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
• Publication Date: 14-Jul-2026
• ICS: 13.060.30 - Sewage water
• Technical Committee: CEN/TC 165 - Waste water engineering
• Drafting Committee: CEN/TC 165/WG 40 - Wastewater treatment plants > 50 PT
• Current Stage: 6060 - Definitive text made available (DAV) - Publishing
• Start Date: 15-Jul-2026
• Due Date: 16-Dec-2025
• Completion Date: 15-Jul-2026
• Ref Project: SIST EN 12255-2:2026 - Wastewater treatment plants - Part 2: Storm water management systems

Overview

EN 12255-2:2026 is a European standard developed by CEN that defines the requirements for storm water management systems specifically within wastewater treatment plants. This standard does not address storm water management in sewer networks or collection systems, but focuses on handling, storing, and treating storm water that reaches the treatment plant itself. The document provides guidance for flow management, separation, storage, treatment, discharge, and return of storm water, helping operators and designers mitigate the impact of storm events on both treatment processes and the environment.

By setting out consistent requirements and recommended practices, EN 12255-2:2026 supports the design, operation, and maintenance of robust storm water management systems at municipal and industrial wastewater treatment plants, promoting safer, more effective handling of storm flows and protecting water quality.

Key Topics

• Scope of Application: Requirements for storm water management apply only within the boundaries of wastewater treatment plants - not to the sewer or conveyance systems.
• Flow Management Systems: Techniques for separating and controlling influent to ensure that treatment capacity is not overwhelmed during storms.
• Storm Tanks: Design and use of storm water storage tanks, including blind storm tanks (units without environmental overflow) and regular tanks, with guidance on sizing, sequencing, and operation.
• Return and Discharge: Procedures for returning stored storm water to treatment after flows subside, and for overflow discharge, including screening and solids removal.
• Tank Cleaning and Maintenance: Recommended cleaning systems and maintenance practices, scaled according to plant size and local requirements.
• Measurement and Control: Guidance on measuring flows, monitoring system status, and applying automation or manual control depending on plant capacity.
• Regulatory Alignment: Reference to related standards such as EN 752 and EN 16933 for integration with external sewer and stormwater systems.

Applications

EN 12255-2:2026 is essential for:

• Municipal and industrial wastewater treatment plants: Ensuring adequate storm water management is in place to handle sudden increases in influent without compromising treatment performance or causing environmental impact.
• Engineering and consulting firms: Providing a technical framework for the planning, design, and specification of storm water management solutions within treatment plant projects.
• Operators and maintenance teams: Supporting the consistent operation and upkeep of storm water tanks, flow control systems, and monitoring equipment.
• Regulatory compliance: Helping plants demonstrate alignment with best practices and national/international regulations for storm water overflows and environmental protection.
• Asset upgrades and retrofits: Guiding modernization and expansion of facilities to meet new storm water management requirements, particularly in response to climate change and increased frequency of extreme weather events.

By following EN 12255-2:2026, stakeholders can improve system resilience, minimize pollutant discharges during storm events, and extend the life and reliability of critical treatment infrastructure.

Related Standards

• EN 752: Drain and sewer systems outside buildings - Management of sewer networks and pumping installations.
• EN 12255-1: Wastewater treatment plants - General construction principles.
• EN 12255-3: Preliminary treatment processes at wastewater treatment plants.
• EN 12255-4: Primary treatment specifications.
• EN 16932: Drain and sewer systems - Pumping systems.
• EN 16933: Drain and sewer systems - Design.
• EN ISO 20456: Electromagnetic flowmeters for liquids.
• ISO 4359, ISO 12242: Flow measurement techniques for treatment plants.

For complete storm water management and compliance at wastewater treatment facilities, EN 12255-2:2026 should be used in conjunction with these and other relevant standards to ensure comprehensive coverage of design, operation, and environmental needs.

Keywords: storm water management, wastewater treatment plants, storm tanks, flow management, overflow control, EN 12255-2, European standard, stormwater systems, treatment plant design, CEN, water quality, environmental protection.

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
...