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

• Refers: SIST EN 16323:2014 - Glossary of wastewater engineering terms
• Effective Date: 09-Sep-2026

SIST EN 12255-2:2026 is the Slovenian standard identical to EN 12255-2:2026 for storm water management systems in wastewater treatment plants. It specifies how excess storm water is separated, stored, treated, discharged, and returned within the plant. The document is aimed at designers, operators, and buyers who need to specify or check storm water handling at domestic or municipal wastewater treatment plants.

What does SIST EN 12255-2:2026 specify?

SIST EN 12255-2:2026 specifies requirements for storm water management systems inside wastewater treatment plants, not for storm water systems in wastewater collection and conveyance networks. It also says these systems can be regarded as part of the sewer system in line with EN 752 and EN 16933.

The document is organised into:

• Scope
• Normative references
• Terms and definitions
• Symbols and abbreviations
• General
• Flow management systems, with subsections on flow separation, storm tanks, return flow, cleaning, and measurement, monitoring and control
• Additional requirements

It also includes three tables that give example methods for flow separation, storm tank cleaning, and storm tank return control.

What are the key requirements of SIST EN 12255-2:2026?

SIST EN 12255-2:2026 sets up storm water systems to take excess inflow when a wastewater treatment plant reaches hydraulic capacity. In practice, that means the plant keeps full treatment for the normal flow and diverts the extra storm flow to storage and partial treatment.

Flow separation and diversion

Clause 6.1 says the plant shall receive its maximum design flow, while excess flow is diverted to storm tanks. Flow to treatment should be controlled, for example with adjustable penstocks, and excess storm flow can be diverted by a weir.

If internal return flows such as wash water cannot go downstream of the full-treatment point, they should be measured separately and included in the flow assessment. That matters because return flows can otherwise overload the treatment line.

Table 1 gives example separation methods for open channels and closed pipes. It links open-channel measurement to ISO 4359 and closed-pipe measurement to EN ISO 20456 or ISO 12242, so the selected instrument must fit the hydraulic arrangement.

Storm tank arrangement, sizing, and design

Clause 6.2.1 requires all storm tanks to be full before any tank overflows to the environment. For plants serving more than 2 000 people, at least two storm tanks should be provided, and at least one should be a blind storm tank.

Storm tanks should be filled sequentially where possible, and all except blind tanks should have overflows and isolation. Floors need at least a 1 % slope for drainage, and floor flushing should follow the approach used for rectangular primary settlement tanks in EN 12255-4.

Clause 6.2.2 recommends modelling the inflow pattern and receiving water impact. If modelling is not feasible, the document allows a time-period approach for minimum storage sizing, which helps when a simple design check is needed.

Clause 6.2.3 says inlets should dissipate energy, avoid short-circuiting, and keep debris from building up. Outlets need a scumboard, a screen, or both, and overflow weirs should be adjustable and instrumented so overflow rate can be calculated.

Return, cleaning, and control

Clause 6.2.4 says stored storm water shall be returned when the flow to full treatment is below design capacity. The return rate should avoid overloading the plant and should empty the tanks before the next storm, normally within 24 to 72 hours.

Clause 6.2.5 requires cleaning systems to avoid dead zones where solids settle permanently. Cleaning equipment should be removable, and process water should be available near the tanks for cleaning and wash-out.

Clause 6.2.6 requires flow to treatment and flow to the environment to be measured, indicated, and recorded. It also gives control logic for starting and stopping storm return and for locating the measurement points relative to inlet screens, grit separators, and storm diversions.

Local and national additions

Clause 6.3 notes that national or local rules may require extra storm treatment, such as finer screening, automatic screen cleaning, particle removal targets, or a specified storm management system to reduce impact on receiving waters.

What terms does SIST EN 12255-2:2026 define?

• Storm tank - a structure for temporary storage of excess wastewater flow during heavy rainfall or snow melt.
• Blind storm tank - a storm tank without a built-in overflow to the environment.
• Balancing - reduction in variations in flow, concentration, temperature, and similar properties; the standard notes that equalisation is deprecated.
• Storm water - foul wastewater mixed with surface water.
• Design capacity - the flow and load of influent that the facilities are designed to treat while meeting specified requirements.
• Design peak flow - the maximum wastewater inflow over a given period that the plant is designed to treat without overflow.
• LMIC - level measurement indication and controller.
• FICR - flow indication controller and recorder.

Who uses SIST EN 12255-2:2026?

SIST EN 12255-2:2026 is used by wastewater engineers, plant designers, operators, and technical buyers working on domestic and municipal wastewater treatment plants. It is also relevant to control-system suppliers, tank and screen equipment suppliers, and authorities reviewing whether storm water handling, measurement, and overflow control match local requirements.

Which standards are used with SIST EN 12255-2:2026?

• EN 16323 - provides the glossary of wastewater engineering terms used for shared terminology.
• EN ISO 20456 - gives guidance for electromagnetic flowmeters in closed conduits carrying conductive liquids.
• ISO 4359 - covers rectangular, trapezoidal, and U-shaped flumes for open-channel flow measurement.
• EN 752 - supports the wider sewer-system context mentioned in the scope and foreword.
• EN 16933 - supports the sewer-system design context mentioned in the scope and foreword.
• EN 12255-1 - gives general construction principles for wastewater treatment plants.
• EN 12255-3 - is used for preliminary-treatment references, especially around screening and sedimentation concepts.
• EN 12255-4 - is used for floor-flushing and rectangular primary settlement tank design references.

What does the SIST EN 12255-2:2026 document contain?

SIST EN 12255-2:2026 contains two schematic figures, three guidance tables, and clause-by-clause design and operation guidance. Figure 1 shows the overall wastewater treatment plant arrangement, while Figure 2 shows an example storm system with process units, flows, and instruments.

Table 1 sets out example flow-separation methods for open and closed systems. Table 2 gives guidance for choosing storm tank cleaning methods by tank volume. Table 3 gives typical control methods for storm tank return by tank size.

The text also covers sizing, inlet and outlet design, return pumping, cleaning cycles, measurement points, and control logic. In practice, that makes the document useful for design specifications, technical review, and checking whether a plant can handle excess storm flow without unwanted overflows.

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
...