EN 12255-4:2023

SLOVENSKI STANDARD
01-maj-2023
Nadomešča:
SIST EN 12255-4:2002
Čistilne naprave za odpadno vodo - 4. del: Primarni usedalnik
Wastewater treatment plants - Part 4: Primary treatment
Kläranlagen - Teil 4: Vorklärung
Stations d’épuration - Partie 4 : Traitement primaire
Ta slovenski standard je istoveten z: EN 12255-4:2023
ICS:
13.060.30 Odpadna voda Sewage water
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN 12255-4
EUROPEAN STANDARD
NORME EUROPÉENNE
March 2023
EUROPÄISCHE NORM
ICS 13.060.30 Supersedes EN 12255-4:2002
English Version
Wastewater treatment plants - Part 4: Primary treatment
Stations d'épuration - Partie 4 : Traitement primaire Kläranlagen - Teil 4: Vorklärung
This European Standard was approved by CEN on 9 January 2023.

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
© 2023 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 12255-4:2023 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 Requirements . 7
5.1 General . 7
5.2 Planning . 8
5.2.1 Required Data and Information . 8
5.2.2 Advantages and Disadvantages of Primary Treatment Systems . 10
5.2.3 Performance . 11
5.3 Design . 12
5.3.1 General . 12
5.3.2 Specifications . 12
5.3.3 Required Capacity . 12
5.3.4 Systems for primary treatment . 12
5.3.5 Materials . 17
5.3.6 Control and Automation . 17
5.3.7 Operation and Maintenance . 17
5.3.8 Health and Safety. 17
Annex A (informative) Illustrations of typical clarifiers . 18
Annex B (normative) Dimensions and tolerances for the structures of clarifiers . 26
Bibliography . 27

European foreword
This document (EN 12255-4:2023) has been prepared by Technical Committee CEN/TC 165 “Waste
water engineering”, the secretariat of which is held by DIN.
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 September 2023, and conflicting national standards
shall be withdrawn at the latest by September 2023.
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.
This document supersedes EN 12255-4:2002.
It is the fourth part prepared by the 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.
The EN 12255 series, Wastewater treatment plants, consists of the following parts:
— Part 1: General construction principles
— Part 2: Storm 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 1 Part 2 is under preparation.
NOTE 2 For requirements on pumping installations at wastewater treatment plants, see EN 752, Drain and
sewer systems outside buildings — Sewer system management and EN 16932 (all parts), Drain and sewer systems
outside buildings — Pumping systems.
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:

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 the design requirements for plant and equipment to remove solids, other than
screenings and grit, from raw wastewater, at wastewater treatment plants for over 50 PT.
It includes primary treatment with sedimentation, fine screens and micro-screens.
NOTE 1 The removal of screenings and grit is covered in EN 12255-3.
NOTE 2 Dissolved air flotation (DAF) is not covered in detail in this document because it is not commonly used
for primary treatment in municipal wastewater treatment plants. It can be used for primary treatment of
industrial wastewater, but then the design is specific to the application.
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 12255-1, Wastewater treatment plants - Part 1: General construction principles
EN 12255-10, Wastewater treatment plants - Part 10: Safety principles
EN 16323:2014, Glossary of wastewater engineering terms
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 16323:2014 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
dissolved air flotation
DAF
separation of solids from wastewater whereby air is dissolved in recirculated wastewater under
pressure, generates micro-bubbles when the pressure is released within a tank, which attach to
particles and floats them to the wastewater surface as scum
3.2
lamella separator
device comprising regularly spaced, inclined plates or tubes designed to increase the effective settling
area
[SOURCE: EN 16323:2014, term number 2.3.2.6]
3.3
micro-screen
screen with a typical mesh size between 0,1 mm and 1,0 mm
3.4
fine screen
screen with a slot or perforation diameter between 1 mm and < 8 mm
[SOURCE: EN 12255-3]
4 Symbols and abbreviations
BOD Biochemical Oxygen Demand
COD Chemical Oxygen Demand
FOG Fat, Oil and Grease
rbdCOD readily biodegradable COD
Ptot Total Phosphorus
TKN Total Kjeldahl Nitrogen
TS Total Solids
TSS Total Suspended Solids
5 Requirements
5.1 General
Primary treatment has the objective of removing solids and the associated organic load from raw
wastewater (solid-liquid separation). Retained solids are removed continuously or on a regular basis in
the form of primary sludge. The kind of solids removed depends on the process employed: primary
clarifiers and lamella separators remove solids depending on their size and density by sedimentation
(settable solids) or by flotation (fat, oil and grease), but they do not effectively remove solids with a
density close to that of the wastewater. Screens remove solids from wastewater based on their particle
size and rigidity, independent of their density.
The type and size of units employed will depend on the overall system, on the inflow and solids load
and their variabilities, on the available space and ground conditions.
Primary treatment can include the following units:
a) Primary clarifier (settling tank or lamella separator):
— upward flow;
— horizontal flow;
— clarifier/sludge storage tank combination;
— lamella separator;
— imhoff tank.
Figure A.1 to A.6 in Appendix A show sketches of typical gravity clarifiers and lamella separators.
b) Micro-screen:
— rotating cylindrical screens with flow from the inside out;
— continuous belt screen which is horizontal or slightly sopped upwards;
— other micro-screens which might be developed.
Figure A.7 in Appendix A shows sketches of typical micro-screens, i.e. of a belt and drum screen.
c) Dissolved air flotation:
— system with or without addition of coagulants or flocculants;
— system with or without lamella separators.
Figure A.8 in Appendix A shows a sketch of a typical dissolved air flotation unit.
...