SIST EN 12255-2:2026

• Abstract:

  This document specifies requirements for storm water management systems on wastewater treatment plants. It does not refer to storm water management systems in wastewater collection and conveyance networks (sewer systems). Regardless of the location and the technical requirements of this document, storm water management systems can be regarded as part of the sewer system in accordance with EN 752 and EN 16933.; This document specifies requirements for separation, storage, treatment, discharge and return of storm water within wastewater treatment plants.; NOTE A storm water management system at the wastewater treatment plant is only required where such a system is not provided within the sewer system, limiting the flow to the wastewater treatment plants see EN 752 and EN 16933 (all parts).

• Status: Published
• Public Enquiry End Date: 18-Sep-2024
• Publication Date: 23-Jul-2026
• ICS: 13.060.30 - Sewage water
• Technical Committee: IOVO - Water supply and waste water engineering
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 23-Jul-2026
• Due Date: 27-Sep-2026
• Completion Date: 24-Jul-2026
• Ref Project: EN 12255-2:2026 - Wastewater treatment plants - Part 2: Storm water management systems

Overview

SIST EN 12255-2:2026 defines the key requirements for storm water management systems within wastewater treatment plants. Developed by the Slovenian Institute for Standardization (SIST), this European standard focuses on the separation, storage, treatment, discharge, and return of storm water that arrives at wastewater treatment facilities. It does not cover storm water management for collection and conveyance networks (sewer systems), which are addressed in standards such as EN 752 and EN 16933.

The objective of compliant storm water management systems at wastewater treatment plants is to mitigate excessive hydraulic loading during storm events, ensure proper treatment before discharge, and minimize environmental impact. These systems are an essential element for plants where upstream sewer networks do not sufficiently control storm water flow.

Key Topics

The standard covers several essential areas for effective storm water management at treatment plants:

• Flow Management Systems: Methods for separating normal and excess flows, including the use of flow measurement devices and bypass systems.
• Storm Tank Design and Operation: Guidelines for the number and types of storm tanks, design features, overflow arrangements, and means of isolating tanks. The importance of floor slopes, cleaning provisions, and sizing methods are also highlighted.
• Return of Storm Water: Procedures for returning stored storm water to the treatment process when capacity allows, including controls to prevent plant overload or untimely return.
• Storm Tank Cleaning: Requirements for cleaning methods according to plant size, including manual and automated options. Ensures prevention of solids accumulation and promotes operational efficiency.
• Measurement and Control: Necessity for monitoring and recording storm water flows, using level sensors, flow meters, and appropriate control systems depending on plant capacity.
• Additional Requirements: Recommendations for further treatment, such as screening and sediment removal, and considerations for minimizing environmental impact.

Applications

Storm water management systems described in SIST EN 12255-2:2026 deliver practical solutions for:

• Wastewater Treatment Plant Operators: Ensuring plant resilience to storm events by managing flows that exceed hydraulic design capacity, thus maintaining effluent quality and protecting downstream ecosystems.
• Design Engineers and Consultants: Providing a framework for planning, sizing, and specifying storm water tanks, return systems, and automation based on plant scale and expected loading conditions.
• Regulatory Bodies: Supporting compliance with environmental protection regulations by enforcing effective management and treatment of storm water within treatment facilities.
• Public Utilities and Municipal Authorities: Guiding investment and operational strategies for infrastructure upgrades, especially where upstream collection networks lack storm flow controls.

The standard is most applicable to treatment plants designed for domestic and municipal wastewater and is particularly relevant for facilities serving populations greater than 2,000, where robust storm water infrastructure is critical.

Related Standards

To complete a robust storm water and wastewater management program, refer to the following standards:

• EN 752: Covers sewer and drainage system management outside buildings, including flow limitation.
• EN 12255 Series: Other parts address general construction, preliminary and primary treatment, lagooning, sludge management, automation, and safety (including EN 12255-1, -3, -4, -10, -11, -12).
• EN 16932 and EN 16933: Offer additional requirements for pumping and design in external drainage and sewer systems.
• ISO Standards: For flow measurement and terminology (e.g., ISO 4359, ISO 12242, EN ISO 20456).

By implementing SIST EN 12255-2:2026 alongside the related standards, organizations can achieve environmentally sound and operationally efficient storm water management at wastewater treatment facilities.

Keywords: wastewater treatment, storm water management, storm tanks, water storage, storm water return, storm water treatment, flow management, environmental protection, SIST EN 12255-2:2026, standardization.

SLOVENSKI STANDARD
01-september-2026
Čistilne naprave za odpadno vodo - 2. del: Sistemi za upravljanje z meteorno vodo
Wastewater treatment plants - Part 2: Storm water management systems
Kläranlagen - Teil 2: Niederschlagsmanagementsysteme
Stations d’épuration - Partie 2 : Systèmes de gestion des eaux pluviales
Ta slovenski standard je istoveten z: EN 12255-2:2026
ICS:
13.060.30 Odpadna voda Sewage water
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN 12255-2
EUROPEAN STANDARD
NORME EUROPÉENNE
July 2026
EUROPÄISCHE NORM
ICS 13.060.30
English Version
Wastewater treatment plants - Part 2: Storm water
management systems
Stations d'épuration - Partie 2 : Systèmes de gestion Kläranlagen - Teil 2:
des eaux pluviales Niederschlagsmanagementsysteme
This European Standard was approved by CEN on 8 June 2026.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2026 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 12255-2:2026 E
worldwide for CEN national Members.

Contents Page
European foreword . 3
Introduction . 5
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Symbols and abbreviations . 7
5 General. 7
6 Flow management systems . 9
6.1 Flow separation . 9
6.2 Storm Tanks . 9
6.2.1 General. 9
6.2.2 Storm Tank Sizing . 10
6.2.3 Storm Tank Design . 10
6.2.4 Storm tank return flow . 11
6.2.5 Storm tank cleaning . 11
6.2.6 Measurement, monitoring and control . 12
6.3 Additional requirements. 13
Bibliography . 14

European foreword
This document (EN 12255-2:2026) has been prepared by Technical Committee CEN/TC 165
“Wastewater engineering”, the secretariat of which is held by DIN.
It is the second part prepared by Working Group CEN/TC 165/WG 40 relating to the general
requirements and processes for treatment plants for a total number of inhabitants and population
equivalents (PT) over 50.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by January 2027, and conflicting national standards shall
be withdrawn at the latest by January 2027.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
EN 12255 with the generic title “Wastewater treatment plants” consists of the following parts:
— Part 1: General construction principles
— Part 2: Storm water management systems
— Part 3: Preliminary treatment
— Part 4: Primary treatment
— Part 5: Lagooning processes
— Part 6: Activated sludge process
— Part 7: Biological fixed-film reactors
— Part 8: Sludge treatment and storage
— Part 9: Odour control and ventilation
— Part 10: Safety principles
— Part 11: General data required
— Part 12: Control and automation
— Part 13: Chemical treatment — Treatment of wastewater by precipitation/flocculation
— Part 14: Disinfection
— Part 15: Measurement of the oxygen transfer in clean water in aeration tanks of activated sludge plants
— Part 16: Physical (mechanical) filtration
NOTE For requirements on pumping installations at wastewater treatment plants see EN 752 and EN 16932
(all parts).
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organisations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia,
Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland,
Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of North
Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and the United
Kingdom.
Introduction
Differences in wastewater treatment throughout Europe have led to a variety of systems being developed.
This document gives fundamental information about the systems; this document has not attempted to
specify all available systems. A generic arrangement of wastewater treatment plants is illustrated below
in Figure 1, storm tanks are not shown here:

Key
preliminary treatment including storm  C discharged effluent
management systems
2 primary treatment  D screenings and grit
3 secondary treatment  E primary sludge
4 tertiary treatment  F secondary sludge
5 additional treatment (e.g. disinfection or removal  G tertiary sludge
of micropollutants)
6 sludge treatment  H stabilized sludge
7 lagoons (as an alternative)  I digester gas
A raw wastewater  J returned water from dewatering
B effluent for re-use (e.g. irrigation)
Figure 1 — Schematic diagram of wastewater treatment plants
Detailed information additional to that contained in this document can be obtained by referring to the
bibliography.
The primary application is for wastewater treatment plants designed for the treatment of domestic and
municipal wastewater.
1 Scope
This document specifies requirements for storm water management systems on wastewater treatment
plants. It does not refer to storm water management systems in wastewater collection and conveyance
networks (sewer systems). Regardless of the location and the technical requirements of this document,
storm water management systems can be regarded as part of the sewer system in accordance with
EN 752 and EN 16933.
This document specifies requirements for separation, storage, treatment, discharge and return of storm
water within wastewater treatment plants.
NOTE A storm water management system at the wastewater treatment plant is only required where such a
system is not provided within the sewer system, limiting the flow to the wastewater treatment plants see EN 752
and EN 16933 (all parts).
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
EN 16323, Glossary of wastewater engineering terms
EN ISO 20456, Measurement of fluid flow in closed conduits — Guidance for the use of
electromagnetic flowmeters for conductive liquids (ISO 20456)
ISO 4359, Flow measurement structures — Rectangular, trapezoidal and U-shaped flumes
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 16323 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at https://www.electropedia.org/
— ISO Online browsing platform: available at https://www.iso.org/obp
3.1
storm tank
structure provided for temporary storage of excess wastewater flow during periods of high rainfall or
snow melt
3.2
blind storm tank
storm tank without a built-in overflow to the environment
Note 1 to entry: The environment includes the sea, a river, land surface etc.
3.3
balancing
equalisation (deprecated)
reduction in variations in flow, concentration, temperature etc
[SOURCE: EN 16323:2014, definition 2.3.2.2]
3.4
storm water
foul wastewater mixed with surface water
Note 1 to entry: The terms “foul wastewater”, “surface water” “design capacity”, “combined system” and
“wastewater treatment plant” are all defined in EN 16323.
3.5
storm
extreme weather event
3.6
scumboard
board partly immersed in the flow to retain floating material
3.7
design capacity
flow and load of influent that the facilities are designed to treat, while conforming to specified
requirements
3.8
design peak flow
maximum wastewater inflow over a given period for which the plant is designed to treat in compliance
with the specified requirements excluding any overflow
4 Symbols and abbreviations
Abbreviations
LMIC level measurement indication and controller
FICR flow indication controller and recorder
5 General
The purpose of storm water management systems is to provide storage for and treatment of the storm
water flow which has exceeded the hydraulic capacity of a wastewater treatment plant. These systems
provide partial treatment of storm water prior to discharging it into a receiving water body. For short
storm events the system will provide sufficient storage volume to prevent wastewater overflow such that
the stored storm water can be returned to the wastewater treatment plant when the incoming flow has
subsided. In case of long and extreme storm rainfall events the system shall prevent some solids from
overflowing by sedimentation and/or screening.
An example storm system is shown in Figure 2.
Key
Process units Process flows Instruments
1 Storm water overflow to A Arriving wastewater LMIC1 Spill to environment event
environment (Total flow to works) duration monitor
2 Storm water screen B Flow to treatment LMIC2 Spill to storm duration
monitor
3 Treatment works screen C Screened flow to works LMIC3 Storm tank water level
4 Grit removal D Screened and de-gritted LMIC4 Spill from storm water tank
flow to works level
5 Overflow to storm water E Flow to full treatment FICR1 Pass forward flow
tanks
6 Storm flow control F Flow to environment
pensto
...