Separator System for Light Liquids


Constant Flow Rain separator system for light liquids in rainwater treatment

Category / type: stormwater, car washing, gas station, warehouse, light liquids, mineral greases and oil.
Application: civil, parceling plan, industrial, impervious surfaces.
Served area: from 100 up to 5000 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).
No energy consumption.

Separator system for light liquids is a constant flow rate stormwater runoff treatment plant working in line on drainage network that treats the whole flow conveyed up to a design value. The excess water passes directly downstream through a by-pass system.
Its installation is required where water contamination is continuous during weather events and does not end in the first foul flush as junkyard.
Stormwater runoff from paved surfaces, parking lots, petrol station, factories, workshops, etc. often contains mineral pollutants as hydrocarbons, grease and mineral oil. Therefore a removal treatment is required as sand trap and oil and light liquid separator.
Separator system treats entire rain flow up to design value. Excess flow is diverted through a by-pass, but it doesn’t stop working after a fixed threshold. There is no water storage as for first rain plant; input and output flow are equal.
System comprises by-pass, sludge trap (sedimentation) and oil, light liquids separator, spillway and sampling points (well).


How It Works

Plant sizing refers to flow calculation; for single component see specific section (sand sludge strap, oil separator).
Nominal size is calculated according to European standard EN 858-2 for stormwater case.
Through a hydrological analysis (local data), project flow is calculated depending on return period – (4.3.5) Rainwater flow rate.
It does not consider the combined drainage of rainwater and wastewater 4.3.3; then impediment factor (fx) is zero as in table 2 in (minimum recommender impediment factors).
Nominal size (NS) depends on the density factor for the relevant light liquids (4.3.1 – General) as in table 3 ( – density factor fd) and Appendix A.
It depends on light liquid density (g/cm3) and composition of plant and presents different values for I class, II class or II – I class.
Components of separator system are: sludge trap (S), separator class II (II or II b for bypass separator), separator class I (I or I b for bypass separator) and sampling shaft (P) as in table 1 (4.2.1).
“The sludge trap is the part of the separator where material settles i.e. sludge, silt and grit. It can be constructed with the separator as a combined unit”.
The value retained for the sizing of the sludge trap can vary according to the standard EN 858-2. (Paragraph 4.4 and table 5)
It defines sludge volume depending of sludge origin being classified in none, low, medium and large: it is directly proportional to the nominal size and inversely proportional to density factor (fd).
European standard also specifies minimum value and restrictions.
Annex B, in table B.1 and B.2 shows minimum plant configuration (to meet required effluent quality) and recommendation for specific application.
Effluent can be discharged if it guarantees the limit of effluent pollutant concentration according to local regulations. The system must be authorized.
Bypass is placed upstream to divert excess flow rates.
For further information about oil and light liquids characteristics as I and II class type, automatic closure device, etc. see oil separator section.


Operation and maintenance

The oil separator system operates by gravity and by a coalescing filter and it doesn’t need any start up operations. Every chamber must be filled up by clear water on normal working level.
Oil and sludge levels are regularly monitored and when storage volume is full, it requires purging.
Higher volume causes the hydraulics residence time reduction and worsen the treatment efficiency.
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

Continuous stormwater flow rate is a particular case where wastewater discharge rate is zero according to EN 858-2. European standard describes configuration and nominal size calculation procedure.
Separator is part of the rainwater plant and it refers to EN 858-1 European standard, certification, class and CE marking.
Separators need a Declaration of Performance (DoP) based on harmonized standard according to Regulation (EU) No 305/2011.
The European standard 858-2 “(…Selection of nominal size, installation, operation and maintenance”) explains how to choose the nominal size depending on users, on separator system components and on their combination and application.
Stormwater or industrial wastewater treatments are subject to prior authorisation by competent authorities.
Local regulations often define application type and design criteria for a rainwater treatment plant.


Features and benefits

Petrol station, covered and uncovered car parkings, hydrocarbons storage areas, urban and logistic areas, roads, highways, ports, etc. require storm water treatment plants according to European standard and local regulations.
Constant flow rate is needed where pollutant can be present not only in the first part but during the whole rainfall event.
According to EN 858-2 waste water discharge (as car-washing – 4.3.3) is allowed combined with rainwater flow.
Refer to specific solution section: Car wash, Filling station, industrial, mechanic’s workshop, etc.
Constant flow rate plants use as excess flow treatment for storage rain treatment as first rain plants.

Advantages (benefits)

  • simply treatment;
  • dangerous pollutants control;
  • improving receiving water quality;
  • no energy consumption and power supply;
  • small required area (underground installation);
  • low operating costs;
  • long useful life.


  • low efficiency on particular pollutants (further treatment may be required);
  • periodic purges, depending on use, by a licensed contractor (vacuum truck);


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