The impact of low-flow season on source drinking water quality, Rosetta branch, Egypt

The water quality of the Rosetta branch is deteriorating continuously due to severe pollution (Abdel-Shafy & Raouf 2002; El Bouraie et al. 2011; El Saadi et al. 2014). Law 48/1982 for the protection of the River Nile and waterways from pollution has established a guideline value for ammonium (not to exceed 0.5 mg/L). This value has been extremely exceeded in the Rosetta branch (Ezzat et al. 2012; Ashry et al. 2013). The highest values of ammonium were observed during the low-flow period and this has affected the aquatic life (Wahaab & Badawy 2004; Dimian et al. 2014). In December 2013, fish were observed floating near drinking water intakes along the Rosetta branch. Pollution indicators had higher values during winter months than in summer months (Yousry et al. 2009).

Decreasing the discharge of fresh water into the Rosetta branch is a seasonal act undertaken by the Ministry of Water Resources and Irrigation (MWRI) during the season of minimum water requirements. It is usually done during the winter months to permit maintenance and construction of structures. The winter closure extends for about 40 days; the exact date and period changes from year to year according to the annual flood stage and the storage behind the Aswan High Dam. The release of water is mainly adjusted for the irrigation sector, while the drinking water sector receives secondary attention. The consequences of low-flow season include lowering the water level, reduction in the water amount, and changes in the water quality. The drinking water treatment plants (DWTPs) are very sensitive to such changes in terms of the source water quantity and quality. Thus, DWTPs face problems regarding the failure of treatment steps.

Ammonium is not of direct importance for health in the concentrations to be expected in the drinking water, thus a health-based guideline has therefore not been derived (WHO 1996). Nevertheless, the presence of elevated ammonium levels in the raw water complicates the conventional treatment steps (Janda & Rudovsky 1994; Hossain et al. 2007). The drinking water treatment steps commonly used in Egypt include pre-chlorination, coagulation, filtration, and disinfection. The presence of relatively high levels of ammonium in raw water increases the chlorine demand to reach ‘breakpoint’. Moreover, it decreases the disinfection efficiency as chlorine is consumed by ammonium, forming the chloramines and thus the appearance of taste and odor problems (Hasan et al. 2011). Consequently, the consumption of chemicals is increased and additional treatment steps are added.

The objectives of this paper are to document the problems raised from the 2014/2015 winter closure and low-flow in the Rosetta branch and to assess their impact on DWTPs and, moreover, to develop potential water management options that are able to deal with such conditions.

The materials for this study include both archival data and water samples at four selected sites out of 25 drinking water intakes existing along the Rosetta branch. Raw water intake points were selected based on the urgency of the problem (high contaminant load during severe low-level seasons) and the frequent interventions done on plant level (such as increasing the chlorine dose) as well as residents' complaints. During the course of the study, we compared between the DWTPs along both eastern and western banks in terms of the number of shutdown episodes and production rate. The four drinking water intakes (1, Foah; 2, Motobas; 3, Sanbada; 4, Edfina) are distributed along both banks; two from each bank as shown by the numbers 1, 2, 3, and 4 in Figure 1. The archival data mainly cover both water level changes in the Rosetta branch (MWRI internal reports) and the mean water quality parameters (Holding Company for Water and Wastewater (HCWW) internal reports) during the high-flow season (summer months). Moreover, interventions done by drinking water companies to deal with the presence of high ammonium in raw water were reviewed.

The water sampling program was set on a daily basis during the low-flow season (October, November, December, January, and February) of the years 2014/2015 at the four drinking water intakes. Nevertheless, there were missing data at all water intakes. The valid numbers of data set range from 322 for electric conductivity (EC) to 139 for phosphate (PO₄) (Table 1). Water sampling and analysis for raw water quality were done according to Standard Methods for the Examination of Water and Wastewater (APHA/AWWA/WEF 2005). EC was measured in the field using WTW 350i multi-parameter sensor. Ammonium (NH₄) was measured using ammonia selective electrode on an ion analyzer. Nitrite (NO₂) and phosphate (PO₄) were measured using ion chromatography. Total organic carbon (TOC) was measured using TOC analyzer. Statistical analyses and graphical presentations of results were done using a statistical program (StatSoft Inc. 1984–2004). Statistical processes included the basic description of central values and dispersion measures as well as linear correlation and principal component analyses (PCA). The presence of significant correlation in the data is necessary for running PCA (Brown 1999).

Summary statistics for the monitored parameters in all DWTPs are shown in Table 1. Looking at the data and statistical evaluation, it can be concluded that the data have a normal distribution. The EC values range from 454 to 1,062 μS/cm and the mean value is 744. Ammonium contents range from 0.38 to 18.5 mg/L and the mean value is 5.45. Nitrite (NO₂) values range from 0.09 to 5.2 mg/L as NO₂ and the mean value is 0.73. The phosphate (PO₄) contents range from 0 to 2.5 mg/L and the mean value is 1.85. The TOC values range from 3.3 to 9.5 mg/L and the mean value is 6.71. The DWTPs that are distributed along the eastern bank of the Rosetta branch are abstracting source water of the relatively higher pollution load than the DWTPs along the western bank (Table 1). This is mainly due to both differential water flow velocity and the contaminant load discharge along each side. Moreover, the eastern bank has shallow water depths, scattered islands and grasses as well as receiving the discharge effluent of an intensive industrial area (Kafr El-Zayiat). The maximum value of ammonium (18.5 mg/L) was recorded in Foah DWTP and its mean value is 12.39 mg/L. The opposite plant (Sanbada) has ammonium values ranging from 0.9 to 5.3 mg/L and the mean value is 3.56 mg/L. The same trend was recorded at the northern DWTPs (Motobas and Edfina), which have mean values of ammonium 5.44 and 3.07 mg/L, respectively. High levels of ammonium in the Rosetta branch have been recorded before (Ashry et al. 2013). The treatment efficiency has decreased, with frequent shutdown, during low-flow periods for DWTPs along both eastern and western banks but the problem was severe along the eastern bank.

It is proven that NH₄, EC, and PO₄ are linked together along the first factor (Table 3 and Figure 4). These parameters are both good indicators for the load of wastewater and probably the worst situation regarding oxygen availability. High ammonium often goes together with anaerobic conditions. The other factor shows that nitrite goes together with the first oxidation of wastewater (microbiological processes). In an ideal situation nitrate will be formed, but under limited oxygen conditions also nitrite. TOC is an indicator for contaminant load and has an effect on the nitrite content.

The potential management options that are able to deal with low-flow conditions and high contaminant load in the source drinking water can be distinguished into present, short-term, and long-term recommendations. The high values of the monitored parameters (NH₄, NO₂, PO₄, and TOC) have caused delay in coagulation, enhanced algae growth, and prevented the breakpoint being reached during the chlorination process. This has been observed before in similar areas (Hossain et al. 2007). Consequently, DWTPs have added more dosages of chlorine to reach the breakpoint. This act, in the presence of high organic load (TOC) will lead to the formation of trihalomethanes (THMs) (Reemtsma & Jekel 2006). The current interventions done by the local companies have included increasing the dose of both chlorine and activated carbon as well as decreasing the flow rate through treatment steps. Moreover, awareness campaigns were accompanied by transporting drinking water to the public from other clean sources. These interventions have increased the overall cost of treatment and decreased the produced drinking water quantities. Eventually, some DWTPs were switched off. This entails water management measures at national level as well as using unconventional techniques for drinking water production (Ghodeif et al. 2016). There is no threshold value for the acceptable raw water quality for the affected treatment steps in the conventional water treatment process. The most acceptable limit of raw water quality in Egypt is cited in law 48/1982, which is concerned with the protection of the River Nile and waterways from pollution (for example, ammonium (NH₄) limit is not to exceed 0.5 mg/L) (MWRI 1982). Surface water protection zones are crucial to keep sustainability of fresh water quality in Egypt (Ghodeif et al. 2013).

The proposed short-term actions include delaying the transport of sewage effluent during the low-flow winter season to coincide with the high-flow summer season. According to the MWRI, the maximum flow in the Rosetta branch during the summer (21 million m³/day) is about 100 times more than the minimum flow during the winter (0.2 million m³/day) (MWRI internal reports). Assuming the same contaminant load entering the branch through drains (total discharge 8.95 million m³/day) during both low-flow and high-flow seasons, releasing more water during high flood and summer seasons will leach, dilute, and improve water quality. Consequently, water management options are very important for water quality improvement in the Rosetta branch. This is especially crucial for El-Rahawy drain that significantly contributes a high load of contaminants into the Rosetta branch. Long-term actions include both treatment of wastewater, before discharge, and continuous monitoring of raw water quality. Moreover, it is proposed to set surface water protection zones along the Rosetta branch. These measures have to be supported by a proper and realistic regulation and laws, including the enforcement of law 48/1982 to protect drinking water sources. Moreover, it is necessary to embed the environmental flow concept within the MWRI strategy of water management.

The impact of low-flow periods on source water quality for DWTPs was investigated along both banks of the Rosetta branch and found to be severe. Gradual increase in contaminant loads (NH₃, NO₂, EC, PO₄, and TOC) was recorded as winter closure starts and low-flow dominates. Microbiological processes and wastewater, as the main source of contaminants, may explain the variations of water quality parameters in the Rosetta branch. The drinking water treatment efficiency is decreased with frequent shutdown for DWTPs during low-flow periods along both eastern and western banks. The problem was severe along the eastern bank. The ammonium measurement is very suitable in the present situation as an indicator for the suitability of the source water for drinking supply (<0.5 mg/L). The maximum flow in the Rosetta branch during the summer is about 100 times more than the minimum flow during the winter. Thus, the MWRI philosophy regarding both the winter closure and low-flow approach needs further revision. It should give priority to protecting source drinking water quality according to the Egyptian legislation standards.

The authors acknowledge the support of the Holding Company for Water and Wastewater and its affiliated companies in Lower Egypt (El-Behira and Kafr El-Sheikh companies). The authors thank the reviewers for helpful comments to improve the paper.