Phosphorus Recovery and Recycling

This chapter will explore whether the differences in the process development for phosphorus recovery and recycling and its implementation in Europe and Japan are linked to the waste flows and the regulative framework. The main waste flows and their qualities are summarized for the two geographical areas. Then a comparative overview of the full-scale applications and their importance in relation to the potential is presented. The drivers for phosphorus recycling and the expected further development in Europe and Japan are described.

In this chapter, phosphorus resource flow was analyzed to determine the extent of the dependency of Asian countries on other countries to meet their phosphorus requirements. Additionally, a virtual phosphorus flow analysis was conducted to determine the future phosphorus demand in various Asian countries. The future phosphorus scenarios in Asia are also discussed based on the findings of these analyses. The findings on the basis of virtual phosphorus analysis indicated that from the life cycle perspective, Asian growing economy became to need more and more virtual phosphorus and many Asian countries rely strongly on phosphorus imports via the international trade supply chain. Thus, they realize that high economic and political risks are associated with the secured supply of phosphorus. Another important finding is that besides population growth, urbanization and economic growth in Asia are likely to be strong driving forces for higher phosphorus demand in the future. Considering the untapped P in waste streams, it was clarified that the steelmaking slag should be one of a good target to introduce P recycling technology for two big Asian steel producing countries, China and India. Approximately 7% and 5% of the virtual P-requirement for plant-based food and feed production was lost in China and India, respectively.

Circular economy for nutrients! How to transfer buzzwords into solid results? So far the potential to recover and recycle phosphorus remains untapped or is just inefficiently used as in the case of sewage sludge, manure and food waste. To provide alternatives to argued traditional nutrient recycling routes, various technical solutions have been developed in recent years. They allow recovery of phosphorus minerals suitable as raw material for industries like fertilizer production or even as ready-to-use renewable or next-generation fertilizer. This contribution focuses on mineral phosphorus-containing materials recovered from wastewater. It discusses legal aspects and market opportunities regarding their valorization in Europe. It has to be kept in mind that there are many other recovery/recycling options out there to allow sustainable nutrient management, especially when it comes to organic wastes and their recycling. A frequently updated inventory is provided on the European Sustainable Phosphorus Platform’s website (http://www.phosphorusplatform.eu/). The current revision of the European fertilizer regulation within the European Commission’s circular economy package provides a concrete example, what issues have to be coped with and what measures have to be taken to create a level playing field for both primary- and secondary-based materials destined for fertilizer use. Some EU member states have started to enforce phosphorus recovery from relevant wastes but are lagging behind in enabling efficient recycling, be it in mineral or organic form. Still, the so-called technical nutrient recovery is missing a demand-side driven market pull for recovered (secondary) nutrients and the biggest challenge will be bridging the gap between supply (recovery) and demand (recycling), especially when it comes to new types of materials or products, not already established on the market. Whereas in the past, the focus of nutrient recovery technologies was laid upon high recovery rates for single nutrients, now energy efficiency, synergies and cost become more and more important. What about value chains? We have to look for easy to implement rather integrative solutions instead of reinventing the wheel, creating fancy parallel (infra)structures.

Recent developments and innovations in the field of P recovery and recycling from municipal sewage sludge claim to provide a sustainable and more efficient alternative to the traditional sludge valorization in agriculture. The method of life cycle assessment (LCA) offers a detailed analysis of the potential environmental impacts associated with different technologies, but it needs to be based on sound definitions and validated input data, not only for the specific technologies but also for the methodological framework. Since the relevant ISO standards 14040/44 provide methodological guidance without specifically fixed definitions, the application of LCA leaves a lot of potential for interpretation of results. Within the European research project P-REX, a methodological framework was developed to assess various available technologies for P recovery from sewage sludge, sludge liquor or incineration ash. Decisive definitions are the setting of adequate system boundaries and functional unit, the selection of LCA indicators and their interpretation. The following chapter discusses important definitions of the LCA methodology and provides recommendations towards a consolidated approach for future LCA studies in the field of P recovery from sewage sludge.

The demand of phosphate fertilizers is growing as a result of a rising population, changing human diets resulting in the increasing (meat) consumption per capita, and an expansion in the production of biofuels. Besides the fertilizer industry, there is a steady growth of using phosphorus compounds in the chemical industry for applications in, e.g., soft drinks, pharmaceuticals, and flame retardants. To meet this growth, it is important to know if the P reserves are sufficient and what kind of processes are used to produce such phosphorus compounds. Reserves are not equally spread around the world, with three-quarters located in Morocco and The Western Sahara. Prices can be volatile, as shown in 2008 with an eightfold price increase. Moreover, the estimated time till depletion of phosphate rock differs substantially between several studies. Therefore, phosphate rock was added on the critical material list of the European Commission. An important aspect for the processing of phosphate rock is the quality of the rock, which is dependent on the ore type (sedimentary or igneous), level of radioactivity, and hazardous metal contents. The main intermediary compounds for phosphorus products are phosphoric acid and white phosphorus. About 95% of the phosphoric acid is made via the wet process: acidulation of phosphate rock to create wet phosphoric acid and the main by-products phosphogypsum and hydrogen fluoride. The purity and thus the reusability of phosphogypsum are dependent on the type of digestion process. However, at the moment, reusing phosphogypsum is not a common practice. Wet phosphoric acid can be purified via several processes. The most common processes are extraction and precipitation. Via extraction, wet phosphoric acid can be purified up to phosphoric acid comparable to that produced with the thermal process. Separation of specific compounds can be done through precipitation. Additionally, cationic impurities can be removed via precipitation, but the product will then be changed into a phosphate salt, which is unfavorable for its use in industrial applications.

Currently available phosphate recycling technologies are compared and rated on the basis of eight criteria. Expectations and predictions regarding implementation are given. Sewage sludge ash-based technologies are identified as overall most promising in terms of recycling efficiency and compatibility with existing industries.

Nippon Phosphoric Acid Co. Ltd. (NPA) started to use sewage sludge ash (SSA) for the industrial manufacturing of phosphoric acid (PA) in 2013. NPA purchases SSA from wastewater treatment plants (WWTP) equipped with a Bio-P process to remove phosphate (P_i) from sewage. In the NPA’s manufacturing plant, SSA is blended with roller-milled P_i rock and is dissolved in concentrated sulfuric acid to generate phosphogypsum slurry. Then the slurry is filtered to separate PA from gypsum which is recycled for the manufacture of plasterboard and cement. Currently, NPA accepts a total of 1300 t/a of SSA for the manufacture of PA and gypsum. In full operation, the manufacturing plant has the capacity to accept 3000 t/a of SSA. The blend ratio of SSA with roller-milled P_i rock is currently limited to a maximum of 2.5% (<97.5% P_i rock). This is needed to guarantee the quality of both PA and by-product gypsum for customers. NPA could accept SSA from WWTP without the need to modify its existing facilities and save material costs by partially replacing P_i rock with SSA.

The alkaline technology has been applied to phosphate (P_i) recovery from sewage sludge ash (SSA) at wastewater treatment plants (WWTP). P_i is extracted from mono-incinerated sewage sludge with NaOH and recovered from the leachate using chemical precipitation with Ca(OH)₂. Approximately 30–40% of P_i could be recovered from SSA as calcium P_i while minimizing the leaching of toxic heavy metals at high pH. The recovered P product can be recycled as a fertilizing material for agriculture.

The full-scale plant for recovering P from sewage sludge ash (SSA) started operation at a wastewater treatment plant in Gifu City, Japan, in April 2010. P is released from SSA using the alkaline (NaOH) leaching technology and recovered as calcium hydroxyapatite (HAP). The full-scale plant recovers approximately 30–40% P_i from SSA and supplies about 300 t/year of by-product P_i fertilizer, named Gifu-no-daichiⓇ, mainly to local farmers. The P_i fertilizer, Gifu-no-daichiⓇ, is sold through the JA-Zen-Noh (National Federation of Agricultural Cooperative Associations) in Gifu City with a favorable reputation from farmers. Gifu City has been increasing the volume of the fertilizer sales year by year and considers it is critical to make the P recovery process more efficient and stable.

The constitutional duty of sustainability in Switzerland requires economical use of valuable and scarce raw materials such as phosphorus. The government of Canton of Zurich recognised this as an opportunity around 10 years ago with respect to the bottlenecks in waste disposal threatening to materialise in 2015 in the existing sewage sludge disposal plan. In 2007, it already gave the Department of Public Works the assignment to design the future sewage sludge disposal so that phosphorus recovery is possible. In 2009, an evaluation of all then-known phosphorus recovery procedures as well as their integration into different sewage sludge disposal pathways showed that the procedures with P recovery from sewage sludge ash are clearly superior to P recovery from sludge and sludge water. The first milestone in the implementation was that it was possible within 6 years and with the involvement of all parties impacted to realign the existing sewage sludge disposal concept completely with respect to the new framing conditions. Since mid-2015, a new central sewage sludge treatment plant at the most optimal location in the Canton has been producing high-phosphorus ash from incinerated sewage sludge. It contains more than 90% of the phosphorus potentials of the entire potential in untreated community waste water from the Canton. By switching over the sewage sludge disposal system from an inefficient, decentralised one to an efficient, centralised system, it has been possible to cut the average sewage sludge treatment costs by more than half, including ash disposal. No modifications to the waste water treatment plants were needed. The Canton has worked with the Foundation ZAR and selected development partners on this implementation of the large-scale engineering of phosphorus recovery from sewage sludge ash since 2011 (Phosphorus-Mining-Project). The initial focus lay on the production of high-grade raw material for fertiliser. This led, among other risks in product sales, to settling on the production of technically pure, conventional phosphoric acid as an already established product. Currently there is development work going on using the Phos4life® procedure by Técnicas Reunidas. The attempts to this point demonstrate a P-recovery rate of >95% from the ash, a material recycling of the minerals and the separated metals as well as the use of iron as a precipitant.

A calcination technology was developed to directly convert sewage sludge ash (SSA) to mineral fertilizer. SSA samples, having a wide range of P₂O₅ content (15.6–36.0 mass%), were collected from ten different wastewater treatment plants. CaCO₃ was added to SSA samples to adjust their CaO content to 45 mass%. The CaO-enriched SSA samples were then heated in an electric furnace at temperatures of 1250 or 1300 °C for 10 min. This could increase the citric acid solubility of P₂O₅ to 80–99%, regardless of the P₂O₅ content of SSA tested. In addition, approximately 75–97% of SiO₂ in SSA became soluble in 0.5 M HCl after being subjected to calcination. The high citric acid solubility of P₂O₅ and HCl solubility of SiO₂ in calcined products were attributable to the generation of silicocarnotite (Ca₅[(SiO₄)(PO₄)](PO₄)) and gehlenite (Ca₂Al₂SiO₇), respectively. The levels of toxic heavy metals such as Cr, Ni, As, Cd, and Hg in the calcined product were below their regulation levels for calcined sludge fertilizer in Japan. The calcination of CaO-enriched SSA is potentially a useful option to increase the solubility of P₂O₅ and SiO₂, thereby improving their plant availability, regardless of the P₂O₅ and SiO₂ contents of SSA.

With the increase in world population causing a rise in food demand, demand for nutrients such as phosphorus, potash, and nitrogen is on an upward trend. To maintain sustainable agricultural production, it is essential in many areas of the world to ensure stable procurement and to utilize these nutrients more efficiently. It is known that phosphorus is concentrated in sewage sludge through the cycle of consumption of food, human living, and sewage treatment systems. Hence, discussions have been held and studies done on ways of recovering phosphorus from sewage sludge and how to utilize it as a fertilizer. On the other hand, in addition to phosphorus, sewage sludge also includes micropollutants and heavy metals. How to separate heavy metals from phosphorus and decompose organic matter are important from the viewpoint of effectively and safely using phosphorus and preventing groundwater contamination. With the above background, we have developed a thermochemical process (melting process) under high temperature (around 1300 °C) that recovers 90% of phosphorus from sewage sludge and converts it into slag. Organic components are utilized as fuel in the furnace and decomposed. Heavy metals are vaporized in the furnace and collected by a gas treatment system. More than 90% of the phosphorus in the slag is citric acid-soluble phosphorus, indicating that plants can utilize the recovered phosphorus in the slag. We have also confirmed the effectiveness and safety of the slag as a fertilizer with a plant cultivation test. In this chapter, we describe the performance of a technology developed to recover phosphorus from sewage sludge based on the results of laboratory and pilot plant tests. We also describe the properties of the slag as a fertilizer based on the results of chemical analysis and the plant cultivation test.

Sewage sludge ash (SSA) often contains high levels of phosphorus (P) and has been considered as one of important secondary P resources. However, SSA also contains toxic heavy metals such as Cd, As, and Hg which can cause contamination problems in the recovery of P from SSA. Alkaline earth metal phosphate salts, which are present in SSA, can be preferentially dissolved by blowing CO₂ into the aqueous suspension of SSA. The dissolution mechanism involves the formation of soluble hydrogen carbonate by the CO₂ blowing which allows alkaline earth metal phosphate salts to form bicarbonate salts having high solubility in water. This chapter describes the selective extraction of phosphate from SSA using the CO₂ blowing method.

Ecophos s.a. has developed a unique modular process for the valorization of low-grade phosphate rock and/or various alternative P resources such as sewage sludge ash on the basis of soft digestion by hydrochloric acid or phosphoric acid. The process is extremely flexible and is, by the modular setup, capable of using several types of raw materials and producing a variety of products (fertilizer-, feed-, and food-grade phosphoric acid (PA), animal feed (DCP and MCP), and liquid NPK, PK, and NP fertilizers). The process has economic and ecological advantages over conventional industrial processes and those in development for valorization of sewage sludge ash, since it is simple, stable, and easy to control without needing expensive chemicals, raw materials, and equipment. The performance has already been tested in industrial plants at Bulgaria, Syria, and Peru as well as pilot- and lab-scale installations. Uptime longer than 7800 h/a is easily reached and the yield on P₂O₅ is 90% or higher. Furthermore the process can use excess HCl in the manufacture of products such as isocyanate, caustic soda, or SOP (Sulfate of Potassium). The energy balance is more positive than competing processes, since PA of high concentration (>42%) can be obtained without evaporation. The process can generate uranium (U)-free fertilizers, while conventional fertilizers generally contain 300–500 mg U/kg P₂O₅. Main by-products include high-purity CaCl₂ (as solution, flakes, or prills), radiation-free gypsum, silicate filter residue, and Fe/Al-chlorides. By applying different modules, most of the by-products can be split into sellable products, thereby minimizing final waste.

Outotec offers the AshDec® process by which inorganic calcined phosphates (thermophosphates) are produced from phosphate-rich ashes remaining from incineration or gasification of sewage sludge, animal by-products, poultry litter, and other nutrient-rich organic waste. In the thermochemical process, solid, potassium and/or sodium-based, alkaline compounds admixed to the ash decompose at a temperature of >900 °C and react with the ash-borne phosphates to form bioavailable (ammonium citrate-soluble) alkaline phosphate compounds. Simultaneously, the toxic arsenic, cadmium, and lead compounds become gaseous and evaporate from the reactor bed. As soon as the process gas is being cooled, the particles condensate and are captured and removed in an electrostatic precipitator as metal concentrate. The process produces a P or PK fertilizer with relevant mass fractions of silicates, sodium, and trace elements. Phosphates are released and taken up by crops when root exudates decompose the Ca-K/Na-PO₄ compounds preventing losses of P in solution if water-soluble P fertilizers are used. The recently explored, partial replacement of sodium sulfates by potassium phosphates avoids high sodium concentrations and leads to a PK 16-7 + 4S fertilizer with >25% total macro nutrient content and >10% sodium/potassium silicates that may enhance crop resilience. In a recent (published 2016) report, the Expert Group for Technical Advice on Organic Production (EGTOP) came to the conclusion to recommend calcined phosphates and struvite for organic production.

Phosphate recycling is an important issue, since phosphate is a finite resource which is essential to food security. The phosphate used in the fertilizer industry, which now solely comes from mining, has to be replaced with so-called secondary phosphates. At ICL Fertilizers, trials have been conducted to investigate the potential implementation of these sources of secondary phosphates into the fertilizer production. Extensive pilot-scale testing and several plant-scale tests have yielded promising results for the use of sewage sludge ash, meat and bone meal ash and struvite. The main issue remaining is the legislation for the use of these sources, as they are currently regarded as waste. Struvite is also suspected to be able to contain contaminants such as pathogens and pharmaceuticals, encapsulated in its crystals. Therefore, further research on this topic is necessary. In the draft of the new Fertilizer Regulations of the European Commission, maximum values for heavy metal content in fertilizer are discussed in greater detail. The first results show that products produced from sewage sludge ash (the big quantity of secondary phosphates) meet some of these demands; however some limits are put (without doing a risk assessment) too low and could block the use of these secondary phosphates in fertilizers. Since heavy metal content in struvite and meat and bone meal ash is low, no problems are expected. The use of secondary phosphate in fertilizer production yields great opportunities; however in parallel ICL is piloting other processes for production of industrial products (elemental phosphorous P₄ and food-grade phosphoric acid). The P₄ route is via the thermal RecoPhos process (inductive heating of ashes and evaporation, cleaning and condensation of the P₄) where no waste what so ever is created, only products with a positive market value. The food-grade phosphoric acid route is via the Tenova process, where ashes are treated with by-product HCl to produce phosphoric acid, which is then purified in several extraction stages. In the coming years, the pilot results will show the economic feasibility of these processes for which ICL has its own captive use in industrial applications. In this way ICL will try to turn the development of a circular economy from a threat into an opportunity.

Struvite recovery is an ideal way to simultaneously recover phosphate (P_i) and ammonia from digested sludge. A full-scale plant to recover struvite directly from digested sludge was implemented at Higashinada Wastewater Treatment Plant (WWTP) having the sewage treatment capacity of 241,500 m3/day in Kobe City. The struvite recovery plant has a capacity of treating digested sludge of 239 m3/day, which is equivalent to a quarter of digested sludge generated at the WWTP. On average, it can recover approximately 40 and 90% of total P and soluble P_i, respectively, from digested sludge. The struvite recovery could reduce the volume of dewatered sludge by 3.3% on average and prevent struvite-scaling problems in the sludge treatment process. Recovered struvite has been registered as a chemical fertilizer and distributed to the Kobe area through fertilizer companies.

In Japan, sanitation for 26% of the population is covered by decentralized treatment facilities called Johkasou and night soil treatment plants (NSTPs). The former is installed to treat black water from small communities or individual households. Johkasou is a general term for compact on-site wastewater treatment unit and/or facility and is applicable to a population of several to several thousands, depending on the installation condition. The latter is installed to treat mainly night soil (human feces) coming from 6% out of the 26% population that uses decentralized treatment facilities. Since the sludge extracted from Johkasou is also treated in NSTPs, they play a key role in the Japanese sanitation system. As part of a social sustainability policy, the “Plan of Sludge Resource Recycling Treatment Center” (SRRTC) was enacted as a bylaw in 1997. It demands that NSTPs be furnished with facilities for resource and/or energy recovery from organic wastes including night soil and Johkasou sludge. Facilities implemented by this plan are categorized as sludge resource recycling treatment centers. This chapter describes the first SRRTC project in which a chemical precipitation process was applied to the recovery of phosphorus as calcium phosphates from night soil and Johkasou sludge.

The Stuttgart Process for nutrient recovery aims to produce struvite as fertilizer from digested sewage sludge from wastewater treatment plants (WWTP) with chemical phosphorus removal. This chapter deals with the detailed description of the experiences with a pilot-scale test plant and its process operation, the latest process optimizations, as well as operational performance data, i.e., phosphorus recovery rates, recyclate product quality, required operational supplements, and costs. The results show that depending on the chemicals used for phosphorus elimination and on the process boundary conditions (especially pH value for dissolving phosphorus from the sewage sludge), different amounts of phosphorus can be recovered. With acidic leaching at pH of approximately 3, it is possible to gain recovery rates of more than 65% as struvite with high purity and very low contents of heavy metals and recalcitrant organic compounds. Additional operating costs for the Stuttgarter process would increase wastewater feed of about 0.15 €/m3.

Germany introduced a legislation to require the recovery of phosphorus from municipal wastewater in 2017. The ExtraPhos® process from chemical company Budenheim is emerging as one of the country’s promising technology options in this area. Budenheim began developing the phosphorus recovery process “ExtraPhos®” in 2010, forming part of a current wave of investigations in the country. Now the company’s process is emerging as one of the promising options for helping meet the recovery target.

Iron is omnipresent in sewage treatment systems. It can unintentionally be present because of, e.g., groundwater seepage into sewers, or it is intentionally added for odor and corrosion control, phosphate removal, or prevention of hydrogen sulfide emissions. The strong affinity of iron for phosphate has advantages for efficient removal of phosphate from sewage, but it is also often considered a disadvantage for phosphate recovery. For instance, the strong affinity between iron and phosphate may reduce recovery efficiencies via struvite precipitation or for some phosphate recovery methods from ash. On the other hand, iron may also have positive effects on phosphate recovery. Acid consumption was reported to be lower when leaching phosphate from sewage sludge ash with higher iron content. Also, phosphate recovery efficiencies may be higher if a Fe-P compound like vivianite (Fe₃(PO₄)₂ 8H₂O) could be harvested from sewage sludge. Developers of phosphate recovery technologies should be aware of the potential and obstacles the iron and phosphate chemistry bears.

Iron (Fe) is a metal element which is most abundantly produced in the world. Steel is an alloy of iron and other elements and plays a crucial role in the development of sustainable society. In the iron manufacturing process, the content of phosphorus (P) in the raw materials is relatively low but is concentrated into molten iron and then removed nearly completely into steelmaking slag. Hence, steelmaking can be viewed as a P enrichment process and generates slag which has a potential to serve as a secondary resource of P. The fundamental system of steelmaking slag is CaO-FeO-SiO₂ which is formed inside the primary 2CaO・SiO₂ region, indicating that this phase precipitates at the early stage of cooling slag. P is unevenly distributed in steelmaking slag and generally concentrated in the 2CaO・SiO₂ phase as a 2CaO・SiO₂-3CaO・P₂O₅ solid solution. This evidence reveals the potential to separate P from steelmaking slag. In this chapter, various technologies for P separation and recovery from steelmaking slag are described. They include magnetic separation, capillary action, dissolution, and carbothermic reduction. Since each method has its own merits and demerits, the best choice of single or combination of the technology options is critical to effective P recovery and separation from steelmaking slag.

In Japan, steelmaking slag is one of the most important secondary phosphorus (P) resources. The annual amount of P removed into steelmaking slag (c. 114 kt P/a) is approximately 2.8 times more than that of P imported as phosphate (P_i) rock to Japan. Increasing attention has been paid to P recovery from dephosphorization slag for mitigating the Japan’s dependency on P import. This chapter describes the smelting reduction technology for extracting P from dephosphorization slag into iron. The potential of this technology has been examined using a pilot-scale test furnace having the capacity to treat dephosphorizing slag at 2 t/day. P could be extracted from dephosphorization slag, forming Fe-P alloy with the P content of approximately 10 wt%.

This chapter examines the role of iron ore phosphorus in the development of steelmaking. The prosperity of steelmaking nations strongly depended on how they strategically dealt with iron ores with various phosphorus contents. At the beginning of steelmaking, the ore’s phosphorus content was immaterial. However, as the quality of steel improved, the effect of phosphorus became obvious. In Europe, iron ores with higher and lower phosphorus content were minable, prompting the development of the Thomas and Bessemer steelmaking processes, respectively. At the dawn of the Industrial Revolution, the main steel producer was the UK using the Bessemer process, but then it shifted to Germany adopting the Thomas process, followed by the USA, which used both open-hearth furnaces for scrap steels and the Thomas process until the mid-twentieth century. In these transitions, Germany found that slag, a steelmaking by-product with higher phosphorus content, could be used as phosphate fertilizer. Japan invented a phosphorus-oriented steelmaking process charging phosphate ore in addition to iron ore; in this process, slag with higher phosphorus was used as fertilizer. After World War II, Japan developed a highly efficient iron ore transpiration system using bulk carrier, and the introduction of the so-called Linz-Donawitz converter process with oxygen gas strongly promoted Japan to the steelmaking mainstream up to the present; most of their products contain extremely low phosphorus. Thus, innovative changes in steelmaking can be explained from the standpoint of the iron ore phosphorus content. From these summaries, future strategies for steel industries are discussed.

Pig farms in Japan tend to concentrate in the countryside owing primarily to livestock feed transport costs. Nearly all livestock waste generated in these areas is converted into composts or dry fertilizing materials according to Japanese agricultural regulation, but because the demand for composts and dry materials of livestock waste arises only in spring and autumn, a substantial need exists for developing new technologies to effectively use surplus pig slurry. This chapter describes the development of a sustainable system for recycling pig slurry into organic fertilizer through carbonization. The Energy-Free Carbonizing for Resource Recovery (EFCaR) system can self-generate enough heat for carbonization and produce biochar that is useful as a fertilizing material for organic fertilizer like steamed bone meal. The biochar generated from pig slurry shows the water solubility higher than those of phosphate rock and pig slurry burning ash (cinder). The EFCaR system is a viable solution to address pig slurry surplus in concentrated swine feeding areas.

To mitigate the risk of the secured supply of phosphate rock, increasing attention has been paid to phosphorus (P) recovery from untapped secondary resources. Animal manure is one of the most important secondary resources for P in light of the large quantity and high P content. This chapter describes the hydrothermal technology to efficiently extract P from animal manure. Laboratory experiments showed that the hydrothermal treatment of pig manure at 180 °C under the oxygen partial pressure of 1 MPa could increase the P extractability by approximately 150% compared to the untreated control. Under this condition, the concentration of P in the eluate reached up to 114 mg/L. The kinetics of extracting P from pig manure by the hydrothermal technology could be given by the first-order reaction equation.

Bench-scale experiments were carried out to examine the potential of recovering calcium phosphates from composted chicken manure. Ca2+ and phosphate ions were eluted from composted chicken manure using HNO₃ that released less K+ from composted chicken manure compared to HCl and H₂SO₄. After filtration, the eluate pH was increased to 6.0 using aqueous ammonia to precipitate calcium phosphates, forming calcium hydroxyapatite which is known as a principal component of phosphate rock. The purity of recovered calcium phosphates was more than 95 mol%, suggesting that composted chicken manure could potentially be used as a secondary source of calcium phosphates for industrial applications. Using this technology, calcium phosphates could also be recovered from industrial wastes such as dephosphorization slag, chemical industry sludge, and spent fluorescent phosphorus powder (called bag powder).

Relevant waste flows like sewage sludge, farmyard manure, digestion residues, and humid residues from food and feed production are known for food safety issues and for environmental and waste management problems. If used as a resource for crop nutrients and soil fertility, distribution is the main issue: urbanization and intensive livestock farming produce mass flows requiring extended cropland typically not available in the densely populated regions of our planet. Thermal conversion is an acknowledged option for concentration and recycling of mineral residues including phosphates, but the typical moisture content of >70 wt% makes it difficult to yield relevant surplus energy flows. This challenge is approached by increasing the efficiency of drying and replacing combustion by gasification, in essence by making effective use of the hydrogen (H₂) molecules of water in the process chain. Outotec’s technology approach aims at keeping H₂ molecules in the loop and eventually using them in the form of a hydrogen-rich gas in a variety of energy and biochemical applications. The approach is intrinsically circular, and the related processes – closed-loop steam drying and steam gasification – are well known but have not been applied to the waste flows and in the configuration as outlined in this book. If successfully implemented, waste flows in the order of 1–1.5 billion cubic meters in the EU28 may be recycled to a relevant building block of a future hydrogen economy with a vast array of applications in the energy and biochemistry sector.

Bone char is the product of a thermochemical conversion of defatted bones. This chapter summarizes the state of the art in the technical pyrolysis process, resulting physicochemical properties and other characteristics of bone chars and possible applications. Special emphasis is put on the solubility of P compounds, which in general characterize bone chars as potentially slow-release P fertilizers. The P release into soil can be improved by an “internal activation” through adsorption of reduced S compounds. Other agronomically relevant properties originate from the porosity that promotes water retention and the habitat function for soil microorganisms. Bone char effects on crop yields are summarized, giving the impression that field crops with long vegetation period and intensive rooting systems benefit most from this material. In conclusion, the carbonization by pyrolysis and formulation of bone char-based products is a reasonable approach in the recycling of P-rich bones from slaughterhouses. However, longer-term agronomic trials are required to fully evaluate the fertilizer potential of bone chars.

Amorphous calcium silicate hydrates (A-CSHs) can be chemically synthesized using abundantly available, inexpensive materials such as siliceous shale and calcium hydroxide. A-CSHs can serve as a bifunctional adsorption-aggregation agent for phosphate (P_i) recovery from aqueous solution. A-CSHs can also be prepared by soaking recyclable calcium silicates such as concrete sludge and steelmaking slag in a dilute hydrochloric acid solution. Since A-CSHs show high P_i removability, settleability, and filterability, they have the potential to offer a simple, cost-effective option to the recovery of P_i from P_i-rich waste streams. On-site experiments using a mobile, pilot-scale plant have showed that A-CSHs can recover approximately 80% P_i from a P_i-rich sidestream in a wastewater treatment plant. This chapter describes a simple process for P_i recovery from aqueous solution using A-CSHs as a bifunctional adsorption-aggregation agent.

This chapter describes the high-performance phosphorus adsorbent, PAdeCS®, which is produced from concrete sludge. The concrete sludge is a slurried, strong alkali waste that is generated in a concrete pole and pile manufacturing plant in Japan. PAdeCS® is a powdery granulate that can be used as a phosphorus-recovery agent for dissolved phosphate. The precipitation of hydroxyapatite is the dominant mechanism for phosphate removal from aqueous solution by PAdeCS®. Because of the high calcium content of PAdeCS®, the recovered phosphorus product can be used as a slow-release phosphorus fertilizer. The phosphorus-removal performance of PAdeCS® was compared with that of conventional phosphorus-removal agents such as polyferric sulfate, aluminum polychloride, and aluminum sulfate. PAdeCS® showed the highest phosphorus-removal performance among them. PAdeCS®, which is produced from concrete sludge, can be used widely as an inexpensive phosphorus-recovery agent.

The recovery and removal of nutrients from the main anthropogenic flows (e.g. urban/industrial wastewater and animal manure) could be crucial to maintain the ecosystems and to secure a renewable source of nutrients. Technology options for recovering phosphorus (P) from wastewater have often been limited by the need to treat large volume of diluted waste streams. The solid-liquid technology, which uses an adsorbent in conventional sorption-regeneration processes, is one of the most realistic solutions for P recovery from wastewater. In particular, the approaches based on a low-cost inorganic adsorbent have attracted increasing attention for recovering P from wastewater treatment plants. This chapter describes the use of coal combustion fly ash and its derivative zeolites as low-cost inorganic adsorbents for P recovery from wastewater. Laboratory experiments showed that the presence of aluminium and iron oxides as well as calcium (Ca)-based minerals was critical to promote the complexation with phosphate (P_i) on the solid structure, thereby forming brushite (CaHPO₄·2H₂O) deposits. Bioavailability assays have demonstrated that the P_i-loaded inorganic sorbents could serve as a slow-release P_i fertiliser in both basic and acidic soils. Furthermore, the use of synthetic zeolite mixtures has demonstrated their ability to simultaneously remove P_i and ammonium from wastewater. The recovered products could be used not only as mineral fertiliser but also as soil amendment, since the zeolite-based particles have a structure capable of improving the water retention capacity of soil. The potential implementation of the zeolite-based sorbents as P_i recovery technology has also been evaluated using a hybrid process consisting of P_i sorption and membrane ultrafiltration.

To remove phosphate (P_i) from wastewater, a novel, low-cost adsorbent was developed by immobilizing copper ions (Cu2+) on a naturally available biopolymer chitosan. The copper ions bound to chitosan through its complex formation with the amino and hydroxyl groups in the polymer chain. This complex formation could reduce the size of the hydrogels and thus increase the density of the natural biopolymer. The chelating interaction between nitrogen and copper enabled the intra- and inter-molecular cross-linking, thereby improving the physical and chemical stability of the polymeric ligand exchanger. Most importantly, Cu2+ ions could serve as the active functional group for P_i removal from aqueous solution. The polymeric ligand exchanger displayed a greater affinity with P_i rather than sulfate due to the electrostatic and Lewis acid/base interactions between immobilized Cu2+ and P_i, regardless of the solution pH. Batch adsorption experiments showed that the polymeric ligand exchanger had the maximum capacity of approximately 85 mg/g in P_i solutions. The polymeric ligand exchanger could also be applied to a fixed-bed column reactor, demonstrating the high performance on the P_i removal from aqueous solutions.

Several chemical industrial processes produce phosphite (Pt) as a by-product. This Pt waste should be recycled and reused as an alternative phosphorus (P) source to reduce the demand for nonrenewable phosphate rock reserves. Nearly all organisms require inorganic phosphate (Pi) or its esters as their P source. Therefore, Pt has been considered a biologically inert P compound, hampering the development of biotechnologies that utilize the Pt waste. During the last decade, the molecular mechanisms involved in the metabolism of inorganic reduced P compounds, including Pt, have been elucidated. Pt dehydrogenase (PtxD) catalyzes the oxidation of Pt to Pi, with a concomitant reduction of NAD+ to NADH, and thus is a promising biocatalyst for developing Pt-based applications. The initial discovery of PtxD, followed by the finding and development of PtxD enzymes with high catalytic activity and thermostability, facilitated the development of several unique biotechnological applications. These applications include (i) a dominant selective cultivation system for microorganisms and plants, (ii) a biological containment strategy for the safe use of genetically modified organisms, and (iii) a cofactor regeneration system for efficient production of chiral compounds by dehydrogenases. This section describes the emerging biotechnology applications that should contribute to the utilization of Pt as a valuable chemical.

Sewage sludge generated in Bio-P processes (also known as enhanced biological phosphorus removal processes) typically contains 3–5% of its dry weight as phosphorus (P). Approximately 60–80% of the P is stored as polyphosphate (polyP) which is a heat-labile polymer of inorganic phosphate (P_i). This chapter describes a simple technology, named “Heatphos,” for leaching polyP from Bio-P sludge by heating at 70 °C for about 1 h and recovering P_i from the leachate by the addition of CaCl₂ without needing to adjust pH to a high value. Heating sludge for polyP leaching can also improve the digestive efficiency and thus biogas productivity in the subsequent anaerobic sludge digestion at both mesophilic (typically 35–40 °C) and thermophilic (50–60 °C) temperatures. The heat energy required for polyP leaching can be supplied from biogas generated by anaerobic sludge digestion. The demonstration plant with a capacity of treating Bio-P sludge of 0.36 m3/day has showed that P could be recovered from Bio-P sludge at a rate of approximately 10 kgP/day (c. 3.5 tP/a).