ISO/TR 22707:2023
(Main)Sludge recovery, recycling, treatment and disposal — Information on the processes and technologies for inorganic substance and nutrient recovery
Sludge recovery, recycling, treatment and disposal — Information on the processes and technologies for inorganic substance and nutrient recovery
This document provides information on the processes and technologies for inorganic substance and nutrient recovery from sludge. This document is applicable to sludge and products from urban wastewater collection systems, night soil, wastewater treatment plants for urban and similar industrial waters. It includes all sludge that can have either similar environmental or health impacts, or both. Hazardous sludge from industry and dredged sludge are excluded from this document.
Valorisation, recyclage, traitement et élimination des boues — Guide sur les procédés et les technologies de récupération des substances inorganiques et des nutriments
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TECHNICAL ISO/TR
REPORT 22707
First edition
2023-07
Sludge recovery, recycling, treatment
and disposal — Information on
the processes and technologies for
inorganic substance and nutrient
recovery
Valorisation, recyclage, traitement et élimination des boues — Guide
sur les procédés et les technologies de récupération des substances
inorganiques et des nutriments
Reference number
ISO/TR 22707:2023(E)
© ISO 2023
---------------------- Page: 1 ----------------------
ISO/TR 22707:2023(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2023
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
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ISO/TR 22707:2023(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Methods of nutrient recovery from sludge . 2
5 Phosphorus recovery .2
5.1 General . 2
5.2 Struvite recovery from either anaerobic digested sludge or filtrate of anaerobic
digested sludge, or both . 3
5.2.1 Principle . 3
5.2.2 Schematic diagram . 4
5.2.3 Operating conditions . 5
5.2.4 Characteristics of recovered products . 5
5.3 Hydroxyapatite recovery . 5
5.3.1 Principle . 5
5.3.2 Schematic diagram . 6
5.3.3 Operating conditions . 6
5.3.4 Characteristics of recovered products . 6
5.4 Phosphorus recovery from incineration ash . 6
5.4.1 Principle . 6
5.4.2 Alkaline treatment . 7
5.4.3 Acidic treatment . 8
5.4.4 Characteristics of recovered products/residues . 8
5.5 Phosphorus recovery from sewage sludge slag . 8
5.5.1 Principle . 8
5.5.2 Schematic diagram . 8
5.5.3 Operating conditions . 9
5.5.4 Characteristics of recovered products . 9
5.6 Other technologies for phosphorus recovery . 9
5.7 Summary . 10
6 Recovery of other nutrients .11
6.1 General . 11
6.2 Nitrogen . 11
6.3 Sulfur . 11
6.4 Potassium . 11
7 Recovery of other inorganics .11
7.1 General . 11
7.2 Metals . 11
Annex A (informative) Sewage sludge composition .13
Annex B (informative) Case studies .14
Bibliography .39
iii
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ISO/TR 22707:2023(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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use
of (a) patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed
patent rights in respect thereof. As of the date of publication of this document, ISO had not received
notice of (a) patent(s) which may be required to implement this document. However, implementers are
cautioned that this may not represent the latest information, which may be obtained from the patent
database available at www.iso.org/patents. ISO shall not be held responsible for identifying any or all
such patent rights.
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 275, Sludge recovery, recycling, treatment
and disposal.
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.
iv
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ISO/TR 22707:2023(E)
Introduction
Inorganics and nutrient recovery is necessary to build a sustainable society; there are many studies
and plants all over the world that demonstrate this concept. Above all, phosphorus recovery systems
to produce fertilizer material are increasingly common and other nutrients recovery systems are now
being developed.
This document provides a selected overview of various technologies and is based on country standards
and guidance documents already in existence or under preparation, and documents provided by
private organizations.
As inorganics and nutrient recovery knowledge and technology is developing rapidly, this document
will therefore be reviewed regularly to reflect the advancing nature of the industry and technology.
Annex A provides examples of sewage sludge composition, which can help determine which element(s)
can be recovered. Annex B provides case studies of nutrient recovery, including practical and emerging
ones.
v
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TECHNICAL REPORT ISO/TR 22707:2023(E)
Sludge recovery, recycling, treatment and disposal —
Information on the processes and technologies for
inorganic substance and nutrient recovery
1 Scope
This document provides information on the processes and technologies for inorganic substance and
nutrient recovery from sludge.
This document is applicable to sludge and products from urban wastewater collection systems, night
soil, wastewater treatment plants for urban and similar industrial waters. It includes all sludge that can
have either similar environmental or health impacts, or both.
Hazardous sludge from industry and dredged sludge are excluded from this document.
2 Normative references
There are no normative references in this document.
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
ammonia stripping
method that removes ammoniacal compounds from water by making it alkaline and of aeration
3.2
calcium phosphate
salts that consist of calcium ions and phosphate ions
Note 1 to entry: Hydroxyapatite (HAP) is a form of calcium phosphate.
3.3
centrate
liquid product from a centrifugal dewatering device
3.4
hydroxyapatite
HAP
sparingly soluble salt that is generated from phosphate and calcium ions
Note 1 to entry: The general chemical formula of HAP is Ca (OH) (PO ) .
10 2 4 6
3.5
incineration ash
residue of combustion
1
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ISO/TR 22707:2023(E)
3.6
nutrient
element required by living organisms throughout the course of their lives in small quantities for a range
of physiological functions
3.7
seed crystal
crystal employed as a nucleus to generate and grow crystals in the crystallization process
3.8
struvite
compound which is precipitated by magnesium addition to water with high concentration of phosphate
and ammonium ions
Note 1 to entry: The chemical formula of struvite is MgNH PO ·6H O.
4 4 2
4 Methods of nutrient recovery from sludge
There are four methods for nutrient recovery from sludge, which are whole use, cleaning, separation
and extraction.
a) Whole use: Whole use of sludge is a simple use method in which sludge, which is typically aerobically
or anaerobically treated (e.g. compost), is directly applied to land as fertilizer or soil improver. This
method can minimize the loss of the nutrients in the treatment process and can achieve the highest
potential of utilizing the nutrients in sludge.
b) Cleaning: Cleaning is the process in which sludge has contaminants such as plastics or heavy metals
removed by mechanical treatment or chemical extraction. The cleaned sludge can be handled in the
same way as whole use.
c) Separation: Separation is the process in which sludge is divided into two or more different parts.
Sludge is separated by physical or chemical parameters such as size, shape, specific gravity
difference and chemical affinity. All or only the least contaminated part of separated sludge can
then be utilized. In this method, sludge contains various nutrients.
d) Extraction: Extraction is the way in which only the target element is taken out as a compound
using chemical actions. Fewer nutrients in sludge are made available or utilized through extraction
processes than in whole use, cleaning and separation methods. However, the process has some
advantages:
— reduces the storage volume of the nutrient;
— prevents contamination of the recovered material by hazardous elements;
— stabilizes the recovered materials as a chemical compound;
— improves the value of the recovered materials.
Precipitation, including stripping processes, can decrease the volatile nutrient content.
This document is focused on nutrients which can be recovered by extraction.
5 Phosphorus recovery
5.1 General
Phosphorus is an essential element for plant growth and is an important ingredient of chemical
fertilizer products. The dry solid contents of sludge normally include more than 1,0 % phosphorus and
it can reach 5,0 % of sludge under certain operating conditions, such as biological dephosphorization or
anaerobic-anoxic-oxic processes.
2
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ISO/TR 22707:2023(E)
On the other hand, the supply of phosphate ore in the global market is strongly influenced by political
and economical issues and often gets unstable, as it is quite unevenly distributed globally. Therefore,
studies and commercialization of phosphorus recovery from sludge is the most progressive area in
inorganic and nutrient material recovery.
Phosphorus can be recovered from sludge using various chemical compounds. The phosphorus recovery
process that is described in Clause 5 is summarized in Figure 1.
For case studies, refer to Clauses B.1 to B.11.
Figure 1 — Summary of phosphorus recovery process
5.2 Struvite recovery from either anaerobic digested sludge or filtrate of anaerobic
digested sludge, or both
5.2.1 Principle
The principle of the struvite recovery process are based on the chemical precipitation carried out in a
crystallizer followed by particle separation. The chemical reaction for struvite is:
3− + 2+
PO + NH + Mg + 6H O → MgNH PO ·6H O
4 4 2 4 4 2
This reaction is the same as the scale formation which is frequently observed in anaerobic sludge
treatment facilities. The difference of struvite recovery from scale formation is well-controlled
chemical dosing, pH control and particle separation. After the application of this process, much less
scale formation is likely to occur in treatment facilities.
Recovered struvite can be used as delayed release fertilizer because of its low solubility.
There are two types of crystallizer processes: agitation by air or mechanical agitation.
Both methods of crystallization are employed in commercial operations. Wastewater employed for
this process is a filtrate of anaerobically digested sludge (ADS) or ADS itself and industrial wastewater
containing phosphate and ammonium. Under optimum operating conditions, dissolved phosphorous
recovery can reach more than 80 % using this process.
3
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ISO/TR 22707:2023(E)
5.2.2 Schematic diagram
Schematic diagrams for an air fluidized crystallizer and a mechanical agitator are shown in Figures 2
and 3. An influent such as a filtrate of ADS, and/or ADS, is mixed in the reactor with magnesium (Mg)
salt and struvite granules (as seed crystal). Alkalising chemicals such as sodium hydroxide solution can
be added for pH control.
Key
1 ADS or ADS liquor 6 treated sludge or liquor
2 phosnix reactor 7 air
3 Mg(OH) 8 rotary sieve
2
4 NaOH 9 struvite
5 liquid cyclone
SOURCE Reference [1]. Reproduced with the permission of the authors.
Figure 2 — Schematic diagram of a fluidized bed reactor
4
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ISO/TR 22707:2023(E)
Key
1 digested sludge 5 struvite separator (cyclone)
2 trash removal equipment 6 treated sludge
3 crystallization reactor 7 washing/drying equipment
4 Mg(OH) 8 recovered struvite
2
SOURCE Reference [7]. Reproduced with the permission of the authors.
Figure 3 — Schematic diagram of a stirred tank reactor
5.2.3 Operating conditions
The key factors influencing struvite recovery rates are inflow concentrations of phosphate and
ammonium, the dosing rate of magnesium ions, alkalinity and pH.
The concentration of phosphate needs to be higher than 50 mg/l of P, preferably over 100 mg/l of P and
ammonia over 300 mg/l of N. From an economical point of view, the pH needs to remain in the range of
7,5 to 9,0. Various Mg compounds can be used as a source of Mg including, Mg(OH) , MgCl and MgSO .
2 2 4
Seawater can also be used as a source of Mg.
5.2.4 Characteristics of recovered products
Recovered struvite is crystalline with few impurities. Its shape depends on the above discussed
operating conditions, including pH, temperature, agitation and the retention time of struvite particles
in the crystallizer.
5.3 Hydroxyapatite recovery
5.3.1 Principle
The principle of hydroxyapatite (HAP) recovery process is based on chemical precipitation carried out
in a crystallizer followed by particle separation. The chemical reaction for HAP is:
2+ 3- -
10Ca + 6PO + 2OH → Ca (OH) (PO )
4 10 2 4 6
HAP recovery systems require well-controlled chemical dosing, pH control and particle separation.
Recovered HAP can be used as raw material for fertilizers.
A crystallizer is a type of mixing reactor employed in commercial operations. The applicable wastewater
for this process will be black water (human faeces and urine), industrial wastewater containing
phosphate and filtrate from sludge treatment processes.
5
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ISO/TR 22707:2023(E)
Under general operating conditions, dissolved phosphorous recovery can reach 70 % through this
process.
5.3.2 Schematic diagram
A schematic diagram for a crystallizer is shown in Figure 4. Influent such as filtration liquid, calcium
chloride and HAP granules (as seed crystal) are mixed in the mixing reactor. Sodium hydroxide solution
is added for pH control.
Key
1 filtration liquid 3 effluent
2 calcium chloride 4 HAP
Figure 4 — Schematic diagram for HAP recovery
5.3.3 Operating conditions
The key factors influencing HAP recovery are inflow concentrations of phosphate, carbonate ion, the
dosing rate of calcium ions, alkalinity and pH.
The concentration of phosphate is required to be around 50 mg/l of P. From an economical point of
view, the pH needs to remain in the range of 7,5 to 9,0.
5.3.4 Characteristics of recovered products
Recovered HAP is a crystalline structure with few impurities. Its shape depends on the above discussed
operating conditions including pH, temperature, agitation and the retention time of HAP particles in
the crystallizer.
5.4 Phosphorus recovery from incineration ash
5.4.1 Principle
When advanced wastewater treatment technologies have been employed, phosphorus tends to
increasingly concentrate in sludge. As a result, incineration ash from sewage sludge contains almost the
same concentration of phosphorus as that of natural phosphate ore, shown in Figure 5. Sewage sludge
ash is expected to be one of the alternative sources of phosphorus for depletion in the future.
6
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ISO/TR 22707:2023(E)
SOURCE Reference [9]. Reproduced with the permission of the authors.
Figure 5 — Composition comparison between phosphate ore and incineration ash
Phosphorus recovery from incineration ash can be achieved by means of a chemical reaction, which is
leaching and precipitation. Leach is performed under both alkaline and acidic condition.
Sewage sludge ash can be used directly as a P fertilizer. This direct utilization, however, is only possible
for sewage sludges with a low level of contamination. See Clause B.8.
5.4.2 Alkaline treatment
5.4.2.1 Schematic diagram
A schematic diagram of the phosphorus recovery process from incineration ash is shown in Figure 6,
which consists of two reaction tanks.
In the first reactor, phosphate is extracted from the incineration ash by using alkaline solution. In the
second reactor, phosphorus-rich sediment is precipitated by adding slaked calcium (Ca(OH) ) into a
2
solution taken from the first reactor. The solution after collecting phosphorus-rich sediment is recycled
and returned into the first reactor.
Key
3-
1 ash 5 PO rich solution
4
2 incinerated ash 6 precipitation
3 extraction 7 circulation of recycled solution
4 neutralized ash
SOURCE Reference [9]. Reproduced with the permission of the authors.
Figure 6 — Schematic diagram of phosphorus recovery from incineration ash
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ISO/TR 22707:2023(E)
5.4.2.2 Operating conditions
In the first reactor, phosphate in the incineration ash is extracted into the solution by controlling the pH
at more than 13, adding sodium hydroxide, maintaining the temperature between 50 °C and 70 °C, and
the retention time between 5 min and 30 min. The chemical reaction is:
− 3−
P O + 6OH → 2PO + 3H O
2 5 4 2
In the second reactor, Ca(OH) is added in a solution from the first reactor and the substances that
2
contain a lot of phosphate are precipitated by controlling temperature between 20 °C to 50 °C, and a
retention time between 6 h and 18 h. The hydroxide ion that is consumed by phosphorus extraction is
replenished by using Ca(OH) as a calcium source. The chemical reaction is:
2
3− −
2PO + 3Ca(OH) → Ca (PO ) + 6OH
4 2 3 4 2
5.4.3 Acidic treatment
Acidification is possible to extract more phosphoric acid and metal components such as iron and
aluminium than alkaline treatment, which can be recovered by precipitation. However, it is necessary to
pay attention to simultaneous extraction of heavy metals. In many cases, sulfuric acid or hydrochloric
acid is used as acid. Many studies on this concept are carrying out.
5.4.4 Characteristics of recovered products/residues
Phosphorus recovery technology from incineration ash can produce the two products, recovered
phosphorus and neutralized ash.
The recovered phosphorus from this technology is a mixture of HAP and calcium phosphate. The
concentration of phosphorus is approximately 30 %. After dewatering and drying, it can be utilized as a
raw material of fertilizer for agriculture, gardening, etc.
Neutralized ash contains fewer amounts of heavy metals than untreated incineration ash because it is
chemically treated for reducing and minimising solubility of heavy metals. After washing and drying, it
can be easily utilized for many applications like cement material, soil rehabilitation, etc.
5.5 Phosphorus recovery from sewage sludge slag
5.5.1 Principle
Melting is a thermochemical process for separating and refining target materials under high
temperature and altered atmospheric conditions. Hazardous heavy metals such as lead, cadmium and
mercury that have lower boiling points than the temperature in the furnace, are evaporated.
Phosphorus generally volatilizes under high temperature and reducing atmospheric conditions.
The melting technology, however, makes it possible to recover phosphorus in the slag in the form of
phosphorus oxide by controlling the air supply into the furnace to maintain oxidation conditions. Ca
and Fe in the sludge effectively influence to fix phosphorus into the slag.
5.5.2 Schematic diagram
A schematic diagram of the phosphorus recovery process from sewage sludge into slag is shown in
Figure 7.
Dewatered sludge, after drying, is supplied to the furnace. Recovered heat from exhaust gas is used for
preheating of the drying and the combustion air. The gas treatment system neutralizes acidic gases and
separates heavy metals from gas as fly ash.
8
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ISO/TR 22707:2023(E)
Key
1 sewage sludge storage tank 6 heat recovery system
2 dryer 7 gas treatment system
3 melting furnace 8 P-rich slag (recovered phosphorus)
4 air 9 fly ash (heavy metals)
5 water bath
SOURCE Reference [3]. Reproduced with the permission of the authors.
Figure 7 — Schematic diagram of phosphorus recovery as slag
5.5.3 Operating conditions
The correct operating temperature in the melting furnace is between 1 250 °C to 1 350 °C. Air levels in
the melting zone in the furnace before secondary combustion is approximately 1,0 %.
If the low heat value (or net calorific value) of dried sludge is high enough, there is no need to supply fuel
in the melting furnace. This results in the following process. In the case of raw sludge with higher high
heat value, sludge is dried to a lower dryness in a dryer in order to achieve self-thermal combustion in
the melting furnace. In the case of digested sludge with lower high heat value, sludge is dried to higher
dryness in a dryer.
5.5.4 Characteristics of recovered products
Based on the above process, the melting process for sewage sludge generates slag which is equivalent
to 90 % to 95 % of ash content in the sludge, and approximately 90 % of phosphorus in the sludge
is recovered in the slag. The main components of the slag are silicon, calcium, phosphorous, iron and
aluminium. The phosphorous concentration in the slag is nearly equal to phosphorous concentration
in sludge incineration ash. Performance data shows that phosphorus concentration in the slag ranges
from 20 % to 30 % of dry P O .
2 5
Slag is a kind of amorpho
...
© ISO TR 22707 – All rights reserved
ISO PDTR /TR 22707:2023(E)
ISO TC 275/WG 7
Sludge recovery, recycling, treatment and disposal — GuidanceInformation on the processes
and technologies on inorganics and nutrients recovery
DTR3 stage – Publication Version
Warning for WDs and CDs
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---------------------- Page: 1 ----------------------
© ISO 2017, Published in Switzerland
First edition
Date: 2023-04-21
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ISO/TR 22707:2023(E)
© ISO 2023
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’sISO's member body
in the country of the requester.
ISO Copyright Office
Ch. de Blandonnet 8 • CP 401 • CH-1214 Vernier, Geneva , Switzerland
Tel. Phone: + 41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.orgFax + 41 22 749 09 47
copyright@iso.org
www.iso.org
Published in Switzerland.
ii © ISO 2023 – All rights reserved
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ISO/TR 22707:2023(E)
Contents
Foreword . v
Introduction. vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
3.1 ammonia stripping . 1
3.2 calcium phosphate . 1
3.3 centrate . 1
3.4 hydroxyapatite (HAP) . 1
3.5 incineration ash . 2
3.6 nutrient . 2
3.7 seed crystal . 2
3.8 struvite . 2
4 Methods of nutrient recovery from sludge . 2
5 Phosphorus recovery . 3
5.1 General . 3
5.2 Struvite recovery from anaerobic digested sludge and/or filtrate of anaerobic
digested sludge . 4
5.2.1 Principles . 4
5.2.2 Schematic diagram . 4
5.2.3 Operating conditions . 6
5.2.4 Characteristics of recovered products. 6
5.3 Hydroxyapatite recovery . 6
5.3.1 Principles . 6
5.3.2 Schematic diagram . 6
5.3.3 Operating conditions . 7
5.3.4 Characteristics of recovered products. 7
5.4 Phosphorus recovery from incineration ash. 7
5.4.1 Principles . 7
5.4.2 Schematic diagram . 8
5.4.3 Operating conditions . 8
5.4.4 Characteristics of recovered products. 9
5.5 Phosphorus recovery from sewage sludge into slag . 9
5.5.1 Principles . 9
5.5.2 Schematic diagram . 9
5.5.3 Operating conditions . 10
5.5.4 Characteristics of recovered products. 10
5.6 Other technologies for phosphorus recovery . 10
5.7 Summary . 11
6 Recovery of other nutrients . 12
6.1 General . 12
6.2 Nitrogen (N) . 12
6.3 Sulphur (S) . 12
6.4 Potassium (K) . 12
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ISO/TR 22707:2023(E)
7 Recovery of other inorganics . 12
7.1 General . 12
7.2 Metals . 13
Annex A (informative) Sewage sludge composition . 14
Annex B (informative) Case studies . 15
iv © ISO 2023 – All rights reserved
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ISO/TR 22707:2023(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 onof 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.htmlthe following URL: .
This document was prepared by Technical Committee ISO/TC 275, Sludge recovery, recycling, treatment
and disposal.
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.
© ISO 2023 – All rights reserved v
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ISO/TR 22707:2023(E)
Introduction
Inorganics & nutrientsand nutrient recovery is necessary to build a sustainable society and; there are
many studies and plants all over the world that demonstrate this concept. Above all, phosphorus
recovery systems to produce fertilizer material are increasingly common and other nutrients recovery
systems are now being developed.
This document ISO provides a selected overview of various technologies and is based on country
standards and guidance documents already in existence (or under preparation), and documents
provided by some private organizations.
It is notedAs inorganics and nutrientsnutrient recovery knowledge and technology is developing
rapidly., this document will therefore be reviewed in timeregularly to reflect the advancing nature of
the industry and technology.
Annex A of this document provides examples of sewage sludge composition, which can help determine
which element(s) can be recovered. Annex B provides case studies of nutrient recovery, including
practical and emerging ones.
vi © ISO 2023 – All rights reserved
---------------------- Page: 7 ----------------------
TECHNICAL REPORT ISO/TR 22707:2023(E)
Sludge recovery, recycling, treatment and disposal —
GuidanceInformation on the processes and technologies on
inorganics and nutrients recovery
1 Scope
This document provides guidance and good practiceinformation on the processes and technologies for
inorganics and nutrients recovery from sludge.
ItThis document is applicable to sludge and products from urban wastewater collection systems, night
soil, wastewater treatment plants for urban and similar industrial waters. It includes all sludge that
maycan have similar environmental and/or health impacts.
Hazardous sludge from industry and dredged sludge already covered by ISO/TC 190 "Soil Quality" isare
excluded from this document.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminologicalterminology databases for use in standardization at the following
addresses:
— IEC Electropedia: available at — ISO Online browsing platform: available at
https://www.iso.org/obp
— IEC Electropedia: available at https://www.electropedia.org/
3.1
ammonia stripping
a method of removingthe removal of ammoniacal compounds from water by making it alkaline, and
aerating.of aeration
3.2
calcium phosphate
salts that consist of calcium ions and phosphate ions
Note 1 to entry: Hydroxyapatite (HAP) is a form of calcium phosphate.
3.3
centrate
liquid product from a centrifugal dewatering device
3.4
hydroxyapatite
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ISO/TR 22707:2023(E)
(HAP)
sparingly soluble salt that is generated from phosphate and calcium ions
Note 1 to entry: The general chemical formula of HAP is Ca10(OH)2(PO4)6.
3.5
incineration ash
residue of combustion
3.6
nutrient
elementselement required by human and otherliving organisms throughout lifethe course of their lives
in small quantities for a range of physiological functions
3.7
seed crystal
crystal employed as a nucleus to generate and grow crystals in the crystallization process
3.8
struvite
a compound which is precipitated by magnesium addition to water with high concentration of
phosphate and ammonium ions
Note 1 to entry: The chemical formula of struvite is MgNH4PO4･·6H2O.
4 Methods of nutrient recovery from sludge
There are four methods for nutrient recovery from sludge, which are whole use, cleaning, separation,
and extraction. The use of these processes must comply with the regulation of each jurisdiction/country.
a) Whole use: Whole use of sludge is a simple use method in which sludge, which is typically
aerobically or anaerobically treated (e.g. compost), is directly applied to land as fertilizer or soil
improver. This method can minimize the loss of the nutrients in the treatment process and may
achieve the highest potential of utilizing the nutrients in sludge.
b) Cleaning: Cleaning is the process in which sludge has contaminants such as plastics or heavy metals
removed by mechanical treatment or chemical extraction. The cleaned sludge can be handled in the
same way as whole use.
c) Separation: Separation is the process in which sludge is divided into two or more different parts.
Sludge is separated by physical or chemical parameters such as size, shape, specific gravity
difference, and chemical affinity. All or only the least contaminated part of separated sludge may
then be utilized. In this method, sludge contains various nutrients.
d) Extraction: Extraction is the way in which only the target element is taken out as a compound using
chemical actions. Fewer nutrients in sludge are made available or utilized through extraction
processes than a)in whole use, b) cleaning or c)and separation, methods. However, the process has
some advantages:
— reduces the storage volume of the nutrient;
— prevents contamination of the recovered material by hazardous elements;
— stabilizes the recovered materials as a chemical compound;
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ISO/TR 22707:2023(E)
— improve the value of the recovered materials.
Precipitation including stripping processes can decrease the volatile nutrients content.
This guidelinedocument is focused on nutrients which can be recovered by extraction.
5 Phosphorus recovery
5.1 General
Phosphorus is an essential element for plant growth and is an important ingredient of chemical
fertilizer products. The dry solid contents of sludge normally include more than 1.,0 % phosphorus and
it maycan reach to 5.,0 % of sludge under certain operating conditions, such as biological
dephosphorization or anaerobic-anoxic-oxic processes.
On the other hand, the supply of phosphate ore in the global market is strongly influenced by political
and economical issues and often gets unstable, as it is quite unevenly distributed globally. Therefore,
studies and commercialization of phosphorus recovery from sludge is the most progressive area in
inorganic / nutrient materials recovery.
Phosphorus can be recovered from sludge using various chemical compounds. The phosphorus
recovery process which describesthat is described in this chapterClause 5 is organised assummarized in
Figure 1.
For case studies, refer to AnnexClauses B.1 to B.11.
Process type Specific process
5.2 Struvite recovery from
anaerobic digested sludge
and/or filtrate of anaerobic
digested sludge
Crystallization
5.3 Hydroxyapatite
recovery
Phosphorus recovery from
sewage sludge
Chemical extraction and 5.4 Phosphorus recovery
precipitation from incineration ash
Thermochemical 5.5 Phosphorus recovery
processing from sewage sludge slag
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ISO/TR 22707:2023(E)
Figure 1 The — Summary of phosphorus recovery process
5.2 Struvite recovery from anaerobic digested sludge and/or filtrate of anaerobic
digested sludge
5.2.1 Principles
The principles of the struvite recovery process isare based on the chemical precipitation carried out in a
crystallizer followed by particle separation. The chemical reaction is shown as follow; for struvite is:
3-− + 2+
PO4 + NH4 + Mg + 6H2O → MgNH4PO4･·6H2O (Struvite)
This reaction is the same as the scale formation which is frequently observed in anaerobic sludge
treatment facilities. The difference of struvite recovery from scale formation is well-controlled chemical
dosing, pH control and particle separation. After the application of this process, much less scale
formation is likely to occur in treatment facilities.
Recovered struvite can be used as delayed release fertilizer because of its low solubility.
There are two types of crystallizer processes - agitation by air or mechanical agitation.
Both methods of crystallization are employed in commercial operations. Wastewater employed for this
process is a filtrate of anaerobically digested sludge (ADS),) or ADS itself and industrial wastewater
containing phosphate and ammonium. Under optimum operating conditions, dissolved phosphorous
recovery maycan reach more than 80 % using this process.
5.2.2 Schematic diagram
Schematic diagrams for an air fluidized crystallizer and a mechanical agitator are shown in Figures 2
and 3. An influent such as a filtrate of ADS, and/or ADS, is mixed in the reactor with magnesium (Mg)
salt and struvite granules (as seed crystal). Alkalising chemicals such as sodium hydroxide solution can
be added for pH control.
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ISO/TR 22707:2023(E)
4
6
3
5
2
8
1
9
7
Key
1 ADS or ADS liquor 6 Treated sludge or liquor
2 Phosnix reactor 7 Air
3 Mg(OH) 8 Rotary sieve
2
4 NaOH 9 Struvite
5 Liquid cyclone
Key
1 ADS or ADS liquor 6 treated sludge or liquor
2 phosnix reactor 7 air
3 Mg(OH)2 8 rotary sieve
4 NaOH 9 struvite
5 liquid cyclone
[1]
Figure 2 — — Schematic diagram -of a fluidized bed reactor
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ISO/TR 22707:2023(E)
5
6
4
2
7
1
8
3
Key
1 Digested sludge 5 Struvite separator (cyclone)
2 Trash removal equipment 6 Treated sludge
3 Crystallization reactor
7 Washing/Drying equipment
4 Mg(OH) 8 Recovered struvite
2
Key
1 digested sludge 5 struvite separator (cyclone)
2 trash removal equipment 6 treated sludge
3 crystallization reactor 7 washing/drying equipment
4 Mg(OH)2 8 recovered struvite
SOURCE: Reference [2]. Reproduced with the permission of the authors.
Figure 3 — Schematic diagram -of a stirred tank reactor
5.2.3 Operating conditions
The key factors influencing struvite recovery rates are inflow concentrations of phosphate and
ammonium, the dosing rate of magnesium ions, alkalinity and pH.
The concentration of phosphate needs to be higher than 50 mg-/l of P/L, preferably over 100 mg-/l of
P/L and ammonia over 300 mg-/l of N/L. From an economical point of view, the pH needs to remain in
the range of 7,5- to 9,0. Various Mg compounds can be used as a Mg source of Mg including, Mg(OH) ,
2
MgCl and MgSO . Seawater can also be used as a Mg source of Mg.
2 4
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ISO/TR 22707:2023(E)
5.2.4 Characteristics of recovered products
Recovered struvite is crystalline with few impurities. Its shape depends on the above discussed
operating conditions, including pH, temperature, agitation and the retention time of struvite particles in
the crystallizer.
5.3 Hydroxyapatite recovery
5.3.1 Principles
The principles of hydroxyapatite (HAP) recovery process is based on chemical precipitation carried out
in a crystallizer followed by particle separation. The chemical reaction is shown as follow: for HAP is:
2+ 3- -
10Ca ＋+ 6PO + 2OH → Ca (OH) (PO ) (HAP)
4 10 2 4 6
HAP recovery systems require well-controlled chemical dosing, pH control and particle separation.
Recovered HAP can be used as raw material for fertilizers.
A crystallizer is a type of mixing reactor employed in commercial operations. The applicable
wastewater for this process will be black water (human fecesfaeces and urine), industrial wastewater
containing phosphate, and filtrate from sludge treatment processes.
Under general operating conditions, dissolved phosphorous recovery maycan reach 70 % through this
process.
5.3.2 Schematic diagram
A schematic diagram for a crystallizer is shown in Figure 4. Influent such as filtration liquid, calcium
chloride and HAP granules (as seed crystal) are mixed in the mixing reactor. Sodium hydroxide solution
is added for pH control.
2
1
3
4
Key
1 Filtration liquid
2 Calcium chloride
3 Effluent
4 HAP
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ISO/TR 22707:2023(E)
Key
1 filtration liquid
2 calcium chloride
3 effluent
4 HAP
Figure 4 — Schematic diagram -for HAP recovery
5.3.3 Operating conditions
The key factors influencing HAP recovery are inflow concentrations of phosphate, carbonate ion, the
dosing rate of calcium ions, alkalinity and pH.
The concentration of phosphate is required to be around 50 mg/l of P/L. From an economical point of
view, the pH needs to remain in the range of 7,5- to 9,0.
5.3.4 Characteristics of recovered products
Recovered HAP is a crystalline structure with few impurities. Its shape depends on the above discussed
operating conditions including pH, temperature, agitation and the retention time of HAP particles in the
crystallizer.
5.4 Phosphorus recovery from incineration ash
5.4.1 Principles
When advanced wastewater treatment technologies have been employed, phosphorus tends to
increasingly concentrate in sludge. As a result, incineration ash from sewage sludge contains almost the
same concentration of phosphorus as that of natural phosphate ore, shown in the Figure 5. Sewage
sludge ash is expected as one of the alternative sources of phosphorus which is concerned for depletion
in the future.
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ISO/TR 22707:2023(E)
Figure 5 — Composition comparison between phosphate ore and incineration ash
Phosphorus recovery from incineration ash can be achieved by means of a chemical reaction, which is
leaching and precipitation. Leach is performed under both alkaline and acidic condition.
Sewage sludge ash can be used directly as a P fertilizer. This direct utilization, however, is only possible
for sewage sludges with a low level of contamination. See AnnexClause B.8.
5.4.2 Alkaline treatment
5.4.2.1 Schematic diagram
A schematic diagram of the phosphorus recovery process from incineration ash is shown in the
Figure 6. It consists of two reaction tanks.
In the first reactor, phosphate is extracted from the incineration ash by using alkaline solution. In the
second reactor, phosphorus-rich sediment is precipitated by adding slaked calcium (Ca(OH) ) into a
2
solution taken from the first reactor. The solution after collecting phosphorus-rich sediment is recycled
and returned into the first reactor.
Key
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ISO/TR 22707:2023(E)
1 ash
2 incinerated ash
3 extraction
4 neutralized ash
3-
5 PO4 rich solution
6 precipitation
7 circulation of recycled solution
Figure 6 — Schematic diagram -of phosphorus recovery from incineration ash
5.4.2.2 Operating conditions
In the first reactor, phosphate in the incineration ash is extracted into solution by controlling pH at
0 0
more than 13, adding sodium hydroxide, maintaining temperature between 50 50° to 70 70°
centigrade, and a retention time of between 5 min and 30 minutes min. The chemical reaction is shown
as follow:
- 3-
P O + 6OH → 2PO + 3H O
2 5 4 2
In the second reactor, Ca(OH) is added in a solution from the first reactor and the substances that
2
0 0
contain a lot of phosphate are precipitated by controlling temperature between 20 20° to 50 50°
centigrade, and a retention time between 6 h and 18 hours h. The hydroxide ion that is consumed by
phosphorus extraction is replenished by using Ca(OH) as a calcium source. The chemical reaction is
2
shown as follow:
3- -
2PO4 + 3Ca(OH)2 → Ca3(PO4)2 + 6OH
5.4.3 Acidic treatment
Acidification is possible to extract more phosphoric acid and metal components such as iron and
aluminumaluminium than alkaline treatment, which can be recovered by precipitation. However, it is
necessary to pay attention to simultaneous extraction of heavy metals. In many cases, sulfuric acid or
hydrochloric acid is used as acid. Many studies on this concept are carrying out.
5.4.4 Characteristics of recovered products/residues
Phosphorus recovery technology from incineration ash can produce the two products, recovered
phosphorus and neutralized ash.
The recovered phosphorus from this technology is a mixture of HAP and calcium phosphate. The
concentration of phosphorus is approximately 30 %. After dewatering and drying, it can be utilized as a
raw material of fertilizer for agriculture, gardening, etc.
Neutralized ash contains fewer amounts of heavy metals than untreated incineration ash because it is
chemically treated for reducing and minimising solubility of heavy metals. After washing and drying, it
can be easily utilized for many applications like cement material, soil rehabilitation, etc.
5.5 Phosphorus recovery from sewage sludge slag
5.5.1 Principles
Melting is a thermochemical process for separating and refining target materials under high
temperature and altered atmospheric conditions. Hazardous heavy metals such as lead, cadmium,
mercury, which have lower boiling point than the temperature in the furnace, are evaporated.
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ISO/TR 22707:2023(E)
Phosphorus generally volatilizes under high temperature and reducing atmospheric conditions. The
melting technology, however, makes it possible to recover phosphorus in the slag in the form of
phosphorus oxide by controlling the air supply into the furnace for keepingto maintain oxidation
conditions, and,. Ca and Fe in the sludge effectively influences to fix p
...
TECHNICAL ISO/TR
REPORT 22707
First edition
Sludge recovery, recycling, treatment
and disposal — Guidance on the
processes and technologies on
inorganics and nutrients recovery
Valorisation, recyclage, traitement et élimination des boues — Guide
sur les procédés et les technologies de récupération des substances
inorganiques et des nutriments
PROOF/ÉPREUVE
Reference number
ISO/TR 22707:2023(E)
© ISO 2023
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ISO/TR 22707:2023(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2023
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
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ISO/TR 22707:2023(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Methods of nutrient recovery from sludge . 2
5 Phosphorus recovery .2
5.1 General . 2
5.2 Struvite recovery from anaerobic digested sludge and/or filtrate of anaerobic
digested sludge . 3
5.2.1 Principles . 3
5.2.2 Schematic diagram . 4
5.2.3 Operating conditions . 5
5.2.4 Characteristics of recovered products . 5
5.3 Hydroxyapatite recovery . 5
5.3.1 Principles . 5
5.3.2 Schematic diagram . 6
5.3.3 Operating conditions . 6
5.3.4 Characteristics of recovered products . 6
5.4 Phosphorus recovery from incineration ash . 6
5.4.1 Principles . 6
5.4.2 Alkaline treatment . 7
5.4.3 Acidic treatment . 8
5.4.4 Characteristics of recovered products/residues . 8
5.5 Phosphorus recovery from sewage sludge slag . 8
5.5.1 Principles . 8
5.5.2 Schematic diagram . 8
5.5.3 Operating conditions . 9
5.5.4 Characteristics of recovered products . 9
5.6 Other technologies for phosphorus recovery . 10
5.7 Summary . 10
6 Recovery of other nutrients .11
6.1 General . 11
6.2 Nitrogen . 11
6.3 Sulfur . 11
6.4 Potassium . 11
7 Recovery of other inorganics .11
7.1 General . 11
7.2 Metals . 11
Annex A (informative) Sewage sludge composition .13
Annex B (informative) Case studies .14
Bibliography .39
iii
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ISO/TR 22707:2023(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 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 275, Sludge recovery, recycling, treatment
and disposal.
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.
iv
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ISO/TR 22707:2023(E)
Introduction
Inorganics and nutrient recovery is necessary to build a sustainable society; there are many studies
and plants all over the world that demonstrate this concept. Above all, phosphorus recovery systems
to produce fertilizer material are increasingly common and other nutrients recovery systems are now
being developed.
This document provides a selected overview of various technologies and is based on country standards
and guidance documents already in existence or under preparation, and documents provided by
private organizations.
As inorganics and nutrient recovery knowledge and technology is developing rapidly, this document
will therefore be reviewed regularly to reflect the advancing nature of the industry and technology.
Annex A provides examples of sewage sludge composition, which can help determine which element(s)
can be recovered. Annex B provides case studies of nutrient recovery, including practical and emerging
ones.
v
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TECHNICAL REPORT ISO/TR 22707:2023(E)
Sludge recovery, recycling, treatment and disposal —
Guidance on the processes and technologies on inorganics
and nutrients recovery
1 Scope
This document provides information on the processes and technologies for inorganics and nutrients
recovery from sludge.
This document is applicable to sludge and products from urban wastewater collection systems, night
soil, wastewater treatment plants for urban and similar industrial waters. It includes all sludge that can
have similar environmental and/or health impacts.
Hazardous sludge from industry and dredged sludge are excluded from this document.
2 Normative references
There are no normative references in this document.
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
ammonia stripping
method of the removal of ammoniacal compounds from water by making it alkaline and of aeration
3.2
calcium phosphate
salts that consist of calcium ions and phosphate ions
Note 1 to entry: Hydroxyapatite (HAP) is a form of calcium phosphate.
3.3
centrate
liquid product from a centrifugal dewatering device
3.4
hydroxyapatite
HAP
sparingly soluble salt that is generated from phosphate and calcium ions
Note 1 to entry: The general chemical formula of HAP is Ca (OH) (PO ) .
10 2 4 6
3.5
incineration ash
residue of combustion
1
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ISO/TR 22707:2023(E)
3.6
nutrient
element required by living organisms throughout the course of their lives in small quantities for a range
of physiological functions
3.7
seed crystal
crystal employed as a nucleus to generate and grow crystals in the crystallization process
3.8
struvite
compound which is precipitated by magnesium addition to water with high concentration of phosphate
and ammonium ions
Note 1 to entry: The chemical formula of struvite is MgNH PO ·6H O.
4 4 2
4 Methods of nutrient recovery from sludge
There are four methods for nutrient recovery from sludge, which are whole use, cleaning, separation
and extraction.
a) Whole use: Whole use of sludge is a simple use method in which sludge, which is typically aerobically
or anaerobically treated (e.g. compost), is directly applied to land as fertilizer or soil improver. This
method can minimize the loss of the nutrients in the treatment process and may achieve the highest
potential of utilizing the nutrients in sludge.
b) Cleaning: Cleaning is the process in which sludge has contaminants such as plastics or heavy metals
removed by mechanical treatment or chemical extraction. The cleaned sludge can be handled in the
same way as whole use.
c) Separation: Separation is the process in which sludge is divided into two or more different parts.
Sludge is separated by physical or chemical parameters such as size, shape, specific gravity
difference and chemical affinity. All or only the least contaminated part of separated sludge may
then be utilized. In this method, sludge contains various nutrients.
d) Extraction: Extraction is the way in which only the target element is taken out as a compound
using chemical actions. Fewer nutrients in sludge are made available or utilized through extraction
processes than in whole use, cleaning and separation methods. However, the process has some
advantages:
— reduces the storage volume of the nutrient;
— prevents contamination of the recovered material by hazardous elements;
— stabilizes the recovered materials as a chemical compound;
— improve the value of the recovered materials.
Precipitation including stripping processes can decrease the volatile nutrients content.
This document is focused on nutrients which can be recovered by extraction.
5 Phosphorus recovery
5.1 General
Phosphorus is an essential element for plant growth and is an important ingredient of chemical
fertilizer products. The dry solid contents of sludge normally include more than 1,0 % phosphorus and
it can reach 5,0 % of sludge under certain operating conditions, such as biological dephosphorization or
anaerobic-anoxic-oxic processes.
2
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ISO/TR 22707:2023(E)
On the other hand, the supply of phosphate ore in the global market is strongly influenced by political
and economical issues and often gets unstable, as it is quite unevenly distributed globally. Therefore,
studies and commercialization of phosphorus recovery from sludge is the most progressive area in
inorganic / nutrient materials recovery.
Phosphorus can be recovered from sludge using various chemical compounds. The phosphorus recovery
process that is described in Clause 5 is summarized in Figure 1.
For case studies, refer to Clauses B.1 to B.11.
Figure 1 — Summary of phosphorus recovery process
5.2 Struvite recovery from anaerobic digested sludge and/or filtrate of anaerobic
digested sludge
5.2.1 Principles
The principles of the struvite recovery process are based on the chemical precipitation carried out in a
crystallizer followed by particle separation. The chemical reaction for struvite is:
3− + 2+
PO + NH + Mg + 6H O → MgNH PO ·6H O
4 4 2 4 4 2
This reaction is the same as the scale formation which is frequently observed in anaerobic sludge
treatment facilities. The difference of struvite recovery from scale formation is well-controlled
chemical dosing, pH control and particle separation. After the application of this process, much less
scale formation is likely to occur in treatment facilities.
Recovered struvite can be used as delayed release fertilizer because of its low solubility.
There are two types of crystallizer processes - agitation by air or mechanical agitation.
Both methods of crystallization are employed in commercial operations. Wastewater employed for
this process is a filtrate of anaerobically digested sludge (ADS) or ADS itself and industrial wastewater
containing phosphate and ammonium. Under optimum operating conditions, dissolved phosphorous
recovery can reach more than 80 % using this process.
3
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ISO/TR 22707:2023(E)
5.2.2 Schematic diagram
Schematic diagrams for an air fluidized crystallizer and a mechanical agitator are shown in Figures 2
and 3. An influent such as a filtrate of ADS, and/or ADS, is mixed in the reactor with magnesium (Mg)
salt and struvite granules (as seed crystal). Alkalising chemicals such as sodium hydroxide solution can
be added for pH control.
Key
1 ADS or ADS liquor 6 treated sludge or liquor
2 phosnix reactor 7 air
3 Mg(OH) 8 rotary sieve
2
4 NaOH 9 struvite
5 liquid cyclone
[1]
Figure 2 — Schematic diagram of a fluidized bed reactor
4
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ISO/TR 22707:2023(E)
Key
1 digested sludge 5 struvite separator (cyclone)
2 trash removal equipment 6 treated sludge
3 crystallization reactor 7 washing/drying equipment
4 Mg(OH) 8 recovered struvite
2
SOURCE Reference [2]. Reproduced with the permission of the authors.
Figure 3 — Schematic diagram of a stirred tank reactor
5.2.3 Operating conditions
The key factors influencing struvite recovery rates are inflow concentrations of phosphate and
ammonium, the dosing rate of magnesium ions, alkalinity and pH.
The concentration of phosphate needs to be higher than 50 mg/l of P, preferably over 100 mg/l of P and
ammonia over 300 mg/l of N. From an economical point of view, the pH needs to remain in the range of
7,5 to 9,0. Various Mg compounds can be used as a source of Mg including, Mg(OH) , MgCl and MgSO .
2 2 4
Seawater can also be used as a source of Mg.
5.2.4 Characteristics of recovered products
Recovered struvite is crystalline with few impurities. Its shape depends on the above discussed
operating conditions, including pH, temperature, agitation and the retention time of struvite particles
in the crystallizer.
5.3 Hydroxyapatite recovery
5.3.1 Principles
The principles of hydroxyapatite (HAP) recovery process is based on chemical precipitation carried out
in a crystallizer followed by particle separation. The chemical reaction for HAP is:
2+ 3- -
10Ca + 6PO + 2OH → Ca (OH) (PO )
4 10 2 4 6
HAP recovery systems require well-controlled chemical dosing, pH control and particle separation.
Recovered HAP can be used as raw material for fertilizers.
A crystallizer is a type of mixing reactor employed in commercial operations. The applicable wastewater
for this process will be black water (human faeces and urine), industrial wastewater containing
phosphate and filtrate from sludge treatment processes.
5
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Under general operating conditions, dissolved phosphorous recovery can reach 70 % through this
process.
5.3.2 Schematic diagram
A schematic diagram for a crystallizer is shown in Figure 4. Influent such as filtration liquid, calcium
chloride and HAP granules (as seed crystal) are mixed in the mixing reactor. Sodium hydroxide solution
is added for pH control.
Key
1 filtration liquid
2 calcium chloride
3 effluent
4 HAP
Figure 4 — Schematic diagram for HAP recovery
5.3.3 Operating conditions
The key factors influencing HAP recovery are inflow concentrations of phosphate, carbonate ion, the
dosing rate of calcium ions, alkalinity and pH.
The concentration of phosphate is required to be around 50 mg/l of P. From an economical point of
view, the pH needs to remain in the range of 7,5 to 9,0.
5.3.4 Characteristics of recovered products
Recovered HAP is a crystalline structure with few impurities. Its shape depends on the above discussed
operating conditions including pH, temperature, agitation and the retention time of HAP particles in
the crystallizer.
5.4 Phosphorus recovery from incineration ash
5.4.1 Principles
When advanced wastewater treatment technologies have been employed, phosphorus tends to
increasingly concentrate in sludge. As a result, incineration ash from sewage sludge contains almost the
same concentration of phosphorus as that of natural phosphate ore, shown in Figure 5. Sewage sludge
ash is expected as one of the alternative sources of phosphorus which is concerned for depletion in the
future.
6
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ISO/TR 22707:2023(E)
Figure 5 — Composition comparison between phosphate ore and incineration ash
Phosphorus recovery from incineration ash can be achieved by means of a chemical reaction, which is
leaching and precipitation. Leach is performed under both alkaline and acidic condition.
Sewage sludge ash can be used directly as a P fertilizer. This direct utilization, however, is only possible
for sewage sludges with a low level of contamination. See Clause B.8.
5.4.2 Alkaline treatment
5.4.2.1 Schematic diagram
A schematic diagram of the phosphorus recovery process from incineration ash is shown in Figure 6. It
consists of two reaction tanks.
In the first reactor, phosphate is extracted from the incineration ash by using alkaline solution. In the
second reactor, phosphorus-rich sediment is precipitated by adding slaked calcium (Ca(OH) ) into a
2
solution taken from the first reactor. The solution after collecting phosphorus-rich sediment is recycled
and returned into the first reactor.
Key
1 ash
2 incinerated ash
3 extraction
4 neutralized ash
3-
5 PO rich solution
4
6 precipitation
7 circulation of recycled solution
Figure 6 — Schematic diagram of phosphorus recovery from incineration ash
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ISO/TR 22707:2023(E)
5.4.2.2 Operating conditions
In the first reactor, phosphate in the incineration ash is extracted into solution by controlling pH at
more than 13, adding sodium hydroxide, maintaining temperature between 50° to 70° centigrade, and a
retention time of between 5 min and 30 min. The chemical reaction is:
- 3-
P O + 6OH → 2PO + 3H O
2 5 4 2
In the second reactor, Ca(OH) is added in a solution from the first reactor and the substances that
2
contain a lot of phosphate are precipitated by controlling temperature between 20° to 50° centigrade,
and a retention time between 6 h and 18 h. The hydroxide ion that is consumed by phosphorus
extraction is replenished by using Ca(OH) as a calcium source. The chemical reaction is:
2
3- -
2PO + 3Ca(OH) → Ca (PO ) + 6OH
4 2 3 4 2
5.4.3 Acidic treatment
Acidification is possible to extract more phosphoric acid and metal components such as iron and
aluminium than alkaline treatment, which can be recovered by precipitation. However, it is necessary to
pay attention to simultaneous extraction of heavy metals. In many cases, sulfuric acid or hydrochloric
acid is used as acid. Many studies on this concept are carrying out.
5.4.4 Characteristics of recovered products/residues
Phosphorus recovery technology from incineration ash can produce the two products, recovered
phosphorus and neutralized ash.
The recovered phosphorus from this technology is a mixture of HAP and calcium phosphate. The
concentration of phosphorus is approximately 30 %. After dewatering and drying, it can be utilized as a
raw material of fertilizer for agriculture, gardening, etc.
Neutralized ash contains fewer amounts of heavy metals than untreated incineration ash because it is
chemically treated for reducing and minimising solubility of heavy metals. After washing and drying, it
can be easily utilized for many applications like cement material, soil rehabilitation, etc.
5.5 Phosphorus recovery from sewage sludge slag
5.5.1 Principles
Melting is a thermochemical process for separating and refining target materials under high
temperature and altered atmospheric conditions. Hazardous heavy metals such as lead, cadmium,
mercury, which have lower boiling point than the temperature in the furnace, are evaporated.
Phosphorus generally volatilizes under high temperature and reducing atmospheric conditions.
The melting technology, however, makes it possible to recover phosphorus in the slag in the form of
phosphorus oxide by controlling the air supply into the furnace to maintain oxidation conditions. Ca
and Fe in the sludge effectively influences to fix phosphorus into the slag.
5.5.2 Schematic diagram
A schematic diagram of the phosphorus recovery process from sewage sludge into slag is shown in
Figure 7.
Dewatered sludge, after drying, is supplied to the furnace. Recovered heat from exhaust gas is used for
preheating of the drying and the combustion air. The gas treatment system neutralizes acidic gases and
separates heavy metals from gas as fly ash.
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ISO/TR 22707:2023(E)
Key
1 sewage sludge storage tank
2 dryer
3 melting furnace
4 air
5 water bath
6 heat recovery system
7 gas treatment system
8 P-rich slag (recovered phosphorus)
9 fly ash (heavy metals)
SOURCE Reference [3]. Reproduced with the permission of the authors.
Figure 7 — Schematic diagram of phosphorus recovery as slag
5.5.3 Operating conditions
The correct operating temperature in the melting furnace is between 1 250 °C to 1 350 °C. Air levels in
the melting zone in the furnace before secondary combustion is approximately 1,0 %.
If the low heat value (or net calorific value) of dried sludge is high enough, there is no need to supply fuel
in the melting furnace. This results in the following process. In the case of raw sludge with higher high
heat value, sludge is dried to a lower dryness in a dryer in order to achieve self-thermal combustion in
the melting furnace. In the case of digested sludge with lower high heat value, sludge is dried to higher
dryness in a dryer.
5.5.4 Characteristics of recovered products
Based on the above process, the melting process for sewage sludge generates slag which is equivalent
to 90 % to 95 % of ash content in the sludge, and approximately 90 % of phosphorus in the sludge
is recovered in the slag. The main components of the slag are silicon, calcium, phosphorous, iron and
aluminium. The phosphorous concentration in the slag is nearly equal to phosphorous concentration
in sludge incineration ash. Performance data shows that phosphorus concentration in the slag ranges
from 20 % to 30 % of dry P O .
2 5
Slag is a kind of amorphous material. Approximately 90 % of phosphorous in the slag is citric acid
soluble. Heavy metal concentration in the slag is low enough to be used as a fertilizer or raw material
fertilizer.
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ISO/TR 22707:2023(E)
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