloride-to-bromide ratio from the TWW zone of this plot. A potential factor responsible for the difference may be the predominant use of thermally desalinated water in Abu Dhabi city as potable water. Thermally desalinated water is produced by an evaporation-condensation process, and, therefore, should have similar characteristics on this plot to precipitation. However, in practice, a small fraction of seawater is blended into the thermally desalinated water in order to carbonize and add hardness to the water (Withers 2005). As for the other types of waters, e.g., seawater and coastal groundwater, which shares characteristics with basin brines, fall into their designated areas. The Mafraq TWW falls at the border between seawater and basin brine areas based on its Cl/Br ratio, indicating the influence of a salinization source with a Cl/Br ratio stronger than regional seawater.
Figure 4

‘Panno’ plot displaying the chloride-to-bromide mass ratio versus the chloride concentration (mg·L−1). This diagnostic plot for aqueous salinity was developed using an extensive dataset (Panno et al. 2005; Kelly et al. 2010). Note that the number in parentheses in the key indicates the number of samples.

Figure 4

‘Panno’ plot displaying the chloride-to-bromide mass ratio versus the chloride concentration (mg·L−1). This diagnostic plot for aqueous salinity was developed using an extensive dataset (Panno et al. 2005; Kelly et al. 2010). Note that the number in parentheses in the key indicates the number of samples.

Close modal

This study relied on available published monitoring data, from both scientific literature and regional project reports, together with inorganic diagnostic ratios, to deduce that wastewater transported to the centralized WWTP in Abu Dhabi city is being impacted by hypersaline coastal groundwater. The high salinity in the wastewater has already been shown to have unintended consequences, such as the production of novel carcinogenic DBPs (Aina & Ahmad 2013), which might have health and environmental impact downstream with TWW reuse. Additional monitoring, such as fluoride as a potential tracer for water supply in the region and isotopic ratios (e.g., deuterium vs. 18O, see Figure S3), can help further confirm the findings of this study and determine the volumetric mixing ratios between the TWW and coastal groundwater salinization. However, the problem of salinity in TWW might be short-lived; the City of Abu Dhabi initiated construction in 2008 of the Strategic Tunnel Enhancement Program (STEP) (Environment Agency – Abu Dhabi (EAD) 2009b), which will have a 41 km-long corrosion-protected gravity sewer tunnel that extends all the way from the island to a 30 m3/s pumping lift station at Al Wathba WWTP. The STEP tunnel is scheduled for commissioning in 2017 in phases to replace the old sewerage system. Once fully operational, the STEP tunnel is likely to reduce salinity impact problems in the sewerage system; however, it might lead to re-optimization of WWTP operations because of the prior acclimation of microorganisms in the activated sludge process to the elevated salinity levels.

Monitoring data demonstrated persistently elevated levels of salinity in the municipal wastewater arriving at the centralized WWTPs from the city. These salinity levels were one and two orders of magnitude higher than unimpacted TWW from a small decentralized WWTP and desalinated water that makes up most of the freshwater supply to the city, respectively. Dilution/concentration analysis using a conductivity vs. chloride plot showed both potential sources, i.e. Arabian Gulf seawater and coastal hypersaline groundwater, as feasible sources of wastewater salinization. Further diagnostic analysis using a Panno Plot indicated that coastal groundwater was the only likely source of salinization. Further confirmation on the identity of the source and the extent of mixing using different lines of evidence is recommended for future study.

This research was funded under research grants 13XAAA2 from the Masdar Institute of Science and Technology.

Additional information as noted in the text is available with the online version of this paper.

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Supplementary data