Study of Total Dissolved Solids (TDS) Concentrations Factor of SWCC Al-Khobar Plant Seawater Intakes ()
1. Introduction
Water is an essential and fundamental resource that is indispensable for the existence of all forms of life. The quality of water plays a critical role in safeguarding human health and preserving the environment. However, with the growing global demand for water, the availability of fresh water has become increasingly scarce in many regions. In order to overcome the water scarcity in areas with the less natural fresh water availability, desalination becomes a reliable and effective tool for providing a sufficient fresh water source [1] . The quality of the water used for desalination is an important factor in determining the process efficiency and product quality [2] . Water with high levels of impurities, such as suspended and dissolved solids, and organic matter can lead to fouling and scaling issues within the desalination system. These fouling and scaling phenomena can reduce the process efficiency, increase energy consumption, and potentially damage equipment, resulting in higher operational costs and decreased product quality [3] .
In particular, the concentration of total dissolved solids (TDS) in seawater plays a vital role in desalination plants, particularly those utilizing Reverse Osmosis (RO) technology [4] . This advanced process involves forcing feedwater through a semi-permeable membrane to eliminate TDS. The performance of the desalination plant is significantly impacted by the TDS concentration in the feedwater [5] . Higher TDS levels pose challenges in removing salts and other substances, requiring more energy to pressurize the water flux through the membranes [6] . Therefore, it is crucial to comprehensively understand, continuously monitor, and effectively control the TDS levels of the feed water source to ensure the efficiency and effectiveness of water production.
In daily operation, the TDS level is typically reported using the electric conductivity (EC) ratio method [7] . The EC measure is the ability of water to conduct electrical charges, which comes from dissolved ions such as salts. It serves as an indicator of ion concentration in the solution. The more ions present in a water sample, the higher the EC results [8] . It is expressed in units of Micro Siemens per meter (µS/m). Total dissolved solids (TDS) combines all dissolved solids in the water, including the majority of salt ions and organic matter. TDS can be measured accurately using the gravimetric method [9] [10] , which involves evaporating a measured volume of water and weighing the residue to determine the TDS level. However, this method is time-consuming and impractical for daily field operating and monitoring. As a result, the TDS level is typically calculated using an empirical factor derived from the relationship between the EC and TDS as mentioned in Rusydi review on 2018.
To further emphasizes the necessity of this research. It is significantly importance to focus on total dissolved solids (TDS) in the Arabian Gulf seawater. For addressing the concern of the gradual increase in TDS levels in Arabia Gulf over time, which has not been overlooked by previous studies. The reliance on the electrical conductivity (EC) factor for TDS measurement may not accurately capture the changes in seawater properties affecting TDS. Particularly for the operation of Reverse Osmosis (RO) plants, understanding and monitoring TDS levels accurately becomes critical.
In Khobar desalination production systems, there are three seawater intake areas: phase-2 (AK2/RO2), phase-3 (AK3), and phase-4 (RO1). The seawater TDS samples value observed to experience a gradually increase. The important of the change has become more essential and highlighted with the start of phase-4 (RO1) plant operation, which is the first RO plant in Al-Khobar plants. Knowing that seawater TDS can be change with time due to climatic and environmental aquatic conditions [11] . Thus, a recent studying of the TDS level in seawater is important for optimizing the design and operation of RO plants. By understanding the TDS level in seawater, it is possible to determine the required appropriate to reduce the TDS level and protect the membranes from fouling and scaling. Which reflects on optimize the operation, reduce maintenance costs, and ensure the production quality.
This paper present recent measurements of total dissolved solids (TDS) utilizing both the gravimetric method and electrical conductivity (EC) in seawater intakes for the Khobar desalination production system intakes; phase-2 (AK2), phase-3 (AK3), and phase-4 (RO1). The collected data is plotted to analyze and observe any trend changes across different locations. Additionally, the paper includes the calculation of the TDS factor, which is used to evaluate and verify the accuracy of the measurement method. By employing both techniques and assessing the TDS factor, the study aims to provide a comprehensive and reliable understanding of TDS levels in the Al-Khobar desalination production system, contributing to improved monitoring and management practices.
2. Methodology
The study analysis conducted by a group sample from Al-Khobar production system water quality section. The study focused on three sampling locations: AK2, AK3, and RO1 intakes sampling points. To ensure accurate and representative samples, continuous flow was maintained at all sample points to prevent contamination. Separate labeled bottles were used for each location, and qualified samplers collected the samples using the grab method. The collected samples were immediately transported to the main Khobar General Laboratory to minimize any physical changes.
Conductivity measurements were performed on the received samples using a calibrated WTW (Cond 7110) instrument. Standard solutions (1413 µS/cm and 12.88 mS/cm) were used to calibrate the instrument and ensure accuracy. To account for instrument precision, duplicate measurements of the samples were taken and averaged.
Before conducting TDS analysis, the samples were filtered using Ashless filter paper (110 mm, Ref. 300010) to remove suspended solids. A known volume (10 ml) of the filtered sample was transferred into a pre-weighed dish. The dish was then heated in an oven at 85˚C for 24 hours until complete dryness. Subsequently, the samples were heated for an additional hour at 180˚C to remove organics. After cooling in a desiccator, the samples were weighed to determine the weight of the residue. The TDS concentration was calculated by dividing the weight of the residue by the sample volume and expressed in parts per million (ppm).
To minimize personal selective errors, all laboratory analyses were performed by the same lab analyst. Additionally, the same instruments and apparatus were used for all samples to minimize errors caused by variations in lab equipment. The EC measurements were based on ASTM D1125-14, and the TDS analysis was conducted using the evaporation (gravimetric) method referenced in ASTM D5907-18 or ABHA 2540. The correlation between TDS and EC calculations was referenced from APHA 1030-E.
To further validate the study data, a portion of the randomly selected samples was sent to the Institute for Research, Water Technology Innovation Institute & Research Advancement (WTIIRA) for analysis, allowing for a comparison of results between the Al-Khobar study lab and WTIIRA, thereby enhancing the credibility of the study’s findings.
3. Data
The study collected seawater samples from routine sample points at the SWCC Al-Khobar plant’s intake for the AK2, AK3, and RO1 locations. Sampling was scheduled on a weekly basis, following the plant’s operational status, with no emergency situations or shutdowns. Sample collection was suspended during vacation periods. All samples were taken in the morning between 9 to 10 AM for consistency across locations. TDS levels were measured using the gravimetric method, while EC measurements were conducted using the WTW (Cond 7110) lab conductivity meter. The TDS factor, or ratio, was calculated by dividing TDS (ppm) by EC (µS/cm).
The measurement results are presented in Table 1, which includes the average, standard deviation, maximum value, minimum value, and range for each measurement. To validate the main data, measurements were also conducted by the WTIIRA, and the corresponding data is listed in Table 2. The comparison between the Khobar lab and WTIIRA lab results is provided in Table 3. The evaluation aimed to ensure the validity of all data, minimize bias, and ensure reliable results.
One notable observation was that the measured TDS level at the RO1 site on July 5th, 2021, was below the seawater TDS level. Considering the EC value at this point, it indicated an invalid relationship between TDS and EC results, suggesting a potential error in the test performed by the analyst. Consequently, this particular data point was excluded from the analysis to ensure the accuracy of the remaining data in representing the seawater’s TDS levels. The EC and TDS values for each site were plotted to assess the strength of the relationship and detect any data disruptions throughout the study period.
4. Result and Discussion
The present study aimed to investigate the levels of total dissolved solids (TDS) in the seawater intake of three locations within the SWCC Al-Khobar production system and analyze any potential differences among them. In this section, we present the results obtained from the TDS measurements conducted at each site. Descriptive statistics such as the mean, standard deviation, and range of the data are provided for each location. Furthermore, the data is visualized through various charts to facilitate a clear understanding and comparison between the three sites.
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Table 1. TDS and EC measurements in SWCC Al-Khobar laboratory.
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Table 2. TDS and EC measurements by Water Technology Innovation Institute & Research Advancement (WTIIRA).
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Table 3. Comparing between Khobar and WTIIRA lab results.
The discussion encompasses an analysis of the potential sources of TDS in each site and how these sources may have influenced the observed TDS levels. By identifying the contributing factors, we can gain insights into the water quality of the three sites and understand the factors influencing TDS levels.
Firstly, the Khobar phase-2 (AK2) seawater plant’s intake is currently the oldest operational intake within the Al-Khobar production system, starting its production phase in 1983. Table 1 shows that the mean TDS value at the AK2 site was 55,922 ppm, with a standard deviation of 2350 ppm. The maximum TDS value was observed in the first week of February and April, reaching 60,000 ppm, while the lowest value of 50,000 ppm was recorded in mid-December. These results indicate significant variations in TDS levels throughout the year. Figure 1 also demonstrates the high fluctuation in TDS values at the AK2 seawater intake. One possible explanation for this instability is that the AK2 intake pipes are located close to the shore, making them susceptible to external activities or effluent discharge from nearby plants. Additionally, the average seawater TDS calculation factor for AK2 was found to be 0.787, the highest factor compared to the other locations. The data disruption shown in Figure 2 indicates a high degree of variability, likely influenced by the natural conditions of the site and its proximity to the coast.
Secondly, the data from phase-3 intake shows relatively lower seawater TDS levels compared to the other intakes, with an average of 55,862 ppm. The maximum TDS value recorded was 59,920 ppm on January 25th, which coincides with the maximum sample value at AK2. Similarly, the minimum TDS value was observed around the same time as AK2, during the first week of October (Table 1). Figure 1 supports the notion that the TDS trends of AK2 and AK3 are similar due to their close proximity. However, AK2’s trend exhibits more fluctuations, which further supports the influence of natural events and other circumstances near the shoreline. The relationship between EC and TDS in AK3 is stronger compared to the other locations, as depicted in Figure 3, which shows a more linear relationship with a slope of 0.7827.
Thirdly, the RO1 intake is approximately 2 kilometers east of the phase, refer to Figure 4, which is a longer distance compared to the other Khobar intakes. This distance is reflected in the RO1 data, resulting in slightly different behavior compared to AK2 and AK3. The average TDS level at RO1 was 56,417 ppm, the highest among the sampled points. The highest TDS value of 61,000 ppm was observed in April, while the lowest concentration of 51,000 ppm was recorded at the end of December. The standard deviation of TDS was calculated to be 2052 ppm. The high seawater TDS level in RO1 can be attributed to location’s geometry falls within a narrow strait, making it more susceptible to the influence of seawater currents and disturbances caused by infrastructure activities and organic sources. Figure 5 illustrates that the pattern reflection change of RO1 TDS occurs earlier than AK2 and AK3, indicating that seawater currents have a greater effect on RO1 before reaching AK2 and AK3. The relationship between EC and TDS in RO1 shows a weaker regression strength compared to AK3. However, the data disruption along the linear trendline is closer compared to AK2, providing further support for the influence of seawater currents on RO1.
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Figure 1. Total dissolved solids (TDS) for Al-Khobar production system intakes.
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Figure 3. Total dissolved solids (TDS) for Al-Khobar production system intakes.
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Figure 4. Al-Khobar approximate intakes location [12] .
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Figure 5. Total dissolved solids (TDS) for Al-Khobar production system intakes.
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Figure 6. Total dissolved solids (TDS) for Al-Khobar production system intakes.
Regarding TDS-to-EC Ratio, electric conductivity is influenced by the type and concentration of ions present in the solution. On the other hand, TDS levels encompass not only ions but also other dissolved solids, including organic compounds, minerals, and other substances that are not electrically charged. Therefore, a solution with a high TDS may not necessarily exhibit high electrical conductivity if the majority of dissolved substances are non-ionic, and vice versa.
To establish the liner relationship between electrical conductivity (EC) and total dissolved solids (TDS), the measured EC data was plotted against TDS levels over time for all Al-Khobar intake locations, as shown in Figure 2, Figure 3, Figure 6. These graphs provide strong visual evidence of the relationship between conductivity and TDS. Consequently, EC is widely considered a reliable means of estimating calculated TDS. The average factor for all Al-Khobar intakes is 0.79, so TDS (ppm) = 0.79 × Conductivity measurement (µS/cm).
From Figure 5, the overall shape of the EC and TDS trends displays a similar pattern. It is observed that the EC patterns show less reaction to high spikes in TDS levels, such as those observed at the beginning of April. This can be explained by the fact that the increase in TDS during that season is primarily due to high concentrations of organic compounds, which are not accurately reflected in EC measurements.
It is important to recognize the limitations of using EC as a proxy for TDS, particularly has limitations due to interference from other ions, temperature dependency, inability to detect non-conductive substances, calibration issues, and variability in sample composition. While EC provides a valuable estimation of TDS, additional analysis and consideration of other factors are necessary to fully understand the composition and sources of TDS in seawater intakes. It is recommended to use EC as a screening tool and confirm TDS measurements using other methods, such as gravimetric analysis, to ensure accuracy.
5. Conclusions
In conclusion, this study aimed to assess the levels of total dissolved solids (TDS) in seawater intakes at three locations within the Khobar desalination production system, AK2 (RO2), AK3, and RO1. The study employed both the gravimetric method and electrical conductivity (EC) measurements to analyze TDS levels and evaluate the accuracy of the measurement techniques. The TDS factor, calculated by dividing TDS by EC, was used to establish the relationship between the two measurements.
The results of the study showed significant variations in TDS levels across the three sampling locations throughout the year. AK2, being the oldest operational intake, exhibited higher TDS levels compared to the other sites, with notable fluctuations. The proximity of AK2 to the shore and potential external activities or effluent discharge from nearby plants could contribute to the observed instability. AK3, located close to AK2, displayed similar TDS trends but with relatively lower levels. The RO1 intake, situated at a greater distance, showed slightly different behavior, with higher average TDS levels, possibly influenced by seawater currents and disturbances from infrastructure activities.
The analysis demonstrated a strong correlation between EC and TDS measurements, with EC serving as a reliable estimation of TDS levels. However, it is important to note that EC may not accurately reflect non-ionic dissolved substances and organic compounds, which can contribute to high TDS levels. Therefore, additional methods, such as gravimetric analysis, should be employed to confirm TDS measurements and ensure accuracy.
The findings of this study provide valuable insights into the water quality of the Al-Khobar desalination production system. The information can be utilized to improve monitoring and management practices, guide decision-making processes related to water treatment strategies, and assess environmental impacts. By understanding the sources and variations of TDS levels, appropriate measures can be implemented to maintain optimal water quality and ensure the efficient operation of the desalination production system.
Acknowledgements
We gratefully acknowledge that the data used in this study is owned by our organization, Saline Water Conversion Corporation, Al-Khobar production system. We appreciate the efforts and resources invested in collecting and maintaining the data, which has been instrumental in conducting this research. This study would not have been possible without the ownership and support provided by our organization.