Life Cycle Assessment of Rooftop Hydroponic Production Systems: A Case Study of ComCrop in Singapore

Abstract

Population growth, limited land, and water scarcity threaten urban food security and the environment. Rooftop farming has emerged as a sustainable way to develop urban roofs, reduce food miles, and increase food access. High-density, land- and water-scarce Singapore has prioritized high-tech farms such rooftop hydroponics to improve self-sufficiency and reduce the environmental impacts of importing food. Research indicates rooftop farming has environmental benefits, including lower CO₂ emissions compared to conventional farming, yet we lack a comprehensive understanding of environmental performance. This study applied a life cycle assessment (LCA) to evaluate ComCrop, a pioneer of rooftop hydroponic production in Singapore, using basil as a commonly consumed import. Results showed that 1 kg of basil emitted 0.30 kg of CO₂. A sensitivity analysis showed renewable energy (e.g., solar) could substantially reduce emissions. Results were compared to conventionally grown USA basil (Singapore’s main source), adding transportation to the LCA. Results showed 1 kg of rooftop basil emitted 0.59 kg CO₂ while 8.90 kg CO₂ was emitted in conventional production. Transportation had the greatest impact. We conclude that rooftop hydroponics is the more sustainable production method based on land, water, packaging, and transportation. However, sustainability can be improved with recyclable materials, water nutrient filtration, and renewable energy sources.

1. Introduction

In urban areas where land values are high, vacant building rooftops constitute substantial under-utilized space. Roofs have the potential to transform for productive uses, such as food cultivation or energy generation, and to mitigate climate-change-related urban environmental problems [1]. Population growth and limited water and land resources bring related food security and environmental issues, and food supply needs are expected to increase [2]. Furthermore, fuels for energy supply and transportation in cities generate most of the total greenhouse gas (GHG) emissions (75.7%) worldwide [3]. Therefore, against the threat of food security and the environment, it is increasingly difficult for countries to rely solely on conventional agriculture. There is an opportunity to adopt technologically advanced sustainable methods such as rooftop hydroponics to maximize natural resources [4].

Rooftop urban agriculture has emerged as an environmentally and economically sustainable way to develop roofs [1] and reduce “food miles” [5]. Food miles, the distance food travels from where it is grown or produced to the consumer, are often used when testing the environmental impact of food and are typically measured as carbon emissions during transportation, varying by mode, e.g., air versus automobile [5]. Rooftop farming has beneficial environmental impacts in terms of reducing water use and greenhouse gas emissions [6,7,8,9], purifying urban air [10], mitigating energy use for artificial cooling [6], water cycle design that can reduce water loss [7,9,10,11], linking plants to increased biodiversity [7,11], and various other environmental benefits [12]. In Singapore, rooftop hydroponics also incorporate nutrient recycling as an alternative source of food [4]. Rooftop hydroponics is a highly efficient utilization for sustainable food production in urban areas because it improves water and nutrient supply with soilless culture [13]. Closed system hydroponics perform better in terms of environmental protection and enhanced product quality [14].

On the other hand, hydroponic production systems have high construction and operation costs [15], creating a substantial barrier in the transition from conventional to rooftop production. For example, in Singapore, commercial-scale hydroponic systems require initial investments of $120,000–150,000 SGD ($92,760–115,940 USD) per unit, with energy contributing to approximately 40% of operational costs. Techno-economic analyses highlight sensitivity to electricity pricing and scale, underscoring the key economic barriers to adoption in urban settings [16,17]. It is unclear whether this type of production system is practical or efficient, or whether its environmental benefits outweigh its energy needs, CO₂ emissions, and construction costs. Research indicates that rooftop farming has lower CO₂ emissions and other environmental benefits compared to conventional farming, yet we lack a comprehensive understanding of environmental performance. More research is needed to comprehensively determine rooftop hydroponics’ environmental benefits through life cycle assessment metrics [18]. We argue that rooftop hydroponic systems are overall more sustainable as compared to conventional production methods due to urban proximity and reduced reliance on land and water resources; however, the technical system can be improved using LCA to identify which variables have the greatest negative environmental impacts.

1.1. Measuring Environmental Impacts of Urban Agriculture

Urban agriculture (UA) is primarily located in and around cities close to consumers. It is a way to reduce food supply distance and cost, but as urban populations grow and demand expands, the large scale of single urban farms puts pressure on the environment. Traditional agriculture accounts for approximately 70% of the global freshwater withdrawn per year [19], ranging from 21% in Europe to 81% in Asia according to 2015 FAO data [20]. Water in soil crops is lost through transpiration, evaporation, runoff, and percolation [21,22], with an estimated inefficiency of water applied to uptake around 56% [19]. Coupled with this, less than 1% of nutrients can be captured in UA due to land restrictions [23]. The expansion of UA for food production creates nutrient pollution when the nutrient recycling is ineffective [24]. Recirculating irrigation water and water treatment can enhance the ability to recover nutrients and reduce the use of groundwater. UA has the potential to increase biodiversity in urban landscapes [25], but widespread monoculture cropping by large-scale industrial farmers has dramatically reduced biodiversity possibility. In contrast, UA can increase city resiliency and self-sufficiency, delivering social and environmental benefits [12]. However, it depends on various factors related to land resources, the type of agriculture (e.g., soil-based, hydroponic, or rooftop greenhouse), and the city’s geographic location [18].

Hydroponics is a soilless method of growing plants by maintaining a layer of nutrient solution around the roots rather than using a water base, soil base, or substrate system [26]. The main reasons for using soilless culture are to reduce substrate pathogens and to improve control over the nutrient and water supply [13]. Important considerations to implement a hydroponic system include growing medium, pots, electricity demand, transportation of raw materials, and product deliveries [27]. A hydroponic farm performs better than cultivation in conventional open-field farms because it is a closed (controlled) system [28]. In an open system, excess irrigation runs off, whereas in a closed system excess irrigation water is collected and recycled back into the system [18]. Functionally, a hydroponics system reduces water demand (with estimates ranging from 33% [28] to 90% reduction [20]) and recovers nutrients by recirculating added nutrients and irrigation water [21]. Closed systems have more efficient environmental performance through precise nutrient dosing to improve product quality [14]. Hydroponic systems make significant savings in terms of fertilizer and water consumption when producing the same number of plants as compared to an open system, and reduce disposal-related costs and environmental impact [29]. At the same time, if the hydroponic system is in an urban location, it can reduce the transport distance, reuse packaging waste, and decrease the wastage of product during the retail transportation stages [30]. The use of recycled construction materials could further reduce environmental impacts [31]. For these reasons, hydroponic UA has been identified as an environmentally friendly and sustainable agricultural practice [32]. As a technologically advanced UA production type, hydroponics has the potential to lower the environmental impact of conventional farming methods. Research indicates rooftop hydroponic farming has lower CO₂ emissions and other environmental benefits compared to conventional farming, yet we lack a comprehensive understanding of its environmental performance. Current studies lack systematic quantification of embodied energy in construction materials, recycling pathways for growth media, and operational carbon-intensity variations across climatic zones. These gaps hinder holistic sustainability assessments of urban hydroponic systems.

Life cycle assessment (LCA) is a systematic evaluation method for assessing the environmental impacts associated with all stages of the life cycle of a product, process, or service [33]. These stages can include the cradle-to-grave process, which means the “life cycle” of the product, from acquisition of raw materials to production, assembly, maintenance, disassembly, to the disposal of the product [34]. LCA was developed by scientists to assess fossil fuel consumption and natural resource losses [35], and comprises a detailed inventory of all inputs and outputs required to produce a product, which is used to calculate the corresponding environmentally relevant emissions. It also indicates the potential accumulated environmental impact to improve the overall environmental condition of the product [36]. By considering the entire production system and avoiding sub-optimization that can occur within individual processes, its results can be used to recommend a comprehensive process with a lower environmental impact [37].

While LCA is a widely adopted method, its application to high-tech urban production systems is limited despite the potential implications for improving environmental impacts [38]. We found several studies using LCA to measure UA generally (for example [39,40]), green roof systems (for example [41,42,43]), and rooftop hydroponic systems (for example [28,44]), yet none were comprehensive. Several assessed crop yields, water use, and fertilizer [39,40,45], whereas others assessed materials and energy inputs [23,38], or packaging- and retail-related emissions [30]. Notably, Mujkic and Andakudi Kesavan [46] found that in rooftop hydroponic systems, the consumption of electricity and distance had the greatest negative impact on environmental conditions. Research indicates that hydroponics has a theoretically lower environmental effect compared to conventional farming methods, but there is a lack of empirical data [47]. Taken together, past research has evaluated many components of the life cycle of high-tech urban production systems, yet none have assessed the entire life cycle in a single study. The gap in research application of a holistic LCA of UA forms, including rooftop hydroponics, needs to be addressed in order to identify which variables have the greatest negative impact and therefore must be targeted to improve the system.

1.2. Singapore and High-Tech Farming

Food security and sustainability are major concerns in Singapore due to limited land sources, rapid urbanization, and low local yields [4,48]. The Singapore Land Authority (SLA), responsible for development and regulation of land resources, is developing a 30 by 30 Vision to grow enough food to meet 30% of demand by 2030 [49]. Arable land continues to be developed, and interest in domestic food production focused on high-tech, high-producing, land-limited or land-less farms (hydroponics and vertical systems) is gaining traction [50]. Furthering the need to move toward the more efficient use of resources in agriculture, Singapore’s total GHG emissions (2022 data) were 58,587 gigagrams (Gg) of CO₂ equivalent. CO₂ accounted for the majority (86%), with the main sources being fossil fuel combustion for energy in the industrial, building, transport, and household sectors. Other significant gases, including methane (CH₄), nitrous oxide (N₂O), and hydrofluorocarbons (HFCs) from the refrigeration and air-conditioning sector, made up the remaining 14% [51].

Singapore offers a critical case study for conducting an LCA of rooftop hydroponics because it is a small, island city state with limited natural resources, including agricultural land, which accounts for less than 1% of Singapore’s total land [52]. As a result, Singapore relies primarily on imported food [53]. Real estate is expensive, and land prices are high. The government has adopted new planning policies to increase self-sufficiency in food production through mixed land uses (e.g., allowing rooftop farming in commercial zones). Singapore has also implemented technological innovations to enhance food security [4]. The reduction in UA land makes hydroponics a feasible production method due to its non-substrate, vertical circulating water system that uses less land and water [18]. As of 2019, there were 79 hydroponic production farms in Singapore, including two on rooftops [52].

One Singapore rooftop farm, ComCrop, is a pioneer in urban hydroponic production, growing and harvesting high-quality pesticide-free produce by repurposing under-utilized rooftop space [54]. ComCrop uses the Nutrient Film Technique (NFT), a hydroponic production system where nursery plants are planted in channels where the root system is kept hydrated by a concentrated stream of water. In an NFT system, plants are quickly harvested, so it is primarily used for leafy greens and herbs, such as basil, grown in limited varieties [55]. Yet Singapore’s investment in hydroponic farming has been challenging due to construction needs, economics, energy inputs, and other multiple technologies needed to make the high-tech system feasible [4]. Despite this, hydroponics has characteristics well-suited to cities and land-scarce areas such as Singapore because it allows for the concentrated production of high-quality plants in mixed land use contexts such as rooftops.

Singapore has recently prioritized high-tech farms such as hydroponics [4]. Hydroponic systems make significant savings in terms of fertilizer and water consumption in production, as well as reducing negative environmental impacts [29,44]. At the same time, the advanced technology of, and high-cost investment of, hydroponic systems [18], as well as the energy demands to run the systems [27], can adversely impact the environment. The aim of this study was to evaluate the environmental impact of an existing rooftop hydroponic production system in Singapore and to compare it with conventional agriculture. In the urban environment, rooftop hydroponics has a significant advantage over conventional agriculture in terms of water and land use, as well as transportation and wastage impacts; however, the high energy demand required for such a high-tech system to grow produce has a significant negative impact as compared to conventional sun-dependent agriculture. By focusing only on ComCrop production, it would be easy to overlook environmental benefits, with is why we choose conventional agriculture in the USA as a comparison. The novelty of this study was to develop and apply a comprehensive LCA to identify which variables of urban rooftop hydroponic farming have the greatest negative impacts so that targeted improvements can be recommended.

2. Materials and Methods

A life cycle assessment (LCA) methodology was adopted to evaluate and identify the advantages and disadvantages of urban rooftop hydroponic production in Singapore by showing the environmental benefits and comparing this with conventionally grown vegetables. The secondary aim was to identify potential environmental shortcomings in the production of hydroponics, such as its energy intensity and material waste production, and recommend improvements for increasing the sustainability of the urban food system. In Singapore, UA is a supplementary source of vegetables, which largely depends on imports. Therefore, the ability of hydroponic production to meet the full urban demand was outside the scope of this study.

An LCA comprises four steps: (1) define the study goal and scope; (2) conduct a Life Cycle Inventory (LCI; data collection); (3) conduct a Life Cycle Impact Assessment (LCIA) comprising data translation into environmental indicators; and (4) interpret and analyze the results [56]. Following this protocol, we first identified an LCA evaluation model to measure the production life cycle of rooftop hydroponic systems. Next, we selected specific measures from the LCA evaluation model related to environmental flows of production and finalized them for use in this case study. Then, we selected specific LCA measures related to transportation and wastage for comparison with imported conventional agriculture. We applied the LCA to evaluate the Singapore rooftop hydroponic farm ComCrop including environmental benefits and the costs of production. Finally, we added transportation to the LCA model and compared ComCrop with imported conventional agriculture.

The LCA software SimaPro 9.I.I.I was used to analyze the data and evaluate environmental benefits versus costs because it has been used most frequently in the study of hydroponic systems. The environmental impact assessment (LCA evaluations) method included ReCiPe and CML 2000. These two methods evaluated several categories: global warming, ozone layer depletion, human health impact, acidification, and ecotoxicity, among others. In case studies, global warming potential, ozone formation, eutrophication, and acidification are the most frequently evaluated impact categories [31] and, therefore, are included in our LCA model.

2.1. LCA Goal and Scope

The first step in an LCA is to define products and their functional units, system boundaries, data requirements, and data assumptions [57]. ComCrop is one of Singapore’s pioneers in urban farming, growing, and harvesting high-quality leafy greens on rooftops near where communities live [54]. ComCrop mainly produces fast-growing crops including sweet basil, Japanese spinach, and mint marketed and sold through local retailers. At the time of this study, ComCrop was producing 1600 plants per day, with sweet basil accounting for more than 60%. Sweet basil was selected for this study because it was one of the most widely sold imported vegetables and the first among those cultivated in the ComCrop rooftop hydroponic system. We defined the functional unit of production as 1 kg of hydroponically produced basil by ComCrop to match the standard unit of measure in SimaPro, the software used to calculate the LCA.

The environmental impact evaluation of rooftop hydroponics was assessed through the five stages of life cycle assessment (LCA): material extraction, production, transportation, construction, operation/maintenance, and disposal [31]. The LCA proposed by this study was defined as an assessment of the environmental impact of the production stage of hydroponically grown basil. Additionally, transportation was evaluated to compare hydroponic production to conventionally grown USA basil, which is Singapore’s main imported source, encompassing 33.75% of overall sales [58]. The production stage of an LCA comprises inputs of energy use, land use, water use, and fertilizer use [59]. It was assumed that conventional basil production was on soil-based land for this study, although we acknowledge that practices can vary by region and farm size.

Figure 1 shows how we defined the system boundaries, the inputs needed, and the outputs produced during the production process for the LCA of ComCrop. The material inputs (green) included a starter cube made of floral foam, basil seeds, and water consumption. The energy inputs (orange) included a hydroponic addition fertilizer, fans for ventilation, and water pumps. It must be noted that while we considered the cost of electricity for electrical equipment, we did not consider the environmental impact of the assembly and recycling of the equipment due to limitations in terms of the access to data and timeframe of the study. Infrastructure materials (gray) included greenhouse construction materials, water pump materials, rubber water tanks, trays, tubes, and land use. Outputs (yellow) included the wastage of basil based on yields per day. A comparison with conventional agriculture expanded to include transportation (blue).

Information on environmental data used in the calculations for infrastructure inputs (aluminum, HDPE, LDPE, and concrete), material inputs (starter cube), and energy inputs (electricity and fertilizer) were obtained from the coinvent v3.7.1 database. Other natural inputs, e.g., seeds, water, and land use, were measured by us using an on-site survey. Outputs comprised yields, dead plants, and transportation emissions. We excluded pest control after the site survey because ComCrop did not use pesticides and insects did not breed on the farm. We also excluded CO₂ fixation by plants because CO₂ was released as biological emissions shortly after harvest. Since fertilizers were not lost through the soil (instead they were retained for plants through the water cycle) we assumed that fertilizer emissions were zero. Additionally, to compare ComCrop to conventional production, product transportation was evaluated for CO₂ emissions and food miles.

Two LCA scenarios were developed (Figure 2). Scenario 1 assessed ComCrop’s rooftop hydroponic basil production and Scenario 2 assessed USA, soil-based, conventionally grown basil production. The production stage of roof hydroponics was calculated based on an on-site survey of ComCrop. The production stage of the USA land-based production was calculated using the Ecoinvent 3.7.1 database for open-field soil-based agriculture in a major production region like California, chosen to represent a typical long-distance import supply chain to Singapore. In Scenario 1, plant wastage of 1 kg of basil was calculated as 0.04 kg, and packaging waste was cut off at 0.06 kg. The primary destination for ComCrop’s basil production was local grocery chain Fairprice’s warehouse in Jurong East Joo Koon, Singapore. In Scenario 2, it was assumed that fresh basil was imported from the USA and locally distributed from Singapore’s Pasir Panjang Wholesale Centre, a major food distribution center located in the logistics area of Singapore. According to Singaporean data, temperature fluctuation, poor handling, and inadequate packaging generated up to 30% wastage in the vegetable supply chain [60].

The data for the distribution stage of conventional agriculture from the USA to Singapore was obtained through a literature review; it included transportation distance and wastage. We calculated the environmental impact of consumption flow, transport requirements, and packaging flow. Transportation data and the packaging system of basil from the distribution center to the retail point were assumed to be the same in both scenarios. Data used in the comparison included data on USA ground basil production, electricity for Singapore, road freight chiller truck transport, aircraft transport (the mode of transport for USA basil to Singapore), and plastic production (packaging). Data were obtained from the Ecoinvent 3.7.1 database.

2.2. Life Cycle Inventory (LCI)

The Ecoinvent 3.7.1 database available in SimaPro provided background inventory including the manufacture of infrastructure inputs. ComCrop’s farm manager provided operational data including the material and energy inputs of the hydroponic system. Through on-site visits, we recorded measurements of detailed infrastructure inputs. From these data, we calculated the lifetime of infrastructure inputs. The annual impact was attributed by dividing the units of inventory of each item by the life of the components. Finally, the input value needed to operate at one functional unit (1 kg of basil) was calculated by LCA to obtain the Life Cycle Inventory list (

Table 1. Life Cycle Inventory (LCI), including variables, definitions, data/measurement and data source.

Variable	Definition	Data/Measurement		Data Source
Material inputs
Seeds	Leafy greens and herbs, as other plants develop extensive root systems that can easily block the channels [49].	Basil number	Seeding time	Farm
Starter cube	The plants in the growing tubes (channel/gully) are typically suspended above the water by placing seedlings in a starter cube	Cost (SGD)		Public
		Dividing unit of inventory item by the life span of the component		Farm
Total water volume	The sources of water loss are evaporation, evapotranspiration, spillage, leakage, and water exchange [47].	Water tank/reservoir capacity (L)		Farm
		Total water volume (L)
Energy Inputs
Water pumping	The whole production system water circulation [47,61].	Spending hours per day/week and cost of electricity (W)(h)		Farm
Ventilation	Fans to alleviate the high temperature	Spending hours per day/week and cost of electricity (W)(h)		Farm
Fertilizer use	Nutrient addition of plants production [62].	Fertilizer		Farm/ Literature
		Nutrient retention time in nutrient solution tank and the floating system unit (h)		Farm
		Use frequency and usage
		Fertilizer nutrient elements
Infrastructure Inputs
PE film/mesh	Infrastructure raw materials [48].	Dividing unit of inventory item by the life span of the component.		LCA software or Farm
Concrete
Aluminum
PE Shade Cloth
HDPE Pipe	Pumping water pipe
Steel Pipe	Pumping water pipe
Rubber tank	Used for water tank
HDPE Tray	Plant/seeding tray
HDPE tray	Plant/seeding tube	Tube length (cm) and growing density (Number of plants)		Farm
Land use	Hydroponic area/greenhouse area	Greenhouse area (m2) Hydroponic area (m2)		Farm
Outputs
Crop Yields	Leafy greens harvesting	Basil (kg) (Number)	Harvesting time	Farm
Wastage	Dead vegetables and waste from picking, packaging, transportation, and selling	Percentage of the wastage of the production and transportation		Farm
Wastage treatment	Dead vegetables and waste from picking, packaging, transportation, and selling	Recycling method		Farm
Transportation
Food miles	The transport distance to a retail outlet or customer	Kilometer (km)		Farm
Emissions	Carbon emissions of transport from farm to retail outlet or customer	GHG Emission		LCA software

).

The LCA was used to calculate the ComCrop Life Cycle Inventory (LCI) for production environmental flows with the LCI list to show the actual amount of all inputs needed for 1 kg of basil production (functional unit) based on midpoint indicators. See

Table 2. Inventory of infrastructure and equipment for one harvest production at ComCrop.

Variable	Material	Lifespan	Units	Quantity per kg
Infrastructure Inputs
Wall	PE	15	kg	5.4 × 10−4
Shade cloth	PE	10	kg	1.0 × 10−5
Support for stalking	Aluminum	50	kg	1.2 × 10−5
Concrete	Concrete	50	m3	2.05 × 10−5
Land use	Greenhouse area (m2) Hydroponic area (m2)	25	m2	14
Tubes	HDPE	25	kg	7.28 × 10−4
Tray	HDPE	25	kg	5.2 × 10−6
Pipe	PE	15	kg	6.0 × 10−6
Water tank	Rubber	25	kg	6.8 × 10−4
Material Inputs
Seeds	Seeds		kg	52
Water	Water		l	0.02
Starter cube	Perlite		kg	7.25 × 10−3
Fertilizers	N		kg	4.8 × 10−4
	P2O5		kg	1.2 × 10−4
	K2O2		kg	1.2 × 10−4
	Ca(NO3)2		kg	6.0 × 10−5
	Mg(NO3)2		kg	3.0 × 10−5
Energy Inputs
Ventilation	Electricity		kw/h	0.0168
Water Pumping	Electricity		kw/h	0.528
Transport Inputs
Transport (seed)	Transport van		tkm	8.32 × 10−6
Transport (fertilizers)	Transport van		tkm	3.42 × 10−6

for the LCI of ComCrop’s production process for one harvest.

ComCrop’s rooftop greenhouse was constructed with an aluminum alloy support structure covered by a PE film and a PE mesh shade cloth, and covered an area of 800 m², including 700 m² for the hydroponic system and 100 m² for walkways and other space. A greenhouse has a 60% depreciation rate, so the greenhouse infrastructure calculation was multiplied by the rate. ComCrop’s greenhouse had six growing beds, and half of the growing beds were harvested daily for basil. The support structure of the planting beds was aluminum with HDPE water pipes. Basil was seeded in nursing trays and then transplanted into 18-hole HDPE tubes, 3.7 m long, and similar in length to the planting bed (Figure 3). The water tank was a 2000 L rubber tank. Water from the tanks was circulated throughout the entire production system with minimal loss through the pumps. In addition to the circulation pipes along the planting beds, smaller drip irrigation pipes were inside the tubes. Environmental data for major infrastructure materials, inputs, aluminum, HDPE, LDPE, and concrete were obtained from the Ecoinvent v3.7.I database.

For ComCrop’s basil production process, plants in growth tubes were usually above the water in 1-inch-long starter cubes for seedlings slotted into a small hole at the top of the test tube. This starter cube was imported from the USA, which provided root support and moisture retention and did not contain nutrients. The cube’s main material was foamed phenolic plastic, for which the basic product information is available in the Ecoinvent v3.7.I database. ComCrop used 1.5 L of water per day to produce 1600 plants, equivalent to 1.0 L of water to produce 50 kg of basil. Singapore imported seeds from the USA with a 96% germination rate. The production process was heat-free, as plant varieties were fast-growing leafy greens suitable for tropical production. The greenhouse had six fans with a power of 35 W to reduce the temperature, which were powered 24 h a day and consumed 0.84 kwh daily. The 1100 W water pump was also powered 24 h a day and consumed 26.4 kwh daily. ComCrop used a 20 kg bag of fertilizer for hydroponics that covered a 60-day growth cycle, calculated as 300 g per day. The fertilizer was a combination of 8% nitrogen (N), 2% phosphorus potassium calcium magnesium phosphate (P₂O₅), 2% potassium (K₂O), 21% calcium (Ca(NO₃)), and 0.05% magnesium (MgO). Information on the content of each fertilizer was obtained from the weight element corresponding to the fertilizer. Emissions from fertilizer production were included in the LCA. For imported materials of starter cube, seed, and fertilizer, we defined the transportation distance from the local distribution center to ComCrop.

This study did not consider the assembly, disassembly, and recycling of infrastructure inputs. Therefore, only the environmental impacts of the materials themselves during the operation were considered (

Table 3. Detailed impact inventory of waste, for LCA comparison of ComCrop with conventional agriculture.

Outputs	Material	Units	Quantity per kg
Wastage	Plant waste	kg	0.04
	Package cut-off	kg	0.1
	Compost	kg	0
Transport (wastage)	Transport vehicle	tkm	0

). This assumption included the fan and pump appliances in the energy input. The output included basil yields, wastage, and greenhouse gas emissions. ComCrop produced an average 50 kg of basil per day, and plant wastage was approximately 2 kg. The first harvest was 10–14 days after transplanting seedlings. The waste treatment was not considered in this study. As mentioned, CO₂ emissions when plants were grown were not considered; only CO₂ generated during transportation was considered. Neighboring farms managed the transportation of vegetables from ComCrop’s farm to retailers; we performed a directed allocation to distribute this process’s impact, and estimated 25 km of freight trips per day. Due to the trips’ length and frequency, no loss of product was assumed at this stage as per the data provided by ComCrop.

2.3. Life Cycle Impact Assessment (LCIA)

SimaPro’s ReCiPe 2016 v1.1 Midpoint method is the most used impact assessment method in hydroponic LCA studies [63]. ReCipe 2016 offers the application of characterization factors at a midpoint level in the cause–effect chain for selected impact categories [64]. We selected the following impact categories: global warming, stratospheric ozone depletion, ozone formation, terrestrial acidification, freshwater eutrophication, marine eutrophication, ecotoxicity, land use, mineral resources scarcity, fossil resource scarcity, and water consumption. Specifically for global models, LCA reliability can be further improved by using larger, global-scale spatial categories and establishing a closer spatial link between every impact [65,66]. It should be noted that the study boundary ignored the potential pollution of hydroponics and the consequent negative effects on human health, although the possibility of occurrence of pollution effects has been ruled out in other studies [40].

2.4. Interpretation and Analysis

The final stage of an LCA is interpretation. The analysis is first performed by integrating the results of the other stages based on the uncertainty of the applied data and then making assumptions throughout the study [67]. The process involved integrating results from the Life Cycle Inventory (LCI) and Life Cycle Impact Assessment (LCIA) phases, explicitly accounting for data uncertainty and study assumptions. The outputs were generated and refined through sensitivity analysis, which measured input uncertainty and tested scenarios, such as replacing the electricity source with solar energy to understand its influence on the environmental impact. Significant results were determined based on the outcomes of the LCI and LCIA phases, which highlighted the most substantial environmental factors. This determination was further validated through sensitivity analysis, where an input was considered significant if altering it (e.g., removing the highest impact factor) caused a major change in the final results.

Scenario sensitivity analysis was conducted for ComCrop’s production stage. The type of input with the largest impact was removed to understand how the scenario would perform in the absence of the maximum impact condition. For this, we replaced electricity with clean energy and calculated the environmental impact. The most efficient source of clean energy in Singapore is solar energy due to its geographical location, climatic conditions, and land area constraints. Solar PY has emerged as a potential renewable energy option for Singapore because of the high average annual solar radiation of about 1580 kWh/m² [68]. Monocrystalline panels have slightly higher conversion efficiency than polycrystalline cells and are the most common solar photovoltaic system panel [69]. We used monocrystalline silicon PV panel, 300 W, 22% efficiency, with standard values from the recent literature as parameters in the sensitivity analysis.

3. Results

3.1. Life Cycle Impact Assessment (LCIA) of Rooftop Hydronics

The Life Cycle Impact Assessment (LCIA) results of ComCrop’s production process identified environmental hotspots. Figure 4 and

Table 4. Life Cycle Impact Assessment (LCIA) results of ComCrop’s rooftop hydroponic basil production using the SimaPro ReCiPe 2016 v1.1 Midpoint method.

Impact Category (Unit)	Total	Basil Production	Infrastructure Inputs								Material Inputs		Energy Inputs
			Greenhouse
			Greenhouse Wall	Shade Cloth	Stalking Support	Concrete	Tubes	Tray	Pipe	Water Tank	Starter Cube	Basil Seeds	Electricity	Fertilizer
Global warming (kg CO2 eq)	2.99 × 10−1	0.00 × 1000	1.38 × 10−3	2.33 × 10−5	1.34 × 10−3	3.00 × 10−3	4.47 × 10−4	3.19 × 10−6	1.44 × 10−5	1.91 × 10−3	2.88 × 10−2	2.32 × 10−5	2.56 × 10−1	5.76 × 10−3
Stratospheric ozone depletion (kg CFC11 eq)	2.21 × 10−7	0.00 × 1000	2.90 × 10−10	5.40 × 10−12	6.65 × 10−10	4.10 × 10−10	3.06 × 10−10	2.19 × 10−12	3.08 × 10−12	1.22 × 10−9	5.70 × 10−9	9.85 × 10−11	9.89 × 10−8	1.14 × 10−7
Ionizing radiation (kBq Co-60 eq)	2.25 × 10−3	0.00 × 1000	5.32 × 10−5	5.31 × 10−7	9.99 × 10−5	1.54 × 10−4	1.30 × 10−4	9.31 × 10−7	3.53 × 10−7	1.51 × 10−4	1.14 × 10−3	1.06 × 10−6	4.10 × 10−4	1.05 × 10−4
Ozone formation, human health (kg NOx eq)	3.12 × 10−4	0.00 × 1000	3.28 × 10−6	4.99 × 10−8	4.59 × 10−6	6.15 × 10−6	5.85 × 10−7	4.18 × 10−9	3.10 × 10−8	5.00 × 10−6	6.17 × 10−5	1.02 × 10−7	2.18 × 10−4	1.24 × 10−5
Fine particulate matter formation (kg PM2.5 eq)	1.40 × 10−4	0.00 × 1000	1.73 × 10−6	2.52 × 10−8	3.50 × 10−6	1.92 × 10−6	3.29 × 10−7	2.35 × 10−9	1.58 × 10−8	3.41 × 10−6	4.53 × 10−5	4.57 × 10−8	7.60 × 10−5	7.60 × 10−6
Ozone formation, terrestrial ecosystems (kg NOx eq)	3.25 × 10−4	0.00 × 1000	3.61 × 10−6	5.33 × 10−8	4.64 × 10−6	6.24 × 10−6	6.04 × 10−7	4.32 × 10−9	3.32 × 10−8	5.43 × 10−6	6.88 × 10−5	1.05 × 10−7	2.23 × 10−4	1.25 × 10−5
Terrestrial acidification (kg SO2 eq)	3.37 × 10−4	0.00 × 1000	3.93 × 10−6	6.33 × 10−8	8.03 × 10−6	5.20 × 10−6	7.32 × 10−7	5.23 × 10−9	3.93 × 10−8	7.40 × 10−6	8.84 × 10−5	1.30 × 10−7	1.98 × 10−4	2.53 × 10−5
Freshwater eutrophication (kg P eq)	2.80 × 10−5	0.00 × 1000	3.22 × 10−7	4.24 × 10−9	1.28 × 10−6	4.35 × 10−7	1.08 × 10−7	7.69 × 10−10	2.67 × 10−9	6.03 × 10−7	9.83 × 10−6	6.43 × 10−9	1.44 × 10−5	1.07 × 10−6
Marine eutrophication (kg N eq)	1.71 × 10−6	0.00 × 1000	3.01 × 10−8	3.12 × 10−10	5.16 × 10−8	2.89 × 10−8	2.01 × 10−8	1.43 × 10−10	2.13 × 10−10	4.37 × 10−8	6.33 × 10−7	3.80 × 10−10	8.10 × 10−7	8.67 × 10−8
Terrestrial ecotoxicity (kg 1,4-DCB)	5.33 × 10−1	0.00 × 1000	2.55 × 10−3	4.97 × 10−5	1.47 × 10−2	9.21 × 10−3	1.82 × 10−3	1.30 × 10−5	2.93 × 10−5	7.08 × 10−3	6.84 × 10−2	1.39 × 10−4	4.03 × 10−1	2.69 × 10−2
Freshwater ecotoxicity (kg 1,4-DCB)	2.08 × 10−2	0.00 × 1000	4.25 × 10−5	7.47 × 10−7	6.24 × 10−4	6.89 × 10−5	4.55 × 10−5	3.25 × 10−7	4.31 × 10−7	1.19 × 10−4	6.77 × 10−4	2.51 × 10−6	1.88 × 10−2	4.27 × 10−4
Marine ecotoxicity (kg 1,4-DCB)	2.57 × 10−2	0.00 × 1000	5.55 × 10−5	9.74 × 10−7	9.13 × 10−4	9.21 × 10−5	5.99 × 10−5	4.28 × 10−7	5.63 × 10−7	1.57 × 10−4	9.26 × 10−4	3.25 × 10−6	2.30 × 10−2	5.51 × 10−4
Human carcinogenic toxicity (kg 1,4-DCB)	4.97 × 10−3	0.00 × 1000	3.94 × 10−5	6.18 × 10−7	2.76 × 10−4	5.16 × 10−5	2.58 × 10−5	1.84 × 10−7	4.89 × 10−7	7.56 × 10−5	1.13 × 10−3	1.01 × 10−6	3.24 × 10−3	1.27 × 10−4
Human non-carcinogenic toxicity (kg 1,4-DCB)	1.53 × 10−1	0.00 × 1000	8.49 × 10−4	1.36 × 10−5	9.72 × 10−3	1.37 × 10−3	8.26 × 10−4	5.90 × 10−6	8.11 × 10−6	2.63 × 10−3	2.76 × 10−2	1.06 × 10−5	1.02 × 10−1	7.47 × 10−3
Land use (m2a crop eq)	4.14 × 10−3	0.00 × 1000	1.33 × 10−5	2.06 × 10−7	3.17 × 10−5	5.30 × 10−5	1.68 × 10−5	1.20 × 10−7	1.22 × 10−7	6.28 × 10−5	3.82 × 10−4	1.56 × 10−5	3.46 × 10−3	1.12 × 10−4
Mineral resource scarcity (kg Cu eq)	5.32 × 10−4	0.00 × 1000	2.71 × 10−6	5.75 × 10−8	7.65 × 10−5	8.94 × 10−6	2.26 × 10−6	1.62 × 10−8	2.92 × 10−8	5.76 × 10−5	5.80 × 10−5	1.35 × 10−7	2.69 × 10−4	5.65 × 10−5
Fossil resource scarcity (kg oil eq)	1.24 × 10−1	0.00 × 1000	9.22 × 10−4	1.64 × 10−5	3.19 × 10−4	3.02 × 10−4	6.87 × 10−5	4.91 × 10−7	1.01 × 10−5	1.18 × 10−3	1.46 × 10−2	6.62 × 10−6	1.06 × 10−1	8.04 × 10−4
Water consumption (m3)	1.05 × 10−3	2.00 × 10−5	1.73 × 10−5	4.62 × 10−7	1.95 × 10−5	7.12 × 10−5	8.05 × 10−6	5.75 × 10−8	1.43 × 10−7	3.10 × 10−5	3.36 × 10−4	8.99 × 10−6	4.83 × 10−4	5.83 × 10−5

show that electricity input (orange bar), infrastructure input (gray bar), and the starter cube (green bar) were the top three factors with the highest negative environmental impacts.

Rooftop hydroponic farming relies on a 24 h water recycling system that requires a constant electricity source. ComCrop’s electricity use (Figure 4, orange bar) was responsible for 90% of GHG emissions, 70% of Ozone formation, 60% of terrestrial acidification, 75% of terrestrial ecotoxicity, and 80% of fossil resource scarcity. In addition to ionizing radiation and freshwater eutrophication, electricity was the most influential factor in this impact category. At the time of this study, 95% of Singapore’s electricity relied on natural gas production. Singapore is deficient in cost-effective and reliable renewable energy sources; however, there is a potential for the application of solar energy due to the intense sunshine in Singapore [70]. The high demand for fuel for natural gas power generation explains why climate warming, air quality, soil acidification, and fossil fuels have a greater impact in this LCIA model. Because of the sourcing of natural gas for energy, electricity use was found to have the greatest environmental impact in hydroponics.

Fertilizer use (Figure 4, light orange bar) contributed to more than 53% of stratospheric ozone depletion, 14% of marine eutrophication, and 10% of mineral resource scarcity. The amount of fertilizer used in hydroponics is less than the amount of waste from field crops due to the recirculating water system, but it still negatively impacts the environment. The release of chemical compounds containing gaseous chlorine or bromine from fertilizers, which react with evaporation into the atmosphere, have a damaging effect on the stratosphere [71]. Chemical elements in fertilizers can also be discharged into the ocean with sewage, causing eutrophication of water bodies. The synthesis of fertilizers also harvests mineral resources.

Starter cubes (Figure 4, green bar) are the primary support for rooftop hydroponic production, from which plants draw water. In our LCIA model, they accounted for 30% of water consumption and produced freshwater eutrophication and ecotoxicity. The starter cube used by ComCrop was a material similar to floral foam, mainly a soaked phenolic resin that does not provide nutrients. ComCrop’s start cubes were used for starting seedlings before transplanting. They served to fix the root system, store water, and absorb nutrients and oxygen from the water for suspended plant roots.

Greenhouse construction materials (Figure 4, gray bar) were primarily made of aluminum, which produced 30% marine ecotoxicity and 14% mineral resource scarcity. The life cycle use of greenhouse infrastructure materials produced 28% of ionizing radiation, 11% of marine eutrophication, and 15% of water consumption. ComCrop used rubber water tanks, which accounted for 10% of mineral resource scarcity. Construction materials also impacted 10% of terrestrial acidification, freshwater eutrophication, and mineral resources scarcity. While the input of infrastructure materials had some environmental impact, this study did not consider the assembly and degradation of the materials but calculated the environmental impact of the materials themselves.

3.2. LCIA of Rooftop Hydronics Versus Conventional Agriculture

The previous LCIA identified which of ComCrop’s production stage process variables created environmental hotspots. Yet the environmental benefits of rooftop hydroponics were not clearly defined. This section describes the specific environmental advantages and disadvantages of rooftop hydroponics by comparing it with conventional agriculture.

Table 5. Comparison of yields, water, consumption, fertilizer use, and land use impacts of the two scenarios (rooftop hydroponics versus conventional production).

Scenario	Yield (kg/m2)	Water Consumption (L/kg)	Fertilizer Use (kg/kg)	Land Use (m2/kg)
Rooftop hydroponic system	890.0	0.02	6.0	0.41
Conventional agriculture	164.3	0.53	28.9	2.22

shows the yield, water consumption, fertilizer use, and land use of the two scenarios, with conventional agricultural substantially using more water, fertilizer, and land than rooftop hydroponic production. Note that while the input of the infrastructure materials of rooftop hydroponics had some environmental impact, this study did not consider the assembly and degradation of the materials but calculated the environmental impact of the materials themselves, which affects interpretation of the comparative model by underestimating the broader environmental impact of rooftop hydroponics. Based on the LCIA in this study, specifically, the water requirement per kg of conventionally grown basil was 26 times that of rooftop, hydroponically grown basil (0.53 vs. 0.02 L/kg). The irrigation system of rooftop hydroponics requires a 24 h, continuously circulating irrigation system, which consumes a high amount of electricity [67]—and, considering the energy sources of electricity in Singapore, most electricity is generated from non-renewable natural gas dominated by 95% sourced from natural gas [72]. Conventional agriculture also consumed 4.8 times the amount of fertilizer of hydroponics (28.9 vs. 6.0 kg fertilizer per kg basil) and 5.4 times the amount of land (2.22 vs. 0.41 m²/kg). Land use differences are due to the difference in yield, where we calculated 164.3 kg of basil produce conventionally on 1 m² of land versus ComCrop’s yield of 890 kg/m². One of the important features of rooftop hydroponic systems is the large amount of green leaf production in a limited space. Moreover, because it is not soil-based, production can be carried out in other areas such as indoors, on abandoned sites, and on walls. A residual effect of hydroponics is that soil contamination and quality do not pose a risk to produce.

Figure 5 and

Table 6. Life Cycle Impact Assessment (LCIA) results of ComCrop’s rooftop hydroponic basil production compared with USA-based conventional agriculture using the SimaPro ReCiPe 2016 v1.1 Midpoint method.

Impact Category	Unit	Basil (ComCrop)	Basil (USA)
Global warming	kg CO2 eq	5.90 × 10−1	8.90 × 1000
Stratospheric ozone depletion	kg CFC11 eq	3.11 × 10−7	3.75 × 10−6
Ionizing radiation	kBq Co-60 eq	1.67 × 10−3	6.38 × 10−2
Ozone formation, Human health	kg NOx eq	9.86 × 10−4	4.33 × 10−2
Fine particulate matter formation	kg PM2.5 eq	5.06 × 10−4	9.11 × 10−3
Ozone formation, Terrestrial ecosystems	kg NOx eq	1.05 × 10−3	4.37 × 10−2
Terrestrial acidification	kg SO2 eq	1.15 × 10−3	2.72 × 10−2
Freshwater eutrophication	kg P eq	1.20 × 10−5	3.83 × 10−5
Marine eutrophication	kg N eq	2.42 × 10−6	4.06 × 10−6
Terrestrial ecotoxicity	kg 1,4-DCB	1.18 × 1000	1.59 × 101
Freshwater ecotoxicity	kg 1,4-DCB	2.59 × 10−4	3.35 × 10−3
Marine ecotoxicity	kg 1,4-DCB	1.04 × 10−3	1.62 × 10−2
Human carcinogenic toxicity	kg 1,4-DCB	4.13 × 10−3	1.09 × 10−2
Human non-carcinogenic toxicity	kg 1,4-DCB	7.66 × 10−2	1.10 × 1000
Land use	m2a crop eq	1.34 × 10−2	2.75 × 10−1
Mineral resource scarcity	kg Cu eq	1.13 × 10−3	4.33 × 10−3
Fossil resource scarcity	kg oil eq	2.90 × 10−1	2.88 × 1000
Water consumption	m3	5.58 × 10−3	1.52 × 10−1

show the results of the LCIA comparison between rooftop hydroponics and conventional agriculture for production, packaging, and transportation processes. ComCrop’s production outperformed the conventional production model across all variables. In particular, the impact of agricultural production far exceeded that of rooftop hydroponics in this case study in terms of climate change and air quality. Regarding freshwater and marine ecotoxicity, and resource scarcity, rooftop hydroponics had a higher percentage. According to the results, for the same production of 1 kg basil, rooftop hydroponics emitted 0.59 kg CO₂ and conventional agriculture emitted 8.9 kg CO₂.

In our case of conventional agriculture, produce was transported by air. The process of transportation impacted on air quality categories including stratospheric ozone depletion, ionizing radiation, ozone formation, and fine particulate matter formation. According to ComCrop’s Life Cycle Impact Assessments of the production process, electricity usage and construction materials were the most affected items. Because cross-country transport vehicles were the most significant influencing factor on the results, it was hard to assess the environmental impact at other stages such as whether rooftop hydroponics was far less environmentally impactful during the production process than conventional agricultural production. Figure 6 and

Table 7. Life Cycle Impact Assessment (LCIA) results of ComCrop’s rooftop hydroponic basil production compared with USA-based conventional agriculture excluding transportation using the SimaPro ReCiPe 2016 v1.1 Midpoint method.

Impact Category	Unit	Basil (ComCrop) Without Transportation	Basil (USA) Without Transportation
Global warming	kg CO2 eq	5.87 × 10−1	1.72 × 10−1
Stratospheric ozone depletion	kg CFC11 eq	3.09 × 10−7	1.49 × 10−6
Ionizing radiation	kBq Co-60 eq	1.87 × 10−2	1.12 × 10−2
Ozone formation, Human health	kg NOx eq	9.66 × 10−4	6.27 × 10−4
Fine particulate matter formation	kg PM2.5 eq	5.01 × 10−4	3.86 × 10−4
Ozone formation, Terrestrial ecosystems	kg NOx eq	1.03 × 10−3	6.39 × 10−4
Terrestrial acidification	kg SO2 eq	1.13 × 10−3	1.35 × 10−3
Freshwater eutrophication	kg P eq	1.03 × 10−4	6.62 × 10−5
Marine eutrophication	kg N eq	1.07 × 10−5	4.41 × 10−6
Terrestrial ecotoxicity	kg 1,4-DCB	1.12 × 1000	7.11 × 10−1
Freshwater ecotoxicity	kg 1,4-DCB	3.11 × 10−2	1.89 × 10−2
Marine ecotoxicity	kg 1,4-DCB	3.91 × 10−2	2.36 × 10−2
Human carcinogenic toxicity	kg 1,4-DCB	1.35 × 10−2	6.18 × 10−3
Human non-carcinogenic toxicity	kg 1,4-DCB	3.46 × 10−1	−3.01 × 10−1
Land use	m2a crop eq	1.30 × 10−2	2.56 × 10−1
Mineral resource scarcity	kg Cu eq	1.12 × 10−3	1.09 × 10−3
Fossil resource scarcity	kg oil eq	2.89 × 10−1	4.74 × 10−2
Water consumption	m3	5.57 × 10−3	1.48 × 10−1

show the results of the LCIA comparison between ComCrop and traditional US land-based production in the production and packaging stages minus international transportation. In this LCIA model, hydroponic production performed lowest in terms of land use and water consumption. In terms of global warming, ozone formation, eutrophication, ecotoxicity, and resource scarcity, rooftop hydroponics had significantly higher impacts than conventional agriculture. For global warming, electricity consumption accounted for 90% of the impact of scenario rooftop hydroponics. Being soil independent, terrestrial acidification for conventional agriculture was slightly better than rooftop hydroponics, probably also due to less land use.

To produce 1 kg basil, rooftop hydroponics emitted 0.59 kg CO₂ and conventional agriculture emitted 0.17 kg CO₂ when transportation was not included in the model. The difference between these two scenarios was 0.42 kg CO₂ with conventional production having a smaller negative impact. However, the results show that conventional agriculture required 20 times more agricultural land, 33 times more water, and produced five times the stratospheric ozone depletion as compared to rooftop hydroponics.

The performance of conventional agriculture was better than rooftop hydroponics in most of the air quality categories except stratospheric ozone depletion. To explain this, note that conventional agriculture was modeled with basil transported by aircraft to consumers in Singapore, with air transport causing stratospheric ozone depletion. In the scenario of conventional agriculture, cultivation, fertilizers, and transportation were responsible for global warming. This is explained by considering the production methods of different systems. Conventional agriculture does not treat excess water and has poor control over pesticide use. At the same time, conventional farms are located farther away from consumers, increasing transport distances. Rooftop hydroponic farms reduce water waste and nutrient use because they do not depend on soil substrate and use a water recycling system. However, they require rooftop greenhouses, vertical structures, and a large amount of electricity for recirculating pumping systems; it simply requires more in infrastructure and energy inputs than other types of farming.

Electricity consumption was one of the main variables contributing to the impact category with rooftop hydroponics substantially exceeding conventional agriculture. Two other major variables were greenhouse construction materials and fertilizers. Regarding eutrophication, ecotoxicity, and mineral resource scarcity, conventional agriculture is not comparable to rooftop hydroponics because some of the fertilizer applied in production is lost through the leaching process [23]. In contrast, hydroponics optimizes nutrient supply to support the production stages of the plants by recycling it through the continuous water flow system.

3.3. Sensitivity Analysis Converting to Renewable Energy

Based on the LCIA of ComCrop’s rooftop hydroponic production of basil, the use of electricity was the most substantial negative variable because it was sourced from natural gas, contributing to 90% of GHG emissions, 70% of ozone formation, 60% of terrestrial acidification, 75% of terrestrial ecotoxicity, and 80% fossil resource scarcity (refer to Figure 7 and

Table 8. Sensitivity analysis of ComCrop’s rooftop hydroponic production replacing natural gas sourced electricity with solar energy.

Impact Category (unit)	Total	ComCrop Basil Production (Solar)	Infrastructure Inputs								Material Inputs		Energy Inputs
			Greenhouse
			Greenhouse Wall	Shade Cloth	Stalking Support	Concrete	Tubes	Tray	Pipe	Water Tank	Starter Cube	Basil Seeds	Photovoltaic Cell	Fertilizer
Global warming (kg CO2 eq)	1.27 × 10−1 ++	0.00 × 1000	1.38 × 10−3	2.33 × 10−5	1.34 × 10−3	3.00 × 10−3	4.47 × 10−4	3.19 × 10−6	1.44 × 10−5	1.91 × 10−3	2.88 × 10−2	2.32 × 10−5	8.41 × 10−2 ++	5.76 × 10−3
Stratospheric ozone depletion (kg CFC11 eq)	1.65 × 10−7 +	0.00 × 1000	2.90 × 10−10	5.40 × 10−12	6.65 × 10−10	4.10 × 10−10	3.06 × 10−10	2.19 × 10−12	3.08 × 10−12	1.22 × 10−9	5.70 × 10−9	9.85 × 10−11	4.27 × 10−8 ++	1.14 × 10−7
Ionizing radiation (kBq Co-60 eq)	8.59 × 10−4 ++	0.00 × 1000	4.77 × 10−6 ++	5.41 × 10−8 ++	9.33 × 10−6 ++	1.60 × 10−5 ++	1.12 × 10−5 ++	8.00 × 10−8 ++	3.50 × 10−8 ++	2.31 × 10−5 ++	1.01 × 10−4 ++	1.82 × 10−7 ++	6.79 × 10−4 (-)	1.40 × 10−5 ++
Ozone formation, Human health (kg NOx eq)	2.95 × 10−4 +	0.00 × 1000	3.28 × 10−6	4.99 × 10−8	4.59 × 10−6	6.15 × 10−6	5.85 × 10−7	4.18 × 10−9	3.10 × 10−8	5.00 × 10−6	6.17 × 10−5	1.02 × 10−7	2.01 × 10−4 +	1.24 × 10−5
Fine particulate matter formation (kg PM2.5 eq)	2.33 × 10−4 (-)	0.00 × 1000	1.72 × 10−6	2.52 × 10−8	3.50 × 10−6	1.92 × 10−6	3.28 × 10−7	2.34 × 10−9	1.57 × 10−8	3.40 × 10−6	4.53 × 10−5	4.56 × 10−8	1.69 × 10−4 (-)	7.51 × 10−6
Ozone formation, Terrestrial ecosystems (kg NOx eq)	3.13 × 10−4	0.00 × 1000	3.61 × 10−6	5.33 × 10−8	4.64 × 10−6	6.24 × 10−6	6.04 × 10−7	4.32 × 10−9	3.32 × 10−8	5.43 × 10−6	6.88 × 10−5	1.05 × 10−7	2.11 × 10−4 +	1.25 × 10−5
Terrestrial acidification (kg SO2 eq)	4.18 × 10−4 (-)	0.00 × 1000	3.93 × 10−6	6.32 × 10−8	8.03 × 10−6	5.20 × 10−6	7.32 × 10−7	5.23 × 10−9	3.93 × 10−8	7.40 × 10−6	8.84 × 10−5	1.30 × 10−7	2.79 × 10−4 (-)	2.50 × 10−5
Freshwater eutrophication (kg P eq)	7.79 × 10−6 ++	0.00 × 1000	3.69 × 10−8 ++	4.67 × 10−10 ++	1.22 × 10−7 ++	4.67 × 10−8 ++	1.20 × 10−8 ++	8.59 × 10−11 ++	2.97 × 10−10 ++	6.43 × 10−8 ++	1.64 × 10−6 ++	1.51 × 10−9 ++	5.66 × 10−6 ++	2.14 × 10−7 ++
Marine eutrophication (kg N eq)	3.38 × 10−6 (-)	0.00 × 1000	1.01 × 10−8 ++	6.24 × 10−11 ++	1.31 × 10−8 ++	3.73 × 10−9 ++	9.59 × 10−9 ++	6.85 × 10−11 ++	5.64 × 10−11 ++	7.14 × 10−9 ++	7.37 × 10−8 ++	1.09 × 10−10 ++	3.21 × 10−6 (-)	5.22 × 10−8 +
Terrestrial ecotoxicity (kg 1,4-DCB)	2.07 × 1000 (-)	0.00 × 1000	2.53 × 10−3	4.94 × 10−5	1.46 × 10−2	9.17 × 10−3	1.79 × 10−3	1.28 × 10−5	2.91 × 10−5	7.04 × 10−3	6.81 × 10−2	1.39 × 10−4	1.94 × 1000 (-)	2.69 × 10−2
Freshwater ecotoxicity (kg 1,4-DCB)	1.25 × 10−4	0.00 × 1000	7.30 × 10−7	1.23 × 10−8	3.66 × 10−6	1.34 × 10−6	4.61 × 10−7	3.29 × 10−9	7.81 × 10−9	2.60 × 10−6	4.24 × 10−5	3.27 × 10−8	6.85 × 10−5	4.89 × 10−6
Marine ecotoxicity (kg 1,4-DCB)	1.98 × 10−3 ++	0.00 × 1000	2.36 × 10−6 ++	4.33 × 10−8	1.29 × 10−4 ++	6.55 × 10−6 ++	1.61 × 10−6 ++	1.15 × 10−8 ++	2.63 × 10−8 ++	8.05 × 10−6 ++	5.60 × 10−5 ++	1.14 × 10−7 ++	1.75 × 10−3 ++	2.28 × 10−5 ++
Human carcinogenic toxicity (kg 1,4-DCB)	1.20 × 10−3 ++	0.00 × 1000	7.88 × 10−6 ++	1.35 × 10−7 ++	6.74 × 10−5 ++	1.02 × 10−5 ++	7.40 × 10−6 ++	5.28 × 10−8 ++	1.07 × 10−7 ++	2.21 × 10−5 ++	3.11 × 10−4 ++	3.09 × 10−7 ++	7.36 × 10−4 ++	3.56 × 10−5 ++
Human non-carcinogenic toxicity (kg 1,4-DCB)	4.45 × 10−2 ++	0.00 × 1000	2.09 × 10−4 ++	3.33 × 10−6 ++	1.47 × 10−3 ++	3.64 × 10−4 ++	9.54 × 10−5 ++	6.81 × 10−7 ++	2.01 × 10−6 ++	5.08 × 10−4 ++	3.56 × 10−3 ++	−2.29 × 10−5 (-)	3.69 × 10−2 ++	1.46 × 10−3 ++
Land use (m2a crop eq)	2.38 × 10−3 +	0.00 × 1000	1.33 × 10−5	2.06 × 10−7	3.17 × 10−5	5.30 × 10−5	1.68 × 10−5	1.20 × 10−7	1.22 × 10−7	6.28 × 10−5	3.82 × 10−4	1.56 × 10−5	1.69 × 10−3 ++	1.12 × 10−4
Mineral resource scarcity (kg Cu eq)	9.33 × 10−4 (-)	0.00 × 1000	2.71 × 10−6	5.75 × 10−8	7.65 × 10−5	8.94 × 10−6	2.26 × 10−6	1.62 × 10−8	2.92 × 10−8	5.76 × 10−5	5.80 × 10−5	1.35 × 10−7	6.70 × 10−4 (-)	5.65 × 10−5
Fossil resource scarcity (kg oil eq)	4.03 × 10−2 ++	0.00 × 1000	9.22 × 10−4	1.64 × 10−5	3.19 × 10−4	3.02 × 10−4	6.87 × 10−5	4.91 × 10−7	1.01 × 10−5	1.18 × 10−3	1.46 × 10−2	6.62 × 10−6	2.20 × 10−2 ++	8.04 × 10−4
Water consumption (m3)	3.31 × 10−3 (-)	2.00 × 10−5	1.73 × 10−5	4.62 × 10−7	1.95 × 10−5	7.12 × 10−5	8.05 × 10−6	5.75 × 10−8	1.43 × 10−7	3.10 × 10−5	3.36 × 10−4	8.99 × 10−6	2.74 × 10−3 (-)	5.83 × 10−5

+ Reduce impact is 5–50% as compared to natural gas; ++ reduce impact is >50% as compared to natural gas; and (-) impact is increased as compared to natural gas.

). We ran an LCA model to replace energy inputs to analyze how the environmental impacts might vary. Considering that Singapore’s only viable, clean energy source is solar energy, we input solar energy to meet the energy demand of hydroponics.

The calculations were performed using solar mono silicon PY panels with a 25-year lifetime, using data obtained from the SimaPro database. The table shows a significant reduction across almost all impact categories with a total overall reduction in impact on global warming of 58%. In particular, the reduced impacts on other categories that were greater than 50% included ionizing radiation (62%), freshwater eutrophication (72%), freshwater ecotoxicity (99%), marine ecotoxicity (92%), human carcinogen toxicity (76%), human non-carcinogen toxicity (71%), and fossil resource scarcity (67%). There were several categories in which solar was calculated to increase the negative environmental impact including fine particulate matter (66%), terrestrial acidification (24%), marine and terrestrial ecotoxicity (98% and 189%), mineral resource scarcity (75%) and water consumption (215%). Despite these increases, solar-based rooftop hydroponics emitted 0.12 kg CO₂ as compared to natural gas rooftop hydroponics, which emitted 0.30 kg CO₂. Therefore, promoting clean and renewable energy sources has the potential to significantly reduce the environmental impact produced by non-renewable sources of electricity, with the caveat that economic and spatial constraints could affect the feasibility of implementation.

4. Discussion

Research indicates that rooftop hydroponic farming has lower CO₂ emissions and other environmental benefits compared to conventional farming, yet we lack a comprehensive understanding of its environmental performance. This study applied a life cycle assessment (LCA) to evaluate the environmental impact of ComCrop, a pioneer of rooftop hydroponic production in Singapore, using basil as a commonly consumed import—a top importer after the USA [73,74]. The LCA was compared to conventionally grown, USA basil, Singapore’s main source. We found that rooftop hydroponics was the more sustainable production method based on land, water, packaging, and transportation. However, sustainability can be improved with recyclable materials, water nutrient filtration, and renewable energy sources with lower lifecycle impacts, such as solar. In this study we found that replacing natural gas with solar equated to a total overall reduced impact on global warming by 58% and more than 50% in other categories including ionizing radiation, freshwater eutrophication, freshwater ecotoxicity, marine ecotoxicity, human carcinogen toxicity, human non-carcinogen toxicity, and fossil resource scarcity. It is evidence that power sources with higher pollution levels (e.g., fossil fuels) significantly exacerbate environmental impacts and that switching to clean energy alternatives could substantially mitigate these effects. Overall, solar-based rooftop hydroponics emitted 0.12 kg CO₂ as compared to natural gas rooftop hydroponics which emitted 0.30 kg CO₂. However, there were several categories in which solar was calculated to increase the negative environmental impact. Additionally, while the input of infrastructure materials of rooftop hydroponics had some environmental impact, this study did not consider the assembly and degradation of the materials but calculated the environmental impact of the materials themselves, which affects the interpretation of the comparative model by underestimating the broader environmental impact of rooftop hydroponics.

While the LCA is a widely adopted method, its application to high-tech urban production systems is limited [38]. Previous studies using LCA to measure UA generally (for example [39,40]), in green roof systems (for example [41,42,43]), and in rooftop hydroponic systems (for example [28,44]) are not comprehensive. This limits the ability to interpret overall environmental impacts. The novelty of this study was to develop and apply a comprehensive LCA to identify which variables of urban rooftop hydroponic farming have the greatest negative impacts so that targeted improvements can be recommended. Our study bridges past studies that have separately assessed crop yields, water use and fertilizer [39,40,45], materials and energy inputs [27,43], and packaging- and retail-related emissions [30]. Additionally, our results support past research that found that rooftop hydroponic systems’ consumption of electricity and distance had the greatest negative impact on environmental conditions [46].

Notably, our results also provide empirical evidence to support past claims that hydroponics has a theoretically lower environmental effect compared to conventional farming methods [47]. When comparing ComCrop’s rooftop hydroponics system with USA-based conventional agriculture, conventional agriculture had significantly higher negative environmental impacts based on the necessity of global transport. Transport emissions are responsible for greenhouse gases and can potentially affect other categories such as eutrophication of water bodies and scarcity of mineral resources due to fossil fuels for transport. In removing the global transport and focusing only on the production stages, rooftop hydroponics had both disadvantages and advantages as compared to conventional agriculture. Conventional agriculture had lower greenhouse gas emissions and less impact on most environment categories for the same amount of basil produced as compared to rooftop hydroponics. But rooftop hydroponics had advantages in that it required much less land use and water consumption than conventional agriculture.

Similarly to other studies [28,46], we found the highest negative environmental impact of rooftop hydroponics was due to its reliance on electricity to power an uninterrupted water recycling system. The environmental impact of electricity depends on its production method, which in this case was non-renewable natural gas. By conducting a sensitivity analysis to replace the source of electricity, we found that using renewable and clean energy sources could significantly reduce the environmental impact associated with electricity use. To provide a more resilient, sustainable, and competitive food system, the use of renewable energy can significantly reduce the environmental impact related to rooftop hydroponic systems. And, with an upgrade to rooftop hydroponic systems, it could facilitate longer-term future development such as a scalable energy policy or planning integration for future development, as well as directly contributing to the urban food supply.

4.1. Limitations

In this study, an LCA was conducted to assess the environmental impact of ComCrop’s rooftop hydroponic system’s production stage. At the same time, the LCA itself is an assessment method closer to the cost and environmental benefits. An LCA is used to analyze impact categories through a quantifiable system; however, not each variable can be calculated to a specific number and inserted into the model—for example, human engagement is difficult to quantify. The reliability and authenticity of data can also lead to inaccuracies. For example, data may be based on imputed values, single sampling, or obsolete results [75]. An LCA also often lacks social and ecological significance. In this study, there remain gaps in studying, for example, biodiversity, community involvement, and others. Finally, while the input of infrastructure materials of rooftop hydroponics had some environmental impact, this study did not consider the assembly and degradation of the materials but calculated the environmental impact of the materials themselves, which affects interpretation of the comparative model by underestimating the broader environmental impact of rooftop hydroponics.

The strength of an LCA is that it can be used to evaluate better processes through comparative scenarios, such as in this study. Nonetheless, due to different system boundaries, different information, and other aspects, research can be weighted to one side in one study and the contrary in another [76]. For this study, infrastructure inputs in the analysis of ComCrop’s rooftop hydroponic system’s production were partially obtained by field measurements and partially by estimation. In the production of basil, daily weight yield was measured. However, the specific number of plants grown per day was estimated based on the average weight per plant. Therefore, calculations could over or underestimate actual production. Furthermore, ComCrop inputs were for all their green leafy plants, so the specific cost of producing basil was estimated based on the percentage of production (in this case 60%). Overall, the data provided by ComCrop was an estimation. For example, use of 20 kg of fertilizer per box was for all plants and adjusted as a percent for the production of basil. Therefore, data may have issues with validity due to including inputs not directly related to basil and reliability due to estimation. Suggested improvement methods include implementing precision monitoring techniques, developing standardized protocols to enhance the data collection framework, expanding the system boundary to include critical elements (e.g., water nutrient runoff tracking), creating localized database development, and applying Monte Carlo analysis to address uncertainty quantification. These approaches maintain methodological rigor while progressively addressing the identified data gaps through practical, implementable solutions.

In defining the boundaries of the LCI, variables outside the production stage, such as disassembly and recycling capital inputs, and uptake and legacy of fertilizers in water, were excluded. In the future development of rooftop hydroponics, it is necessary to improve the filtration method of water nutrients, recycled construction materials, and clean energy such as solar energy. Additionally, this study only considered fresh basil, but the traditional land-based production of imported USA basil is mostly found in processed products such as Italian pesto. The environmental impact of green leaf waste, packaging losses, processing, and increased transport distances would likely be greater than what is reported in this study. However, even if fresh basil was produced locally in Singapore, these additional LCA components would be similar in impact across the two systems. Therefore, focusing only on production is more useful for comparison.

Finally, the conventional agriculture dataset in SimaPro may have bias because it was based on the LCI dataset representing land-based production in the USA. While the authors of the dataset confirmed that their data was likely representative of similar cultivation in the USA, the lack of actual investigation into specific production, transportation, and selling processes did not allow these values to ensure the characteristics of specific production practices of USA-imported basil. In conclusion, the data for this experiment were obtained partly through field measurements, partly from ComCrop, and partly from SimaPro databases, which could influence the quantitative nature of the findings.

4.2. Recommendations

This study was an attempt to assess the environmental benefits of existing rooftop hydroponics in Singapore. The study results showed that capital input and energy expenditure were the main environmental impact factors of rooftop hydroponics. It was also found that hydroponic systems use very little water and the closed water cycle system itself is a way to conserve and protect productive water resources. The use of urban rooftop production not only saves agricultural land use, but, when producing for local markets, it greatly reduces the distance of food transportation and consequent environmental impact compared to imported produce. By replacing the source of electricity with a renewal source such as solar energy, the corresponding environmental impact can be significantly reduced.

In a high-density urban environment such as Singapore, rooftop hydroponics, a closed and efficient agricultural production cycle, has viable environmental benefits. Traditional agriculture is not possible due to the urban context, making transport unavoidable. Rooftop hydroponics mean there is less wastage in packaging and transportation, which decreases the environmental impact. Therefore, it is recommended for the Singapore government to continue to promote and support local agriculture for sustainable development, reduced environmental impact, and food safety.

4.3. Suggestions for Future Research

Rooftop hydroponic production, because it is on the rooftop of a building, can positively affect the energy cycle of the whole building, such as lowering the building temperature and reducing air conditioning usage. Moreover, rooftop production in the city is a multiple land use in a high-density city, improving land use efficiency. Future LCAs could confirm the increased benefits related to land use. Rooftop farms also increase the greening rate but do not necessarily increase biodiversity or connect urban green patches. This is a gap in practical and theoretical knowledge. Since this research, ComCrop has moved from its original city-center location to an industrial area in the north—from a farm with more community involvement to a more productive one—consequently reducing its accessibility for residents. Singapore’s urban agriculture production is generally located at the periphery rather than the center of the urban area, making it less accessible and more reliant on local transport. Enhanced community awareness of green urban output and increased participatory production activities can positively affect urban agriculture production. Combining an LCA with other methods can allow a broader perspective on the environmental, economic, ecology, and social impacts of agriculture. But can hydroponic systems improve ecological biodiversity, or are they designed in a way that contradicts or excludes biodiversity?

Finally, the infrastructure material inputs of rooftop hydroponics generated environmental emissions when the disassembly and recycling process was excluded in the calculation. It would be possible to assess whether replacing them with recycled materials could mitigate the environmental impact for future hydroponic production system improvements.

5. Conclusions

In high-density cities such as Singapore, where there is a severe shortage of land, rooftop hydroponics is a viable method of increasing green leaf production. In this study, we found that, based on empirical data, it had the potential to produce the same amount of produce as conventional agriculture (based on modeled data) but with minimal land use, a reduced amount of water flowing into the system, and reduced food miles. A hydroponics system is a recycling water production mode. Because it does not rely on soil substrates, it depends on a constant use of electric pumps to circulate the water system and add nutrients to the water, which eliminates nutrient runoff and reduces resource depletion. Thus, electricity and fertilizer are the main energy inputs that impact the environment. Meanwhile, hydroponic production requires infrastructure investment, including rooftop greenhouses, vertical plastic structures, and a circulating irrigation system. These infrastructure materials require more capital investment and affect the environment over the long-term.

The LCA in this study identified significant environmental burdens primarily from resource extraction and energy-intensive operations. Specific impacts include the fossil depletion, human toxicity, and waste disposal of non-recyclable growth media. Prioritizing recycled materials and renewable energy would mitigate these hotspots. By comparing imported conventional agriculture, rooftop hydroponics uses less land and water in the production process, and it substantially reduces food miles and GHG emissions of cross-country transportation, and the consequent environmental pressure. In the future development of rooftop hydroponics, it is necessary to address the filtration method of water nutrients, recycle construction materials, and use clean energy such as solar energy. In future studies, we need to demonstrate the preservation of high-value land for other uses, the benefits related to building energy, ecological biodiversity improvements, and social benefits of rooftop hydroponics.