ISO 23044:2020

INTERNATIONAL ISO
STANDARD 23044
First edition
Guidelines for softening and
desalination of industrial wastewater
for reuse
PROOF/ÉPREUVE
Reference number
ISO 23044:2020(E)
©
ISO 2020

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ISO 23044:2020(E)

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© ISO 2020
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Published in Switzerland
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ISO 23044:2020(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and abbreviated terms . 1
3.1 Terms and definitions . 2
3.2 Abbreviated terms . 3
4 General . 4
5 Requirements for influent quality . 5
6 Softening process . 7
7 Desalination process . 9
Annex A (informative) Pre-treatment process .12
Annex B (informative) Typical performance of desalination technologies .13
Bibliography .15
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ISO 23044:2020(E)

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 on 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 the following
URL: www .iso .org/ iso/ foreword .html.
This committee responsible for this document is Technical Committee ISO/TC 282, Water reuse,
Subcommittee SC 4, Industrial water reuse.
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.
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ISO 23044:2020(E)

Introduction
With the development of society and economy, the contradiction between water shortage and
industrial growth is becoming increasingly acute. Industrial wastewater reclamation and reuse could
[4]
be an effective way to alleviate this contradiction by improving the water utilization efficiency .
Industrial processes such as oil extraction, chemicals production, printing and dyeing, pharmaceuticals
[5]
manufacturing and food processing produce the wastewater containing total dissolved solids. In
order to reuse these wastewater, total dissolved solids need to be removed by using water softening
[6]
and desalination technologies .
Currently, wastewater softening and desalination processes are based on chemical precipitation,
ion exchange, nanofiltration (NF), evaporation, reverse osmosis (RO), electrodeionization (EDI),
electrodialysis (ED), membrane distillation (MD), and so on, see References [7] to [10]. Each technology
has different applicable conditions and operational costs. The absence of an international standard to
provide guidance on the selection of wastewater softening and desalination processes makes it difficult
to determine the most appropriate softening or desalination technology for industrial enterprises.
Therefore, it hinders industrial wastewater reclamation and reuse. Six technologies have been selected
for consideration under this document, including chemical precipitation, ion exchange, nanofiltration
(NF), reverse osmosis (RO), electrodialysis (ED), electrodeionization (EDI), and there are other
technologies that could be similarly considered for future updates. It should be noted that mechanical
vapour recompression (MVR) and multi-effect evaporation (MEE) are mainly used for evaporation and
crystallization to acquire salts, not for the purpose of water reuse.
Based on the specific inorganic ion species and their concentration in influent, appropriate effluent
quality can be obtained using the recommended technologies that meets the requirement for hardness,
alkalinity and salinity for potential reuse applications.
This document is an innovative standard in the field of industrial wastewater reclamation and reuse.
It can help enterprises, engineers, operators and other stakeholders, who engage in designing or
operating in industrial saline wastewater reclamation and reuse, choose the technologies applying in
the process, and evaluate the treatment effects. As a result, the reuse of industrial saline wastewater
can be promoted and utilization of water can be improved.
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INTERNATIONAL STANDARD ISO 23044:2020(E)
Guidelines for softening and desalination of industrial
wastewater for reuse
1 Scope
This document provides guidance on, the evaluation and comparison of wastewater softening and
desalination processes for industrial wastewater reclamation and reuse with specific consideration for
the following six: 1) chemical precipitation; 2) ion exchange; 3) nanofiltration (NF); 4) reverse osmosis
(RO); 5) electrodialysis (ED) and 6) electrodeionization (EDI). This document provides guidance on the
characterisation of both influent and effluent quality (e.g. hardness, alkalinity, etc.) and the effects of
these processes on those constituents. The purpose of softening and desalination is only for the reuse
usages that have requirements for hardness and salinity, such as cooling circulating water, boiler water,
production process water, and cleaning water.
This document includes the following sub-processes of wastewater softening and desalination
processes:
a) wastewater softening processes based on chemical precipitation, ion exchange and NF, which aim
2+ 2+
to remove hardness ions, such as Mg and Ca ;
b) desalination processes based on ion exchange, RO, ED, EDI and NF, which aim to remove the most of
total dissolved solids (TDS).
This document is applicable to:
a) industrial saline wastewater, which has been pre-treated to remove most of the organic matters if
necessary;
b) the selection or design of wastewater softening and desalination processes for reuse of wastewater
from industries.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. 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
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 http:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
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ISO 23044:2020(E)

3.1 Terms and definitions
3.1.1
regeneration
process of restoring an ion-exchange resin after use to its operationally effective state
Note 1 to entry: Two types of generation can occur: co-current regeneration and counter-current regeneration.
Co-current regeneration is original downflow process where both input water and regeneration chemicals flow
in the same direction, while counter-current regeneration is upflow process where input water and regeneration
chemicals flow in different directions.
3.1.2
electrodeionization
method for removing ions by combination of mixed bed ion exchange and electrodialysis in an
electrodialyser , where the fresh water chamber is filled with mixed bed ion exchange resin, and the ion
exchange resin can be electrochemically regenerated by polarization during the electrodialysis process
Note 1 to entry: Generally, it is a polishing process for production of ultrapure reclaimed water and used after
reverse osmosis.
3.1.3
electrodialysis
process used for the deionization of water in which ions are removed, under the influence of an electric
field, from one body of water and transferred to another across an ion-exchange membrane
[SOURCE: ISO 6107-1: 2004, 32]
3.1.4
industrial saline wastewater
industrial wastewater that contains high concentration of inorganic ions
3.1.5
ion exchange
process by which certain anions or cations in water are replaced by other ions by passage through a
bed of ion-exchange material
[SOURCE: ISO 6107-1: 2004, 46]
3.1.6
mechanical vapour recompression
use of the heat of the secondary steam as a heat source instead of fresh steam by raising its temperature,
with a part of the compressor working to achieve cyclic evaporation
3.1.7
membrane distillation
separation process where a micro-porous hydrophobic membrane separates two aqueous solutions at
different temperatures
3.1.8
microfiltration
type of physical filtration process by pressure driven where a contaminated liquid is passed through
a special pore-sized membrane (0,1-1 µm) to separate microorganisms and suspended particles from
process liquid
3.1.9
multi-effect evaporation
use of microporous membranes with a filtration accuracy of 0,01-0,1 μm for the separation of
microorganisms, large molecules or very finely divided suspended matter from water by filtration,
often by means of applied differential pressure
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ISO 23044:2020(E)

3.1.10
nanofiltration
membrane separation technology with a filtration accuracy of 0,001-0,01 μm to separate proteins and
low molecular organic compounds
3.1.11
precipitation
chemical reaction in solution resulting in the formation of a solid product
[SOURCE: ISO 11074: 2015, 6.4.30]
3.1.12
pre-treatment
treatment process or processes carried out before the softening and desalination processes
3.1.13
reverse osmosis
flow of water through a membrane with a filtration accuracy of 0,000 1-0,001 μm, from a more
concentrated to a less concentrated solution, as a result of applying pressure to the more concentrated
solution in excess of the normal osmotic pressure
Note 1 to entry: The filtration accuracy of membrane is added.
[SOURCE: ISO 6107-1: 2004, 61]
3.1.14
softening
partial or complete removal from water of calcium and magnesium ions which are responsible for
hardness
Note 1 to entry: In this context, not only calcium and magnesium ions are removed, other inorganic ions and
cations are also included.
[SOURCE: ISO 6107-1: 2004, 68]
3.1.15
ultrafiltration
use of microporous membranes with a filtration accuracy of 0,01-0,1 μm for the separation of large
molecules or very finely divided suspended matter from water by filtration, often by means of applied
differential pressure
Note 1 to entry: The filtration accuracy of microporous membranes is added.
[SOURCE: ISO 6107-6: 2004, 100]
3.2 Abbreviated terms
BOD biochemical oxygen demand after 5 days
5
COD chemical oxygen demand
DO dissolved oxygen
ED electrodialysis
EDI electrodeionization
MF microfiltration
NF nanofiltration
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ISO 23044:2020(E)

RO reverse osmosis
TDS total dissolved solids
TOC total organic carbon
TSS total suspended solids
UF ultrafiltration
MD membrane distillation
MEE multi-effect evaporation
MVR mechanical vapor recompression
4 General
Water quality indicators should include TSS, TOC, COD, pH, temperature, TDS, the species and
concentrations of ions.
The product water from wastewater softening and desalination processes is recommended to be reused
for urban non-potable water, environmental water, and as pure or ultrapure water for cooling water,
[4]
boiler feed water, process water, rinse water, and so on .
The process selection of wastewater softening and desalination processes should be determined after
technical and economic comparison based on factors such as influent quality, product quality, quantity
requirements, site conditions and environmental protection requirements.
The wastewater needs to be pre-treated if necessary, before being fed into softening and desalination
devices.
The selection of pre-treatment process should consider the quality of wastewater, influent quality
requirements for softening and desalination processes, water treatment volume and test data. Besides,
the operational experience of similar projects should be referred, combined with local conditions.
Finally, users can determine which technology to adopt through technical and economic comparison.
Minimizing the discharge quantity of waste acid, waste alkali, waste residue and other harmful
substances are important in the selection of softening and desalination processes or device. Measures
for treating and disposing these wastes should be taken to meet the relevant environmental protection
requirements.
Waste liquid (e.g., regeneration liquid of ion exchange resin process, concentrate of reverse osmosis
process, etc.) disposed from the softening and desalination processes should be collected separately
according to the characteristics of wastewater quality.
Process flow diagram of industrial saline wastewater treatment for reuse is shown in Figure 1.
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ISO 23044:2020(E)

Figure 1 — Process flow diagram of industrial saline wastewater treatment for reuse
5 Requirements for influent quality
Influent quality requirements for softening and desalination device are shown in Table 1. It is noted that
data provided in this table is all in advisory typical ranges, which is suggested to be applied according
to specific conditions, as well as the manufacturer's specifications. The parameters listed in Table 1 are
also illustrated as follows to show its effect on softening and desalination devices.
a) Silt density index (SDI) reflects the content of particles, colloids, and other objects in influent that
can block softening and desalination devices. SDI values higher than the limit can easily block the
membrane which will lead to fouling, thereby shortening the operating life of the membrane.
b) Turbidity represents the concentration of undissolved matters in influent that reduce transparency.
These undissolved matters can adhere to surface of ion exchange resin, and then block the exchange
channel or pollute resin. It can also cause membrane fouling.
c) Water temperature can affect ion exchange rate and ion absorption ability of resin. It also can affect
membrane flux and TDS removal ability of membrane.
d) pH can affect TDS removal ability of membrane and shorten its operating life if exceed typical range.
e) Chemical oxygen demand refers to organic matters which can easily pollute anion exchange resin,
because it is difficult to precipitate after the reaction with the anion exchange resin.
f) Appropriate residual chlorine can ensure the sterilization ability for water quality. However,
resin is combined with macromolecular organic compounds those can be easily oxidized by
high concentration of chloride to break the chemical structure, and then ion exchange ability of
resin would be weakened. High residual chlorine can also oxide membrane element and make an
[11]
irreparable damage .
g) Iron and manganese can be intercepted by resin to form adsorbent that is not easy to wash off. The
resin would lose function as the reaction is not reversible. In addition, both iron and manganese can
accelerate the oxidation of the membrane and cause irreversible damage to the membrane element.
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ISO 23044:2020(E)

h) Lower electrical conductivity reflects lower ion content, which is beneficial to form a larger electric
potential gradient. More cations and anions would be generated along with the increasing of water
dissociation degree. Then the regeneration ability of resin can keep well.
i) High total exchangeable anions can reduce the resistivity of effluent, and then larger running
current should be set. However, larger running current can increase the system current and
residual chlorine, which is not beneficial to the membrane.
j) High hardness (>200-500 mg/l CaCO ) can cause fouling in EDI units. Low hardness (<200 mg/l
3
CaCO ) can extend the cleaning cycle and improve the water utilization rate of EDI system.
3
k) Both CO and SiO are weakly ionized substances, which can lead to deterioration of water quality.
2 2
Decarbonator is suggested to be installed to remove CO .
2
Table 1 — Influent quality requirements for softening and desalination devices
Nanofiltration or Electro-
Parameter Ion exchange Electrodialysis
reverse osmosis deionization
a
Silt Density Index (SDI ) <5 <5 <5 <1
15
Counter-
current <2
Turbidity
regeneration
<1 <1 <1
b
(FTU)
Co-current
<5
regeneration
c d
Water temperature (°C) 5~40 5~35 5~40 5~40
e
pH (25 °C) — 3~11 4~10 5~9
Chemical oxygen demand
<50 — <10 —
(mg/l) (K Cr O )
2 2 7
Residual chlorine (mg/l) <0,1 <0,1, control to 0 <0,2 <0,05
<0,05
f
Fe (mg/l) <2 <0,3 <0,01
g
(DO > 5 mg/l)
Mn (mg/l) — <0,3 <0,1 <0,01
Electrical conductivity
h i
— — >10 000 <40
(25 °C, μS/cm)
a
RO membrane manufacturers recommend that the RO feed water should have an SDI < 5. However, through long term
[12]
operational experience, many operators now recommend having an SDI < 3 .
b
FTU: formazan turbidity units.
c
Higher water temperature can increase ion exchange rate, but if water temperature is too high, the ion absorption
ability will weaken. Besides, resin may deteriorate and radical group for exchange may degrade with too high temperature.
The influent temperature for alkali II resin and acrylic resin is suggested to be lower than 35 °C.
d
Higher water temperature can increase membrane flux, but TDS removal ability will be worse if water temperature is
too high. Optimum water temperature for reverse osmosis devices is suggested to be in the range of 20~25 °C.
e
The symbol “—” means that any value for the given parameter can be applicable.
f
The influent iron concentration of ion exchange resin device, which is regenerated by hydrochloric acid and sulfuric
acid, is suggested to be less than 2 mg/l, and the amount of iron contained in the influent of sodium ion exchange resin
device for softening is suggested to be less than 0,3 mg/l.
g [13]
The oxidation rate of iron depends on the iron content, the concentration of DO in water and the pH value of water .
2+
When the pH is less than 6, DO is suggested to be less than 0,5 m g/l, and maximum Fe is suggested to be less than 4 mg/l.
h
It is not economical to utilize electrodialysis units if the conductivity of the influent is less than 10 000 μS/cm.
i
The influent into electrodeionization device is suggested to be the effluent from reverse osmosis device, whose expected
value of conductivity (25 °C), including the equivalent conductivity of carbon dioxide, should be less than 20 μS/cm.
j
When the hardness is greater than 500 mg/l (CaCO ), the exchange capacity of ionic resin may reach saturation fast.
3
k
When the hardness is greater than 200 mg/l (CaCO ), scaling may generate in reverse osmosis membrane. When the
3
hardness is greater than 500 mg/l (CaCO ), scaling may generate in nanofiltration membrane.
3
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ISO 23044:2020(E)

Table 1 (continued)
Nanofiltration or Electro-
Parameter Ion exchange Electrodialysis
reverse osmosis deionization
Total exchangeable anions
— — — <25
(mg/l, CaCO )
3
<200 (RO)
j
Hardness (mg/l, CaCO ) <500 <10 <1
3
k
<500 (NF)
CO (mg/l) — — — <5
2
SiO (mg/l) — — <20 ≤0,5
2
a
RO membrane manufacturers recommend that the RO feed water should have an SDI < 5. However, through long term
[12]
operational experience, many operators now recommend having an SDI < 3 .
b
FTU: formazan turbidity units.
c
Higher water temperature can increase ion exchange rate, but if water temperature is too high, the ion absorption
ability will weaken. Besides, resin may deteriorate and radical group for exchange may degrade with too high temperature.
The influent temperature for alkali II resin and acrylic resin is suggested to be lower than 35 °C.
d
Higher water temperature can increase membrane flux, but TDS removal ability will be worse if water temperature is
too high. Optimum water temperature for reverse osmosis devices is suggested to be in the range of 20~25 °C.
e
The symbol “—” means that any value for the given parameter can be applicable.
f
The influent iron concentration of ion exchange resin device, which is regenerated by hydrochloric acid and sulfuric
acid, is suggested to be less than 2 mg/l, and the amount of iron contained in the influent of sodium ion exchange resin
device for softening is suggested to be less than 0,3 mg/l.
g [13]
The oxidation rate of iron depends on the iron content, the concentration of DO in water and the pH value of water .
2+
When the pH is less than 6, DO is suggested to be less than 0,5 m g/l, and maximum Fe is suggested to be less than 4 mg/l.
h
It is not economical to utilize electrodialysis units if the conductivity of the influent is less than 10 000 μS/cm.
i
The influent into electrodeionization device is suggested to be the effluent from reverse osmosis device, whose expected
value of conductivity (25 °C), including the equivalent conductivity of carbon dioxide, should be less than 20 μS/cm.
j
When the hardness is greater than 500 mg/l (CaCO ), the exchange capacity of ionic resin may reach saturation fast.
3
k
When the hardness is greater than 200 mg/l (CaCO ), scaling may generate in reverse osmosis membrane. When the
3
hardness is greater than 500 mg/l (CaCO ), scaling may generate in nanofiltration membrane.
3
Industrial wastewater should be pre-treated before being fed into the softening and desalination
devices to improve water quality by removing particles, TSS, organic matters, etc. Pre-treatment
processes consist of conventional treatment and tertiary treatment. Popular pre-treatment processes
are shown in Annex A. Combination of those technologies can be adopted according to the quality of
wastewater, influent quality requirements of softening and desalination processes, technical feature,
cost and so on. Besides, experimental data from lab scale study or similar engineering experience also
should be referred.
6 Softening process
Based on hardness and alkalinity of the influent and the water quality requirements of the effluent,
softening process can adopt chemical precipitation, ion exchange resin or a combination of the two
[9]
technologies . Recommended softening processes are illustrated as follows. Table 2 can also be
[6][10]
referred for softening process selection .
a) The effects of ion exchange methods are stable and accurate, including single-stage sodium resin,
two-stage sodium resin, H-type resin in series with sodium resin, and weak acid cation resin, etc.
b) As to chemical precipitation, the most common precipitants are lime, soda ash and sodium
hydroxide.
c) Single-stage sodium resin cannot remove alkalinity; thus, it is suitable for treatment of wastewater
with low alkalinity and hardness.
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d) Neither can two-stage sodium resin remove alkalinity. The two-stage sodium resin is applicable to
treat wastewater with low alkalinity but high hardness.
e) Lime-soda ash softening is an option. Lime softening can remove carbonate alkalinity and soda ash
can remove non-carbonate alkalinity.
f) Lime softening combined sodium resin can simultaneously remove hardness and alkalinity, so
it is suitable for wastewater with high hardness of carbonate, low excess alkalinity or no excess
alkalinity. Lime softening treatment has the advantage of low cost, but there are shortcomings like
labour intensity, poor working conditions and that non-carbonate calcium hardness is not affected
by treatment with lime softening alone.
g) H-type resin in series with sodium resin is suitable for wastewater with high hardness and high
alkalinity.
h) H-type resin in parallel with sodium resin can simultaneously remove hardness and alkalinity, so it
is suitable for wastewater with high hardness of carbonate and high alkalinity.
i) Weak acid cation resin in hydrogen form can remove alkalinity but not suitable for the removal of
hardness.
j) Nanofiltration membranes can intercept calcium and magnesium ions in water, radically reducing
the hardness. However, it has strict requirements for the pressure of influent and high investment
and operation cost.
Table 2 — Softening process selection
Influent quality Effluent quality
Ratio of
Total Carbonate
Process name and Hardness
carbonate
hardness hardness Alkalinity
Number
code
hardness to
[mg/l
[mg/l [mg/l [mg/l (CaCO )]
total 3
(CaCO )]
3
(CaCO )] (CaCO )]
3 3 hardness
a
1 Ion exchange <500 <350 — <20 5
Chemical
2 — — — ≥35 ≥135
b
precipitation
lime softening in
series with
3 — >150 >0,5 <2 40~60
sodium resin
(CaO-Na)
4 Nanofiltration <500 <350 — <20 0
a
Table symbol: Na – Sodium ion exchanger; CaO – Lime
...