Separator Systems for Light Liquids ^†

Abstract

This article deals with separator systems for light liquids that can be used in the management of rainwater from hard and polluted surfaces. Attention is focused on the material of the separators, the smallest nominal width and the types of separators. The procedure used for determining the type and size of a light liquid separator and its operation and maintenance is presented in this article as well. An example of a light liquid separator design for an industrial area is introduced in the experimental part.

1. Introduction

Separators for light liquids are an integral part of the process of managing rainwater from polluted hard surfaces contaminated with oil. The areas that are significantly contaminated and used for capturing rainwater include industrial sites, especially their parking lots and roads [1,2,3,4].

Furthermore, light liquid separators are used in the petroleum industry, in oil extraction, to separate oil and water in order to reduce extraction costs. Axial separators with multi-stage separation are used here with water flow rates ranging from 3 to 7 m³/h and an oil input fraction below 10% [5,6,7]. Due to the fact that the process of the separation of light liquids from water is not simple, simulation models are developed in order to increase the separation efficiency and to select the right type of separator [8,9,10].

Our paper deals with the design of light liquid separators as devices to be used in the management of rainwater that is polluted with oil.

According to the CSN EN 858-1 standard [11], a light liquid separator can be defined as a device used for the treatment of industrial wastewater or rainwater containing light liquids that are not emulsified. These are liquids with a density lower than or equal to 0.95 g/cm³ (e.g., petroleum substances). These may be present in wastewater from vehicle parking areas, garages, refuelling points, car wash boxes or industrial plants. The separator prevents such polluted water from entering the receiving stream or sewer network.

2. Structural Composition

From the design point of view, the structural composition of the separator comprises one or more tanks in which the technological equipment of the separator is installed—separating the space, sludge trap and sampling point. The tank may be of circular or rectangular design. It may be constructed on-site or prefabricated.

According to

Table 1. Classes of light liquid separators.

Class	Maximum Permissible Residual Oil Content (mg/L)	Typical Relieving Procedure
I	5.0	Coalescing separators
II	100.0	Gravity separators

, separators can be divided into two classes based on the maximum permissible residual oil content at the outlet. In this article, the focus is on light liquid separators that belong to the first class.

The recommended nominal sizes for light liquid separators are 1, 3, 5, 6, 10, 15, 20, 30, 40, 50, 65, 80, 100, 125, 150, 200, 300, 400 and 500.

The smallest permissible nominal width of the inlet, outlet and eventual interconnecting pipes has to adhere to the respective nominal size value of the light liquid separators, as presented in

Table 2. Minimum nominal width.

Nominal Width (NS)	DNmin (a)
Lower or equal to NS 3	100
Higher than NS 3     up to NS 6	125
Higher than NS 6     up to NS 10	150
Higher than NS 10    up to NS 20	200
Higher than NS 20    up to NS 30	250
Higher than NS 30    up to NS 100	300
Higher than NS 100	400

^(a) Nominal width can be applied to either the inner diameter or the outer diameter.

. Provisions must be made to ensure that the inlet, outlet and interconnecting pipes can withstand any movement or settlement of the soil.

The most commonly used materials for the construction of separation facilities include the following:

• Concrete—plain concrete, concrete with dispersed reinforcement and reinforced concrete;
• Metallic materials—cast iron, steel and stainless steel;
• plastic—polyethylene, polypropylene and glass fibre-reinforced plastic.

In the case of sealing materials, the use of elastomers (rubber) and permanently elastic sealing materials is allowed. The use of cement mortar and similar sealing materials is prohibited.

The use of other materials is permitted provided that all requirements of the relevant standards are met. All materials that come into direct contact with wastewater shall be chemically resistant to its action or shall be otherwise suitably protected.

3. Separator with Sludge Trap and Separating Space

The principle is based on the different densities of light liquids and water. Wastewater contains, in addition to light liquids and suspended sediment (e.g., sand and mud). These settle at the bottom of the tank in the so-called sludge trap after the wastewater flows into the tank. The flow through the sludge trap is directed by a flow diverter. From the sludge trap, the mechanically cleaned wastewater flows into the separating space. A coalescing filter is located here, which separates light liquids from wastewater—the coalescing gravity principle. The separated light liquids are collected near the surface in an area designed to capture them. The wastewater treated in this way is discharged through a drainage channel into the discharge pipe.

There is a safety element in the form of a float cap in the space behind the coalescing filter. The cap is used to close the drainage channel when the maximum height of captured light liquids is exceeded. The closure occurs automatically due to the difference in density of the separated light liquids and water.

The sludge trap and the separating space may be in one common tank or divided into several separate tanks.

In some cases, a sludge filter is fitted between the sludge trap and the separating space.

Figure 1 shows a diagram of a light liquid separator with a sludge trap and a separating space.

4. Separator with Sludge Trap, Separating Space and Final Separating Stage

Compared to the above-mentioned type, the separator is equipped with the so-called final separating stage. A sorption filter filled with sorbent (e.g., Fibroil) is fitted here. It is used to capture the residual amount of light liquids that have not been separated in the separating space. It is used in particular when the quality requirements for the runoff water are increased—it guarantees a concentration of light liquids at an outflow of 0.2 to 0.5 mg/L. The treated water flows into the outlet pipe (see Figure 2).

The sludge trap, separating space and final separating stage can be in one common tank or divided into several separate tanks.

5. Procedure Used for the Equipment Size Designing Process

The procedure used for the light liquid separator size designing process consists of 6 points:

A. Location and catchment area

First, it is important to determine the location of the site of interest and its relevant catchment area.

B. Rainfall intensity at the given periodicity, n

This is a tabular value available in the reference sources—e.g., short-term rain intensities in the Elbe, Oder and Morava catchment areas by Josef Trupel. It is determined on the basis of the measured results of an ombrographic station. The station is selected as it is close as possible to the site of interest.

C. Runoff coefficient, φ, and calculation of the drainage area, A

The determination of the runoff coefficient is based on CSN 75 6101 [12]. It is determined according to the type of development, the type of land and the slope of the terrain. The runoff coefficient is indirectly related to the calculation of the hard drainage area from which the rainwater will be drained.

D. Maximum rainwater runoff, Q_r

The maximum rainwater runoff is calculated using the following formula:

$$Q_r=\phi ·i·A$$

(1)

where,

Q_r—maximum rainwater runoff [l/s];

φ—runoff coefficient [–];

i—rain intensity [l/s·ha];

A—drainage area [ha].

E. Maximum wastewater runoff, Q_s

The maximum wastewater runoff is used only in the case of the treatment of water other than rainwater polluted with oil substances. Most often, this is the treatment of industrial wastewater from factories, car washes or fuel filling stations.

It is calculated using the following formula:

$$Q_s=Q_{s1}+Q_{s2}+Q_{s3}+\cdots$$

(2)

where,

Q_s—maximum wastewater runoff [l/s];

Q_s1—wastewater runoff from all runoff sites [l/s];

Q_s2—wastewater runoff from car wash facilities [l/s];

Q_s3—wastewater runoff from high pressure cleaning facilities [l/s].

F. Nominal size of NS separator

This parameter is calculated using the following formula:

$$NS=\left(Q_r+\left(f_x·Q_s\right)\right)·f_d$$

(3)

where,

NS—nominal separator width [–];

Q_r—maximum rainwater runoff [l/s];

f_x—aggravating coefficient depending on the type of runoff [–];

Q_s—maximum wastewater runoff [l/s];

f_d—density coefficient for the light liquid in question [–].

Both the aggravating coefficient and the density coefficient are tabular values available in CSN EN 858-2 [13].

6. Operation and Maintenance

According to the legal regulations of the Czech Republic, a light liquid separator is a water management facility. In order to be inspected by the water authorities and to ensure the proper functioning of the separator, it is necessary to comply with the conditions and procedures developed for its operation and maintenance. The equipment must be monitored and maintained in accordance with the manufacturers’ instructions, and maintenance must be carried out in a timely and planned manner. The necessary information is given in the documents included in the technical documentation and is usually also provided by the manufacturers of the separators. These are the operating rules and the operating logbook.

Operating rules

This is basically an instruction manual for the operation of the equipment. Operation and maintenance are carried out by the equipment user and its operator. The user manages the operation of the separator, decides on major operational measures relating to the actual separation process, keeps records of laboratory monitoring and material consumption and also provides training for the operators. The operators carry out periodic tasks and keep detailed records in an operating logbook.

The activities performed by the operators can be divided according to intervals into the following:

• Once per week—visual inspection;
• Twice a month—the cleaning of coalescing and sorption filters, and the checking of the sludge trap;
• Once per six months—the removal of trapped light liquids;
• Once per year—the cleaning of the sludge trap, replacement of the sorption filter cartridge and inspection;
• No interval/as needed—sampling, and other activities listed above are performed as needed.

Operating logbook

This is used together with the operating rules. Entries are made in the operating logbook by the equipment operators on individual faults and malfunctions at the time of their occurrence and rectification, on spare parts and maintenance or maintenance needs. Entries are also made in the logbook in the event of handling. Handling in this case means, for example, the removal of sludge or the taking of control samples. In the case of sludge removal, the date of sludge removal, the amount of sludge removed in m³, the description of the maintenance activity and who performed the operation are usually written in the record. If necessary, the logbook must be shown on request.

7. Practical Example of the Design of a Light Liquid Separator Tank

This chapter is focused on a practical example of the design of a light liquid separator system for rainwater from an industrial site in the Liberec Region. The theoretical designing procedure is presented in Section 5. The rainfall intensity for the industrial area in the Liberec Region was determined from Trupl tables. A 15 min long rainfall period with a periodicity of 0.2, or 1 × in 5 years, was chosen as the initial value for calculation. The periodicity of 0.2 is typically used for industrial areas. The f_x coefficient was chosen to be zero according to the tables. There is no need to account for adverse conditions for the separation or treatment of multiple types of wastewater. The sewerage system option proposed for the site only deals with rainwater management.

Rainwater calculation:

Runoff coefficient, φ: 0.8   (Asphalt areas)

Runoff coefficient, φ: 0.6   (Concrete pavement)

Rainfall intensity, i: 152 l/s·ha   (loc. Liberec, periodicity 0.2)

Area, A: 3726 m²        (Asphalt areas—based on the situation)

Area, A: 2141 m²        (Concrete pavement—based on the situation)

$$Q_r=\phi ·i·A=\left(0.8·152·0.3726\right)+\left(0.6·152·0.2141\right)=64.84 \mathrm{l}/\mathrm{s}$$

Wastewater calculation:

$$Q_s=0 \mathrm{l}/\mathrm{s}$$

Water from parking area drainage is not polluted from car washes or high-pressure cleaning equipment.

Choice of nominal size of separator:

Coefficient f_x:     0

Coefficient f_d:     1 (do 0.85 g/cm³)

Rainwater Q_r:       64.84 l/s

Polluted water      Q_s: 0 l/s

$$NS=\left(Q_r+\left(f_x·Q_s\right)\right)·f_d=\left(64.84+\left(0·0\right)\right)·1=64.84$$

From the resulting value, NS = 64.84, and Table 2, it can be seen that the smallest permissible nominal width of the inlet pipe and outlet pipe is DN300.

Technical specification:

The light liquid separator from ASIO spol. s r.o. has been chosen on the basis of the calculation of the nominal tank size. The chosen type was AS-TOP 80 RC/EO PB PP. It is a class I gravity-coalescing separator of with sludge and separating space. The sludge area is designed for small quantities of sludge (100 × NS). The separator technology is designed for an influent water contamination value of C10-C40 < 4000 mg/L. The parameters of the treated water at the outlet then reach the value C10-C40 = 2–5 mg/L. This meets the discharge limit requirements for class I separators.

The separator is a plastic–concrete construction. The tank consists of a double-walled shell made of polypropylene (PP). The intermediate shell is factory-fitted with a B500B, Ø12 concrete reinforcement. At the installation site, the intermediate shell is subsequently filled with concrete of the strength class C 35/45. The plastic shell ensures the watertightness of the tank and protects the concrete filling from the negative effects of the surrounding environment. The outer shell therefore protects, for example, against the negative effects of groundwater or the ingress of ballast water. The inner shell protects against the aggressiveness of oily water. The construction of the tank does not require additional protection against corrosion.

The ceiling slab of the tank must be insulated with IPA 400H PE. The insulation layer prevents the penetration of ground moisture, and of surface and groundwater into the intermediate shell.

The basic technical parameters of the tank are presented in

Table 3. The basic parameters of the designed light liquid separator tank.

Nominal Width NS	80
Maximum flow rate (l/s)	80
Max. quantity of captured light liquids (l)	871
Sludge trap volume (m3)	9.88
Outer tank diameter (mm)	3430
Outer tank height (mm)	2220
Inlet height (mm)	1650
Outlet height (mm)	1550
DN inlet, outlet	300
Volume of concrete necessary for the intermediate tank shell (m3)	4.61
Transport weight of the tank (kg)	2343

.

The technological baffles inside the tank are made of polypropylene. Coalescing filter inserts are made of polyurethane foam mounted in stainless steel frames. The float cap is also made of stainless steel. The technical parameters of the coalescing filter are presented in

Table 4. Technical parameters of the coalescing polyurethane filter.

Bulk Density of Polyurethane Foam	25 kg/m3
Tensile strength	120–135 kPa
Thermal resistance	−40 až +10 °C
Compressibility	40% of compression at 5.0 kPa
Expansibility	80–100%

.

The design is a single cylindrical tank that is intended to be stored below ground level. The selected tank type does not allow the tank to be placed in areas with groundwater levels below the foundation slab. The tank is self-supporting after installation without the need for concrete application.

According to the manufacturer’s requirements, the maximum foundation base depth must be 5000 mm below the prepared terrain. The tank must be placed on a reinforced concrete slab with adequate bearing capacity and a flatness of ±5 mm.

From the static point of view, the tank ceiling is designed for the ground load of the road structure with vehicle traffic—a random load from the vehicle to the centre of the hatch; F = 50 kN.

8. Conclusions

This paper presents the theoretical basis for light liquid separators as one of the construction objects in the field of the management of rainwater polluted with oil substances. The theoretical part describes the design solution of light liquid separators. The focus is on the classes, the smallest nominal width according to the NS and the material. Two basic types of light liquid separators are also described in this section.

The experimental part is focused on an example of light liquid separator design for an industrial site in the Liberec Region. There is a procedure of the hydro-technical calculation of the separator design, based on which the min. DN of the piping at the inlet and outlet of the light liquid separator is designed. The AS-TOP 80 RC/EO PB PP type of light liquid separator from ASIO, an a.s. company, was selected and recommended on the basis of the determination of the min. DN. It includes the technical and design parameters of the tank as well.