FprEN 12255-2
(Main)Wastewater treatment plants - Part 2: Storm water management systems
Wastewater treatment plants - Part 2: Storm water management systems
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).
Kläranlagen - Teil 2: Niederschlagsmanagementsysteme
Dieses Dokument legt die Anforderungen an Systeme zur Bewirtschaftung von Mischwasserüberläufen für Kläranlagen fest. Es bezieht sich nicht auf Systeme zur Bewirtschaftung von Mischwasserüberläufen in Abwassersammel- und -beförderungsnetzen (Kanalisationen). Unabhängig vom Standort und den technischen Anforderungen in diesem Dokument können Systeme zur Bewirtschaftung von Mischwasserüberläufen als Teil des Kanalisationssystems nach EN752 und EN16933 angesehen werden.
Dieses Dokument legt Anforderungen für die Abscheidung, Lagerung, Behandlung, Ableitung und Rückführung von Mischwasser in Kläranlagen fest.
ANMERKUNG Ein System zur Bewirtschaftung von Mischwasserüberläufen in der Kläranlage ist nur dann erforderlich, wenn ein solches System nicht in der Kanalisation vorhanden ist, wodurch der Volumenstrom zu den Kläranlagen begrenzt wird, siehe EN752 und EN16933(alleTeile).
Stations d’épuration - Partie 2 : Systèmes de gestion des eaux pluviales
Le présent document spécifie les exigences relatives aux systèmes de gestion des eaux pluviales dans les stations d’épuration. Il ne fait pas référence aux systèmes de gestion des eaux pluviales des réseaux de collecte et de transport des eaux usées (réseaux d’assainissement).
Le présent document spécifie les exigences relatives à la séparation, au stockage, au traitement, au rejet et à la recirculation des eaux pluviales dans les stations d’épuration.
NOTE Un système de gestion des eaux pluviales au niveau de la station d’épuration est uniquement exigé lorsqu’un tel système n’est pas prévu dans le réseau d’assainissement, ce qui limite l’écoulement vers les stations d’épuration, voir l’EN 752 et l’EN 16933 (toutes les parties).
Čistilne naprave za odpadno vodo - 2. del: Sistemi za upravljanje z meteorno vodo
General Information
- Status
- Not Published
- Publication Date
- 01-Jul-2026
- Technical Committee
- CEN/TC 165 - Waste water engineering
- Drafting Committee
- CEN/TC 165/WG 40 - Wastewater treatment plants > 50 PT
- Current Stage
- 5020 - Submission to Vote - Formal Approval
- Start Date
- 12-Mar-2026
- Due Date
- 26-Aug-2025
- Completion Date
- 12-Mar-2026
Overview
FprEN 12255-2 is a European standard developed by CEN for storm water management systems at wastewater treatment plants. This standard outlines requirements specifically for the management of stormwater within the confines of treatment facilities, focusing on the separation, storage, treatment, discharge, and return of stormwater. It deliberately excludes storm water management in wastewater collection and conveyance networks (sewer systems), which are covered by standards such as EN 752 and EN 16933. Proper adherence to this standard supports compliance, system reliability, environmental protection, and optimal plant operation during high-flow events caused by rainfall or snowmelt.
Key Topics
- Scope and Exclusions
- Applies only to stormwater systems within wastewater treatment plants
- Excludes management systems for collection and conveyance networks (see EN 752, EN 16933)
- Storm Water Separation
- Methods for separating excess flows, using measurement and control to protect treatment capacity
- Storm water Storage and Tanks
- Requirements for sizing, designing, and sequencing storm tanks, including blind and overflow tanks
- Guidance on tank cleaning, floor slope, and sediment management
- Treatment and Discharge
- Principles for partial treatment, appropriate screening, and overflow management to limit environmental impact
- Control Systems and Monitoring
- Mandates on flow measurement, system automation, and return flow management after storms
- System Operation
- Manual and automated protocols for tank filling, emptying, and cleaning
- Return of stored stormwater and integration with plant flow rates
- Environmental and Regulatory Compliance
- Addresses particle retention, automatic screening, and adaptation to local regulations
Applications
Storm water management systems following FprEN 12255-2 are critical to wastewater treatment plants facing variable inflows due to storm events:
- Peak Flow Management: Helps plants manage sudden surges by diverting and storing excess stormwater, thereby preventing overloading and protecting downstream treatment processes.
- Environmental Protection: Ensures stormwater is treated and monitored before being discharged, reducing the risk of pollution in receiving water bodies.
- Plant Operation Continuity: Allows for controlled return of stored storm flows to the plant when capacity permits, minimizing treatment interruptions.
- Compliance: Supports fulfillment of municipal, regional, or national requirements for stormwater handling and overflow prevention.
- Scalable Solutions: The standard provides guidelines for a range of plant sizes, from small facilities serving under 1,000 inhabitants to large municipal plants.
Practically, these requirements help transform stormwater from a potential operational hazard into a managed resource, while contributing to the sustainability of public water infrastructure and the protection of watercourses.
Related Standards
For a comprehensive and compliant approach to wastewater and stormwater management, consult these key related standards:
- EN 752: Sewer system management for collection and conveyance networks
- EN 12255 series: Other parts covering general construction, preliminary and primary treatment, operation, instrumentation, safety, and automation
- EN 16932 (all parts): Pumping systems in drain and sewer infrastructure
- EN 16933 (all parts): Design of drain and sewer systems
- EN 16323: Glossary of wastewater engineering terms
Adopting FprEN 12255-2 alongside these standards ensures robust, harmonized solutions for both routine operation and extreme weather events, supporting regulatory compliance and operational excellence in storm water management at wastewater treatment facilities.
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Frequently Asked Questions
FprEN 12255-2 is a draft published by the European Committee for Standardization (CEN). Its full title is "Wastewater treatment plants - Part 2: Storm water management systems". This standard covers: 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).
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).
FprEN 12255-2 is classified under the following ICS (International Classification for Standards) categories: 13.060.30 - Sewage water. The ICS classification helps identify the subject area and facilitates finding related standards.
FprEN 12255-2 is available in PDF format for immediate download after purchase. The document can be added to your cart and obtained through the secure checkout process. Digital delivery ensures instant access to the complete standard document.
Standards Content (Sample)
SLOVENSKI STANDARD
oSIST prEN 12255-2:2024
01-september-2024
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: prEN 12255-2
ICS:
13.060.30 Odpadna voda Sewage water
oSIST prEN 12255-2:2024 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
oSIST prEN 12255-2:2024
oSIST prEN 12255-2:2024
DRAFT
EUROPEAN STANDARD
prEN 12255-2
NORME EUROPÉENNE
EUROPÄISCHE NORM
June 2024
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: Systeme zur Bewirtschaftung von
des eaux pluviales Mischwasserüberläufen
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee
CEN/TC 165.
If this draft becomes a European Standard, 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.
This draft European Standard was established by CEN 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.
Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are
aware and to provide supporting documentation.
Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a European Standard.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2024 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 12255-2:2024 E
worldwide for CEN national Members.
oSIST prEN 12255-2:2024
prEN 12255-2:2024 (E)
Contents Page
European foreword . 3
Introduction . 4
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Symbols and abbreviations . 6
5 General. 6
6 Flow management systems . 8
6.1 Flow separation . 8
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
oSIST prEN 12255-2:2024
prEN 12255-2:2024 (E)
European foreword
This document (prEN 12255-2:2024) has been prepared by Technical Committee CEN/TC 165
“Wastewater”, the secretariat of which is held by DIN.
This document is currently submitted to the CEN Enquiry.
It is the eighth 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. 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).
oSIST prEN 12255-2:2024
prEN 12255-2:2024 (E)
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:
Key
1 preliminary treatment
2 primary treatment
3 secondary treatment
4 tertiary treatment
5 additional treatment (e.g. disinfection or removal of micropollutants)
6 sludge treatment
7 lagoons (as an alternative)
A raw wastewater
B effluent for re-use (e.g. irrigation)
C discharged effluent
D screenings and grit
E primary sludge
F secondary sludge
G tertiary sludge
H stabilized sludge
I digester gas
J returned water from dewatering
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.
oSIST prEN 12255-2:2024
prEN 12255-2:2024 (E)
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).
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 16323:2014, Glossary of wastewater engineering terms
ISO 4359, Flow measurement structures — Rectangular, trapezoidal and U-shaped flumes
ISO 12242, Measurement of fluid flow in closed conduits — Ultrasonic transit-time meters for liquid
EN ISO 20456, Measurement of fluid flow in closed conduits — Guidance for the use of electromagnetic
flowmeters for conductive liquids (ISO 20456)
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 terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp
— IEC Electropedia: available at https://www.electropedia.org/
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 an 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]
oSIST prEN 12255-2:2024
prEN 12255-2:2024 (E)
3.4
storm water
foul wastewater mixed with surface water in excess of the design capacity of the combined system or
wastewater treatment plant
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
[SOURCE: EN 16323:2014, definition 2.3.8.18]
3.7
design capacity
flow and load of influent that the facilities are designed to treat, while conforming to specified
requirements
[SOURCE: EN 16323:2014, definition 2.3.10.7]
3.8
design peak flow
maximum flow of influent over a specified period that the facilities are designed to treat, while
conforming to specified requirements
[SOURCE: EN 16323:2014, definition 2.3.10.9]
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.
oSIST prEN 12255-2:2024
prEN 12255-2:2024 (E)
Key
Process units Process flows Instruments
1 Storm water overflow to A Arriving LMIC1 Spill to environment event
environment wastewater duration monitor
(Total flow to
works)
2 Storm water screen B Flow to treatment LMIC2 Spill to storm duration
monitor
3 Treatment works screen C Screened flow to LMIC3 Storm tank water level
works
4 Grit removal D Screened and de- LMIC4 Spill from storm water tank
gritted flow to level
works
5 Overflow to storm water E Flow to full FICR1 Pass forward flow
tanks treatment
6 Storm flow control F Flow to
penstock environment
7 Measurement flume G Excess storm water
flow
8 Blind storm water tank H Return of retained
storm water
9 Storm water tank
10 Storm water return
pumping station
Figure 2 — Example of a typical storm system
Storm water management systems at wastewater treatment plants vary in configuration and Figure 2
should only be taken as an example.
Arriving wastewater (A) shall only be diverted to storm tanks (8 and 9) when the flows, being received
at the wastewater treatment plant exceed the capacity to achieve full treatment. Storm tanks shall not
be used for general flow balancing.
Storm tanks, unless they are blind storm tanks (8), shall be designed like sedimentatio
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