i.WET is a practical concept for heat and water reuse that promotes a nature-based urban water cycle and is characterized by an improved energy balance compared to conventional water infrastructure. It was developed at Fraunhofer ISI as part of the BMBF research project TWIST++.
In the i.WET concept, low-contaminated partial flows such as graywater and stormwater are decoupled from municipal wastewater. These partial flows can be used for heat recovery to increase energy efficiency and/or reused for other purposes within the household, depending on the level of contamination and the treatment required. Water that is not reused in the household flows into the so-called “Energy Alley“ – a green strip designed as a soil filter that can also serve as roadside greenery and absorb street runoff.
In this way, i.WET promotes a near-natural urban water cycle, serves as a barrier against nutrients and pollutants, improves resource efficiency, and provides additional ecosystem services enhancing the urban landscape.
The path toward sustainable and resilient cities requires new approaches on many levels, including infrastructure design and provision. Infrastructure secures a society’s basic services and directly shape resource consumption, environmental quality, and thus quality of life. Due to major transformation processes such as climate change, demographic shifts, and technological and social change, infrastructure systems face substantial demands for adaptation. This translates into significant technical and economic challenges, particularly for pipe-bound, long-lasting systems such as water infrastructure.
Water infrastructure in German cities faces major challenges: Climate change is leading to more extreme rainfall events and altered precipitation patterns; demographic change is resulting in a different population structure; and increasing scarcity of raw materials is creating a need to improve resource efficiency. Water infrastructure must be adapted to these new conditions.
The transition of water infrastructure toward resilient, resource-efficient systems – capable of responding more flexibly to adaptation needs – requires concepts that can be gradually integrated into the existing infrastructure. Furthermore, these concepts must be able to function as hybrids alongside conventional system components – whether such components are retained or adapted – while reliably fulfilling all required functions.
In response to the challenges described (climate change, demographic change, and technological and societal change), Fraunhofer ISI developed the i.WET concept (integrated water-energy transition concept) as part of the BMBF research project TWIST++. The key innovation of this approach lies in the combination of two complementary water reuse pathways into a practical, holistic concept characterized by high sustainability and flexibility.
i.WET is based on the selective separation of low-contaminated partial flows from municipal wastewater – graywater, stormwater, and, where applicable, street runoff – before these streams are mixed together. This separate treatment enables optimized, demand-based processing while keeping effort to a minimum.
The “blue“ reuse pathway combines proven individual technologies – such as rainwater harvesting, graywater recycling, and heat recovery from graywater – that have, however, seen little application in municipal contexts to date. This combination ensures a continuous supply of service water and thermal energy.
The “green“ pathway uses nature-based modules to functionally integrate the urban landscape as “green infrastructure“. The central element is the so-called Energy or i.WET Alley, a combination of a vegetated soil filter and a drainage element. The i.WET Alley consists of a multi-layered soil structure that is sealed at the bottom. Excess stormwater and rainwater, as well as street runoff, are distributed (underground) across the surface and directed vertically through the soil filter body. The filtration effect, complemented by sorption and transformation processes in the biofilms, enables substantial removal of pollutants, nutrients, and pathogens.
Additional functions of the i.WET Alley may include: retention of hydraulic peaks during heavy rainfall events, a cooling effect in urban neighborhoods through increased evapotranspiration, biomass production, and the provision of ecosystem services.
Due to their high design flexibility, i.WET Alleys can be arranged along streets, at property boundaries, or as design elements within green spaces. They thus integrate seamlessly into a wide range of urban contexts and enable multifunctional use of existing land. A conceptual illustration is provided below.
The strength of the i.WET concept lies not in the individual technologies, but in their intelligent interconnection:
A key feature of i.WET is its potential for gradual implementation, aligned with the long renovation cycles of pipe-bound infrastructure systems:
This three-stage strategy avoids lock-in effects and enables adaptive infrastructure development without abrupt system changes. Importantly, i.WET can also be implemented as a standalone solution in existing neighborhoods without necessarily progressing through all three transition steps.
The innovative combination of the “blue“ and “green“ reuse pathways creates a practical, sustainable, and flexible comprehensive concept that delivers positive effects across all three dimensions of sustainability:
The i.WET concept thus represents an integrative, multifunctional approach that combines technological innovation with nature-based elements to sustainably address the complex challenges of urban water infrastructure.
Fraunhofer Institute for Systems and Innovation Research ISI