**4. Results**

#### *4.1. Energy Consumption and GHG Emissions*

The energy consumption and GHG emissions of the Chinese seawater desalination are determined for the period of 2006–2016 (Figure 6). The annual energy consumption increased in the 11 y from 81 MWh/y to 1,561 MWh/y, with an increasing rate of 182%. The GHG emissions increased from 85 MtCO2eq/y to 1,628 MtCO2eq/y.

**Figure 6.** Overall seawater desalination energy consumption and GHG emissions from 2006–2016.

The breakdowns of GHG emissions by province and desalination processes are estimated for the year of 2016, as shown in Figure 7.

**Figure 7.** GHG emissions of seawater desalination: Regional distribution and breakdowns by processes.

The map in Figure 7 showed the east costal line in China and marks the provinces with seawater desalination plants. The deeper color indicates higher provincial seawater desalination GHG emissions. The total GHG emissions of the seawater desalination plants in China in 2016 are 9,409 MtCO2eq. The provinces of Tianjin, Hebei, and Shandong are the middle three contributors. The GHG emissions of these three provinces are 7,359 MtCO2eq, which is 78.2% of the total seawater desalination GHG

emission in China. Regarding the desalination process, MED plants contribute the most to the GHG emissions. Tianjin has the highest GHG emissions of 4,142 MtCO2eq in China, and the MED plants contributed more than 88.0% (3,645 MtCO2eq) of the total seawater desalination GHG emissions of Tianjin. This is a major contributor to the overall emissions of all seawater desalination plants in China in 2016. Shandong Province has the second largest seawater desalination capacity, but the GHG emissions are much lower (1,035 MtCO2eq) compared to Hebei, which has larger emissions (2,183 MtCO2eq) with smaller desalination capacity. The main reason is that the seawater desalination plants in Shandong are all RO plants. For Hebei Province, about 65% of the total desalination capacity is MED, which is more energy and GHG emission intensive. On the other hand, the south part of China has relatively lower GHG emissions, due to a smaller capacity and less energy intensive processes.

#### *4.2. Unit Product Cost*

The Unit Product Cost (UPC) is correlated with the desalination processes and the capacity of the plant, the type of energy used for the plant, etc [30]. In this study, the energy consumptions of all processes are converted to electricity, therefore the impact of energy source is not analyzed. The UPC of seawater desalination plants in different provinces in 2016 is determined as shown in Figure 8, and the desalination process is specified.

The UPC of RO desalination plants shows a slight difference. Firstly, the UPC of MED, MSF, and ED is much higher than RO, which is inconsistent with the conclusion of other studies. For example, a case study of Qingdao [14], Shandong Province, China showed the average economic cost of seawater desalination process is 8 CNY/ m<sup>3</sup> (approx. 1.16 USD based on the current exchange rate 0.15), with an RO plant capacity of 3 × 10<sup>3</sup> m3/d. Hainan has the highest UPC for RO seawater desalination of 1.3 USD, Shandong and Zhejiang have the lowest UPC for RO seawater desalination of 0.8 USD. For MED seawater desalination, Hainan has the highest UPC of 3.6 USD, while Tianjin has the lowest UPC of 2.0 USD. The only province with MSF process, Tianjin, has a UPC for MSF desalination of 3.0 USD. The UPC for ED process desalination of Fujian and Hainan are 1.9 and 1.7 USD. It can be seen that for different processes, the UPC in increasing order is RO < MED < ED < MSF, with RO as the cheapest and most applied desalination process. For the same process, the price varies within a reasonable range, e.g. the UPC of RO process desalination plants in the selected provinces in increasing order are: Shandong = Zhejiang < Tianjin = Hebei = Liaoning = Guangdong < Fujian < Jiangsu < Hainan.

#### **5. Discussion and Future Directions**

## *5.1. Discussion*

Seawater desalination has high water supply potential, but high energy consumption, GHG emissions, and cost. Water desalination has a considerable water supply potential due to the abundance of its water source, and the desalination technology is improving. On the other hand, this type of water supply is still supported by higher cost and intensive energy consumption and GHG emission. Even though the cost and energy use are decreasing [7], seawater desalination is still an energy intensive and costly approach compared to other freshwater alternatives. The World Resources Institute investigated the energy consumption of water desalination plants in Qingdao, Shandong province in China. The results showed that electricity is the main energy used for water desalination [15], and the SEC of different water supplies are shown in Table 5.


**Table 5.** Energy demand and water supply potentials of water supply alternatives (from [15]).

It showed that for the various water supply alternatives, surface water has the lowest energy demand for per unit water (about 10% of seawater desalination), and at the same time with the lowest water supply potential (37% of seawater desalination). On the other hand, seawater desalination (RO process) has the highest energy demand with the highest water supply potential. For other desalination processes, the energy demand is even higher.

This indicates that with the current situation of techniques, water desalination is still an option with higher cost. Considering the continuously increasing population and the demand for freshwater, seawater desalination has the potential of providing a stable amount of fresh water and should be viewed as a crucial component in the future development of non-conventional water supplies [31]. The energy demand and thus GHG emissions are highly related to the capacity and techniques applied in the plant [30]. The study of [4] showed that for a water desalination plant with capacity from 5 to 15 m3/d, the total electricity consumption would vary from 14.45 to 21.35 kWh/m3, and the MED process has the highest energy demand, which is inconsistent with the results of this study (Figure 8). This indicates the potential for energy consumption and GHG emission reduction can be optimized with improving the combination of capacities and processes. According to the results of [14], a case study of Qingdao, the overall cost of different water supplies from the lowest to the highest are local surface water < groundwater < reclaimed water < brackish water desalination < seawater desalination. Seawater desalination is still currently a more expensive approach for producing freshwater compared to other water supply alternatives.

Determining the energy consumption, GHG emissions, as well as the cost of seawater desalination would be helpful to identify the potential of improving the energy efficiency and productivity of seawater desalination. New and advanced technologies, e.g. low energy reverse osmosis membranes, improved energy recovery devices, highly energy efficient pumps, and optimized pre-treatment systems, can enhance the energy efficiency of seawater desalination. The application of energy recovery units in the desalination processes can also highly increase the energy use efficiency [3]. Increasing the efficiency of the process and the application of energy recovery system will reduce its energy consumption and thereby its CO2 emissions.

The energy consumption, GHG emissions, and the cost of seawater desalination in China are higher than the average values of the major desalination contributors in East Asia, e.g. Singapore. Singapore has the second largest seawater desalination capacity of 0.45 × 10<sup>6</sup> m3/d [32], which is 36% of the seawater desalination capacity of China. The average UPC of the two large scale RO seawater desalination plants is 3.5 kWh/m<sup>3</sup> and the cost is estimated as 0.75 USD/m<sup>3</sup> [33], and the price is estimated to increase due to the higher price of energy. The energy consumption and cost of per m<sup>3</sup>

desalinated water in Singapore are lower than in China. However, as these indicators are affected by the specific process, type, quality, and price of the energy, as well as the location of the plants, etc. The comparison between different countries would be limited to provide insightful information, but a comprehensive analysis of different water supply alternatives, or different desalination processes in the same region would facilitate regional water use management.

#### *5.2. Future Works*

The correlation between the UPC and the capacity needs further investigation. In this study, the UPC and capacity of seawater desalination plants showed a very obvious correlation. Identifying this correlation would be helpful for the optimization of seawater desalination. Figure 9 showed the trend line of the UPC-availability plots. As MSF and ED are rarely applied, there is not enough sample data, and only RO and MED are discussed. For RO plants, the correlation between the UPC and the capacity fits the logarithmic correlation with a R<sup>2</sup> of 0.9847 and 0.9359 for MED plants.

**Figure 9.** The correlation between unit product cost (UPC) and the seawater desalination capacity.

The figure showed a decreasing UPC with the capacity increasing. According to a statistic of China seawater desalination [14], in 2016, large plants (capacity > 10<sup>4</sup> m3/d), the average cost is 6.22 CNY/m<sup>3</sup> (0.90 USD), and the average of desalination cost in medium plants (10<sup>4</sup> < capacity < 103) is 7.20 CNY/ m<sup>3</sup> (1.05 USD). In this study, Shandong has the lowest UPC (0.8 USD) with the highest RO capacity of 0.28 × 10<sup>6</sup> m3/d. Following is Zhejiang, with the same UPC and a RO capacity of 0.22 × 10<sup>6</sup> m3/d Tianjin. For MED the trend is not clear. The correlation analysis between the capacity and the UPC should be further investigated with more sufficient data.

Potentials for energy consumption and GHG emission reduction in seawater desalination. The water-energy integration in seawater desalination, e.g. seawater desalination plants utilizing renewable energy [34], low potential heat integrated with seawater multiple-effect thermal desalination [35], which are worth further investigation. The implementation of the integration of energy and seawater desalination still needs a comprehensive review.

In addition to energy consumption and GHG emission, there are other potential environmental issues of seawater desalination. Along with the desalted seawater, a remarkable amount of brine is also produced from the process. A common approach is currently to dispose the brine back to the sea, which might cause harm to the regional aquatic life due to high salinity [36]. The utilization and treatment technologies of the brine is still. Similarly, chemical and thermal pollutions in the near-plant areas, should be further investigated in the future works.

A limitation of this work, which should form the potential future works, is less investigation of the energy consumption by different forms of energy, e.g. heat and electricity in different processes. The usage of renewable energies should also be further reviewed. To cover the whole life cycle, the waste generated during the desalination process, including brine, wastewater, and waste heat should be investigated.
