SIST EN 12255-4:2023
(Main)Wastewater treatment plants - Part 4: Primary treatment
Wastewater treatment plants - Part 4: Primary treatment
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, are covered in EN 12255, Part 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 may be used for primary treatment of industrial wastewater, but then the design is specific to the application.
Kläranlagen - Teil 4: Vorklärung
Dieses Dokument legt die Auslegungsanforderungen für Anlagen und technische Ausrüstung zum Entfernen von anderen Feststoffen als Rechengut und Sandfanggut aus Rohabwasser in Kläranlagen für über 50 EW fest.
Dies schließt Vorklärung durch Sedimentation, Feinsiebe und Mikrosiebe ein.
ANMERKUNG 1 Die Entfernung von Rechengut und Sandfanggut ist in EN 12255 3 beschrieben.
ANMERKUNG 2 Die Entspannungsflotation (DAF, en: dissolved air flotation) ist in diesem Dokument nicht im Detail beschrieben, da sie bei der Vorklärung in kommunalen Kläranlagen nicht gebräuchlich ist. Sie kann bei der Vorklärung von Industrieabwasser eingesetzt werden, wobei die Auslegung dann spezifisch an diese Anwendung angepasst wird.
Stations d’épuration - Partie 4 : Traitement primaire
Le présent document spécifie les exigences de conception des installations et des équipements destinés à éliminer les matières solides, autres que les refus de dégrillage et le sable, des eaux usées brutes, dans les stations d’épuration de plus de 50 équivalents-habitants (EH).
Il inclut le traitement primaire par sédimentation, dégrilleurs fins et micro-tamis.
NOTE 1 L’élimination des refus de dégrillage et le dessablage sont traités dans l’EN 12255, Partie 3.
NOTE 2 La flottation à l’air dissous (FAD) n’est pas détaillée dans le présent document car cette technique est peu employée pour le traitement primaire dans les stations d’épuration des eaux usées municipales. Elle peut être utilisée pour le traitement primaire des eaux usées industrielles mais la conception est, dans ce cas, spécifique à l’application.
Čistilne naprave za odpadno vodo - 4. del: Primarni usedalnik
Ta dokument določa zahteve v zvezi s projektiranjem čistilne naprave in opreme za odstranjevanje trdnih nečistoč, ki ni vključuje precejanja ali posipavanja, iz neobdelane odpadne vode v čistilnih napravah za odpadne vode za več kot 50 populacijskih ekvivalentov (PE).
Vključuje primarno obdelavo s sedimentacijo, finim precejanjem in mikro precejanjem.
OPOMBA 1: Odstranjevanje elementov za precejanje in posipavanje je zajeto v 3. delu standarda EN 12255.
OPOMBA 2: Flotacija z raztopljenim zrakom (DAF) ni podrobno zajeta v tem dokumentu, saj se je ne uporablja pogosto pri primarni obdelavi čistilnih naprav za komunalne odpadne vode. Ta postopek se lahko uporablja za primarno obdelavo industrijskih odpadnih voda, vendar pa je zasnova v tem primeru specifična.
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
SIST EN 12255-4:2023
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
SIST EN 12255-4:2023 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST EN 12255-4:2023
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SIST EN 12255-4:2023
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.
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SIST EN 12255-4:2023
EN 12255-4:2023 (E)
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
2
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SIST EN 12255-4:2023
EN 12255-4:2023 (E)
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
3
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SIST EN 12255-4:2023
EN 12255-4:2023 (E)
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.
4
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SIST EN 12255-4:2023
EN 12255-4:2023 (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.
5
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SIST EN 12255-4:2023
EN 12255-4:2023 (E)
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
6
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SIST EN 12255-4:2023
EN 12255-4:2023 (E)
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.
7
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EN 12255-4:2023 (E)
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.
Primary treatment is not required where secondary wastewater treatment and simultaneous aerobic
sludge stabilization is achieved. However, upstream preliminary treatment with fine screens for the
removal of solids and grit removal is always required if there is no primary treatment with
sedimentation.
Primary treatment can be enhanced through coagulant and flocculant (polymer) addition.
5.2 Planning
5.2.1 Required Data and Information
Performance requirements shall be determined considering the following factors:
— the nature and quantity of flow, including its variation (in particular storm events);
— chemical Oxygen Demand (COD) and Suspended Solids (TSS) loads and their variations;
— ratio of dissolved (or particulate) COD to total COD;
— ratio of readily biodegradable COD (rbdCOD) to total COD; knowledge of this ratio is required
where biological Nitrogen removal is performed during subsequent biological treatment; the ratio
of Biochemical Oxygen Demand (BOD) to COD may be used as a substitute;
— total Kjeldahl Nitrogen (TKN) load; this parameter is also required where biological Nitrogen
removal is performed during subsequent biological treatment;
— slot or perforation width of the headworks screens; lamella separators and micro-screens require
fine screens (see Part 3) with a maximum slot width of 4 mm or a perforation diameter of
maximum 6 mm;
— the quantity and quality of primary sludge generated, including its peak during storm events, which
depends on the quality and operation of the sewer system;
— type of sludge stabilization and requirements concerning the primary sludge solids concentration
(gravity or mechanical sludge thickening is usually needed);
— the quantity and quality of the effluent and its variation; where nitrogen removal is required, the
COD/TKN-ratio of the effluent should not be below 6:1 so that enough rbdCOD for denitrification
8
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EN 12255-4:2023 (E)
remains in the effluent. More specifically, the ratio of rbdCOD to TKN in the effluent should not be
lower than 1:1,2;
— redundancy requirements;
— health and safety requirements;
— concentrations of Sulfide and Chloride in the influent (they can cause concrete and metal
corrosion).
Figure 2 shows the composition of COD and TSS in raw wastewater. Typical average specific loads are
shown, but they can vary depending on local conditions. It should be noted that 85-Percentiles of the
specific loads are about 20 % higher.
Figure 2 — Typical composition of COD and TSS
9
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SIST EN 12255-4:2023
EN 12255-4:2023 (E)
5.2.2 Advantages and Disadvantages of Primary Treatment Systems
Lamella separators have the following pros and cons in comparison with conventional clarifiers:
Pros:
— their footprint is about half that of primary clarifiers;
— savings of construction costs;
— better flow distribution.
Cons:
— additional costs of mechanical equipment and its maintenance;
— need for regular cleaning;
— inconsistent performance due to more frequent blockages from fat and rags which can be mitigated
by good preliminary treatment (see Part 3).
Lamella separators are typically taken into consideration where:
— existing primary clarifiers need upgrading; or
— where the available footprint for conventional primary clarifiers is insufficient.
Micro-screens have the following pros and cons in comparison with conventional clarifiers:
Pros:
— they require only about 10 % of the footprint of primary clarifiers;
— savings of construction and investment costs;
— smaller variation of removal ratios depending on flow [1];
— removal ratios can be adjusted by changing the mesh;
— removal of fibres and thus protection of subsequent treatment stages from cording;
— removal of micro-plastic;
— can be used to thicken primary sludge (saving pre-thickening storage and separate thickeners).
Cons:
— micro-screens typically generate a head loss of up to 0,4 m and thus can need inflow pumping (this
can be comparable to a flow splitter chamber and traditional clarifiers); however, pumping offers
the benefit of flow control;
— power consumption is higher;
— need for redundancy: Failure of one unit shall not result in total loss of primary treatment capacity;
— increased reliance on mechanical equipment which requires maintenance.
10
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EN 12255-4:2023 (E)
Micro-screens are typically taken into consideration where:
— an existing primary clarifier needs upgrading; instead a portion of the inflow can be mechanically
treated by micro-screening;
— there is a lack of available space for a conventional clarifier;
— a wastewater plant with aerobic sludge stabilization is changed into a plant with anaerobic sludge
stabilization and it is difficult to add a primary clarifier (lack of space or requirement for pumping);
— sea or river outflows need mechanical treatment (with or without prior coagulation or
flocculation).
5.2.3 Performance
In a primary clarifier the removal rate depends on the mean retention period, as shown in Table 1.
Table 1 — Removal Efficiencies of primary clarifiers (DWA-A 131 modified)
Removal effectivity η in %
Retention period in primary
clarifier calculated with the 0,75 h − 1 h 1 h − 1,5 h 1,5 h − 2 h 2 h − 2,5 h > 2,5 h
average dry weather flow
COD 30 % 32,5 % 35 % 37,5 % 40 %
Particulate COD 45 % 50 % 55 % 57,5 % 60 %
TSS 50 % 55 % 60 % 62,5 % 65 %
TKN 10 %
P 10 %
tot
Where nitrogen is to be removed during subsequent biological treatment, the mean retention period
during dry weather conditions in a clarifier should be between 0,5 h and 1,5 h. Where this is not the
case, a dry weather retention period between 1,5 h and 2,5 h may be used.
Primary clarifiers for wastewater treatment plants serving a total population of up to 1 000 PT should
have a retention period, based on the design flow, of minimum 2 h. In such cases the primary clarifier
can also serve as primary sludge storage tank. Its volume shall be calculated accordingly. The primary
sludge should be removed in such intervals that it does not become septic. The pr
...
SLOVENSKI STANDARD
oSIST prEN 12255-4:2021
01-oktober-2021
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: prEN 12255-4
ICS:
13.060.30 Odpadna voda Sewage water
oSIST prEN 12255-4:2021 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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oSIST prEN 12255-4:2021
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oSIST prEN 12255-4:2021
DRAFT
EUROPEAN STANDARD
prEN 12255-4
NORME EUROPÉENNE
EUROPÄISCHE NORM
August 2021
ICS 13.060.30 Will supersede 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 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, Turkey 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
© 2021 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 12255-4:2021 E
worldwide for CEN national Members.
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oSIST prEN 12255-4:2021
prEN 12255-4:2021 (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 Requirements . 6
5.1 General. 6
5.2 Planning . 7
5.2.1 Required Data and Information . 7
5.2.2 Advantages and Disadvantages of Primary Treatment Systems . 8
5.2.3 Performance . 10
5.3 Design . 11
5.3.1 General. 11
5.3.2 Specifications . 11
5.3.3 Required Capacity . 11
5.3.4 Systems for primary treatment . 11
5.3.5 Materials . 15
5.3.6 Control and Automation . 15
5.3.7 Operation and Maintenance. 15
5.3.8 Health and Safety . 15
Annex A (informative) Illustrations of typical clarifiers . 16
Annex B (normative) Dimensions and tolerances for the structures of clarifiers . 24
B.1 Circular clarifiers with scrapers . 24
B.2 Rectangular clarifiers with scrapers . 24
Annex C (normative) Wall tracks . 25
Annex D (normative) Scraper design . 26
Bibliography . 27
2
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oSIST prEN 12255-4:2021
prEN 12255-4:2021 (E)
European foreword
This document (prEN 12255-4:2021) has been prepared by Technical Committee CEN/TC 165 “Waste
water engineering”, the secretariat of which is held by DIN.
This document is currently submitted to the CEN Enquiry.
This document will supersede 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 parts of the series are as follows:
• Part 1: General construction principles
• Part 2: Storm management systems
• Part 3: Preliminary treatment
• Part 4: Primary settlement
• 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, Drain and
sewer systems outside buildings — Sewer system management and EN 16932 (all parts), Drain and sewer systems
outside buildings — Pumping systems.
3
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oSIST prEN 12255-4:2021
prEN 12255-4:2021 (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:
Key
1 preliminary treatment
2 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 digested 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 may be obtained by referring to the
bibliography.
The primary application is for wastewater treatment plants designed for the treatment of domestic and
municipal wastewater.
4
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oSIST prEN 12255-4:2021
prEN 12255-4:2021 (E)
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, are covered in EN 12255, Part 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 may 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
EN ISO 14122-2:2016, Safety of machinery - Permanent means of access to machinery - Part 2: Working
platforms and walkways (ISO 14122-2:2016)
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 terminological databases for use in standardization at the following addresses:
• ISO Online browsing platform: available at http://www.iso.org/obp
• IEC Electropedia: available at http://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
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4 Symbols and abbreviations
BOD Biochemical Oxygen Demand
COD Chemical Oxygen Demand
FOG Fat, Oil and Grease
rbCOD readily biodegradable COD
SS Suspended Solids
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 may 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:
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• 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.
Primary treatment is not required where secondary wastewater treatment and sludge stabilization is
achieved by extended aeration. However, upstream preliminary treatment with fine screens for the
removal of solids and grit removal is always required.
Primary treatment can be enhanced through coagulant and flocculant (polymer) addition.
5.2 Planning
5.2.1 Required Data and Information
Performance requirements shall be determined considering the following factors:
• the nature and quantity of flow, including its variation (in particular storm events);
• chemical Oxygen Demand (COD) and Suspended Solids (SS) loads and their variations;
• ratio of dissolved (or particulate) COD to total COD;
• ratio of readily biodegradable COD (rbCOD) to total COD; knowledge of this ratio is required where
biological Nitrogen removal is performed during subsequent biological treatment; the ratio of
Biochemical Oxygen Demand (BOD) to COD may be used as a substitute;
• total Kjeldahl Nitrogen (TKN) load; this parameter is also required where biological Nitrogen
removal is performed during subsequent biological treatment;
• slot or perforation width of the headworks screens; lamella separators and micro-screens require
fine screens (see Part 3) with a maximum slot width of 4 mm or a perforation diameter of
maximum 6 mm;
• the quantity and quality of primary sludge generated, including its peak during storm events, which
depends on the quality and operation of the sewer system;
• type of sludge stabilization and requirements concerning the primary sludge solids concentration
(gravity or mechanical sludge thickening is usually needed);
• the quantity and quality of the effluent and its variation; where nitrogen removal is required, the
COD/TKN-ratio of the effluent should not be below 6:1 so that enough rbCOD for denitrification
remains in the effluent. More specifically, the ratio of rbCOD to TKN in the effluent should not be
lower than 1:2;
• redundancy requirements;
• health and safety requirements;
• concentrations of Sulfide and Chloride in the influent (they can cause concrete and metal
corrosion).
Figure 1 shows the composition of COD and SS in raw wastewater. Typical average specific loads are
shown, but they can vary depending on local conditions. It should be noted that 85-Percentiles of the
specific loads are about 20 % higher.
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Figure 2 — Typical composition of COD and SS
5.2.2 Advantages and Disadvantages of Primary Treatment Systems
Lamella separators have the following pros and cons in comparison with conventional clarifiers:
Pros:
• their footprint is about half that of primary clarifiers,
• savings of construction costs,
• better flow distribution.
Cons:
• additional costs of mechanical equipment and its maintenance,
• need for regular cleaning,
• inconsistent performance due to more frequent blockages from fat and rags which can be mitigated
by good preliminary treatment (see Part 3).
Lamella separators are typically taken into consideration where:
• existing primary clarifiers need upgrading or
• where the available footprint for conventional primary clarifiers is insufficient.
Micro-screens have the following pros and cons in comparison with conventional clarifiers:
Pros:
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Micro-screens require only about 10 % of the footprint of primary clarifiers;
• savings of construction and investment costs;
• smaller variation of removal ratios depending on flow [1];
• removal ratios can be adjusted by changing the mesh;
• removal of fibres and thus protection of subsequent treatment stages from cording;
• removal of micro-plastic;
• can be used to thicken primary sludge (saving pre-thickening storage and separate thickeners).
Cons:
• micro-screens generate a head loss of up to 0,4 m and thus may need inflow pumping (this may be
comparable to a flow splitter chamber and traditional clarifiers); however, pumping offers the
benefit of flow control;
• power consumption is higher;
• need for redundancy: Failure of one unit shall not result in total loss of primary treatment capacity;
• increased reliance on mechanical equipment which requires maintenance.
Micro-screens are typically taken into consideration where:
• an existing primary clarifier needs upgrading; instead a portion of the inflow can be mechanically
treated by micro-screening;
• there is a lack of available space for a conventional clarifier;
• a wastewater plant with aerobic sludge stabilization is changed into a plant with anaerobic sludge
stabilization and it is difficult to add a primary clarifier (lack of space or requirement for pumping);
• sea or river outflows need mechanical treatment (with or without prior coagulation or
flocculation).
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5.2.3 Performance
In a primary clarifier the removal rate depends on the mean retention time, as shown in Table 1.
Table 1 — Typical removal performance of primary clarifiers during average daily dry weather
flow [2 modified]
Retention time during dry weather flow
0,75 to 1,25 h 1,25 h to 2,25 h > 2,25 h
Parameter Typical removal performance in %
COD or BOD 30 35 40
Particulate COD 45 55 60
SS 50 60 65
TKN 10 10 10
P (total phosphorus) 10 10 10
tot
The maximum COD removal ratio depends on the ratio of particulate COD to total COD.
Where nitrogen is to be removed during subsequent biological treatment, the mean retention time
during dry weather conditions in a clarifier should be between 0,5 h and 1,5 h. Where this is not the
case, a dry weather detention time between 1,5 h and 2,5 h may be used.
Primary clarifiers for wastewater treatment plants serving a total population of up to 1 000 PT should
have a detention time, based on the design flow, of minimum 2 h. In such cases the primary clarifier can
also serve as primary sludge storage tank. Its volume shall be calculated accordingly. The primary
sludge shall be removed in such intervals that it does not become septic. The primary sludge may be
transferred to a larger plant where it can be stabilized (e.g.in an anaerobic digester).
Micro-screens with a mesh size of 0,3 mm and without prior flocculation can remove about 70 % of the
TSS and 50 % of the COD [1]. After prior flocculation up to 90 % of the TSS can be removed
Micro-screens can also be used for mechanical treatment at sea and river outfalls. In such cases prior
flocculation should be provided.
Where no subsequent nitrogen removal is required, the SS- and COD-removal rate should be as high as
reasonably possible and therefore mesh sizes between 0,1 mm and 0,25 mm should be used. This raises
the ratio of primary to secondary sludge, increasing the gas production of anaerobic digesters, reduces
the volatile solids ratio of stabilized sludge, and improves its dewaterability.
However, where subsequent biological nitrogen removal is required, the denitrification capacity
depends on the availability of rbCOD and the COD/TKN-ratio should not drop below 6:1. In such cases
the mesh size of very fine screens should be in a range between 0,2 mm and 0,5 mm, so that about 25 %
of the COD and about 60 % of the SS are removed.
Where only a single clarifier or micro-screen is provided,
...
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