ISO 16075-5:2021

INTERNATIONAL ISO
STANDARD 16075-5
First edition
2021-06
Guidelines for treated wastewater use
for irrigation projects —
Part 5:
Treated wastewater disinfection and
equivalent treatments
Lignes directrices pour l'utilisation des eaux usées traitées dans les
projets d'irrigation —
Partie 5: Désinfection des eaux usées traitées et traitements
équivalents
Reference number
©
ISO 2021
© ISO 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved

Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, and abbreviated terms . 1
3.1 Terms and definitions . 1
3.2 Abbreviated terms . 3
4 Wastewater pathogenic contaminants and their inactivation or removal .4
4.1 General . 4
4.2 Type and occurrence of pathogens in wastewater . 4
4.3 Reduction of pathogenic microorganisms in various stages of wastewater treatment . 5
4.4 Reduction of pathogenic microorganisms by different disinfection methods . 6
5 Disinfection . 7
6 Chemical disinfection . 8
6.1 General . 8
6.2 Disinfection by chlorine/bromine compounds . 8
6.2.1 General. 8
6.2.2 Reactions of chlorine/bromine with ammonia . 9
6.2.3 Definition of the halogenated disinfection residuals .10
6.2.4 Breakpoint reaction.10
6.2.5 CT values of chlorine/bromide and their compounds .12
6.2.6 Chlorinated compounds for TWW disinfection .12
6.2.7 Advantages, disadvantages and technical considerations of chlorine
biocides-based disinfection method .13
6.2.8 Chlorination process .15
6.2.9 Brominated compounds for TWW disinfection .15
6.2.10 Advantages, disadvantages and technical considerations of brominated
biocides-based disinfection method .17
6.3 Ozone .18
6.3.1 Chemistry of ozone disinfection .18
6.3.2 Direct ozone reaction .18
6.3.3 Indirect ozone reaction .19
6.3.4 Advantages, disadvantages and technical considerations of Ozone
disinfection method .20
6.3.5 System configuration .20
6.3.6 Monitoring of ozonation .21
6.4 Environmental impacts of chemical disinfection .21
6.4.1 Environmental impacts of chlorination/bromination disinfection .21
6.4.2 Environmental impacts of ozonation disinfection .22
7 UV disinfection .22
7.1 General .22
7.2 UV light technologies and how they work .23
7.2.1 General.23
7.2.2 UV disinfection system components .23
7.3 UV source .24
7.3.1 General.24
7.3.2 UV source protector .25
7.4 Disinfection chamber .25
7.5 Sensors .25
7.5.1 UV intensity sensors .25
7.5.2 UV transmittance sensors .26
7.6 Ballasts .27
7.7 UV validation .27
7.8 The effectiveness of a UV disinfection system .29
7.9 Cleaning .29
7.10 Environmental impacts of UV disinfection .29
7.11 Advantages, disadvantages and technical considerations of UV disinfection method .30
8 Removal of pathogens by membrane methods .30
8.1 General .30
8.2 Membrane system .30
8.3 Pathogen removal by membrane filtration .31
8.4 Considerations for operation and maintenance .31
8.5 Monitoring .31
8.6 Environmental impacts of membrane systems.32
8.7 Advantages, disadvantages and technical considerations of pathogens removal by
membrane systems disinfection method .32
Annex A (informative) Infection agents potentially present in untreated (raw) wastewater .33
Annex B (Informative) Microbial removal performance by various membrane filtration .35
Annex C (Informative) Bromine further compounds .36
Annex D (informative) Factors in operation, maintenance and monitoring of membrane system .37
Bibliography .40
iv © ISO 2021 – All rights reserved

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 282, Water reuse, Subcommittee SC 1,
Treated wastewater reuse for irrigation.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
Introduction
Disinfection of treated wastewater (TWW) is a critical phase in the process of TWW use. Its purpose is
to reduce or eliminate major health risks to the wastewater treatment plant's operators and to anybody
who may come in contact with TWW or with crops that were irrigated with TWW.
This document provides a guideline for the available methods of disinfection, their effectiveness and the
factors impacting those methods, along with their advantages and disadvantages, regarding technical
and environmental aspects and effective inactivation or removal of various pathogens in wastewater
and TWW for use in irrigation.
vi © ISO 2021 – All rights reserved

INTERNATIONAL STANDARD ISO 16075-5:2021(E)
Guidelines for treated wastewater use for irrigation
projects —
Part 5:
Treated wastewater disinfection and equivalent
treatments
1 Scope
This document provides a guideline for the application of various available methods of treated
wastewater (TWW) disinfection for an effective inactivation or removal of pathogens from TWW,
which is intended for irrigation purposes.
This document deals with:
— chemical and physical technologies, principles of operation, and establishment of effective doses to
be applied, possible interferences, and technical guidance for design and monitoring;
— comparison of the advantages and disadvantages of various disinfection methods suitable for TWW;
— potential environmental effects of the disinfection methodologies and ways to minimize those
impacts;
— disinfection at different locations in the TWW use system, including in the wastewater treatment
plant, within the distribution system and at the point of use.
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.
ISO 20670, Water reuse — Vocabulary
3 Terms, definitions, and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 20670 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1.1
advanced oxidation process
AOP
process that generates hydroxyl radicals in sufficient quantity to remove organics by oxidation
3.1.2
ballast
unit inserted between the supply and one or more discharge lamps, which by means of inductance,
capacitance, or a combination of inductance and capacitance, serves mainly to limit the current of the
lamp(s) to the required value so as to convert and regulate incoming power to UV lamps to produce UV
light
Note 1 to entry: The ballast provides the proper voltage and current required to initiate and generate UV photons.
3.1.3
fouling
process leading to deterioration of membrane flux due to surface or internal blockage of the membrane
[1]
Note 1 to entry: See AWWA B130-13 .
3.1.4
pore size
size of the opening in a porous membrane
Note 1 to entry: Pore sizes are expressed either as nominal (average) or absolute (maximum), typically in terms
of μm.
[1]
Note 2 to entry: See in AWWA B130-13 .
3.1.5
reduction equivalent dose
RED
dose of UV in a given device which is determined by biodosimetry
Note 1 to entry: See UV dose (3.1.9) and “biodosimetry”
Note 2 to entry: This UV dose (3.1.9) is determined by measuring the inactivation of a challenge microorganism
after exposure to UV light in a UV unit and comparing the results to the known UV dose response curve of the
same challenge organism determined via Bench scale collimated beam testing.
3.1.6
ultrafiltration
UF
Note 1 to entry: pressure driven process employing semipermeable membrane under hydraulic pressure gradient
for the separation components in a solution
Note 2 to entry: The pores of the membrane are of a size smaller than 0.1μm, which allows passage of the
solvent(s) but will retain non-ionic solutes based primarily on physical size, not chemical potential.
Note 3 to entry: See in ASTM D6161-10.
3.1.7
UV disinfection system
combination of UV disinfection units (3.1.8) with associated controls and instrumentation
3.1.8
UV disinfection unit
independent combination of single or multiple bank(s) in series with a common mode of failure (e.g.,
electrical, cooling, cleaning system, etc.)
3.1.9
UV dose
UV fluence
amount of UV energy given as the time integral of the fluence rate or irradiance (W/m )
2 2
Note 1 to entry: This is given in units of mJ/cm or J/m
2 © ISO 2021 – All rights reserved

3.1.10
UV intensity sensor
UV irradiance meter or radiometer instrument to measure UV irradiance
3.1.11
UV transmittance
fraction of photons in the UV spectrum transmitted through a material such as water or quartz
Note 1 to entry: It is preferable that an online UVT sensor be installed and used to verify UVT.
Note 2 to entry: The wavelength of the UVT (%) should be specified, often using a path length of 1 cm. The
measurement is calibrated compared to ultra pure water (ISO 3696 grade 1 or equivalent).
Note 3 to entry: UVT is related to the UV absorbance (A) by the following formula (for a 1 cm path length): % UVT
-A
= 100 × 10 .
3.2 Abbreviated terms
A254 absorbance at 254
CT product of the total residual chlorine and contact time
DBP disinfection by-products
EPA Environmental protection agency
DOC dissolved organic carbon
DVGM German Technical and Scientific Association for Gas and Water (deutsher veriein des gas-
und wasserfaches e.v.)
LP low pressure
LPHO low pressure high output
LRV log removal value
MF microfiltration
MP medium pressure
MWCO molecular weight cut off
NOM natural organic matter
ONORM Austrian Standard (Österreichisches Normungsinstitut)
QA/QC quality assurance/quality control
RED reduction equivalent dose
RO reverse osmosis
TDS total dissolved solids
THM trihalomethanes
TMP trans membrane pressure
TOC total organic carbon
TWW treated wastewater
UF ultra-filtration
UV ultraviolet
UVT ultraviolet transmittance
WW wastewater
4 Wastewater pathogenic contaminants and their inactivation or removal
4.1 General
The most critical objective in a TWW reuse programme should be public health.
To achieve the main objective, other equally important objectives should be considered, including:
— environmental protection,
— aesthetics (odour and colour); and
— ability to meet irrigation requirements.
To protect public health and prevent environmental degradation, the TWW quality characteristics and
pathogenic microorganisms contained in the wastewater should be assessed and consideration given
to appropriate treatment to reduce the risk of negative impacts.
There are a wide range of technology options available to meet the water quality goals and to reduce
the risk of disease transmission from pathogenic microorganisms that can be present in TWW and to
meet the water quality goals.
In regular wastewater treatment plants, the two main processes that reduce the concentrations of
pathogenic microorganisms in the water should be:
— the wastewater treatment process itself, which is intended mainly to reduce concentrations of
suspended and dissolved organic matter;
— the process of disinfection of the TWW.
4.2 Type and occurrence of pathogens in wastewater
Urban wastewater intended for agricultural irrigation or for other purposes contains a variety of
pathogenic microbial contaminants that can pose a risk to public health.
The type and number of pathogenic microorganisms in urban wastewater varies between countries
and cities and with time/season (wet and dry), epidemics etc. When selecting disinfection method(s) the
range of microorganisms that can be present should be considered, including parasites eggs, bacteria,
amoebas and other protozoa, Giardia and viruses. Common infectious agents, associated diseases, and
[2]
potential numbers of microorganisms found in domestic wastewater are shown in Table 1 (for the
complete table see Table A.1).
[2]
Table 1 — Infectious agents potentially present in untreated (raw) wastewater
Numbers in
Pathogen Disease raw wastewa-
ter (per litre)
Shigella` Shigellosis (bacillary dysentery) Up to 10
4 © ISO 2021 – All rights reserved

Table 1 (continued)
Numbers in
Pathogen Disease raw wastewa-
ter (per litre)
Salmonellosis, gastroenteritis (diarrhoea, vomiting, fever), reactive
Salmonella Up to 10
arthritis, typhoid fever
Vibro cholera Cholera Up to 10
Campylobacter Gastroenteritis, reactive arthritis, Guillain-Barré syndrome Up to 10
Enteroviruses (polio,
echo, coxsackie, new Gastroenteritis, heart anomalies, meningitis, respiratory illness, nerv-
Up to 10
enteroviruses, sero- ous disorders, others
type 68 to 71)
Respiratory disease, eye infections, gastroenteritis (serotype 40 and
Adenovirus Up to 10
41)
Rotavirus Gastroenteritis Up to 10
Entamoeba Amebiasis (amebic dysentery) Up to 10
Giardia Giardiasis (gastroenteritis) Up to 10
Cryptosporidium Cryptosporidiosis, diarrhoea, fever Up to 10
Ascaris Ascariasis (roundworm infection) Up to 10
Ancylostoma Ancylostomiasis (hookworm infection) Up to 10
Trichuris Trichuriasis (whipworm infection) Up to 10
The practical measurement of all pathogenic pollutants in TWW is almost impossible.
The main reasons are:
— low concentrations of the pathogenic contaminants in the TWW;
— limitation of present technology, to detect pathogens when they are present in low numbers;
— testing for pathogenic contaminants in the laboratory is lengthy and expensive.
Consequently, the control and monitoring of pathogenic microorganisms should be done by testing for
indicator microorganisms, which are feasible and simple to measure as a result of their much larger
numbers, and based on the premise that factors and treatment affecting their removal similarly affect
the pathogens of interest.
4.3 covers the effect of the first process (WW treatment and the reduction of the concentration of
contaminants). 4.4 covers the effects of the disinfection of the TWW.
4.3 Reduction of pathogenic microorganisms in various stages of wastewater treatment
Although wastewater treatment is mainly intended to eliminate suspended and dissolved organic
matter, independent of disinfection, the treatment process can reduce the number of pathogenic and
indicator microorganisms present in the wastewater. The degree of removal can depend (in part) on
[2]
the type of treatment process, as illustrated in Table 2 .
Table 2 — Indicative log removals of indicator microorganisms and enteric pathogens during
[2]
various stages of wastewater treatment
Indicator microorganisms Pathogenic microorganisms
Escheri- Phage
Clostrid- Enteric bacteria Crypto-
chia coli (indi- Enteric Giardia Hel-
ium per- (e.g., Campylo- sporidium
(indicator cator viruses lamblia minths
fringens bacter) parvum
bacteria) virus)
Bacteria X X X
Protozoa
and hel- X X X
minths
Viruses X X
Indicative log reductions in various stages of wastewater treatment
Secondary 0,5 to
1 to 3 0,5 to 1 0,5 to 2,5 1 to 3 0,5 to 2 0,5 to 1 0 to 2
treatment 1,5
Dual media
0 to1 0 to 1 1 to 4 0 to 1 0,5 to 3 1 to 3 1,5 to 2,5 2 to 3
a
filtration
Reservoir
1 to 5 N/A 1 to 4 1 to 5 1 to 4 3 to 4 1 to 3,5 1,5 to >3
storage
Key
N/A not available
NOTE 1 Reduction rates depend on specific operating conditions, such as retention time, contact time and concentrations
of chemicals used, pore size, filter depths, pretreatment, and other factors. Ranges given should not be used as design or
regulatory bases—they are meant to show relative comparisons only.
NOTE 2 See Table 3.
a
Including coagulation.
As the reduction presented in the table for each type of treatment is only indicative, the exact values
of pathogen reduction should be determined for each situation taking into account both the type of
treatment and the environmental and operating conditions such as temperature, organic matter,
turbidity, pH, ammonia, alkalinity, of each system.
4.4 Reduction of pathogenic microorganisms by different disinfection methods
The purpose of disinfecting TWW should be to remove or inactivate pathogenic microorganisms
that remain in the TWW at the end of the standard treatment process. As complete inactivation is
not always feasible or involves investment in methods which could make the required treatment
unpractical, pathogenic microorganisms should be brought to low levels that will not cause significant
health damage when the wastewater is used for irrigation. Reduction of pathogenic microorganism
concentrations may be integrated with additional control strategies that can prevent health impact
such as setting limitations to irrigation with TWW based on the quality achieved.
The practical measurement of all pathogenic microorganisms in TWW is almost impossible for reasons,
and indicator microorganisms are used (see 4.2).
The reduction of indicator and pathogenic microorganisms in TWW by different disinfection methods
[2]
is indicated in Table 3 .
6 © ISO 2021 – All rights reserved

Table 3 — Indicative log reductions of indicator microorganisms and enteric pathogens by
[2]
various methods of disinfecting TWW
Indicator microorganisms Pathogenic microorganisms
Enteric
Crypto-
Escherichia Phage bacteria
Clostridium Enteric Giardia sporidi-
coli (indica- (indicator (e.g., Cam- Helminths
perfringens viruses lamblia um par-
tor bacteria) virus) pylobac-
vum
ter)
Bacteria X X X
Protozoa and
X X X
helminths
Viruses X X
a
Indicative log reductions by various disinfection methods
Membrane
filtration (UF, 4 > 6 > 6 2 > 6 > 6 2 > 6 > 6 4 > 6 > 6
b
NF, and RO)
Ozonation 2 to 6 0 to 0,5 2 to 6 2 to 6 3 to 6 2 to 4 1 to 2 N/A
UV disinfec-
2 > 6 N/A 3 > 6 2 > 6 1 > 6 3 > 6 3 > 6 N/A
tion
Advanced
> 6 N/A > 6 > 6 > 6 > 6 > 6 N/A
oxidation
0,5 to
Chlorination 2 > 6 1 to 2 0 to 2,5 2 >6 1 to 3 0 to 0,5 0 to 1
1,5
Key
N/A not available
a
Reduction rates depend on specific operating conditions, such as retention times, contact times and concentrations
of chemicals used, pore size, filter depths, pretreatment, and other factors. Ranges given should not be used as design or
regulatory bases—they are meant to show relative comparisons only.
b
Removal rates vary dramatically depending on the installation and maintenance of the membranes.
5 Disinfection
A partial removal of microorganisms may be obtained in various treatment stages, while disinfection is
the main process for microorganisms’ inactivation or removal from the TWW.
A TWW reuse for irrigation scheme should include disinfection to reduce pathogenic microorganisms;
it is one of the main barriers, compulsory for some uses and an option for others.
NOTE the process of disinfection reduces the number of microorganisms to the analytical detection limit
but does not eliminate them. Complete destruction can only be done by the process of sterilization.
Disinfection of TWW may be achieved with the use of a variety of methods presented in Clauses 6 to 8,
including:
— chemical disinfection,
— ultraviolet light, and
— membrane filtration.
The action of disinfectants on microorganisms is a result of various mechanisms occurring
[3]
simultaneously or separately :
— changes in DNA structure that thwart reproduction and thus infectivity,
— damage to cell wall,
— alteration of cell permeability,
— alteration of the colloidal nature of the protoplasm, and
— inhibition of enzyme activity.
An exception is the use of membrane methods; in this case, none of the mechanisms detailed in the
previous bullets work. In this case the membrane separates the microorganisms from the TWW and
concentrates in the reject brine.
When applied, the chemicals used as disinfectants should be available in large quantities, reasonably
priced, noncorrosive and non-staining. However, equipment used in chemical disinfectant application
should be resistant and/or adapted to these chemicals – when concentrated or diluted.
The scheme may also include some types of disinfection as a means to prevent the formation of
biofouling in the water distribution pipelines and irrigation equipment.
6 Chemical disinfection
6.1 General
[4]
The function of disinfectants in the use of TWW for irrigation is well described in ISO 16075-2 .The
use of halogenated chemicals in water transportation and distribution systems has the purpose of
[5]
protecting irrigation systems from biofouling which means slime growing in pipes and tubes .
Various chemicals may be used for disinfection of TWW. The two most common are halogenated
oxidizing chemicals (chlorine and bromine and its compounds) and ozone, as described in 6.2 and 6.3.
[3]
TWW chemical disinfectant should have the following characteristics :
— effective with minimum alteration of the water characteristics such as increasing the total dissolved
solids (TDS) or changing the pH,
— uniform in composition and low loss of germicidal action during storage or idle periods,
— soluble in water and pass through cell tissues,
— having a capacity to penetrate through particle surfaces,
— unadsorbable by organic matter other than bacterial cells,
— nontoxic to humans and other animals and safe to transport, store, handle, and use,
— toxic to target microorganisms and effective at high dilutions in the ambient temperature range.
6.2 Disinfection by chlorine/bromine compounds
6.2.1 General
The disinfection of water and TWW by halogenated based chemicals is extensively used and is
considered to be an effective as well as an easy to use method for disinfection. The addition of these
types of chemicals into water results with 2 reactions: hydrolysis forming hypohalous acid Formula 1
[6]
and dissociation to hypohalite ion Formula 2, while X= Cl, Br.
+ -
X + H O → HOX + H + X (1)
2 2
8 © ISO 2021 – All rights reserved

+ -
HOX ↔ H + OX (2)
Dissociation of the hypohalous acids is a function of pH, as shown in Table 4 and Figure 1. Hypohalite
[6]
anions are up to 2 orders of magnitude less effective biocides than the acid form .
[6]
Table 4 — Distribution between hypohalous acid and hypohalite anions as a function of pH
Chlorine Bromine
pH
- -
% HOCl % OCl % HOBr % OBr
7,5 50 50 94 6
8,0 24 76 83 17
8,5 9 91 60 40
9,0 3 97 33 67
The dissociation curve of the hypohalous acids is given in Figure 1:
Key
X pH
Y % of active acid
1 HOCl
2 HOBr
Figure 1 — Dissociation curves of the hypohalous acids
6.2.2 Reactions of chlorine/bromine with ammonia
Untreated WW and TWW contain different concentrations of ammonia that cause difficulties in water
disinfection. The reaction of the ammonia with the added chlorine or bromine creates compounds with
a much lower disinfectant ability than free chlorine or bromine.
[3]
The possible reactions between HOX and ammonia are presented in Formula 3 to Formula 5, while
X=Cl or Br.
NH + HOX  H O + NH X → (Monochloramine/bromamine) (3)
3 2 2
NH + 2HOX   2H O + NHX → (Dichloramine/bromamine) (4)
3 2 2
NH + 3HOX   3H O + NX → (Nitrogen Trichloride/Tribromide) (5)
3 2 3
These reactions occur essentially instantaneously and are pH dependent. For instance, for chlorine
compounds at pH levels above 8,5, only monochloramine is formed; below this level mixtures of mono
and dichloramine exist; and below pH 4,2 only nitrogen trichloride exists.
6.2.3 Definition of the halogenated disinfection residuals
The chlorine in chloramine compounds is defined as combined available chlorine. Combined available
chlorine has a lower disinfection capacity than free chlorine, however the combination of free chlorine
and combined available chlorine (chloramines) that are present in the water create the ability to
disinfect the water.
The residual chlorine is available in three forms:
— chloramines: a form of combined chlorine,
— chloro-organic compounds: a weak form of combined chlorine, and
— free chlorine: the strongest form of residual for disinfection.
Chlorine demand should be calculated as the difference between total chlorine added into the water and
residual chlorine. It is the amount that reacts with the substances in water, leaving behind an inactive
form of chlorine.
The sum of the chlorine demand and the residual chlorine should determine the chlorine injected into
the water.
Added Chlorine = Chlorine Demand + Residual Chlorine
Residual Chlorine = Combined Available Chlorine + Free Chlorine
The same applies to bromide.
6.2.4 Breakpoint reaction
A reaction between chlorine and ammonia causes a unique phenomenon, which has a great effect on
chlorine disinfection process. When chlorine is added to TWW (that contains ammonia) a stepwise
reaction occurs, as described below (Figure 2).
10 © ISO 2021 – All rights reserved

Key
X Chlorine dose, mg/l
Y Chlorine residual, mg/l
A to B Combined residual
C Breakpoint
C to D Free and combined residual
A Destruction of chlorine residuals by reducing compounds
B Formation of chloro-organic and chloramine compounds
C Destruction of chloramines and chloro-organic compounds
D Formation of free chlorine and presence of chloro-organic compounds not destroyed
E free residual
F Combined residual
[3]
Figure 2 — Breakpoint chlorination
— Step one: destruction of residual chlorine by reducing compounds. Inorganic reducing materials
commonly found in wastewater that take precedence in reacting with chlorine can be: hydrogen
2+ 2+ -
sulfide (H S), ferrous iron (Fe ), manganese (Mn ), nitrite (NO ). These reducing materials
2 2
consume at the first step the residual chlorine added to the water, up to point A.
— Step two: formation of chloro-organic and chloramine compounds. As described in 6.1 ammonia
(NH ) is found in wastewater and is the second level of reaction with chlorine. It combines with
chlorine to form one of three forms of chloramine. Organic compounds are the last to react with
available chlorine in the wastewater and form chlororganic compounds (chlorine demand between
points A to B). In this range, the mole ratio of chlorine to ammonia is equal to 1, and increases
toward point B.
— Step three: destruction of chloro-organic and chloramines compounds. Between point B and the
breakpoint (point C), some chloramines will be converted to nitrogen trichloride, the remaining
chloramines will be oxidized to nitrous oxide (N O) and nitrogen (N ), and the chlorine will be
2 2
reduced to the chloride ion. The mole ratio at the breakpoint is equal to 1,5 to 1.
— Step four: formation of free chlorine and presence of chloro-organic compounds not destroyed.
Continued addition of chlorine past the breakpoint (C), resulting in a directly proportional increase
in the free chlorine. At this stage, combined and free chlorine (residual chlorine) are present in the
water.
The breakpoint can be described as the point at which the demand for chlorine has been fully satisfied
(i.e. the chlorine has reacted with all of the reducing agents, organics and ammonia). Ammonia nitrogen
can disappear completely at the breakpoint, or be reduced to only a trace, especially at neutral pH
values; but in practice an “irreducible minimum” residual chlorine (also referred to as a “nuisance
residual”) remains, typically a few tenths of a milligram per litre as Cl .
6.2.5 CT values of chlorine/bromide and their compounds
The factors that determine the efficiency of disinfection include the type of disinfectant, its
concentration in water and the time it acts on the pathogens in water.
The disinfection efficiency for each type of disinfectant is calculated by CT value. CT value is obtained
by multiplying the residual concentration of the disinfectant (C) after a given contact time (T) by
the contact time. CT values should be used for the calculation of disinfectant dose needed for the
disinfection of TWW, and be expressed in units of mg.min/l.
6.2.6 Chlorinated compounds for TWW disinfection
[3]
6.2.6.1 Chlorine (Cl )
Chlorine disinfection is the most common method for disinfection of water and TWW due to the
simplicity of the method, equipment and operation.
Chlorine (Cl ) can be present as a gas or a liquid (in pressure tanks). When added to water, two reactions
take place, as explained generically in 6.2.1 for halogenated oxidants: hydrolysis to form hypochlorous
-
acid (HOCl) Formula 6 and ionization to hypochlorite ion (OCl ) Formula 7:
+ -
Cl + H O → HOCl + H + Cl (6)
2 2
+ -
HOCl → H + OCl (7)
The two reactions can decrease the pH of the water and change the solubility of salts affected by the
water's pH. These can cause problems when the ions concentration of the ion is close to the solubility
limit in the water.
The relatively distribution in the water of the two species (Formula 6 and 7) is important as the
-
germicide efficiency of HOCl is significantly higher than that of OCl .
Dissociation of HOCl in water has been shown to be temperature dependent, and the pKa (dissociation
constant) for HOCl is in the range of 7,49 to 7,82 between 0 ºC to 30 ºC. At 20 ºC and pH = 7,58 there is
50 % of each species (see Figure 1 and Table 4).
–
The OCl and HOCl compounds are commonly referred to as free chlorine, (see 6.2.3), which is extremely
reactive with numerous components of the bacterial cell.
6.2.6.2 Sodium and calcium disinfection agent hypochlorite (“liquid bleach” and “chlorine
powder”)
Sodium hypochlorite and calcium hypochlorite are chlorine compounds formed by the reaction of
chlorine with hydroxides. The application of hypochlorite to water systems produces the hypochlorite
ion and hypochlorous acid, Formula 8 to Formula 11:
- +
NaOCl → OCl + Na (8)
- + + -
OCl + Na + H O → HOCl + Na + OH (9)
- 2+
Ca (OCl) → 2 OCl + Ca (10)
12 © ISO 2021 – All rights reserved

- 2+ 2+ -
2 OCl + Ca + 2 H O → 2 HOCl + Ca + 2OH (11)
While a slight contribution of alkali when a hypochlorite is added cannot make a big difference in the
final pH of the water, due of the buffering capacity of the TWW, some pH increase could contribute to
the precipitation of insoluble salts.
6.2.6.3 Chlorine dioxide (ClO )
Chlorine dioxide is an efficient disinfection agent. Its advantages include efficiency in a wide range of
pH values and elevated temperatures. It is effective against a wide range of organisms (including cysts
and protozoa) and virus.
Another advantage of the use of chlorine dioxide, compared to chlorine, is that chlorine dioxide does
not form compounds with ammonia, which reduces the chlorine disinfection efficiency.
Chlorine dioxide is an unstable and explosive gas and should be generated on site by reacting sodium
chlorite (NaClO ), Formula 12:
2 NaClO + 2 Cl → 2 ClO + 2 NaCl (12)
2 2 2
6.2.6.4 Chlorine production on site
When small quantities of wastewater are to be disinfected in remote areas, the transportation costs of
the disinfectants sometimes exceed the price of the disinfectant.
In this case solid chlorine tablets may be used or several methods exist to produce relatively small
amounts of active chlorine (sodium hypochlorite) on site, such as chlorine production from salt or from
solution of salty water.
This process is based on the production of hypochlorite by running an electric current through salt
water. When this happens, sodium hypochlorite, as well as hydrogen gas (H ), are produced on site.
Only sodium chloride (NaCl) is added to the water, or sea or saline water are used.
The reaction is described in Formula 13:
NaCl + H O + Electric current → NaOCl + H (13)
2 2
For efficient operation the system should be fed with softened water.
For safety operating reasons, a dedicated system should be operated to control and treat the hydrogen
gas concentration.
Chlorine can also be produced by on site electrochlorination (OSEC) of sodium hypochlorite solution
at concentration of 0,8 % to 1 %. Hypochlorite solution is always available at stable con
...