The suggested approach starts with defining the desalination goal and scope to establish the system boundaries needed for the phases of the sustainability evaluation. The purpose of the desalination process may be established by determining the characteristics of the incoming seawater and the characteristics of the primary outcome, which is the desalinated water. As a result, these outputs will be valuable for determining whether a desalination plant satisfies the required characteristics.

The scope of desalination means identifying the local circumstances and components affecting the desalination plant that can be utilised to evaluate its sustainability. These conditions vary between environmental, technical, economic, social, and institutional setup aspects. Identifying project stakeholders is also crucial to the scope definition because they will be essential in assessing the sustainability indicators. The outputs from the scope definition will be primarily utilised to create a list of sustainability indicators with associated quantification units for use in the assessment process.

Identifying which indicators to involve under the proposed five sustainability categories primarily results from the opinions of local experts, in addition to the published literature. Several documents and studies on identifying and selecting indicators were reviewed for consideration. Some of these studies are the Water Research Foundation Report [24], the UNEP Handbook of Environmental Impact Assessment of desalination [25], the Desalination Journal paper on environmental and energy issues relevant to water desalination [26], and other sources. The sustainability indicators for evaluating desalination plants used in previous studies have been studied and identified to assess the performance of these plants and determine the various environmental, social, and economic impacts [27].

Table 1 shows the selected 27 sustainability indicators that have been carefully chosen to ensure a comprehensive framework for evaluating the desalination plants in the Gaza Strip.

The following points have been taken into consideration in the proposed framework when selecting sustainability indicators:

• The importance of this indicator and its relationship with the desalination process,

• Measurability,

• Inclusiveness,

• Availability of sufficient data for quantification purposes.

In summary, consideration of large numbers of sustainability indicators will complicate the proposed framework, as well as the reluctance of water and desalination experts involved in evaluating and giving the relative weights to these indicators, which will lead to a lower response rate and thus, the accuracy of the final results of the study. So, many of the less important indicators were excluded.

The values assigned to the sustainability indicators are collected through experts' interviews, field visits, visual inspections, and interviews with key operation and maintenance staff and the plants' operators. Therefore, the questionnaire was provided to 30 responders who represent desalination plant stakeholders, such as PWA, Coastal Municipalities Water Utility (CMWU), and Municipalities, in addition to local water desalination experts in the academic field.

Since the measurement units of these indicators differ, as shown in Table 1, the standardisation of their units was made through the normalisation process to facilitate the process of their integration. Table 1 presents a proposed list that can be utilised as a starting point in formulating the relevant SI, with the option of adding other indications that may arise during the selection process.

The relative relevance of the primary sustainability categories and their indicators is represented by their weights in the proposed framework. In general, the two following methods represent the most important methods used to determine the relative weights:

This study used this approach to achieve transparency, rationality, and validity in this framework. All relative weights of the key sustainability categories and indicators were identified through a survey in which questionnaires were distributed to local water and desalination experts. Local specialists have been selected to reflect local perceptions in deciding on the sustainability of the Gaza Strip's desalination plant. In this study, the relative weights of the various sustainability indicators were calculated using the swing method. In contrast, the relative weights of the major sustainability categories were determined and calculated using the hierarchical analysis technique.

Thirty experts were selected from governmental entities and academic fields, which included CMWU, PWA, EQA, and academics from local universities. These experts are highly experienced in strategic planning and the water desalination field. A field survey was conducted by distributing a questionnaire asking all experts to evaluate the importance of the main categories and their indicators.

From the above, the sustainability indicators are different from each other in terms of measurement units, as shown in Table 1, which makes it impossible to compare, collect, or directly integrate, making the need to standardise these different units by using a unified scale that eliminates these units, and therefore obtaining dimensionless indicator. A linear utility function with a domain from zero to one is the method adopted for this purpose and is applied to each indicator separately. A value of one represents the highest value of the indicator, while a value of zero represents the lowest value. As shown in eq. (1) the middle values will depend on the preferred order, while eq. (2) will determine the lowest values representing the desirable option [18].

$Y\left(i\right)=\left\{\begin{array}{ll}0,\hfill & ifi=LowestValue\hfill \\ \frac{i-l}{H-L},\hfill & ifL\text{ < }i\text{ < }H\hfill \\ 1,\hfill & ifi=Highestvalue\hfill \end{array}\right.$ (1)

$Y\left(i\right)=\left\{\begin{array}{ll}1,\hfill & ifi=Lowest\text{ Sub}\hfill \\ \frac{H-i}{H-L},\hfill & ifL\text{ < }i\text{ < }H\hfill \\ 0,\hfill & ifi=Highest\text{ Sub}\hfill \end{array}\right.$ (2)

The units utilised were classified into two types: quantitative and qualitative. Figures and values like volumes, rates, areas, concentrations and so on express quantitative units. However, because qualitative units describe uncountable elements, the units employed for this kind are the same as in a Likert-type scale, as shown in Table 2, to reflect the range of linear utility function.

Indicator weighing techniques can be broadly categorised into equal weighting, weighting based on statistics, and weighting based on public or expert opinion. Giving each indicator the same weight is called "equal weighting". In statistically based weighing, weights are calculated from the statistical characteristics of the data. As opposed to equal and statistic-based weighting, public/expert opinion-based weighing is based on feedback from the participating public or experts, whose opinions ultimately determine the weights assigned to individual indicators [29].

The weights of the primary categories and indicators were established after normalising all indicators. As mentioned, the analytical hierarchy approach will determine the sustainability category weights. In contrast, the weights of the indicators will be determined using the swing technique.

Choosing the right weighting and aggregation techniques for a particular sustainability assessment project is important and tricky. The weighted summation approach is the most used aggregation technique for preference determination [36]. It is used in this research to combine the normalised indicators and their weights into a single score to calculate the desalination plant's global sustainability index (GSI).

Firstly, it was necessary to integrate the normalised indicators and the corresponding weights to obtain the sustainability index for the desalination plant. The next step was to combine them with the weights assigned to the major sustainability categories to determine the final score. The weighted summation method was used in this research to get the overall score as it is the most common MCDA aggregation technique, eq. (12).

$Sf={\displaystyle \sum _{i=1}^I{w}_i\times Is}$ (12)

Where: Sf - the overall score of each category, w_i - the indicator's assigned weight, Is - the indicator normalised score.

The desalination plant's overall score GSI is derived by adding the weighted values of each primary category, eq. (13).

$GSI={\displaystyle \sum _{k=1}^K{w}_k\times Sf}$ (13)

Where: GSI - the total score of the desalination plant under assessment, W_k - the weight assigned to each category, Sf - the category score.

The weights suggested by respondents for each sustainability category are shown in Table 5. These weights were used to calculate the plant's sustainability index, and it was calculated for the Gaza southern desalination plant and cannot be used for other desalination plants.

The Figures from 1 to 5 show the weights of each sustainability indicator in each sustainability category.

Relative weights of technical category indicators.

As illustrated in Figure 1, the highest indicator weight in this category is the amount of energy consumed per cubic meter; this indicator represents the energy required for plant operation and maintenance. The second-highest indicator is the quality of water produced, followed by the recovery rate indicator, and the lowest indicator weight in this category is the ratio of renewable energy used in the plant.

Figure 1.

Technical indicators' relative weights

Relative weights of environmental category indicators.

As illustrated in Figure 2, The highest indicator weight in the environmental category is the groundwater effects, followed by land used, effects on the marine environment, salinity increase, air emissions, and noise level. The lowest indicator weight in this category is temperature increase due to discharged brine.

Figure 2.

Environmental indicators' relative weights

Relative weights of economic category indicators.

As illustrated in Figure 3, The highest indicator weight in the economic category is the unit cost of desalinated water measured as USD/m³, followed by the financial revenue rate indicator, fuel cost, spare parts cost, and human resources cost. The lowest indicator weight in this category is the chemicals' cost.

Figure 3.

Economic indicators' relative weights

Relative weights of social category indicators.

As illustrated in Figure 4, the level of water quality acceptability gets the highest weight under this category. It is followed by affordability, willingness to purchase, and level of community awareness, while the provision of employment gets the lowest weight under this category.

Figure 4.

Social indicators' relative weights

Relative weights of institutional setup category indicators.

As illustrated in Figure 5, the presence of government support gets the highest weight under this category. It is followed by knowledge of employment staff and institutional compatibility. The training and development of the staff gets the lowest weight under this category.

Figure 5.

Institutional setup indicators’ relative weights

Table 6 shows all weights results of sustainability categories expressed by w1, the relative importance weight of the indicators under each category expressed by w2, and the final weight W of each indicator measured by w1×w2.

Table 6.

Summary of sustainability categories and indicators weighting

Sustainability Categories	Category weight w1	Indicator weight w2	Final weight W=w1×w2
Technical	0.277
Energy consumed [kW/produced m3]		0.291	0.081
Recovery rate %		0.237	0.066
Quality of water produced		0.257	0.071
Ratio of renewable energy used		0.215	0.060
Environmental	0.085
Land used "the possibility of expansion"		0.160	0.014
Groundwater effects (in case of beach well intakes)		0.161	0.014
Salinity increase		0.127	0.011
Temperature increase		0.087	0.007
Effects on the marine environment (alga, topography, seawater quality, etc.)		0.128	0.011
Carbon dioxide emission “CO2” [kg/m3]		0.118	0.010
NOX& SOx emissions [g/m3]		0.104	0.009
Level of noise		0.114	0.010
Economic	0.309
Human resourses cost		0.140	0.043
Fuel costs		0.180	0.056
Spare parts cost		0.156	0.048
Chemicals usage cost		0.134	0.041
Unit cost of desalinated water [USD/m3]		0.210	0.065
Financial revenue rate		0.181	0.056
Social	0.093
Level of water quality acceptability		0.222	0.021
Level of community awareness		0.196	0.018
Provision of employment		0.153	0.014
User Ability to Pay “Affordability”		0.213	0.020
Willingness to purchase		0.216	0.020
Institutional set up	0.236
The knowledge of the operation staff (Level of education)		0.242	0.057
Training and development of the staff		0.235	0.055
The presence of government support		0.284	0.067
Institutional compatibility		0.239	0.056
Sum	1.000		1.000

The standardisation procedure is defined for each indicator once the indicators and their respective weights have been established to provide a standard measuring scale for all indicators with varying unit types to calculate the overall score. The measurement unit type and the indicator's nature are the key determinants of standardisation methodology.

All resulting criteria are quantified based on each indicator unit. The figures were driven from the data collected from the operation and maintenance staff of the southern short-term low-volume (STLV) desalination plant in addition to the design report and the EIA study conducted for the desalination plant.

By using eq. (1) and eq. (2) for the indicators that were measured quantitatively, and the Likert-type scale presented in Table 2 for the indicators measured qualitatively, Table 7 summarises the normalised scores for all selected indicators.

The total score of each category was measured by multiplying the weight assigned to each indicator under each category by the indicator normalised score, eq. (12). The results show that the technical category has a sustainability index of 71%, indicating that this plant's desalination process is going well because the water quality is within WHO guidelines. In addition, the average energy consumption per cubic meter and the recovery rate are within acceptable ranges. Regarding the environmental category, the results show that this category gets a sustainability index of 72% since the significant environmental risks are low from the desalination plant project.

Due to the resultant index of 68% in the economic area, the southern STLV desalination plant is regarded as only partially sustainable. The primary markers of this score are the high relative weight of the economic category and the lack of sustainable financial resources for the operation and management of desalination. The large decline in the revenue rate is another crucial metric that aids in the partial economic viability of a plant.

The results showed that the institutional category got a sustainability index of 56%. In comparison, the social category got the lowest sustainability index of 49%. Figure 6 shows the total score for the five sustainability categories. Each category defines the strength of choice according to sustainability ranking, as shown in Table 8.

Figure 6.

Main sustainability categories' total scores

These classifications and degrees were proposed based on similar literature. These indexes were used by Salem [37] to evaluate the sustainability of the north Gaza wastewater treatment plant. Another similar classification was used by Balkema et al. [38] in proposing a general assessment methodology that builds on multi-objective optimisation and a complete set of sustainability indicators.

Finally, the southern Gaza desalination plant global sustainability index score (GSI) was obtained by applying the weighted summation method, eq. (13), which integrates the sustainability categories scores with the weight of each category, the result of the GSI of the desalination plant is 65%, and according to the framework and as indicated in Table 8, the plant is considered a partially sustainable plant in the ranking scale of sustainability.