Evaluating the environmental impacts of nutrient film technique hydroponic systems with alternate water sources: a life cycle assessment perspective

Currently, meeting the global water demand with freshwater remains a significant challenge due to the escalating depletion of water quantity and degradation of its quality (Lyu et al. 2016). Key drivers of this global water demand include water-intensive agriculture, industrialization, rapid urbanization, and evolving living standards (Hanjra et al. 2012). The excessive utilization of freshwater by these factors poses unprecedented challenges to humanity in the near future. By 2050, the global and urban populations are projected to rise to around 9–10 billion and 6.4 billion, respectively, accompanied by a rapid increase in water consumption rates (Evely 2010; FAO 2017). Consequently, the demand for irrigation water is expected to surge, estimated to reach 2.9 thousand km3 by 2050 (FAO 2017; Liu et al. 2017). By the end of 2050, global water usage is anticipated to escalate by 20–30% from current levels (Wada et al. 2016). Predictions suggest that global water demand may surge by 20%, potentially reaching 5,440 km³ by 2050 under a sustainability scenario, with an annual increase ranging between 360 and 660 km³ (Wada et al. 2016). To address the challenge of feeding this expanding population amidst escalating water scarcity, transitioning to modern agriculture practices that incorporate water reuse is imperative. Techniques such as irrigation utilizing brackish water, virtual water trade, rainwater harvesting, hydroponics, and controlled environment agriculture offer avenues to enhance food production while minimizing water usage (Kannan & Anandhi 2020; Preite et al. 2023).

Embracing an array of innovative and eco-friendly technologies, hydroponics has gained traction as a sustainable and reliable food production method that addresses the pressing concerns of food scarcity and water scarcity (Li et al. 2018). Notably, hydroponics surpasses traditional soil-based agricultural practices in optimizing resources, making it an efficient approach for large-scale crop production without exacerbating water scarcity challenges (Muller et al. 2017). Despite criticisms from proponents of agro-economic approaches, hydroponics presents a promising soil protection technique due to its minimal soil usage, aligning well with the goals of sustainable agriculture while mitigating impacts on water resources (National Organic Standards Board 2010). The appeal of hydroponics lies in its ability to offer both environmental and production benefits, supporting the well-being of farmers and communities while contributing to water resource management (Li et al. 2018).

Different types of water used for water reuse in agriculture include secondary-treated wastewater, chlorine-treated wastewater, and green wall-treated greywater (Fernandes et al. 2023). Reclaimed water, which includes brackish/saline water and treated sewage effluent (TSE), is also used as an alternative water source for agriculture (Fernandes et al. 2023). In the Mediterranean region, reclaimed water is used for irrigation purposes, and it has been found to have beneficial effects on soil microorganisms and their activities (Leogrande et al. 2022). In context of hydroponics different types of waters have been tried. For instance, hydroponic systems can use various types of water, including freshwater, reclaimed water, deionized water, and desalinated water (Lee & Lee 2015). Relying on a single water source is not considered as a sustainable option. Therefore, the assessment of the environmental burden of alternate water sources is necessary. The Nutrient Film Technique (NFT) is a hydroponic system where a thin film of nutrient-rich water is continuously circulated over the roots of plants. This system involves placing plants in channels or tubes where their roots are exposed to a shallow stream of nutrients, water, and oxygen. The nutrient solution is pumped from a reservoir to one end of the channel and flows back into the reservoir, creating a closed-loop system. This method ensures efficient nutrient uptake and oxygenation of the roots, promoting healthy plant growth. NFT systems are highly water efficient as they recycle the nutrient solution, reducing water wastage. Additionally, the controlled environment minimizes the need for pesticides, which lowers the release of harmful chemicals into the ecosystem (Mohammed 2018). However, the system's reliance on continuous electricity for pumping the nutrient solution can lead to a higher carbon footprint, particularly if non-renewable energy sources are used (Wibisono & Kristyawan 2021).

Compared to other hydroponic techniques, NFT offers a balance between water efficiency and simplicity but comes with certain environmental drawbacks. For instance, while NFT is more water-efficient than Deep Water Culture (DWC), DWC systems generally require less energy since the roots are submerged in nutrient-rich water, eliminating the need for constant water circulation (Lennard & Leonard 2006). However, NFT systems are more prone to issues during power outages as the roots can quickly dry out without a constant water flow, unlike in Ebb and Flow systems where plants are periodically submerged in water (Silva et al. 2021). Moreover, the use of plastic components in NFT systems can contribute to environmental waste if not managed properly (da Silva Cuba Carvalho et al. 2018). Despite these challenges, NFT remains a popular hydroponic method due to its water efficiency and effectiveness in nutrient management.

The environmental impacts of different water types used in NFT hydroponic systems vary significantly, each presenting unique challenges and benefits in terms of sustainability and ecological footprint. Conventional water, often the most commonly used, leads to higher environmental burdens, including significant freshwater consumption and potential ecotoxicity impacts, as highlighted by life cycle assessment (LCA) (Nadeem et al. 2024).

Desalinated water, though a solution to freshwater scarcity, is energy-intensive to produce, contributing to greenhouse gas emissions and disrupting marine ecosystems due to brine disposal. Deionized water, while offering purity, also requires substantial energy for production and lacks essential nutrients, necessitating additional fertilizer use, which can result in runoff issues. Reclaimed water, on the other hand, demonstrates lower environmental impacts compared to conventional sources, with significant reductions in climate change potential and freshwater consumption. It also effectively recycles nutrients, making it a sustainable choice for hydroponics (Tran et al. 2024).

LCA is a tool which quantifies the environmental burden of a product or the process throughout its life cycle (van der Giesen et al. 2020). Specific steps to perform LCA are (1) goal and scope definition; (2) life cycle inventory (LCI); (3) LCIA; and (4) life cycle interpretation.

Since hydroponics is a sustainable option, several studies have been done to assess sustainability using LCA given in Table 1. LCA emerges as a crucial tool in the context of agricultural production, facilitating the identification of more sustainable options (Roy et al. 2009; Meier et al. 2015).

There is no such study to assess the environmental impacts of hydroponics using different types of water. Therefore, the aim of this study is to quantify the environmental burden of hydroponics using conventional water, desalinated water, deionized water, and reclaimed through the LCA approach.

The detailed methodology with data acquisition, study area and LCA is given in the following.

Data for the LCA were gathered from a hydroponic farm located at Mian Nawaz Sharif Agriculture University in Multan, Pakistan, covering operations throughout 2022. The farm was selected based on its consistent use of various water sources in hydroponic systems, allowing for a comparative assessment of environmental impacts associated with reclaimed, desalinated, deionized, and conventional water. Data were collected directly from farm operations to enhance accuracy and reliability, as recommended in prior LCA studies (Klopffer & Grahl 2014). This primary data collection process included documenting precise quantities and qualities of each input, such as water, nutrients, and energy, thus reflecting real-life farm practices in the study's inventory (see Table 2 in Section 2.3.3).

The study's goal was to evaluate the environmental impacts associated with different water sources in hydroponic systems by assessing various impact indicators. This assessment provides insights into the sustainability of each water source, aiding policymakers and hydroponic practitioners in making environmentally responsible decisions. The scope covers the operational phase of hydroponic farming, excluding upstream and downstream processes such as water extraction, transportation, or disposal, thus adhering to a gate-to-gate boundary definition (Wimmerova et al. 2022). This focused scope allows the study to emphasize the environmental burdens directly tied to water source selection within the hydroponic system.

The functional unit chosen for this study is the use of 2,000 l of water within a hydroponic system across a single growing cycle, a choice that reflects typical operational needs and allows for comparative analysis across water sources (Klopffer & Grahl 2014). By setting the system boundary from ‘gate-to-gate,’ this study restricts its analysis to only the operational life of the system – specifically, the input of water, electricity, and materials within the hydroponic setup. This gate-to-gate boundary excludes broader lifecycle phases like resource extraction and end-of-life disposal to concentrate on the environmental implications during active farm operations (Muthu 2020).

The inventory includes all material and energy inputs essential for hydroponic operations, specifically detailing components with significant environmental impacts. Major inputs are documented in Table 2.

Inventory data were quantified and recorded according to standard LCA guidelines to ensure comparability (Curran 2016; Muthu 2020). This direct data acquisition, outlined in Table 2, provides a clear and replicable foundation for the impact assessment phase.

LCI analysis is defined by ISO as the phase of LCA involving the compilation and quantification of inputs and outputs for a product throughout its life cycle (Muthu 2020). In this study, the inventory includes the main components, energy usage, and water usage as shown in.

For evaluating environmental impacts, the study used the CML 2001 method, which assesses indicators such as abiotic depletion potential (ADP), acidification potential (AP), eutrophication potential (EP), and global warming potential (GWP). This method is widely applied in agricultural LCAs to capture the diverse environmental burdens associated with resource use, emissions, and waste (Poopak & Agamuthu 2011; Ozturk & Dincer 2019).

• ADP: Measures resource depletion through indicators like fossil and mineral use.

• GWP: Assesses greenhouse gas emissions, aligning with global climate change metrics.

• AP: Indicates impacts on acid rain, affecting soil and aquatic ecosystems.

• EP: Evaluates nutrient release, which can lead to harmful algal blooms.

Using the GaBi software with the CML 2001 impact categories, this study conducted a detailed analysis across all impact categories, enabling a robust comparison of the environmental costs associated with each water type.

Life cycle impact assessment (LCIA) is a method used in LCA to evaluate the potential environmental effects of a product throughout its entire life cycle (Sanyé-Mengual et al. 2023). It considers the environmental impacts associated with resource acquisition, production, consumption, and waste management (Azmi et al. 2023). LCIA aims to quantify and analyze the environmental impacts of a product or process, such as GWP, acidification, eutrophication, and depletion of the ozone layer (Noviani et al. 2023; Halvaei Khankahdani et al. 2024). In this study CML 2001 impact assessment method is used for the quantification of environmental burden. The CML 2001 methodology is used to identify and assess various impact categories, such as global warming, acidification, eutrophication, ozone layer depletion etc. (Poopak & Agamuthu 2011). The CML 2001 impact assessment method evaluates environmental impacts such as depletion of abiotic resources, eutrophication, global warming, marine sediment, and aquatic ecotoxicity (Ozturk & Dincer 2019). The categories chosen for your LCA using the CML 2001 impact assessment method are essential for capturing a broad range of environmental impacts, including resource depletion, pollution, and toxicity. These categories, such as AD, AP, EP, and GWP, among others, provide a comprehensive understanding of the ecological footprint. This approach aligns with the study by (Poopak & Agamuthu 2011; Murphy et al. 2013; Klopffer & Grahl 2014; Sanyé-Mengual et al. 2023), who used a similar impact assessment method under comparable conditions, affirming the relevance of impact assessment selection.

Life cycle interpretation is the final phase of a LCA study, where the results of the previous phases are analyzed and evaluated considering uncertainties and assumptions made throughout the study (Laurent et al. 2020; Sala et al. 2020). It involves a critical assessment of the LCI and LCIA phases, considering the goals and scope of the study (Sala & Andreasson 2018). The interpretation phase includes several steps, such as completeness check, consistency check, sensitivity check, identification of significant issues, and drawing conclusions, limitations, and recommendations (Hauschild et al. 2017). The goal of interpretation is to ensure the relevance, soundness, and credibility of the LCA study. It helps in identifying hotspots, understanding uncertainties, conducting sensitivity analysis, and comparing results with other disciplines. Appropriate interpretation of LCA results is crucial for decision support in both business and policy contexts. For this purpose, a detailed section of results and discussion is given in the following.

Detailed results of the environmental burden of different water sources only using LCA with each impact assessment category are given in the following.

The findings regarding the environmental impacts of different water sources, particularly the use of reclaimed water in hydroponic systems, highlight several considerations for expanding its use in regions facing water scarcity. Reclaimed water offers significant advantages, such as reducing the pressure on freshwater resources and providing a consistent water supply for agricultural and hydroponic applications, even in arid regions. Studies show that using reclaimed water can mitigate water scarcity while also supporting sustainable agricultural practices by supplying essential nutrients and reducing the need for chemical fertilizers (Ricart et al. 2021). However, there are environmental and health concerns associated with the presence of contaminants such as antibiotics, antibiotic-resistant bacteria, and heavy metals in reclaimed water, which can have detrimental effects on both the environment and public health if not properly managed (Christou et al. 2017).

In terms of cost, the use of reclaimed water is generally more economical in the long term, especially when considering the reduced need for chemical inputs and the ability to sustain crop production in regions with limited water availability. However, the initial investment in infrastructure for treating and distributing reclaimed water can be high, and the economic viability depends on the scale of operations and the crop types being irrigated. Additionally, there are ongoing concerns about the public perception of using reclaimed water, particularly regarding food safety and the long-term impacts on soil health. Despite these challenges, when environmental and non-market benefits, such as reduced freshwater use and improved water security, are factored into the cost-benefit analysis, reclaimed water often emerges as a favorable option for regions facing water scarcity (Alcon et al. 2013). Therefore, the expansion of reclaimed water use in hydroponics and agriculture is a viable strategy for sustainable water management, provided that proper treatment and monitoring systems are in place to address potential risks.

The comparison of various environmental impact metrics across different water sources in hydroponic systems highlights significant differences in their sustainability profiles. Reclaimed water consistently emerges as the most environmentally favorable choice supported by Lopez-Galvez et al. (2014), which shows decreasing values across multiple indicators including ADP, AP, EP, FAETP, HTP, MAETP, and TETP. Its recycled nature contributes to reduced resource consumption, lower risk of acidification, nutrient enrichment, and toxicity to both aquatic and terrestrial ecosystems.

Deionized water follows reclaimed water in terms of environmental sustainability, exhibiting moderate impacts across most indicators. Desalinated water and freshwater consistently show greater environmental burdens across various metrics, indicating greater resource depletion, AP, EP, and toxicity to aquatic and terrestrial ecosystems. Additionally, freshwater has greater contributions to the GWP and ODP. Various water sources, used globally for various purposes, are underutilized in many areas (Shahid et al. 2022). The potential for irrigation, industry, and groundwater replenishment depends on treatment levels, which address water demands (Escobar 2010). Recent advancements have ensured water quality and safety (Donnaz 2020). In water-scarce regions such as Israel and Spain, treated wastewater supports irrigation, potentially meeting 4% of global demand (Fridman et al. 2023). Florida and California have safely used reclaimed water for more than 40 years, regulated with no food safety issues (Parsons et al. 2010) however feasibility relies on considering the costs, availability and benefits (Garcia & Pargament 2015).

The comparative analysis reveals that reclaimed water is the most sustainable option for hydroponic systems, showing lower impacts in multiple indicators. Specifically, reclaimed water demonstrates the lowest ADP (2.11E-05 kg Sb eq.), GWP (2.14 kg CO₂ eq.), and FAETP (0.0165 kg DCB eq.), indicating reduced resource use and greenhouse gas emissions. Desalinated water also performs well in EP (0.00189 kg Phosphate eq.) and Marine Aquatic Ecotoxicity (250 kg DCB eq.), although it has higher energy requirements. In contrast, conventional freshwater consistently shows the highest environmental burdens across most indicators.

These findings underscore reclaimed water's potential for reducing environmental impacts and supporting sustainable hydroponic agriculture. To maximize these benefits, stakeholders should consider investment in water recycling infrastructure and policies that incentivize reclaimed water use. This approach could significantly contribute to resource conservation and reduced ecological impact in hydroponic systems, particularly in water-scarce regions.

While the study's outcome holds significance, it is important to acknowledge certain limitations. The research was conducted with limited inventory data, therefore, the system boundary was confined to operational stage only (gate to gate) excluding the extraction, transportation, and end of life phases. Hence, it is recommended to use these water sources as an alternative to conventional water; however, a thorough evaluation of the water source must be done based on the availability, utilization, and environmental impacts and available data inventory.

A.N., M.A.U.R.T., and K.A. contributed to conceptualization, writing – original draft, writing review & editing, formal analysis, and methodology. A.N. and K.A. contributed to data curation, writing review & editing, formal analysis, and investigation. A.N. and M.A.U.R.T. contributed to supervision and project administration.

No external funding.

The submitted work is original and has not been published elsewhere in any form or language.

All the authors agree with the participation of this article.

All the authors agree with the publication of this article.

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

The authors declare there is no conflict.