A strategic approach to sustainable wastewater management and reuse in Saudi Arabia

The Kingdom of Saudi Arabia (KSA) faces challenges in protecting its limited natural water resources. The centralized sewage treatment plants (STPs) contribute to conserving natural water resources in various regions of KSA. For sustainable wastewater management strategies in arid environments, the performance of STPs needs to be assessed for various beneficial uses in addition to the existing regulations. The present study used extensive effluent quality data of four STPs to develop a performance assessment approach using multi-criteria and principal component analyses. The performance levels were derived from the compliance rate of effluent standards with different reuses and the efficacy of biosolids management. The proposed approach evaluated the STPs based on the planned applications of treated effluents, considering the land use characteristics of each plant’s disposal point. The factors loadings obtained for the STPs, encompassing total suspended and dissolved solids, and total dissolved solids, oxidizing demanding pollutants (BOD and COD), and nutrients (NO3 and PO4), with 3 to 4 principal components demonstrating above 60% of the effluent monitoring data, established the importance of the effluent quality parameters. The effluent quality index (EQI) was developed for existing and potential reuses, e.g., irrigation, landscaping, fishery, and recreation, during dry (summer) and wet (winter) periods. All STPs showed high EQI for both periods for the present reuse standards of unrestricted irrigation and landscaping. The overall performance index, aggregating EQI and the efficacy of sludge management practices, illustrated declining performance, suggesting staged improvements (tertiary-level treatment, air floatation, sludge digestion, and waste-to-energy) through a periodic assessment process. STPs must enhance their sludge management efficacy to produce Class-A biosolids and waste-to-energy. The proposed approach will help make strategic decisions regarding improvements for STPs and the allocation of financial resources to protect natural resources in various regions across KSA.


Introduction
Ever-growing population and climate change inexplicably impact the curbed freshwater resources, particularly in dry climatic regions like the Kingdom of Saudi Arabia (KSA) [1].Receiving treated effluents from Sewage Treatment Plants (STPs), low-flow and dry rivers in the Kingdom of Saudi Arabia (KSA) serve various intended uses without continuous flows [2].For developing effective circular economy-based water resources conservation strategies, wastewater reuse's socio-economic and environmental benefits establish the desired STPs' effluent quality in a city or a subregion [3].An increasing gap between water availability and demand due to low groundwater recharge rate and increasing water demands has signified the environmental value of treated assessment framework appraising seasonal variations in effluent quality and various beneficial uses of dry rivers, (ii) identify the significant variables and corresponding weights for dry and wet periods using PCA, and (iii) develop EQI, SEI, and OPI for dry and wet periods using Technique for Order Preference by Similarity to the Ideal Solution (TOPSIS) to provide aggregated performance score for inter-city performance assessment of four STPs operating in Wadi Rumah catchment in Qassim Province of KSA.The present study's findings will help prioritize the STPs to plan improvement in performance efficiency or additional processes for augmenting reuse applications in future years.

Performance assessment framework
Figure 1 presents the performance assessment methodology developed to meet the objectives of the present study.The process begins with defining the study area boundary, which depends on the size and importance of the STPs in the region.Each STP needs to be evaluated for the reuse criteria, followed by effluent standards for each potential reuse application.Effluent water quality monitoring was categorized for summer and winter season data sets.Effluent quality was evaluated regarding compliance rate by comparing the measured concentrations with the applicable reuse limits.PCA estimated the variability of the data sets, and the importance weights of water quality parameters were estimated using PCA results for both seasons.Next, the multi-criteria analysis methods aggregated the scores and importance weights to generate EQI and OPI for each reuse application.Based on the reuse criteria, STPs were assessed for potential reuse applications, and improvement actions were recommended for water resource protection and effective biosolids management.The following sub-section explains the details of each component of the framework.

Description of study area and potential reuses
Wadi Rumah is the longest (∼ 2,000 km) river in Saudi Arabia, with two distinct dry and wet seasons in Wadi flow (figure 2).The present study encompassed the 100 km reach of the Wadi, passing through the Qassim Region, well-known for its extensive agriculture and livestock in the KSA.The region experiences two seasons concerning Wadi: the dry or summer season from June to October and the wet or winter season from November to May.Most of the year, the Wadi remains dry, with wastewater ponds collecting outfall overflows (treated effluent) after meeting agricultural demands.Winter depicts a wet season with low-intensity to high rainfalls, occasionally resulting in flash floods.The study area consists of the section of the Wadi receiving treated effluent from four central STPs covering significant cities of the Qassim Region.Figure 2(b) defines the treatment processes at the STPs.
A greener landscape of the Qassim Region enhances the potential for recreational activities (e.g., fishing and boating) along the Wadi's front, particularly during the wet season.As the wet season lies in winter, the potential of swimming is almost negligible.Figure 2 shows that STP-1 serves Buraydah (the Capital of Qassim with a 714,000 population), and STP-2 receives wastewater from another city -Unayzah, near Buraydah.The outfall of STP-3 is located on the northwest side of Al Rass City and of STP-4 at the southeast of Al Bukayriah City.STP-4 service area covers the governorates of two cities, Al Khabra and Riyadh Al Khabra, in addition to Al Bukayriah's population.Around 90% of the treated effluent meets Al Bukayriah's agricultural and landscaping demands.Municipality and private vehicles buy the treated wastewater at a meager cost from the outfall's management, as shown on the right side of figure 2(b).Meanwhile, the excess effluent, particularly during the wet (winter) period, is discharged directly to the Wadi Rummah joining river water.Figure 2(b) illustrates a typical process flow diagram of the four STPs in the study area.

Performance indicators
The performance assessment process aims at continuous improvement by inviting service providers in a region operating in more or less similar conditions, such as the organization's size, personnel, environmental conditions, and financial resources [21].Although the sizes of the STPs are different in terms of flow, as shown in table 1, other conditions are similar.Therefore, their inclusion for assessing their performance based on effluent compliance for multiple reuse applications is reasonably rational.Identifying the potential uses for each STP is crucial for continuous improvement-based performance assessment.Table 2 outlines the criteria used to recognize potential uses for each treatment facility.
The concept of continuous improvement for urban water infrastructure, as developed by Bereskie et al [22], appraises the performance (mainly in indices) against a predefined level of service.In the performance assessment of STPs, the level of service refers to the regulatory effluent standards for different reuse applications (see table 2).The methodology for performance assessment in the present study compares the performance of all four STPs in terms of the effluent quality index (EQI) for existing (2023) and potential beneficial uses in the future to develop effective wastewater management strategies.Notably, the performance of an STP remains constant in the case of no additional beneficial use, e.g., the existing reuse of unrestricted irrigation (URI) is the only reuse application.The concerned organizations, such as the National Water Company (NWC) and municipalities, can allocate funds to the underperformers with the potential to reuse treated effluents for multiple reuses requiring higher treatment levels.The assessment interval can be defined based on the participant's willingness and available reuses.For instance, the National Water and Wastewater Benchmarking Initiative (NWWBI), the first Canadian Infrastructure Benchmarking Initiative, is organized annually [23].Regarding STPs in Saudi Arabia, the stakeholders can decide on the assessment intervals, e.g., NWC, Desert Well Water Treatment Company, and participating municipalities.Without local reuse criteria for fishery, livestock, and recreation in KSA, table 2 presents the reuse criteria values obtained from the literature for other world regions.

Effluent quality compliance indicators
The treated effluent quality data was collected from the STP administrations for (i) physical parameters, including total dissolved solids (TDS), total suspended solids (TSS), (ii) chemical and biological parameters: pH, 5-day biological oxygen demand (BOD 5 ), chemical oxygen demand (COD), ammonia-nitrogen (NH 3 -N), nitrate-nitrogen (NO 3 -N), phosphates (PO 4 -P), residual chlorine (Cl 2 ), total coliforms (TC), and fecal coliforms (FC).The plants have frequently monitored these parameters due to their environmental significance and health-related concerns.Table 2 presents the effluent quality standards for potential reuses and the associated potential adverse environmental and health impacts.
The adverse impacts of the reuse of treated effluents originate through soil, groundwater, and surface water contamination, eventually affecting plants' growth, aquatic life, and human health through direct consumption of the food chain [34].As the blue-green algae's blooms can develop in almost stagnant treated water reservoirs, combined levels of NO 2 -N and NO 3 -N must be maintained below 100 mg L −1 to protect livestock drinking [32,33,35,36].Human health can be exposed to direct (recreation: swimming and boating) and indirect (consumption of raw vegetables and drinking of shallow groundwater) risks associated with reusing treated effluent.The absence of indicator organisms, total coliforms (TC) and fecal coliforms (FC), minimizes these health risks from various exposures [37].As wild animals and free-grazing livestock can assess the ponds formed from effluent discharge during dry periods, removing microbiological contaminants is essential for their protection [38].Polluted water can affect the quality of milk produced by livestock and, eventually, human health [39].Table 2 shows higher stringency in effluent standards with an increase in number of reuse applications.
Effluent quality was tested following the standard methods for the Examination of Water and Wastewater Analysis developed by the American Public Health Association (APHA) [40].The following equipment  • high TDS can impact soil salinity at the application point [24].
• TDS < 1000 mg l −1 does not burden livestock or poultry [26].monitored various parameters: TDS and pH (HACH 440d multi-parameter meter), TSS (gravimetric method), BOD5 (standard dilution method), COD (reactor digestion method), NH 3 -N (HACH-DR 5000TM UV-vis spectrophotometer), NO 3 -N and PO 4 -P (acid persulfate digestion), and coliforms using Quanti-Tray [40].Due to a lack of monitoring observations and the complete absence of available data, coliforms were not considered in PCA and the development of EQI.

Sludge management efficacy indicator
The existing sludge management process at the STPs consists of gravity thickeners and drying beds.Nevertheless, the facilities are motivated to use effective sludge treatment to produce Class-A biosolids and to follow KSA's vision 2023 sustainability targets [29].The performance assessment process proposed in the present study appraised the existing and future sludge management practices using a knowledge-based scoring scheme.The scheme was established based on expert opinion, considering the environmental benefits of sludge management.The facilities with the current sludge treatment process, including sludge thickening, conditioning, and dewatering, will get a performance score up to 50 (depending on system efficiency) out of 100 as the sludge that goes to the sludge disposal area affects the natural environment less than the land disposal of untreated sludge does.Additional treatment processes, air floatation, and sludge digestion can produce Class-B or Class-A biosolids, which would get a score between 50 and 80, and the facilities with a complete biogas collection and energy generation process would be able to secure performance scores between 80 and 100.

Principal component analysis
The principal component analysis (PCA) is a well-recognized multivariate technique used to minimize the dimensionality of extensive data and identify the most dominant parameters in a given water source [16,41].As the process interprets the principal components established by basic variables (water quality parameters), the resultant matrix consisting of factor loadings represents the more rational weights of the basic parameters for EQI estimation than expert judgment [42].
The principal component analysis (PCA) assessed the importance of different quality parameters in the three-year effluent data obtained from the STPs in the study area.PCA is a well-known multivariate statistical analysis method for minimizing the correlated data into fewer uncorrelated components [43].The process develops a correlation matrix to calculate the principal components.The present study used Varimax orthogonal rotation with Kaiser Normalization, which has been considered simpler than more complex rotation methods [44].The correlated variables were orthogonally multiplied with the eigenvectors to obtain the uncorrelated variables or principle factors [45].Statistical Package for the Social Sciences (SPSS) performed PCA in the present study.

Effluent quality index
The following steps developed the OPI for the performance assessment of STPs using the Technique for Order Preference by Similarity to the Ideal Solution (TOPSIS) [46]: Step 1: The weights of each effluent quality parameter 'w j ' were estimated using the results of the rotated component matrix.
Step 2: Calculate the compliance rate of meeting the desired standards for STPs' effluents.The exceedance rate (Er) represents the number of times the value of any effluent quality parameter surpassed the desired reuse criteria.Er values of all the parameters were estimated for both seasons as: Where 'm' is the rank of the sorted monitored data and 'n' represents the total monitoring data points.As the performance score should have a positive polarity, the estimated Er was transformed into the compliance rate with the reuse criteria (C c ) with the help of equation (2).
= - As Cc values range between 1 and 100, the data need not be normalized.
Step 3: The weighted matrix, consisting of all the effluent parameters for each reuse application, was developed using the following equation: where v ijk is the weighted performance score of each STP' i' for each resue application 'k', x ijk represents the probability of meeting the reuse criteria for each effluent parameter 'j', and w j is the corresponding importance weight estimated in step 1.
Step 4: Using the concept of the distance of performance from the positive ideal solution (PIS) defined as X * and the negative ideal solution (NIS) defined as X − , equation (4) estimated X * and X − .
Step 5: The n-dimensional Euclidean distances of weighted performance scores were measured regarding effluent quality compliance with the desired standards.Equation (5) defined the distance of each parameter from Where W 1 and W 2 are the corresponding weights, the present research used 0.8 for W1 and 0.2 for W 2 .A higher weight was allocated to EQI because of its direct association with the conservation of high-value groundwater and environmental protection -the main objectives of STPs.
Step 8: Assess water quality for future improvement actions.Table 3 defines the classification for EQI and OPI used for this study for performance assessment of STPs in desert climatic regions.

Water quality compliance
Figure 3 illustrates the calculated compliance rate (Cc) with the three reuse criteria (RC) in table 2. Three variations in Cc values affirm the need for performance assessment: (i) parameter variations in complying with multiple reuses for each plant, (ii) seasonal variations for each plant, and (iii) performance variations amongst the participating STPs.Cc values less than 0.9 (90%) suggest a need for improvement.Consider the case of STP-1 for the dry period in figure 3(a), showing excellent compliance (Cc > 0.9) for RC-1, medium (Cc < 0.7) for RC-2 due to non-compliance of TDS, NH 3 , and very low (Cc < 0.5) for RC-3 due to failure of complying with desired guideline values for TDS, BOD 5 , and PO 4 .Considering the temporal variations in the performance of STP-1 for RU-2, figure 3(a) illustrates that the plant performed low to medium (Cc = 0.696) in the dry period and very high (Cc = 0.981) during the wet period, showing complete nitrification during the wet period.Figures 3(b) to (d) illustrate similar findings for other STPs.Variations in compliance levels across the participating STPs are also evident in the figure.For instance, STP-2 shows higher Cc values to meet effluent quality criteria for RC-2 in dry and wet periods.

Principal component analysis
Table 4 and table 5 present the results of PCA in the form of rotated component matrices for STPs during dry and wet periods.The present study adopted varimax with the Kaiser normalization rotation method.Three principal components with eigenvalues greater than '1' were extracted for the dry season of STP-1 (table 4).These three principal components collectively accounted for more than 63% of the variability in the dataset.PC1 accounted for 28.4% of the data variance with positive loading of BOD 5 , COD, and NH 3 -N and strong negative loading of NO 3 -N.Thus, PC1 is suitable for describing the variation of organic and nitrogen compounds in the treated water.There is a tendency for higher concentrations of BOD5, COD, and NH 3 -N, while NO 3 -N concentrations decrease.PC2 accounted for 18.9% variance with strong positive loading of PO 4 -P, indicating the phosphorous variations in the treated water.Finally, PC3 accounted for 16.7% variance with strong positive loading of TDS and TSS PC3 is associated with variations in ion activity components, as indicated by TDS, and particulate matter, as indicated by TSS.The PC analysis for STP-1 in the wet season extracted 3 PCs, accounting for 60.5% of the variances.Like the dry period, PC1 has a strong positive loading of TSS, BOD 5, and COD, PC2 has a strong positive loading of NO 3 -N and PO 4 -P, and PC3 has a strong positive loading of NH 3 -N.Based on the PCA results, the treatment performance of STP-1 is consistent between the wet and dry seasons, as indicated by the similar loading patterns of the principal components.Overall, the variation of organic and nutrient concentrations significantly influences the plant's effluent.
In the dry season, the PCA of STP-2 four principal components and accounted for a 65.4% variance in the dataset.PC1 accounted for a 24.82% variance with a positive loading of TDS and PO4-P.Meanwhile, PC2 accounted for a 16.87% variance with a significant loading of BOD 5 and COD.PC2 is attributed to the organic properties of treated effluent.A significant loading of NO 3 -N in the PC 4 is associated with the nutrient removal performance of the STP.The PCA results for the wet season dataset revealed 62% total variance with 3 PCs.PC1 is significantly loaded with PO4-P, TSS, and TDS, while PC3 has strong BOD 5 and COD loadings.In contrast to the dry season, the wet season PCA did not show significant loadings of NO 3 -N and NH 3 -N in any of the PCs.These results suggest that during the wet season, the STP's performance may not be influenced by these nutrient removals to the same extent as observed in the dry season.
Tables 4 and 5 present the PCA results of the STP-3 for dry and wet periods.For the dry season, four extracted PCs accounted for 63.97% of the variance in the data set.PC1 accounted for 23.16% of the variance and has significant TDS loadings, indicating the ion activity association in the treated water.PC2 accounted for 16.37% variances and has significant loadings for BOD5 and COD, indicating the association of organic quality of the treatment plant effluent.PC3 accounted for 14.4% variance and had loadings for NO 3 -N, while PC4 accounted for 10.4% variance and had loadings for NH 3 -N and PO 4 -P.The results suggest that during the dry season, TDS (PC1), organic quality (PC2), and nutrient concentrations (PC3 and PC4) significantly influence the effluent quality of STP-3.For the wet period, four PCs were extracted with a total of 65.12% variance.PC1 accounted for a 20.54% variance with positive loading for BOD 5 and COD.PC2 accounted for 17.13% variances and has significant loading NO 3 -N and PO 4 -P.PC3 accounted for 14.52% of the variance, with a considerable loading for TDS, while PC4 contributed 12.94% of the variances and a significant loading for NH 3 -N.The overall results suggest that during the wet season, the effluent quality of STP-3 is significantly influenced by variations in organic pollutants, nutrients, and ion concentrations.
In the case of STP-4 during the dry season, three PCs accounting for 48.55% of the data variance were extracted.Similar to STP-3, PC1, with 23.46% variance and significant TDS loading, indicated an association of ion activity with treated effluent.PC2 accounted for 12.92% variances and has significant loadings for BOD5 and NH 3 -N, indicating the association of organic parameters to the effluent.PC3 accounted for a 12.16% variance with significant NO 3 -N loading.Likewise, three PCs were extracted for STP-4 for the wet period with a 45.84% variance.PC1 accounted for a variance of 21.65% with positive loading for TDS and PO 4 -P.PC2 accounted for 12.84% variances and has significant loading TSS, COD, and NH 3 -N.PC3 accounted for an 11.35% variance, showing a substantial loading for NO 3 -N.As per the overall results for the wet season, the STP-4's effluent quality is significantly influenced by variations in organic pollutants, ion levels, and nutrients.
For a clear perspective of the characteristics of STPs' effluents, principal components' score plots of the first three PCs for the entire study period were calculated and analyzed.Figure 4 presents the principal component scores for all plants during the dry and wet periods.For STP-1, high fluctuation of effluent quality was observed throughout the study period (figures 4(a) and (b)).PC1 was related to the organic and nutrient quality for both seasons, while PC2 was initially associated with the PO 4 -P in the dry season and TDS in the wet season.PC3 was related to the TDS, TSS, and BOD 5 in the dry season and NH 3 -N in the wet season.A positive score of PC1 (figure 4(a)) was mainly observed during the early dry season (Jun-Sep) and the late wet season (Jan-May), indicating a higher concentration of organic and nutrients during these periods.On the contrary, the negative PC score was found during the late dry and early wet seasons, indicating better effluent quality in organic and nutrient concentrations.The positive score of PC2 (figure 4(a)) during the late dry season (Jul-Sep) and the early wet season indicates higher TDS levels in the effluent during these periods, whereas the concentrations of TDS in the effluent reduced during the early dry season and the late wet season, as indicated by the negative scores of PC2.The high variability in PC3 scores indicates a dynamic effluent quality profile at STP-1.Fluctuations in TSS, TDS, and BOD 5 during the dry period and NH 3 -N during the wet period contribute to the ununiformed removal performances of these parameters.
Figures 4(c) and (d) shows the PC scores of STP-2 for the entire dry and wet period.PC1 scores related to the TDS and PO 4 -P in the dry period and TSS, TDS, and PO 4 -P in the wet period show high fluctuation except during the late wet season (Mar-May).Results suggested that concentrations of TDS and PO 4 -P of this plant varied significantly throughout the year, which may be unsuitable for specific recycling purposes such as fisheries.The negative score of PC1 in the late wet period indicates that TSS, TDS, and PO 4 -P were relatively lower in the plant effluent during this period.PC2 was initially related to the BOD 5 and COD in the dry seasons, while PC3 was associated with BOD 5 and COD in the wet period.The positive scores of PC2 and PC3 during both early dry and early wet periods suggest higher levels of organic pollutants (BOD 5 and COD) in the effluent.The negative scores during late dry and late wet periods indicate a reduction in organic content, possibly due to treatment methods or environmental factors (figure 4(b)).
The observation of highly fluctuating PC scores for STP-3 and STP-4 during both dry and wet periods suggests significant variability in the effluent water quality, including TDS, organic, and nutrient concentrations (figures 4(e)-(h).Fluctuations could be attributed to variations in the efficiency or performance of the treatment processes at STP-3 and STP-4.Environmental conditions, such as variations in flow rates, or influent composition, may contribute to the observed fluctuations.The highly fluctuating PC scores indicate the need for further investigation into the factors contributing to the variability in effluent water quality.Identifying the sources of fluctuation can inform strategies for improving treatment efficiency and maintaining more stable effluent characteristics.

Effluent quality index
The step-by-step procedure outlined in section 2.5 estimated the EQI for all plants' dry and wet periods.First, the weights of each parameter 'w j ' were estimated from the rotated component matrix.For each effluent quality parameter, the maximum loadings obtained from PCA for all the plants and the loadings were translated into importance weights using the normalization method (see first row in table 6).Nutrients (NO 3 and PO 4 ) obtained the highest weights, followed by BOD, COD, and Ammonia, and TSS and TDS at the last.Microbiological parameters (TCs and FCs) were not included in the PCA due to a lack of data; consequently, Cl 2 was also excluded.At the same time, effluent flow is a function of the city's size, and pH and temperature are almost consistent across the study area; therefore, they are excluded from developing the EQI.The next step estimated the Cc, using equation (2) values as given in table 4 and table 5 for dry and wet periods.In the subsequent step, equation (3) developed the weighted matrices for all the reuse applications in both seasons.The results with PIS and PNS are presented in table 6.After calculating separation measures using equation (4) and n-dimensional Euclidean distances from equation (5), and similarities to the positive ideal solution using equation (6), EQI was estimated with the help of equation (7), and the OPI using equation (8). Figure 5(a) presents the performance assessment results for the four STPs based on their water quality compliance for three wastewater reuses defined in table 1 and table 2. Considering the sludge management efficacy index (SEI ik ), equation (8) estimated the OPI ik for the STPs, and the results are illustrated in figure 5(b).
Figure 5(a) shows that the STPs' performance is very high, considering water quality compliance with the existing reuse standards (RU-1: Unrestricted irrigation and landscape applications) during dry and wet periods.Shortly, STP-1 will be completely renewed and relocated to primarily serve long-term agricultural and ecological applications.Therefore, its performance will remain consistent without complying with more strict reuse standards (RU-2 and RU-3).Treated effluent of STP-2 irrigates a large landscape area of Unayzah City, and some amount is used for cooling in the local industry.Considering a periodic increase in wastewater flows during the near future (the next three-year assessment period), the STP-2 does not need to improve its treatability performance to comply with RU-2.
Nevertheless, in the long-term, STP-2 may need to progress towards RU-3 in the wet period with process enhancement, e.g., reverse osmosis, to meet TDS criteria of less than 450 mg L −1 .STP-3 and STP-4, serving relatively smaller cities of the Qassim Region, discharge their effluents to Wadi Rumah in a wet period.The low scores during the wet period reflect inadequate compliance with the existing treatment processes: fair for RU-2 and marginal for RU-3.
Figure 5(b) demonstrates the performance assessment results for the OPI of the STPs.The current sludge management practices are not comprehensive, as described in section 2.3.2,reflected by good performance scores.The results suggest improving sludge management practices for all STPs in the Qassim Region.The case of STP-1 is a practical example.A new plant with complete sludge collection, treatment, and disposal processes, including gravity thickeners, dissolved air floatation, homogenization tank, thermophilic digestion, sludge buffer tank, centrifuge decanters, and solar drivers, will replace the existing basic level sludge management facility in the coming year.The plant will also be equipped with biogas recovery and energy generation units.The red hollow bars represent the expected OPI for STP-1 during future performance assessment.The OPIs for other plants eventually decline with their marginal performance regarding sludge management practices.

Discussion
The performance assessment process provides valuable and crisp information to regulators, policymakers, and Wadi users [4].A long stretch of Wadi Rumah adds to landscape and agricultural benefits while traversing the Qassim Province.The residents of smaller towns and some villages transport the domestic sewage, stored in septic tanks in their houses, to the nearest centralized STPs [47].The four STPs in the present study are of primary significance regarding regional wastewater management.The present study presented a performance assessment framework for improving the sustainability of water resources in the region.Extended aeration-type activated sludge STPs' efficiently meet promulgated wastewater reuse standards for unrestricted irrigation and landscaping in the Qassim and other regions in KSA [48,49].Considering the water scarcity in the country, the study proposed periodic upscaling of the STPs to comply with other reuses with stricter effluent quality criteria.In addition, the performance assessment framework addressed the sludge management-related inadequacies.The present study developed two types of performance indices.The first index is the EQI, aggregating the water quality compliance of different effluent quality parameters (i.e., performance indicators).Generally, an index contains performance scores and relative weights of performance indicators.Performance scores were estimated through exceedance probabilities for each parameter over the required guideline value.The weight distribution obtained from PCA appears rational, giving relative importance to all parameters without significant differences (Coefficient of Variation ≈10%).As the importance of parameters may change with stringent reuse criteria for each coming assessment period, such weighting schemes help establish benchmarks with continuous improvement objectives.TOPSIS method aggregated the scores and importance weights to generate the EQI.In a similar recent study, Arabzadeh et al [50] evaluated the reliability of treated effluent for wastewater reuse with the help of EQI in Bushehr Province of Iran.PCA has frequently been used to assess the water quality of surface water systems [17][18][19] and effluents of STPs [15,51,52].
The present study considered all the PCs significant with an Eigenvalue of more than 1.As a result, three PCs were selected for STP-1 and four PCs for STP-2, STP-3, and STP-4 in the dry period, representing 63.7%, 65.4%, 64%, and 56.7% data.For the wet period, four PCs were considered for STP-1 and STP-3, accounting for 70.8% and 65.2% of the data, while three PCs were found significant for STP-2 and STP-4, characterizing 62% and 45% of data.The PCs in the dry period showed different behaviors across the STPs.For instance, PC1 in STP-1 highlighted BOD5, COD, and NH 3 -N, while in the case of STP-2, TSS, TDS, and PO4 were the parameters with the most significant variation.STP-3 again signified the importance of BOD5 and COD in PC1.Similar findings can be seen in table 5 for other PCs.TSS is an important parameter to ensure the efficient working of sprinkler irrigation systems, particularly in landscaping applications [53].TDS influences multiple reuse applications, such as irrigation, livestock drinking, and recreation.The PCA also identified BOD5, COD, NH 3 -N, NO 3 , and PO 4 as important parameters, which is consistent with the past studies using PCA by Bayo and López-Castellanos's [54] for an STP in Spain and Ebrahimi et al [20] in Louisville, Kentucky, Ohio, USA.
The weighting scheme used for OPI estimation seems predisposed towards EQI due to its prominent contribution to the main objectives of STPs in the country-groundwater conservation and protection of surface water sources [2,3].The framework proposed in this study provides a strategic approach for STPs in arid regions toward continuous improvement for wider reuse applications.For the framework's applications in other areas, the proposed weighting scheme can be revised for site-specific objectives.The present study primarily evaluated the STPs based on their effluent quality compliance and biosolids management practices.The operational cost is supposed to increase by enhancing the treatability of STPs for future reuse applications, and minimizing operational cost is another critical objective that should be considered for the holistic assessment of STPs.Vrecko et al [55] developed, and Flores-Alsina [56] applied an operational cost index for assessing the objective of minimizing the operational cost of STPs.
Appraising seasonal variations in effluent quality gains special attention in desert environments, like the KSA, where almost all the treated effluent is utilized (or will be utilized by 2030) in the dry (summer) season, primarily for irrigation and landscape applications.In the wet (winter) season, when natural rainfall meets some of the irrigation and landscape requirements, the STPs' overflows discharge in the Wadi Rumah.Despite flash floods, short or long ponds formed by mixed-treated effluents and surface runoff also have multiple reuses, such as seasonal fish, livestock drinking, and recreation [2].In the case of more extended rainfall events resulting in flowing conditions, these overflows should not affect the water quality after mixing with the rainwater.Nevertheless, during minimal rainfall events, the highly treated effluent can be stored for beneficial uses instead of mixing rainwater with agricultural runoff.Considering the water scarcity and the sustainable utilization of natural resources, the existing plants should be upgraded to meet multiple reuse criteria in the future.Plant management needs to monitor the performance of their treatment systems carefully.The study identified the need for substantial improvement in sludge management practices.

Conclusions
Water resources in arid lands need cautious efforts in addition to meeting basic reuse applications and sludge management practices.The performance assessment process allows sewage treatment plants to compare their performance regarding water quality compliance and sustainability of sludge management practices.Principle components analysis and TOPSIS can effectively develop an EQI based on compliance of various parameters to given reuse standards and an OPI, including the efficacy of the sludge management process for a treatment plant.For the dry period, three PCs were selected for STP-1 and four PCs for STP-2, STP-3, and STP-4 in the dry period, representing around 60% data.For the wet period, four PCs were considered for STP-1 and STP-3, accounting for more than 65% of data, while three selected PCs for STP-2 and STP-4, characterized 62% and 45% of data, respectively.
The selected components effectively covered solids, oxidizing demanding pollutants, and nutrients in the treated effluents.Factor loadings assessed by PCA established the importance of the effluent parameters for developing the EQI.The study found that all four participating plants obtained excellent performance scores (EQI > 95) in complying with promulgated reuse standards for unrestricted irrigation and landscaping applications.Nevertheless, some plants (with very low to low EQI) would need to improve their tertiary treatment processes to comply with potential future reuse applications, such as fishery and livestock drinking.
All the plants need to enhance their sludge management efficacy to achieve long-term sustainability objectives of producing Class-A biosolids and waste-to-energy.The proposed performance assessment framework will help decision-makers define short-and long-term improvement actions for wastewater treatment facilities in KSA and other arid lands.Disseminating assessment results can motivate underperforming entities and enhance public awareness of sharing responsibilities for conserving limited natural resources.

Figure 1 .
Figure 1.Performance assessment methodology for sewage treatment plants in desert environments.

Figure 2 .
Figure 2. Study area: (a) map showing the location of sewage treatment plants in Wadi Rumah's catchment, (b) treatment process flow diagram.

Figure 4 .
Figure 4. Temporal variability of principle components score plots (a) STP-1 in the dry period, (b) STP-1 in the wet period, (c) STP-2 in the dry period, (d) STP-2 in the wet period, (e) STP-3 in the dry period, (f) STP-3 in the wet period, (g) STP-4 in the dry period, (h) STP-4 in the wet period.

Table 1 .
Existing and potential beneficial uses for continuous improvement of the four STPs in the Qassim Region.

Table 2 .
Effluent standards for potential wastewater reuse applications in Qassim.

Table 4 .
Rotated component matrix for STPs during the dry period.

Table 5 .
Rotated component matrix for STPs during the wet period.

Table 6 .
Weighted matrices for four STPs during the dry period.