**2. Coagulation**

Graham, in 1833, in which he proposed the division into orthophosphates, pyrophosphates,

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

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

P associated with Fe, Al, Ca, or even residual forms [30].

and metaphosphates [26].

520 Biofuels - Status and Perspective

### **2.1. Chemical coagulation/flocculation**

Many commercialized processes for phosphorus removal from wastewater use chemical coagulation/flocculation methods by dosing divalent or trivalent metal cations (e.g., [Al(H2O)6] 3+ and [Fe(H2O)6] 3+, abbreviated as Al3+ and Fe3+ for convenience) via chemicals such as ferric chloride, ferric sulfate, aluminum sulfate, alum [KAl(SO4)2⋅12H2O] and poly-alumi‐ num chloride [44]. Coagulation process removes not only phosphorus in the form of phos‐ phate, but also dissolved organic matter, colloids and particulates which are prone to be coagulated [45]. Coagulation is different from crystal precipitation process (crystallization) in which by adding lime or magnesium compounds to wastewater precipitates of calcium phosphate (e.g., hydroxyapatite) or struvite are formed and gravitationally separated from liquid fraction [8]. But when multivalent metal cations of coagulants are added, only a portion of phosphate is precipitated with these ions to serve to the phosphate removal from liquid. Further information about precipitation is described in the section of "Struvite precipitation" in this chapter. It is suggested by some authors that the real mechanisms in the coagulation/ flocculation are complex, and it may include effects induced by other processes such as hydrolysis, complexation, crystallization, precipitation, adsorption (Figure 1), reductionoxidation, other types of interactions (e.g., ligand competition) among different ions, etc. It is not surprising that a study with emphasis on theoretical aspect of a process may have little indication of the real significance in experimental observations [46].

**Figure 1.** Schematic illustration of the charge neutralization by aluminum species [45].

The stability of colloidal system in wastewater is maintained principally by two ways: the coverage of the negative charge on particulate surface so that a repelling electrostatic force counteracts with van der Waals force and separates particulates from each other; and the hydration of the surface layers of colloids [46]. Destabilization of colloidal systems is the first step for coagulation. The efficacy of aluminum and iron coagulants principally originates from their ability to form multi-charged poly-nuclear complexes that enhance their adsorption capability [46]. Hydrated metal ions (Al3+ and Fe3+) with one or more hydroxyl ions are observed to substantially improve absorptivity and coagulation, but it is not clear through what mechanisms the hydrolysis and the poly-nuclear complexation improve adsorption (sweep flocculation) [45, 46]. The above processes for aluminum species are illustrated in Figure 1, while ferric species follow a similar way. The formation of insoluble amorphous metal hydroxide precipitate also provides an important way as adsorbents for phosphate to attach [44, 47]. Generally, the hydrolyzed cations generated from dosed chemicals (applied in solid or solution forms depending on the chemicals used) will provide positive charges in bulk liquid and neutralize the surface charge of colloids, causing an increased attraction among colloids and destabilizing the colloidal system. The destabilized particles are followed by flocculation process through metal hydroxo complexes to form larger agglomerates, eventu‐ ally forming larger particles of flocs. This mechanism can be especially useful because it is well accepted that the majority of manure phosphorus already exists in the colloidal form rather than dissolved phosphate anions, so crystallization of phosphate salts may be unnecessary prior to coagulation. Precipitation of insoluble phosphate salts, e.g., FePO4 and Fe5(PO4)2(OH)9 when ferric is added, is another significant factor contributing to phosphate removal [48]. When chemical dosing is high, insoluble metal hydroxides are precipitated from liquid, which will also enmesh particulate materials by a sweep action. After flocculation the wastewater can be directly subjected to some physical separation processes, such as floating, gravitational sedimentation, screw press, or filtration.

[Al(H2O)6]

522 Biofuels - Status and Perspective

3+ and [Fe(H2O)6]

3+, abbreviated as Al3+ and Fe3+ for convenience) via chemicals such

as ferric chloride, ferric sulfate, aluminum sulfate, alum [KAl(SO4)2⋅12H2O] and poly-alumi‐ num chloride [44]. Coagulation process removes not only phosphorus in the form of phos‐ phate, but also dissolved organic matter, colloids and particulates which are prone to be coagulated [45]. Coagulation is different from crystal precipitation process (crystallization) in which by adding lime or magnesium compounds to wastewater precipitates of calcium phosphate (e.g., hydroxyapatite) or struvite are formed and gravitationally separated from liquid fraction [8]. But when multivalent metal cations of coagulants are added, only a portion of phosphate is precipitated with these ions to serve to the phosphate removal from liquid. Further information about precipitation is described in the section of "Struvite precipitation" in this chapter. It is suggested by some authors that the real mechanisms in the coagulation/ flocculation are complex, and it may include effects induced by other processes such as hydrolysis, complexation, crystallization, precipitation, adsorption (Figure 1), reductionoxidation, other types of interactions (e.g., ligand competition) among different ions, etc. It is not surprising that a study with emphasis on theoretical aspect of a process may have little

indication of the real significance in experimental observations [46].

**Figure 1.** Schematic illustration of the charge neutralization by aluminum species [45].

The stability of colloidal system in wastewater is maintained principally by two ways: the coverage of the negative charge on particulate surface so that a repelling electrostatic force counteracts with van der Waals force and separates particulates from each other; and the hydration of the surface layers of colloids [46]. Destabilization of colloidal systems is the first step for coagulation. The efficacy of aluminum and iron coagulants principally originates from their ability to form multi-charged poly-nuclear complexes that enhance their adsorption capability [46]. Hydrated metal ions (Al3+ and Fe3+) with one or more hydroxyl ions are observed to substantially improve absorptivity and coagulation, but it is not clear through what mechanisms the hydrolysis and the poly-nuclear complexation improve adsorption (sweep flocculation) [45, 46]. The above processes for aluminum species are illustrated in Figure 1, while ferric species follow a similar way. The formation of insoluble amorphous metal hydroxide precipitate also provides an important way as adsorbents for phosphate to attach [44, 47]. Generally, the hydrolyzed cations generated from dosed chemicals (applied in solid Both Fe3+ and Al3+ salts (sulfate and chloride salts in solid or solution form) are widely used for coagulation for phosphorus removal, and the dose in liquid manure should reach 2 to 3 for the molar ratio of metal to phosphorus in order to achieve over 95% removal. Aluminum salts usually outperform iron salts at anaerobic condition because part of ferric is reduced to ferrous which is less effective in coagulation. A study evaluated the phosphorus removal by adding iron salts to simulated cattle manure (dissolving back dry solids in water): ferric salts were found much more effective than ferrous salts, and ferric chloride was more effective than ferric sulfate. Adding calcium oxide (CaO) removed additional amount of phosphorus [49]. Ferric chloride, ferric sulfate, aluminum chloride, and aluminum sulfate were assessed in jar test for phosphorus removal from liquid dairy manure [50]. Compared to the removal efficiency of 50% by natural sedimentation, 100 mg-Fe/L by ferric chloride slightly reduced the removal by 2%. Further increase of Fe (ferric chloride) to 200 mg/L, 300 mg/L, 400 mg/L, 500 mg/L, and 600 mg/L achieved removal efficiencies of 71%, 82%, 89%, 89%, and 92%, respectively. Ferric sulfate addition achieved very close removal efficiency for phosphorus, and removal efficiency was slightly improved from 82% to 88% when ferric was increased from 300 mg/L to 600 mg/ L. Aluminum chloride obtained much better phosphorus removal: 80%, 85%, and 99% efficiency at 100, 200, and 300 mg-Al/L. Aluminum sulfate was comparable to the chloride salt: 60%, 75%, 89%, 95% and 100% removals were achieved at 59, 119, 179, 239, and 300 mg-Al/L [50]. 200 mg/L of flocculant (polyacrylamide) dosing with ferric (300 mg/L) and aluminum (180 mg/L) achieved over 98% of phosphorus removal. The total cost of P removal (84%) and transportation of 5 mile was calculated to be \$4.09/m3 (\$0.02/gal). More data based on liquid cattle/dairy manure expressed in molar units are presented in Table 1.

Another study compared the coagulation/flocculation performance in terms of phosphorus removal from liquid swine manure by coagulants/flocculants of FeCl3, FeCl2, FeClSO4, poly aluminum chloride (PAC), and sodium aluminate solutions [54]. The manure was liquid/solid separated and treated by activated sludge in a reactor for 30 days. The resulting total phos‐ phorus was 322 mg/L. The ranges for the chemical dosing (mmol-metal/L) and removal efficiencies were listed as follows: FeCl3, 3.3 to 16.3, 39% to 95%; FeCl2, 2.1 to 10.6, 17% to 78%; FeClSO4, 3.3 to 16.7, 26% to 91%; poly aluminum chloride (PAC), 4.5 to 23, 23% to 91%; and sodium aluminate, 5.7 to 28.7, 14% to 34%. FeCl3 had the best performance of 95% removal, and the corresponding molar ratio of Fe to total-P was 1.57. Based on extrapolated data, in order to achieve total phosphorus down to the 2 mg/L from 322 mg/L, 27 mmol/L of FeCl3 should be dosed to manure, corresponding to a molar ratio of 2.6 [54]. Performance variance in different studies (Table 1) indicates that further investigation is needed for optimal phos‐ phorus removal and a reduced chemical cost.


**Table 1.** Performance of chemical coagulation on total phosphorus removal from liquid dairy manure

### **2.2. Electrochemical Coagulation (electrocoagulation; EC)**

Electrochemical coagulation (EC) is an alternative to chemical coagulation. The main mecha‐ nism responsible for coagulation is similar in electrocoagulation and chemical dosing, except the self-generation of metal cations by anode oxidation. The electrocoagulation offers some advantages over chemical dosing: it has simple equipment requirement and can be readily automated; reduces the chemical cost by using cheaper materials; gas bubbling provides gentle mixing that promotes coagulation and helps form bigger flocs; and gas bubbling carries some particles up to the top of liquid in a way of flotation, which may be easily separated. So electrocoagulation is not only an alternative to the conventional way, but also a promising method due to its effectiveness and low cost [55, 56], which has been used for phosphate removal from drinking water [57], turbidity reduction [58], and wastewater remediation [59, 60].

The application and performance of EC for animal manure treatment has been reported in several recent publications [61-63]. EC was explored to remove solids from digested and screw pressed manure [62]. Before the pre-processed manure entered the EC system, the screw pressed digestate contained 4.2% TS and 3.0% VS. The EC effluent had a decreased TS and VS to 0.89% and 0.28%, corresponding to a removal efficiency of 79% and 89%, respectively. A parallel EC yielded a comparable results in effluent, with only 0.62% TS and 0.13% VS. Electrode materials, configurations, operating conditions, and phosphorus removal were not reported in the literature. Another field EC test reported total and dissolved phosphorus removal from lagoon effluent which was chemically pretreated and centrifuged before entering EC unit [61]. The dissolved and total phosphorus was reduced from 0.15 and 7 mg/L to 0.01 and 2.1 mg/L respectively, with limited description of the EC reactor configuration and operating conditions in the literature [61]. The preprocessing of lagoon effluent may generate effluent that did not reveal the total capability of EC for P removal. 304 stainless steel was used as EC electrodes for dairy manure treatment [63] with 1 A (at a voltage of about 6 V) for 500 mL manure. Within the first 30 min, 83% TP was removed but the ensuing current application seemed not to substantially improve removal efficiency. TSS removal reached 88% after 20 min EC operation. In a continuous mode, 53% to 78% TP was removed at current density of 3 A to 5.5 A after 30 min operation. Based on these results, EC can be a method to treat liquid discharge from dairy farm, but more studies must be conducted before its field application in order to articulate the interfering factors, appropriate electrode materials and configuration, and operating conditions.
