**1. Introduction**

### **1.1. Anaerobic digestion**

Anaerobic digestion (AD) is a commercially available industrial process that generates biogas (roughly consisting of 60% of CH4 and 40% of CO2) and breaks down organic materials by anaerobic microbes. It is a process that can greatly reduce the amount of organic matter which might otherwise be destined to be land filled or burnt in an incinerator, both scenarios generating strong public concerns. The use of AD is suitable for most types of organic wastes such as livestock manure, waste paper, grass clippings, municipal waste, food and fruit/ vegetable processing waste etc. AD provides benefits including substantial odor reduction, production of a renewable energy source (biogas), reduction of greenhouse gas (GHG) emissions, potential pathogen reduction, minimization of solid waste for disposal, and enhanced nutrient management (Borowitzka 1999). Different groups of microorganisms are working together as a food chain to degrade the organic materials to produce methane as the final product. Briefly, insoluble organic material is hydrolyzed to produce simple soluble materials such simple sugar, amino acid and long chain fatty acid. Acidogenic bacteria degrade them to produce volatile fatty acid (VFA) and hydrogen, which is called acidogenesis. Then, acetogenic bacteria produce acetate from VFA and solvents in acetogenesis. There is a group of acetogenic bacteria which can synthesize acetate from hydrogen and carbon dioxide, referred as homoacetogenesis. And finally methanogens use acetate or hydrogen to produce methane as the final product. There are also other bacteria groups involved in the AD, for example sulfate reduction bacteria. Of all the current bioenergy options, AD is a well-estab‐ lished technology in Europe with large scale systems developed primarily in countries such

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as German and Denmark. In the US, large dairy farms are the leading users of AD in agricul‐ ture. EPA's AgSTAR program estimates that 137 dairy farms and 23 swine operations are using anaerobic digestion in the US. The current rate of anaerobic digestion deployment contrasts with AgStar's estimates of 2,645 dairy and 5,596 swine operations that could use anaerobic digestion with current designs.

While AD is the widely recommended technology for bioenergy production and nutrient management for animal wastes, the literature on AD is vastly focused on the influence of reactor configuration and manure characteristics based on four goals: solids destruction, biogas production, odor reduction, and pathogen reduction [1]; and few has been done on regards to P dynamics and mechanisms of transformation during AD. It is commonly believed that organic phosphate is degraded during the anaerobic degradation and therefore converted to inorganic phosphate. Gerritse and Vriesema [2] worked on the fractionation of P into organic and inorganic portions, as well as phase distribution, i.e., dissolved vs. particulate P, in liquid cow manure samples, either raw and digested. Their results confirm the hypothesis that most P found in manure is inorganic (around 90%), and overall, the inorganic:organic ratio does not change significantly after digestion – having found inorganic P at 86% of total P in their digestate samples. Similar qualitative conclusions are found in other literatures [3-5]. In regards to AD on phosphorus availability, discussions are very discrepant among studies. While some state that this degradation process increases the nutrient availability for plants [6] or that it does not have direct effects [7], some state that AD has potentially the opposite influence, i.e., it decreases P availability for plants [8-10]. It is known that pH influences the solubility of P and micronutrients; e.g., raising the pH moves the chemical equilibrium toward the formation of dissociated phosphate ion, which facilitates the precipitation of such ion as insoluble Ca and Mg phosphates. Some other concerns, such as the binding form of other elements, as Fe, may be regulated by AD [11]. Also, during AD, the fraction of dissolved P becomes mineralized and it becomes associated with suspended solids [12]. Also, the waterextractable P-fraction decreases substantially during AD. Struvite formation, which will be further discussed in this chapter, is very likely to be formed and crystallized, due to a combi‐ nation of mineralization of P, N, and Mg during AD, being regulated by many ionic species found in the digestate media, e.g., Ca2+, K+ , CO3 2- [13]. P loss, i.e., loss of phosphorus due to leaching, retention, etc, during AD are also in a wide range of results in the literature, being reported as smaller than 10% [14] or as high as 36% [15]. This is due to many factors, since AD systems are different among them with different operational conditions, which also include partial retention in the digesters during P precipitation [16]. Möller and Müller [7] suggested some losses could also occur via leaching and runoff after field application, even though no data has been published by then.

Since AD does not show any significant effects on phosphorus (P) removal [17], AD effluent still contain a high level of phosphorus (either organic or inorganic phosphate) that, when directly discharged, has a potential to cause various environmental issues, for instance, eutrophication, which severely damages aquatic ecological systems. In terms of manure management, large portions of the phosphorus can be removed by the solid–liquid separation of manure, which was proposed mostly in alignment with the AD of dairy manure. The phosphorus is relatively enriched in the solid phase while the liquid phase primarily only contains the soluble phosphate. The solid-liquid separation can be installed before the AD process and separating the solids from the liquid manure makes the liquid easier to pump and handle. However, it may significantly decrease the amount of organic materials available for the biogas production. The solid-liquid separation can also be installed after the AD process, where the solids can be used as the animal bedding materials. Numerous technologies have been developed to effectively and efficiently separate the solid and the liquid portions of the manure [18-23]; however, it is still considered as a pretreatment or as a first stage decontami‐ nation in highly polluted effluents with high content of particulate phosphorus. Even with the solid-liquid separation, the liquid waste still contains high concentration of phosphorus, while the phosphorus content of the solids is very low and the transportation of manure solids is still limiting the applications of phosphorus in a wider region.

As the animal industry is developing to larger production and operation to meet the increasing food and meat consumption as a result of the population growth and higher living standard [24], animal wastes, mainly animal manure in liquid, slurry, or solid forms, need more appropriate handling and utilization. Research on AD is boosted to a new level because of the recent research highlights on bioenergy and biofuel, so studies on methods for phosphorus removal and recovery from AD effluent needs investigation of the same level of attentions. In fact, recovery and recycle of P from AD effluent would offer a sustainable way of producing P fertilizer compared to the current approach that P is unsustainably mined from phosphate rock which according to some estimation that the reserve would be depleted within a century [25]. This book chapter will elucidate the mechanisms, processes and performances of some of the currently available P recovery technologies for manure and AD effluent, including chemical, electrochemical and biological methods.

### **1.2. Phosphorus in manure**

as German and Denmark. In the US, large dairy farms are the leading users of AD in agricul‐ ture. EPA's AgSTAR program estimates that 137 dairy farms and 23 swine operations are using anaerobic digestion in the US. The current rate of anaerobic digestion deployment contrasts with AgStar's estimates of 2,645 dairy and 5,596 swine operations that could use anaerobic

While AD is the widely recommended technology for bioenergy production and nutrient management for animal wastes, the literature on AD is vastly focused on the influence of reactor configuration and manure characteristics based on four goals: solids destruction, biogas production, odor reduction, and pathogen reduction [1]; and few has been done on regards to P dynamics and mechanisms of transformation during AD. It is commonly believed that organic phosphate is degraded during the anaerobic degradation and therefore converted to inorganic phosphate. Gerritse and Vriesema [2] worked on the fractionation of P into organic and inorganic portions, as well as phase distribution, i.e., dissolved vs. particulate P, in liquid cow manure samples, either raw and digested. Their results confirm the hypothesis that most P found in manure is inorganic (around 90%), and overall, the inorganic:organic ratio does not change significantly after digestion – having found inorganic P at 86% of total P in their digestate samples. Similar qualitative conclusions are found in other literatures [3-5]. In regards to AD on phosphorus availability, discussions are very discrepant among studies. While some state that this degradation process increases the nutrient availability for plants [6] or that it does not have direct effects [7], some state that AD has potentially the opposite influence, i.e., it decreases P availability for plants [8-10]. It is known that pH influences the solubility of P and micronutrients; e.g., raising the pH moves the chemical equilibrium toward the formation of dissociated phosphate ion, which facilitates the precipitation of such ion as insoluble Ca and Mg phosphates. Some other concerns, such as the binding form of other elements, as Fe, may be regulated by AD [11]. Also, during AD, the fraction of dissolved P becomes mineralized and it becomes associated with suspended solids [12]. Also, the waterextractable P-fraction decreases substantially during AD. Struvite formation, which will be further discussed in this chapter, is very likely to be formed and crystallized, due to a combi‐ nation of mineralization of P, N, and Mg during AD, being regulated by many ionic species

, CO3

leaching, retention, etc, during AD are also in a wide range of results in the literature, being reported as smaller than 10% [14] or as high as 36% [15]. This is due to many factors, since AD systems are different among them with different operational conditions, which also include partial retention in the digesters during P precipitation [16]. Möller and Müller [7] suggested some losses could also occur via leaching and runoff after field application, even though no

Since AD does not show any significant effects on phosphorus (P) removal [17], AD effluent still contain a high level of phosphorus (either organic or inorganic phosphate) that, when directly discharged, has a potential to cause various environmental issues, for instance, eutrophication, which severely damages aquatic ecological systems. In terms of manure management, large portions of the phosphorus can be removed by the solid–liquid separation of manure, which was proposed mostly in alignment with the AD of dairy manure. The

2- [13]. P loss, i.e., loss of phosphorus due to

digestion with current designs.

518 Biofuels - Status and Perspective

found in the digestate media, e.g., Ca2+, K+

data has been published by then.

Phosphorus (P) is one of the most abundant elements in the Earth's crust and it occurs in a large variety of forms, either in organic or inorganic forms, and also as monomeric (phos‐ phates) or as constituent part of macromolecules (polyphosphates). Its discovery dates back to 1669, by Hennig Brand of Hamburg, through the distillation of urine. Its history proceeds to further characterization of some phosphorus compounds and production of phosphoric acid in the 18th century by Boyle. It was during the first half of the nineteenth century that some scientists, especially Liebig and Lawes, made very significant advances in the science of plant nutrition, and the first studies on the utilization of phosphates as fertilizers were recorded [26]. Some remarkable work in the 20th century in the field of biochemistry has been developed upon the understanding of phosphorus in biological systems, such as the discovery of adenosine triphosphate (ATP) in 1929; the concept of high energy phosphate bonds in 1941; and the elucidation of the molecular structure of nucleic acids (DNA and RNA) by Crick and Watson in 1953. These findings, and other outstanding results, led to the understanding that phosphorus plays a vital role in living processes. Phosphorus is usually not found free in nature and mostly occurs in the fully oxidized state as phosphate; and phosphates can be classified according to their molecular structure. The first attempt to classify them was introduced by Graham, in 1833, in which he proposed the division into orthophosphates, pyrophosphates, and metaphosphates [26].

Phosphorus is present in agricultural residues and wastes, for instance, animal manure and litter. These residues are used as inexpensive fertilizer to improve soil quality. Soil tests have been conducted to estimate how much nutrient may be available for plant uptake during growth (Corbridge, 1985). For phosphorus characterization in soil, the values are aimed at identifying the labile P fraction, i.e., the fraction that is readily available for plants. From these observations, significant advances have been achieved over the last 65 years. The utilization of manure as fertilizer brought a different scenario for the analysis, due to the rich phase of phosphorus present in several samples reported in the literature [27]. Analytical techniques for phosphorus have transformed from simple gravimetric and volumetric titration methods to advanced new applications of 2D-NMR, chromatograph‐ ic, spectroscopic, and microscopic methods. However, for several studies, understanding the chemical behavior of phosphorus is more relevant than predicting its molecular properties, like on sediments, soils, and residual materials. On these studies, the different phosphorus forms are usually categorized within their capability of being recovered by some physiochemical methods. The inorganic fraction are usually categorized under the following groups: (i) adsorbed by exchange sites; (ii) associated with iron, aluminum and manganese oxides; (iii) associated with carbonate; (iv) associated with calcium as apatite; and (iv) bound in a crystalline mineral form. The organic fraction is divided into: (i) labile organic substances; (ii) organic phosphorus associated with humic and fulvic acid; (iii) acidsoluble components; and (iv) residuals consisting of phosphate esters and phosphonates [28, 29]. The method of phosphorus fractionation relies on sequentially extracting com‐ pounds from a sample with selective solvents, that are able to isolate P pools of different solubility and of different chemical behavior. The major drawback of such analysis is that, it is unable to isolate discrete chemicals, though sophisticated methods of fractionation is used. Hence, extractants are usually designated to solubilize groups of minerals defined as P associated with Fe, Al, Ca, or even residual forms [30].

On regards to the variety and solubility of manure phosphorus, it must be understood that manure is a complex system and there are numerous interactions between the organic and inorganic phases within its matrix. It has been stated that manure relies on a sensitive and balanced dynamic equilibrium where minor changes, such as through chemical, physical or biological processes, affect the matrix as a whole [31]. Researchers defined the characteristics of four types of manures based on the different phosphorus contents and their characterization: i) Swine; ii) Beef and Dairy; iii) Chicken and Turkey; and iv) other species [27]. SEM images of swine manure samples revealed the presence of MgNH4PO4⋅6H2O (struvite) and trace amounts of MgKPO4⋅6H2O (K-struvite); and found that these forms of struvite were in chemical equilibrium with beta-tricalcium phosphate (beta-TCP), and CaHPO4⋅2H2O (brush‐ ite) [32]. It was also found that swine manure has significant portions of brushite and Al‐ PO4⋅2H2O (variscite) [33]. The majority of analysis shows that struvite and brushite are commonly present in swine manure [27]. Similar to swine manure, cattle manure analysis provides a range of phosphate minerals; struvite and CaHPO4 (dicalcium phosphate, DCP) were found to be the main mineral forms of manure inorganic phosphorus as detected by SEM and X-ray diffraction [1, 31]. It was reported that [33] dairy manure predominantly has struvite and brushite, like swine manure. Chicken and turkey manure, shows significant amounts of brushite, beta-TCP, DCP, and struvite, but they vary with different studies [27]. Studies on other manures, such as horse, deer, sheep, and goat, have been reported as being constituted by a mix of struvite and at least one form of Ca-phosphate mineral, usually brushite [26].

Land application of manure has been a more sustainable practice to provide an alternative source for nutrients in order to improve agricultural crop production. Over the past few decades, livestock production has undergone an industrial revolution, resulting in the largescale generation of livestock manure [34]. Recent developments in corn ethanol production are also transforming the feed industry for wide applications of corn ethanol coproducts such as dry distiller's grains with solubles (DDGS) and corn gluten feeds (CGF) in animal diets. These new feeding materials are causing an increase in P excretion in animal manure [35]. There are multiple barriers to the land application of manure for P utilization and uptake by plants. First, the application is limited to the site close to the livestock, due to the low nutrient content (less than 1% of P in dry volatile solids, and solid content of swine manure around 6%) and subsequent high transportation cost. With the increasing size of livestock farms, especially in the areas where animal farming is highly concentrated, tremendous amounts of surplus manure must be discharged while the land in the surrounding area is oversaturated with P. Second, land application of animal manure is limited by its composition. For example, nutrients such as nitrogen (N) and P are present in swine manure in N:P ratios ranging from 1:1 to 2:1, while the N:P ratios needed by crops are between 3:1 to 15:1 [36, 37]. Therefore, when manure, especially swine manure, is applied to supply the crops demand for N, it results in the overdose of P. P-based application of manure is proposed as a new practice; however, this practice will not only result in under-application of N in most cases, but also will require more land to apply the same volume of manure [38, 39]. Over-application of P leads to its accumu‐ lation in soils; in turn, soils with high levels of P have been linked to environmental problems such as eutrophication of water bodies [40]. The land application of manure, especially swine manure, is considered an important contributor of P entering surface waters. Finally, once commercial chemical P fertilizer and manure are applied to soil, a large portion of soluble inorganic P is rapidly converted into insoluble forms by adsorption to the surface of soil particles, reacting with soil cations (such as calcium, iron, and aluminum), or immobilized into organic P by microorganisms in soil [41, 42]. Although the total P content in soil (average 0.05% w/w) is sufficient for plant growth, only 0.1% of the total P in soil is available to plants [43]. Overall, phosphorus separation from animal manure and digestate is a critical step in the sustainable utilization of the nutrient and in the healthy development of livestock industry. Some common phosphorus separation methods for liquid manure and digestate are discussed in the following sections.
