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.
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.
RESULTS AND DISCUSSION
Salinity levels in the central WWTP
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.