Closing the phosphorus loop: circular economy approaches for phosphorus recovery from agricultural wastes in India

Abstract

Phosphorus (P) is an important macronutrient vital for plant development, energy transfer, and food production, making its steady availability essential for agricultural yields and food security. Nonetheless, its availability mainly hinges on limited phosphate rock deposits and fertilizer systems that heavily depend on imports. In India, issues like reliance on fertilizers, varying levels of fertility across many regions, and the rise in agricultural waste production highlight the necessity for implementing more circular P management methods. Existing farming methods, such as residue burning and poor nutrient cycling, worsen P depletion and harm the environment. This review examines major waste streams in India that have potential for P recovery, including crop residues, livestock waste, fish processing waste and food processing waste. It also highlights key recovery methods like struvite crystallization, adsorption, thermochemical processing, chemical extraction, biological treatment and hybrid approaches. These waste streams hold significant quantities of P, which if properly harnessed, can lessen dependence on limited phosphate rock supplies and minimize nutrient leakage into the environment. Research shows that recovery efficiency differs significantly depending on the feedstock type, processing conditions and operational scale, indicating a requirement for customized extraction methods to enhance P recovery. Recovered P can aid in replacing P fertilizers, thus minimizing environmental losses and improving nutrient use efficiency. Nevertheless, widespread adoption in India encounters obstacles including gaps in collection and segregation, contamination risks, elevated capital and operational costs, and insufficient integration into value chain, alongside societal pushback against altering practices such as residue burning. Enhancing policy backing, performing localized techno-economic evaluations, establishing incentive structures, and crafting market routes for recovered P products will be vital for broadening circular P systems and boosting long-term nutrient security, while easing environmental and economic stresses.

1 Introduction

Phosphorus (P) is crucial for global agriculture and food security as it is vital for the growth of both plants and animals, yet its main commercial source, mined phosphate rock, is a non-renewable geological resource limited to a few countries and is prone to depletion. All the plants and animals need P, a non-renewable macronutrient, for basic metabolite functions like energy transfer, nucleic acid synthesis, and membrane function. The availability of P in soils directly affects agricultural productivity, and deficiencies limits crop yields and food output and maintaining a sufficient supply of P is essential to food security (Cordell et al., 2009). Managing P fluxes has become a strategic necessity for India, a country with one of the largest agricultural sectors in the world and a constantly expanding population. Mined rock phosphates are the main source of commercial P fertilizers. In accordance with global assessments, phosphate rock is a limited, non-renewable resource, and the present reserve and demand trends raise concerns regarding sustainability of the supply over the long term (Cordell et al., 2009, 2011; Cordell and White, 2011). India’s agriculture is heavily dependent on imported P. As evidenced by the country’s mineral P import reliance ratio of roughly 93% (Keil et al., 2018), it reflects the scarcity of the high-grade phosphate rock formations in India. Only roughly 35% of the country’s need can be satisfied by India’s 15.3 million tonnes of native high-grade rock phosphates reserves (Rao et al., 2015).

Following China, India ranks as the second largest consumer of P worldwide (IFA, 2023; Wentworth et al., 2024). Due to the rising population and a larger P footprint per person caused by dietary changes (Gandhi and Zhou, 2014; Metson et al., 2012), India’s P demand is set to keep increasing. Moreover, more than 90% of the represented districts exhibits low to medium fertility (Sanyal et al., 2015). Significant amounts of P contained in agricultural residues, livestock manure, slaughterhouse by-products, and industrial effluents are lost from productive cycles and represent squandered possibilities for nutrient recovery and reuse.

The world’s human population is likely to increase to nearly 9.7 billion by 2050 (World Population Prospects, UN, 2024). The increased population across the globe has led to an increased demand for food, thereby resulting in the production of a large amount of agricultural residues and biomass wastes (Li et al., 2021). A substantial amount of agricultural residues is either burnt or used as animal feed, a process which results in environmental pollution. The depletion of non-renewable resources has triggered a lot of interest in the conversion of agricultural wastes into industrial products or new carbon resources. Agriculture plays a vital role in the Indian economy and supports a large population of the country. In India about 140 million hectares are under cultivation with a cropping intensity of about 136%. The rice-wheat system is one of the most common practices in the northern regions of India, including the states of Punjab, Haryana, Uttar Pradesh, and Madhya Pradesh. However, the intensification of the rice-wheat system has led to the production of a large quantity of residues. These residues are a rich source of organic carbon and nutrients such as nitrogen (N), P, potassium (K), and sulphur (S) (Singh et al., 2020). As per the Indian Government’s National Policy for Management of Crop Residues (NPMCR), approximately 500 Mt of crop residue is generated each year in India (Government of India, 2014; Wentworth et al., 2024). Out of the total crop residue, 18.5% is incinerated, 9.57% is retained in the soil, and 71.93% is utilized (Bhuvaneshwari et al., 2019a; Government of India, 2014) and 25% of the total P in burned residue is lost (Mandal et al., 2004). Rice straw residues contribute to approximately 40% of total residues burnt in open fields, whereas wheat residues contribute to 22% and sugarcane trash contributes to 20% (Jain et al., 2014; Rajitha et al., 2025). The burning of one tonne of residues, generates approximately 3 kg of particulate matter, 60 kg of carbon monoxide (CO), 1460 kg of carbon dioxide (CO₂), and 2 kg of sulphur dioxide (SO₂), along with trace amounts of other gases and aerosols, released into the atmosphere (Meena et al., 2020). Apart from air pollution, crop residue burning is responsible for soil pollution through increased soil temperature and destruction of soil microorganisms. For instance, one tonne of rice straw contains approximately 5.5 kg nitrogen, 2.3 kg P₂O₅, 25 kg K₂O, and 1.2 kg Sulphur along with trace amounts of other nutrients (Meena et al., 2020). These residues can be recycled through various technologies and utilized in nutrient cycling. According to 19^th Livestock Census, livestock population in India is 512.05 million which produces 1095 million MT dung per year (Prasad et al., 2014). Waste from livestock and poultry industry includes a mixture of excreta, bedding material or litter, waste feed, dead animals or birds, broken eggs and farm sweep outs (Parihar et al., 2019). Food processing wastes is another major challenge that the food-production and food-processing sector faces. Cereal waste in India ranges between 3.89% and 5.92%, whereas losses from fruit and vegetables range from 4.87% to 15.05%. Food losses in the milk, meat, poultry and marine fisheries industries, ranging between 1% to 6%, have also been reported (pib.gov.in; 2022). Aquaculture is also a vital sector that plays a significant role in ensuring food and nutritional security at the global level that is witnessing rapid growth and contributing to agricultural waste. India, the world’s second-largest aquaculture producer, generates over 2 million metric tons of fish processing waste annually, with significant contributions from Gujarat, Maharashtra, and Kerala. This waste, often 30-85% of total fish weight, is mostly discarded, creating pollution risks (Sumithra and Amala, 2020). The demand for seafood is projected to increase by 60% with rise in global population, which is expected to reach 9.8 billion in 2050 (FAO, 2020; European Commission, 2014).

The interrelation of necessity, limited global supplies, regional concentration, and India’s 93% reliance on imports presents a strong argument for investigating domestic P recovery and circular economy methods as strategic complements to conventional mineral imports. The P circular economy seeks to close nutrient loops by recovering P from agricultural and agro-industrial waste, reintegrating it into agricultural systems. This method not only preserves an essential resource but also reduces environmental hazards, improves nutrient use efficiency (NUE), and bolsters agricultural sustainability. Although P recovery studies has been reported widely, most of the studies are fragmented across individual technologies, emphasizing on water- based systems. This review aims to synthesize the major P recovery technologies for different streams of agricultural wastes in India, where there is distinct mix of crop, livestock, aquaculture and food processing wastes. This review will focus on the relative strengths and limitations of different P recovery technologies, address feedstock specific opportunities, practical constraints and highlight research and policy implementation gaps for scalable circular P management in India.

2 Agricultural wastes as source of Phosphorus

2.1 Crop residues

Crop residues are rich in organic matter and is an important secondary source of P, which can be converted into valuable products through various technologies (Xie et al., 2023). Understanding the transformation dynamics of organic matter and P during processing of waste is essential for enhancing the utilization efficiency of the products. Consequently, clarifying these transformation traits is an essential prerequisite for the essential recovery and valorization of agricultural organic waste. Of the total P resources used in agriculture, which includes both mined and naturally occurring sources, only about 13 – 17% is eventually consumed as food (Vaccari et al., 2019). The P content and speciation in agricultural residues greatly affect the immediate availability of P and its later interactions within the soil matrix. The characteristics and levels of P in residues are influenced by environmental conditions, soil attributes and the crop’s physiological maturity when harvested. The P concentration in residues is a critical determinant of P mineralization potential; residues with P concentrations exceeding 3 mg g⁻¹ typically facilitate mineralization, whereas concentrations below this threshold may lead to microbial immobilization due to microbial demand outstripping supply (Kwabiah et al., 2003; Iqbal, 2009).

Phosphorus in crop residues exists in two primary forms: water-soluble inorganic P (Pi), which is rapidly leachable, and organic P (Po), which decomposes more slowly. Studies have reported that Pi constitutes approximately 40–60% of total P in mature residues and 60–80% in green residues (Noack et al., 2012; Ha et al., 2008). Root residues exhibit comparable P release kinetics compared to above-ground biomass. The existence of residues reduced the adsorption of P from a soil lacking adequate P supply (Thibaud et al., 1988). Within plant tissues, P is stored in vacuoles as Pi and maintained in cytoplasm at concentrations typically between 0.1–0.8 mg g⁻¹, both of which are readily bioavailable upon residue decomposition (Soumya et al., 2022).

2.2 Livestock wastes

Livestock manures, including those from cattle, swine, and poultry, are valuable organic P sources with varied chemical compositions. Contrary to crop residues, 70–80% of the P contained in livestock manure represents a substantial secondary P resource for agriculture (Karunanithi et al., 2015). Pig slurry, which contains 8–10% solids, consists of both inorganic and organic P, mostly bound to colloidal and suspended particles. Mechanical separation yields a P-rich solid fraction and a nutrient-diluted liquid fraction. The solid portions hold the majority of the P and organic materials, while the liquid portion comprises most of the soluble salts, which include nitrogen. The solid fraction may be utilized directly for enhancing soil or for bio-energy generation via incineration or pyrolysis, followed by the extraction of P from ashes and biochar (Burton and Turner, 2003; Schoumans et al., 2011). The inorganic P content in composted manures differs by species: cattle (90%), swine (93%), and poultry (83%) (Komiyama and Ito, 2019). The readily soluble phosphate diminished as the composting duration increased. As the composting duration extended, the development of insoluble phosphate enhanced. The highest percentage of water soluble and easily soluble inorganic phosphate was found in cattle manure compost with rice hull, while the lowest was seen in the compost with bark or sawdust (Yokota et al., 2003). P speciation in manure also includes significant proportions of organic P, particularly phytate. Broiler and turkey manure contain 26–56% of total P as phytate (Maguire et al., 2004; Leytem et al., 2006). Swine manure generally exhibits lower phytate content (Turner and Leytem, 2004). The compositional differences in both the concentration and speciation of P in animal manures are predominantly determined by the P content of the diet (Fuentes et al., 2006). In feed formulations, P is introduced either as inorganic orthophosphate salts or via plant-based ingredients, wherein P occurs in organically bound forms such as adenosine triphosphate (ATP), nucleic acids, phospholipids, phosphoproteins, and various phosphoglucides. Within the phosphoglucide fraction, myo-inositol hexakisphosphate (phytic acid) constitutes the principal storage form, representing approximately 60–80% of total grain P. This dominance has significant consequences for P bioavailability and the following transformation routes in manure – soil systems.

2.3 Agro-industrial and food processing waste

Each year, the worldwide seafood sector generates 6–8 million tonnes of crustacean shells, comprising byproducts from the processing of shrimp, lobster, oyster and crab (Vicente et al., 2022). Disposing these food wastes in landfills can result in soil and groundwater pollution, as well as the proliferation of pest and pathogens (Nor Faiza et al., 2019). Byproducts of food processing such as peels, seeds, and pomace also serve as renewable P sources. With 50–90% of cereals diverted to animal feed, only 10–40% of ingested P is retained by livestock, and the remainder is excreted. Effective manure management is essential to harness this P resource, yet in many regions, less than half of manure is collected and returned to fields. In China, for example, an estimated 45% of manure-derived P enters surface waters (Schröder et al., 2011). Specific plant residues, such as dried papaya peel, have demonstrated appreciable P content, potentially contributing over 10% of the recommended daily intake from a 10 g sample (Wadhwa et al., 2016). Globally, around 10 million metric tons of biomass are burned annually for purposes such as land clearing and domestic energy, with humans responsible for over 90% of such activity (Levine, 1990). The ash, comprising 0.1–1.5% P by weight, often remains in-situ, though incomplete combustion can result in higher ash yields.

Phosphorus in anaerobic digestates is predominantly present in inorganic forms complexed with Fe, Al, Mn, Mg, and Ca ions, constituting approximately 80–90% of the total P content. Digestates derived from the anaerobic fermentation of agricultural and food wastes, particularly fruit and vegetable residues, exhibited the lowest total P concentrations (Tuszynska et al., 2021). In the solid fraction of digestates, highly labile P; comprising P associated with organic matter and bound to Al-, Fe-, Mg-, and Mn-bearing oxides and hydroxides; represented 30–70% of total P. In contrast, the liquid fraction contained a markedly higher proportion of labile P, accounting for 80–90% of the total P, indicating a greater immediate bioavailability potential in the aqueous phase.

2.4 Fishery and aquaculture waste

Aquatic wastes are rich in organic content along with nitrogen and P. The by-products from fisheries aquaculture, such as fish viscera and processing waste, offer a concentrated geographically dispersed source of P, necessitating strategic decentralized recovery systems. These are high in particularly organic P, which present significant scope for circular nutrient management if stabilization methods are taken up to address rapid breakdown of protein components (Jupp et al., 2020). The primary contributors to high level of organic P are feed residues, excreta, which are mostly bound within complex biological structures. The fish processing effluents however have significant amounts of inorganic P, such as orthophosphates, which are easily accessible, but highly mobile. These wastes are a potential threat to aquatic life if discharged into aquatic resources without proper treatment. Inappropriate management of wastes generated during aquaculture activities may result in environmental degradation. Due to the fragmented nature of these waste streams, especially in inland aquaculture centers integrated recovery models that combine onsite stabilization with nutrient extractions technologies are essential for capturing their value while reducing environmental pollution due to discharge into water bodies (Wentworth et al., 2024). The amount of fish sludge that enters the aquatic system is 7-10% of the total amount of fish food supplied as input into the system (Strauch et al., 2018). Aquaculture sludge, which accumulates P from leftovers and waste, is an emerging resource which upon controlled processing can greatly decrease reliance on mineral fertilizers (Mukherjee et al., 2015). The varying forms of P in different sources of waste necessitates integrated treatment systems for enhancing its recovery efficiency (Jupp et al., 2020; Mukherjee et al., 2015). Therefore, conversion of fish wastes into valuable products like fish silage, fish protein hydrolysates, animal feed, and organic fertilizers has gained more importance as a means of environmental sustainability (De et al., 2020). The comparative assessment of the different agri waste streams are present in Table 1.

3 Agricultural waste as central to circular P systems

The European Commission presented the Circular Economy concept as a solution to environment and social issues arising from the swift increase in human population (Chojnacka et al., 2020). By-products from a single production process serve as secondary raw materials in another, replacing raw materials with leftover biomass, such as postharvest waste, livestock production residues, fish by-products and waste from slaughter and food processing. Fertilizer production is a very energy-intensive process, relying on fossil fuels (e.g.: Nitrogen fertilizers) (Sigurnjak et al., 2019) or fossil ore sources (e.g.: Phosphate rock) (Ahuja et al., 2020; Chojnacka et al., 2020; Zhang et al., 2023).The circular economy seeks to encourage the use of recycled materials and to complete the nutrient cycle by preventing nutrient loss and harmful environmental impacts (Egan et al., 2022).

The circular economy represents an essential shift from the conventional linear model to a regenerative system that preserves resource value over a longer duration (Nenciu et al., 2022). Given the finite reserves of rock phosphate and substantial dependence on imported P fertilizers, developing circular economy model for P via enhanced nutrient recovery and recycling (Nesme and Withers, 2016) is a practical approach to guarantee food security. The circular nutrient economy model has been well-known in Indian agriculture for a while. Traditional approaches have emphasized resource recycling through the integration of crops and livestock, with animal manure improving soil health and crop residues feeding livestock. These nutrient cycles in a closed loop maintained soil fertility for an extended duration (Eghombi et al., 2022).

A circular system for P in agriculture can be described in three phases:

• Reduction of losses at source through effective fertilizer use and preventing practices such as burning residues or mishandling manure. In addition to individual farms, regional nutrient recycling systems are essential for reducing P losses. For instance, extensive livestock farms typically produce an excess of nutrients, while adjacent crop farms may encounter shortages. Enabling the transfer of manure, compost or processed fertilizers between farms promotes a more even nutrient distribution, lessens localized nutrient excess, and bolsters resilience in P availability. Rittmann et al. (2011) estimated that about 46% of all loss of mined P, occurs through agricultural runoff. Additionally, only 16% of the P applied as fertilizer to crops, remains in the final food product, due to losses at all stages in the food system. Cancellier et al. (2025) reported castor oil-based polyurethane coating-controlled release fertilizers of MAP with suitable release rates can enhance P fertilizer efficiency in high-P fixing soils. A study by Chowdhury and Zhang (2021) indicated the high potential for improving PUE in the agricultural production system of Asian countries particularly through diverting manure-bound P from non-agricultural land or unproductive use to agricultural lands. The retention or incorporation of the residues in soil can contribute to closing the P loop in agriculture systems.

• Recycling and recovery from waste streams such as agricultural residues, animal manure, agro-industrial processing waste and sewage sludge and wastewater effluents: Crop residues can contain significant amounts of P - ranging from 1.0–11.5 kg P/tonne depending on the crop and residue type, where straws contain less P and cover crops as well as residues like sugarcane trash contain higher P content. The different recovery pathways are anaerobic digestion, composting, struvite precipitation, pyrolysis, chemical treatment, thermochemical treatment.

• Reuse through reintegration including returning recovered P into soils: This is a critical strategy for creating a circular economy and ensuring the long-term sustainability of agriculture. Damon et al. (2014), report that residue-derived P recovered in plants generally ranges from 5-40%. Mau et al. (2021) reported that P from algal biomass is available to wheat seedlings upon its application and is released gradually over time and showed similar growth as with fertilizer. In the rice-wheat cropping system, integrating a suitable P reuse strategy with struvite as a P fertilizer may be sustainable, resulting in enhanced crop P uptake, P use efficiency, while preserving comparable yields (Wang et al., 2023). Biochar can serve as P fertilizer if the feedstock was high in P, like poultry manure. Its primary benefit, however, is its capacity to adsorb and hold soluble P, functioning like a sponge to inhibit leaching and ensuring its availability for plant uptake over time. Compost serves as a natural fertilizer and soil additive that improves the chemical and physical characteristics, while supplying P. During the comparison of composts prepared from fish waste, sewage sludge, green waste, and horse manure, it was noted that fish waste compost had relatively high proportion of labile P, while, sewage sludge compost had the highest overall P concentration but labile P was only 6% (Lanno et al., 2021).

4 Methods of Phosphorus recovery

P can be recovered from agricultural wastes through a range of chemical, biological and thermochemical processes that convert P in wastes into fertilizers or phosphate materials like struvite, biochar P and hydroxyapaptite.

4.1 Precipitation and crystallization method

P from manures, digestate or fish processing wastewater is crystallized with Mg and NH₄⁺ to form struvite (MgNH₄PO₄.6H₂O), a slow-release NP fertilizer. Struvite precipitation is the most widely studied and commercially implemented method of P recovery from wastes (Yang et al., 2012; Yilmazel and Demirer, 2011; Zhang et al., 2014; Westerman et al., 2010; Kim et al., 2017). Struvite precipitate extracted from piggery wastewater comprised approximately 94.1% PO₄^3- - P (Kim et al., 2023). Recovery efficiency ranges from 70-90% depending on waste characteristics and operating conditions like (pH, molar ratios, reaction time, temperature) (Yang et al., 2012; Yilmazel and Demirer, 2011; Zhang et al., 2014; Kim et al., 2017; Ryu et al., 2020). Struvite precipitation is implemented in liquid phase of anaerobic digester effluents, showing recovering efficiencies of 95.1% PO₄^3– P (Yilmazel and Demirer, 2013). Nitrogen and P can be recovered simultaneously through this method, facilitating production of high value slow release fertilizer (Ryu et al., 2020; Kataki et al., 2016; Lorick et al., 2020). Beal et al. (1999) reported that struvite precipitation from raw swine waste led to 92% phosphate recovery, a level that increased to 98% after prior treatment through anaerobic digestion. Under ideal conditions, the recovery of N and P as NH₄-N and PO₄-P from anaerobic digestion effluents sourced from swine manure were 74% and 83% (Ryu et al., 2020).

The significant P concentration, along with the considerable amounts generated by intensive poultry farming, establishes poultry waste as a key focus for P recovery through precipitation (Fiameni et al., 2021; Chang et al., 2022; Fahimi et al., 2025; Luo et al., 2022). Yang et al. (2012) reported an 88.7% efficiency in P removal at pH 10.5 with a Mg:N:P molar ratio of 1.2:1:1. Yilmazel and Demirer (2011) obtained 85-90% P recovery from the anaerobic digestion byproducts of poultry manure through struvite precipitation. Struvite precipitation achieved recovery efficiency rates of 80-95% of soluble P with optimized pH and Mg addition, under varying effluent type (livestock slurry, wastewater, food industry effluents) (Achilleos et al., 2022).

In the absence of previous biological P enrichment, merely 10-40% of the P found in the aqueous phase can be retrieved, especially when phosphate is already attached to metals or biomass (Salkunić et al., 2022). Additionally, the procedure is financially viable only if the liquid phase has a minimum of 50–60 mg L-1 of phosphate and sufficient amounts of essential ions like Mg2+, Ca2+ and NH4+ to enable large scale precipitation (Salkunić et al., 2022). Furthermore, effective struvire precipitation necessitates accurate Mg dosing and exact pH control (around 8), as variations from the ideal Mg/N/P stochiometric ratio considerably lower P recovery efficiency and the quality of the recovered product (Salkunić et al., 2022).

4.2 Adsorption and ion exchange

Adsorption and ion exchange method use solid sorbents to selectively capture phosphate ions from solution followed by desorption and recovery. Pyrolyzed crop residues or manures produce biochar, which can be metal modified with Mg, Al, Fe and serves as effective phosphate adsorbent. The recovery efficiency of this method is approximately 95%. Ferric oxide hydrate/biochar composites have been studied for phosphate adsorption from swine manure, achieving significant P removal (Zhang et al., 2016). The advantage of this method is their selectivity and ability to treat dilute P streams. Co-pyrolysis of nutrient rich biomass feedstocks, like crop residues has been studied to create biochar products with enhanced P and K content (Rajput et al., 2024).

Biochar products offer further advantages beyond P availability, such as soil carbon storage better water retention, and increased microbial activity in the soil. Co-pyrolysis methods that improve biochar to increase its P and K levels result in multifunctional soil enhancers (Rajput et al., 2024). Nevertheless, the immediate P availability from biochar is typically less than that from struvite or traditional fertilizers, rendering biochar more appropriate for long-term soil improvement instead of meeting immediate nutrient needs for crops. Biochar (created via the pyrolysis of agricultural waste and manure) captures nearly 100% P from the raw materials and stabilized as Ca/Mg/Fe phosphates depending on the pyrolysis parameters (Xu et al., 2016).

P recovery methods reliant on adsorption and ion exchange are affected by numerous operational factors, such as absorbent type, surface area, pH, contact time and adsorbent amount, complicating process optimization, scaling up and consistency when feedstock charactertics differ (Salkunić et al., 2022; González et al., 2025). While agricultural waste derived adsorbents that have been thermally valorized offer a low cost and sustainable option, their capacity for P adsorption is typically moderate (around 55–60 mg P g-1), neccessating relatively large amounts of adsorbents to realize significant P recovery (González et al., 2025). Moreover, the existence of competing ions diminishes phosphate adsorption effectiveness and makes adsorbents regeneration more complex (Salkunić et al., 2022). The long-term stability, resistance to fouling, selectivity and mechanical durability of ion-exchange materials are also not adequately characterized in practical operating conditions (Salkunić et al., 2022). While alumina-based adsorption systems can reach P extraction efficiencies over 91% with limited heavy-metal transfer, they necessitate substantial quantities of adsorbents and do not have thorough assessments of reusability, economic viability, and handling of used adsorbents and leachates (Liu et al., 2021). Moreover, ion – exchange systems incur considerable operational expenses linked to cleaning, maintenance and chemical dosing needs, restricting their widespread use (Salkunić et al., 2022).

4.3 Thermochemical treatments

Thermochemical method includes pyrolysis, hydrothermal carbonization (HTC) and combustion (Fiameni et al., 2021; Rajput et al., 2024; Fahimi et al., 2025). Pyrolysis involves heating biomass in absence of oxygen at temperatures ranging from 300-700°C. Pyrolysis of swine manure at 500°C produced biochar with 5-10% P (dry wt), representing 2–3-fold increase in concentration compared to the original manure (Azuara et al., 2013). The P in these biochar exist as calcium phosphates and Mg phosphates with plant availability depending on the specific mineral forms and processing condition. HTC processes biomass in hot compressed water under autogenous pressure producing hydrochar with concentrated nutrients. HTC of animal manures showed P recovery of 80-95% in solid hydrochar product (Heilmann et al., 2014). Combustion and incineration at high temperatures (800°C) produce P rich ash with P content often exceeding 10-20% (Fiameni et al., 2014; Fahimi et al., 2025). Poultry litter ash contains 15-25% P as various Ca and Mg phosphates. Thermochemical method offers advantage of significant volume reduction and stable products. However, P in these products are easily available to plants compared to struvite and requires acid extraction to improve its solubility (Fiameni et al., 2021; Fahimi et al., 2025). Products treated thermochemically might demonstrate greater plant-available phosphorus compared to struvite, as thermal treatment can decompose organic matrices and change mineral associations, resulting in more labile phosphate forms that dissolve easily in soil. Struvite is more stable and dissolves more slowly; therefore, its phosphorus release tends to be lower unless acidic soil conditions facilitate it. Acid extraction can enhance the solubility of thermochemical products by freeing phosphate that is attached to calcium, iron, or aluminum phases (Khomenko et al., 2023; Nanzer et al., 2014). Co-pyrolysis of nutrient rich biomass feedstocks has been used to develop biochar products with enhanced P and K content (Rajput et al., 2024). Burning poultry litter produces ash that includes 15-25% P, mainly in the form of calcium phosphates (Fiameni et al., 2021; Fahimi et al., 2025). Fiameni et al. (2021) examined the concurrent extraction of amorphous silica and P from ash derived from rice husk poultry litter, highlighting the possibility for multi-nutrient recovery of about 16.17% of P₂O₅ and 19.66% of SiO₂. Microwave treatment resulted in the creation of NaCaPO₄ crystals, accompanied by a considerable rise in P solubility, which is vital for plant absorption (Fahimi et al., 2025). Thermochemical processing is potentially best suited for crop residues, as it focuses P while generating biochar that has soil amendment qualities.

Phosphorus in biochar and ash produced thermodynamically mainly occurs as calcium phosphates and various mineral forms with differing plant availability (Heilmann et al., 2014; Azuara et al., 2013; Rajput et al., 2024). The availability of the plant is influenced by the specific mineral phases, particle size and soil conditions. Microwave-assisted treatment and additional post-processing techniques can increase P solubility in ash materials, thereby enhancing their value as fertilizers (Fahimi et al., 2025).

The main constraints of this process are the significant capital and operational expenses, which are influenced by the energy-intensive requirements for drying feedstock and the high-temperature reactors (Aghili et al., 2026; Fiameni et al., 2022). The method accumulates dangerous heavy metals and pollutants in the final solids, necessitating costly secondary leaching or purification processes to fulfil stringent agricultural safety regulations (Adam et al., 2009). Furthermore, extreme temperatures frequently amalgamate P into stable, vitrified mineral forms or insoluble ash phases that demonstrates very low availability for plants (Adam et al., 2009; Huang and Tang, 2015), whereas techniques like hydrothermal carbonization separate the nutrient into distinct solid and liquid phases, making full extraction more challenging and diminishing overall yield efficiency (Huang and Tang, 2015; Li et al., 2023).

4.4 Chemical treatment

Chemical precipitation methods can recover P as various phosphate minerals. Calcium phosphate precipitation produces hydroxy apatite, which occur in high Calcium wastes. The formation of calcium phosphates vs struvite depends upon the relative concentration of Ca and Mg, pH and other solution conditions. A different approach is acid leaching, which involves the chemical dissolution of P from manure, ash or fish waste utilizing mineral or organic acids (such as sulfuric acid, hydrochloric acid, nitric acid, citric acid and formic acid) followed by a precipitation process (Xu et al., 2024a). This treatment can recover 80-90% of solubilized P. This technique is straightforward, adaptable and yields high purity products when used with adsorbents (Xu et al., 2024b).

Electrodialysis is a process for recovering P from household wastewater, relying on the selective extraction that separates anions (PO₄^3-, SO₄^2-, Cl^-) and cations (NH₄⁺, K⁺, Na⁺, Ca²⁺, Mg²⁺), concentrating them into distinct solutions. This takes place via ion-exchange membrane, propelled by an electric field applied between electrodes, resulting in 95% P recovery at a pH of 2.0. This anaerobic method facilitated the extraction of heavy metals Cd (31%), Cr (6%), Cu (22%), Ni (56%), Pb (1%) and Zn (85%), due to reduced adsorption sites, enhancing P availability and extraction ability (Carrillo et al., 2020). Ebbers et al. (2015) and Zhang et al. (2013) utilized electrodialysis for recovering P from wastewater, achieving efficiencies of 85% and 93%.

The electrochemical process is a technique for accumulating phosphate and subsequently recovering it, relying on the conversion of chemical energy found in organic matter into electrical energy via microorganism. The anode and cathode individual chambers may be divided by an ion exchange membrane to avoid the blending of oxidation and reduction products (Ichihashi and Hirooka, 2012). Ye et al. (2019) investigated nutrient recovery within a double-chamber microbial fuel cell, attaining a P recovery efficiency of 94%, where P comprised 2.6% of the overall precipitate mass. Huang et al. (2016) reached 95.7% purity at a current density of 2mA/cm² while examining the purity of struvite obtained at various current densities of 1–5 mA/cm², whereas Kruk et al. (2014) obtained 90% purity in precipitated struvite utilizing a magnesium sacrificial anode.

P recovery from agricultural waste through chemical extraction faces various technical and financial constraints. Pretreatments with chelating agents such as EDTA, Na₂-EDTA, Na₄-EDTA, NTA, Na₂-oxalate and EDTMP can extract 9-37% of P; however, they can only eliminate up to 33% of the majority of heavy metals and metalloids (excluding As), leading to restricted depollution alongside the need for expensive chemical inputs (Liu et al., 2021). The effectiveness of chemical extraction significantly relies on the feedstock used; for instance, the SEAL-phos method is suitable solely for Al-rich minerals, attaining P extraction efficiencies of as much as 78%, while acid-base leaching can boost P recovery to around 86% but necessitates extra chemicals for pH modifications and stays selective toward particular feedstocks (Liu et al., 2021). Multi-stage extraction methods intended for materials high in Al and Fe enhance P recovery to approximate 80%, but require increased chemical usage, energy needs, specialized machinery and in certain instances, resin regeneration to achieve high-purity outputs (Liu et al., 2021). While acidic extraction at a pH lower than 2 can nearly achieve full P solubilization, it also co-exerts heavy metals like As, Cd, Cu, Pb and Zn, requiring extra depolluting measures that raise process complexity and expenses (Liu et al., 2021). On the other hand, alkaline extraction at pH 12 or above reduces heavy-metals leaching but is efficient solely for feedstocks high in non-apatite inorganic P, thus restricting its use to a limited variety of agricultural residues (Liu et al., 2021). Moreover, the handling and disposal of ash byproducts and water from post-treatments processes are still unresolved for many chemical extraction techniques, such as EDTA-based, SESAL-phos, acid-base and multi-step extraction methods, with thorough techno-economic evaluations still absent (Liu et al., 2021). Moreover, EDTA-based extraction typically attains only moderate P recovery (around 73%) and exhibits relatively low product purity, while the disposal options for leftover ash and process water still not clearly defined (Liu et al., 2021).

4.5 Biological and process integrated methods

This approach leverage microbial process to transform P into more recoverable forms or to accumulate P in microbial biomass. Aerobic microbial pre-treatment of livestock manure increased recoverable phosphate content by 92% by solubilizing organically bound and mineral associated P (Rabinovich, 2021). Microorganism that solubilize phosphate, including phosphate solubilizing bacteria (PSB) like Bacillus, Pseudomonas, Rhizobium, and Burkholderia/Prarbulkholderia; phosphate solubilizing fungi (PSF) such as Aspergillus, Penicillium, and Trichoderma; mycorrhizal fungi like Glomus; and cyanobacteria such as Anabaena/nostoc, contribute to the release of organic acids, extrusion of protons, hydrolysis by phosphatase/phytase, and mediation by siderophores, transforming both insoluble mineral and organic P into plant-accessible orthophosphate (Mukherjee et al., 2015; Oubohssaine et al., 2025).

Anaerobic digestion (AD), primarily used for biogas production, serves as an important pre-treatment for P recovery by converting organic P to orthophosphates and increasing ammonium concentration, both of which are favorable for struvite formation (Yilmazel and Demirer, 2011; Yilmazel and Demirer, 2013; Ryu et al., 2020; Valverde-Vozmediano et al., 2025). A microbial pre-treatment using cellulose/lignin-degrading consortia (frequently Trichoderma + cellulolytic Bacillus) speeds up hydrolysis and enhances the yield of P-rich digestate; following AD, most input P is preserved in the residue, but only about 10-20% exists as soluble plant available P unless a downstream conditioning process is executed (Oubohssaine et al., 2025). The digestate from anaerobic digestion of animal manures and crop residues represent an ideal feedstock due to its high orthophosphate and ammonium content (Martín-Sanz-Garrido et al., 2025). Spectroscopic analyses have identified struvite (MgNH₄PO₄·6H₂O) and dicalcium phosphate anhydrous (DCPA; CaHPO₄) in raw dairy manure, while anaerobic digestion increases struvite formation and promotes hydroxyapatite accumulation (Güngör et al., 2007). Trotta et al. (2023) examined different process parameters such as pH, Mg:P ratio and reaction duration, obtaining P recovery efficiencies of 65-70% from an anaerobic digestate of cattle manure. Enhanced biological P recovery is commonly used for waste water treatment. This process facilitates accumulation of P in microbial biomass as polyphosphate and can be potentially used for agricultural waste water treatment. The viability of the enhanced bio-acidification method for P recovery, particularly in pig manure and sewage sludge samples, demonstrated the greatest P recovery rates of 65.7% and 69%, respectively (Valverde-Vozmediano et al., 2025).

Aerobic microbial pretreatment raised recoverable phosphate by 92% (Rabinovich, 2021), whereas hydrothermal pretreatment improved the efficiency of struvite precipitation by 15-25% (Munir et al., 2017). Anaerobic digestion acts as an energy recovery method and a pretreatment that transforms organic P into orthophosphate, enhancing later chemical precipitation (Yilmazel and Demirer, 2011; Yilmazel and Demirer, 2013; Ryu et al., 2020). Composting is the aerobic biological breakdown of organic materials into a stable, humus-like end product, viewed as an economically and environmentally sustainable method to manage organic-rich waste (Ahuja et al., 2020). In India, composting is a prevalent practice; nonetheless, it is marked by a lower concentration of nutrients. The addition of rock phosphate to compost provides dual advantages by enhancing nutrient levels and increasing the solubilization of phosphate minerals (Illmer and Schinner, 1995). Aerobic microbial pretreatment of cattle manure can boost recoverable phosphate levels by 92%, highlighting the significance of biological methods in improving P accessibility (Rabinovich, 2021). This pretreatment dissolves organically bound and mineral -associated P, rendering it more suitable for subsequent chemical precipitation upon requirement. The digestate produced by anaerobic digestion (manure, agro-industrial waste) retains approximately 100% of the initial P in the feedstock, with the soluble P fraction increasing during digestion (Campos et al., 2019). P Uptake efficiency of algal cultivation in wastewater varied from 57.9% to 100%, influenced by the algal strain and system (Göncü et al., 2025).

The recovery of biological P from agricultural waste faces numerous technical, operational and economic challenges. The recovery of P from the liquid phase via crystallization significantly relies on previous enhanced biological P removal (EBPR), since merely 10-40% of the P in the aqueous phase can be retrieved without biological enrichment (Salkunić et al., 2022). In the systems using microalgae, the economic viability of P recovery is heavily reliant on the presence of value-added uses for the collected biomass, including biofuels, animal feed, or biochemicals; without these, the recovery method is probably not cost-effective (Salkunić et al., 2022). Furthermore, biological recovery techniques such as composting and microbial P solubilization are highly sensitive to feedstock characteristics, such as the C/N/P ratio, moisture content, indigenous microbial communities, and seasonal variations, make it challenging to standardize procedures across different types of agricultural waste (Oubohssaine et al., 2025). The proper operation of biological reactors requires precise control of pH, dissolved oxygen, and nitrogen compounds, as elevated levels of NH₄⁺ and NO₃^- often found in agricultural waste can impair P recovery efficiency and selectivity (Salkunić et al., 2022). Moreover, P products derived from biological processes continue to face regulatory challenges related to microbiological safety, organic contaminants, product traceability and public acceptance, limiting their broad commercialization (Oubohssaine et al., 2025). The absence of strategically crafted P-solubilizing microbial groups continues to be a significant obstacle for enhancing the effectiveness and resilience of biological P recovery systems (Oubohssaine et al., 2025).

4.6 Hybrid method

Hybrid and combined approach integrate several technologies to maximize P recovery and product value. One combined approach is combining hydrogen peroxide digestion for P solubilization followed by struvite crystallization for primary recovery and then ferric oxide biochar adsorption to capture residual P (Zhang et al., 2018). This kind of hybrid approach achieved high overall P recovery from swine manure by treating different P fractions with appropriate technologies. Aggressive chemical oxidation with hydrogen peroxide converts organic P to orthophosphate, followed by struvite crystallization yielding 50% recovery of P from sludge. Integrating biological, chemical and physical techniques with thermal processes seems to be the most efficient approach for recovering P from wastewater sludge (Witek-Krowiak et al., 2022). Pyrolysis followed by acid extraction can recover P from biochar in a more plant available form. Thermochemical treatment of poultry litter improved P solubility promoting subsequent extraction and recovery of P (Fahimi et al., 2025). Among the different technologies, struvite precipitation is the most widely studied and implemented technology (Yang et al., 2012; Kim et al., 2023; Yilmazel and Demirer, 2011; Yilmazel and Demirer, 2013; Westerman et al., 2010; Kim et al., 2017) with recovery efficiency ranging from 70-95% across various waste types (Witek-Krowiak et al., 2022), with significant variation depending on waste characteristics and process conditions. Zhang et al. (2014) showed that integrating H₂O₂ digestion, struvite crystallization and adsorption resulted in over 90% total P recovery from swine manure. Catalytic-thermal hydrolysis combined with struvite crystallization attained a recovery rate of 85-95% (Zhang et al., 2020). Microbial aids (gluconic, citric, oxalic) release P from both insoluble mineral and organic matrices; the resulting leachates, rich in orthophosphate, can be treated with a Mg source to facilitate struvite precipitation. When digestate from vermicompost is used as the feed, earthworm assisted enzymatic mineralization concurrently frees NH₄⁺-N and PO₄³—P, preparing the solution for the struvite formation (Oubohssaine et al., 2025). Hybrids of this kind transform agricultural waste organics into a recovered, compact, transportable mineral product with plant-aligned stichometry. These studies indicate that combined methods might be essential to optimize P recovery from intricate waste materials.

The environmental advantages of recovering P encompasses resource preservation, lowered eutrophication risk, diminished greenhouse gas emission, and enhanced waste management. Studies on life cycle assessment have typically indicated positive environmental impacts for P recovery technologies, especially with virgin phosphate rock (Vaish et al., 2020; Kataki et al., 2016). Hybrid P recovery techniques offer improved recovery efficiency, but they still hindered by various technical and financial challenges. Although sequential extraction methods are effective for different feedstocks and can achieve P recovery rates as high as 91% with minimal heavy-metal transfer, these integrated approaches lack comprehensive economic evaluations at both pilot and full scales, resulting in uncertainty regarding their commercial feasibility (Liu et al., 2021). Moreover, hybrid systems integrate various treatment stages, leading to increased overall expenses related to extraction chemicals, resin regeneration, pH modifications, struvite or calcium phosphate precipitation, and subsequent drying and product development (Liu et al., 2021). Struvite and calcium phosphate are favored recovery products due to their high availability of P for plants (>89% of total P), but additional process optimization is necessary to enhance cost-effectiveness and the purity of the product still relies on the upstream extraction method (Liu et al., 2021). The combination of several unit operations heightens process complexity by necessitating the concurrent management of essential parameters, such as pH, temperature and the levels of competing ions like Ca₂⁺, Mg₂⁺, Fe₃⁺, Al₃⁺, NH₄⁺ and NO₃^-, thus amplifying monitoring demands and operational knowledge (Liu et al., 2021). Additionally, hybrid recovery systems typically use more energy and chemicals than single-step methods due to extra thermal drying, controlled cooling, pumping and chemical dosing, which may diminish the overall sustainability advantages of circular P recovery (Liu et al., 2021). Phosphor-Sulpho-Nitrocompost (RP+pyrite+PSB) composting, along with bioleaching through microbial acids and subsequent struvite crystallization (85-99% lab recovery at Mg:P = 1.2-1.3:1, pH 8.5-9.0, microbial pretreatment prior to AD, biochar-microbe composites, and immobilized cell inoculant carriers, transform agricultural waste organics into slow-release, plant-compatible fertilizer formulations (Oubohssaine et al., 2025). Microbial inoculants applied at the field level have shown an increase of up to 25% in P bioavailability, alongside yields improvements and approximately a 30% reduction in soil pathogen as documented in experiments, with biochar-enriched compost systems recycling about 50% of C, N, P and K in Asian research initiatives (Oubohssaine et al., 2025). The mechanism of P capture by biochar (including physical adsorption, ion exchange, surface complexation involving carboxyl/hydroxyl groups and the precipitation of Ca/Mg phosphates) work in conjunction with biological activity. After application, soil PSMs and mycorrhizal fungi engage with P bound to biochar and slowly release it for use (Oubohssaine et al., 2025).

The different methods of P recovery from varying agricultural wastes are tabulated in Table 2 and schematically represented in Figure 1.

Figure 1

5 Valorized products developed from the agri wastes

5.1 Enriched composts

Composting, vermicomposting, mycorrhization of organic wastes, direct application of residues are some of the commonly used valorization pathways that produce stable organic amendments (Almutari, 2023; An X. et al., 2021). Composting facilitates conversion of organic to inorganic P. The composting and vermicomposting (crop residues, manure, agro-industrial waste) retains nearly 100% of the original P and demonstrate P enhancement after composting, reaching up to 327%. Moreover, P in the resulting compost transforms into more plant available forms (Hwang et al., 2020). The bioavailable P from different composts are given in Figure 2. The mature Phospho-sulpho-nitrocompost (PSNC) had 2.5–4.2% P and 1.4–2.3% N, showing C/N ratios of 8.2–21.7, water-soluble carbohydrates content of 0.23–0.43%, and increased CEC/TOC and lignin/cellulose ratios indicative of maturity (Manna et al., 2001; Manna et al., 2003). The PSNC dose of 5 t ha⁻¹ corresponded to a mineral regime of 25 kg N + 60 kg P₂O₅ ha⁻¹ for soybean (Manna et al., 2003); while the 10t ha⁻¹ PSNC dose was equivalent to 26.2 kg P ha⁻¹ from SSP on soybean in semi-arid Vertisol (Manna et al., 2001). In an alkaline sandy clay loam soil (pH 7.6) with an initial available phosphorus of 8.6 mg kg⁻¹, the application of rock phosphate-enriched compost, inoculated with phosphate-solubilizing microorganisms at an optimal rate of 800 kg ha⁻¹ (applied 7 days prior to sowing), resulted in a 20.8% increase in grain yield, a 35.3% rise in nodules, and a 26.7% growth in nodule dry weight compared to the recommended phosphorus fertilizer, while 1000 kg ha⁻¹ yielded the highest field response in chickpea. The treatment improved P accumulation by 12.9% in straw and 4.3% in grain, suggesting better phosphorus availability and absorption in P-deficient conditions (Ditta et al., 2018). In an alkaline sandy loam Inceptisol (pH 8.0) with an initial Olsen-P of 22.1 kg ha⁻¹ (0–15 cm), the application of 5 t ha⁻¹ enriched compost along with 50% of the recommended fertilizer dose in a wheat–soybean cropping system raised available P to 28.8 kg ha⁻¹ following wheat and 25.8 kg ha⁻¹ following soybean, which corresponds to increases of 68.8% and 72.0%, respectively, compared to the unfertilized control. The treatment additionally improved saloid-P, Fe-P, Al-P, Ca-P, microbial biomass, and phosphatase activity, indicating a lasting enhancement in soil P availability and crop yield (Meena and Biswas, 2014).

Figure 2

Billah and Bano, 2015, Moharana et al., 2020; Gaind, 2014; Moharana et al., 2020; Wei et al., 2015).

5.2 P loaded biochar

Fertilizer solutions (NPK) can be intercalated into the porous biochar synthesized from organic wastes to develop enriched biochar fertilizers and other variants with humic acid or seaweed extract. Enriched biochar formulations can contain NPK grades (eg.6-6-4) (Mohamad Sarbani et al., 2025). An et al. (2021) reported tropical biochar with 46.5% C, 3.8%N, 0.6% P and CEC of 46.3 cmol/kg. Mohamad Sarbani et al. (2025) found that enriched biochar formulations enhanced fertilizer use efficiency (29.5%, 11.5% and 22.9% for N, P and K respectively) and improved crop response compared to conventional fertilizers in direct seeded rice. Magnesium-modified corn-stalk biochar co-adsorbed NH₄⁺ and PO₄³⁻ with Langmuir–Freundlich maxima of 37.72 and 73.29 mg g⁻¹ respectively, with the gradual release mechanism linked to struvite and Mg-P dissolution; the P-loaded magnesium-modified corn stalk biochar enhanced the growth of Zea mays and Lolium perenne (He et al., 2022). The effectiveness of maize was evaluated with varying combinations of biochar, lime, and P application rates in field conditions. Rice husk biochar (10 t ha⁻¹) was specifically used alongside 75% of the necessary lime and three levels of phosphorus fertilizer (100%, 75%, and 50%). The findings indicated that the addition of biochar and lime, regardless of P application rates, markedly enhanced soil nutrient (nitrogen and P) availability, while concentrations of aluminum (Al) and iron (Fe) in the soil were diminished. Additionally, maize production rose by 62.38% compared to the control when biochar was used with a reduced quantity of lime (75% of the suggested amount) and half of the necessary phosphorus (Mosharoff et al., 2022). Poultry-litter biochar applied at an equivalent total P rate as inorganic P during a 2-year rye-corn-sorghum rotation resulted in comparable or notably higher cumulative biomass yields than inorganic P, with the residual impact varying by soil type (Freitas et al., 2025). P-loaded Mg-Al Layered Double Hydroxide-biochar and Layered Double Hydroxide-hydrochar greatly enhanced the available P, dry matter of maize shoots and roots, and P absorption in calcareous soil, liberating 18, 22, and 27% of adsorbed P into 0.03 M KCl and 52, 63, and 66% into 0.1 M Na₂CO₃ (Azimzadeh et al., 2021). Sandy/light textures: Poultry-litter biochar surpassed inorganic P with more significant residual impacts than Entisols (Freitas et al., 2025); P-Salt + biochar was especially beneficial on light sandy soils because of lesser leaching (Ehmann et al., 2017). 4. Dose-response evidence suggests a decline in the ideal P application rate when using biochar carriers, with P-enriched biochar providing greater yields than SSP at a reduced kg-P-per-hectare rate (Arwenyo et al., 2022; Rodrigo et al., 2025). The dependence on soil type is significant: P-fixing acidic soils and P-deficient sandy textures gain the most benefit, while calcareous and P-rich soils exhibit smaller or inconsistent responses, reinforcing the idea that biochar-assisted P delivery is most effective in soils where traditional P is least impactful (Mosharoff et al., 2022; Freitas et al., 2025).

5.3 Slow/controlled release fertilizer

The calcium-based phosphates and hydroxyapatites synthesized from agricultural wastes by precipitation, thermal or hydrothermal methods can be used as slowly available P fertilizers or as engineered nano fertilizers. These can also be synthesized by combining recovered P source with sportive carriers like biochar or clays or by encapsulating them in polymers matrices to reduce ingression of water and enhance controlled diffusion. Zhang et al. (2014) reported that a hydroculture bioassay demonstrated that struvite positively influenced the initial growth of wheat. The P levels in both root and shoot tissues for struvite treatment exceeded those of soluble P by more than two-fold, Slow release composites or coated fertilizers can significantly reduce leaching losses of P. Kassem et al. (2022) reported that carboxymethyl cellulose biochar coating reduces P release by 82% over a period of 30 days in comparison to uncoated Triple Super Phosphate. Engineered biochar coating was found to reduce P leaching by approximately 44% over a period of 80 days. Slow-release biochar-based fertilizers has been reported to reduce P loss and improve nutrient uptake in cereals and horticultural crops (Azimzadeh et al., 2021; Zhao et al., 2025; Kassem et al., 2022).

Struvite is a thoroughly defined slow- release fertilizer known for its recognized agricultural uses and market worth (Cañas et al., 2023; Ryu et al., 2020; Kataki et al., 2016; Lorick et al., 2020). It delivers P, nitrogen and magnesium, classifying it is a multi-nutrient fertilizer. Struvite forms characteristic rod-shaped crystals with approximately 6% N and 13% P for pure struvite, which varies with recovered solids due to co-precipitates or impurities (Westerman et al., 2010). Different P rich minerals like struvite, octa calcium phosphate, hydroxy apatite can be synthesized from the by-products of livestock industry (Figure 3). Field trials have confirmed struvite’s effectiveness in agriculture and its beneficial role in reducing water pollution (Kim et al., 2023). Struvite’s gradual release characteristics is due to its low solubility in water, which reduces nutrient loss through leaching and guarantees a steady supply of nutrients for plants. These traits are particularly advantageous in regions with heavy precipitation or in sandy soil where conventional water-soluble fertilizers are prone to leaching losses. Struvite recovery extracts dissolved P from wastewater, mitigates eutrophication and repurpose P for agricultural applications (Zhang et al., 2017). Mixed P rich mineral products (struvite+ apatite) can enhance P recovery from dairy waste water and function as fertilizer blend with value in agriculture (McIntosh et al., 2022). The production of Vivianite (Fe₃(PO₄)₂·8H₂O) for P recovery is more economical because Fe (II) chemicals are less expensive than magnesium salts. The produced vivianite minerals demonstrate their significance as slow-release fertilizers for agriculture (Du et al., 2023). Wang et al., 2023 noted that with the same P application rate, in a rice-wheat crop rotation, the yield and aboveground biomass for the struvite substitution of 100%P (SP) treatment were marginally greater than the conventional fertilizer treatment, while crop P uptake, PUE, and soil available P content saw significant increases. The struvite substitution treatment (RSP) with a 50% P reduction did not decrease yield and notably enhanced the PUE and the soil’s available P content. The SP and RSP treatments with struvite substitution also showed increased crop N uptake and NUE. Kokulan et al., 2024 noted that both struvite (11.4 Mg ha⁻¹) and blend (11.5 Mg ha⁻¹) treatments yielded higher grain outputs compared to the control (10.1 Mg ha⁻¹) in maize. Grain yields from treatments with struvite and a struvite/monoammonium phosphate (MAP) blend were statistically comparable to those of MAP (10.7 Mg ha−1). Struvite works best in acidic soils (Ultisols, Oxisols, certain Andisols/Allophanic soils), where the high proton/H⁺ concentration promotes dissolution at its pH-of-minimum-solubility (~9.0–11) (Hall et al., 2020). Meyer et al. (2018) measured that the effectiveness of struvite dropped by approximately 40% as soil pH rose, although struvite remained the most effective recycled phosphorus source in calcareous soil (75% relative effectiveness) and significantly surpassed Calcium silicate hydrate phosphate, Sewage sludge ash, and Rock phosphate in the same substrate.

Figure 3

5.4 Polymer composites

Polymers regulate nutrient flux by reducing water absorption and increasing the diffusion pathway. The incorporation of clay or biochar increases the tortuosity and the strength of the composite thereby improving the binding strength of the nutrients and slowing P release. P extracted from agricultural and fishery or aquaculture by-products can be incorporated into polymer-based composite fertilizers to develop slow- or controlled-release formulations. Feedstocks abundant in phosphorus, struvite obtained from landfill leachate or treatment wastewater (Darwish et al., 2021) and hydroxyapatite sourced from fish bones (Swart, Bordoloi and Goosen, 2019), constitute the fundamental mineral element of these composites. This connection completes the nutrient cycle by transforming fishery and agricultural waste into valuable agricultural inputs, diminishing dependence on traditional mineral fertilizers while minimizing the ecological effects of untreated waste disposal (Rahman et al., 2004). For instance, struvite can be integrated into PVA-chitosan-lignin dual-crosslinked hydrogels (Khan et al., 2025), and starch–alginate hydrogel beads have been designed to provide phosphate via induced gelation (Tiamwong et al., 2023). Nanomaterials like hydroxyapatite sourced from fish bones or other biogenic materials can be created or integrated in situ within a polymer structure, as observed in cellulose-graft-poly(acrylamide) hydrogels, to align nutrient release with crop requirements (Rop et al., 2018). Matrices frequently integrate polymers with minerals such as zeolite, attapulgite, or biochar to improve phosphorus retention. Biochar–attapulgite composites utilize the mineral fraction to slow P release through diffusion (Kaur et al., 2025); nano zeolite-coupled biochar-based fertilizers release just 29.4% of P in 24 hours, contrasting with 54.3% for traditional chemical P (Dong et al., 2025). Chitin and chitosan, which can be derived from crustacean processing byproducts (Periyannan et al., 2023), can function as both the delivery matrix and a bio stimulant (Mujtaba et al., 2020), integrating the feedstock and the carrier. P-loaded chitosan-carboxymethyl cellulose (CMC) hydrogels have shown a slow release of nutrients over a period of 20 days, promoting plant growth using merely 25% of the usual fertilizer amount (Priya et al., 2025). Polymer coatings or matrix embedding protect P from these soil components; for instance, biodegradable maleic–itaconic polymer coatings enhanced P uptake by 66.7% in calcareous soil (Khalid et al., 2024). Regulatory frameworks like the European Commission’s proposal and EN 17033 increasingly mandate that polymer coatings achieve 90% biodegradation within 48 months in natural soil settings (Šerá et al., 2020). Polymer composites provide a technically sound method to supply recovered phosphorus from fishery and agricultural waste to crops, although enhancing the primary recovery phase (e.g., acidic leaching or precipitation) to align with the production capacity of existing Indian fishery waste streams is essential for practical implementation (Swart, Bordoloi and Goosen, 2019; Darwish et al., 2021).

Some of the polymer based encapsulated systems studied are given in Table 3.

5.5 Carbon dots

Carbon dots (CDs) are nanomaterials characterized by a particle size under 10 nm (Maholiya et al., 2023). Carbon dots, being a novel form of carbon-based material, offer substantial benefits over conventional carbon-based materials (Xia et al., 2019; Singh et al., 2023) enhance nutrient delivery and plant growth. Various plant species serve as renewable carbon sources for green CD production. The most frequently utilized components of the plants are the leaves (Jiang et al., 2019), flowers (Murugan and Sundramoorthy, 2018), or roots (Sobhani et al., 2019a). Additional types of plant biomass consist of shells (Xue et al., 2016), seeds (Amin et al., 2018), bark (Qin et al., 2013), stems (Shi et al., 2017) and skins (Lu et al., 2012). The beneficial effects of CDs on various plants suggest their significant application potential in agriculture, which is crucial for enhancing crop development and boosting the sustainability of agricultural practices (Li et al., 2023). The main physiological roles of CDs include facilitating seed germination and root development, enhancing nutrient uptake in plants, stimulating growth, boosting biomass production, improving photosynthesis and augmenting carbohydrate levels in plants, as well as enhancing tolerance to abiotic stress and disease resistance; which are vital for plant development and crop yield (Li et al., 2020; Maholiya et al., 2023; Li et al., 2019). Studies on coriander indicated that the application of 40 mg L^-1 of CDs enhanced the levels of K, Ca, Mg, P, Mn and Fe by 64.3%, 21.0%, 26.2%, 12.8%, 56.0% and 125%, respectively (Hu et al., 2022). When 0.02 mg L^-1 of CDs was applied to lettuce, the plants N, P and K level rose by 4.4%, 10.8% and 16.5%, respectively (An Z. et al., 2021). CDs can also be synthesized from different agricultural wastes and utilized to enhance P uptake in plants.

A recent study by Priya et al. (2025) showcased a holistic circular economy approach by transforming rice straw into cellulose nanofiber/carboxymethyl cellulose (CNF/CMC) hydrogel infuse with iron hydroxide nanoparticle (Fe (OH)₃@CNF/CMC) aimed at recovering phosphate from secondary treated wastewater. The adsorbent efficiently captured phosphate via inner-sphere complexation, reaching a peak adsorption capability of 3mg P g^-1. In actual secondary treated wastewater with 4.63 mg P L^-1, phosphate removal efficiencies rose from 39.5% to 81% as the adsorbent dosage increased from 1 to 5 g L^-1, while optimized synthetic wastewater conditions allowed up to 90% phosphate removal. In addition to effective nutrient recovery, the hydrogel demonstrated outstanding water retention capabilities by taking in nearly 40 times its dry weight and slowly releasing water over a span of 7–8 days, while experiencing about 80% biodegradation within 3 months when buried in soil. The phosphate-rich hydrogel was utilized as a slow-release fertilizer, yielding wheatgrass growth similar to commercial single superphosphate (SSP) while providing merely 5% of the P delivered via SSP, demonstrating significantly improved P-use efficiency. From an economic viewpoint, the technology is appealing as it employs affordable rice straw as the main feedstock and integrates solvent recycling (DMSO, ethanol, and toluene), thus minimizing raw material usage and manufacturing expenses. Additionally, the straightforward reuse of the phosphate-saturated adsorbent as fertilizer removes the necessity for chemical regeneration, thereby lowering operational expenses, chemical use, and waste management needs. An initial techno-economic evaluation suggested that the manufacturing expenses of the phosphate-loaded FCC adsorbent is similar to that of commercially available slow-release phosphate fertilizers, with expected further cost savings arising from large-scale production and economies of scale. Together, this technology showcases significant environmental and economic promise by concurrently extracting P from secondary wastewater, utilizing agricultural byproducts, saving irrigation water, reducing dependence on fertilizers and generating a value-added slow-release fertilizer, positioning it as a viable method for sustainable P management and the promotion of a circular bioeconomy.

6 Challenges and limitations

6.1 Technical and logistical challenges

A major challenge in managing agricultural waste in India is the lack of sophisticated technical infrastructure for effective waste collection, processing and conversion. Inadequate waste segregation and ineffective waste collection systems lead to improper management and disposal of agricultural waste (Kumar et al., 2017). Moreover, current technologies for converting agricultural waste, like biogas facilities and composting systems, frequently face inefficiencies and challenges related to scalability (Das, 2020). Additionally, the elevated moisture levels and varied characteristics of agricultural waste present technical difficulties in its processing and transformation into valuable products. Even with significant biomass power potential, various challenges impede its utilization, such as the difficulties in biomass collection and elevated moisture levels. Tackling these issues necessitates choosing suitable technologies, discovering synergies among thermochemical, biological, biochemical processes and instituting supportive policies for the utilization of biomass energy (Singh, 2017). Additionally, the availability of feedstock for different valorization processes fluctuates based on regional differences (Punnathanam and Shastri, 2022). India’s waste varies in P concentrations and chemical forms in which it exists, which governs the different recovery technologies to be adopted, pre-treatments, which thereby increases technical complexities and scale up challenges. Several technologies like struvite precipitation, adsorption, thermochemical treatments, biological treatments exist, but each demands specific feedstock qualities and multi-step processing which complicates its operation. Heterogenous feedstock necessitate appropriate pre-treatment as P occurs in various organic and inorganic forms in the different agri waste, thereby influencing its recovery process. Suitable feedstock is required since process like struvite crystallization requires ammonia rich, low Ca digestates, whereas hydrothermal process suits solid organic wastes and produce different products with distinct agronomic characteristics. The quality of the fertilizer generated from the crystallization method can be reduced due to contamination of the waste derived products. Implementation of the different recovery methods depends on suitable balance of investment, operating costs and market value for the recovered products. Inadequate waste collection and processing infrastructure, low value of the recovered products and uncertainty in its prices constrain the commercial viability of the different methodologies. Most of the waste management systems rely on dumping, burning or informal handling, which makes centralized recovery plants to run consistently. Most of the recovery systems often require reactors, pH control, Mg sources membrane units, thermal treatment, which add to the capital and operating costs. Sewage sludge studies from India showed lower P content compared to several European studies (Breitenmoser et al., 2026), which thereby reduces P recovery and increases chemical demand. A closed loop recovery of P from wastes like poultry litter or any other concentrated wastes demand substantial capital and operational costs and skills, which hinders its adoption by small scale operators.

Product quality due to risk from contamination is another issue that needs to be addressed. Contamination risk varies among the different agri-waste streams based on the source, handling, and the recovery method. Crop residues generally have low contamination risk among the four waste streams discussed in this study, when they come from field straw, stalk or leaves. Some of the concerns could be pesticide residues, fungal spores or soil dust (Shinde et al., 2022; Verma et al., 2024). The livestock wastes may contain bacteria, pathogens, antibiotics, drug residues and heavy metals indirectly from feed additives, bedding material. So these may require better sanitation and quality control before utilizing the wastes. Agro-industrial wastes, however require feedstock specific treatment as the contaminants vary depending on the type of processing industry. These may be contaminated with residues from washing, cleaning or processing chemicals, heavy metals and microbial loads (Golovko et al., 2022). The residues from feed, antibiotics, and pathogens, organic loads can be a source of contamination for aquaculture wastes, there by necessitating careful pre- treatment prior to use (Minhas et al., 2022). To address these safety barriers, standardized certification protocols need to be created to guarantee that recovered materials satisfy certain quality standards for heavy metal levels and pathogen presence (Zheng et al., 2022). Moreover, rigorous standardized evaluations need to be performed to assess P bioavailability, as current precipitation technique often yield highly crystalline forms that impede rapid uptake by plants (Stávková and Maroušek, 2021). Further, integrating these products into formal fertilizer regulations is essential for creating a stable market and providing the regulatory framework necessary to compete with conventional fertilizers (Melia et al., 2017). Establishing a robust value chain, in which recovery techniques consistently meet agricultural demands, is crucial for making these technologies economically viable enough to overcome the current reliance on informal disposal and incineration (Jupp et al., 2020; Bagheri et al., 2024).

6.2 Economic challenge

Financial limitations also intensify these technical difficulties. The initial investment required to set up advanced waste management infrastructures, including biorefineries and thermochemical conversion facilities, is significant. Small and marginal farmers, who represent the majority of the agricultural sector in India, frequently do not have the financial means to invest in such technologies (Vij, 2012). Moreover, collecting residues and moving them to treatment facilities and storage sites often incurs significant financial expenses. The economic viability of these technologies depends on variable government subsidies and backing, introducing further uncertainties and influencing implementation.

To improve market competitiveness, it is crucial to establish “End-to-waste” criteria that unify the quality of secondary raw materials, facilitating their transition into traditional commercial fertilizers (Leinweber et al., 2017). These regulatory actions can aid in minimizing market volatility that is hindering the adoption of recycled nutrient fertilizers, which often struggle to match the pricing and performance reliability of conventional synthetic alternatives (Cordeiro and Sindhøj, 2024a). Moreover, cooperation among industry players and research organizations is essential for enhancing nutrient recovery techniques, as current technical issues, such as low extraction rates and high energy consumption, must be addressed to achieve viability at an industrial scale (Bernal, 2017; Chojnacka, 2023). Further, it is crucial to harmonize international safety standards with domestic legislation to enable the unrestricted movement of pre-treated bio-based fertilizers without being obstructed by differing local regulations (Malila et al., 2019).

6.3 Social and regulatory challenge

Traditional methods, like incineration of crop leftovers in fields, are deeply ingrained in the agricultural community as they are considered simple and cost effective (Vij, 2012). Changing these established methods requires significant efforts in increasing awareness and teaching individuals about the health and environmental effects of these practices (Kumar et al., 2017). Increasing awareness in agricultural communities regarding residue or farming waste management is crucial for promoting its sustainable handling (Lohan et al., 2018). Additionally, the uptake of new technologies and practices is often hindered by lack of trust and understanding among farmers. Consequently, it is crucial to carry out thorough outreach and demonstration initiatives to showcase the benefits of modern waste management techniques. Improving organizational frameworks and governance systems is essential for effective agricultural waste management in India.

Developing localized extension services and promoting farmer producer organizations are crucial for bridging national policy with practical implementation (Rajendran et al., 2025). Furthermore, encouraging the adoption of technologies like pyrolysis or hydrothermal carbonization through specific subsidies can lower the significant energy cost associated with transforming biomass into stable soil amendments such as biochar (Peng et al., 2023). Furthermore, augmenting these modifications with targeted microbial inoculants can remedy natural composition deficiencies, thereby boosting their agricultural efficacy and overall market value (Bhattacharjya et al., 2019). Alongside these technical advancements, regulatory bodies should mandate clear product labelling regarding feedstock origin and nutrient composition to reduce consumer uncertainty (Bouhia et al., 2022). Quality management systems need to include proactive governmental measures, such as capacity-building programs and demonstrations farms, to bridge the knowledge divide between innovative bio-based solutions and traditional agricultural practices (Venkatramanan et al., 2021).

7 Policy and social support

Policy and economic factors are crucial for speeding up the implementation of circular economy practices. Support for organic fertilizer processing, incentives for biogas facilities, tougher rules on waste disposal, and market strategies for recycled fertilizers can influence farmer practices. Due to gravity of residue burning problem, the Government of India has enacted several policy initiatives, such as offering subsidies for purchasing residue management and agricultural equipment, setting up custom hiring centers for farming tools, and imposing a ban on the burning of crop residues. The Indian government unveiled a new initiative called the ‘PM Programme for Restoration, Awareness, Nourishment and Amelioration of Mother Earth’ (PM-PRANAM) in 2023-2024, designed to encourage states and union territories to pursue alternatives to mineral fertilizers and thereby reduce subsidy expenditures (Wentworth et al., 2024). States and union territories will receive 50% of the savings to create alternative fertilizer systems and incentivize farmers and groups to decrease fertilizer usage (Ministry of Agriculture and Farmers Welfare, 2023). Despite rising rates of mineral fertilizer application in India, the P fertility of the majority of the soils in the country remains very low (Das et al., 2022). In India, government programs like the GOBAR-DHAN scheme under Swachh Bharat Mission already support biogas and composting initiatives, showing the potential for scaling nutrient recovery systems. With the aim to combat stubble burning, the Crop Residue Management (CRM) Scheme provides machines at subsidized rate that incorporate residue into the soil or balers to collect straw for off-site use. Globally, policy frameworks such as the European Union’s Circular Economy Action Plan recognize P recovery from wastewater and manure as a strategic priority.

In addition, awareness and capacity building programs focusing on farmers, farmer cooperatives and Self-Help Groups to promote the adoption of agricultural waste recycling to reduce dependence on chemical P fertilizers and creation of a circular P economy should be implemented. Public–private partnerships for waste collection and processing infrastructure also need to be focused up on, to overcome and find solutions to technical limitations of recovery processes and high upfront costs for advanced technologies. A successful transition to a circular economy demands a cohesive policy framework, the creation of a market for reclaimed products, cooperation across value chain from waste generation to agricultural use, and technological innovations.

For recovered P products to achieve market viability, the emphasis must shift from just technical potential; to establishing formal certification and quality labelling standards. These standards are essential to guarantee nutrient levels, bioavailability and safety, thereby addressing farmer doubts and encouraging uptake (de Boer et al., 2018; Breitenmoser et al., 2026). Even with programs like GOBAR-DHAN, the Crop Residue Management Scheme and PM-PRANAM aimed at reducing waste and enhancing subsidies, their success in fostering a circular P economy relies on integrating these approaches into a broader framework that supports the development of high-quality, standardized fertilizer substitutes (Sharma, 2024; Breitenmoser et al., 2026). Effectively expanding these solutions requires policies that encourage not just infrastructure improvement but also comprehensive educational initiatives and transparency vial for convincing end-users of the economic and agricultural advantages of recovered products. This alignment is essential for coordinating local efforts with the strategic priorities outlined in global frameworks, like European Union’s Circular Economy Action Plan (Jupp et al., 2020). To establish a domestic market for circular P, Indian policy frameworks must transition from focusing solely on waste management to implementing robust regulatory pathways for certifying recovered products, guaranteeing their compliance with defined standards for safety, nutrient quality and economic feasibility. By linking initiatives like PM-PRANAM and GOBAR-DHAN to market-focused standards that transparently express nutrient efficacy, application methods and yield benefits to farmers, the government can effectively address concerns regarding recycled fertilizers and encourage widespread acceptance. Moreover, formalizing “End-of-Waste” criteria will deliver essential regulatory clarity to align product quality with agricultural safety requirements, successfully positioning secondary phosphorus on an equal footing with fossil-based mineral fertilizers (Leinweber et al., 2017). To guarantee that these policy frameworks effectively promote market adoption, they must include market-based tools, like compulsory sales quotas for secondary phosphorus, to encourage producers to incorporate recovered materials into national supply networks. In addition to concentrating on technical possibilities, expanding the circular phosphorus economy necessitates a transition to demand-side pull strategies that specifically tackle barriers to farmer adoption via evidence-based validation. Specifically, programs like PM-PRANAM and GOBAR-DHAN must focus on creating demonstration networks that clearly convey the nutrient effectiveness, safety, and economic benefits, including cost competitiveness and yield improvements of recovered products compared to mineral options. Requiring transparency in product certification and labeling, along with supporting thorough educational outreach that delivers standardized application guidelines, allows governments to successfully alleviate farmer doubts and stimulate the real commercial demand crucial to convert recovered phosphorus from a theoretical option to a common agricultural practice (Leinweber et al., 2017; Jupp et al., 2020).

8 Future thrust area

The conversion of nearly 500 million tons of agricultural waste produced each year in India into products with added value is greatly limited by various technical, financial, socioeconomic, institutional, and logistical challenges (Reddy et al., 2025). tackling the technical, financial, social, cultural and institutional obstacles is essential for the effective execution of sustainable waste management. The implementation of advanced waste management methods is frequently hindered by the absence of formal credit opportunities for smallholder farmers, insufficient cooperation among agencies and the logistical difficulties related to managing biomass resources that are disperse both spatially and temporally (Bhuvaneshwari et al., 2019a). The main emphasis should be on creating affordable and effective methods for transforming various agricultural byproducts into useful products. Future studies ought to concentrate on enhancing current techniques, like anaerobic digestion, pyrolysis, while also examining options for scaling up, in addition to exploring novel methods for waste valorization, including uses in building materials. Future studies must be concentrated on the interactions and trade-offs between end uses like energy, biofuels and soil amendments, considering local feedstocks characteristics. Research ought to concentrate on creating integrated solutions that merge emerging technologies such as Remote Sensing and GIS systems with artificial intelligence and machine learning for effective waste management. Remote sensing technology can detect crop residues, burning hotspots, land use patterns and seasonal biomass accessibility, helping to identify regions with high concentrations of P rich residues and guiding collection efforts (Martín-Hernández et al., 2022). GIS can integrate spatial layers such as crop intensities, livestock populations, transportation availability, fertilizer needs, soil P levels and waste generation to pinpoint recovery centers and mange logistics for decentralized P recycling (Wentworth et al., 2024). Machine learning can forecast P recovery efficiency based on feedstock characteristics and processing conditions, minimizing trial-and-error in methods such as hydrothermal treatment, struvite recovery, and biomass conversion (Tong et al., 2024). AI-driven decision tools can integrate technical, economic, and environmental information to prioritize waste streams and recovery alternatives based on feasibility, cost, and anticipated P yield, which is particularly beneficial when feedstocks differ significantly throughout India (Sheik et al., 2024). Data fusion models can integrate remote sensing, field sampling, and laboratory analysis to create decision maps at the district or state level for circular nutrient management (Wu and Zhao, 2023).

Creating interlinked frameworks is crucial for transforming agricultural by-products from environmental burdens into sustainable resources that offer rural communities’ additional sources of income and energy security (Khan and Akram Ahmad, 2025). Through the integration of technological progress with institutional support, India can shift from harmful disposal practices like residue burning toward adopting circular, value-oriented production systems (Kaur and Singh, 2022). Achieving this objective requires to concentrate on developing high efficient biorefinery systems capable of processing multiple products, enhancing the economic viability of agricultural waste through simultaneous biochemical and thermochemical (Polipalli et al., 2025; Thaha et al., 2025). Furthermore, conducting comprehensive life cycle assessments for these integrated systems is vital to determine their real environmental impact and to ensure the enduring sustainability of the bioenergy supply chain (Begum et al., 2024; Osman et al., 2024). Future research should emphasize prolonged monitoring of decentralized systems to verify their efficiency across diverse climate conditions, making certain technological adaptability meets the specific requirements of India’s varied agricultural landscape (Pual, 2026). Extended research monitoring the social acceptance and continuous adoption rates of residue management technologies among smallholder farmers could guide the development of more effective extension strategies. Furthermore, research should focus on standardizing modular reactors designs to lower capital expenses and enhance the market feasibility of smaller-scale plants (Rout et al., 2022). Comprehensive techno-economic assessments at both pilot and industrial scales are crucial for optimizing material and energy flows, ensuring the sustainability of these systems throughout the entire supply chain (Kumar et al., 2021). Joint efforts between public and private sectors cam significantly support financing and improving waste management initiatives. Collaborative ventures between the public and private sectors can greatly aid in funding and enhancing waste management programs. Programs aimed at enhancing skills and training for farmers and local communities should be essential components of policy efforts to encourage widespread acceptance and success.

9 Conclusion

With the increase in the Indian population, the demands for food will also rise, leading to a greater necessity for P, a limited resource that is mostly imported. Adopting a circular economy model centered on recovering and reusing P from waste could address the P access insecurity and its status as limited resource, while also decreasing environmental pollution related to P loss. Improved efficiency coupled with P recovery and reduced dependence on P fertilizers could drive India towards a closed-loop model. Among the different recovery pathways, suitable technology need to be selected based on specific agriculture waste stream, local availability, collection feasibility and regional infrastructure. Expanding these technologies demands not just funding, improvements in technology, and changes in infrastructure, but also major changes in social behavior. Decentralized, modular recycling systems customized to local conditions and waste types provide opportunities for enhancing P-recovery on a broader scale. Strategies for managing agricultural waste need to be customized to particular areas and should be directed to technologies that can shift them from specialize uses to common methods. The policy support must emphasize prioritizing quality standards, incentive systems, integration of nutrient recovery into fertilizers and waste management strategies. A joint effort among researchers, technology developers, farmers, policymakers and other stakeholders to create fundamental institutional frameworks can significantly improve the efficiency and sustainability of agricultural waste management practices in India. A circular P system could thereby reduce India’s dependence on volatile global phosphate markets, recycle and reuse domestic organic wastes into P inputs, lower environmental pollution, making it a core part of India’s fertilizer security.

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