Evaluation of the quality of water produced by a commercial atmospheric water generator (AWG) for its suitability for drinking and irrigation in South Africa Open Access

Water is essential for the preservation of life on our planet, serving as a fundamental component for the sustenance of plant, animal, and human life (Matchawe et al. 2022). In addition to its vital functions in human life, water is essential for the irrigation of crops, animal, and fish production (Hannemann 2015). Globally, water can effectively support both drinking and irrigation needs when it adheres to the World Health Organization Guidelines for drinking water (World Health Organization (WHO) 2022). Ensuring compliance with these standards helps protect public health and promotes sustainable agricultural practices. Furthermore, access to plentiful, uninterrupted, clean, potable, and affordable water is an essential right for personal and family use. Therefore, the effective management of water resources plays a crucial role in fostering economic growth and poverty reduction (Matchawe et al. 2022).

Water scarcity is the inadequate availability of safe water supplies or the lack of access to sufficient water (Dos Santos et al. 2017), and globally approximately 2.7 billion people are currently experiencing severe water shortages and approximately 663 million individuals worldwide lack access to safe water (Dos Santos et al. 2017; Armah et al. 2018), with a particularly high incidence among those residing in developing countries and regions (Zhang et al. 2022). Water scarcity may arise from various factors, including human factors such as population increase, urbanisation of major cities, and agriculture and natural factors (climate change, drought, and rainfall deficit) (Dos Santos et al. 2017; Ngene et al. 2021).

Water scarcity in Africa has led to an increase in waterborne diseases that can also be caused by microbial contamination in drinking water (Nedelkova et al. 2019). The consumption of contaminated water, which may contain pathogenic bacteria and viruses, poses a significant risk for the development of various infectious diseases, including malaria, typhoid, cholera, diarrhoea, dysentery, hepatitis A, typhoid fever, and poliomyelitis, resulting in numerous deaths (Armah et al. 2018; Matchawe et al. 2022). Moreover, an estimated 829,000 individuals are projected to die annually from diarrhoeal disease due to inadequate access to potable water, sanitation and poor hand hygiene, which could be alleviated through the enhancement of water sources (Armah et al. 2018).

South Africa also faces significant water scarcity, recording an average annual rainfall of 464 mm, compared to the global average of around 850 mm. This places it among the 30 driest nations (South African Government 2024). Moreover, between 37 and 42% of drinkable water is lost due to leaks, waste, and unauthorised connections (South African Government 2024). As a result, many public water supply systems and reservoirs experience water shortages, which also affect the agriculture sector. South Africa's agriculture sector contributes roughly 2.4% to the nation's GDP (Department of Agriculture Land Reform and Rural Development 2022). While this percentage may be small compared to the overall GDP, primary agriculture is vital to the country's economy. It provides essential employment opportunities, especially in rural regions, and is a significant source of foreign exchange revenue (Department of Agriculture Land Reform and Rural Development 2022).

Water scarcity is a pressing global issue that led the United Nations to adopt Sustainable Development Goal (SDG) target 6, which needs to be achieved between 2015 and 2030 (Matchawe et al. 2022). This goal prioritises universal access to water and sanitation, ensuring a secure supply and sustainable management for all. However, despite the relative achievement of the water targets set by the Millennium Development Goals in various regions across the globe, it is notable that half of the population consuming water from unsafe sources resides in Africa (Matchawe et al. 2022).

Diversifying water sources beyond surface water is a critical and essential approach to enhance urban water reliability (Zhang et al. 2022). According to Jasim et al. (2016), Jung et al. (2015), and López Zavala et al. (2018), various alternative water sources, such as rainwater harvesting, sewage reuse, inter-basin water transfer, and seawater desalination, can be employed to complement surface waters and bolster water supply resilience. However, some of these approaches have limitations. For instance, rainwater and sea desalination may not be viable for inland cities facing severe water shortages (Wu et al. 2020). Additionally, inter-basin water transfer is vulnerable to water quality and ecological safety issues (Wu et al. 2020). Furthermore, these methods cannot be relied upon to address water supply emergencies, during which bottled water is typically the preferred option (Wang et al. 2019).

However, there is another commonly overlooked renewable and sustainable source of water known as atmospheric water, which refers to the moisture present in the air (Yang et al. 2021). The Earth's atmosphere holds approximately 12,900 trillion litres of renewable water, which accounts for roughly 10% of the planet's surface water reserves (Fathy et al. 2020). In addition, even in arid deserts, the air contains significant moisture (Wang et al. 2017). As a result, the concept of atmospheric water production using atmospheric water generation (AWG) has been conceptualised and developed to complement the presently available water sources, such as surface and groundwater sources.

The AWG device effectively converts atmospheric moisture into potable water. This device operates on the principle of latent heat, facilitating the transformation of water vapour molecules into liquid water droplets (Tripathi et al. 2016). The production rate of water by an AWG is influenced by several factors, including relative humidity, ambient air temperature, and the size of the compressor (Jahne et al. 2018). AWG exhibits enhanced effectiveness as relative humidity and air temperature increase. It is important to note that AWGs' cooling condensation generally does not function efficiently when the temperature falls below 18.3 °C (65°F) or when relative humidity declines below 30%. Furthermore, the cost-effectiveness of an AWG is determined by the capacity of the machine, the prevailing local humidity and temperature conditions, as well as the expense associated with powering the unit (Tripathi et al. 2016).

Ensuring high water quality is essential for domestic, agricultural, and industrial use (Ibrahim et al. 2019). To achieve this goal, various stringent standards have been established to protect and enhance water quality. The WHO, the Environmental Protection Agency (EPA), and the South African Water Quality Guideline (SAWQG) have played a vital role in adopting these standards (South African National Standard (SANS) 2015; World Health Organization (WHO) 2022). By adhering to these guidelines, sustainable water management can be promoted, contributing to overall public health and environmental well-being.

Hence, the present study aimed to assess the quality of treated/filtered atmospheric water produced by a commercial AWG situated in an industrial area as an additional water source for drinking, domestic, and irrigation purposes. The objective of this study was to use the WHO (World Health Organization (WHO) 2022) and SAWQG (South African National Standard (SANS) 2015) water standards as the basis for evaluating the suitability of treated AWG water for drinking purposes.

Moreover, the potential of treated AWG water for various domestic applications, including irrigation, was assessed through a range of classification models focusing on total dissolved solids (TDS), electrical conductivity (EC), and pH levels. These assessments will help ensure that the treated AWG water is suitable for diverse uses, promoting adequate water resources. No literature was found on the suitability of treated atmospheric water generated by a commercial AWG for drinking and irrigation uses under African tropical conditions.

The atmospheric water was sampled over 12 months, from November 2022 to October 2023. Twelve (12) water samples were collected directly from the AWG-filtered water storage tank using 500 ml polyethene bottles with lids. The bottles were handled with the utmost sterility and caution to prevent external contamination during collection and transportation to the Aqua Air Africa (Pty) Ltd laboratory.

Each atmospheric water sample was accompanied by a blank sample containing 500 ml of distilled water only in a tightly sealed bottle. The blank samples were treated and tested the same way as the AWG water samples.

A water analysis was performed to determine the filtered AWG water quality and identify any potentially harmful elements. Once the water bottle samples arrived at the Aqua Air Africa laboratory, a chemical analysis of major elements (calcium, potassium sulphate, magnesium, chlorine-free, chloride, and alkalinity) and trace elements (aluminium, copper, iron, manganese, nickel, silica, zinc, fluoride, nitrate, and ammonia) was conducted using the HANNA H183399 Multiparameter Photometer (HANNA, South Africa).

Physical water parameters, such as pH, EC, and TDS, were measured using the pH/conductivity/TDS bench meter (HANNA, South Africa). The turbidity was measured using the HANNA H198703 Turbidimeter (HANNA, South Africa). The colour of the atmospheric water was determined using the HANNA Multiparameter Photometer. All the analyses were conducted according to the manufacturer's protocol (HANNA manual). In addition, comprehensive bacterial analyses were carried out at Waterlab (Pty) Ltd in Pretoria, South Africa.

Multivariate statistical analysis was conducted using IBM^® SPSS^® Statistics 29 to examine various physicochemical parameters in atmospheric water. These parameters included calcium, potassium, sulphate, magnesium, chlorine-free, chloride, alkalinity, aluminium, copper, iron, manganese, nickel, silica, zinc, fluoride, nitrate, ammonia, pH, EC, TDS, turbidity, and colour of water. The multivariate statistical method employed in this study was univariate analysis, which was used to perform descriptive statistics on all physicochemical parameters of the filtered atmospheric water samples. The results of the descriptive statistics were then utilised to classify and standardise the filtered water for comparison purposes.

The physicochemical data of filtered AWG water were compared to water standards to evaluate whether the water quality meets the criteria for domestic use.

The guidelines used for classifying and evaluating treated AWG water in the study area are

• Guidelines for drinking water quality, WHO (World Health Organization (WHO) 2022)

• Domestic water uses SAQWG (South African National Standard (SANS) 2015)

• Microbiological analysis (SANS 2015) and (WHO 2022)

• Water classification based on TDS (Davis & Dewiest 1966)

• Water classification based on TDS (Freeze & Cherry 1979)

• Salinity hazard classification based on EC (Richards 1954; Wilcox 1955).

The descriptive statistics of the physicochemical parameters of the AWG-filtered water are summarised in Table 1.

A statistical summary of the physicochemical analyses of the sampled filtered AWG water in the study area is presented in Table 1. The drinking water standards established by the SAQWG and the WHO (Table 1) were used to evaluate the suitability of AWG-filtered water for drinking and other domestic use. The pH of the filtered AWG water varied from 6.5 to 9.62, with a mean value of 7.44, which fell within the permissible limits of the SAQWG, and 1 out of 12 AWG water samples exceeded the WHO drinking water guidelines.

EC varied from 3.4 to 17.76 μs/cm, with a mean value of 10.15 μs/cm. All EC values were within the permissible limits of the SAQWG and WHO drinking water guidelines. According to Arega et al. (2019), EC values within drinking water limits indicate that the water has no risk for human consumption; however, EC values above 250 μs/cm may cause gastrointestinal irritation in humans (Ramesh & Elango 2012).

The concentration of TDS in AWG-filtered water varied from 1.7 to 8.71 mg/l, with an average value of 4.90 mg/l. All the TDS concentrations were well below the SAQWG and WHO drinking water guidelines. Additionally, classification methods by Davis & Dewiest (1966) and Freeze & Cherry 1979) were developed to assess the suitability of TDS in the AWG-filtered water (Table 2).

According to Davis & Dewiest (1966), 100% of the AWG-filtered water has a TDS concentration of less than 500 mg/l. Therefore, the water is classified as desirable for drinking and can further be used for domestic purposes. The TDS classification based on Freeze & Cherry (1979) showed that 100% of the AWG-filtered water samples can be classified as fresh water; thus, they are suitable for drinking and domestic purposes. Elevated TDS concentrations in water can lead to gastrointestinal discomfort and may cause staining on fabrics that have been washed with it (Ayodele & Aturamu 2011; Ibrahim et al. 2019).

All the AWG-filtered water samples had turbidity concentrations within the SAQWG and WHO drinking water guidelines. The concentrations varied between 0.10 and 0.35 NTU, with a mean of 0.21 NTU (Table 1). Furthermore, the colour of the AWG water varied between 0 and 14 with a mean of 2.17, thus complying with both SAQWG and WHO drinking water guidelines. The colouration of water plays a significant role in its aesthetic appeal, according to Amfo-Otu et al. (2014). Thus, it is essential for the colour of drinking water to be within permissible limits for consumers to desire to drink or use it for domestic purposes.

Ammonia concentrations ranged from 0 to 8.40 mg/l, with a mean of 2.73 mg/l. This exceeded the SAQWG permissible limits in 5 out of 12 water samples. However, all ammonia concentrations complied with the WHO drinking water guidelines. Both Inbar et al. (2020) and Kaplan et al. (2023) reported ammonia concentration in AWG-produced water, indicating that ammonia is prevalent in AWG water due to its common atmospheric origin. However, according to the SAQWG, ammonia in drinking water is not considered a direct health risk but, rather, an aesthetic; thus, the water is still suitable for drinking and domestic purposes.

Calcium concentrations ranged from 0 to 106 mg/l, with a mean of 51 mg/l. Magnesium concentrations ranged from 0 to 32 mg/l, with a mean of 6 mg/l. Furthermore, potassium concentrations ranged from 0 to 0.8 mg/l, with a mean of 0.34 mg/l. All three of these concentration ranges complied with the SAQWG and WHO drinking water guidelines. According to Yasmin et al. (2019), elevated levels of magnesium can lead to a laxative effect in the body. Therefore, it is essential to monitor magnesium intake to maintain optimal digestive health.

Both chloride and chlorine complied with the SAQWG and the WHO drinking water standards. Chloride ranged from 0 to 0.70 mg/l, with a mean average of 0.23 mg/l. Having high levels of both chloride and chlorine can create a noticeable salty taste, which may have a laxative effect on some individuals (Adebayo et al. 2021). Thus, the AWG-treated water is a good alternative choice for drinking and domestic purposes. Furthermore, nitrate and sulphate complied with the SAQWG and the WHO drinking water permissible limits; thus, the water is safe for drinking and domestic purposes. Sulphate ranged from 0 to 5 mg/l, with a mean of 1.17 mg/l.

The fluoride concentration in AWG water ranged from 0 to 0.30 mg/l, with a mean of 0.10 mg/l, thus complying with the SAQWG and the WHO drinking water permissible limits. High levels of fluoride can cause dental fluorosis and skeletal fluorosis, whereas low levels of fluoride may cause dental decay (Scholz et al. 2015).

The AWG-filtered water was evaluated for trace elements such as copper, manganese, zinc, aluminium, nickel, iron, and silica (Table 1). Trace elements play a crucial role in our nutrition and can be classified into three distinct groups based on their nutritional functions (Cannas et al. 2020):

• potentially toxic elements, e.g., aluminium and silica;

• elements of probable physiological importance, e.g., manganese and nickel;

• essential elements, e.g., copper, zinc, and iron.

According to Zoroddu et al. (2019), essential elements can be toxic to humans if the dose is too high or too low, even resulting in death. Some characteristic deficiency symptoms of copper are artery weakness, liver disorder, and secondary anaemia. Zinc deficiency can cause skin damage and stunted growth, and iron deficiency may cause anaemia and immune system disorders (Zoroddu et al. 2019). Furthermore, according to Mohammadi et al. (2019), ingesting high levels of potentially toxic elements such as aluminium and silica may cause cancer and Alzheimer's disease, as well as chronic neurological disorders such as dialysis dementia.

The AWG water contained aluminium levels ranging from 0 to 0.03 mg/l, with a mean of 0.01 mg/l. Silica levels ranged from 0.04 to 0.21 mg/l, with a mean of 0.11 mg/l. These levels were within the acceptable limits (Table 1). Manganese concentrations ranged from 0 to 0.03 mg/l, with a mean of 0.01 mg/l. Nickel concentrations ranged from 0 to 0.010 mg/l, with a mean of 0 mg/l. Copper concentrations ranged from 0 to 0.060 mg/l, with a mean of 0.01 mg/l. Zinc concentrations ranged from 0.04 to 0.61 mg/l, with a mean of 0.27 mg/l. Iron concentrations ranged from 0 to 0.09 mg/l, with a mean of 0.01 mg/l. All trace elements complied with the SAQWG and WHO drinking water permissible limits, thus making the AWG-filtered water suitable for drinking and domestic use.

According to the pH values, the AWG-filtered water in the study area is neutral to alkaline (Ayers & Westcot 1985); thus, the water can be used for domestic and irrigation purposes. Eight out of 12 samples were neutral, two were slightly alkaline, and the last two were alkaline (Table 3).

Moreover, AWG-filtered water is suitable for irrigation, as certain plants, including carrots, tomatoes, and cucumbers, demonstrate tolerance to acidic water conditions (Adebayo et al. 2021). In contrast, species, such as beans, cabbage, and celery, are sensitive to pH variations and may suffer adverse effects with any further decrease in pH levels (Adebayo et al. 2021). According to Keesstra et al. (2012), irrigating land with water with a high pH can impact soil fertility negatively. Therefore, it is crucial to conduct further evaluations of the AWG-filtered water to ensure its suitability for irrigation. This proactive approach will help maintain land productivity and health.

Parameters used to determine water quality for irrigation involve assessing the total salt concentration, measured by EC (Nolakana et al. 2017). Therefore, Table 4 displays the classification of water based on EC (Richards 1954; Wilcox 1955) for evaluating the salinity hazard of AWG-treated water. Table 4 shows that all AWG-treated water samples are below the 250 μs/cm range, indicating that they are classified as low saline hazard (Richards 1954; Wilcox 1955). To promote healthy plant growth, it is essential to manage salinity levels effectively. High salinity increases the osmotic pressure of soil water, which can impede plant roots from absorbing water and lead to physiological drought (Nolakana et al. 2017).

Keeping salinity below the 250 μs/cm threshold can create a more favourable environment for plants to thrive. The low salinity of AWG water suggests that the AWG treatment/filtration process effectively manages salinity levels, contributing to the overall quality and safety of the water. Furthermore, Table 4 shows that the AWG-filtered water in the study area is entirely safe and, thus, can be utilised for irrigation purposes due to its low salinity hazard.

The TDS classification in Table 4 shows that all the AWG-filtered water samples are desirable for drinking and fall under the freshwater classification. Thus, the water is suitable for both drinking and irrigation purposes. According to Robinove et al. (1958), this water is considered suitable for irrigation purposes.

Additionally, the AWG-filtered water exceeded the SAQWG drinking water standard for ammonia; however, ammonia serves as a fertiliser for crops in the agricultural industry, making the water suitable for irrigation (Ghavam et al. 2021).

Table 5 displays the chloride classification of water based on Bauder et al. (2014), and all AWG-filtered water samples fell below the 70 mg/l range. Therefore, all the AWG-filtered water samples are safe for all plants. Some chloride tolerance levels of selected crops, according to Bauder et al. (2014), are as follows (from low to high tolerance): dry bean, onion, carrot, lettuce, pepper, corn, potato, alfalfa, sudangrass, zucchini squash, wheat, sorghum, sugar beet and barley. Thus, the AWG-treated water will be a good alternative water source for irrigation, especially in water-scarce areas.

Calcium, magnesium, and potassium all complied with the SAQWG and WHO drinking water guidelines; thus, the AWG-filtered water is also good for irrigation purposes. Nagaraju et al. (2014) suggested that managing magnesium levels in irrigation water is important, as excessive magnesium can impact crop yields and increase soil alkalinity. Farmers can promote healthier crops and maintain balanced soil conditions by monitoring and adjusting these levels. In addition, a study conducted by Oster et al. (2016) indicated that managing potassium levels in irrigation water is crucial, as elevated potassium concentrations may hinder infiltration, water availability, and plant growth.

The moderate levels of magnesium and calcium found in the studied water positively contribute to its suitability for irrigation, according to Ogunfowokan et al. (2013). Their higher proportion relative to sodium plays a beneficial role in enhancing soil permeability and improving the rate of water infiltration (Ogunfowokan et al. 2013). This suggests that utilising this water for irrigation could effectively support soil health and agricultural productivity. Furthermore, sulphate complies with the SAQWG and WHO drinking water guidelines, and sulphate in irrigation water has fertility benefits, making AWG water a good alternative source for irrigation.

The maximum allowable concentration of fluoride in irrigation water can differ greatly between countries and depends on the specific irrigation conditions (Scholz et al. 2015). To ensure optimal agricultural practices, it is essential to first assess the soil type in the area before determining the appropriate fluoride levels. However, the fluoride levels in all AWG-treated water samples fell within the SAQWG and WHO drinking water guidelines; thus, the water can be utilised for irrigation after further assessment of the soil types. Furthermore, managing fluoride levels in vegetation is essential, as excessive accumulation can result in visible leaf damage, affect the quality of fruits, and lead to changes in overall yield (Fluoride in Soil & Plant 2016).

All trace elements (aluminium, nickel, copper, iron, manganese, silica, and zinc) complied with the SAQWG and WHO permissible limits for drinking water, thus making the AWG-filtered water suitable for irrigation use. However, the way plants respond to metals varies according to the amount to which they are exposed (Reichman 2002). Understanding this dose-dependent relationship can help better manage plant health and environmental conditions. For essential metals such as copper, zinc, and iron, the response addresses the phases from deficiency to sufficiency/tolerance and toxicity. In contrast, only the phases of tolerance and toxicity are relevant for non-essential metals such as aluminium and silica (Reichman 2002).

Crops cultivated in contaminated soil can accumulate significant amounts of heavy metals, leading to negative health effects for people consuming these crops (Ahmed et al. 2018). Understanding metal toxicities can help us identify a wide range of plant symptoms, providing valuable insights for accurate diagnosis and effective management. Copper, zinc, and manganese toxicity can lead to chlorosis and reddening of younger leaves, often resulting in necrotic lesions in severe cases (Reichman 2002). Furthermore, according to Liu et al. (2005), heavy metals are known to accumulate more easily in the edible parts of leafy vegetables than in the fruits or grains of crops.

Table 6 provides a detailed analysis of the microbiological parameters, specifically total coliform bacteria and Escherichia coli. This assessment was conducted in accordance with the SAWQG and WHO drinking water risk assessment guidelines, ensuring a rigorous evaluation of water quality.

According to Table 6, all AWG-filtered water samples were less than 1 for total coliform bacteria; therefore, all the water samples were within the permissible limits of the SAQWG and WHO drinking water guidelines with no consumption risks. Furthermore, E. coli was not detected in all the AWG-filtered water samples; therefore, the water is safe for drinking. The results presented in Table 6 demonstrate that both total coliform bacteria and E. coli were absent in all samples of AWG-filtered and disinfected water, which can be attributed to the effectiveness of the UV steriliser.

The microbiological analysis confirms that the disinfection process has successfully removed pathogenic bacteria, which are known to pose a significant risk of infectious diseases to humans (Nedelkova et al. 2019). Given the importance of safeguarding public health from the impacts of faecal water contamination (Bumadian et al. 2013), the disinfection of AWG-filtered water is an effective method for ensuring safe and clean water for the consumers.

The descriptive statistics for all the physicochemical parameters of the AWG-filtered water indicate that all the water samples complied with the SAQWG and WHO drinking water guidelines, with only two variables falling outside the permitted ranges. The two variables of concern were pH and ammonia; therefore, further monitoring is recommended for these variables. Furthermore, the safety of drinking the AWG-filtered and disinfected water was proven by the absence of total coliform bacteria and E. coli in all of the analysed water samples.

The analysis of irrigation water, focusing on key physicochemical parameters such as TDS, EC, pH, and chloride levels in AWG-filtered water samples, reveals that these samples are deemed suitable for irrigation use. This positive assessment supports the potential application of AWG-filtered water in agricultural practices. The quality of irrigation water available to farmers is crucial in determining the range of plants that can be successfully cultivated. By ensuring high-quality water, farmers can enhance plant productivity, improve water infiltration, and positively influence the physical condition of the soil, leading to more sustainable agricultural practices and better yields.

The authors express their gratitude to the University of South Africa for the financial support provided to conduct this study. Additionally, Aqua Air Africa (Pty) Ltd is appreciated for granting permission to collect data on their premises.

This study was funded by the University of South Africa Postgraduate Bursary Fund.

Material preparation, data collection, and analysis were performed by A.M. The conceptualisation of the study was done by A.M., L.L.S., M.M., and T.S.M. The first draft of the manuscript was written by A.M. Writing, review, and editing were done by A.M., T.S.M., M.M., and L.L.S. All authors read and approved the final manuscript.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.