6 November 2023

First rain plants: identikit of a rainwater purification system

In the context of sustainable rainwater management, first rain systems are one of the most innovative technical solutions. In a period of water crisis, these systems allow the collection of rainwater, to treat it and allow its reuse, or the discharge on soil or in surface water.

In this way, hydropower companies will not only become more environmentally sustainable. They can also save on the ever-increasing costs of water bills, with considerable image and economic advantages. And the environment, in all this, thanks.


First rain plants: types of treatment plants

Impianti Di Prima Pioggia

We have already talked about the operation of the first rain plants. The plants of first rain allow in the first step to separate the water of second rain, less polluted, thanks to a drain, then channeling the water of first rain towards the sand remover and the deoliator coalescence.

The passage from the drain to the sanding chamber can be directed, or the water can be channeled into a series of tanks where the suspended materials carried by the rain settle. To this we owe the difference between plants of first rain in accumulation and plants in continuous.


First rain plants in storage

The first accumulating rain plants include a first rain tank, to allow rainwater to separate from suspended materials, before proceeding with purification. The plant for the treatment of first accumulating rain water must therefore have a sufficient volume to collect and treat the first 4 or 5 millimeters of precipitation, depending on the regional legislation of reference. The calculation for the sizing of the first rainwater tanks must therefore take into account the draining surface (S) and the amount of mud capable of raising the volume of water (a coefficient Q = 100 is assumed when the amount of sludge is reduced, 200 when it is average, and 300 when it is high).

Given the constant of the intensity of precipitation 0.0056 l/s x sqm (given the 5 mm/sqm in 15 minutes, or 20 mm/sqm in 1 hour), can be calculated

  • the volume of first rain Vpp = S x 5mm (Ts)
  • the flow rate Q= S x i = S x 0,0056 l/s x mq
  • the sludge sedimentation volume Vsed = Q x Cf

The total volume of the first rain tank should therefore be greater than the sum of the first rain volume and the sedimentation volume.


Plants of first continuous rain

In systems of first continuous rain, after separation from the water of second rain, the water of first rain is channelled directly into a sand chamber, where the separation between rainwater and suspended matter takes place, containing a coalescence filter.

The total volume of the tank must therefore take into account the volume of separation in the sanding zone, and the volume of sedimentation of the portion dedicated to the de-oiler. The latter is influenced by the time (Ts) necessary for the oil components in suspension to divide from the water, and therefore by their density (for example, Ts = 16.6 minutes with a density of oily substances up to 0.85 g/cmc, while it will be higher, equal to 30 minutes, for heavy sands or 45 minutes for light powders).

Therefore, always taking into account the draining surface (S) and the constants of the precipitation intensity, and the sludge sedimentation coefficient, at this point it is possible to calculate:

  • the flow rate Q = S x i = S x 0,0056 l/s x mq
  • the volume of the sedimentation or sand removal tank Vsed= 100*Q / 1000 or Vsed = 200*Q / 1000 or Vsed = 300*Q / 1000
    the volume of the separation chamber Vsep = P*Ts

The total volume of the tank of the first continuous rain system must therefore be at least Vpp = Vsep + Vsed.

But the reference millimetres for separating first and second rain water, as well as the treatments provided for rainwater, also change in relation to the regional legislation of reference. For your first rain systems in accumulation or continuous, therefore, and to be sure of their size and compliance with the law, contattate quindi Di Camillo Serbatoi.

The technicians of Di Camillo, with almost 60 years of experience in the production of tanks and water purification plants even in the first rain, will be able to respond to all your needs, with customized plants and the most suitable tools for all your needs.