EN 12255-13:2023

SLOVENSKI STANDARD
01-september-2023
Nadomešča:
SIST EN 12255-13:2003
Čistilne naprave za odpadno vodo - 13. del: Kemijsko čiščenje odpadnih vod z
obarjanjem/kosmičenjem
Wastewater treatment plants - Part 13: Chemical treatment - Treatment of wastewater by
precipitation/flocculation
Kläranlagen - Teil 13: Chemische Behandlung - Abwasserbehandlung durch
Fällung/Flockung
Stations d'épuration - Partie 13: Traitement chimique - Traitement des eaux usées par
précipitation/floculation
Ta slovenski standard je istoveten z: EN 12255-13:2023
ICS:
13.060.30 Odpadna voda Sewage water
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN 12255-13
EUROPEAN STANDARD
NORME EUROPÉENNE
July 2023
EUROPÄISCHE NORM
ICS 13.060.30 Supersedes EN 12255-13:2002
English Version
Wastewater treatment plants - Part 13: Chemical
treatment - Treatment of wastewater by
precipitation/flocculation
Stations d'épuration - Partie 13: Traitement chimique - Kläranlagen - Teil 13: Chemische Behandlung -
Traitement des eaux usées par Abwasserbehandlung durch Fällung/Flockung
précipitation/floculation
This European Standard was approved by CEN on 24 April 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 12255-13:2023 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 . 8
4.1 Symbols . 8
4.2 Abbreviations . 8
5 Requirements . 9
5.1 General. 9
5.2 Regulation . 9
5.3 Phosphorus removal strategies . 9
5.4 Design considerations . 10
5.5 Chemical background and process options . 11
5.6 Storage preparation and dosing of chemicals . 15
5.7 Dosing equipment . 15
5.8 Silos, tanks and pipes . 16
5.9 Mixing . 17
5.10 Control systems for dosing of chemical . 17
5.11 Flocculation reactors . 19
5.12 Sedimentation tanks . 20
5.13 Flotation . 20
5.14 Physical filtration . 20
5.15 Sludge . 20
Annex A (informative) Precipitation chemicals . 22
Bibliography . 24

European foreword
This document (EN 12255-13:2023) 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 January 2024, and conflicting national standards shall
be withdrawn at the latest by January 2024.
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 12255-13:2002.
This is the thirteenth 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.
The EN 12255 series with the generic title “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 Part 2 is under preparation.
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 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
The primary application is for wastewater treatment plants designed for the treatment of domestic and
municipal wastewater.
NOTE For requirements on pumping installations at wastewater treatment plants see EN 752, Drain and
sewer systems outside buildings and EN 16932, Drain and sewer systems outside buildings — Pumping systems:
— Part 1: General requirements;
— Part 2: Positive pressure systems;
— Part 3: Vacuum systems.
1 Scope
This document specifies the requirements for chemical treatment of wastewater by
precipitation/flocculation for removal of phosphorus and suspended solids.
The application of polymers is not described in this document.
This document has not attempted to specify all available practices.
NOTE Chemical treatment can be performed in combination with primary and more commonly with
secondary treatment, but it can also be performed as separate tertiary treatment, usually in combination with
filtration (see EN 12255-16). Chemical treatment can provide a potential contribution to the circular economy
through the recovery of materials, such as phosphorus, from wastewater or sludge.
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 12255-11, Wastewater treatment plants - Part 11: General data required
EN 16932-1, Drain and sewer systems outside buildings - Pumping systems - Part 1: General requirements
EN 16932-2, Drain and sewer systems outside buildings - Pumping systems - Part 2: Positive pressure
systems
3 Terms and definitions
For the purposes of this document, the following terms and definitions 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
chemical treatment
process involving the addition of chemicals to achieve a specific result
Note 1 to entry: For wastewater, typical chemical treatments comprise: coagulation/precipitation with metal
salts (including lime) or organic polymers in order to remove inorganic and organic phosphorus compounds or
suspended solids and colloids.
[SOURCE: ISO 6107:2021, definition 3.108 modified – Note 1 to entry added]
3.2
chemical precipitation
conversion of components dissolved in water into an undissolved form by chemical reaction with a
precipitant
[SOURCE: EN 16323:2014, 2.3.5.8]
3.3
precipitant
chemical used to bring about precipitation
[SOURCE: EN 16323:2014, 2.3.5.45]
3.4
coagulation
destabilisation of undissolved and colloidally dispersed matter to allow aggregation, usually by addition
of coagulants
[SOURCE: EN 16323:2014, 2.3.5.9]
3.5
coagulant
chemical added to destabilise suspensions or emulsions
[SOURCE: EN 16323:2014, 2.3.1.21]
Note 1 to entry: A coagulant is normally based on an aluminium or iron salt (inorganic
coagulant) but could also be a polymer (organic). In water treatment, the coagulant always has a
positive charge
3.6
flocculation
formation of separable flocs by aggregation of small particles
[SOURCE: EN 16323:2014, 2.3.5.19]
Note 1 to entry: Micro-flocculation can be achieved by destabilization and coagulation/aggregation, macro-
flocculation can be achieved by addition of bridge building polymers.
3.7
destabilisation
compensation of negative charges on the surfaces of particles by addition of positively charged bivalent
or trivalent metal ions to achieve particle aggregation
Note 1 to entry: Destabilization is performed by reduction of the zeta-potential.
3.8
tertiary treatment
additional treatment processes which result in further purification than that obtained by primary and
secondary treatment
[SOURCE: EN 16323:2014, 2.3.5.51]
3.9
aerobic
dissolved oxygen is present
[SOURCE: EN 16323:2014, 2.3.1.1]
Note 1 to entry: The terms oxic and aerobic are synonyms.
3.10
anaerobic
absence of dissolved oxygen, nitrate, nitrite and sulphate
[SOURCE: EN 16323:2014, 2.3.1.2]
Note 1 to entry: It can be important that other oxidising chemicals are also absent.
3.11
anoxic
absence of dissolved oxygen but presence of nitrite or nitrate
[SOURCE: EN 16323:2014, 2.3.1.3]
4 Symbols and abbreviations
4.1 Symbols
C concentration of the dosed iron or aluminium salt (kg/m )
Dos
C arriving P-concentration (mg/l)
P,in
C remaining P-concentration (mg/l)
P,out
L removed phosphorus load (kg/h)
P,rem
Q wastewater flow (l/s)
Q dose rate (m /h)
Dos
ω specific molar weight of the chemical (g/mol)
ρ density of the chemical (kg/m )
β ratio between the dosed precipitant (e.g. iron) to the stoichiometrically required precipitant
for the phosphorus to be removed
4.2 Abbreviations
ADP adenosine-di-phosphate
ASP activated sludge plants
ATP adenosine-tri-phosphate
DS dry solids
EBPR enhanced biological phosphorus removal
FPD flow proportional dosing
FST final settlement tanks
P phosphorus
TSR tertiary solids removal
5 Requirements
5.1 General
Chemical treatment of wastewater can be divided into two processes:
— a reaction phase, that consists of precipitation of dissolved phosphates, destabilization of colloids
and the formation of flocs; and
— a separation phase, in which the flocs are separated from the water.
The reactors and floc separators (sedimentation tanks, flotation or filtration units etc.) for the chemical
treatment can be integrated with the other parts of the wastewater treatment plant (pre-precipitation
as part of primary treatment, simultaneous precipitation as part of secondary treatment, see 5.5.2.1 and
5.5.2.2) or be a separate part of the treatment plant (post precipitation, direct precipitation, i.e. tertiary
treatment, see 5.5.2.3, or direct precipitation).
The water level in the chemical reactors and tanks may be controlled by fixed or adjustable weirs. It is
particularly important where there are multiple parallel reactors.
The design of the process shall take into account variations in flow and load as stipulated in EN 12255-1
and EN 12255-11.
5.2 Regulation
National or local regulations or the relevant authority can set requirements for phosphorus removal,
recovery and re-use. This might also include a limit for the metal (iron or aluminium) used in the
precipitation/flocculation of phosphorus in its various forms.
Limits would typically be on the remaining concentration of cations and anions (e.g. chloride, sulfate or
aluminium) in the final effluent or its pH range. The mechanism and risk of over-dosing should be
considered in the design phase.
5.3 Phosphorus removal strategies
5.3.1 General
Phosphorus can be removed by primary treatment followed by biological or chemical treatment and
may require tertiary (solids and phosphate removal) to meet tighter consents.
The Total Phosphorus permit limits imposed on sites can typically range from 2 mg/l down to 0,1 mg/l.
The recommend strategy for achieving these permits varies depending on a number of factors:
— the total Phosphorus permit;
— the site’s biological treatment stage;
— the site’s final effluent solids removal performance;
— the precipitant chemical chosen to achieve Phosphorus removal;
— the means by which the precipitant is added and mixed;
— the characteristics of the influent (e.g. organic load, pH, industrial discharges etc)
Table 1 outlines an approach for sites with new total phosphorus permits which have not previously
had final effluent total phosphorus permits and do not have existing tertiary solids removal (TSR) e.g.
through effluent filtration:
Table 1 — Recommended strategies for meeting required total phosphorus permits
Total phosphorus permit
Biological
treatment stage
≥ 1 mg/l 0,75 mg/l to 1 mg/l 0,5 mg/l to 1 mg/l < 0,5 mg/l
Activated sludge Single point dosing Single point dosing Dual or single point Dual or single
plant with TSR optional dosing with TSR point dosing
optional with
TSR
Trickling filters Single point dosing Single point dosing
NOTE 1 Table 1 assumes good performance of existing final or humus tanks with respect to solids removal.
NOTE 2 The total P concentrations in Table 1 are based on the yearly average of 24-h composite samples as
common in most EU countries. However, other national regulations can exist.
5.3.2 Primary treatment
In primary treatment, phosphorus associated with settleable particles is removed (typically 5 % to
15 % of the total influent phosphorus depending on the character of the wastewater).
5.3.3 Biological treatment and enhanced biological phosphorus removal
In the biological treatment a certain amount of phosphorus is consumed at the microbial synthesis of
new cellular material (10 % to 30 % of the influent phosphorus). By introducing anaerobic zones in
activated sludge systems where phosphates are released an increased biological removal of total
phosphorus can be reached without addition of chemicals (40 % to 70 % of the influent concentration).
The uptake of Phosphorus in the anoxic and aerobic zones is thus increased (so-called luxury uptake).
This additional process in activated sludge systems with incorporated anaerobic zones is called
“enhanced biological phosphorus removal” (EBPR).
Chemical precipitation of orthophosphate PO -P requires the addition of chemical at single or dual
dosing points.
5.3.4 Tertiary treatment
If an assessment of the site indicates that final effluent suspended solids is high (>10 mg/l on average),
TSR technology shall be considered at less stringent total phosphorus permits than Table 1
recommends.
Where the existing phosphorous or solids removal performance of the plant is poor, or the permit has
been made more stringent, refurbishment of assets to improve the performance should be considered
as an alternative option to the installation of new TSR technology. This review should consider data
from as long a period as possible.
Tertiary solids removal (e.g. by sedimentation, flotation or filtration) may be required downstream of
the final settlement tanks (FSTs) to meet permit requirements. This assessment must be made by the
designer on a case by case basis.
5.4 Design considerations
5.4.1 Chemical
The reactions involved in the precipitation of phosphate shall be considered in light of the many
competing reactions and the associated equilibrium constants.
Various parameters (e.g. pH value, alkalinity or the solids concentration in the final effluent), affect a
plant's ability to meet a total phosphorus permit and shall therefore be considered. Appropriate means
to prevent overdosage of chemicals shall be provided.
At the commencement of any projects involving chemical phosphate removal, it is essential to establish
the level of total phosphorus within the wastewater to be treated. If high levels of phosphorus are
present (greater than 20 mg/l), an audit to understand the source(s) should be undertaken to establish
if source reduction offers a better option.
Jar tests can be performed early in the design to establish if chemical precipitation is feasible for the site
and which chemical precipitant gives optimum performance. These tests should be conducted under a
range of flow conditions to ensure the widest range of influent P concentrations and shall also consider
metal concentrations and pH values. Jar tests will have limited significance for complex activated sludge
processes.
For works with low or very low total phosphorus permit limits, onsite trialling of precipitant dosing
should be considered.
Industrial discharges can affect the effectiveness of wastewater treatment plants and their units. This
shall be considered during design. Where the quality of considerable portions of the raw wastewater
differs from the characteristics of common municipal wastewater, pilot testing of treatment systems
over a representative time period is recommended.
5.4.2 Interchangeability
Tanks need to have external fixings to foundations. EN 12255-1 contains requirements for
interchangeability of tanks.
Other design requirements can be found in EN 12255-1.
5.5 Chemical background and process options
5.5.1 Chemical process
Most, but not all phosphorus in the raw wastewater is in the form of phosphates, generally
orthophosphate. Industrial effluents may contain more or less other phosphorus forms. Biological
wastewater treatment may transform other forms to orthophosphate because this is the form bacteria
can take up. The phosphorus in cells is predominantly in the form of adenosin-di- and adenosin-tri-
phosphate (ADP and ATP). The latter serves as an energy reservoir. While the bacteria have sufficient
substrate and oxygen, they build up their ATP reservoir, but when they are under stress, they reduce it
and release phosphate. For this reason surplus sludge shall be removed from aerobic or anoxic zones.
In order to obtain precipitation and coagulation a cationic chemical shall be added to the wastewater.
Most commonly this is a salt of aluminium or iron. Lime can also be used. When using lime an additional
neutralization after precipitation will be needed. If improvement/enhancement of flocculation is
required, a polymeric flocculant can be added.
Precipitants react not only with phosphate but also with other substances (e.g. iron reacts with
sulphide). For this reason, a surplus of precipitants needs to be dosed, i.e. more than the stoichiometric
ratio relative to the phosphate concentration. The ratio is described with a factor β which is usually
between 1,2 and 2,0.
The addition of magnesium hydroxide (Mg(OH) ) can be used for promoting a controlled phosphate
precipitation as struvite, in liquids containing high concentrations of phosphate and ammonium (e.g.
return flow from digested sludge dewatering or anaerobically digested sludge).
2+ 3+ 3+
Salts of Fe (e.g. FeSO ) or Fe (e.g. FeCl ) or of Al (e.g. AlCl ) or of polyaluminium can be used as
4 3 3
2+ 3+
precipitants and coagulants. If Fe is used as a precipitant it will be oxidized to Fe in aerated reactors
therefore increasing the oxygen consumption.
Phosphorus can be present in the wastewater in the following forms:
a) organically bound phosphorus,
b) inorganic phosphorus,
— orthophosphate,
— polyphosphate,
— nonreactive phosphorus compounds.
Polyphosphates are eventually converted to orthophosphates and the organically bound phosphorus
can be converted to orthophosphate during biological treatment.
In the chemical precipitation, a precipitation agent (such as aluminium sulphate, polyaluminium
chloride, ferric and ferrous chloride, ferric chloride sulphate, ferric and ferrous sulphate, ferric chloride
and blends of aluminium and iron salts or calcium hydroxide) is added to the wastewater.
3+ 2+ 3+
Orthophosphate is precipitated as metal- orthophosphate. Al Fe and Fe are multivalent ions which
act as coagulants by destabilization. The solubility of the precipitates is pH-dependent.
Organic polyelectrolytes are used as flocculation agents for colloidal and suspended matter.
Chemical preci
...