Is collecting rainwater in cities legal? Regulations and rules for rainwater harvesting
Yes, in general, collecting rainwater is legal, but in urban environments, reuse is not without regulations. Legislation often distinguishes between stormwater collected from roofs and runoff water from yards, paved areas, or traffic surfaces, which may require separate collection and dedicated treatment, particularly for the treatment of first-flush stormwater.
For this reason, an effective project should consider three key elements from the outset: the regulatory framework (regional requirements and permits), an assessment of the volumes that can realistically be recovered, and the selection of the most suitable system (collection, storage, filtration, pumping and, where necessary, first-flush treatment).
In urban settings, rainwater is typically used for non-potable applications such as toilet flushing, irrigation, and technical washing operations, with a separate distribution network and safety devices designed to prevent cross-connections.
At IDRO Group, we design solutions for stormwater management that include storage and non-potable reuse, as well as dedicated first-flush treatment systems where required by local conditions and discharge requirements. If you are developing an urban project, contact us for a preliminary technical assessment.
Rainwater harvesting systems: components, layouts, and the difference between roofs and paved areas
In urban environments, an efficient rainwater harvesting system is much more than a simple storage tank; it is an integrated process that makes water usable and manageable over time.
In roof rainwater collection systems, the typical layout includes collection from downpipes, pre-filtration for leaves and solids, storage sized according to rainfall and demand, pumping and pressurisation systems, and filtration designed for non-potable uses such as toilet flushing, irrigation, and technical washing.
When rainwater originates from yards, roads, parking areas, or potentially contaminated surfaces, the approach is different. The focus shifts to managing the first-flush runoff, as this fraction tends to concentrate pollutants accumulated on the surface. Consequently, storage tanks, separation systems, and treatment processes must comply with local regulations.
Put simply, first-flush runoff is the most heavily contaminated portion, while subsequent flows are generally easier to manage and can be treated or discharged according to project requirements and applicable regulations.
Among IDRO Group's rainwater harvesting references is a stormwater treatment system with partial reuse for toilets and irrigation. Water collected from downpipes undergoes high-efficiency dual pre-filtration, is stored in a tank equipped with a calming chamber and protected overflow, then pumped to end users and disinfected using UV treatment before non-potable reuse.
Another example involves first-flush treatment systems serving public ecological collection areas for CEM Ambiente. These underground installations feature automated post-event management designed to restore tank capacity within approximately 96 hours. Treatment includes sedimentation and grit removal, as well as oil and hydrocarbon separation through an oil separator sized according to EN 858, with indicative working volumes ranging from 5 to 50 m³, depending on site requirements.
Rainwater harvesting calculations: sizing and water savings in urban applications
To properly size an urban rainwater harvesting system, it is generally better to avoid relying on a single “automatic” formula and instead evaluate the factors involved, since performance depends on several interrelated variables.
During the preliminary phase, it is advisable to follow three key steps:
- Reconstruct local rainfall patterns and the effective catchment area, taking materials and collection losses into account.
- Estimate the volume that can realistically be recovered, including initial losses and filtration performance.
- Compare the recovery potential with the expected non-potable demand (for example, toilets and irrigation), while considering seasonal consumption patterns.
Selecting the storage volume requires finding the right balance between autonomy and water turnover. Oversized tanks can reduce water renewal and increase management and maintenance complexity, while undersized tanks may limit savings during critical periods.
For this reason, storage systems are typically sized according to a period of use consistent with the urban context and consumption profiles, while also considering available space, maintenance accessibility, overflow management, and integration with any first-flush treatment systems.
For yards and trafficable surfaces, first-flush tank sizing is not based on annual calculations but rather on event-based criteria and regional requirements. The objective is to intercept and treat the most contaminated initial runoff and rapidly restore system capacity before the next rainfall event.
Hydraulic invariance and stormwater discharge management
In modern urban developments, rainwater harvesting is increasingly integrated with the principle of hydraulic and hydrological invariance, which requires that post-development discharge flows do not exceed pre-development conditions.
This creates the need to control peak runoff flows generated by the impermeabilisation of surfaces through storage systems and regulated discharge mechanisms.
In this context, storage tanks can perform a dual function, contributing both to water reuse and to flow attenuation, while excess volumes can be managed through controlled discharge devices or infiltration systems.
The integration of rainwater reuse and flow attenuation not only helps ensure compliance with local regulatory requirements but also improves the overall sustainability of the project, making the stormwater management system more resilient and efficient over time.
IDRO Group can also support designers in the sizing and supply of flow attenuation systems in accordance with applicable regulations.
Cost-benefit analysis of rainwater harvesting: when it makes sense and how to build a strong business case
The economic viability of rainwater harvesting and reuse depends on three key variables: the volume of potable water replaced, the costs avoided (water supply and, where applicable, discharge and management charges), and indirect benefits such as urban resilience and contributions to ESG strategies and environmental project requirements.
For public authorities and designers, the most robust way to make the investment defensible is to prepare a concise but comprehensive dossier including:
- Water balance before and after implementation
- Estimates of recoverable water volumes
- System layout and process scheme
- Maintenance and water quality management plans
- Regulatory framework assessment
- Verification of local compliance requirements
If you are considering an urban rainwater harvesting solution, IDRO Group can support you from design through turnkey implementation and operational management, in full compliance with applicable regulations.
