CAS INSIGHTSSUSTAINABLE
AGRICULTURE

Innovation in fertilizer production

SUSTAINABLE AGRICULTURE INSIGHT REPORT | 1

Introduction
Modern agricultural methods are struggling to meet
future food demand imposed by the accelerating
growth of the world’s population, set to reach 8.5
billion by 2030, and 9.7 billion by 2050.1 Furthermore,
the total global food demand is expected to increaseby 35% to 56% between 2010 and 2050.2
With mounting pressures come economic
consequences. According to reports from The Foodand Agriculture Organization of the United Nations
and the Intergovernmental Panel on Climate
Change Special Report on Climate Change andLand, agricultural emissions are estimated to range
between 10.7 and 12.0 gigatonnes of CO2 equivalent(GtCO2e) year. 3
The most important nutrients for fertilizers arenitrogen, phosphorus, and potassium, while others,
such as calcium, magnesium, and sulfur, tend
to be less frequently used.4–7 Synthetic fertilizers
use phosphorus mined from phosphate rock,potassium mined from potash ores, and ammonia
from atmospheric nitrogen.8 It has been calculated
that 147 million tons of ammonia, 219 million tonsof phosphate, and 44 million tons of potassium
were produced in 2020.9–11 Despite these substantial
amounts, nearly half of the global population in 2015
(approximately 3.5 billion) were thought to be reliant
on food produced by nitrogen fertilizers alone.12

Organic fertilizers include manure from various
animals, alfalfa meal, blood meal, fish meal, andwood ash, as well as wastewater or sewage.13 Manureand other agricultural wastes can be bulky andexpensive to treat, transport, and dispose. The globalorganic fertilizer market is expected to reach $19.88
billion by 2029, at a compound annual growth rate of
11.6% during the forecast period 2022–2029.14
China is the world's largest consumer of fertilizers,
having consumed more than 45 million metric tons
worth of nutrients in 2019.15 India and the U.S. comein second and third, with consumption recorded
at 29.4 and 20.4 million metric tons, respectively.15
Together, these statistics suggest that fertilizer
demand is far outweighing supply. Escalating
fertilizer costs further compound the risk of global
food security. Following an 80% increase in fertilizer
prices in 2021, a further 30% increase has beenrecorded since the start of 2022, driven by higher
production costs, supply disruptions due to conflicts
including the Russia–Ukraine war, sanctions, andexport restrictions from China.16

The carbon footprint for fertilizer production
While millions of tons of fertilizers are produced andconsumed, existing methods for fertilizer production
are not sustainable in the long term. Phosphate rockis a finite, non-renewable source that may be depleted
in the next century, with the majority (70%) being
concentrated in Morocco, followed by China (5%), Syria
and Algeria (3%), Russia, South Africa, the U.S., Egypt
and Jordan (2% each).17,18 Synthesis of ammonia from
nitrogen by the Haber-Bosch process (not all of whichis converted to fertilizer) is another energy-intensive
process estimated to emit 500 tons of CO 2 annually, andis responsible for 1.8% of global total energy emissions.19
The use of ammonia-based fertilizers in soil can leadto the formation of nitrous oxide, a potent greenhousegas.19 Both phosphate rock mining and nitrogen fixationstrongly depend on fossil fuels such as oil and naturalgas. Potassium in the form of potash is derived fromsoluble salts mined in the upper northern hemisphere,
and while its supply is still sufficient, 92% of reserves areconcentrated in only four countries: Canada (who owns
53% of global reserves), Russia, Belarus, and Germany. 20

Nutr
as ienfer

We are reaching a tipping point. Radical transformations
must be made to counteract food shortages and combat
climate change. The agricultural industry must work
towards a system that is sustainable, through efficientuse of water, energy and nutrient resources, a reductionof environmental impacts, maintaining economic
strength, and minimizing dependence on finite nonrenewable resources preserving the ability of bothcurrent and future generations to thrive (Figure 1). 24

Fertilizer

Soil

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Nutrient

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ew ve
at ry
er

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ti

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ta
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Most organic fertilizers are bulky and thus impracticaland costly to transport over long distances; therefore,
production is restricted to local sites, in turn leading tochallenges with upscaling production. Pharmaceutical,
pathogen, and heavy metal contamination can befound in wastewater and sewage, 21 and require testing
and mitigation to avoid transferring contaminants tofarmland and crops. 22 There is also significant uncertainty
around the true scale of CO 2 emissions associated withthe use of organic and synthetic fertilizers. 3,23

Waste
water

nt
rie trt
Nu andFigure 1. Circulation of nutrients in sustainable agriculture
SUSTAINABLE AGRICULTURE INSIGHT REPORT | 3

Let’s not waste our waste: opportunities for nutrient recovery
The term circular bioeconomy has recently gained
popularity as a way of transforming and managing
our land, food, health, and industrial systems via the
management of biological resources (plants, animals,
micro-organisms, and derived biomass, includingorganic waste), with the goal of achieving sustainable
wellbeing in harmony with nature. 25,26
Traditionally, solid and biosolid wastes have been
dumped in landfills or the ocean, or applied to land,
while biosolid wastes are also incinerated, which reduces
waste volume but still produces ash. 27–29
Overapplication of manure or treated biosolids toland can result in a build-up of nutrients such as
nitrogen and phosphorus, resulting in eutrophicationof surface waters, which can be damaging to aquatic
ecosystems. 30–32 To address these problems, alternative
systems and processes are being developed to provide
new uses for waste while reducing contamination, andthus their harm, to human and environmental health.
Sewage sludge generated by wastewater treatment
presents significant potential for repurposing waste
through various processes, depending on sludge type.
Sewage sludge can undergo thermal decomposition to
produce biochar, hydrochar, or ashes, which are rich inenergy and nutrients for use in fertilizer production. 21,33–35

Other common methods for nutrient recovery fromwaste are summarized in Table 1.
Potentially sustainable methods include:– Eco-friendly technologies: Composting,vermicomposting, composting with biochar, anaerobic
digestion, and pyrolysis using green energy– New emerging technologies: Electrolysis andforward osmosis– Smart nano-fertilizers: A combination of
nanotechnology with controlled nutrient releasethrough degradable delivery systems36– Biorefineries: A combined production of sustainableenergy, biofuels, food, chemicals, and nutrientrich fertilizers that utilize renewable biomass asa feedstock, 37 which show added environmental
benefits following integration with other systems,e.g., a mixed crop-livestock system38– Struvite: Struvite (MgNH 4 PO4) is a crystalline
mineral composed of magnesium, ammonium, andphosphate, and is considered an adequate source ofnutrients and a sustainable means for phosphorus
recovery from wastewater, but can also be used either
in fertilizer formulations or as a slow-release fertilizer39

Using manure and agricultural wastes in organic fertilizer
formulations or deriving chemical nutrients fromagricultural wastes and many types of wastewaters could
reduce transport and disposal costs while supplementing
the available chemical nutrient supply.
Table 1. Overview of commonly used nutrient recovery processes from waste

MethodDescription

Biological

Anaerobic
digestion– The natural process in which micro-organisms break down organic materials in closed spaces
where there is no air (or oxygen)40– Preferred method for treating organic fractions of agricultural, industrial, and municipal solid waste– Methods: Hydrolysis (polymer decomposition), acidogenesis (volatile fatty acid production),
acetogenesis (acetic acid production), and methanogenesis (methane production)40– Products: Digestate (a by-product of biogas production)40

Composting– An aerobic, thermophilic, micro-organism-mediated bioconversion of organic matter into humic
substances41– Product: Compost

Vermicomposting– A bioconversion method that utilizes microbes as well as earthworms for the decomposition ofsolid organic wastes into useful organic fertilizer– Can be used to convert waste from food, plants, animals, pharmaceuticals, and sewage42–44– Product: Vermicompost

Method

Description

ChemicalChemicalprecipitation andcrystallization– The most common chemical technology for phosphate recovery from municipal wastewater– The formation of struvite has been commercialized as a treatment process for phosphorous
and ammonia recovery from wastewater sludge dewatering*45– For phosphate recovery, the crystallization of calcium phosphate has been performed using
calcites as seeds46– Products: Ca5(OH)(PO4)3 (hydroxyapatite) and NH4MgPO4.6H2O (struvite)

Ion-exchange
membraneelectrolysis (ED)– Extraction of nutrients from wastewater occurs via the application of ion-exchange
membranes47– Advanced methods such as selective ED and simultaneous anionic/cationic selective ED areeven more efficient48,49– Products: (NH4)+, K+, Ca2+, Mg2+ and (PO4)3-

PhysicalIncineration,
gasification, andpyrolysis ash
nutrient recovery– Incinerator ash is rich in nutrients such as phosphorus and can be used as a part of fertilizer
feedstock– Pyrolysis includes waste heating under a limited supply of oxygen. It is a very important process
to manage livestock waste.50 Bio-oil and biochar or charcoal are produced from pyrolysis– Gasification is a form of pyrolysis that applies to higher temperatures producing mainly gases
and a small amount of biochar51– Products: Ash; and bio-oil, biochar or charcoal (pyrolysis)

Forward Osmosis(FO)– FO is a technique that uses an osmotic pressure gradient as a driving force and semipermeable membranes to separate dissolved solutes from water52,53– The use of selective osmotic membranes improves phosphate and ammonium nutrient
recovery from wastewater52,53– Products: Phosphate and ammonium nutrients

Adsorption,
absorption, andsorbents– Natural adsorbents such as zeolites, clays, biopolymers, and biochar have been investigated for
nutrient recovery54–56– Biochar was applied to recover 96% of ammonium and phosphate from swine wastewater54– Na- and K-zeolites were used for ammonium and phosphorus recovery in conjunction with
MgO to stabilize bobierrite or struvite,55 while Ca-zeolites facilitated the recovery of phosphates
from wastewater via formation of Ca-phosphates56– Products: Struvite and calcium phosphate

Membrane
filtration– Useful for nutrient recovery from anaerobically digested slurries– The combination of microfiltration, ultrafiltration, and nanofiltration membranes allows
recovery of >94% of nitrogen from digestate57– Recovery of both ammonia (30–36%) and phosphate (83–95%) has also been demonstrated bynanofiltration58– Products: Phosphate and ammonium nutrients*Struvite precipitation can be classed as a biological and physical method of nutrient recovery.

SUSTAINABLE AGRICULTURE INSIGHT REPORT | 5

Highlights and trends in nutrient recovery
The CAS Content Collection™ is an expert-curated resource that was used to evaluate methods of nutrient
recovery and the concepts driving innovation to foster a more circular economy.
Publications and topics
A broad fertilizer search retrieved 121,213 patents
and 125,228 journal publications over the 2001–
2021 period. The number of journal and patentpublications grew steadily and following a peak in
2017, has since been decreasing (Figure 2). Trends
in patent publications during this period appeared
to be influenced primarily by China, who was
responsible for a substantial number of patents
in the field.

Topics associated with journals were related tothe effects of fertilizers on crop plant growth,
biological responses, and soil fertility, with some
focusing on nutrient removal (in terms of their roleas potential pollutants) or recovery for fertilizer use.
Topics associated with patents focused strongly
on organic substances and processes associatedwith fertilizer nutrient recovery and formulations,
including bio-waste-related topics, e.g., manure,ashes, and fermentation.

24K
Journals

Patents

22K
20K
18K

Number of Publications

16K
14K
12K
10K
8K
6K
4K
2K
0K

2001

2003

2005

2007

2009

2011

2013

2015

2017

2019

2021

Year
Figure 2. Journal and patent publication numbers (2001–2021) for a broader search of fertilizers, sustainability, recycling,
and recovery topics

Substance classes that were discussed for nutrient
recovery included “elements”, “oxides” and“hydroxides”, “metal salts”, “minerals”, and “polymers”.
Carbon was the most popular element, featuring
in almost 14,000 publications. Oxides were mostly
represented by silicon (1,214 publications), calcium (917publications), and magnesium (882 publications) oxides,
while the most frequently cited hydroxides included
calcium, magnesium, and aluminium hydroxides.
Calcium carbonate (1,820 publications) was the mostfrequently reported compound, used as both aprecipitating agent and sorbent.

Nutrient recovery from waste and wastewater
Findings showed that biological processes were themost prominent methods for nutrient recovery fromwastewater, having increased by 40% between 2001
and 2018, while physical and chemical methods wereassociated with a 20% and 27% increase in publications,
respectively, during the 2002 to 2012 period.
Patents comprised a significant proportion — 53% — of
publications on physical methods, while making up
49.5% of publications for chemical methods and 37% of
publications for biological methods.

The number of documents concerning nutrient
recovery from wastewater treatment sludge, biochar,
and ashes increased in number during the periodof study, while the number associated with sewage
sludge fertilizers remained low. A clear increasing
trend in charcoal/biochar topics was noted in patents
and journals, with journal publications demonstrating
continued growth despite a slight drop in 2019.
Patent publications have grown, especially since2013, although the numbers year on year have beensomewhat variable (Figure 3).

90
Journals

Patents

80

Number of Publications

70
60
50
40
30
20
10
0

2003

2005

2007

2010

2012

2014

2016

2018

2020

Year
Figure 3. Patents and journals (2000–2021) including the CAS term for biochar for narrower search of fertilizers, sustainability,
recycling, and recovery topics related to wastes and wastewaters. 2001, 2002 and 2008 had values of zero (not shown)

SUSTAINABLE AGRICULTURE INSIGHT REPORT | 7

Struvite publications increased substantially, where
hexahydrate [(NH 4)Mg(PO 4).6H 2 O] was the dominant
form studied — little has been published on potassium
struvite (MgK(PO 4).6H 2 O). “Precipitation wastewater
treatment” was a major concept relating to struviteA

production, comprising >90% of all concepts listed,followed by “crystallization wastewater treatment”.
Both concepts are directly related to struvite
recovery (Figure 4).

B
Neutralization wastewater treatment
Reductive wastewater treatment
Acidification wastewater treatment

18%

Wastewater treatment hydrolysis
Wastewater treatment coagulation

46%

Oxidative treatment hydrolysis
Chemical wastewater treatment

36%

Ion exchange wastewater treatment
Electrochemical wastewater treatment
Crystallization wastewater treatment
Precipitation wastewater treatment
PhysicalChemical

Biological

0

50

100

150

Number of Publications

Figure 4. A) Publications on biological, chemical, and physical methods associated with wastewater treatment for struvite precipitation
and B) Chemical treatment concepts for struvite production

Green ammonia synthesis
The process of making ammonia 100% renewable andcarbon-free, using renewable energy, nitrogen, andwater — was discussed primarily in journal publications,with patent documents reaching 20% of the total
publication volume in 2020. There was a dramatic growthin substances with a role in catalytic green ammonia
synthesis from 2017 to 2021; numbers increased from lessthan 100 distinct substances in 2017 to nearly 500 distinct
substances in 2021.
Specific subsets of substance classes, such as inorganic
materials, organic/inorganic small molecules, elements,
and co-ordination compounds make up many of the newcatalysts used in green ammonia synthesis (Figure 5).
“Electrochemical reduction” and related concepts wereamong the most frequently occurring concepts noted
in green ammonia and nitrogen reduction reaction,with “photocatalysts” occurring less frequently. The
proportion of journal publications featuring

photocatalytic or electrocatalytic nitrogen reduction
grew from 1% in 2001 to 25% in 2021.
In terms of catalyst evaluation, 52% (1,167/2,256) of
documents discussed reaction “rate,” “yield,” or“mechanism.” More than 40% of 1,500 documents with afocus on electrocatalysis discussed faradaic efficiencies,
consistent with an emphasis on the potential efficiencyof ammonia production. Only 4.1% of the 2,256
documents discussed the use of 15N or other isotopicallylabeled reagents; the use of these reagents is important
to determine the source of the ammonia produced bya method and its reproducibility, implying that moreconsistent use of control experiments is likely needed.

500

450

400

Number of Publications

350

300

250

200

150

100

50

0

2001

2005

2007

2009

2011

2013

2015

2017

2019

2021

Year
Substance ClassAlloy

Element

Org/Inorganic Small Molecule

Salts/Compounds with Salts

Coordination Compound

MineralPolymer

Tabular Inorganic

Figure 5. Publication trends and distinct substances used for catalysts by year (2001–2021) in green ammonia
synthesis research. Data for 2003-2004 is not available

SUSTAINABLE AGRICULTURE INSIGHT REPORT | 9

Looking to the futureThis analysis using the CAS Content Collection serves asa snapshot of popular and emerging methods to recover,recycle, and reuse fertilizer nutrients from various wastes
and wastewater generated by humans in food, livestock,and industrial production.
Proactive recovery of fertilizer macronutrients from waste
would reduce energy consumption and eliminate future
concerns regarding finite global reserves. Alternative“greener” processes for fertilizer production include
green ammonia synthesis and recovery of fertilizer
nutrients from phosphorus-containing wastes. The
feasibility of integrated systems has been demonstrated,
where multiple processes can be used together toproduce a variety of products with minimal waste.
For example, during industrial algae production as a“biofuels crop”, nutrients are required for microalgal
growth in the cultivation media to produce biomass foruse in the biorefinery system for biofuels production. 59
Animal-derived wastewaters and anaerobically digested
manures have been used to supply nutrients to systems
such as one using lipid-rich pig wastes, to grow alipid-accumulating algal biomass that can be used
for biodiesel production. 60 Aquaculture wastewaters
integrated into hydroponic or aquaponic plant growthsystems is another example of such systems, which canbe used for the combined production of plant biofuels
or food products. 61
Several processes involving phosphorus and/or nitrogenrecovery have been commercialized.

These include:– AirPrex® Patented Sludge Optimization Process(CNP Cycle GmBH) removes struvite from biosolids62– Ostara’s proprietary Pearl® and WASSTRIP®
technologies are designed to recover phosphorus andnitrogen to create an environmentally responsible
fertilizer (Crystal Green®) 63– Metso Outotec's AshDec® Thermochemical
P-Recovery recovers phosphorus from sewage
sludge ash 64– RecoPhos have developed the thermoreductive
RecoPhos-Process for phosphorus recovery fromsewage sludge and sewage sludge ashes65
These examples of waste recovery innovations are proof
that sustainable agriculture will be a key contributor
to the circular bioeconomy by minimizing energy
consumption and protecting the finite nature of global
phosphorus, nitrogen, and potassium reserves.
As an expert-curated resource, the CAS Content
Collection was employed to perform a quantitative
analysis of fertilizer and waste recovery processes overtime, across countries/regions, specific research areasand substances. It is hoped that by highlighting thesetrends in nutrient waste recovery, researchers in relevant
industries will feel equipped to augment fertilizers
with alternative nutrient resources while improving theefficiency and sustainability of waste management.

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For more details on thecurrent challenges andresearch landscape ofsustainable fertilizers,
see our publication at
cas.org/sustainable-ag

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