First Rain Plant


First Rain Plant for first foul flush in runoff treatment

Category / type: stormwater runoff treatment plant.
Application: gas station, commercial warehouse, car park, urban and logistic area, road, highway, impermeable surface.
Served area: from 1 up to 50000 m2.
Flow rate: from 1 up to 100 m3 per hour.
Normative references: EN 858-1 and 858-2, local and regional regulations.
Material: concrete, polyethylene, polypropylene, glass-reinforced plastic (GRP).
Pollution control.

First rain plant treats stormwater run-off from paved areas, such as urban areas, car parking, warehouse, gas station, roads, etc.
The first rain defines the initial surface runoff of a rainstorm, that is the most polluted part of entire rainfall: it’s called the first foul flush. During the first foul flush, pollutants, that has been settled in the previous dry period all over the urban area, are washed away from impermeable surfaces. In the first rain period water pollutants are typically more concentrated compared to the remaining of the storm.
First flow must be stored and treated by the mean of a first sedimentation phase and then by a light liquid separator. Second flow will pass through a by-pass to receiving water.
Conversely, small storm can’t remove large amount of pollutant unlike most intense.
Also in mixed drainage systems, where a part of rainwater enters to municipal treatment plant, a significant partition of excess flow contains dangerous pollutant (oil, hydrocarbons and heavy metals) that pass before peak discharge flows.
The by-pass system must be sized very carefully, in fact low flow values cause the increase treated wastewater quantity with the decreasing of organic matter concentrations; the contrary high flow values can cause to discharge an high quantity of dangerous pollutant.


How It Works

The first rain plant treats only the most polluted waters, those that arise from the first downpour of rain.
The first foul flush volume store guarantees to intercept most of the pollutants but not to limit the peak flow. For this scope it’s use a detention basin.
The plant is always composed by a spillway to divert excess flow, a storage tank, a sedimentation chamber (often included in storage tank) and an oil and light liquid separator.
Pump, level sensor, coalescing filters (in the separator), electric panel, rain sensor, alarms and remote controls complete the system equipment.
Proper storage tank sizing must consider a rain flow analysis to optimize pollutant removal. After settlement period, wastewater is pumped to an oil separator section. The oil separator sizing is quite similar to oil removal system, but in this case flow is fixed by pump data.
Upstream drainage network before the spillway, by-pass and downstream network must be sized considering the maximum flow rate (peak) which is calculated through an hydrological analysis.
In order to increase the effluent quality for waterways discharge, irrigation, reuse, etc, further treatments can be added, as sand and carbon filter, bio filtration or physical-chemical plant.
In particular cases, when even the second part of rain flow is polluted, a separator system for light liquids for constant flow rate should be installed.


Operation and maintenance

First foul flash requires a periodical maintenance for tank and mechanical parts, pumps and control system. In case of high polluted area a frequent cleaning operation is needed for light liquid and sand disposal. They can contain dangerous pollutant as hydrocarbons, heavy metals and pathogen.
Maintenance and cleaning operation must be performed only by licensed firms able to follow the procedures for the correct disposal of hazardous materials.
After light liquids and sand removal, the oil removal system is filled again with clean water up to working level.


Regulatory framework

Local regulations often specify where a first rain plant is required by law as impermeable surface dimension, user served (car park, storage area, road, gas station etc.) and first rain ratio to be treated.
Prefabricated oil and light liquid separators must refer to EN 858-1 for testing, certification and CE marking and to ENE 858-2 for NS calculation.
Local regulations also state where first rain plant or constant flow rain plant is required.


Features and benefits

The first rain plant main propose is pollutant control from impermeable surface. It is not a peak flow mitigation solution (detention basin).
Used within medium and large municipal separated or mixed sewerage, it works on stormwater line or downstream to excess flow spillway.
In small drainage system, local regulations frequently require runoff treatment plants upstream to public sewer.
It’s a very efficient solution for pollutant control, but it requires a further flow division. Duplicate networks is a disadvantage for investment costs increasing, but it also offers important advantages: clean water is not channeled into sewer and it avoids to grow the peak flow rate, reducing in the same time the flood risks and the sizing of the tanks. Impermeable polluted areas flow as car park, storage zone, gas station, warehouse etc. is sent to treatment system, whereas water form roof, porches, green areas is directly discharged into receiving water or onto the ground.
It also offers the opportunity to recycle water coming from roofs for non-potable uses, such as irrigation or water closet cistern.

Advantages (benefits)

  • pollutant control;
  • receiving water improving quality;
  • simply treatment;
  • low energy consumption;
  • small footprint (underground);
  • long useful life.


  • periodical maintenance;
  • pump required;
  • low efficiency for eventually water reuse;
  • maintenance by skilled personnel and authorized companies (vacuum truck).


Products in category First Rain Plant