SELECTION OF THE PETROLEUM SEPARATOR FOR YOUR OWN INSTALLATION

SELECTION OF THE PETROLEUM SEPARATOR

Application of separators

Oil separators are devices of sanitary installations, the task of which is to treat rainwater discharged from parking lots, gas stations or roads. Separators also clean technological wastewater, which come, for example, from car washes, workshops, and vehicle storage. The most durable oil separators are made of polyethylene rotomoulding, which enables long-term tightness and ease of maintenance.

The principle of operation of separators

In the separator, the contaminated water is cleaned of petroleum substances and solid contaminants (sand, earth, gravel, etc.). The first stage of wastewater treatment consists in separating solid pollutants with the help of a settling tank (so-called sand trap) in the process of gravitational sedimentation. In the second phase, the wastewater flows into the separation chamber, where, with the help of special filters, the separation and then collection of petroleum particles takes place. In this phase, contaminated water flows through a special coalescing filter, which is designed to facilitate the flow of petroleum substances to the surface, where the separated particles clump into larger clusters. In the third stage, i.e. the last stage of treatment, the sewage flows out of the device through the outflow to the sewage system or drainage ditch. The outflow is equipped with an automatic float closure which closes the tank when it is overfilled with accumulated petroleum substances. The current market offers by-pass separators. They are designed especially for large drainage areas and fulfill their function mainly during heavy rains. For larger flows, rainwater can be diverted by a storm bypass. Thanks to the by-pass system, excessive amounts of rainwater do not disturb the operation of the separator.

Selection of separators of petroleum substances

For the separator to function properly, it must be properly selected. According to PN-EN 858: 2005, the selection of a separator depends on the water flow rate through the separator.

General formula for selecting separators according to PN-EN 858:

NS=(Qr + fx ∙ Qs) ∙ fd

where:
NS - flow rate
Qr - nominal flow of rainwater in l / s
Qs - maximum flow of process wastewater in l / s
Fd - density coefficient (usually we take the value 1)
fx - separation difficulty factor (usually we take the value 1)

Calculation of the volume of process wastewater (e.g. car washes)

QS=QS1 + QS2 + QS3 +…

where:
QS1 - wastewater from intake points (table 1.)
QS2 - wastewater from car washes and / or self-service washing stands (check washer flow)
QS3 - wastewater from high-pressure cleaners and cleaning units

Table 1 The volume of wastewater produced from the intake points depending on the valve diameter

Qs2 I Qs3 - Car washes - for each car wash should be taken 3l / s, and for each subsequent station 1l / s of sewage.

Flow calculation for rainwater

Qr = (FZ ∙ Ws) ∙ q ∙ a

where: 
Qr - rainwater flow in l / s
Fz - catchment area
Ws - surface runoff coefficient selected according to tab.2
q - rainfall intensity l / s / ha (typically 150 l / s / ha except mountainous areas)
a - delay factor (the most common value is 1)

Installation of separators

When selecting the location where the separator will be located, familiarize yourself with the water and soil conditions. This research is necessary to prevent possible displacement of the reservoir by groundwater. The separator should be located as close as possible to the source of contamination, and must also be accessible to the service, i.e. the vehicle cleaning the separator. The separators are installed at a level that allows the connection of the distribution and discharge pipes, in a way that allows gravity flow. The device should not be installed deeper than required. If there is a need for deeper foundation, you should obtain superstructures. The groundwater level must be below the outlet of the separator. Separators can be placed at high groundwater levels. In such situations, it is necessary to make foundations, anchor the separator, and cover the tank with stabilized sand (50 kg of cement per 1 m3 of sand). The separators are delivered to the construction site as complete devices ready for installation. The installation of the tanks consists in:

 • placing a separator in the excavation,
 • connecting the separator nozzles with the rainwater or technological sewage pipes,
 • careful filling of the separator with stabilizing sand or cement.

Foundation in a passable place

The trench must be sufficiently deep, ensure the depth of overburden and stratification according to the type of pavement and load. Asphalt and concrete foundations must protrude at least 300 mm horizontally behind the device in all directions. Carry out a foundation with a thickness of at least 200 mm from a semi-dry concrete mix. When the concrete of the foundation initially hardens (usually after one day), balance the tank by pouring about 0.5 m of water into it. Backfill the space around the tank with compacted soil. The backfill material should be free of organic material, large stones, bricks or sharp objects. Backfilling is carried out in layers so that no voids are formed under and around the sides of the reservoir and there is no local stress concentration. Gradually fill the tank with water to a level above the filling, which will help stabilize the stresses in the tank. Then, the tank should be secured with a relief plate from above and cast iron hatches of appropriate strength should be selected. The cover frames should be installed in a concrete slab resting on the edges of the excavation.

Deep foundation in a passable place

If there is a need for deeper foundation of the reservoir below passable traffic, cover the reinforced concrete relief slab with compacted soil depending on the required foundation depth. A relieving ring should be placed under the cast iron hatch to distribute the stresses that have arisen.

Maintenance of separators

Due to the fact that petroleum substances (according to the Ordinance of MOŚZNiL of 24.12.97 Journal of Laws No. 162 item 1135) are classified as hazardous substances, maintenance and servicing of separators may be carried out only by a qualified company. Maintenance of petroleum separators consists in periodic emptying of waste retained in the device and cleaning the inside of the separator. The periodicity of cleaning depends on the rate of waste accumulation (at least once a year). The cleaning should be performed when the tank is full of sediments to half its volume or in 4/5 of the maximum oil holding capacity. After cleaning, fill the separator with water and set the float valve to its working position. Currently, there is a wide range of different oil separators on the market. Before buying a separator, we should consult the seller to choose the correct separator for our needs. Only a correctly selected separator will properly fulfill its function.