First-flush rainwater is the rain that falls during the early stages of a rainfall event and, as it runs off impermeable surfaces such as roads, industrial yards and roofs, picks up large quantities of pollutants. This water may contain hydrocarbons, heavy metals, suspended solids, mineral oils and other harmful substances, making it necessary to treat it appropriately before it is discharged or reused.

Stages of Treatment

1. Separation of Suspended Solids and Sedimentation
The water is channelled into a collection basin or tank where the heavier solids (sand, soil, inert materials) settle by gravity, thereby reducing the subsequent pollutant load.
2. Removal of Oils and Hydrocarbons
The water then passes through an oil separator, a system fitted with coalescing filters that facilitate the aggregation and separation of oils and hydrocarbons, which are then collected and disposed of separately.
3. Physical-chemical treatment or treatment using adsorbent filters
To remove residual contaminants such as heavy metals and organic compounds, the following methods can be used:

• Activated carbon filters, effective at absorbing organic substances and hydrocarbons.
• Ion exchangers or zeolites, to remove metals such as lead, copper and zinc.
• Chemical oxidation, for the breakdown of substances that are difficult to biodegrade.

4. Water Disposal or Reuse

After treatment, the water can be discharged into water bodies or the sewerage system, provided it complies with regulatory limits, or reused for industrial purposes, irrigation or washing.

Benefits of First-Flush Rainwater Treatment

• Reduction of environmental pollution in receiving water bodies.
• Compliance with emissions regulations, avoiding penalties and negative consequences.
• The possibility of reusing treated water, thereby reducing the consumption of drinking water.
• Protection of wastewater treatment plants, preventing overloads and inefficiencies.

The adoption of efficient systems for treating first-flush rainwater ensures the sustainable management of water resources, helping to protect the environment and optimise operating costs for industry and urban infrastructure.