prEN 12255-2

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).
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).
Dieses Dokument legt Anforderungen für die Abscheidung, Lagerung, Behandlung, Ableitung und Rückführung von Niederschlagswasser 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 EN 752 und EN 16933 (alle Teile).

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: 17-Jun-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: 4599 - Dispatch of FV draft to CMC - Finalization for Vote
• Start Date: 15-Dec-2025
• Due Date: 17-Jun-2025
• Completion Date: 15-Dec-2025
• Ref Project: oSIST prEN 12255-2:2024 - Wastewater treatment plants - Part 2: Storm water management systems

Overview

The CEN draft standard prEN 12255-2:2024 titled Wastewater treatment plants - Part 2: Storm water management systems establishes essential requirements for managing storm water flows within wastewater treatment plants (WWTPs). It specifically focuses on systems designed to separate, store, treat, discharge, and return storm water that exceeds the hydraulic capacity of these plants. This standard does not cover storm water management in sewer collection and conveyance networks but rather systems located directly at the WWTP. Storm water management is critical to ensure that treatment plants operate effectively during intense rainfall events or snowmelt, preventing untreated discharges into the environment.

Key Topics

• Scope & Applicability
  prEN 12255-2 applies exclusively to storm water management at wastewater treatment plants serving populations greater than 50 person equivalents (PE). It defines criteria for temporary storage, partial treatment, controlled discharge, and recirculation of storm water, emphasizing cases where sewer systems do not provide sufficient storm flow control.

• Storm Water Separation and Flow Control
  Efficient flow separation is vital to limit influent flows to the treatment plant within its design capacity. The standard highlights control mechanisms such as adjustable penstocks and overflow weirs that divert excess flow to storm tanks, with flow measurement in compliance with ISO and EN standards.

• Storm Tanks Design and Operation
  Storm tanks provide temporary storage and partial sedimentation of excess storm water. Requirements include:

  • Minimum of two tanks for plants serving over 2000 PE, with at least one tank as a blind storm tank (without environmental overflow).
  • Full filling of tanks before overflow occurs, and design as primary settling tanks with sedimentation capability.
  • Provision for sludge collection, tank isolation, floor slope for drainage, and cleaning measures.
  • Proper inlet/outlet design to avoid short circuiting and ensure flow dissipation.
  • Return of stored storm water for treatment post-storm via pumping or gravity systems.

• Flow Measurement and Monitoring
  Flow to full treatment must be closely monitored and controlled-targeting less than 8% deviation from design peak flow. Technologies for flow measurement include electromagnetic and ultrasonic meters compliant with relevant ISO standards.

• Sizing and Modelling
  Storm tank capacity can be determined by modelling inflow patterns to protect receiving water bodies or by prescriptive volume calculations based on population equivalents or retention time during peak flows.

Applications

prEN 12255-2 is essential for engineers, consultants, and operators involved in designing, retrofitting, or managing wastewater treatment facilities, especially where storm water inflows challenge treatment capacity. Its application ensures that:

• WWTPs maintain optimal operation during storm events, avoiding untreated discharges that can cause environmental pollution or regulatory noncompliance.
• Storm water is temporarily stored and partially treated, minimizing solids overflow and protecting downstream aquatic ecosystems.
• Flow measurement and control systems are properly integrated, enabling informed management decisions and regulatory reporting.
• Facilities serving medium to large populations implement robust storm water management without depending solely on upstream sewer infrastructure.

Related Standards

To complement the requirements of prEN 12255-2, the following standards offer additional guidance:

• EN 752 and EN 16933 – Cover storm water management within sewer systems and pumping installations, informing separation and conveyance outside the treatment plant.
• EN 12255-1 to EN 12255-16 – Address related wastewater treatment plant components such as construction principles, preliminary treatments, activated sludge processes, sludge handling, and safety.
• EN 16323 – Provides glossary and terminology for wastewater engineering.
• ISO 4359, ISO 12242, EN ISO 20456 – Define flow measurement methodologies crucial for monitoring flows in storm water management.

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By adopting the specifications set forth in prEN 12255-2, wastewater treatment plants can enhance their resilience and compliance when managing storm water, ensuring sustainable water quality protection and regulatory adherence across Europe and beyond.

Standards Content (Sample)

SLOVENSKI STANDARD
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
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

DRAFT
EUROPEAN STANDARD
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.

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

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).
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.
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]
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.
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
...