Introduction | Stormwater can no longer be managed the way it was before

For decades, urban stormwater management followed a simple principle: collect it, channel it, and discharge it as quickly as possible. A pipe-and-drain approach that remained effective as long as cities stayed within the limits of what their infrastructure could handle.

Today, that model is reaching its limits. On impermeable urban surfaces, around 55% of rainwater becomes runoff, while only 15% infiltrates into the ground. On natural soils, the opposite occurs: runoff drops to around 10%, while approximately 50% infiltrates naturally. At the same time, climate change is intensifying rainfall events and increasing the frequency of urban flooding.

In response, a different vision of urban water management has emerged: the sponge city. A city designed to absorb, slow, filter, and return water to the ground rather than evacuate it immediately.

Nature-based solutions are central to this transformation. Instead of fighting water, sponge cities work with natural hydrological processes to restore the infiltration, evapotranspiration, retention, and purification functions that urbanization has disrupted.

But this transition raises a critical question that cannot be ignored: is the water being infiltrated clean enough to avoid contaminating soils and groundwater? Urban runoff carries hydrocarbons, PAHs, and heavy metals. Clean infiltration is therefore essential.

This is precisely where OSMORIA® depolluting aquatextiles fit within the broader nature-based stormwater management approach.

Part 1 | Sponge cities and nature-based solutions: principles and systems

What are nature-based solutions?

Nature-based solutions (or Sustainable drainage system,SuDS) are approaches that rely on natural ecological processes to address urban challenges such as water management, thermal regulation, and biodiversity.

In stormwater management, SuDS aim to restore the hydrological functions naturally performed by permeable soils: infiltration, evapotranspiration, water retention, and pollutant filtration. The European Union strongly supports the deployment of nature-based solutions through its Biodiversity Strategy 2030 and broader climate adaptation policies.

Nature-based solutions in practice

Organizations such as ADOPTA (France) summarize the principle clearly: rather than discharging stormwater into pipes, water should infiltrate as close as possible to where it falls, restoring space for water at the surface in a way that mimics natural hydrological networks.

Several complementary infiltration systems support this approach:

Vegetated swales and drainage ditches

Shallow vegetated channels collect runoff water, slow flows, and promote gradual infiltration. They have become common features in business parks, industrial areas, and residential developments.

Infiltration trenches

These systems operate underground using porous materials that temporarily store runoff before gradually infiltrating it into the natural soil.

Permeable pavements

Permeable pavers, porous asphalt, and permeable gravel stabilization systems such as Bidim® Accorder® allow water to infiltrate directly through traffic surfaces rather than becoming surface runoff, making them particularly suitable for parking lots and low-traffic roads.

Open infiltration basins

Designed to receive large runoff volumes during intense rainfall events, these systems also provide landscape and ecological benefits.

Reservoir pavements and underground storage systems

These underground hydraulic structures temporarily store water within porous aggregate layers before infiltration. Because they remain invisible at surface level, they are particularly well suited to dense urban environments with limited available land.

Why nature-based solutions embody the sponge city approach

The sponge city is not simply an aesthetic or landscaping concept. It is a systemic approach where every surface, public space, and hydraulic structure contributes to slowing, storing, and infiltrating water instead of evacuating it immediately.

Its benefits are significant:

• Reduced urban flood risk through runoff peak attenuation

• Improved groundwater recharge

• Enhanced urban environments and biodiversity

• Reduced pressure on aging sewer infrastructure requiring costly upgrades

Part 2 | The limitation nature-based solutions alone cannot fully address: pollution

Runoff water is rarely clean

While nature-based solutions manage runoff volumes effectively, they do not always adequately address water quality. Yet urban and peri-urban runoff consistently carries pollution.

Three major pollutant categories are commonly identified in roadway and parking area runoff:

Hydrocarbons

Generated by engine leaks, brake system residues, and asphalt-based materials, hydrocarbons are the dominant pollutant on high-traffic surfaces. Their concentrations are generally low but continuous, creating chronic pollution conditions that conventional systems often struggle to manage effectively.

PAHs

Produced through incomplete fuel combustion and tire wear, many PAHs are classified as probable carcinogens, even at very low concentrations. Difficult to biodegrade, they are easily mobilized during the first flush of rainfall events. Research cited by GRAIE shows that stormwater discharges can contribute more than 50% of annual PAH loads entering receiving environments.

Heavy metals

Zinc, copper, lead, and cadmium are non-biodegradable and accumulate over time within sediments, soils, and groundwater systems.

The limits of oil separators

Oil separators remain one of the most commonly specified stormwater treatment systems. Yet according to GRAIE, these systems “may be useful for containing accidental spills, but they are largely ineffective for treating stormwater runoff pollution.”

There are two main reasons for this limitation:

• First, maintenance is often inadequate. Insufficient cleaning, lack of inspection, and saturated tanks significantly reduce treatment efficiency over time.

• Second, most separators include bypass systems. During major rainfall events, precisely when pollutant loads are highest, runoff volumes exceed treatment capacity and bypasses activate. Polluted water is then discharged untreated downstream in a single event.

Infiltration without treatment simply transfers pollution

A nature-based solution infiltrating untreated runoff in areas exposed to hydrocarbons and PAHs does not eliminate pollution. It transfers it into soils and groundwater systems. Groundwater contamination is particularly problematic because it is difficult to detect, extremely expensive to remediate, and potentially irreversible.

Regulation increasingly makes this obligation explicit. The EU Water Framework Directive (2000/60/EC) establishes binding objectives for achieving good chemical and ecological status across all water bodies, including groundwater. For water intended for human consumption, Directive 98/83/EC, reinforced by the recast Drinking Water Directive 2020/2184/EU, sets strict thresholds of:

• 0.1 µg/L per individual PAH substance

• 0.5 µg/L total PAH concentration

Yet measured PAH concentrations in urban runoff regularly range between 0.03 and 6 µg/L depending on the site.

Across the EU, national regulations increasingly require stormwater systems to address not only hydraulic performance, but also water quality.

Regulatory pressure will intensify further through the revised Urban Wastewater Treatment Directive (UWWTD 2), adopted in December 2024. By 2035, large agglomerations will be required to implement integrated stormwater management plans, significantly increasing expectations around qualitative stormwater treatment at source.

Part 3 | Depolluting aquatextiles: the missing qualitative layer for nature-based solutions

Aquatextiles: a dedicated treatment technology

OSMORIA aquatextiles are innovative environmental technologies specifically developed to address a gap conventional stormwater systems do not fully cover: treatment of diffuse runoff pollution during infiltration itself.

Integrated directly into nature-based stormwater systems such as swales, infiltration trenches, basins, and permeable pavements, aquatextiles occupy a unique position within the treatment chain. They are neither heavy civil engineering structures nor conventional filters requiring regular replacement, but active treatment materials positioned directly between runoff water and the natural soil.

Their operating principle combines two complementary mechanisms:

Adsorption

Hydrocarbons and PAHs are captured and irreversibly retained within the textile structure, preventing migration toward underlying soils and groundwater.

Biodegradation

Microorganisms naturally present in soils and water colonize the textile and progressively biodegrade the adsorbed pollutants. This biological activity continuously regenerates the treatment capacity of the material without human intervention.

This dual mechanism reproduces and enhances natural soil purification processes while providing a controlled and measurable treatment environment.

Urban soils are inherently heterogeneous. Their treatment capacity varies significantly depending on soil composition, previous contamination, construction disturbance, and geology. Relying solely on the natural purification capacity of soils means relying on a treatment mechanism that is neither controlled nor predictable.

OSMORIA Geoclean: independently verified performance

Solmax developed OSMORIA specifically to address hydrocarbon pollution in stormwater infiltration systems. The biodegradation performance of OSMORIA Geoclean Origin was independently verified through Environmental Technology Verification (ETV) according to EN ISO 14034, the international reference framework for validating the environmental performance of innovative technologies through transparent and third-party verification.

Positioned as the first layer in contact with the natural soil, OSMORIA integrates into a wide range of infiltration systems including:

• Open infiltration basins

• Vegetated swales and ditches

• Permeable parking systems

• Underground basins

• Reservoir pavements

This compatibility allows engineering consultants to integrate a qualitative treatment layer into projects without redesigning hydraulic infrastructure.

Complementary to nature-based solutions, not a replacement

Depolluting aquatextiles do not replace nature-based solutions. They complement them.

Nature-based solutions manage runoff volume, slow flows, contribute to biodiversity, and improve urban resilience. Aquatextiles ensure that infiltrated water is treated before entering soils and groundwater systems.

This complementary approach is central to the Solmax philosophy: supporting modern stormwater management strategies while adding the qualitative treatment layer necessary to make source infiltration a complete and sustainable solution.

By integrating a depolluting aquatextile with defined, homogeneous, and independently verified adsorption and biodegradation properties directly within infiltration systems, treatment performance becomes active, repeatable, and independent of underlying soil variability.

Conclusion | Sponge cities cannot afford to become polluted cities

The transition from pipe-and-drain infrastructure toward sponge cities is already underway. The technologies exist, regulations increasingly support this direction, and urban development professionals now have access to proven stormwater management solutions.

But a sponge city infiltrating polluted water is not a resilient city. It is simply transferring pollution into groundwater systems and future generations.

For sponge cities to deliver on both environmental and regulatory objectives, clean infiltration is essential.

OSMORIA aquatextiles provide a durable and reliable qualitative treatment solution integrated directly into existing infiltration structures and compatible with a wide range of projects, from industrial sites and logistics platforms to urban roads, parking areas, and redevelopment projects.