FAQ - Ecol-Unicon

How to select a Horizontal Sedimentation Tank?

Using the graph (Fig.), one can determine the internal diameter D_w of the sedimentation tank with horizontal flow for the required degree of suspended solids reduction (efficiency) and the design wastewater flow rate. The given approximate efficiency pertains to tanks with inlet and outlet located on the axis. In the case of deviation of the inlet pipe, the efficiency will be lower. The graph has been developed considering data related to the degree of reduction of overall suspended solids and the shape of OS sedimentation tanks.

To properly select a horizontal sedimentation tank, one should:

Determine the flow that requires preliminary treatment and the required efficiency of suspended solids removal.
Read the optimal diameter from the sedimentation tank series from the graph (Fig. 2) or individually calculate the sedimentation tank surface area A_p
Calculate the active volume V_cz of the sedimentation tank based on the maximum inflow to the system (to protect the device against washing out the suspended solids).

Having calculated the parameters, one can select a horizontal sedimentation tank from the catalog or individually by using the offered wells and tanks (section: Concrete Wells and Tanks). A detailed algorithm for calculating A_p and V_cz is provided.

Selection of a Sedimentation Tank based on the appropriate series

Selection of a Sedimentation Tank based on the appropriate series is made using two parameters:

The hydraulic capacity of the device, which should not be less than the maximum flow directed to the device (Q_max of the catchment area)
The efficiency of suspended solids removal required for the nominal flow directed to the device (Q_nom of the catchment area).

Knowing the flow values from the catchment area Q_max and Q_nom, one can preliminarily select a sedimentation tank from the catalog while adhering to the principle: Q_max of the catchment area < Q_max of the device. The EOW vortex sedimentation tanks demonstrate an 80% efficiency in removing suspended solids for the nominal flow of the device Q_nom (80%). In the case of a different required suspended solids removal efficiency, the device should be selected using the graph (Fig. 3), from which the appropriate flow for the given efficiency can be read.

Example: For a required efficiency of 69%, a double nominal flow of 2 x Q_nom (80%) is possible, which means the possibility of selecting a lower model of the sedimentation tank.

Selection of separators

Selection of separators involves matching the series of the separator from the catalog to the calculated flow values from the catchment area (Q_nom and Q_max) while meeting specific hydraulic conditions. The device flow rate Q_nom of the separator should be equal to or greater than the calculated flow values from the catchment area.

Hydraulic conditions are determined depending on the type of separators and their intended use:

Coalescence separators (ESK, PSK II) are used for all types of catchment areas

Separators with stormwater flow capability (ESL, ESK‐B) are used for all types of catchment areas, except for impermeable surfaces of fuel storage and distribution facilities

Pre-treatment units with individually designed overflow

According to the Regulation of the Minister of the Environment dated November 18, 2014 (Journal of Laws 2014 item 1800), in the case of using an overflow (discharging part of the rainwater without treatment), the treatment device (sedimentation tank and separator) should be protected from inflow greater than its nominal capacity. Typical solutions guaranteeing the protection of devices against hydraulic overload are flow regulators installed at the inlet to the technological sequence. The use of these devices allows meeting the conditions of the Regulation.

Coalescence Separators for Pre-treatment of Technological Wastewater

Technological wastewater containing petroleum substances is mainly generated in facilities such as car washes and repair workshops. The treatment of technological wastewater should be performed across the full range of flows.

Grease separator selection

In the case of wastewater entering the grease separator contaminated with suspended solids, a separator with a settling tank should be used. Based on the nominal inflow value of wastewater to the separator NS, the capacity of the settling tank V_os should be selected.

Table: selection of settling tank capacity based on the nominal inflow value of wastewater NS depending on the location of the grease separator

Grease Separator Selection Scheme

(0.16 MB)

If two or more draw-off taps are intended only for cleaning purposes and are not connected to any equipment, the values given in the table are applied to these same taps.

The need to design a retention tank

The need to design a retention tank is largely dependent on the available methods of managing rainwater in the catchment area. Retention tanks are most commonly designed for catchment areas with a high degree of sealing or in catchments covered with impermeable soils, where the outflow to natural receivers is limited, for example, by water permits or the decision of the stormwater network administrator regarding the conditions of connection to the network. In such cases, the retention tank should be equipped with a flow regulator (e.g., a vortex flow regulator) or a pumping station serving a regulatory function. Undoubtedly, the greatest advantage of vortex flow regulators is their maintenance-free and cost-free operation.

Sometimes the possibility of discharging rainwater into the network or a natural watercourse is limited, e.g., due to the great distance to the nearest receiver. In such cases, underground tanks with an infiltration module or open tanks, so-called infiltration-evaporation tanks, are often designed. It is important that before choosing the soil as a recipient of rainwater, geological surveys of the subsoil are necessary, as it often turns out that the soil under the planned tank does not have a sufficiently high filtration coefficient, which determines the maximum infiltration capacity of the soil (it is considered that the limit value for the profitability of using the infiltration system is kf = 10^-6 m/s).

If it is not possible to discharge water into the ground and the use of evaporation to discharge water into the environment is also not profitable, it may be necessary to use a rainwater pumping station in the retention tank and discharge rainwater via a pressure rainwater drainage system from the catchment area to the nearest receiver. The use of a retention tank will reduce operating costs, as it will allow the use of a lower power pumping system.

The design of a tank is also necessary in investments where higher fire safety standards must be met, and the capacity of the water supply system cannot provide the required flow. In such cases, the designer may propose a fire tank supplied with mains water or a retention tank, in the stormwater drainage system, with a constant fire reserve. The situations described above, in which the design of a retention tank is necessary, result from the occurrence of excessive amounts of rainwater.

However, it should be remembered that water, as a natural resource, is becoming increasingly scarce due to climate change. Therefore, it should be stated today that the design of a retention tank is necessary in any case where the possibility of water retention in the natural catchment area is limited, as water should be treated as a resource that should be used before being discharged into the aquatic environment.

Reference to PN-EN 752 standard for retention tank design

To find the answer to this question, one should refer to the PN-EN 752 standard.

For a sewered catchment area (less than 200 hectares) or systems with a flow time of less than 15 minutes, it is assumed that the tank should account for the reception of rainwater or stormwater that will fall on the given area within a specified time, minus the amount of water that will flow out of the tank or infiltrate into the ground. However, the PN-EN 752 standard does not specify the exact value of rainfall intensity and duration to be used for calculations.

Assistance in obtaining the above data is provided by the Tank Selection Calculator – a tool that allows for the quick and easy selection of appropriate parameters for a retention tank using the most reliable rainfall data from the Polish Atlas of Rainfall Intensities developed by RetencjaPL.

What data is necessary for designing a retention tank?

A retention tank offers a wide range of possibilities for utilizing rainwater for economic, industrial, or even household purposes. The main advantage of retention tanks is their ability to balance the irregularity of rainfall, meaning that collected water can be used even during periods without rain. Rainwater stored in the retention tank usually comes from sealed surfaces; therefore, before designing a retention tank, it is necessary to have data about the catchment area, including its location, surface area, degree of sealing, and land use. Information about the land use of the catchment area will help in selecting an appropriate pre-treatment system. However, the planned use of rainwater is also important, as installations for irrigation or car washing often require high parameters of pre-treated rainwater. Knowing the location of the planned investment is important due to the need to determine the design rainfall intensity, which is closely related to geographical location. Knowing the degree of sealing and the surface area of the catchment area allows for the determination of surface runoff from the catchment area. The planned method of utilizing rainwater and the operation principle of the retention tank are significant in the tank design process.

To prevent the occurrence of rainwater shortages, it is necessary to design a sufficiently large tank volume. Therefore, when determining the volume, information is needed about the maximum inflow of rainwater, water demand, and the maximum possible discharge to a receiver not owned by the investor.

Depending on the planned method of rainwater utilization, various data need to be collected to select the appropriate installation. In the simplest, yet often justified, method of utilization, which is mere retention aimed at reducing runoff to the receiver during intense rainfall, it is necessary to know the maximum allowable runoff to the receiver.

If you intend to discharge water into the ground, geological surveys must be conducted to determine the soil’s infiltration capacity. When designing a tank with an infiltration function, it is essential to limit the maximum ponding height in the tank, as high pressure can accelerate the process of colmatation, or the self-sealing of the first layer at the water-soil interface. When intending to use rainwater for irrigation during dry periods, it is necessary to determine the method of water distribution, required flow rate, pressure, number of draw-off points, and the method of controlling the irrigation system. A site plan of the area covered by the installation and the planned route of the water distribution pipes are also needed. When using rainwater for municipal needs (e.g., street cleaning), it is necessary to collect information about the planned frequency of water withdrawal from the tank. In some cases, when water is to be used for technological processes in production, ensuring the appropriate water quality is necessary, but the proper method of treatment depends on the conditions in the catchment area and individual requirements.

To facilitate the design process of retention tanks, we encourage the use of the Tank Selection Calculator. This tool allows for the quick and easy selection of appropriate tank parameters using the most reliable rainfall data from the Polish Atlas of Rainfall Intensities developed by RetencjaPL.

How can retention tanks help with ecological certification?

Hydrozone tanks have been analyzed for their impact on the environment, human health, and surroundings. The Ecocard clearly indicates that incorporating the device into a project contributes to earning points in the building certification process. The information contained in the Ecocard is useful for obtaining certifications such as LEED, BREEAM and DGNB.

“The product can contribute to earning points in LEED certification under credits SS 6.1 and 6.2 by reducing the disruption of natural hydrology through decreasing impermeable cover, increasing on-site infiltration, reducing stormwater runoff pollution, and eliminating contaminants.”

“The product can contribute to earning from 1 to 3 points in the BREEAM certification process under credit LE06 by collecting rainwater for reuse. Rainwater collection systems are designed and specified according to national best practice standards (to ensure consideration of both demand and building performance in tank selection). The collection area is measured according to national best practice standards.” Retention tanks can be helpful in ecological certification.