Long-term desalinated water demand and investment requirements: a case study of Riyadh

The Kingdom of Saudi Arabia (KSA) is situated in an arid region and faces a chronic challenge to meet its increasing water demand. Riyadh is the capital of KSA and home to about six million people. The water demand is mostly met by groundwater resources (up to 48%), while the desalination plants cover the rest of the water supply requirements. There is a potential risk of a significant gap in water demand–supply due to the retirement of old desalination plants. This study, therefore, developed a probabilistic model to forecast desalinated water demand in Riyadh for domestic purposes up to the year 2040 based on three scenarios: low growth, the most likely (mean), and high growth scenario. The results showed that an investment of about US$6.24, 11.59, and 16.04 billion is required to meet the future domestic water demand of the city for the next 25 years based on low, mean, and high growth scenarios, respectively. Moreover, a strong commitment to public–private partnership is required to remove the fiscal budget burden related to the desalination along with public awareness campaigns to reduce per capita water consumption, upgrading the water tariff system and using renewable energy to run desalination plants. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/wrd.2017.107 om https://iwaponline.com/jwrd/article-pdf/8/3/432/240804/jwrd0080432.pdf 2020 O. K. M. Ouda Prince Mohammad Bin Fahd University, Al-Khobar, Saudi Arabia Y. Khalid Institute of Energy Technologies, Polytechnic University Catalonia, Barcelona, Spain A. H. Ajbar I. Wazeer Department of Chemical Engineering, King Saud University, Riyadh, Saudi Arabia M. Rehan K. Shahzad A. S. Nizami (corresponding author) Center of Excellence in Environmental Studies (CEES), King Abdulaziz University, Jeddah, Saudi Arabia E-mail: nizami_pk@yahoo.com


INTRODUCTION
Scarce water supply in the Middle East region is a strategic and chronic problem that can be traced back to the early 1970s (Allan ; UNDP ).The Kingdom of Saudi Arabia (KSA) struggles with water scarcity, as it is located in the driest spot of the Middle East and has extremely hot summers and dry winters (SGS ).The country has no perennial lakes or rivers; the only reliable natural water source for domestic water supply is groundwater (World Bank ; Ouda ).In 2010, the total natural water resources that could be used in the country for all purposes, including agriculture, industrial, commercial, and domestic was around 188 m 3 per capita.This figure is far below the threshold limit of 500 m 3 per capita per year, set by the World Health Organization (WHO) for water stress countries ( Jagannathan et al. ; UNDP ).
An enormous socio-economic development has occurred in KSA over the last few decades, as a result of revenues generated from high crude oil production (World Bank , ; Nizami et al. a, b; Shahzad et al. ).In the early 1970s, KSA's population was 7 million.
By 2010 it had increased to around 27 million, with an average growth rate of 3.4% (Ismail & Nizami ; Nizami et al. a, b).Urbanization levels also increased from 50 to 80% during this period (CDSI ; Ouda et al. ; Nizami et al. ).Consequently, the water demand in all sectors of KSA rose substantially (Abderrahman ; Ouda a, b).The water demand-supply gap in 2010 was around 11.5 billion m 3 (Table 1), which was mainly bridged by depletion of groundwater resources (UNDP ).
A significant increase in water demand by domestic, industrial, and agriculture sectors will exert additional pressure on water resources and challenge the socio-economic development plans of the KSA's government (Ouda et al. ).The government has considered desalination of seawater as a strategic option to meet the growing domestic water demand throughout the country (Al-Ibrahim The future estimations of water demand are critical for water system planning and design, water utilities asset management, and water resources management.These forecasts, along with the role of demand management policies, become more critical in areas of scarce water supplies such as the city of Riyadh (Almutaz et al. ; Rahmanian et al. ).However, despite the extreme importance of water forecasting, there is a very limited amount of literature and data available in KSA that tackles this topic.In fact, the little that exists is mostly outdated (Almutaz et al. ).The scientific literature documents various models for water demand forecast based on deterministic or probabilistic approaches (Arbues et al. ;Davis ;Worthington & Hoffman ;Alshawaf 2008).The selection of a forecast model is driven mainly by the data quality and its level of certainty.The deterministic model will be very efficient when the data are both high quality with high certainty levels (Cheng & Chang ).However, in situations where the main explanatory factors or variables are uncertain, the effectiveness of such deterministic models will be limited (Khatri & Vairavamoorthy ).This applies in particular for countries like KSA, where the temperature is the only variable (Almutaz et al. ).Other

METHODOLOGY Water demand forecast using probabilistic model
The development of a probabilistic forecast model for any city relies on the selection of explanatory variables followed by the assignment of probability density functions to each selected variable (Almutaz et al. ).The forecast time frame is set to the year 2040.This matches with the average useful life of a desalination plant which is 25 years.The average household income is the first selected explanatory variable (Table 2).In some countries, the water bills are a significant fraction of household income.However, in the case of KSA, where the government largely subsidizes water cost, the household income has a minimum indirect effect on water consumption.For example, higher income or wealth can be used for luxury goods such as dishwashing machines and swimming pools that lead to higher water consumption.Moreover, households with high income possess large houses that tend to consume larger quantities of water.The average household income was assumed to be normally distributed with an average growth rate of 2.2%, while the standard deviation is restricted to 10% of the mean (Table 2).
The water forecast model also depends on population size and its growth.Riyadh city attracts both national and international immigrants.Immigration from outside the country, in particular, depends on the economic opportu-

Mathematical expressions and models' variables
The total water use was estimated by the following standard functional population model: Q y ¼ Nq where Q y is the total annual water use in year (y), N the population at year (y), and q is the water use per capita The water use (q) per capital was assumed to depend on the following explanatory variables: whereas, income is the normal distribution with an average growth rate of 2.2%, while the standard deviation is restricted to 10% of the mean.Household size, including the average number of persons per household, is triangular probability distribution.
The mean is assumed to increase by an annual growth rate of 2.4% from the base value of 5.86.The minimum and maximum values of ±1.5% of mean.Temperature is normal distribution with a standard of deviation of 1 W C The model parameters were determined using @Risk.The population (N) forecast was based on the following methodology: The population growth rate is based on the data for annual growth rate for Saudi and non-Saudi population, currently the annual growth rate of Saudi's population is 2.95%, while it is 2.895 for non-Saudi's population.The change in annual growth rate of Saudi's population is 0.16%, while it is 0.26% for non-Saudi's population Given the above nominal data, the growth rate of Saudi was computed (resp.non-Saudi) at year_i through the following formula: Growth rate of Saudi at year_i ¼ Annual growth rate for Saudi -Change in growth rate for Saudi*(year_i -reference year) The total population of Saudi (resp.non-Saudi) at year _i is then calculated by: Total population of Saudi at year_i  This has been increased to 2,818,000 m 3 /day by 2014 due to the inclusion of the RAK plant.Afterwards, the water supplies will be reduced to the amount of 1,884,826 m 3 /day with the retirement of old MSF plants.This trend will remain constant until 2040.

Desalination technologies
The desalination technologies (Figure 2 3 and 4, based upon three sets of criteria: merits and limitations, computable (quantitative), and non-computable (qualitative) criteria.

RESULTS AND DISCUSSION
Water demand-supply gaps The results of water demand forecast for Riyadh city are presented in Figure 3, which shows the annual water demand for the 5th (low-growth scenario) and 95th (high-growth growth scenario) percentiles and the most likely scenario (the mean).The most likely projected water demand for the year 2040 will be about 2,846 × 10 3 m 3 /day.This value lies between the limits of 2,414 × 10 3 m 3 /day (low-growth scenario) and 3,240 × 10 3 m 3 /day for 95% (high-growth Under high salt concentrations and impurities, the process reliability is affected scenario).The above-mentioned projected water demand can also be calculated using the water deficit between available water supplies and projected demand.(Figure 4).

Suitable desalination technologies
In cross comparison (Tables 3, 4 and Figure 3 and 4).However, in many cases, and specifically for larger plants, the lowest cost is obtained by MED when joining power and water production (Tables 3   and 4).
Seawater desalination together with power generation provides a promising and better usage of fuel (Ouda , 2016).In this regard, MSF desalination plants can allow cogeneration of water and electricity.Therefore, the cost of the plant can be distributed to products such as water and electricity, if power generation is considered from MSF (Tables 3 and 4); although the unit operating    4).
Therefore, MSF will provide more technical and economic advantages for serving Riyadh city's desalination needs to fulfill the water deficit for years to come.
Advances in technology and the maximum number of running units determine the size and number of units for respective desalination technologies.In comparison, only MSF units are becoming bigger in size and unit capacity (Table 4).The RAK desalination plant is one of the latest plants, with a unit size of 16.7 million imperial gallons per day (MIGD) that is equivalent to 75,898 m 3 /day.Therefore, it was assumed as a benchmark for developing new desalination units, as it is of good standard size and proven to work for a longer period of time in KSA (Water Technology ).
The location of desalination plants is very critical.According to Tsiourtis (), a plant should be developed in an area where interconnections to power and water supply networks are technically possible and economically affordable.
In this regard, the current infrastructure of roads can be utilized along with existing fuel network facilities.

Economic analysis
Due to the extra advantages, the total investment required  Based on this, MSF desalination costs (Table 5) and the water demand-supply data, the required total investments for the three scenarios were calculated and are summarized in Table 5. Considering the three water demand forecast scenarios of low, mean, and high, the water demand of 531,000, 981,000, and 1,366,000 m 3 /day, respectively, was predicted for Riyadh city for the next 25 years.To meet this demand for all three scenarios, 7, 13, or 18 MSF units, respectively, would need to be installed in Riyadh city (Table 5).As the total cost of each MSF unit was estimated to be US$891.3million, the KSA government would need to invest about 6.24, 11.59, and 16.04 billion US$ to meet the future water demand of Riyadh city through MSF desalination technology for the next 25 years, based on low, mean, and high water demand scenarios, respectively (Table 6).
This is a substantial amount of money, given the country's high dependency on crude oil revenue and the drastic decrease in crude oil prices (Demirbas et al. a, b, c).Furthermore, the public has very little knowledge about the water shortage in the country and other related challenges, including the cost of water production and distribution, and the government's huge subsidies (Ouda et al. ; Ouda b).These conditions diminish the potential for water conservation and put the operational productivity and financial stability of the water desalination plants at stake (Ouda ).Therefore, there is a high need for longlasting and effective awareness campaigns among communities and institutions for promoting water conservation in the country (Ouda ).

Desalination technologies and climate change
The impact of desalination technologies on climate change and how the research and development trends influence the overall water management systems with time must be considered in water policy-making models (Table 7).Currently, desalination process for cleaning water is one of the most energy-intensive processes and consumes more energy for producing each liter of water than most other water treatment and supply methods in KSA (Ouda ).
For example, in RO desalination technology, around 70% of total energy intake is by the RO process, and the remaining 30% is consumed for water pumping and pre-and posttreatments.Desalination plants use around 15,000 kWh per million gallons or 4 kWh per m 3 of water produced (Foster & Loucks ).However, these estimated energy consumption values are based on a fixed set of conditions in ideal scenarios, but the actual operating conditions are usually not so ideal due to various process limitations and losses (Napoli & Rioux ).In KSA, currently, the energy consumed by desalination plants is mainly generated by fossil fuels that result in greenhouse gas (GHG) emissions and potential climate change (Table 7).
Since the understanding of challenges and risks associated with climate change for ecosystems and human

Future perspective
The ongoing slump in global oil prices is changing the way governments and economies in the Middle East work, and water is no exception to the trend.In the petro-economies that make up some of the world's biggest markets for desalination, such as KSA, Iran, Qatar, and Abu Dhabi, oil-driven national budgets have been savaged by an oil price that has decreased to less than half over the last 12 months.Moreover, the dwindling water supplies and uprising water demand has adversely affected the mega cities around the world in general ).Currently, around half of the domestic water demand in KSA is fulfilled by 30 different desalination plants (SWCC , ).These desalination plants are developed in the Red Sea and the coast of the Arabian Gulf, under the authority of the Saline Water Conversion Corporation (SWCC) (SWCC , ).The production capacity of these desalination plants has significantly increased from around 200 million m 3 /year in 1980 to around 1,050 million m 3 /year in 2010 (SWCC , ; MWE ; Ouda ).
nities of the oil-rich country.The uncertainties incorporated in local population growth are therefore justified.Currently, the Saudi population is growing at an annual rate of 2.95% (Almutaz et al. ).Planning authorities predict a decline in population due to several socioeconomic factors.Data from the Central Department of et al. , ).After forming the forecast model, Monte Carlo simulation (MCS) was employed to determine the point demand model with specified variable uncertainties, which made a probabilistic demand model to obtain the total water consumption distribution.The @Risk software package, version 6.0 by Palisade Corporation (www.palisade.com)was used to carry out the MCSs.This software can be used for a number of probability distribution functions.It allows the simulations to generate random numbers and specify sampling rules, together with setting graphics and other scenario options.A confidence interval of 90%, in which potential water demand would fall, was projected and formed with 5th and 95th percentile values.The 5th, 95th, and mean values roughly represent the low, high, and most likely growth scenarios, respectively.Water demand-supply gap The typical lifetime of a desalination plant ranges between 25 and 30 years, depending on the type of technology (Raluy et al. ).The existing old plants, if refurbished, will provide an additional 15 years' lifetime to the plant (Schiffler ).Accordingly, the useful life of a desalination plant was adopted to estimate the water demand-supply gap, while the gap in the water supply was assessed based on an estimation of the available desalinated water quantity and the model feeding of future demand (Sommariva et al. ; IAEA ; Raluy et al. ).The water supply capacity of the existing desalination plant was calculated based on the following considerations.In the years, 2023 and 2024, respectively, both desalination plants MSF I and MSF II will complete their lifetime with a total useful life of 40 years from the date of operation and will maintain the same production rate until complete amortization.Second, the amount of water, 906,600 m 3 /day, to be supplied by the local groundwater resources, was assumed to remain the same until the year 2040.The reason for this includes the ongoing debate on whether the groundwater in KSA can be considered as a renewable or not(Foster & Loucks ).If the groundwater is renewable, then an increasing use of groundwater will be assumed.While, if the groundwater is not considered as a renewable, then a decreasing use of groundwater will be assumed.Therefore, a constant supply of groundwater for the future was assumed until the year 2040.Third, the RAK plant (900,000 m 3 /day) will supply the desalinated water for a total duration of 40 years from the year 2014 onward.Fourth, the amount of water, 78,182 m 3 /day, to be supplied by RO plant, was also assumed to remain the same until the year 2040.Total water supply from 2012 until 2040 is shown in Figure1.Initially, the water supply for all scenarios was around 1,918,000 m 3 /day in 2011.
) are classified as thermal or membrane-based technologies.The membranebased desalination technologies include RO and electrodialysis (ED), whereas the thermal based desalination technologies include MSF, vapor compression (VC), and multieffect distillation (MED) (Al-Karaghouli & Kazmerski ).RO technology is a pressurized filtration process, where a semi-permeable membrane filter is used to allow only water to pass through (Figure 2).The products of RO are fresh water as well as a concentrated solution that remains on the membrane side with high pressure.The process is completed in four steps: (1) pretreatment, (2) highpressure pump, (3) membrane, and (4) post-treatment.RO is widely used for the treatment of saline groundwater and seawater and can be expanded easily from house level to commercial scale (El-Dessouky & Ettouney ; Al-Karaghouli & Kazmerski ).

Figure 1 |
Figure 1 | Projected water supply for Riyadh up to year 2040.
The total water demand-supply gaps for all three scenarios are presented under two different time period zones, as shown in Figure4.During the first time period zone, the water supply would meet the water demand, whereas the water demand will exceed the water supply during the second time period zone.This change will start from the year 2024, where the demand and supply curves intersect with each other, as shown by an arrow in Figure4.The water deficit for 2024 was 30% for a 95% demand scenario that increased to 72% (about 1,366 thousand m 3 /day) in 2040.Similarly, there will be a water deficit of 20% in 2024 for a 5% demand scenario that further increased to 28% (531 thousand m 3 /day) in 2040.Regarding the mean (the most likely scenario), the deficit value for 2024 is 25%, increasing to 51% (981 thousand m 3 /day) by 2040 2), RO is preferred over MED and MSF, as there is a better understanding today of the pretreatment requirements of the RO process.However, RO's ability to handle feed-in seawater with variable quality is still a major drawback.This is particularly the case for Arabian Gulf water with high level of harmful algal blooms (HABs) and salt concentration.The HABs tend to cause membrane fouling with an accumulation of organic and particulate materials and, ultimately, results in operational problems of the RO plants.On the other hand, high thermal efficiencies, savings in fuel costs, operation at a low temperature to avoid corrosion and scaling, and ability to operate with feed-water that has a large salt concentration are some of the advantages of the MED process (Tables

Figure 3 |
Figure 3 | Projected water demand for Riyadh up to year 2040.

a
Annual operating cost ¼ (Unit operating cost) (Plant capacity).b OPEX ¼ (Annual operating cost) (25 years); (Plant life cycle is considered to be 25 years (Mabrouk 2013).c CAPEX ¼ (Unit capital cost) (Plant capacity).O. K. M. Ouda et al. | Long-term desalinated water demand and investment requirements Journal of Water Reuse and Desalination | in press | 2017 Uncorrected Proof (US$1.07/m 3 ) and annual operating (US$26.6 million) costs of MSF are relatively higher than MED and RO's unit operating (US$0.83and 0.76/m 3 ) and annual operating costs (US$20.66 and 18.92 million), respectively (Table 4).Considering each plant life cycle to be 25 years, the total cost of MSF plants would be about US$800.75 million as compared to US$636.5 and 585.5 million for MED and RO desalination plants, respectively.However, the proven track record reliability of operation over a longer period of time (more than 30 years) coupled with construction in large capacities are special features that give added advantages to MSF plants than RO and MED desalination plants (Almutaz et al. , ).MSF, therefore, is a commercially viable and dominant desalination technology in the Middle East region, with a market share of about 70% (Mabrouk ).Moreover, MSF plants provide a significant benefit in reusing the old MSF plants' infrastructures in building new similar plants.The MSF operation is also slightly influenced by the feed-in water quality (Table and detailed economic analysis for the next 25 years was carried out only for MSF desalination technology.The overall capital and operational costs of any desalination plant depend upon many factors, including feed water features, quality of produced water, plant capacity, local conditions and labor costs, and energy sources and requirements.A single unit of 75,898 m 3 /day capacity was selected as a benchmark capacity for MSF evaporators.The annual operating cost of US$29.64 million) for each MSF unit included the cost of parts (US$0.01/m 3 ), chemicals (US$0.05/m 3 ), labor (US$0.08/m 3 ), amortized capital cost (US$0.42/m 3), thermal energy (US$0.31/m 3 ), electrical energy (US$0.20/m 3 ), and

Figure 4 |
Figure 4 | Projected demand-supply curve for three scenarios up to year 2040.
and KSA in particular(Ouda ).Riyadh, being KSA's most densely populated city consumes an incredible amount of water per day.The bulk of desalinated water is supplied through desalination plants located in the Eastern Province of KSA(Almutaz et al. ).The older plants will complete their lifespans in the near future, and there will be a huge water deficit once these plants stop working(Almutaz et al.   ).In this regard, the present study presents a foundation for selecting the most suitable desalination technologies.MSF technology seems to be a promising choice to fulfill Riyadh city's desalination needs.However, the determinations of water demand and water forecast are critical and challenging tasks due to the uncertainties in population and economic growth coupled with fluctuations in international crude oil prices.Finally, research initiatives towards improving desalination technology efficiency and minimizing its potential environmental impacts are greatly needed (Table7).Nonetheless, the decision to implement any desalination technology needs further in-depth technical, economic, social, and environmental investigations using life-cycle assessment and life-cycle thinking methodologies(Nizami & Ismail ; Nizami et al. , b; Rathore   et al. ).KSA should take the lead in this aspect, given the country's high dependency on desalination plants for domestic water usage.

Table 1 |
KSA sustainable water resources yields vs. water demand in 2010(Ouda 2015) a Variable depending on rainfall pattern.bThetotalnon-conventionalsourcesincludedesalinatedwater,andtreatedwastewateravailable in the country every year.developnewplans.Therefore, this study aims to determine the long-term investment needs for seawater desalination in Riyadh city for the next 25 years to meet the domestic water supply, with an ambition to maintain a sustainable water supply through a network of desalination plants.The study includes, in particular, the forecast of Riyadh's water domestic water demands up to the year 2040, assessment of the water supply options to meet the demands, suggestions for the appropriate desalination technologies based on the current practices and standards, and estimations of the necessarily required investments.Case study -Riyadh cityRiyadh city has a population of about six million which constitutes more than one-fifth of the total population of KSA (CDSI ).The city has a very harsh climate with an averthe city's water resources(Ouda ).The water supply for Riyadh city comes from local groundwater (about 48%) and desalination plants (about 50%) that are installed on the Arabian Gulf, about 450 km to the east of the city(Almutaz et al. ).The remaining minimal amount (about 2%) comes from treated wastewater that is only used for landscape irrigation(Almutaz et al. ;

Table 3 |
Comparisons of selected desalination technologies based on their merits and limitations(Napoli & Rioux 2016)

Table 4 |
Comparison among the dominant desalination technologies(Ghaffour et al.

Table 6 |
Number of units (based on 75,898 m 3 /day unit capacity) and total cost

Table 5 |
Cost summary for the 75,898 m 3 /day MSF plant, adapted from GWI/WDR (2012), Ouda et al. | Long-term desalinated water demand and investment requirements Journal of Water Reuse and Desalination | in press | 2017 Rehan et al. ; Sadef et al. ).Moreover, a Cost is based on an oil price of US$60 per barrel (GWI 2012).11O.K. M.change (Table 7).KSA's government has recently launched a special program, the King Abdullah City for Atomic and Renewable Energy (KACARE) to generate about 72 GWe from renewable energy sources, including nuclear, solar, the wind, waste-to-energy (WTE), and geothermal, by 2032 (Nizami et al. a; Demirbas et al. a; Miandad et al. b;

Table 7 |
The recommendations and future perspectives of water management in KSA Recommendations 1 One of the most obvious recommendations would be to make all possible efforts for maximum water conservation, including moving towards more efficient systems and reducing the current huge government water subsidies 2 The negative impact of current desalination processes due to high energy consumption and brine disposal on local environment should be studied, aimed at helping reduce water consumption and moving towards more sustainable and environment friendly technologies 3 Currently, KSA desalination processes are running on nonrenewable fossil fuel-based energy leading to negative environmental impact.The possibilities and sustainability of using renewable energy including water-to-energy should be investigated 4 More funding should be allocated for research and development work, to reduce the cost of clean water and negative environmental impact of desalination technologies, and to study feed water pre-treatment, sustainable ways of managing brine and other waste, more energy efficient systems and potential of combinations of renewable energy and distillation technologies 5 All stakeholders including academics, environmentalists, policy-makers, government institutions, etc. should collaborate with each other to tackle this serious worldwide issue of clean water production and distribution 6 Public awareness, concerning the importance of sensible water usage and the challenges and costs associated with cleaning and distribution of clean water, should be created through seminars, education, media, and public events 7 The recent advancements and emerging trends in desalination technologies, especially in design and development of renewable energy powered desalination plants, can significantly reduce GHG emissions and their impact on climate change