First FlushTanks

Modular or monolithic rainwater harvesting tanks are used for collecting the first 5 mm of runoff evenly covering a surface.

The purpose of such tanks is that of storing polluted water and avoiding it immediately flows into the drain. Later, the water held by the tanks is directed, by means of a discharge pump, to the municipal sewage system or, if containing hydrocarbons, surfactants, settleable solids, heavy metals, etc, undergoes treatment in specific systems (e.g. oil separation plants and/or chemical-physical plants).

First flush water should be allowed to flow into the municipal system within 24-48 hours of the end of the meteorological event and, anyhow, according to the limits set forth by the entity running the sewer system and the water treatment plant. Such rule applies because, in the event of precipitation, municipal water treatment plants are overloaded with incoming water from mixed sewage and would hence have to purify huge quantities exceeding their treating capacity.

First flush tanks
First flush tanks

Sizing criteria

This system is made up of a precast modular or monolithic tank containing a party wall and, if applicable, by other containers in the same size in the number needed to reach the storage volume, calculated by applying the 5 mm of evenly distributed rain on a paved surface parameter.

The main module is fitted with an electronic device placed by the inlet, which signals start and end of precipitation and, following a set time of beginning of the same, activates a pump for discharging the water collected so far, directing it to the sewer system, or to the treatment plant if applicable. The tanks are fitted with a standard first flush flow stop float, which acti- vates when the same are full.

Running and maintenance

The sensor must be periodically checked and, if needed, freed from sediments that may affect its correct operation.

Also tank discharge pump operation must be verified.

Products and technical features

Tank size cm 300x200x200 h, is available in both modular (50 cm height modules) and mono- lithic version with 200 cm height and 16 cm thickness. Cm 400-500x200x200 h tanks are only monolithic. Monolithic tanks are single-cast, according to reinforced concrete standards, and bear up to 2000 kg/m² uniformly distributed load.

All systems are fitted with water-tight electronic rain detector and IP65 and PLC protection electrical panel controlling the various functions.