Performance, Salinity Constraints, and Agricultural Reuse Potential of Treated Wastewater in a Hyper-Arid Oasis: A Case Study of the Timimoun WWTP, Southern Algeria

Abstract

Today, the reuse of treated wastewater is considered an important and strategic key driver of integrated and sustainable water and soil management in extremely arid desert regions, where significant constraints due to water scarcity, soil salinization, and the fragility of agricultural ecosystems within palm oases place a strain on all sustainable development policies. Through this study, we conducted a comprehensive evaluation of the performance of the treatment, as well as the constraints related to salinity and the implications for land management of the activated sludge wastewater treatment plant located in the Timimoun desert oasis in southern Algeria. Through monthly monitoring over a 12-month period, we conducted an analysis of physicochemical, nutritional, and microbiological parameters, as well as a seasonal analysis, in addition to calculating irrigation suitability indicators using first-order kinetic modeling of COD degradation. The results showed high reduction rates for COD (90%), BOD₅ (90.5%), and TSS (93.8%), confirming the resilience and effectiveness of biological treatment under very difficult and hostile climatic conditions. Furthermore, the ultraviolet disinfection process ensures microbiological quality that enables the reuse of treated water for agriculture. Despite this, the treated wastewater exhibited moderate salinity and sodicity levels, reflected by EC values ranging from 2.4 to 2.8 dS/m and an SAR value of 6.2, which remain important limiting factors for the long-term sustainability of wastewater reuse. Therefore, this study provides valuable scientific data for developing sound and sustainable water and land management policies in the harsh climate of Saharan oases.

1. Introduction

Sustainable wastewater management in hyper-arid regions poses a major challenge to water security, soil conservation, and the resilience of agricultural ecosystems in oases. Extreme evaporation, the near absence of groundwater recharge, and the natural accumulation of salts in desert environments impose significant biogeochemical constraints that directly and effectively impact the functioning of biological purification processes [1,2]. In this context, the reuse of treated wastewater is considered a key strategy to reduce the depletion of deep aquifers and to support agricultural production in oases [3,4].

Most studies conducted in arid regions have shown that wastewater treatment processes, particularly intensive biological and plant-based systems, can maintain high productivity despite extreme climatic conditions, thanks to microbial adaptation, natural aeration processes, and rhizosphere interactions [4,5]. However, these environments are characterized by several challenges, including high salinity and temperature fluctuations, which can lead to changes in soil structure, in addition to the risks of sodification and a significant progressive decrease in soil permeability. This requires careful monitoring of electrical conductivity, major ions, and sodicity [6,7].

Research on irrigated agriculture using treated wastewater in harsh and arid environments in North Africa has also highlighted the crucial importance of assessing the long-term effects of this water on soil, plant productivity, and nutrient dynamics [8]. Furthermore, the quality of the water used for irrigation and the treatment performance stability are factors strongly influenced by the anthropogenic and climatic pressures [9]. In this context, analyzing the operation and efficiency of the wastewater treatment plant in the Timimoun oasis is of particular importance to ensure the sustainable management of scarce water resources and to support agricultural development in a desert environment subject to significant constraints.

Therefore, this study aims to provide a comprehensive evaluation of the wastewater treatment performance of the Timimoun plant within the broader framework of scientific knowledge on wastewater treatment in extremely arid and hostile environments.

Most previous studies focused mainly on treatment efficiency without fully addressing salinity constraints and agricultural reuse [10,11]. Most previous work did not integrate treatment performance with irrigation suitability indicators.

This study evaluates treatment performance, salinity constraints, and agricultural reuse suitability under hyper-arid conditions. Unlike previous studies primarily focused on treatment efficiency, this work integrates operational performance, irrigation suitability indicators, and the practical implications of wastewater reuse. It also provides field data from a Saharan oasis environment.

In order to address these gaps, this study is guided by the following research questions: (1) Can the activated sludge system maintain high treatment efficiency under extremely arid climatic conditions? (2) Does seasonal variation, especially extreme summer temperatures, have a significant impact on treatment performance? (3) According to irrigation standards, is the treated wastewater suitable for agricultural reuse? (4) In the long term, to what extent does residual salinity affect the feasibility of treated wastewater reuse?

In accordance with these questions, this study tests the following hypotheses: (H1) the treatment efficiency (removal of COD, BOD₅ and TSS) remains high despite the harsh climatic conditions; (H2) seasonal variations have a limited or moderate impact on the system’s performance; (H3) the treated wastewater meets the requirements for agricultural reuse in terms of organic and microbiological quality; and (H4) residual salinity is the most important limiting factor for sustainable reuse in hyper-arid environments.

2. Materials and Methods

2.1. Study Area and Environmental Context

The Timimoun oasis (southern Algeria) is located in the hyper-arid Sahara region, characterized by an annual rainfall of less than 80 mm, with temperatures exceeding 45 °C in summer and among the highest potential evaporation rates worldwide. These extreme climatic conditions significantly affect the performance of biological treatment processes and the quality of effluents intended for agricultural reuse [12,13]. The wastewater treatment plant serves approximately 5000 population equivalents (PE). It primarily receives domestic wastewater with a moderate organic load but a high mineral content, typical of desert environments [8,9].

2.2. Description of the Treatment Process

The wastewater treatment plant in the Timimoun oasis operates using a conventional activated sludge process adapted to the harsh climatic conditions of hyper-arid desert environments. The treatment system ensures stable organic matter removal while maintaining satisfactory microbiological quality despite extreme temperatures, high evaporation rates, and the elevated mineral content characteristic of wastewater in desert regions.

As illustrated in Figure 1, the treatment process includes preliminary treatment, biological treatment, secondary clarification, UV disinfection, and sludge management. During the preliminary stage, coarse and fine screening are applied to remove large suspended solids and floating debris, followed by grit and oil removal to protect downstream equipment and improve process stability.

The biological treatment stage is based on an extended aeration activated sludge system that promotes organic matter biodegradation and partial nitrification. Previous studies conducted in similar environments have shown that activated sludge biomass can progressively adapt to high temperatures and elevated salinity levels while maintaining relatively stable treatment efficiency [4,5]. Secondary clarification ensures the gravity separation of biological sludge from the treated effluent before tertiary treatment.

As shown in Figure 1, ultraviolet (UV) disinfection is applied as the final treatment stage to reduce microbiological contamination and improve the sanitary quality of treated wastewater intended for agricultural reuse. UV treatment is particularly suitable for arid environments because it avoids the formation of chlorinated disinfection by-products commonly associated with chemical disinfection methods [14,15].

Table 1. Main operational parameters of the wastewater treatment plant.

Parameter	Value/Range	Unit	Description
Hydraulic Retention Time (HRT)	18–24	h	Aeration tank residence time (varies seasonally)
Sludge Retention Time (SRT)	15–20	days	Maintains stable biological activity
Influent Flow Rate	Variable	m3/d	Moderate seasonal variation
Organic Loading Rate	Typical range	kg COD/m3·d	Consistent with conventional activated sludge
Sludge Recirculation Ratio	50–100	%	Ensures sufficient biomass concentration
Aeration Strategy	Continuous	—	Increased oxygen demand in summer
UV Dose	30–40	mJ/cm2	Effective pathogen removal
Sludge Wasting	Periodic	—	Prevents biomass overaccumulation

summarizes the main operating conditions of the wastewater treatment plant. The hydraulic retention time ranged from 18 to 24 h, while the sludge retention time varied between 15 and 20 days depending on seasonal conditions and influent flow variations. Continuous aeration was maintained to compensate for the reduced oxygen solubility observed during high summer temperatures. The sludge recirculation ratio was maintained between 50 and 100% to ensure sufficient biomass concentration and process stability. Excess sludge generated during clarification was periodically removed and naturally dried under desert climatic conditions before being reused for agricultural applications according to local management practices [7,16].

This treatment configuration allows the plant to maintain high removal efficiencies under extreme desert conditions while supporting the safe reuse of treated wastewater in oasis agriculture.

2.3. Sampling Strategy

To ensure more representative monitoring under severe drought conditions, sampling was conducted over a 12-month period from January to December 2025 according to a structured protocol. Twenty-four composite samples (two samples per month) were collected at each sampling point (P1, P2, P3, and B1).

Table 2. Sampling points.

Sampling Point	Description
P1	Influent wastewater (raw wastewater)
P2	Secondary effluent
P3	Final effluent after UV disinfection intended for agricultural reuse
P4	Stabilized sludge intended for agricultural recovery

shows the locations of the sampling points. Each sample consisted of a time-proportional composite collected over a 24 h period. Each composite sample was prepared by combining four sub-samples collected at 6 h intervals (06:00, 12:00, 18:00, and 00:00) to account for diurnal variations, which are particularly prominent in very dry climates due to extreme temperature fluctuations and high evaporation rates. In order to reduce temporal variability and to provide a more representative assessment of the daily wastewater characteristics, composite sampling was preferred over random sampling.

Samples were collected according to standard APHA methods [12] and stored at 4 °C prior to analysis. To ensure data quality and integrity, they were analyzed within the recommended retention periods.

The sampling design was chosen to achieve a balance between statistical representation and logistical constraints in the presence of highly variable climatic conditions, consistent with monitoring strategies reported in the literature on wastewater in arid regions [11,12].

2.4. Physicochemical Analyses

The analyzed parameters included: COD, BOD₅, TSS, TDS, pH, electrical conductivity (EC), ammonium (NH₄⁺), nitrate (NO₃⁻), Kjeldahl nitrogen (TKN), total phosphorus (TP), and major ions (Na⁺, Ca²⁺, Mg²⁺, Cl⁻).

Table 3. Parameters analyzed and associated methods.

Setting	Method	Reference
COD	Dichromate	APHA 5220 D [12]
BOD5	Incubation period: 5 days	APHA 5210 B [12]
TSS	GF/C Filtration	APHA 2540 D [12]
TDS	Gravimetric method	APHA 2540 C [12]
NH4+	Indophenol	APHA 4500-NH3 [12]
NO3−	UV 220 nm	APHA 4500-NO3 [12]
TP	Ascorbic acid	APHA 4500-P [12]
EC	Conductimetry	ISO 7888 [17]
pH	Potentiometry	ISO 10523 [18]
Na+	Atomic Absorption Spectrometry (AAS)	APHA 3111 B [12]
Ca2+	Atomic Absorption Spectrometry (AAS)	APHA 3111 B [12]
Mg2+	Atomic Absorption Spectrometry (AAS)	APHA 3111 B [12]
Cl−	Argentometric titration	APHA 4500-Cl− B [12]
Total Coliforms	Membrane filtration	APHA 9222 B [12]
Fecal Coliforms	Membrane filtration	APHA 9222 D [12]
E. coli	Membrane filtration	APHA 9222 E [12]

summarizes the analytical methods used.

Physicochemical and nutrient parameters were analyzed using standardized APHA and ISO methods to ensure data quality and comparability.

Quality assurance and quality control (QA/QC) procedures included the analysis of blanks and duplicate samples to ensure analytical accuracy and data reliability.

The physicochemical and microbiological parameters were selected based on their relevance for evaluating treatment efficiency, salinity constraints, irrigation suitability, and sanitary safety under hyper-arid reuse conditions.

2.5. Calculation of Irrigation Suitability Parameters

To evaluate the suitability of treated wastewater for agricultural reuse under hyper-arid conditions, irrigation quality indicators were calculated using the average physicochemical values obtained during the 12-month monitoring period.

2.5.1. Sodium Adsorption Ratio (SAR)

The sodium adsorption ratio (SAR) was calculated to evaluate the sodicity hazard associated with irrigation water according to Equation (1) [19]:

$$SAR={\displaystyle \frac{{Na}^{+}}{\sqrt{\frac{{Ca}^{2+}{+Mg}^{2+}}{2}}}}$$

(1)

where:

${Na}^{+}$, ${Ca}^{2+}$, and ${Mg}^{2+}$ are the concentrations of sodium, calcium, and magnesium, respectively (meq/L).

2.5.2. Residual Sodium Carbonate (RSC)

Residual sodium carbonate ($RSC$) was determined according to Equation (2) [20]:

$$RSC=\left({CO}_3^{2-}+{HCO}_3^{-}\right)-\left({Ca}^{2+}{+Mg}^{2+}\right)$$

(2)

where:

${CO}_3^{2-}$ and ${HCO}_3^{-}$ are carbonate and bicarbonate concentrations (meq/L),

${Ca}^{2+}{ \mathrm{a}\mathrm{n}\mathrm{d} Mg}^{2+}$ are calcium and magnesium concentrations (meq/L).

An $RSC$ > 2.5 meq/L indicates a high risk of sodification [1,21].

2.5.3. Risk Index for Soil Permeability (ISP)

The soil permeability index ($ISP$) was calculated according to Equation (5) [20]:

$$ISP={\displaystyle \frac{{Na}^{+}+\sqrt{{HCO}_3^{-}}}{{Ca}^{2+}{+Mg}^{2+}+{Na}^{+}}}\times 100$$

(3)

where all ionic concentrations are expressed in meq/L

These standards are essential in desert regions where salinity levels are naturally high [6,7].

To ensure consistency, we converted the ion concentrations from mg/L to meq/L using the following relationship (4):

$${\displaystyle \frac{meq}{L}}={\displaystyle \frac{\frac{mg}{L}}{Equivalent weight}}$$

(4)

The equivalent weight is calculated as (5):

$$Equivalentweight={\displaystyle \frac{Molecularweight }{Valence}}$$

(5)

This approach is widely adopted in assessing the quality of irrigation water as it allows comparison with FAO guidelines and international standards [19,20].

2.6. Microbiological Analyses

The microbiological indicators measured included total coliforms, fecal coliforms, and E. coli determined by membrane filtration (APHA 9222 D/E) [22].

The effectiveness of UV disinfection was evaluated by comparing the bacterial concentrations before and after exposure.

Microbiological analyses were performed regularly throughout the 12-month monitoring period using membrane filtration (APHA 9222 D/E). The microbiological parameters analyzed and their corresponding analytical methods are summarized in Table 3. The limit of detection was approximately 1 colony-forming unit (CFU/100 mL). Samples were collected before and after UV disinfection and handled according to recommended holding times. The UV system operated at a dose of 30–40 mJ/cm², with contact times consistent with standard UV disinfection practices.

2.7. Kinetic Modeling

A first-order model was used to study the kinetics of COD and nitrogen decomposition (6) [23]:

$${\displaystyle \frac{dC}{dt}}=-k.C \Rightarrow C=C_0{e}^{-kt}$$

(6)

where C₀ and C are the P1 raw wastewater and P3 treated wastewater COD concentrations (mg/L), k is the apparent first-order rate constant (day⁻¹), and t is the hydraulic residence time (HRT, days).

The hydraulic residence time (HRT) was calculated as (7):

$$HRT={\displaystyle \frac{V}{Q}}$$

(7)

where V is the effective volume of the aeration tank (m³), and Q is the influent flow rate (m³/day). Based on operational data from the treatment plant, the hydraulic retention time ranged from 18 to 24 h, depending on seasonal flow variations.

The kinetic constant k was estimated using nonlinear regression (OriginPro 2023) by matching experimental COD data with the exponential decay model. The obtained k values were subsequently compared with values previously reported in the literature for activated sludge systems operating under warm and arid climatic conditions. The model performance was evaluated using the coefficient of determination (R²).

To account for temperature effects on reaction kinetics, the Arrhenius-type relationship was considered (8):

$$k_t=k_{20}·{\theta }^{(T-20)}$$

(8)

where k_t represents the kinetic constant at temperature T (°C), k₂₀ the reference constant at 20 °C, and θ the temperature coefficient (often 1.02–1.07 for biological processes).

This method is commonly used in biological processes in arid regions, particularly in nature-based and hybrid treatment systems [24,25].

2.8. Statistical Analysis

Statistical analysis was performed using R 4.3 and OriginPro 2023:

-

Normality tests (Shapiro-Wilk)

-

Analysis of variance (ANOVA) of seasonal variations

-

Pearson correlation analysis between salinity, sodium, and electrical conductivity;

-

kinetic modeling using nonlinear regression

-

Statistical significance was set at p < 0.05.

3. Results and Discussion

3.1. Overall Treatment Performance of the Timimoun Wastewater Treatment Plant Under Hyper-Arid Conditions

Table 4. Average wastewater treatment performance expressed as mean ± standard deviation (SD).

Setting	P1 Raw Wastewater (mg/L)	P3 Treated Wastewater (mg/L)	Yield (%)
COD	780 ± 60	75 ± 10	90%
BOD5	420 ± 35	40 ± 5	90.5%
TSS	320 ± 28	20 ± 3	93.8%
NH4+	58 ± 6	10 ± 2	82.7%
TKN	68 ± 5	18 ± 3	73%
TP	11 ± 3	4 ± 1	64%

presents the average treatment performance of the Timimoun plant. The high removal rates observed for COD (90%), BOD₅ (90.5%), and TSS (93.8%) indicate high treatment efficiency of the activated sludge system despite the severe climatic constraints of the hyper-arid environment. Similar results have been reported for wastewater treatment plants operating in hot and dry climates, demonstrating the robustness and efficiency of conventional biological treatment systems under appropriate operational conditions [4,26,27]. The main physicochemical parameters and treatment performances are summarized in Table 4.

All values are presented as mean ± standard deviation (SD).

TSS visually illustrates the results obtained in Table 3, which clearly demonstrate the overall stability of the treatment, a result also observed in studies recently published in several journals [28,29].

Figure 2 illustrates the seasonal variation of removal efficiencies for the main wastewater quality parameters, while detailed numerical values are summarized in Table 4.

Figure 3 illustrates the monthly variation of TSS concentrations in influent and effluent. Despite fluctuations in P1 raw wastewater concentrations, effluent exhibits relatively low and stable concentration levels, indicating effective secondary settling and good floc stability [30]. These results are consistent with what has been observed and reported for biological systems operating under high-temperature conditions [31].

Figure 4 illustrates the temporal variation of total dissolved solids (TDS). The removal efficiency is moderate (40–55%), reflecting the inherent limitations of biological processes for dissolved constituents. This finding has been widely documented in recent studies, as TDS is a key parameter in evaluating the feasibility of effluent reuse in arid regions [31,32].

Figure 5 and Figure 6 illustrate the monthly variation of BOD₅ and COD, showing significant and relatively stable removal rates, often exceeding 85%. These results confirm the stability of biological treatment under hyper-arid conditions [33].

The reduction in BOD is particularly high, ranging between 85% and 95%. This trend was observed even during peak periods in July/August, where the treatment system maintained relatively low effluent concentrations of biodegradable organic matter and stable removal efficiencies, confirming the resilience of activated sludge biomass under increased organic loading [34].

Values represent monthly mean concentrations. P1 corresponds to raw wastewater and P3 to treated wastewater after UV disinfection.

The high removal efficiencies observed for TSS, BOD, and COD confirm the effectiveness of biological treatment under hyper-arid conditions [19,35]. In contrast, the slight decrease recorded in the total dissolved solids (TDS) is typical of conventional biological processes [36]. This underscores the need to use advanced technologies such as nanofiltration or reverse osmosis to remove dissolved constituents [36]. The removal efficiencies obtained are comparable to those observed in constructed wetlands in arid regions [5] and in wastewater treatment plants in hot climates [37]. In addition to conventional organic pollution indicators, the nutrient and salinity parameters presented in Table 4 were further analyzed in relation to agricultural reuse suitability and environmental constraints under hyper-arid conditions.

Nitrogen and phosphorus removal efficiencies were lower than those observed for organic matter and suspended solids. The high NH₄⁺ removal efficiency (82.7%) indicated effective nitrification despite elevated temperatures, whereas the lower total nitrogen and total phosphorus removal efficiencies may be attributed to limited biological nutrient removal and the absence of advanced tertiary treatment. Nevertheless, the residual nutrients may provide agronomic benefits for wastewater reuse in oasis agriculture [21,38]. Residual nitrogen and phosphorus concentrations reported in Table 4 may contribute to nutrient recycling and reduce fertilizer demand in oasis agriculture.

3.2. Seasonal Variability of Treatment Performance

Seasonal variations in treatment performance were analyzed to assess the impact of the extreme climatic conditions typical of hyper-arid environments. As illustrated in Figure 7, a slight decrease in COD removal efficiency was observed during the summer period, when temperatures exceeded 40 °C. Variability of key parameters over the monitoring period is illustrated using time-series and boxplot representations, showing the stability of treatment performance despite seasonal fluctuations. Despite this, a one-way ANOVA analysis revealed that this variation was statistically significant (p = 0.018), indicating that extreme seasonal conditions, especially high temperatures in the summer, have a measurable impact on treatment performance, even though the system remains operationally stable.

Boxplots were used to illustrate seasonal variability to illustrate the seasonal variation of key parameters (COD, BOD₅, and TSS). The results indicate limited variability, confirming the overall stability of treatment performance despite seasonal fluctuations (Figure 8).

In addition, Figure 9 illustrates the relationship between variation in physicochemical parameters and temperature fluctuations. Despite the extreme temperature conditions, the concentrations of suspended solids and organic matter showed limited variation, highlighting the robustness of the activated sludge system. These results are consistent with previous studies showing that biological treatment systems operating in arid and hyper-arid environments, due to microbial adaptation and process flexibility, can maintain stable performance [37,38,39].

The slight decrease in treatment performance observed at high temperatures can be attributed to the partial inhibition of microbial activity, particularly nitrogenous bacteria, as well as increased osmotic stress associated with high salinity during summer [6,7,40].

Despite statistically significant seasonal variation (p = 0.018), the treatment process maintained high treatment efficiency, indicating strong operational stability under combined thermal and salinity stress, which is essential for sustainable wastewater reuse in hyper-arid environments [13,26].

3.3. Salinity of Effluents and Suitability for Agricultural Reuse

Based on established irrigation suitability criteria, treated wastewater quality was assessed using key salinity and sodicity indicators.

Table 5. Irrigation suitability parameters.

Setting	Measured Value	FAO Threshold	Interpretation
EC	2.4–2.8 dS/m	<3	Acceptable
SAR	6.2	<10	Low risk
RSC	1.1 meq/L	<1.25	Satisfying
Cl−	5.4 meq/L	<7	Tolerable
Boron	0.65 mg/L	<1	Low risk

summarizes the measured values and corresponding guideline limits.

As shown in Table 5, the treated wastewater exhibits electrical conductivity (EC) values ranging from 2.4 to 2.8 dS/m, indicating moderate salinity levels. Although these values remain within the FAO guidelines for irrigation (<3 dS/m), under hyper-arid conditions, they represent a critical threshold, as the limited natural leaching and high evaporation rates favor salt accumulation in the root zone [28,41].

The elevated TDS and conductivity values reported in Table 4 reflect the mineralization characteristics commonly observed in treated wastewater from arid environments.

Based on the sodium adsorption ratio (SAR = 6.2), the treated wastewater presents a low sodicity risk according to FAO standards; however, with repeated irrigation, even moderate SAR values may gradually lead to an increase in soil sodification [42]. This process may promote clay dispersion, deterioration of soil structure, reduced aggregate stability, and ultimately a decrease in infiltration capacity and hydraulic conductivity [6].

In addition to measuring electrical conductivity (EC) and sodium uptake ratio (SAR), analysis of major ions provides insight into potential irrigation hazards. While the chloride concentration (5.4 meq/L) remains within acceptable limits, long-term accumulation may induce ion-specific toxicity, particularly in chloride-sensitive crops. Boron (0.65 mg/L) also remains within acceptable limits, although long-term irrigation may cause a gradual accumulation in the soil, potentially affecting plant metabolism over time [4].

The combined effect of salinity (EC) and sodicity (SAR) is particularly important in desert soils, where buffering capacity is low, and evapoconcentration processes are intense [43]. In such conditions, even moderately saline water can cause osmotic stress in plants, which reduces water uptake efficiency and negatively affects crop productivity over time.

From an agricultural perspective, these salinity levels limit the suitability of treated wastewater use for moderately to highly salt-tolerant crops. Suitable crops include date palms (Phoenix dactylifera), forage crops such as alfalfa (Medicago sativa), and barley (Hordeum vulgare). However, sensitive crops may experience reduced growth and yield under such conditions.

Regarding soil management, long-term irrigation with this water may lead to a gradual deterioration of its physical properties, including decreased permeability, surface crusting, and reduced aeration. The high evaporation rates characteristic of extremely arid environments exacerbate these effects, as salts concentrate in the upper soil layers.

Therefore, although the treated wastewater complies with both FAO guidelines and Algerian national standards for its reuse in irrigation, its sustainable use requires appropriate management strategies, including:

-

The application of leaching fractions (10–20%) to control salt accumulation,

-

Continuous monitoring of soil salinity and sodicity,

-

The use of efficient irrigation techniques such as drip irrigation,

-

The selection of salt-tolerant crops.

These results confirm that salinity remains the principal limiting factor for the reuse of treated wastewater in hyper-arid oasis systems, not due to immediate non-compliance with regulations, but as a result of long-term risks related to soil degradation and reduced agricultural productivity.

3.4. Effluent Microbiological Quality and Sanitary Safety

Table 6. Microbiological quality of wastewater at sampling points P1 and P3 before and after UV disinfection.

Indicator	P1—Influent Wastewater (Raw Wastewater) (CFU/100 mL)	P3—Final Effluent After UV Disinfection Intended for Agricultural Reuse (CFU/100 mL)	Reduction
Total coliforms	>106	<102	4 log10
E. coli	>105	<10	4–5 log10
Fecal coliforms	>106	<102	4 log10

presents the results of microbiological analyses obtained before and after UV disinfection. Significant reductions demonstrate the high effectiveness of UV treatment, with reductions ranging from 4 to 5 log₁₀ for coliforms and E. coli. These results are consistent with previous studies for wastewater treatment plants intended for agricultural reuse, where microbiological risk control has been identified as a fundamental condition for sanitary safety [38].

As shown in Table 6, microbial concentrations decreased from more than 10⁵–10⁶ CFU/100 mL in raw wastewater to 10–10² CFU/100 mL after UV treatment, corresponding to a reduction of 4 to 5 log₁₀. Indicate high disinfection efficiency and consistent performance throughout the monitoring period.

Complies with international guidelines (such as WHO/FAO) for restricted agricultural reuse, particularly for crops not consumed raw and for controlled irrigation practices. Therefore, the treated effluent can be considered suitable from a microbiological perspective for the intended reuse category under controlled conditions.

According to World Health Organization (WHO) and Food and Agriculture Organization (FAO) guidelines for treated wastewater reuse in agriculture, treated effluents intended for restricted irrigation should not exceed 10³ fecal coliforms per 100 mL under controlled irrigation conditions. The microbiological concentrations measured after UV disinfection remained within the recommended limits.

3.5. Kinetic Modeling of COD Degradation

The application of a first-order kinetic model shows good agreement with experimental data (R² > 0.90), indicating the suitability of this approach under hyper-arid conditions. The experimentally determined average kinetic constant was estimated at k ≈ 0.32 d⁻¹. This value is consistent with kinetic coefficients previously reported for activated sludge systems operating under warm climatic conditions [38].

Figure 10 compares the measured COD concentrations and the model predictions, which demonstrates the robustness of the biological process despite fluctuations in influent quality.

Seasonal analysis revealed moderate variation in the kinetic constant. Partial inhibition due to high temperatures of microbial activity and increased osmotic stress due to high salinity may have resulted in slightly lower values being recorded during peak summer temperatures (> 40 °C). Despite the extremely harsh climatic conditions recorded, the system maintained stable performance, demonstrating strong microbial resilience, as reported in previous studies in arid regions.

3.6. Implications for Land Management and Reuse Policies

The results indicate that the feasibility of reusing treated wastewater in desert environments requires a comprehensive approach that combines treatment performance, salinity management, and appropriate agricultural practices. This approach is fully consistent with the guidelines proposed in recent studies, which recommend the implementation of flexible regulatory frameworks, adaptable to specific soil and climate conditions, and integrated into sustainable soil management strategies [23,44].

Residual salinity (EC = 2.4–2.8 dS/m) and sodicity (SAR = 6.2) are the main obstacles to the reuse of treated wastewater. Under these conditions, treated wastewater is suitable for the cultivation of moderately salt-tolerant crops, especially date palms (Phoenix dactylifera) and barley (Hordeum vulgare), as well as forage crops such as alfalfa (Medicago sativa). To protect the soil from degradation, drip irrigation with a leaching fraction of 10–20% is recommended to control salt accumulation. These results suggest that sustainable wastewater reuse may be feasible in oasis systems when appropriate crops are selected, and appropriate irrigation management is implemented [45].

3.7. Transferability to Other Arid and Hyperarid Oases

Although this study focuses on a single wastewater treatment plant, it provides insights applicable to other oases in the Sahara and similar arid regions. This is consistent with recent studies, which confirm the importance of these integrated approaches for improving the sustainability of treated wastewater reuse strategies in water-scarce environments [24].

Compared to wastewater treatment plants operating in several other arid regions (

Table 7. Comparative performance of wastewater treatment plants in arid and hyper-arid regions [4,20,37].

Site/Country	Climate	Processing Method	Influent COD (mg/L)	Effluent COD (mg/L)	COD Yield (%)	CE Effluent (dS/m)	Agricultural Use Possible
Timimoun—Algeria	Saharan hyper-arid	Activated sludge + UV	780	75	90	2.4–2.8	Salt-tolerant crops
Al-Ahsa—Saudi Arabia	Hyper-arid	Prolonged activated sludge	720	85	88	2.6–3.1	Yes (strict management)
Ouargla—Algeria	Hyper-arid	Lagooning + planted filters	650	110	83	3.2	Limit
Negev—Palestine	Extreme arid	MBR	600	30	95	1.8	Yes (broad spectrum)
Al-Qassim—Saudi Arabia	Hyper-arid	Conventional activated sludge	700	95	86	2.9	Yes (forage)
Rajasthan—India	Hot arid	Activated sludge + ponds	680	120	82	3.4	Restricted
Southern Tunisia—Tunisia	Arid	aerated lagoon	620	100	84	3.0	Yes (salt tolerance)

), the Timimoun plant exhibits high COD removal efficiency, comparable to or higher than conventional biological systems operating under similar climatic conditions. Unlike membrane processes, residual salinity remains the main limiting factor for expanding agricultural reuse, confirming that treatment performance alone is insufficient to guarantee the sustainability of wastewater reuse in desert environments [40].

As shown in Table 7, COD removal efficiency and salinity levels, a comprehensive assessment should also consider the process type, climate, influent characteristics, and reuse constraints [46]. Activated sludge systems in hyper-arid regions demonstrate high removal efficiency (>85%), but their high salinity (EC > 2.4 dS/m) limits their reuse for irrigating salt-tolerant crops. Lagoon systems exhibit lower efficiency and higher salinity. In contrast, membrane bioreactor systems achieve lower salinity and broader reuse potential, but at a higher cost. Finally, comparisons between studies should be interpreted with caution due to methodological differences. Overall, in arid regions, salinity remains the primary limiting factor for sustainable reuse.

Under hyper-arid conditions, the operation of activated sludge systems is associated with increased energy demand due to decreased oxygen solubility at high temperatures (>40–45 °C), requiring increased blower operation and aeration intensity. resulting in higher specific energy consumption than in temperate climates. Furthermore, due to evaporation, open aeration tanks and sedimentation basins experience significant water loss, which can lead to increased salt concentration and a reduction in the net volume available for reuse. To improve transferability and sustainability, optimization strategies should be considered, such as microbubble aeration systems, intermittent aeration control, and, if possible, partial covering of tanks to reduce evaporation. Integrating renewable energy sources, such as solar power, can also help reduce the increasing energy demand in desert environments. These approaches can support the long-term sustainability of wastewater treatment and reuse systems in arid regions [47].

4. Conclusions

This study demonstrates that the wastewater treatment plant in the Timimoun oasis maintains high treatment efficiency under hyper-arid desert conditions and severe climatic constraints. High removal efficiencies were achieved for BOD₅, COD, and TSS, confirming the stability and robustness of the activated sludge process under extreme temperatures and very high evaporation rates. Seasonal analysis revealed moderate variations in treatment performance, particularly during the summer months when temperatures exceeded 40 °C. Despite this, the treatment system remained stable throughout the monitoring period, demonstrating strong microbial resilience and adaptability to combined thermal and salinity stress conditions.

The analysis of treated wastewater showed satisfactory microbiological and physicochemical quality for safe reuse in agriculture. Furthermore, UV disinfection ensured a significant reduction in pathogens, while irrigation suitability indicators, such as SAR and RSC, remained within acceptable guidelines. These results demonstrated that treated wastewater reuse is feasible for irrigating moderately salt-tolerant crops.

Nevertheless, residual salinity remains the primary limiting factor affecting the long-term sustainability of treated wastewater reuse in hyper-arid oasis systems. EC values ranging from 2.4 to 2.8 dS/m may gradually contribute to soil salinization and reduced agricultural productivity under desert conditions. Therefore, sustainable reuse requires integrated management strategies such as drip irrigation, leaching practices, continuous soil monitoring, and the selection and cultivation of salt-tolerant crops.

Overall, this study highlights the importance of combining treatment performance assessment with salinity management and sustainable agricultural practices to support long-term wastewater reuse in Saharan oasis environments and other arid regions worldwide.