Rainwater collection systems: tanks and rainwater treatment

A first flush treatment system is designed to intercept, store and treat the first few millimetres of rainfall that wash over yards, car parks and impervious surfaces — i.e. the potentially most polluted fraction of a rainfall event, carrying hydrocarbons, mineral oils, sand and suspended solids. This category includes precast reinforced concrete and polyethylene first flush tanks, diversion chambers, coalescence oil separators and complete turnkey systems, sized in accordance with Italian Legislative Decree 152/2006 and the applicable regional regulations.

What is first flush water and how the system works

Italian legislation defines first flush water as the first 5 mm of rainfall (4 mm in some regions, such as Abruzzo) uniformly distributed over the drainage surface, referring to the first 15 minutes of the rainfall event and preceded by at least 48 hours of dry weather. These initial waters carry almost the entire pollutant load washed off the paved area, whereas the subsequent rainfall is conventionally regarded as clean and can be discharged directly to the final receiving body.

A storage-type (discontinuous) first flush system operates in four stages:

1. Diversion chamber — separates first flush water from subsequent rainfall by means of a weir or a stainless-steel float check valve.
2. Storage tank — temporarily collects the first flush, allowing sand and sludge to settle on the bottom.
3. Time-delayed discharge — 48 to 96 hours after the end of the rainfall event, a submersible pump slowly empties the tank and feeds the water to the downstream treatment stage.
4. Oil separator (light liquid separator) — removes hydrocarbons and mineral oils through a coalescence filter, in compliance with EN 858-1, producing water that meets the limits of Table 3, Annex 5 of Legislative Decree 152/06.

A sampling chamber is always installed downstream of the treatment for inspection by the relevant authorities.

Available configurations

Our online shop offers a range of first flush systems and tanks, sized according to drainage surface and regional requirements:

• Precast vibrated reinforced concrete first flush tanks — for medium to large surfaces, available with D400 load-class trafficable covers.
• Polyethylene (PE) first flush tanks — lightweight, monobloc units, ideal for fast installation and reduced excavation volumes.
• First flush systems with storage (discontinuous) — featuring diversion chamber, storage tank, time-controlled pump, PLC electrical panel and oil separator.
• Continuous-flow first flush systems with by-pass — an alternative to storage systems where space is limited.
• Stormwater settling and storage tanks — for detention and reuse.
• Diversion chambers, coalescence oil separators, first flush kits and accessories (rain sensors, level probes, control panels, alarms).

Reference regulations

At national level, the key reference is Italian Legislative Decree 152/2006 (Consolidated Environmental Act), in particular Articles 113 and 133 and Annex 5 to Part III, which regulate the discharge of stormwater runoff. Alongside these, regional regulations vary significantly in terms of rainfall thresholds, runoff coefficients and the categories of activity covered: R.R. Lombardia 4/2006, D.G.R. Emilia-Romagna 1860/2006, L.R. Abruzzo 31/2010, R.R. Puglia 26/2013, P.T.A. Veneto, and so on. All our systems are sized in compliance with the regulations applicable to the installation site and come with technical specifications and downloadable drawings.

Applications

First flush treatment systems are mandatory — or strongly recommended — for industrial and commercial yards, logistics and storage areas, medium-to-large car parks and rest areas, service stations and fuel distributors, car washes and auto repair shops, body shops, recycling centres and municipal waste collection sites, scrap and aggregate storage yards, port areas, airports and new urban developments with impermeable surfaces exceeding 2,000 m².

Request a tailored quote

To correctly size a first flush tank, we need the drainage surface, the region of installation, the type of activity carried out on the yard and the characteristics of the final receiving body. Our technical department is available to help you choose the right model, provide DWG/BIM drawings and prepare the technical specification.