Linking Wastewater Treatment Performance to River Ecosystem Health: Insights from Atlantis WWTW (Wesfleur) in Western Cape ^†

Abstract

The semi-arid nation of South Africa deals with critical water shortages and deteriorating water quality because of rising population numbers as well as expanding urban areas and industrial operations. The protection of water resources depends heavily on wastewater treatment, yet treated wastewater continues to serve as a significant pollution source. The study assesses environmental effects from the Atlantis (Wesfleur) Wastewater Treatment Works (WWTW) located in the Western Cape while studying its pollution impact on the Donkergat River. The research team obtained water samples from the treatment plant effluent and river water downstream between October and November 2023 to measure the COD, TSS, nitrates, orthophosphates, chloride, conductivity, sulphates, and pH levels. The Atlantis WWTW effluent met all the requirements set by the Department of Water and Sanitation (DWS) and the National Water Act (NWA, 36 of 1998). The COD, TSS, and nutrient measurements in the treated water stayed within the established limits. The river water quality showed high chloride (264 mg/L) and sulphate (1543 mg/L) levels during multiple sampling events because of wastewater discharge accumulation and suspected human-made pollution sources. The WWTW achieves satisfactory treatment performance, but the research demonstrates that ongoing surveillance, facility enhancements, and environmental protection strategies are essential to protect water bodies. The results support the ongoing discussion about achieving proper wastewater treatment and environmental protection in areas with limited water resources.

1. Introduction

South Africa is classified as a water-scarce country, with increasing pressure on limited freshwater resources driven by population growth, urbanisation, industrial development, and climate variability. These pressures have intensified the need for sustainable water resource management strategies, including wastewater treatment and reuse as key mechanisms for environmental protection and water security. Wastewater treatment works (WWTWs) play a critical role in reducing pollutant loads prior to discharge into receiving water bodies; however, even treated effluent may contain residual nutrients, salts, and other contaminants capable of influencing downstream water quality and ecosystem health [1,2,3]. In semi-arid regions such as the Western Cape, surface water systems are particularly sensitive to both hydrogeological controls and anthropogenic inputs. Understanding the interaction between treated effluent and receiving rivers is therefore essential for informed catchment management and regulatory compliance.

The Donkergat River in Atlantis receives continuous discharge from the Wesfleur WWTW and represents a vulnerable aquatic system influenced by both wastewater inputs and regional hydrogeochemical processes. Despite the importance of this system, limited site-specific data are available to characterise effluent quality and its relationship with downstream river chemistry. Accordingly, this study undertakes a preliminary assessment of the physico-chemical characteristics of the Wesfleur WWTW effluent and evaluates its potential influence on the water quality of the Donkergat River. This baseline investigation provides initial insight into treatment performance, salinity dynamics, and potential environmental risks, thereby establishing a foundation for future long-term monitoring and integrated water quality management in semi-arid urban catchments.

South Africa receives an average annual rainfall of less than 500 mm, which is approximately half of the global average [4]. Consequently, the country relies heavily on surface water resources to meet domestic, agricultural, and industrial demands [5]. This dependency places increasing strain on rivers, dams, and associated wastewater infrastructure. To meet rising water demands and protect environmental quality, wastewater treatment plants (WWTPs) have become essential components of urban water management systems [6]. Nevertheless, many existing treatment technologies face challenges related to ageing infrastructure, population growth, and increasing hydraulic and pollutant loads [7,8].

The primary objective of wastewater treatment is to reduce or remove physical, chemical, and biological contaminants from influent wastewater prior to environmental discharge [2]. When wastewater is inadequately treated, its release into natural watercourses can significantly degrade water quality, posing risks to aquatic ecosystems and human health [9]. Municipal wastewater effluents are recognised sources of nutrients, trace organic compounds, and dissolved salts that may accumulate in receiving waters and sediments [10]. In contrast, when properly managed and regulated, the controlled treated effluent can contribute to water resource augmentation while minimising ecological risks [1]. Balancing these dual resource recovery and environmental protection remains a central challenge in semi-arid regions where water scarcity and ecological vulnerability coexist.

2. Materials and Methods

2.1. Study Area

This study was limited to Atlantis WWTP which is situated in the Atlantis Industrial Area approximately 5 km outside of Atlantis Town, a suburb that was established in the 1970s and serves a population of approximately 67,491 residents [11]. The plant is designed with two separate process streams, namely the industrial and domestic WWTWs. The domestic WWTWs is designed to treat 8M L/day average dry weather flow (ADWF). The treated effluent from the Atlantis domestic WWTW is split in two (2), with one part routed to the ponds that recharge the Atlantis aquifer, from which potable water is extracted; and the other part discharged as a continuous flow to the Donkergat River, a non-perennial river that flows only in winter [12], and receives a low volume of effluent in summer. The treated effluent from the industrial plant is discharged to the coastal ponds to create a buffer that prevents salty water ingress. In addition, it’s estimated that on average there is approximately 7500 m³/d of storm water and wastewater [12,13]. Available evidence has reported that an estimated volume of 3000 m³/d to 2000 m³/d of effluent is discharged in the Donkergat River and its vicinity in early and late summer [12], respectively.

2.2. Water and Effluent Sampling

Samples were collected according to the suggestions provided by [14]. In this research, water samples are collected from two main locations: the Atlantis WWTP (Wastewater Treatment Plant) domestic effluent discharge point to the river and 3.5 km downstream Donkergat River. The Atlantis WWTP represents domestic wastewater after treatment, while the Donkergat River serves as a natural water source. These specific sites provide a comprehensive perspective on water quality, encompassing both treated wastewater and natural river conditions. The sampling from these distinct locations contributes to a more nuanced understanding of wastewater parameters and their potential impact on the receiving environment. General equipment required for successful collection of wastewaters included those recommended by the South African Medical Research Council, or SAMRC [15], such as: a sampling device that can reach up to 2 m; ±20 L container; a cooler box; ice packs (no ice); paper towels; waterproof markers; a field notebook and pen; ample labels; a 70% v/v ethanol spray bottle and a sealed container for contaminated gloves. It should be indicated that the sampling campaign was conducted over a period of approximately three weeks, from 30 October to 17 November 2023. This monitoring period was designed as a preliminary assessment to establish baseline water quality conditions and identify potential hydrochemical patterns associated with wastewater discharge and downstream river response. While this timeframe provides valuable insight into short-term system behaviour, it does not capture seasonal variability, and therefore the results should be interpreted within the context of an initial investigation rather than a comprehensive long-term assessment.

2.3. Parameters Analysed and Materials Used

A detailed summary of all the analytical parameters and associated materials is presented in

Table 1. Summary of analytical parameters and required materials.

Parameter	Materials Required
Chemical Oxygen Demand (COD)	Concentrated Sulphuric Acid (H2SO4); Heating block for sample digestion (Gerhardt digestion unit); Colorimeter or spectrophotometer for COD measurement.
Nitrate	Palintest Nitratest Powder (Spoon Pack); Palintest Nitratest Tablets; Palintest Nitricol Tablets; Palintest Nitratest Tube, 20 mL (PT 526); Palintest Automatic Wavelength Selection Photometer; Round Test Tubes, 10 mL (PT 595).
Orthophosphates	Palintest Phosphate No. 1 LR Tablets; Palintest Phosphate No. 2 LR Tablets; Palintest Automatic Wavelength Selection Photometer; Round Test Tubes, 10 mL glass (PT 595).
pH	pH meter; Five 50–100 mL beakers; Three 50 mL polyethylene bottles with caps; pH buffer solutions (pH 4, 7, 10); 100 mL graduated cylinder; Paper towels; Soft tissues; Distilled water (squeeze bottle); Stirring rod or spoon; Masking tape; Permanent marker; Latex gloves; Safety goggles.
Conductivity	Total dissolved solids (TDS) tester or conductivity meter; Standard solution; Distilled water; Squeeze bottle; Soft tissue; Three 50–100 mL beakers; Jewellery screwdriver (for calibration).
Total Suspended Solids (TSS)	Hach 10 mL matched sample cells; DR3900 spectrophotometer; Measuring syringe; Iodised water.
Chloride	Palintest Acidifying CD Tablets; Palintest Chloridol Tablets; Palintest Automatic Wavelength Selection Photometer; Round Test Tubes, 10 mL glass (PT 595); Measuring syringe, 1 mL (PT 361); Sample containers (100/50/10 mL plastic, PT 510).
Sulphate	Palintest Sulphate Turb Tablets; Palintest Automatic Wavelength Selection Photometer; Round Test Tubes, 10 mL glass (PT 595).

. This study used Palintest Methods, as previously discussed, used by [16,17]. The procedure for colourimetric water analysis involved six sequential steps. A suite of physico-chemical parameters was analysed to evaluate both treatment performance and downstream river water quality. These included: Chemical Oxygen Demand (COD), Nitrate (NO₃⁻), Orthophosphate (PO₄³⁻), pH, Electrical Conductivity (EC), Total Suspended Solids (TSS), Chloride (Cl⁻) and Sulphate (SO₄²⁻).

First, water samples were collected in pre-cleaned, contamination-free containers to maintain sample integrity. The appropriate Palintest reagent tablet(s) or liquid reagent was then added according to the manufacturer’s specifications for the target analyte. Samples were subsequently mixed by gentle shaking or stirring to ensure complete dissolution of the reagent and initiation of the chemical reaction. Following mixing, samples were allowed to stand for the prescribed reaction period to permit full colour development. The developed colour was measured using a Palintest photometer, colour comparator, or visual reference chart, depending on the parameter under investigation. Finally, the observed colour intensity was interpreted using the corresponding calibration curve or reference scale to determine the analyte concentration.

In addition, Figure S1 was created on 19 February 2026, with the assistance of OpenAI ChatGPT 5.2, using the prompt “replace osmotic biodiversity with reduced biodiversity, suggested in reviewer response” [18].

3. Results and Discussions

3.1. Regulatory Framework and Compliance Benchmarks

The interpretation of the measured water quality parameters was guided by the National Water Act (NWA, 36 of 1998) wastewater discharge limits (see Table S1) for key physico-chemical indicators [19]. These standards define acceptable thresholds for effluent discharge into receiving water bodies and provide a benchmark for assessing compliance and potential environmental risk.

3.2. Effluent Quality from the Wesfleur WWTW

Laboratory analysis of final effluent samples collected between 30 October and 17 November 2023 indicates that the Wesfleur WWTW produced generally stable and compliant effluent across most parameters as seen in

Table 2. Laboratory results of effluent samples from Atlantis WWTP discharge point into Donkergat River.

Date	COD (mg/L)	Nitrate (mg N/L)	Orthophosphate (mg P/L)	pH	Conductivity (mS/m)	TSS (mg/L)	Chloride (mg/L)
30 October 2023	23	12.2	3.3	7.5	75	<5	111
31 October 2023	30	11.7	2.7	7.6	73	<5	117
1 November 2023	33	12.5	3.7	7.6	71	<5	115
2 November 2023	22	12.1	4	7.3	72	<5	108
3 November 2023	20	10.3	3.1	7.6	73	<5	109
6 November 2023	25	10	3.3	7.5	71	<5	107
7 November 2023	25	8.9	3.7	7	69	<5	103
8 November 2023	37	11.6	4.6	7.5	69	<5	98
9 November 2023	27	9.9	5.3	7.6	72	<5	113
10 November 2023	35	10.8	5.3	7.9	75	<5	100
13 November 2023	38	9.6	6.2	7.8	75	11	104
14 November 2023	33	9.5	6	7.6	78	<5	146
15 November 2023	52	9	5.9	7.6	73	5	118
16 November 2023	30	10.3	5.9	8	76	6	124
17 November 2023	30	10.3	5.9	8	76	6	124

. The COD concentrations ranged from 20 to 52 mg/L, remaining well below the DWAF limit of 75 mg/L, indicating effective organic matter removal. TSS values were consistently below 5 mg/L, with only minor exceedances on isolated days (maximum 11 mg/L), demonstrating excellent solids separation. Similar COD and TSS performance have been reported for well-functioning municipal WWTPs in South Africa and elsewhere [8,20].

Nitrate concentrations in the effluent ranged from 8.9 to 12.5 mg N/L, remaining below the regulatory limit of 15 mg N/L. Orthophosphate ranged between 2.7 and 6.2 mg P/L, also well below the DWAF threshold of 10 mg P/L. These nutrient levels are typical of secondary treatment systems and comparable to values reported in other South African treatment plants [1,20].

The effluent pH remained stable between 7.0 and 8.0, within the acceptable regulatory range, indicating effective buffering and chemical stability. The conductivity values ranged from 69 to 78 mS/m, well below the limit of 250 mS/m, suggesting moderate ionic content. The chloride concentrations were relatively stable, ranging from 98 to 146 mg/L, remaining below the regulatory limit of 200 mg/L.

Overall, these results confirm that the Wesfleur WWTW is operating efficiently and producing effluent that is largely compliant with national discharge standards, consistent with findings from other well-managed wastewater treatment systems [1,2].

3.3. Downstream Water Quality in the Donkergat River

In contrast to the stable effluent signal, the Donkergat River exhibited substantial variability, particularly in salinity-related parameters. Inorganic nitrate in the river ranged from 2.8 to 11.2 mg N/L, which is generally lower than effluent concentrations (see

Table 3. Laboratory results for downstream sampling of Donkergat River.

Date	Inorganic Nitrate (mg N/L)	Orthophosphate (mg P/L)	pH	Sulphate (mg/L)	Chloride (mg/L)
30 October 2023	4.2	0.9	7.5	83	194
31 October 2023	2.9	0.7	7.6	242	1543
1 November 2023	4.4	0.4	7.5	125	204
2 November 2023	3.4	0.6	7.5	230	1049
3 November 2023	4.2	0.3	7.3	131	188
6 November 2023	3.3	1.2	7.4	189	605
7 November 2023	2.8	1.2	7.3	264	1111
8 November 2023	9.7	2.9	7.4	103	131
9 November 2023	11.2	4.3	7.3	99	164
10 November 2023	5.8	3.2	7.3	124	292
13 November 2023	7.9	3.8	7.4	107	126
14 November 2023	10	4.4	7.3	91	144
15 November 2023	9.3	4	7.5	93	142
16 November 2023	9	4.2	7.4	96	150
17 November 2023	10.4	5	7.4	95	160

). Orthophosphate ranged from 0.3 to 5.0 mg P/L, which is again lower than effluent levels. This suggests that nutrient inputs from the WWTW are subject to dilution, assimilation by aquatic biota, and biogeochemical processing, a pattern widely observed in receiving water systems [9,10]. However, chloride showed extreme fluctuations, ranging from 126 mg/L to a maximum of 1543 mg/L, while sulphate ranged from 83 to 264 mg/L. Figure S2 depicts that river ionic chemistry is driven by catchment-level hydrogeology, not only WWTW effluent. These values far exceeded effluent concentrations and, in several cases, surpassed typical freshwater thresholds. Such elevated chloride and sulphate concentrations have been associated with saline groundwater intrusion, geological mineral dissolution, evapoconcentration, and urban runoff in semi-arid environments [1,10]. Similar salinity patterns have been reported in other South African rivers influenced by groundwater–surface water interactions [20,21].

3.4. Effluent–River Chemical Comparisons

Direct comparison between effluent and river chemistry reveals important differences in the behaviour and likely sources of key dissolved constituents. Effluent chloride concentrations remained relatively stable throughout the monitoring period, ranging between 98 and 146 mg/L, while river chloride concentrations exhibited extreme variability, with peak values reaching 1543 mg/L. These values exceed effluent concentrations by more than an order of magnitude, clearly indicating the presence of additional chloride sources beyond the WWTW. Such elevated concentrations are consistent with hydrogeological influences, including saline groundwater intrusion, mineral dissolution, and evapoconcentration processes, which are characteristic of semi-arid catchments.

Sulphate concentrations in the Donkergat River were consistently elevated and exhibited variability similar to chloride, reinforcing the interpretation that salinity dynamics are strongly influenced by catchment-scale hydrochemical processes. The strong positive correlation observed between chloride and sulphate further supports the likelihood of a shared geochemical origin, rather than isolated point-source inputs. These findings are consistent with previous studies demonstrating that groundwater–surface water interactions and geological controls are major contributors to river salinity in South African semi-arid environments [1,10].

However, it is important to acknowledge a limitation of the present study. Sulphate concentrations were not measured in the WWTW effluent during the sampling campaign. While the analytical programme prioritised parameters commonly used to assess treatment performance and regulatory compliance, including COD, TSS, nutrients, chloride, conductivity, and pH, the absence of effluent sulphate measurements prevents direct quantification of the wastewater contribution to downstream sulphate levels. As a result, although the magnitude, variability, and correlation structure of the river salinity indicators strongly suggest dominant hydrogeochemical controls, the potential contribution of effluent-derived sulphate cannot be fully excluded.

In contrast to salinity parameters, nutrient concentrations were generally higher in the effluent than in the river, indicating that dilution, biological uptake, and natural attenuation processes occur downstream of the discharge point. This pattern is typical of receiving water systems, where effluent-derived nutrients are rapidly assimilated or dispersed. Overall, these observations indicate that while the WWTW contributes measurable chemical inputs to the Donkergat River, the dominant drivers of elevated salinity appear to be catchment-scale hydrogeological and geochemical processes. Future studies incorporating comprehensive salinity profiling of both effluent and receiving waters will enable more precise quantification of source contributions.

3.5. Correlation Structure and Salinity Dynamics

Correlation analysis revealed a strong and statistically significant positive relationship between chloride and sulphate (ρ = 0.711, p < 0.01). This indicates that both ions increase simultaneously, suggesting a shared source or common controlling process. The strong chloride–sulphate relationship is characteristic of salinity-driven systems, where groundwater seepage or mineral dissolution dominates ionic composition [10]. Similar correlation structures have been reported in rivers affected by saline aquifers and evaporative concentration [1,21]. The scatterplot analysis (see Figure S3) further confirms this relationship, showing that extreme chloride peaks (>1000 mg/L) coincide with elevated sulphate levels, reinforcing the interpretation that hydrogeological processes, rather than wastewater discharge, dominate salinity dynamics in the Donkergat River.

3.6. Implications for Ecosystem Health

The elevated chloride and sulphate concentrations observed in the Donkergat River have important implications for aquatic ecosystem health. High salinity is known to cause osmotic stress in aquatic organisms, reduce biodiversity, alter species composition, and impair ecosystem functioning [9,10]. Chronic exposure to elevated salinity can also affect sediment chemistry, promote sulphate reduction, and lead to the formation of hydrogen sulphide (H₂S), which is toxic to aquatic life [1]. These processes may result in root zone anoxia, fish mortality, and long-term degradation of riverine habitats, as illustrated in Figure S1. However, the evidence from this study indicates that the Wesfleur WWTW is not the primary driver of these salinity stresses. Instead, the dominant controls appear to be catchment-scale hydrogeochemical processes, particularly groundwater interactions.

3.7. Overall Synthesis

The combined statistical, graphical, and hydrochemical analyses demonstrate that the Wesfleur WWTW produces stable, compliant effluent with moderate nutrient and ionic loads. While the WWTW contributes to nutrient levels in the Donkergat River, it does not explain the extreme salinity variability observed. The river system is primarily shaped by hydrogeological and geochemical processes, with groundwater–surface water interactions exerting the strongest influence on water quality. These findings are consistent with regional and international studies that highlight the dominance of diffuse and geological drivers over point-source wastewater inputs in semi-arid environments [1,21].

4. Conclusions and Future Work

The combined statistical, graphical, and hydrochemical analyses demonstrate that the Wesfleur Wastewater Treatment Works (WWTW) produces consistently stable effluent quality, with most measured parameters remaining within the regulatory discharge limits. This indicates that the treatment system is functioning effectively in terms of organic matter removal, solids reduction, and general chemical stability. In contrast, the Donkergat River exhibits pronounced salinity variability, primarily driven by groundwater interactions and geochemical processes. The extreme fluctuations observed in chloride and sulphate concentrations suggest that catchment-scale hydrogeochemical controls, rather than wastewater discharge, dominate downstream water quality. Overall, while the Wesfleur WWTW contributes measurable nutrient and ionic loads to the river, its influence is secondary relative to natural hydrogeological processes. These findings highlight the complexity of managing water quality in dynamic semi-arid river systems, where multiple interacting drivers influence ecosystem health.

Although the effluent quality is largely compliant, occasional deviations and anomalous trends observed in downstream water quality indicate the need for enhanced and continuous monitoring programmes. Future research should focus on:

• Conducting long-term and seasonal monitoring to capture hydrological variability and extreme events.
• Identifying and quantifying alternative salinity sources, particularly groundwater inflows and geological contributions.
• Implementing a comprehensive assessment of the WWTW treatment processes, including advanced nutrient and salinity removal options.
• Adopting an adaptive water management approach, whereby monitoring results are regularly used to update operational strategies and mitigation measures.

Such actions are essential for improving the understanding of effluent–river interactions and for supporting evidence-based decision-making aimed at safeguarding river ecosystem health under increasing environmental pressures.

It is also important to recognise that the monitoring period was limited to a short-term sampling campaign of less than one month. While this approach allowed for the identification of key water quality characteristics and potential salinity drivers, it does not fully capture seasonal and hydrological variability. As river systems in semi-arid environments are strongly influenced by temporal fluctuations in groundwater interactions, rainfall, and evapoconcentration, extended monitoring is necessary to establish fully representative trends. This present study therefore represents an initial baseline assessment that provides a foundation for future long-term investigations.