Decoupled Aquaponics Makes Local Rooftop Farms Greener Than Imports

The way we grow and transport food has a profound impact on the environment. In Northern Europe, fresh vegetables like tomatoes and lettuce often travel thousands of kilometers from warmer regions like Spain or Italy before reaching supermarket shelves in cities like Berlin.

This long journey involves refrigerated trucks, energy-intensive greenhouses, and water-heavy farming practices, all of which leave a significant environmental footprint.

A recent study published in the Journal of Cleaner Production offers a promising alternative: decoupled multi-loop aquaponics (DAPS)—a system that combines fish farming (aquaculture) and soil-free plant cultivation (hydroponics) while recycling water and nutrients in separate loops. This innovative approach drastically reduces resource use and emissions.

The Environmental Challenges of Traditional Vegetable Production

Northern Europe’s cold climate makes it difficult to grow vegetables like tomatoes and lettuce year-round without relying on energy-intensive greenhouses. For example, heating a typical tomato greenhouse in Germany consumes about 1.38 gigajoules (GJ) of energy per square meter annually—eight times more than what’s used in Spain.

A gigajoule is a unit of energy equivalent to one billion joules, roughly the energy needed to power a mid-sized car for 1,000 kilometers. On the other hand, Southern Europe faces its own set of challenges.

Farms in Spain and Italy often rely on water-intensive practices, including desalination (removing salt from seawater) or overpumping groundwater, which strains local ecosystems. To make matters worse, transporting these vegetables to Northern markets adds to their environmental cost.

Refrigerated trucks traveling from Spain to Berlin result in a 7.5% loss of produce due to spoilage, alongside significant carbon emissions.

The study highlights that Berlin’s current supply of tomatoes and lettuce—a mix of local and imported produce—comes with hidden environmental costs. For instance, 58% of tomatoes and 39% of lettuce in Berlin’s supermarkets are imported, primarily from Spain, Italy, and the Netherlands. While these imports may seem affordable, their true cost includes high water use, fossil fuel consumption, and greenhouse gas emissions.

Introducing Decoupled Multi-Loop Aquaponics (DAPS)

Traditional aquaponics systems, which combine fish farming (aquaculture) and hydroponic plant cultivation (growing plants in nutrient-rich water without soil), have been around for decades.

However, they often struggle with balancing nutrients between the fish and plants, leading to lower crop yields or poor-quality produce. Decoupled multi-loop aquaponics (DAPS) solves this problem by separating the fish and plant systems into independent loops.\

This allows farmers to tailor water quality and nutrient levels specifically for each system, maximizing efficiency. In a DAPS setup, fish like tilapia are raised in recirculating aquaculture systems (RAS)—closed-loop systems that filter and reuse water to minimize waste.

The water in RAS is kept at a warm 30°C, ideal for fish growth. The nutrient-rich water from the fish tanks is then treated to remove waste and adjust pH levels before being sent to the hydroponic plant system. Any missing nutrients, such as iron or potassium, are added to ensure optimal plant growth.

Meanwhile, solid fish waste is processed in a sludge bioreactor—a tank where microorganisms break down organic matter—which converts it into biogas (a renewable energy source composed mainly of methane and carbon dioxide). This closed-loop approach minimizes waste, recycles water, and reduces the need for synthetic fertilizers.

The study tested DAPS in four configurations: ground-based hydroponics, ground-based DAPS, rooftop DAPS with passive energy exchange, and rooftop DAPS with active waste heat reuse. The goal was to compare these systems against traditional imports and local hydroponics in terms of energy use, water consumption, and emissions.

How the Study Evaluated Sustainability

Researchers used a detailed simulation model to analyze tomato and lettuce production over a 10-year period (2009–2018). This model accounted for variables like local climate conditions, energy use, and water consumption in four regions:

1. Germany,
2. the Netherlands,
3. Italy, and
4. Spain.

For example, tomato greenhouses in Germany were set to maintain temperatures between 18–20°C, while lettuce farms operated at 16–18°C. The study also factored in technologies like LED lighting, which is used in Northern Europe to compensate for shorter daylight hours during winter. LED lights are energy-efficient bulbs that emit specific light wavelengths to boost plant growth.

A life cycle assessment (LCA)—a scientific method to evaluate the environmental impact of a product from raw material extraction to disposal—was then conducted to measure the sustainability of each production method.

This assessment covered 12 categories, including global warming potential (GWP₁₀₀) (the heat-trapping ability of greenhouse gases over 100 years), fossil fuel scarcity (depletion of non-renewable energy sources like oil and coal), water consumption, and mineral resource depletion (overuse of finite minerals like copper and phosphorus).

However. the focus was on two key products: a 500g package of tomatoes and a 150g bag of lettuce, delivered to a supermarket in central Berlin.

Local DAPS Outperforms Imports

The results revealed that locally produced vegetables using DAPS—especially when integrated into urban rooftops—significantly outperformed imported and traditional hydroponic methods. For tomatoes, the most striking improvement was in water use.

While imported tomatoes required 14.2 liters of water per 500g package, DAPS systems in Berlin achieved a net water saving of -10.1 liters through recycling.

This means the system returned more water to the environment (via rainwater harvesting or reduced groundwater extraction) than it consumed—a rare feat in agriculture.

Similarly, lettuce grown in rooftop DAPS with active waste heat reuse (capturing excess heat from building systems like refrigeration) used 30% less energy than traditional hydroponics.

Emissions also dropped dramatically. For lettuce, the global warming potential (GWP₁₀₀) decreased by 49.9% compared to the imported mix. Tomatoes saw a smaller but still meaningful reduction of 8.7%.

Fossil fuel scarcity, another critical metric, improved by 44.2% for lettuce and 4.7% for tomatoes. These gains were driven by the system’s ability to recycle resources, use waste heat from buildings, and minimize transportation.

Yield and quality improvements further underscored DAPS’s advantages. Lettuce grown in DAPS systems saw a 25% increase in yield compared to standard hydroponics, thanks to optimized nutrient levels.

Tomatoes benefited from fewer quality issues, with a 99% reduction in blossom-end rot—a common problem caused by calcium deficiency in traditional systems that leads to dark, sunken spots on the fruit.

The Role of Urban Rooftop Farming Integration

One of the study’s most innovative aspects was its exploration of urban integration. By placing DAPS systems on rooftops, researchers tapped into underutilized city spaces while leveraging existing infrastructure.

For example, supermarkets generate significant waste heat from refrigeration units (systems that cool stored food), which rooftop greenhouses can redirect to maintain optimal growing temperatures. In Berlin, this active heat reuse met 18% of the heating needs for tomatoes and 30% for lettuce, slashing energy costs.

Rooftop systems also reduced land use—the area of Earth’s surface required for farming. Traditional farming requires vast agricultural areas, but urban DAPS facilities can operate on a fraction of that space.

For tomatoes, land use dropped by 82.7%, while lettuce production required 71.3% less land. Additionally, organic waste from the system, such as tomato leaves and stems, was converted into biogas. This biogas offset 39.1% of the electricity needs in Berlin’s DAPS facilities, creating a self-sustaining energy loop.

Berlin’s 5,000 m² Rooftop Farm

To put these findings into perspective, the study modeled a 5,000 m² rooftop DAPS facility in Berlin. This facility produced 180 cubic meters of tomatoes and 112 cubic meters of lettuce annually.

Compared to imported tomatoes, the local DAPS-grown tomatoes had a carbon footprint of 0.066 kg CO₂ equivalent per 500g package—a 26% reduction. Lettuce fared even better, with emissions dropping from 0.066 kg CO₂ eq. (hydroponics) to 0.048 kg CO₂ eq.

Economically, the upfront costs of such a system are high. Advanced technologies like LED lighting, desalination units (systems that remove salt from water), and biogas reactors require significant investment.

However, the long-term savings are substantial. For instance, recycling fish waste reduced fertilizer costs by 60%, while eliminating transportation expenses for local sales cut logistical overhead.

The study suggests that government subsidies (financial support from public funds) or public-private partnerships (collaborations between governments and businesses) could help scale these systems, making them accessible to more cities.

Despite its promise, DAPS faces hurdles. The initial investment is a major barrier for small-scale farmers, and the system’s efficiency depends on a reliable energy mix.

Currently, many regions still rely on fossil fuels (non-renewable energy sources like coal and oil) for electricity, which undermines some of the environmental benefits. Transitioning to renewable energy sources like solar or wind could further enhance the system’s sustainability.

Future research could explore expanding crop diversity. While tomatoes and lettuce were the focus of this study, DAPS could potentially grow herbs, leafy greens, or even fruiting plants like strawberries.

Policy changes will also play a critical role. Incentives such as tax breaks (reductions in taxes for specific activities) for rooftop farms or grants (funding awards) for biogas infrastructure could accelerate adoption.

Conclusion

The study’s findings make a compelling case for reimagining urban spaces as hubs of sustainable food production. By adopting decoupled aquaponics, cities like Berlin can reduce their reliance on imported vegetables, cut emissions, and conserve water—all while producing fresher, higher-quality food.

The road ahead requires collaboration between governments, businesses, and communities, but the payoff is a healthier planet and more resilient food systems. As climate change intensifies, solutions like DAPS offer a blueprint for sustainable living.

They prove that innovation, when rooted in circular economy principles (reusing resources and minimizing waste), can turn environmental challenges into opportunities. From rooftops to dinner tables, the future of farming is local, efficient, and green.