A comprehensive analysis of institutional wastewater for irrigating ornamental plants

Water, paramount to life, has remained a focal point on the global agenda for decades. It is an indispensable natural resource, as it is fundamental for sustaining life, facilitating industrial operations, and enabling irrigation activities (UN World Water Development Report 2023). Despite this attention, the supply of freshwater worldwide is diminishing steadily, largely due to the substantial agricultural need for irrigated lands (Boretti & Rosa 2019). Consequently, there is a pressing need for greater efficiency in water usage, alongside an increased utilization of non-conventional water sources, such as treated wastewater (TW) (Amali et al. 2021). Wastewater can be repurposed for various applications, including agricultural and landscape irrigation, industrial processes, groundwater replenishment, urban tasks like street cleaning and firefighting, as well as ecological and recreational purposes (Nas et al. 2020). Notably, compared to other sectors, the utilization of wastewater for irrigation is more widely accepted (Pescod 1992; Ofori et al. 2021). Many nations have opted to repurpose wastewater for irrigation, aiming to alleviate urban water demands and combat water scarcity (Bauer et al. 2020).

The utilization of TW for irrigating field crops and landscaping plants addresses the increasing water scarcity challenges faced by many regions, ensuring sustainable water management practices, especially in dry areas (Jiménez & Asano 2008). Hashem & Qi (2021) presented a comprehensive review on the reuse of TW for irrigation. Wang et al. (2017) present a comprehensive review of using reclaimed water for agricultural and landscape irrigation in China. TW contains essential nutrients, contributing to improved soil physicochemical properties, fertility, nutrient status, and plant growth (Abdel-Aziz 2015; Al-Busaidi & Ahmed 2017). Additionally, employing TW helps mitigate environmental pollution by reducing the discharge of pollutants into natural water bodies. However, the inherent nature of this water source raises significant concerns regarding its suitability for irrigation, including potential health risks, salinity accumulation, and toxicity hazards (Libutti et al. 2018; Hussain & Qureshi 2020). Therefore, correct methods must be followed in the treatment and use of wastewater for irrigation. Various national and international agencies provide guidelines for irrigation water quality and set limits on the allowable concentrations for various parameters in irrigation water, particularly for agricultural purposes, to ensure human health and environmental protection (Shoushtarian & Negahban-Azar 2020).

Lubello et al. (2004) investigated the feasibility of utilizing tertiary-treated municipal wastewater incorporating filtration and disinfection using peracetic acid and ultraviolet for irrigating six nursery ornamental plants, including Cupressus sempervirens, Juniperus horizontalis, Myrtus communis, Arbutus unedo Spiraea japonica, and Weigelia florida. Their findings revealed comparable plant growth between TW and fertigated water, satisfying soil nutrient requirements and irrigation regulations regarding bacteria removal. Gori et al. (2008) explored the potential for reusing well water, reclaimed textile wastewater, and a mix of both for irrigating container-grown ornamental shrubs, i.e., Buxus, Photinia, Pistacia, and Viburnum. They found that a combined treatment of peracetic acid and ultraviolet for disinfection was highly effective in inactivating E. coli.Najafi et al. (2014) examined the impact of urban TW irrigation on Laurus Nobilis L. and Buxus Sempervirens L., utilizing various irrigation methods, including furrow, drip, and sub-surface drip irrigation methods. Their findings demonstrate that wastewater irrigation positively influenced the growth of both plant species.

Younis et al. (2015) evaluated the effects of sewage water and recycled TW on three ornamental plant species (Umbrella plant, Euonymus, and Dracaena), finding improved growth and development, but higher lead concentrations in plants irrigated with sewage water after recycling. El Khoumsi et al. (2024) evaluated the impact of TW irrigation versus fresh water on the growth of two ornamental plants Rosa Sinensis Hibiscus and Lantana Camara, finding that TW generally enhances agronomic (height, number of branches, leaves, and flowers) and aesthetic parameters for both plants. Anangadan et al. (2024) investigated the use of greywater and freshwater for irrigating the ornamental plant Ruellia Tuberosa and compared their effects on growth, health, and soil properties. They observed that greywater enhances nutritional and soil conditions and proposed its direct use for green wall irrigation in water-scarce regions as an alternative to energy-intensive greywater treatment systems.

The studies mentioned above and many others (Areola et al. 2011; Castro et al. 2011; Minhas et al. 2022; Yadav et al. 2023) underscore the potential for using domestic, municipal, and industrial wastewater in agriculture, highlighting the need for proper wastewater treatment, irrigation method, and soil quality monitoring to mitigate adverse effects. The discussion above also indicates that the reuse of TW for irrigating ornamental plants is a subject of growing interest in sustainable irrigation water management. While existing research explores the reuse of domestic, municipal, and industrial wastewater irrigation on various ornamental plants, there is a dearth of studies focusing on the utilization of institutional wastewater. Moreover, a notable gap exists in studies specific to certain ornamental plants, i.e., Dianthus barbatus, Dianthus chinensis, and Phlox drummondii. This underscores the necessity for a comprehensive investigation into the feasibility and impact assessment of TW irrigation on these plants.

This work aimed to evaluate the suitability of treated and untreated institutional wastewater compared to traditional borewell water sources for irrigating three container-grown ornamental plants. The specific objectives are:

• 1. To analyse the characteristics of institutional wastewater, secondary TW, tertiary TW, ozonated TW, and borewell water used for irrigating three ornamental plants.

• 2. To assess the impact of different irrigation treatments on the growth of these ornamental plants.

The experiments were conducted on the campus of Shoolini University of Biotechnology and Management Sciences (hereinafter referred to as Shoolini University), which is located in the district of Solan, Himachal Pradesh, India. The elevation of the study location is 1,350 m above mean sea level, and the geographical coordinates are 30.9084° N 77.0967° E. Solan experiences a subtropical highland climate with warm summers and chilly winters. The average temperature in Solan is 10 °C, ranging from −4 °C in January to 34 °C in June. The average annual rainfall at the study location is 1,500 mm.

The WWTP at the university is a recycled activated sludge plant, where the hybrid sewage undergoes a rigorous treatment process comprising several sequential operations. Initially, raw hybrid sewage is collected and stored in sewage storage tanks, allowing for the accumulation and regulation of influent flow. The sewage is then subjected to preliminary screening using closely spaced vertical bars to remove large debris such as debris, plastics, and other floating materials. Following this, the wastewater passes through an oil and grease trap to separate hydrophobic substances. Subsequently, the pre-treated wastewater enters an equalization tank to balance flow rates and pollutant concentrations, ensuring consistent treatment performance. The wastewater then undergoes aerobic biological treatment in an aeration tank, facilitated by the introduction of air through mechanical aerators. During this process, the microorganisms present in the wastewater metabolize organic pollutants, converting them into simpler, less harmful substances.

Next, the wastewater flows into a tube settler unit for sedimentation, where solids are separated from the TW. The settled solids form a sludge layer at the bottom of the tank, while clarified water rises to the top. A sludge pump is utilized to remove sludge from the tube settler, which is then recycled to a filter press and further processed through sludge drying methods to reduce moisture content and facilitate disposal or reuse. After sedimentation, the clarified water undergoes chlorination to achieve disinfection, effectively eliminating pathogenic microorganisms and ensuring the microbiological safety of the treated sewage. This chlorination was carried out using sodium hypochlorite at a dose of 40 mg/L. A filter pump is then employed to direct water to sand and carbon filters for further purification before storage in a treated water tank. These filters further remove suspended solids, residual contaminants, and odours from the treated water, enhancing its quality. From the treated water tank, water is pumped to an ozonation unit, where ozone oxidizes remaining organic compounds, micropollutants, and pathogens, further improving water quality before discharge or reuse. Throughout the treatment process, an air blower provides aeration in the equalization tank, aeration tank, chlorination tank, and filter press, which promotes the growth of aerobic microorganisms, enhances treatment efficiency, and aids in the removal of dissolved gases and odours.

The TW is used for irrigation (both ornamental and field plants), construction, and washing activities of the university campus. To optimize the use of TW, approximately 80% (280 m³/day) is designated for irrigation purposes, which has been derived through a comprehensive analysis based on the irrigation needs of the ornamental and field plants. This allocation is determined by the specific water requirements of the various plant species and green areas on campus, ensuring that the landscaping is adequately maintained without overuse of the treated water resource. The TW is pumped thrice from the WWTP as per the laid-down timings and is stored in storage tanks. This helps in regulating the flow and ensuring that there is a steady supply of water available for irrigation throughout the day, accommodating variations in demand. A dedicated pumping and distribution network ensures that TW is delivered to different parts of the campus efficiently. However, during extreme winter conditions and rainy seasons, the utilization of TW is adjusted on an as-needed basis, and any surplus TW is diverted to the nearby forest area. There is no additional investment for water quality improvement in this study, as the treatment processes (including secondary, tertiary and ozone treatment) at the WWTP of the institution are continuously in operation.

The TW from the chlorination tank, treated water tank, and ozonation unit is designated as secondary, tertiary, and ozone-treated wastewater, respectively, in this study.

The agronomic experiment used three distinct types of flowering plants: Sweet William (Dianthus Barbatus), Annual Phlox (Phlox Drummond), and Rainbow Pink (Dianthus Chinensis). These are popular ornamental garden plants indigenous to the southern regions of Asia and are widely used in the institution's landscaping activities. The plants were planted on 23 November 2022 and grown in containers placed outdoors in an open area near the WWTP. The experimental observations continued for about 8 months until 18 July 2023. The containers had dimensions of 35 cm in height, with an upper diameter of 32 cm and a lower diameter of 20 cm. The containers were filled with 11.25 kg of soil from the surrounding area, thoroughly mixed, removing all rocks, vegetation, and other contaminants.

The standard practices for irrigation and pest control in commercial nursery production were adhered to. The surface irrigation method was adopted rather than sprinkler irrigation to avoid potential negative impacts on plants (Gori et al. 2008). The irrigation was applied from 9:00 to 10:00 h using a watering can based on the visual appearance of soil and plants. The irrigation amount per plant varied between 0.5 and 1 L, and the frequency varied between 7 and 15 days, depending on the growth stage of plants and prevailing climatic conditions. During the initial growth phase (November–December), plants received 0.5 L of water every 7 days. However, from January to February, the irrigation interval was extended to 15 days while maintaining the same volume. During the flowering stage (March–May), plants were irrigated with 1 L of water at an interval of 7 days. Post-May, as plants approached maturity, the irrigation interval was again adjusted to 15 days, maintaining the increased volume. The process of irrigating all the plants took approximately 15–20 min. It should be noted that all three ornamental plant species were uniformly irrigated simultaneously to prevent any discrepancies in the subsequent analysis of irrigation treatments and their impact on plant growth parameters, including height, leaf, and flower growth, which were observed during the experimental period.

The wastewater quality parameters of irrigation treatments T1, T2, T3, T4, and T5 were monitored and investigated throughout the experimental period. The water samples were collected during each irrigation event and analysed at the Shoolini Life Sciences (www.shoolinilifesciences.com) laboratory on the Shoolini University campus. The parameters were pH, electric conductivity (EC), total suspended solids (TSS), turbidity, total hardness, ammonia, colour, odour, residual free chlorine (RFC), total alkalinity, dissolved oxygen (DO), biological oxygen demand (BOD), chemical oxygen demand (COD), potassium, sodium, calcium, boron, E. coli, total coliform, and faecal coliform. A detailed description of these parameters can be found in standard textbooks on water and wastewater engineering, such as Metcalf et al. (1991) and Punmia et al. (2022). All samples for physical–chemical–biological analysis were collected during irrigation and immediately analysed according to the methods indicated in Table 1. These parameters were compared among different treatments and with irrigation water quality standards. All the data were subjected to analysis of variance (ANOVA) using SPSS, with a 0.05 significance level.

The main chemical, physical, and biological parameters of the water sources used in the experiment are summarized in Table 1. These parameters play a crucial role in determining soil health, nutrient availability, and plant productivity. The mean values of parameters across three replications, their standard deviation, and methods used for determining these values are given in Table 1. Understanding the variations in these parameters across different water sources is essential for making informed decisions regarding irrigation practices and optimizing agricultural sustainability (Ara et al. 2021). Therefore, a comparative analysis of these values for different irrigation water sources is presented and discussed. The statistical differences between different irrigation water treatments (T1–T5) were evaluated using ANOVA.

The pH values of water sources used for irrigation vary significantly (p < 0.05) across the treatments. T1 (7.40) and T4 (7.42) exhibit neutral to slightly alkaline pH levels, which is ideal for irrigating most ornamental plants, as it provides a balanced environment for nutrient uptake and microbial activity, which promotes healthy root development and overall growth. T2 (6.90) and T3 (7.27) show slightly acidic to neutral pH, indicating the presence of acidic compounds or contaminants, which can hinder nutrient absorption and root function and possibly make plants more susceptible to certain diseases or stress conditions (Kidd & Proctor 2001). This necessitates monitoring before irrigation. T5 demonstrates a moderately alkaline pH (7.70), which can benefit soil health and plant growth. The higher standard deviation observed in T2 and T3 samples suggests significant variability in pH values. This is likely attributed to the diverse composition of sewage, including organic matter, chemicals, and microbial activity. This variability underscores the complexity of untreated wastewater and highlights the need for effective treatment strategies to mitigate environmental impacts. The relatively low standard deviation suggests a consistent pH profile within T1, T4, and T5 samples, reflecting stable environmental conditions and minimal anthropogenic influences.

The electrical conductivity of irrigation water varies significantly (p < 0.05) across treatments. T2 exhibits the highest conductivity (1,148.67 μS/cm), indicating elevated levels of dissolved ions, salinity, and potential contaminants, which could potentially impact plant growth adversely. On the other hand, T1 shows comparatively lower conductivity (558.67 μS/cm), suggesting fewer dissolved solids, lower ion concentration, and salinity levels. Treatments T3 (815.44 μS/cm), T4 (806.22 μS/cm), and T5 (791.78 μS/cm) fall within intermediate conductivity ranges.

TSS is a crucial indicator of water clarity and can affect plant health and growth by interfering with light penetration and nutrient availability. There was a significant difference (p < 0.05) in the concentration of TSS across the irrigation treatments. T1 (0.00 mg/L) displayed negligible TSS levels, indicating excellent water clarity and minimal particulate matter, which minimizes the risk of clogging irrigation systems and ensures unimpeded water uptake by plant roots. T2 exhibits the highest TSS level (281.11 mg/L), which suggests potential contamination with organic and inorganic particles, potentially reducing water penetration into the soil. Treatments T3 and T4 displayed intermediate TSS levels, 222.22 and 169.56 mg/L, respectively. Among TW, T5 demonstrates the lowest suspended solids (145.88 mg/L), indicating effective WWTP treatment.

Turbidity levels vary significantly (p < 0.05) across different irrigation treatments. T1 exhibits negligible turbidity, indicating clear and high-quality water, which ensures optimal light penetration into the soil, promoting photosynthesis and healthy plant growth. T2 shows the highest turbidity (145.34 NTU), suggesting significant particulate matter and potential contamination with suspended solids, organic matter, and pollutants, which confirms the visual observations. This can lead to reduced light penetration and potentially hinder photosynthesis. Treatments T3 and T4 display lower turbidity levels, 57.22 and 48.36 NTU, respectively, which indicates improved treatment and filtration. T5 demonstrates the lowest turbidity (36.93 NTU) and reflects effective purification processes. Still, however, the presence of suspended particles in T3, T4, and T5, albeit at lower levels, can affect light penetration and nutrient availability in the soil.

The total hardness of irrigation water can impact plant growth by affecting nutrient availability, soil structure, and root development. There was a significant difference (p < 0.05) in the total hardness values across irrigation treatments. T1 exhibits relatively high hardness (251.67 mg/L), which is common in groundwater sources. These hardness levels provide essential minerals, such as calcium and magnesium, which can benefit plant growth. However, excessively hard water may lead to the build-up of mineral deposits in the soil and irrigation system. T2 demonstrates slightly higher levels (267.89 mg/L), possibly due to mineral leaching from sewage pipes. The presence of other contaminants and pollutants in sewage water may offset the positive effects of hardness, leading to nutrient imbalances and adverse effects on plant health. TW treatments show reduced hardness: T3 (204.78 mg/L), T4 (179.89 mg/L), and T5 (172.00 mg/L), which indicates effective removal of mineral ions from the wastewater through proper filtration and treatment. With relatively lower total hardness levels, T5 provides softer irrigation water than other sources.

The colour and odour of irrigation water can potentially impact plant growth by indicating the presence of contaminants and organic matter, which may affect soil health and plant development. The colour of irrigation water sources was significantly different (p < 0.05) across the treatments. Similarly, water odours varied across irrigation sources. T1 with a low colour intensity (1 Hazen units) and agreeable odour suggests relatively clean and uncontaminated water suitable for irrigation. T2 exhibits the highest colouration (60 Hazen units) and disagreeable odour, indicating the presence of significant organic and inorganic contaminants in the institutional wastewater, which may pose risks to plant growth and soil health due to potential nutrient imbalances. T3 (40 Hazen units) exhibits a lower colour intensity compared to T2, suggesting some improvement in water quality through treatment processes. However, a disagreeable odour indicates the persistence of organic compounds, reflecting contamination. A considerable reduction in colouration was observed and recorded in T3 (40 Hazen units), T4 (40 Hazen units), and T5 (25 Hazen units). T4 with a similar colour intensity to T3 but an agreeable odour reflects the effective removal of organic matter and contaminants during treatment. T5 (25 Hazen units) displays a considerable reduction in colouration among the water sources, indicating high water clarity and minimal presence of organic matter and contaminants. Additionally, the agreeable odour suggests improved water quality, with reduced potential risks to plant growth and soil health.

Monitoring RFC aids in ensuring water safety for irrigation as its presence can have varying impacts on plant growth, depending on the concentration and the sensitivity of the plant species to chlorine. RFC levels significantly varied (p < 0.05) across different irrigation treatments. T1 displayed minimum RFC levels (0.42 mg/L), possibly from natural disinfection. RFC at low levels can help control pathogens and algae in the water without significantly harming the plants. T2 with no detectable RFC may indicate the absence of chlorine-based disinfection treatments or complete chlorine degradation. While chlorine-free water eliminates the risk of chlorine toxicity to sensitive plants, sewage water may contain other contaminants and pathogens that could adversely affect plant growth. T3 containing RFC at relatively higher concentrations (2.76 mg/L) may have a more pronounced impact on plant growth and can be detrimental to sensitive plants, causing leaf burn, stunted growth, and nutrient uptake inhibition. T4 and T5 exhibit moderate RFC levels of 1.63 and 1.35 mg/L, respectively, indicating strong disinfection processes being followed in the WWTP but may pose some risk to plant growth, particularly for chlorine-sensitive species.

The total alkalinity of irrigation water can impact plant growth by influencing soil pH stability and nutrient availability. Total alkalinity values varied significantly (p < 0.05) among irrigation water sources. T1 exhibits relatively high alkalinity (263.61 mg/L), which is typical in groundwater sources. Such high levels may contribute to maintaining stable soil pH, which is beneficial for plant growth. Adequate alkalinity can help buffer against fluctuations in pH, providing a favourable growing environment for ornamental plants. However, excessively high alkalinity levels may lead to alkaline soil conditions, potentially affecting nutrient availability and plant health. T2 shows slightly lower alkalinity levels (243.94 mg/L) compared to T1, which may be possible due to dilution but still contributes to soil pH stability. TW sources display further reduced alkalinity levels: T3 (207.72 mg/L), T4 (197.39 mg/L), and T5 (181.83 mg/L), reflecting the use of specific control agents in treatment processes adopted in WWTP, thereby indicating reduced buffering capacity. While still contributing to pH stability, lower alkalinity levels may require additional amendments to maintain optimal soil pH and nutrient availability for ornamental plant growth.

The DO levels in irrigation water are critical for supporting plant growth. The DO levels of irrigation water sources differed significantly (p < 0.05) across the treatments. T1 exhibits the highest DO (8.54 mg/L), typical in clean groundwater. Relatively high DO levels provide favourable conditions for root respiration and soil microbial activity, supporting healthy plant growth. T2, with the lowest DO (6.61 mg/L), indicates moderate organic pollution and reduced oxygen availability in the soil, potentially leading to oxygen stress in plant roots and soil microorganisms. Inadequate oxygen levels can inhibit root growth and nutrient uptake, compromising plant health and development. Treatments T3 and T4 display adequate DO values of 7.87 and 7.80 mg/L, respectively, indicating that the aeration process has effectively improved the DO of wastewater. Adequate oxygen availability in the soil promotes nutrient uptake and root development, contributing to overall plant vigour and vitality. T5, with relatively high DO levels (8.19 mg/L), provides optimal conditions for plant growth, similar to T1. These values reflect the effectiveness of the ozonation process.

The levels of BOD and COD in irrigation water indicate the amount of oxygen required for microbial degradation of organic matter and pollutants, respectively. BOD and COD levels were significantly different (p < 0.05) across the irrigation treatments. T1 exhibits the lowest values of BOD (0.96 mg/L) and COD (15.33 mg/L), suggesting minimal organic and inorganic pollution, suggesting clean and suitable water for irrigation. Low levels of organic pollutants reduce the oxygen demand in the soil. T2 shows the highest BOD (2.24 mg/L) and COD (54.00 mg/L), indicating significant organic and inorganic contamination of the wastewater, posing risks to plant growth. Increased oxygen demand from organic decomposition can lead to oxygen depletion in the soil, negatively impacting root health and nutrient availability. TW sources display reduced BOD and COD: T3 (1.99 and 40.00 mg/L), T4 (1.76 and 32.78 mg/L), and T5 (1.40 and 30.67 mg/L). These values reflect the effectiveness of the treatment processes in removing organic and inorganic contaminants from institutional wastewater. While improved compared to hybrid sewage, these levels may still pose risks to plant growth. Reduced oxygen demand and pollutant concentrations in T5 may support healthier root development and nutrient uptake, contributing to enhanced plant growth and vitality.

The presence of ammoniacal nitrogen (⁠⁠) in TW can indicate potential issues in the biological treatment processes, specifically in the nitrification stage. The presence of toxic substances or high levels of certain chemicals in the influent wastewater can inhibit the activity of nitrifying bacteria, which can lead to incomplete nitrification and higher ammoniacal nitrogen concentrations in the effluent. The levels of ammoniacal nitrogen in irrigation water can significantly impact plant growth. It is a nitrogen source for ornamental plants but can also be toxic at high concentrations. The concentration of ammoniacal nitrogen varied significantly (p < 0.05) across irrigation treatments. T1 exhibits minimal values, which suggests minimal ammoniacal nitrogen contamination (0.26 mg/L), and can serve as a nitrogen source, providing a suitable irrigation source for ornamental plants. T2 with elevated levels indicates significant nitrogen contamination (20.43 mg/L), potentially exceeding the tolerance levels of ornamental plants. High concentrations can inhibit root growth, disrupt nutrient uptake, and lead to leaf burn or discolouration, adversely affecting plant health and growth. Treatments T3 (1.32 mg/L) and T4 (1.08 mg/L) exhibit moderate levels, suggesting some reduction in nitrogen contamination through treatment processes. While lower than T2, these concentrations may still pose risks to plant growth if not adequately managed. T5 displays the lowest levels (0.88 mg/L) among the treated irrigation sources, suggesting effective treatment and minimal nitrogen contamination. Lower ammoniacal nitrogen concentrations reduce the risk of toxicity and promote healthier root development.

The concentrations of minerals, such as potassium, sodium, calcium, and boron, in irrigation water can significantly impact plant growth, as these elements are essential for various physiological processes.

Sodium levels were found to vary significantly across irrigation water sources. T1 exhibits the lowest sodium content (53.24 mg/L), while T2 has the highest (165.62 mg/L). The elevated sodium in hybrid sewage suggests potential soil salinity issues, emphasizing the importance of effective water treatment in irrigation to avoid adverse effects on plants and soil health. TW sources, including T3, T4, and T5, show intermediate sodium levels.

Boron is essential for plant cell wall formation, pollen germination, and carbohydrate metabolism. However, excessive boron levels can be toxic to plants, leading to stunted growth and leaf necrosis. There was a significant difference in boron levels among different irrigation treatments. T1 exhibits the lowest boron content (0.77 mg/L), followed by T5 (1.40 mg/L), while T2 shows the highest level (11.08 mg/L). Treatments T3 and T4 display moderate but undesirable boron levels in irrigation water. Boron levels impact plant growth, and their elevated levels in TW sources emphasize the need for careful irrigation management to prevent boron accumulation in soils.

Potassium is essential for plant growth and is crucial in enzyme activation, osmoregulation, and photosynthesis. Potassium levels were significantly different among the irrigation treatments. T2 has the highest potassium content (25.87 mg/L), whereas T1 exhibits the lowest (1.84 mg/L). TW, including T3, T4, and T5, show intermediate potassium levels in irrigation water. Higher potassium in wastewater indicates potential nutrient imbalance in irrigation, necessitating careful management to prevent soil salinity and optimize plant growth and yield.

Calcium is vital for cell wall formation, root development, and nutrient uptake in plants. Calcium levels varied significantly among irrigation water sources. T1 exhibits the highest calcium content (155.01 mg/L), followed by T2, T3. Treatment T4 and T5 show relatively lower calcium levels, indicating potential differences in water treatment processes and calcium content, which could affect soil structure and plant growth.

The microbial contamination of water sources was different across irrigation treatments. T1 shows the absence of E. coli, total coliform, and faecal coliform, indicating low microbial contamination. T2 exhibits the presence of E. coli, total coliform, and faecal coliform, reflecting untreated sewage contamination. TW sources, including T3, T4, and T5, display absence of any microbial contamination, which indicates effective treatment. These findings underscore the importance of water treatment processes in mitigating microbial risks, ensuring safe irrigation practices, and preventing potential health hazards and plant contamination in agricultural settings.

The discussion above and the results in Table 1 indicate that a comprehensive treatment process is followed in the WWTP considered for the study. This ensures high-quality treated water is produced in compliance with regulatory standards, promoting environmental sustainability within the institution's infrastructure.

In Sweet William, the plants irrigated with T4 displayed better growth during the initial stage, followed by T5. However, during the later stages, the plant growth was better in T5, followed by T4. The maximum average height of the plants was also observed in treatment T5. The first flowering commenced in treatment T4 during April, i.e., after 5 months of planting. However, the number and growth of flowers in T5 were relatively better than other treatments. T1 and T2 did not contribute much to the growth & flowering of Sweet William plants. About 8 months after the seeding, the leaves started shedding in all the treatments, causing a significant reduction in flowering and consequently resulting in the dying of plants.

In Annual Phlox, the maximum average height of the plants was observed in treatments T3 and T5. During the initial stages, the plants irrigated with T3 exhibited better growth, which was followed by T4 and T5. However, during the later stages, T5 irrigated plants were found to have better growth, followed by T4 and T3. The flowering of plants was first observed in T5 during the last week of March (after 4 months of planting). During April and May, the plant growth and flowering were significantly good in T4 and T5 compared to other treatments. After 7 months of seeding, the growth of plants started reducing in all treatments, with significant shedding of leaves and fading of flowers. It was observed that, in the case of T2, the flower petals started fading much earlier and faster than other treatments.

In Rainbow Pink, it was observed that the average height of the plants was maximum in treatments T4 and T5. During the initial stages, T3 irrigated plants exhibited better growth, which T4 and T5 followed. However, during the later stages, T4 and T5 irrigated plants were found to have better growth, followed by T3. The first flowering commenced during March (3 months after planting) in treatment T1, followed by T3. During April, the growth and flowering of plants were excellent in all treatments. Treatments T4 and T5 showed good but slightly reduced flowering in May, with two-thirds of flowers surviving. However, in other treatments, a considerable reduction in flowering was observed. After 8 months of planting, a significant observation was made about regrowth during July. Treatments T4 and T5 exhibited considerable regrowth of leaves and flowers; however, this regrowth was limited in T3. Insignificant regrowth was observed in T1 and T2.

Rainbow pink exhibited the best growth during the initial stages, followed by Annual Phlox. Sweet William plants had the slowest growth during the initial stages. A similar pattern was observed in the case of flowering. The first flowering commenced after 3 months in all the plants of Rainbow Pink. This was followed by Annual Phlox and Sweet William, where the flowers emerged during the fourth and fifth months of planting. Sweet William plants displayed the slowest growth of flowers. The Sweet William plants were tall and steadily grew during all the stages. However, Annual Phlox plants were widely spread and showed limited growth. It was observed that the growth and density of leaves of Annual Phlox were better than other plants. The Rainbow Pink plants displayed the best flowering among different samples. The reduction in leaves and fading of flowers was first observed in Annual Phlox plants, which was followed by Sweet William and Rainbow Pink. Both Annual Phlox and Sweet William died after shedding leaves and flowers. However, Rainbow Pink experienced the regrowth of stems, leaves, and flowers. It was also observed that Annual Phlox plants were more sensitive to climatic conditions and irrigation water sources; however, low sensitivity was observed for other ornamental plants.

The results indicate no significant limitations to using TW as an irrigation source for the ornamental plants considered in this study. In fact, the nutrient content of the TW was sufficient to support healthy plant growth. It is clear from the above discussion that the ornamental plants irrigated with T4 and T5 exhibited enhanced growth than other treatments. The pH levels in all the treatments were in the ideal range (6.5–7.5) for irrigating most ornamental plants, providing a balanced environment for nutrient uptake and microbial activity, which promotes healthy root and overall development, avoiding stunt growth and leaf chlorosis (Zhao et al. 2013). All treated irrigation sources contained higher levels of TSS and total hardness, which can clog irrigation systems and cause scaling in irrigation equipment. This was not the case with the present study, as surface irrigation was adopted. However, higher levels of TSS and total hardness in irrigation water impede nutrient uptake by plant roots and were observed in all three ornamental plants irrigated with T2. Relatively lower levels, as in the case of T4 and T5, reduced the risk of mineral build-up and potential nutrient imbalances and supported healthier plant growth.

According to the World Health Organization, for irrigation water, the ammoniacal nitrogen concentration should generally not exceed 1.5 mg/L to prevent plant toxicity and soil degradation (WHO 2006). Except T2, all other treatments exhibited lower levels. The exceptionally higher level of ammoniacal nitrogen in T2 might have caused earlier fading of flower petals in Annual Phlox plants. Guo et al. (2022) observed that ammoniacal nitrogen can change the amino acid profiles in flower petals which in turn bring changes to petal colours. It was also observed that high concentrations of ammoniacal nitrogen can negatively affect the root growth (Lubello et al. 2004). The moderate levels of potassium, sodium, calcium, and boron in treatments T4 and T5 can possibly be linked with the regrowth of leaves and flowers in the Rainbow Pink plant. However, higher levels of total alkalinity in T1 and T2 might have caused the nutrient lockout, potentially contributing to the insufficient regrowth.

The higher levels of DO ensure aerobic conditions in the soil, promoting root health. This was particularly evident during the initial growth stages of all three ornamental plants irrigated with T3, T4, and T5. The TW irrigation sources (T3, T4, and T5) were free from any microbial contamination and do not pose any health risk if the ornamental plants are handled or used in public spaces or recreational areas of the institution. However, the odour from the treatment T2 was disagreeable (Table 1), which could negatively impact the health of the institutional population (students, teachers, staff, etc.).

The study comprehensively analysed the characteristics of treated and untreated institutional wastewater and presented the results of experiments carried out to assess the impact of reusing institutional wastewater for irrigating container-grown ornamental plants (Sweet William, Annual Phlox, and Rainbow Pink) under subtropical highland climatic conditions. The following conclusions are drawn from the study:

• The wastewater treatment plant considered in the study followed a comprehensive treatment process that ensures the production of high-quality TW in compliance with regulatory standards and promotes environmental sustainability within the institution's infrastructure.

• The tertiary and ozone-TW showed favourable physical and chemical characteristics for irrigation; however, hybrid sewage and secondary TW reflected sub-optimal characteristics.

• The secondary, tertiary, and ozone-treated institutional wastewater sources display no microbial contamination, which indicates effective treatment in mitigating microbial risks, ensuring safe irrigation practices, and preventing potential health hazards and plant contamination in landscaping settings.

• The statistical analysis indicated significant differences (p < 0.05) in plant height across irrigation treatments for Sweet William and Annual Phlox but not for Rainbow Pink. However, the Annual Phlox plants exhibited higher sensitivity to climatic conditions and irrigation water sources than other ornamental plants.

• The best plant growth and flowering among Sweet William, Annual Phlox, and Rainbow Pink was observed in the treatments irrigated with tertiary and ozone TW.

Based on the above conclusions, tertiary and ozone-TW irrigation can be considered an important source of nutrients and minerals for ornamental plants. This practice not only reduces reliance on synthetic fertilizers but also offers positive economic and environmental advantages, making it a recommended option, especially as an emergency water supply.

Considering the aforementioned findings, it is recommended that further experimental research be carried out to investigate the suitability of institutional wastewater on other ornamental plants and crops across varying climatic conditions. However, special focus needs to be placed on removing ammoniacal nitrogen from the wastewater by adopting cost-effective and environmentally friendly methods. This study can serve as a prototype for other institutions aiming to utilize TW for their irrigation requirements, demonstrating the feasibility and benefits of such an approach.

We would like to extend our sincere gratitude to the estate wing, landscaping staff, and sewage treatment plant staff of Shoolini University for their invaluable assistance and support in this research endeavour.

The research work did not receive any external funding or support.

KSN conceptualized the whole article, developed the methodology, rendered support in formal analysis, investigated the article, and wrote the original draft. NK supervised the work, rendered support in project administration, validated the data, wrote the review and edited the article. AP supervised the work, rendered support in project administration, validated the data, wrote the review and edited the article.

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

The authors declare there is no conflict.