SIST-TP CEN ISO/TR 22707:2026
(Main)Sludge recovery, recycling, treatment and disposal - Information on the processes and technologies for inorganic substance and nutrient recovery (ISO/TR 22707:2023)
Sludge recovery, recycling, treatment and disposal - Information on the processes and technologies for inorganic substance and nutrient recovery (ISO/TR 22707:2023)
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.
Schlammrückgewinnung, -verwertung, -behandlung und -entsorgung - Informationen über Verfahren und Technologien zur Rückgewinnung anorganischer Stoffe und Nährstoffe (ISO/TR 22707:2023)
Valorisation, recyclage, traitement et élimination des boues - Guide sur les procédés et les technologies de récupération des substances inorganiques et des nutriments (ISO/TR 22707:2023)
Predelava, recikliranje, obdelava in odlaganje blata - Informacije o postopkih in tehnologijah za pridobivanje/predelavo anorganskih snovi in hranil (ISO/TR 22707:2023)
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
kSIST-TP FprCEN ISO/TR 22707:2025
01-maj-2025
Predelava, recikliranje, obdelava in odlaganje blata - Informacije o postopkih in
tehnologijah za pridobivanje/predelavo anorganskih snovi in hrani l(ISO/TR
22707:2023)
Sludge recovery, recycling, treatment and disposal - Information on the processes and
technologies for inorganic substance and nutrient recovery (ISO/TR 22707:2023)
Valorisation, recyclage, traitement et élimination des boues - Guide sur les procédés et
les technologies de récupération des substances inorganiques et des nutriments
(ISO/TR 22707:2023)
Ta slovenski standard je istoveten z: FprCEN ISO/TR 22707
ICS:
13.030.20 Tekoči odpadki. Blato Liquid wastes. Sludge
kSIST-TP FprCEN ISO/TR 22707:2025 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
kSIST-TP FprCEN ISO/TR 22707:2025
kSIST-TP FprCEN ISO/TR 22707:2025
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)
kSIST-TP FprCEN ISO/TR 22707:2025
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
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ISO copyright office
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Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
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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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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
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notice of (a) patent(s) which may be required to implement this document. However, implementers are
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database available at www.iso.org/patents. ISO shall not be held responsible for identifying any or all
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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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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
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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.
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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.
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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
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
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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
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.
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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 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.
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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
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
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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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
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:
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.
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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 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.
5.6 Other technologies for phosphorus recovery
Other than the processes and technologies discussed in this subclause, additional processes and
technologies are available, including:
— the reductive melting process, a technology for yellow phosphorus recovery from sludge incineration
ash where yellow phosphorus is evaporated from melting ash under reductive atmosphere;
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— the heating process, which solubilizes excess sludge by heating and where eluted phosphate is
precipitated in the adsorption step;
— the adsorption process, which employs specific adsorbent to recover phosphorus with high
selectivity from centrate.
5.7 Summary
Table 1 shows a summary of high-level technologies for phosphorus recovery. It is noted that processes
and results can vary considerably depending on many conditions, such as the wastewater source, the
wastewater treatment process and sludge quality. The table therefore is read assuming all input factors
are the same.
Before the installation of any P recovery system, consideration of capital expenditure (CAPEX) and
operating expense (OPEX) is undertaken to ensure installation is financially viable. Financial viability
is highly dependent on the condition of the existing facilities and the circumstances.
Table 1 — Summary of phosphorus recovery
Process type
Chemical
Thermochemical
Crystallization extraction and
processing
precipitation
Filtrate of ADS, Filtrated liquid,
Input material Ash Ash, dried sludge
ADS return sludge
Characteristics of recovery system
Input material Soluble P Soluble P Solid P Solid P
Recovered material Struvite Hydroxyapatite Ca salt Slag
Use of recovered P Fertilizer Fertilizer Fertilizer Fertilizer
Comparative recovery
Low Very low High High
ratio of input P
a b b
Dosing chemicals Mg compounds Ca compounds Ca compounds None
Comparative amount
of dosing alkali for pH Low Low High Nil
control
Installation in sludge Need an anaerobic Need a filtration Need a melting
Need an incinerator
process flow treatment system system facility
Anti-scaling
Less moisture
Advantages Anti-scaling Decrease waste Decrease waste
content of
dewatering sludge
Requires attention Requires attention
Need excess High temperature
Disadvantages on heavy metals in on heavy metals in
ammonium ion dependence
sludge sludge
c
Implementation Most Most Less Least
a
Compounds which release Mg or Ca ions. Appropriate chemical can be selected depending on the process.
b
The necessity of Ca compounds depends on the process or plants.
c
Refer to Reference [12].
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6 Recovery of other nutrients
6.1 General
Sludge include various nutrients which is needed of growth of microorganism. A lot of recovery
technologies of these are still undergoing research and are not yet in commercial operation.
6.2 Nitrogen
Nitrogen (N) is an important nutrient for plants and fertilizer to increase the level of a crop’s production.
Sludge includes large amount of nitrogen and can reach 3 % on average and, in some cases, even more.
Studies for nitrogen recovery systems are underway and there are a few recovery plants in the world.
However, composting is much more economically effective than inorganic nitrogen recovery at present.
An ammonia stripping system is sometimes applied for centrate, industrial wastewater, etc. Recovered
ammonia is absorbed in acid liquids such as sulfuric acid and used as raw material for chemicals.
However, recovered ammonia can be released to the air as nitrogen gas after it is broken with a catalyst
under high temperature.
6.3 Sulfur
Much of organic matter includes sulfur (S) and sulfate is one of the most common acids used by industry.
Sludge can include 2 % to 3 % of sulfur.
Some researchers have studied a sulfur recovery process using bacteria from wastewater including
[4]
sulphide.
Contacted bacteria can oxidize sulphide to sulfur with aeration under controlled pH and ORP. However,
almost all sulfur is produced as a by-product of removing sulfur-containing contaminants from fossil
fuel and mine ore.
6.4 Potassium
Potassium (K) is one of the basic elements for plant growth, which is used as chemical fertilizer and
pesticide, etc. There are fewer studies developing potassium recovery from sludge as there is currently
no shortage of the resource.
Sludge can include 0,2 % to 0,5 % of potassium.
Potassium can be recovered as MgKPO from wastewater with high concentration of potassium and
phosphate dosing Mg salts under pH 11. This high alkalinity can strip ammonia from liquid to air and
prevent the generation of MgNH PO .
4 4
7 Recovery of other inorganics
7.1 General
Wastewater collecting system can gather not only wastewater but a lot of elements, which can be
considered a resource. Nowadays, sludge is being considered the urban mine of various elements.
7.2 Metals
Sludge is being considered the municipal mine of various metals, like Ag, Mg, Al and Fe. Much research
on metal recovery has progressed in the light of the circular economy principle, but they are currently
only emerging technologies.
On the other hand, there are some examples of metal recovery which generally happen as a by-product.
One example is gold recovery in a particular area in Japan. It is thought that trace amounts of gold, which
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comes from hot spring water and plating industrial wastewater, can be accumulated in incineration ash
of sludge.
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Annex A
(informative)
Sewage sludge composition
Table A.1 shows typical inorganic composition of sewage sludge.
Table A.1 — Inorganic composition of sewage sludge
a
Composition
Element % of dry solids
b c d[5] e[6]
China Finland Germany Japan
Nitrogen 2,96 4,0 5,0 5,67
(N) (0,84 – 5,03) (2,6 – 5,5) (0,2 – 14,7) (3,61 – 8,43)
Sulfur 0,858
No data No data No data
(S) (0,50 – 1,81)
Potassium 0,583 0,15 0,26 0,353
(K) (0,14 – 1,22) (0,08 – 0,5) (0,00 – 2,45) (0,188 – 0,558)
Phosphorus 2,22 3,0 2,5 2,75
(P) (0,6 – 3,74) (1,9 – 3,8) (0,05 – 5,2) (1,69 – 4,14)
a
Values are shown as an average (minimum minus maximum).
b
Samples come from sludge of 98 municipal sewage treatment plants (almost none of industrial wastewater included).
The wastewater treatment process includes activated sludge process, anaerobic-anoxic-oxic process, anaerobic-oxic
process and membrane bioreactor (MBR).
c
The number of samples is in the several hundreds. The samples are mainly dewatered sludge cake by centrifuge, dry
solid (DS) about 20 %. These data are from research reports, written in Finnish. The wastewater treatment processes are
based on activated sludge process in Finland.
d
The number of samples is 704. The samples are municipal sewage sludge for agricultural utilization (2017/2018). Data
have been queried in the federal states of Germany which have to report the quality of agricultural utilized sewage sludge
annually. Some data are obviously implausible or runaway values.
e
The number of samples is 32. The samples include sludge cake dewatered by centrifuge, screw press, multidisc screw
press, belt press or rotary press. The wastewater treatment process includes activated sludge process, anaerobic-anoxic-
oxic process, anaerobic-oxic process, modified Ludzack-Ettinger (MLE) process, step-feed MLE process and oxidation ditch
process.
kSIST-TP FprCEN ISO/TR 22707:2025
ISO/TR 22707:2023(E)
Annex B
(informative)
Case studies
B.1 Phosphorus recovery from centrate ‘Phosnix’
1)
For information on phosphorus recovery from centrate ‘Phosnix’ , see Table B.1, and Figures B.1 and
B.2.
Table B.1 — Technical overview
General data
Type of process Crystallization
Filtration liquid, Dewatered sludge, ADS, Filtrate of ADS,
Input material Incineration ash, Dried sludge, Industrial wastewater,
Others ( )
Phosphorus: Struvite, HAP, CaPO , Others( )
Product
Nitrogen, Metals ( ), Others ( )
Wastewater treatment plant (WWTP) with P removal with coagulant and
Type of plant
anaerobic digestion
T-P recovery rate —
Dissolved P recovery rate 60 % to 70 %
P-concentration of product 27 % P O of dry matter (DM)
2 5
Average total electricity de-
(2,5 to 4,0) kWh/kg-P
recovered
mand
(2,5 to 3,0) kgMg(OH) /kg-P
2 recovered
Average chemical demand
(0,8 to 1,1) molar ratio Mg: P
recovered
(as 100 % concentrate)
(0,8 to 1,2) molar ratio Mg: P
dissolved
Reference
Filtration liquid, Dewatered sludge, ADS, Filtrate of ADS,
Raw materials for recovery Incineration ash, Dried sludge, Industrial wastewater,
Others ( )
Reuse of recovered materials Fertilizer Raw material of chemicals Others ( )
Country Japan
In order to keep the water body quality of Lake Shinjiko, the local govern-
ment must decrease total phosphorus discharge of STP. Along with the global
Background
demand for sustainable society, phosphorus recovery from the sewage sludge
had been started.
Objective Resource recovery and improvement of sewer effluent quality
Amount of recovered struvite: 0,6 kg/m -centrate
Result
Effluent of T-P: 1 mg/l → 0,4 mg/l without coagulant dosing into water treat-
ment process
1) Phosnix is an example of a suitable product available commercially. This information is given for the convenience
of users of this document and does not constitute an endorsement by ISO of this product.
kSIST-TP FprCEN ISO/TR 22707:2025
ISO/TR 22707:2023(E)
TTabablele B B.11 ((ccoonnttiinnueuedd))
This plant is the one of the earliest commercialized plants in the world for
phosphorus control and recovery from STP. One reactor system for crystalli-
Description
zation and solid separation make small footprint. Application of magnesium
(differentiation from others)
hydroxide as chemical can reduce operation cost.
Not only centrate but sludge can be treated in almost same process.
[Project outline]
Location: Shinjiko East STP (Shimane)
Capacity: 1 150 m -centrate/day
Main components and
Operation start: 1998
specification
[Main equipment]
Crystallizer/fluidized bed: φ3,6 m /1,42 m × H 7 m
Struvite separator: φ300 mm × l 1 000 mm
Source See Reference [1]
Organization: Japan Sewage Works Agency
Contact details
Email: js -international@ jswa .go .jp
Key
1 water treatment process 6 Phosnix
2 excess sludge 7 centrate
3 anaerobic digester 8 struvite
4 digested sludge 9 cake
5 dewatering facility
Figure B.1 — Process flow diagram
kSIST-TP FprCEN ISO/TR 22707:2025
ISO/TR 22707:2023(E)
Figure B.2 — Commercial plant in Shimane
B.2 Struvite recovery from digested sewage sludge
For information on struvite recovery from digested sewage sludge, see Table B.2, and Figures B.3 and
B.4.
kSIST-TP FprCEN ISO/TR 22707:2025
ISO/TR 22707:2023(E)
Table B.2 — Technical overview
General data
Type of process Crystallization
Filtration liquid, Dewatered sludge, ADS, Filtrate of ADS,
Input material Incineration ash, Dried sludge, Industrial wastewater,
Others ( )
Phosphorus: Struvite, HAP, CaPO , Others( )
Product
Nitrogen, Metals ( ), Others ( )
Type of plant WWTP with P removal with coagulant and anaerobic digestion
T-P recovery rate 30 % to 40 %
Dissolved P recovery rate More than 80 %
P-concentration of product 26 % P O of DM
2 5
Average total electricity demand Not detected (ND)
)Average chemical demand
ND
(as 100 % concentrate)
Reference
Filtration liquid, Dewatered sludge, ADS, Filtrate of ADS,
Raw materials for recovery Incineration ash, Dried sludge, Industrial wastewater,
Others ( )
Reuse of recovered materials Fertilizer Raw material of chemicals Others ( )
Country Japan
Removal and reuse of phosphorus are very important theme in sewerage
Background treatment. Therefore, a more effective phosphorus recovery process from
digested sewage sludge has been developed.
Objective Removal and reuse of phosphorus in sewage
T-P removal from the returned water: more than 85 %
Result
Quantity of struvite: more than 1,4 times than a conventional method
This technology can recover more phosphorus from the digested sludge
D e s c r ip t ion
than struvite recovery methods applied to the filtrate of anaerobic digested
(differentiation from others)
sludge.
[Project outline]
Location: Higashinada WWTP (KOBE)
Capacity: 239 m /d (digested sludge)
Operation start: 2013
Main components and spec
...
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