EN 18087:2026
(Main)Devices for in-situ generation of biocides - Chlorine dioxide generated from sodium chlorite by acidification or oxidation
Devices for in-situ generation of biocides - Chlorine dioxide generated from sodium chlorite by acidification or oxidation
This document specifies requirements for dosing systems for chlorine dioxide generation according to the chlorite-chlorine gas process, the chlorite-acid process and the chlorite-sodium peroxodisulphate process, which are used for the disinfection and oxidation of substances in water.
The chlorine dioxide (ClO2) solution is produced on site (in situ) by automated mixing of chemical precursors.
NOTE According to EN 12671, chlorine dioxide is suited for the use of the treatment of water intended for human consumption (drinking water).
Anlagen zur In-Situ-Erzeugung von Bioziden - Chlordioxid, hergestellt aus Natriumchlorit durch Ansäuern oder Oxidation
Dieses Dokument legt Anforderungen an Dosiersysteme zur Chlordioxid-Erzeugung nach dem Chlorit-Chlorgas-Verfahren, dem Chlorit-Säure-Verfahren und dem Chlorit-Natriumperoxodisulfat-Verfahren, die für die Desinfektion und die Oxidation von Substanzen in Wasser verwendet werden, fest.
Die Chlordioxid-(ClO2-)Lösung wird vor Ort (in situ) durch automatisiertes Mischen von chemischen Ausgangsstoffen (Präkursoren) hergestellt.
ANMERKUNG Nach EN 12671 ist Chlordioxid für die Aufbereitung von Wasser für den menschlichen Gebrauch (Trinkwasser) geeignet.
Équipements pour la production in situ de biocides - Dioxyde de chlore produit par acidification ou oxydation de chlorite de sodium
Le présent document contient des exigences relatives aux systèmes de dosage permettant la production de dioxyde de chlore selon le procédé chlorite-chlore gazeux, le procédé chlorite-acide et le procédé chlorite-peroxodisulfate de sodium, qui sont utilisés pour la désinfection et l’oxydation des substances présentes dans l’eau.
La solution de dioxyde de chlore (ClO2) est produite sur place (in situ) par mélange automatisé de précurseurs chimiques. Le présent document s’applique au traitement de l’eau destinée à la consommation humaine, à l’eau de rinçage des filtres des piscines et des bassins de baignade, ainsi qu’à d’autres usages (par exemple, eau de refroidissement, eau de traitement, etc.).
Naprave za proizvodnjo biocidov na kraju samem - Klorov dioksid, proizveden iz natrijevega klorida z acidifikacijo (nakisanjem) ali oksidacijo
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
oSIST prEN 18087:2024
01-september-2024
Naprave za proizvodnjo biocidov na kraju samem - Klorov dioksid, proizveden iz
natrijevega klorida z acidifikacijo (nakisanjem) ali oksidacijo
Devices for in situ generation of biocides - Chlorine dioxide generated from sodium
chlorite by acidification or oxidation
Anlagen zur In-Situ-Erzeugung von Bioziden - Chlordioxid, hergestellt aus Natriumchlorit
durch Ansäuren oder Oxidation
Équipements pour la production in situ de biocides - Dioxyde de chlore produit par
acidification ou oxydation de chlorite de sodium
Ta slovenski standard je istoveten z: prEN 18087
ICS:
13.060.20 Pitna voda Drinking water
71.100.80 Kemikalije za čiščenje vode Chemicals for purification of
water
oSIST prEN 18087:2024 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
oSIST prEN 18087:2024
oSIST prEN 18087:2024
DRAFT
EUROPEAN STANDARD
prEN 18087
NORME EUROPÉENNE
EUROPÄISCHE NORM
June 2024
ICS 71.100.80
English Version
Devices for in situ generation of biocides - Chlorine dioxide
generated from sodium chlorite by acidification or
oxidation
Équipements pour le production in situ de biocides - Anlagen zur In-Situ-Erzeugung von Bioziden -
Dioxyde de chlore produit par acidification ou Chlordioxid, hergestellt aus Natriumchlorit durch
oxydation de chlorite de sodium Säurezugabe oder Oxidation
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee
CEN/TC 164.
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 18087:2024 E
worldwide for CEN national Members.
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prEN 18087:2024 (E)
Contents Page
European foreword . 4
Introduction . 5
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 7
4 Processes for the preparation of chlorine dioxide solutions . 9
4.1 General. 9
4.2 Properties . 9
4.3 Requirements for the generation of chlorine dioxide . 11
4.4 Chlorite-Acid-Process (Chlorine dioxide generated from sodium chlorite by
acidification) . 12
4.4.1 Reaction conditions for the generation of chlorine dioxide. 12
4.4.2 Selection of the system . 12
4.5 Chlorite-chlorine gas process and chlorite-sodium peroxodisulphate process
(chlorine dioxide generated from sodium chlorite by oxidation) . 17
4.5.1 Chlorite-chlorine gas process . 17
4.5.2 Chlorite sodium peroxodisulphate process . 20
4.6 Requirements for the chlorine dioxide dosing point of continuously operating
systems . 28
4.7 Buffer tank . 29
4.7.1 General. 29
4.7.2 Requirements for the buffer tank . 29
4.7.3 Measures against release of gaseous chlorine dioxide . 29
4.7.4 Requirements for the chlorine dioxide dosing device . 30
4.8 Safety bunds . 30
4.9 Prevention against backflow . 31
4.10 Purification process . 31
4.10.1 General. 31
4.10.2 Technical execution . 31
4.10.3 Requirements for purification processes . 32
5 Materials for chlorine dioxide systems . 33
6 Equipment for the housing or area for the installation of the chlorine dioxide system . 33
7 Operation and maintenance . 33
8 Documentation . 34
9 Test requirements . 35
9.1 General. 35
9.2 Scope of testing . 35
9.3 System documentation . 35
9.4 Chemical characterization . 35
9.4.1 General. 35
9.4.2 Sampling . 36
9.4.3 Determination of pH value and temperature . 38
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9.4.4 Chemical characterization of the chlorine dioxide solution generated by the chlorite-
acid and chlorite-chlorine-gas processes. . 38
9.4.5 Chemical characterization of the chlorine dioxide solution prepared by the chlorite-
sodium peroxodisulphate process . 45
9.5 Yield of chlorine dioxide generation . 47
9.5.1 General . 47
9.5.2 Calculation of the yield . 47
Annex A (informative) Decomposition conditions for chlorine dioxide in solution . 48
Annex B (informative) Determination of chlorine dioxide in solutions in the concentration
range 0,05 mg/l to 10 mg/l . 49
Annex C (informative) Determination of chlorine dioxide in solutions within the
concentration range 10-30 mg/l . 51
Bibliography . 53
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European foreword
This document (prEN 18087:2024) has been prepared by Technical Committee CEN/TC 164 “Water
supply”, the secretariat of which is held by AFNOR.
This document is currently submitted to the CEN Enquiry.
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prEN 18087:2024 (E)
Introduction
In respect of potential adverse effects on human and animal health and the environment, caused by the
product covered by this document, it provides no information as to whether the product may be used
without restriction in any of the Member States of the EU or EFTA.
NOTE while awaiting the adoption of verifiable European criteria, attention is also drawn to national
regulations that can apply.
Systems according to this document may be used in different fields of application, e.g. drinking water,
swimming pool water, wastewater, air treatment, surface disinfection, etc. Additional requirements to
this document should be observed, where appropriate for the specific application.
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1 Scope
This document contains requirements for dosing systems for chlorine dioxide generation according to
the chlorite-chlorine gas process, the chlorite-acid process and the chlorite-sodium peroxodisulphate
process, which are used for the disinfection and oxidation of substances in water.
The chlorine dioxide (ClO ) solution is produced on site (in situ) by automated mixing of chemical
precursors. This document applies to the treatment of water for human consumption, rinsing water for
filters for swimming and bathing pools as well as for other uses (e.g. cooling water, process water, etc.
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 901, Chemicals used for treatment of water intended for human consumption — Sodium hypochlorite
EN 937, Chemicals used for treatment of water intended for human consumption — Chlorine
EN 938, Chemicals used for treatment of water intended for human consumption — Sodium chlorite
EN 939, Chemicals used for treatment of water intended for human consumption — Hydrochloric acid
EN 1717, Protection against pollution of potable water in water installations and general requirements of
devices to prevent pollution by backflow
EN 12671, Chemicals used for treatment of water intended for human consumption — Chlorine dioxide
generated in situ
EN 12926, Chemicals used for treatment of water intended for human consumption — Sodium
peroxodisulfate
EN 15363, Chemicals used for treatment of swimming pool water — Chlorine
EN ISO 3696, Water for analytical laboratory use — Specification and test methods (ISO 3696)
EN ISO 10304-4, Water quality — Determination of dissolved anions by liquid chromatography of ions —
Part 4: Determination of chlorate, chloride and chlorite in water with low contamination (ISO 10304-4)
EN ISO 12100, Safety of machinery — General principles for design - Risk assessment and risk reduction
(ISO 12100)
EN IEC 60751, Industrial platinum resistance thermometers and platinum temperature sensors (IEC
60751)
ISO 3165, Sampling of chemical products for industrial use — Safety in sampling
ISO 6206, Chemical products for industrial use — Sampling — Vocabulary
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3 Terms and definitions
For the purposes of this document, the following terms and definitions 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
chlorine dioxide system
ClO system
treatment system in which various chemical precursors are introduced by means of dosing devices in
fixed or controlled proportions into a mixing chamber contained in the system with the intention of
producing chlorine dioxide in situ and making it available for short-term use
3.2
precursor
substance that is fed to the chlorine dioxide system for production of the biocidal active substance
3.3
mixing chamber
component of a chlorine dioxide system in which the precursors are mixed and converted to ClO
3.4
bypass
partial flow to dilute chlorine dioxide solutions to lower concentrations, which can also be achieved by
dilution with feed water
3.5
outgassing
release of chlorine dioxide from its aqueous solution
3.6
purification process
process for purifying the freshly produced chlorine dioxide from impurities from precursors and
synthesis, e.g. via a gas phase transfer
3.7
reaction time
retention time of the precursors after complete mixing in the mixing chamber under operating
conditions to achieve the nominal capacity
3.8
nominal capacity
maximum chlorine dioxide output of a chlorine dioxide system determined under specified conditions
and declared by the manufacturer
3.9
metering device
device or assembly for dosing precursors, including dosing pump or injector, dosing line, mixing device,
injection point and fittings
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3.10
expert
person who, due to their technical scientific training, work experience and knowledge of applicable
standards and regulations, is capable to assess a chlorine dioxide system with regard to functions and
safety
Note 1 to entry: This person can be from the manufacturer or an independent third-party organization (such as a
test institution) without limitations, an inspector according to EN ISO/IEC 17020, Type C [17], fulfills this
criterion.
3.11
chlorine dioxide dosing
controlled process of adding chlorine dioxide solutions
3.12
dosing capacity
flow rate per time or flow rate of the metering device
3.13
feed water
water supplied to the system in accordance with the chemical requirements of the corresponding
manufacturer’s specifications which, in addition to diluting the product or the precursors, can also be
supplied to the reaction or used as motive water
3.14
dilution water
feed water supplied to the system, which serves to dilute the chlorine dioxide produced or the
precursors
3.15
buffer tank
tank for temporary provision of the generated chlorine dioxide solution that is intended for the
application
Note 1 to entry: There can be one or several tanks.
3.16
yield
indication of how completely the reaction shown in the chemical reaction equation proceeds under
practical conditions
Note 1 to entry: The difference between the actual yield and 100 % yield is due to unreacted precursors or side
reaction products that were not represented in the chemical reaction equation
EXAMPLE If 9,5 g of chlorine dioxide are obtained in a reaction batch instead of according to the reaction
equation
5 NaClO + 4 HCl → 4 ClO + 5 NaCl + 2 H O
2 2 2
of the expected 10 g chlorine dioxide, the yield would be 95 %.
3.17
turnover rate
indication of the percentage of molecules of chlorine dioxide that is formed from one molecule of the
starting component according to the chemical reaction equation (i.e. the stoichiometric ratio)
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EXAMPLE The chlorite-acid process is based on the following reaction equation:
5 NaClO + 4 HCl → 4 ClO + 5 NaCl + 2 H O
2 2 2
From 5 molecules of the precursor sodium chlorite, 4 molecules of chlorine dioxide are formed, the turnover rate
is therefore 80 %.
3.18
stability curve
representation of the concentration of chlorine dioxide and by-products at the specified temperature
over time
Note 1 to enty: In systems with buffer tanks, this forms a basis in order to ensure that the active substance
content and the amount of by-products such as chlorate in the generated chlorine dioxide solution in the buffer
tank do not change unacceptably during the time between generation and metering the chlorine dioxide solution
into the application
4 Processes for the preparation of chlorine dioxide solutions
4.1 General
Chlorine dioxide is used in water treatment not only for disinfection but also for pre-oxidation, among
other things, because its high normal potential oxidises phenols, other organic substances and complex-
bound metals without forming chlorinated by-products. EN 12671 shall be applied to the treatment of
water for human consumption.
4.2 Properties
Chlorine dioxide is a yellow-orange substance with a boiling point of 11 °C and a melting point of −59 °C
at atmospheric pressure. Unlike chlorine, for example, it dissolves in water as a gas without dissociation
(see Henry’s law) and therefore tends to outgas strongly. At 25 °C and under atmospheric pressure, the
chlorine dioxide concentration in the aqueous phase is approx. 25 times higher than in the supernatant
gas phase (see Don Gates et al. [3]).
Chlorine dioxide is not stable. When heated, under pressure or under the influence of light, it may
explosively decompose into chlorine and oxygen. Gaseous chlorine dioxide is explosive above a
concentration of 300 g/m . The concentration of chlorine dioxide solutions with excess gas space shall
therefore be kept below the explosion limit (see Figure 1).
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Key
X chlorine dioxide in water
Y1 chlorine dioxide in air (Vol% at 1 013 mbar)
Y2 chlorine dioxide in air (partial pressure)
Y3 chlorine dioxide in air (concentration at 1 013 mbar and 293 K)
A range of unsafe handling
B range of safe handling
Figure 1 — Solubility of chlorine dioxide in water (see Gates et al. [3] and Ishi [12])
In a solution containing 8 g/l chlorine dioxide, there is a risk of the formation of an explosive
atmosphere in the supernatant gas space at a temperature of more than 20 °C.
Depending on the pressure and temperature, liquid chlorine dioxide can precipitate in aqueous
solutions. Pure chlorine dioxide in liquid status explodes above −40 °C with an explosive force
equivalent to about 1/3 that of TNT (2,4,6-trinitrotoluene) (see Sattelberger et al. [9]).
− −
In aqueous solutions, chlorine dioxide decomposes to chlorate (ClO ), (chlorite ClO ) and chloride
3 2
−
(Cl ) depending on temperature, light irradiation, pH-value and impurities. The rate of decay is
essentially determined by the initial concentration of chlorine dioxide. While the chlorine dioxide
concentration in solutions of 20 g/l at room temperature, for example, produced by the hydrochloric
acid process, decreases by half within hours, diluted solutions can remain stable over longer periods of
time (see also Annex A). Concentrated solutions are therefore either diluted to a concentration < 3 g/l
immediately after preparation or dosed immediately into the water to be treated without intermediate
storage. Further information on the decomposition reactions can be found in Annex A.
NOTE Solutions with a chlorine dioxide concentration < 3 g/l according to the CLP Regulation (Regulation
(EC) 1272/2008 amended by Regulation (EU) 2018/1480 of 04. 10. 2018) do not require labelling.
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Chlorine dioxide gas is very toxic and causes extremely life-threatening lung damage when inhaled.
Therefore, when handling chlorine dioxide solutions, care shall be taken to ensure gas tightness. The
occupational exposure limit of chlorine dioxide is 0,28 mg/m (0,1 ppm) and is below the odour
perception limit.
Information on the determination of chlorine dioxide in the respective application concentration is
given in Annex B and Annex C.
4.3 Requirements for the generation of chlorine dioxide
When designing chlorine dioxide systems, it shall be ensured that the precursors are completely mixed
quickly and that the reaction time required for a high yield is maintained. Particularly in systems with
alternating output, the residence time of highly concentrated chlorine dioxide solution in the mixing
chamber shall be restricted in order to prevent the decomposition of the chlorine dioxide.
The quality of the water and the precursors for the production of the chlorine dioxide solutions shall
not have any adverse effects on the production of chlorine dioxide. To this end, the purity criteria of the
following document shall be met: among others, chlorine according to EN 937, sodium chlorite
according to EN 938, sodium hypochlorite according to EN 901, hydrochloric acid according to EN 939
and sodium peroxodisulphate according to EN 12926.
When water is added together with precursors inside the mixing chamber by a metering device in order
to achieve safe chlorine dioxide concentration, it shall be ensured that a sufficient dosage of water is
added to prevent a potentially explosive chlorine dioxide liquid phase. Hence a proved safety system to
monitor the water dosage shall be ensured, and if a risk assessment is performed according
EN ISO 12100, the severity and extend of the harm shall be estimated to the highest foreseeable
severity, e.g. “death of several persons” (see chapter 5.5.2.2. in EN ISO 12100:2010).
Chlorine dioxide is not a stable substance, but decomposes depending on temperature, pH value and the
− −
concentration of the chlorine dioxide solution to form chlorite (ClO ) and chlorate (ClO ):
2 3
− − +
2 ClO + H O → ClO + ClO + 2 H (1)
2 2 2 3
Higher pH values, exposure to light, heat and even traces of impurities reduce the stability of the
chlorine dioxide solution produced. Further information on the decomposition of chlorine dioxide to
chlorate is given in Annex A. When selecting the production process and the conditions for dosing the
chlorine dioxide or making it available for dosing, the shelf life shall be taken into account, including the
application-specific requirements. To achieve the highest possible yield with minimum by-product
formation at the same time, the following conditions shall be fulfilled:
— optimized reaction time (depending on temperature, reaction process, precursors used);
— the highest possible reaction concentration (depending on the production process);
— rapid and complete mixing of the precursors;
— immediate consumption in the case of direct dosing or immediate rapid dilution of the highly
concentrated mixing chamber effluent for more stable provision in the buffer tank.
The following processes are used for the in situ production of chlorine dioxide:
1) chlorite-acid process (chlorine dioxide generated from sodium chlorite by acidification);
2) chlorite chlorine gas process and chlorite sodium peroxodisulphate process (chlorine dioxide
generated from sodium chlorite by oxidation).
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4.4 Chlorite-Acid-Process (Chlorine dioxide generated from sodium chlorite by
acidification)
4.4.1 Reaction conditions for the generation of chlorine dioxide
The chlorite acid process is based on the reaction of sodium chlorite solutions according to EN 938 with
hydrochloric acid according to EN 939 in excess.
+ 4 HCl → 4 ClO + 5 NaCl + 2 H O (2)
Reaction equation: 5 NaClO
2 2 2
Above a concentration of e.g. c ≥ 8 g/l at 20 °C, an explosive atmosphere of more than 300 g/Nm
chlorine dioxide develops in the gas space above the solution. No gas cushion may form in the mixing
chamber. Immediately after preparation, the solution is usually diluted to a safe concentration of max.
3 g/l or added directly to the water to be treated without intermediate storage.
NOTE 1 In practice, a threefold molar amount of hydrochloric acid (related to the chlorite in reaction
Formula 1a) has proven to be effective, e.g. by using commercial 9 % hydrochloric acid and 7,5 % sodium chlorite
solution in a mixing ratio of 1 : 1.
NOTE 2 Other strong mineral acids such as sulphuric acid can also be used. However, a lower yield and
increased chlorate formation is expected with these acids.
A common chlorine dioxide concentration in the mixing chamber is 20 g/l. Higher concentrations are
possible, provided that a suitable combination of pressure and temperature ensures that no pure liquid,
highly explosive chlorine dioxide can separate in the aqueous solution (see also 4.2). In particular, when
using concentrated precursors such as e.g. 25 % to 36 % hydrochloric acid and 25 % to 31 % sodium
chlorite, safe conditions for the chlorine dioxide concentration in the mixing chamber shall be ensured.
A functional safety required for this shall be achieved e.g. by an intrinsically safe pre-dilution of
concentrated precursors.
The temperature for preparing a solution of, for example, 20 g/l is at least 10 °C and should not exceed
35 °C. The reaction time is strongly temperature-dependent and decreases with increasing
temperature. An increase in temperature leads to increased formation of reaction by-products,
therefore low temperatures are preferable. The reaction time should be at least at the following
temperatures in the mixing chamber:
— 10 °C to 15 °C: 15 minutes to 10 minutes;
— 15 °C to 25 °C: 10 minutes to 7 minutes;
— 25 °C to 35 °C: 7 minutes to 3 minutes.
NOTE 3 Systems working with other concentrations might have different reaction times at different
temperature ranges and different maximum temperature limits.
4.4.2 Selection of the system
4.4.2.1 General
A distinction is made between systems:
a) with diluted or concentrated precursors;
b) with continuous or discontinuous operation;
c) with direct dosing of the prepared solution and systems with a buffer tank.
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The precursors shall meet the purity criteria according to EN 938 for sodium chlorite (NaClO ) and
EN 939 for hydrochloric acid (HCl). Systems working with diluted chemicals typically use 7,5 % sodium
chlorite solution and 9 % hydrochloric acid. As higher safety requirements have to be observed when
handling concentrated precursors, systems operating with these are mainly used for higher chlorine
dioxide production rates. Typical concentrations are 25 % to 31 % for sodium chlorite solution and
25 % to 36 % for hydrochloric acid.
Continuously operating systems shall be operated in the range between 20 % and 100 % of their
nominal capacity in order to minimize excessive retention time and the associated decomposition of the
chlorine dioxide to the by-product chlorate. Information on the decomposition of chlorine dioxide to
chlorate is given in Annex A. These limits are referred to the nominal capacity of the working mixing
chamber. However, a system may work with wider range if equipped with selectable mixing chambers
of different or variable capacity. For the same reason, interruptions in production during intended
operation shall be kept as short as possible. In the case of interruptions, it shall be assessed for each
individual case whether the lower chlorine dioxide concentration and the higher concentration of by-
products (chlorate) can be tolerated in the corresponding application or whether the aged solution in
the mixing chamber shall be discarded before the chlorine dioxide is used. Continuously operating
systems shall be designed according to the maximum demand for chlorine dioxide prevailing in the
specific application.
Discontinuously operating systems are designed for production interruptions of a few hours to a few
days in the intended use and operate with a buffer tank in which a chlorine dioxide solution of lower
concentration and thus longer storability is kept. Discontinuously operating systems can be designed
according to the average demand for chlorine dioxide prevailing in the application, provided that the
buffer tank and the dosing capacity are designed sufficiently to be able to serve peak loads.
Buffer tanks can also be filled with a continuously operating system, but care shall be taken in the
design that sufficient chlorine dioxide solution is taken from the buffer tank to minimize interruptions
in the operation of the chlorine dioxide production.
4.4.2.2 Continuously operating systems
4.4.2.2.1 General
In these systems, the precursors are continuously fed into the mixing chamber and the chlorine dioxide
produced is diluted in a water stream outside the mixing chamber to a safely manageable concentration
according to Figure 1. A typical structure of such systems is shown in Figure 2, whereas the mixing
chamber 3 may also be positioned directly in the dilution water stream or in the water to be treated.
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Key
1 precursor metering device
2 dilution water
3 mixing chamber
5 ClO dosing point or buffer tank
Figure 2 — Typical block diagram of a chlorine dioxide system operating continuously according
to the chlorite-hydrochloric acid process
4.4.2.2.2 Requirements for the precursor metering device
The precursor metering device shall meet the following requirements:
a) precise and reproducible dosing performance: the addition of precursors shall be adjustable in such
a way that precise chemical dosing takes place. Automatic restart after interruptions of chlorine
dioxide production shall not change the dosing. The system manufacturer shall specify a
maintenance interval within which the dosing rate is to be checked and readjusted;
b) monitoring of chemical addition: it shall be ensured that the precursors are fed to the mixing
chamber in the required quantity at all times. For example, in the case of diaphragm dosing pumps
that can be calibrated, this can be done by single-stroke monitoring; in the case of other pumps or
injector systems, it can be done with flow or quantity monitoring. Deviations from the set flow rate
shall be recognized as a fault condition within the measuring accuracy of the monitoring device and
lead to the system being switched off;
c) interlocking with the dilution water: dosing of the precursors may only take place if a sufficient
quantity of dilution water is available at the same time. This shall be interlocked with a flow rate
monitoring of the dilution water.
4.4.2.2.3 Requirements for dilution
The following requirements shall be met when adding dilution water:
a) purity: the dilution water shall be largely free of particles and shall be at least of drinking water
quality if the use is in the field of drinking water disinfection. Impurities can react with chlorine
dioxide and reduce both yield and purity;
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b) protection against uncontrolled venturi effect: the dilution water line shall be at such a pressure to
prevent uncontrolled suction of precursors through the mixing chamber. Suitable pressure control
shall be put in operation to avoid this condition. If such a condition is possible due to peripheral
hydraulic conditions, a vacuum breaker may be installed in the dilution water line, for example;
c) protection against running dry: the dilution water line shall always be completely filled with water
to prevent the formation of explosive gas phases in the mixing chamber.
4.4.2.2.4 Requirements for the mixing chamber
The mixing chamber shall meet the following requirements:
a) mixing and reaction time: the reaction flow shall be hydraulically designed in such a way that the
precursors mix quickly and completely, and their homogeneous mixture is then achieved as far as
possible by uniform flow with the required reaction time (compare 4.4.1) through the container;
b) backflow prevention: to prevent the acidic chlorine dioxide solution from returning into the
precursors’ feed system, a backflow prevention device shall be provided;
c) pressure: the mixing chamber can be operated under pressure and under negative pressure. In
both modes of operation, the formation of gas phases shall be precluded, if there is a gas phase in
the mixing chamber in accordance with Figure 1 dangerous chlorine dioxide concentration can
form. When operating under pressure, the chlorine dioxide concentration, pressure and
temperature shall be coordinated in such a way that the formation of highly explosive liquid
chlorine dioxide is excluded.
4.4.2.3 Discontinuously operating systems
4.4.2.3.1 General
In discontinuously operating systems, a specific quantity of precursors is mixed in the mixing chamber.
After complete conversion to a safely manageable concentration according to Figure 1 it is diluted and
held in the buffer tank (see Figure 3).
oSIST prEN 18087:2024
prEN 18087:2024 (E)
Key
1 precursor metering device
2 feed water
3 mixing chamber
4 buffer tank
5 dosing device chlorine dioxide
6 device for preventing the escape of chlorine dioxide into ambient air
Figure 3 — Typical block diagram of a chlorine dioxide system operating discontinuously
according to the chlorite-hydrochloric acid process
4.4.2.3.2 Requirements for the precursor metering device
The precursor metering device shall meet the following requirements:
a) precise and reproducible dosing performance: the addition of the precursors shall be adjustable in
such a way that an exact chemical dosing takes place. Automatic restart after interruptions of
chlorine dioxide production shall not change the dosing. The system manufacturer shall specify a
maintenance interval within which the dosing rate is to be checked and readjusted;
b) monitoring of chemical addition: it shall be ensured that the precursors are added to the mixing
chamber in the required quantity. For example, in the case of diaphragm dosing pumps that can be
calibrated, this can be done by means of a single-stroke counting system; in the case of other pumps
or injector systems, it can be done by means of a flow and quantity monitoring system. Deviations
from the target addition quantity shall be detected as a fault condition within the scope of the
measuring accuracy of the monitoring device and lead to the system being switched off.
4.4.2.3.3 Requirements for the mixing chamber
The mixing chamber shall fulfil the following requirements:
a) mixing and reaction time: the reaction chamber shall be hydraulically designed in such a way that
the precursors mix quickly and completely and that the reaction mixture is immediately diluted
with feed water to a safe concentration according to Figure 1 after the required reaction time
(compare 4.4.1). In the case of installations with an unsafe reaction concentration according to
Figure 1, the reaction shall take place in a room without gas phase;
oSIST prEN 18087:2024
prEN 18087:2024 (E)
b) backflow prevention: in order to prevent the acidic chlorine dioxide solution from getting back into
the precursors supply, a backflow prevention device shall be provided;
c) pressure: the mixing chamber is operated without pressure and, together with the buffer tank, is
open to the atmosphere via the device to prevent the escape of chlorine dioxide into the ambient
air.
4.4.2.3.4 Requirements for the feed water
The following requirements shall be met when adding feed water:
a) purity: the feed water shall be largely free of particles and shall be at least of drinking water quality
if the use is in the field of drinking water disinfection. Impurities can react with chlorine dioxide
and reduce both yield and purity;
b) protection against backflow: the feed water line shall be protected against backflow from the
mixing chamber;
c) feed water used as dilution water: if the correct quantity of dilution water is necessary for safe
operation, control of the dilution water dosed into the mixing chamber and/or into the buffer tank
is essential and the system shall isolate the complete chlorine dioxide generation process in case of
a failure situation.
4.5 Chlorite-chlorine gas process and chlorite-sodium peroxodisulphate process
(chlorine dioxide generated from sodium chlorite by oxidation)
4.5.1 Chlorite-chlorine gas process
4.5.1.1 General
The chlorite-chlorine gas process describes the use of the reaction of aqueous solutions of sodium
chlorite with chlorine gas (Cl ) Formula (3) or hypochlorous acid (HOCl) (Formula (3a) and
Formula (3b)) on the one hand and sodium chlorite (NaClO ) with sodium hypochlorite (NaOCl) and
hydrochloric acid (HCl) on the other Formula (3c).
Reaction equations:
2 NaClO + Cl → 2 ClO + 2 NaCl (3)
2 2 2
Cl + H O ↔ HOCl + HCl (3a)
2 2
2 NaClO + HOCl + HCl → 2 ClO + 2 NaCl + H O (3b)
2 2 2
2 NaClO + NaOCl + 2 HCl → 2 ClO + 3 NaCl + H O (3c)
2 2 2
The chlorine solution shall be prepared from chlorine gas according to EN 937 and EN 15363. The
sodium chlorite solution according to EN 938 is fed from the precursor tank, e.g. with a dosing pump or
with a vacuum system, and fed to the mixing chamber at the same time as the chlorine solution (see
Figure 4).
oSIST prEN 18087:2024
prEN 18087:2024 (E)
Key
1a chlorine gas
1b mixing and metering device hypochlorous acid
1c metering device NaClO
2 feed water
3 mixing chamber
4 buffer tank
5 dosing device chlorine dioxide
6 device for preventing the escape of chlorine dioxide into ambient air
Figure 4 — Typical block diagram of a chlorine dioxide system operating according to the
chlorite-chlorine gas process
Alternatively, the chlorine solution can be prepared by acidifying sodium hypochlorite solution with
hydrochloric acid (see reaction Formula (3 b)).
Sodium hypochlorite solution is subject to decomposition due to storage. This shall be taken into
account in order to maintain the quantity ratios of the precursors used. Furthermore, when using this
process, special attention shall be paid to the chlorate content in the produced dosing solution and in
the treated water. Further information on the decomposition of chlorine dioxide to chlorate is given in
Annex A.
The pH value in the reaction mixture shall be below pH < 3,5. (Gordon et al. [1]) This can be achieved by
a chlorine concentration of at least 2 g/l Cl of the prepared chlorine solution and a 1,3-fold
stoichiometric chlorine quantity related to reaction Formula (2) (2 NaClO + 1,3 Cl → 2 ClO +
2 2 2
2 NaCl + 0,3 Cl ). Depending on the use, the excess chlorine can be adjusted between 0 % and 300 %.
For reaction temperatures between 10 °C and 30 °C, a reaction time of at least 4 minutes shall be
specified (Gordon et al. [1]).
The concentration of chlorine dioxide after completion of the reaction is 5 g/l to 8 g/l, the pH value is in
the range of 2 to 3.
In order to achieve a constant product concentration, the chlorine dioxide solution produced is made
available at a concentration between 1 g/l and 3 g/l chlorine dioxide in a buffer tank after leaving the
mixing chamber and adding dilution water. This ensures a constant back pressure for the chlorine gas
injector and for the dosing device of the sodium chlorite solution. The buffer tank allows multiple
oSIST prEN 18087:2024
prEN 18087:2024 (E)
dosing points to be supplied from a single preparation unit and allows dosing at very high back
pressures above the maximum allowable pressure for the preparation unit.
NOTE There are systems that are suitable for single point applications without a buffer tank. These include
processes in which undiluted starting materials are reacted mixed by means of an injector in the negative
pressure range and immediately diluted to a concentration of less than 3 g/l.
4.5.1.2 Requirements for the precursor metering device
The precursor metering device shall meet the following requirements:
a) precise and reproducible dosing performance: the addition of the precursors shall be adjustable so
that precise chemical dosing takes place. Automatic restart after interruptions of chlorine dioxide
production shall not change the dosing. The system manufacturer shall specify a maintenance
interval within which the dosing rate is to be checked and readjusted;
b) monitoring of chemical addition: it shall be ensured that the precursors are supplied to the mixing
chamber in the required quantity at all times. For the chlorine gas supply, an automatic switchover
from empty to full containers shall be ensured. To maintain a constant chlorine addition, constant
operating conditions of the injector dosing system shall be ensured. In the case of precursors in
liquid
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