Decentralised Wastewater Systems Builds Sustainable Cities

Human consumption of energy and resources is proving unsustainable, prompting a re-evaluation of how essential systems function—including the way water is managed in urban areas.

Supplying fresh water to cities is highly energy-intensive, while the wastewater produced by households, businesses, and industries offers a valuable yet often overlooked resource.

From wastewater, it is possible to recover water, energy, and essential nutrients to fertilise plants. However, conventional centralised wastewater treatment systems are energy-intensive and typically discard much of this potential, focusing instead on disposal rather than recovery.

With the growing challenges of climate change, finite resources, and rising demand, more sustainable and resilient systems are necessary. One promising approach is the use of decentralised systems.

Decentralised water and wastewater systems: A sustainable alternative

A recent study, “Using water and wastewater decentralisation to enhance the resilience and sustainability of cities”, published in Nature Water Journal in September 2024, evaluated the technical and economic feasibility of decentralised systems, as well as their environmental impact.

To maximise the benefits of recovered resources, the researchers integrated decentralised water systems with vertical farming, solar energy, and rainwater harvesting. Their findings indicate that such systems can significantly enhance resource efficiency and increase cities’ resilience.

The model applied source separation, dividing wastewater into:

• Yellow water (urine)
• Brown water (faeces and flush water)
• Grey water (everything else)

To illustrate the concept, the authors modelled a city block of 2,000 residents in eight medium-rise apartments. Each building collected rainwater from a 670 m² roof area. Nutrients from treated wastewater supported vertical farming, either inside or outside the buildings, while a shared basement housed the treatment system, occupying just 8% of the space.

How the system works

• Nutrient Recovery: Yellow water was processed to extract nitrogen and phosphorus, providing 45% of the nitrogen and 7% of the phosphorus needs for the vertical farm.
• Energy Generation: Brown water, mixed with food waste, was fed into an upflow anaerobic sludge blanket (UASB) reactor. This process recovered phosphorus and generated biogas, covering more than half of the treatment system’s energy needs.
• Water Reuse: Grey water was treated through a membrane bioreactor (MBR), producing safe, reusable water that met international safety standards, including those set by the World Health Organisation, the European Union, and the US Environmental Protection Agency.

Together, these elements demonstrated how wastewater can be transformed into resources, supporting both circular economies and sustainability goals.

Key benefits and findings

The study highlights several advantages of decentralised systems:

• Higher efficiency: Resource recovery improves significantly, with 40–50% more energy recovered, 85% less reliance on utility water, and much greater nutrient capture (nitrogen by 50%, phosphorus by 90%).
• Cost-effectiveness: While initial investment is slightly higher (around 4% more than conventional infrastructure), integration with vertical farming and solar panels provides a payback period of 10–12 years
• Local food and energy: Vertical farms supply fresh, nutrient-rich produce and contribute to economic returns, while solar panels pay for themselves within 5–10 years.
• Water security: Up to 85% of water demand can be met by recycled wastewater, reducing dependence on conventional water supplies.

The authors also stressed that stakeholder engagement—including regulators, utilities, and the public—is critical to overcoming implementation challenges and ensuring widespread adoption.

Towards resilient and circular cities

This study demonstrates that decentralised water and wastewater systems, when combined with vertical farming and renewable energy, can transform urban sustainability.

They offer a pathway to reducing costs, improving resilience, and creating resource-efficient cities ready to face the challenges of climate change.

For a deeper dive, you can read the complete study: “Using water and wastewater decentralisation to enhance the resilience and sustainability of cities”.