**3. Struvite precipitation**

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‐

**total P**

**Reference**

**Coagulant Fe/Al dose Total solids Initial total P Final total P Ratio of Fe/Al to**

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

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

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

mmol/L % mg/L mg/L Molar ratio FeCl3 5.0 1 104 12.6 1.5 [51] Fe2(SO4)3 4.4 1 104 16.4 1.3 [51] FeCl3 5.4 0.9 110 19.8 1.5 [50] FeCl3 10.7 0.9 110 8.8 3.0 [50] Fe2(SO4)3 5.4 0.9 110 19.8 1.5 [50] Fe2(SO4)3 10.7 0.9 110 13.2 3.0 [50] AlCl3 3.7 0.9 110 22.0 1.0 [50] AlCl3 11.1 0.9 110 1.1 3.1 [50] Al2(SO4)3 6.6 0.9 110 12.1 1.9 [50] Al2(SO4)3 11.1 0.9 110 0.0 3.1 [50] Al2(SO4)3 8.0 1.6 106 8.4 2.3 [52] FeCl3 16.0 1.6 106 29.6 4.7 [52] Al2(SO4)3 5.9 1.1 143 25.0 1.3 [53] Al2(SO4)3 11.7 1.1 143 4.4 2.5 [53] FeCl3 3.4 1.1 143 25.9 0.7 [53] FeCl3 6.7 1.1 143 16.0 1.5 [53]

phorus removal and a reduced chemical cost.

524 Biofuels - Status and Perspective

The crystallization technology was originally developed primarily because of the more stringent phosphorus removal requirements to further decrease the phosphorus (P) level of effluent in wastewater treatment plants and produce more valuable and sustainable endproducts, such as struvite (magnesium ammonium phosphate, MgNH4PO4⋅6H2O) and calcium phosphate (e.g., hydroxyapatite, Ca5(PO4)3OH) [17]. Struvite is a mineral crystalline substance consisting of equal molar magnesium, ammonium, and phosphate, and it can potentially be an excellent slow-release fertilizer that has numerous potential uses in agricul‐ ture and horticulture. Its precipitation reaction can be expressed as Eq. (1):

$$\text{Mg}^{2+} + \text{NH}\_4^+ + \text{HPO}\_4^{2-} + 6\text{H}\_2\text{O} \rightarrow \text{MgNH}\_4\text{PO}\_4 \cdot 6\text{H}\_2\text{O} \cdot \text{4H}^+\tag{1}$$

Precipitation of struvite often occurs in wastewater when ammonium, phosphate, and magnesium ions exceed the struvite solubility limit. The solubility product constant of struvite is 10-13.26 [64]. Struvite precipitation from digested manure has attracted tremendous attention recently as a method of P removal and recovery [65-68], although the reusability of this precipitated phosphate salt is debatable [66, 69], especially when it is applied to the soil with higher pH due to the low solubility of the struvite in the alkaline conditions [70]. Manure sources to apply this technology include calf manure [71], swine wastewater [72-80], poultry manure wastewater [81], and dairy manure [82-85]. Struvite precipitation can be separated into two stages: nucleation and growth. Nucleation occurs when constituent ions combine to form crystal embryos. Then, they quickly form a larger nucleus of crystals and crystal growth continues until equilibrium is reached [86]. Struvite precipitation is influenced by type of reactors, pH, temperature, Mg:P ratio, chemicals added, reaction time and the presence of other ions in solution such as calcium.

### **3.1. Crystallizer**

Several types of crystallizer have been developed to remove phosphorus from manure wastewater by struvite precipitation. Bowers and Westerman [87] developed a cone-shaped fluidized-bed for struvite precipitation from swine wastewater. This design provided a high ratio of crystal surface area to reactor volume and it struvite seed to promote the growth of struvite crystals within the reactor. Field tests with pH adjustment and magnesium amend‐ ment demonstrated dissolved reactive phosphorus (DRP) removal efficiencies ranging from 70% to 82%, and total phosphorus (TP) reductions ranging from 63% to 80% [88]. However, high content of solids in the influent slurry could lead to washout of fine bed particles, resulting in lower phosphorus removal. The solids also could interfere with the bed particles and inhibit the struvite crystal growth. Shepherd et al. [89] developed a bench-scale continuous flow air sparged tank reactor (ASTR) for struvite precipitation in swine manure slurries. This system used air sparging for both pH adjustment and mixing, and used a peristaltic pump to contin‐ uously inject MgCl2 for struvite precipitation. The bench-scale ASTR system provided DRP reductions of 78% and 95% in swine manure slurry collected from a shallow under floor pit collection system and from a concrete storage tank with a permeable cover, respectively. However, separation of precipitated struvite for TP reduction was not achieved with an upflow clarifier operated in continuous flow mode. Then, they tested the performance of a pilotscale ASTR-hydrocyclone system [90]. The pilot-scale ASTR-hydrocyclone system provided a 92% reduction of DRP in manure slurry from a swine finishing facility concrete storage tank and a 91% reduction of DRP in manure slurry collected from a swine finishing facility deeppit under floor collection system. The pilot-scale ASTR-hydrocyclone system removed 18% of TP in swine manure from a concrete storage tank and 9% to 14% of TP in swine manure slurry from a deep-pit under floor collection system. The low TP recovery was attributed to the hydrocyclones inability to provide effective struvite separation as operated. Although ASTR was simple in design, fabrication, and operation, economics analysis indicated that this ASTRhydrocyclone system in swine finisher manure slurries was not currently economically viable. The third design presented the reactor for removing and recovering phosphorus from swine wastewater with dual functions of crystallization through aeration and separation of formed struvite by settling [78]. The swine wastewater was fed to the aeration column of the reactor continuously. Air was aerated to diffuser units via a stainless steel air tube. The crystals formed in the aeration column were settled and withdrawn from the bottom of the reactor. The maximum yield of struvite was 171 g/m3 swine wastewater, and the purity of the recovered struvite was approximately 95% without washing [79].

### **3.2. Effect of pH**

is 10-13.26 [64]. Struvite precipitation from digested manure has attracted tremendous attention recently as a method of P removal and recovery [65-68], although the reusability of this precipitated phosphate salt is debatable [66, 69], especially when it is applied to the soil with higher pH due to the low solubility of the struvite in the alkaline conditions [70]. Manure sources to apply this technology include calf manure [71], swine wastewater [72-80], poultry manure wastewater [81], and dairy manure [82-85]. Struvite precipitation can be separated into two stages: nucleation and growth. Nucleation occurs when constituent ions combine to form crystal embryos. Then, they quickly form a larger nucleus of crystals and crystal growth continues until equilibrium is reached [86]. Struvite precipitation is influenced by type of reactors, pH, temperature, Mg:P ratio, chemicals added, reaction time and the presence of other

Several types of crystallizer have been developed to remove phosphorus from manure wastewater by struvite precipitation. Bowers and Westerman [87] developed a cone-shaped fluidized-bed for struvite precipitation from swine wastewater. This design provided a high ratio of crystal surface area to reactor volume and it struvite seed to promote the growth of struvite crystals within the reactor. Field tests with pH adjustment and magnesium amend‐ ment demonstrated dissolved reactive phosphorus (DRP) removal efficiencies ranging from 70% to 82%, and total phosphorus (TP) reductions ranging from 63% to 80% [88]. However, high content of solids in the influent slurry could lead to washout of fine bed particles, resulting in lower phosphorus removal. The solids also could interfere with the bed particles and inhibit the struvite crystal growth. Shepherd et al. [89] developed a bench-scale continuous flow air sparged tank reactor (ASTR) for struvite precipitation in swine manure slurries. This system used air sparging for both pH adjustment and mixing, and used a peristaltic pump to contin‐ uously inject MgCl2 for struvite precipitation. The bench-scale ASTR system provided DRP reductions of 78% and 95% in swine manure slurry collected from a shallow under floor pit collection system and from a concrete storage tank with a permeable cover, respectively. However, separation of precipitated struvite for TP reduction was not achieved with an upflow clarifier operated in continuous flow mode. Then, they tested the performance of a pilotscale ASTR-hydrocyclone system [90]. The pilot-scale ASTR-hydrocyclone system provided a 92% reduction of DRP in manure slurry from a swine finishing facility concrete storage tank and a 91% reduction of DRP in manure slurry collected from a swine finishing facility deeppit under floor collection system. The pilot-scale ASTR-hydrocyclone system removed 18% of TP in swine manure from a concrete storage tank and 9% to 14% of TP in swine manure slurry from a deep-pit under floor collection system. The low TP recovery was attributed to the hydrocyclones inability to provide effective struvite separation as operated. Although ASTR was simple in design, fabrication, and operation, economics analysis indicated that this ASTRhydrocyclone system in swine finisher manure slurries was not currently economically viable. The third design presented the reactor for removing and recovering phosphorus from swine wastewater with dual functions of crystallization through aeration and separation of formed struvite by settling [78]. The swine wastewater was fed to the aeration column of the reactor continuously. Air was aerated to diffuser units via a stainless steel air tube. The crystals formed

ions in solution such as calcium.

**3.1. Crystallizer**

526 Biofuels - Status and Perspective

pH is an important role for struvite production because struvite plays slightly soluble under neutral and alkalinic conditions but readily soluble in acid [91]. At 25°C, the solubility of struvite is 0.018 g/100 mL in water, and increases to 0.033 g/100 mL in 0.001N HCl and 0.178 g/100 mL in 0.01N HCl [92]. Therefore, optimized struvite precipitation in manure slurries generally requires increasing the slurry pH. The minimal pH required for effective precipita‐ tion of struvite from anaerobically digested cattle manure effluents is about 8 and the operation pH should be controlled between 8.5 and 9.5 [83]. Miles and Ellis [76] reported that the optimum pH was about 9.5 for struvite precipitation in anaerobically digested piggery wastes. The experiments using anaerobic swine lagoon liquid for struvite precipitation showed that that the minimum concentrations of DRP occurred between pH 8.9 and 9.25 at all Mg:P ratios [77]. Burns et al. [74] also found that 96%-98% of the DRP was removed from swine manure slurries at a pH of 8.6 with a 10-minute reaction time and MgCl2 addition. However, a higher operation pH concurrently increases the caustic consumption and the cost of neutralizing the final treated effluents, even though it promotes better phosphate removal efficiency. Air sparging to elevate pH may be an advantageous approach for P recovery at larger scales because it would avoid liabilities from alkali additions [84]. Suzuki et al. [93] confirmed that aeration was effective for elevating the pH of swine wastewater to 8.5, and 65% removal of orthophosphate was achieved.

### **3.3. Effect of magnesium salts**

The Mg2+/PO4 3−/NH4 + molar ratio is 1:1:1 in struvite based on the molecular formula (MgNH4PO4⋅6H2O). The concentration of soluble P in animal manure is normally much higher than that of magnesium ion but much lower than ammonium nitrogen, which results in that magnesium ion becomes the limiting constituent for struvite precipitation from manure. Without magnesium addition, only 20–30% of soluble phosphorus was removed from centrifuged digested cattle manure effluent at pH 7.8–9.0 as both struvite and calcium phosphates due to the low levels of the initial Mg2+ and Ca2+ in the effluent [83]. In order to increase phosphate removal efficiency through struvite precipitation, more Mg2+ would be needed. Phosphate removal efficiency greater than 80% was achieved when the molar ratio of Mg2+/PO4 3- in swine wastewater was between 1.1 and 1.6 [74, 77]. However, the requirement of higher Mg2+/PO4 3- molar ratio was also observed by other studies [83]. The phosphate removal efficiency just increased to 50% at a Mg2+/PO4 3− molar ratio larger than 5 and reached 73% at a Mg2+/PO4 3− molar ratio up to 22:1. Struvite formation could be hindered by high Ca2+ concentration, ionic strength, alkalinity, suspended solids content, or a combination of these [85, 93]. Huchzermeier and Tao [85] reported that Ca2+ inhibited the struvite formation by blocking active struvite growth sites and competing for orthophosphate to form calcium phosphates. They also found that calcium can be effectively removed from anaerobically digested dairy manure through precipitation of calcium carbonate at pH 9 to 10 while retaining magnesium and orthophosphate.

Technically, many magnesium sources that produce Mg2+ ions can be used for the struvite precipitation process. MgCl2, MgSO4, MgO, Mg(OH)2 and other low-cost sources of magnesi‐ um, such as bittern and seawater, were tried as magnesium amendment for struvite precipi‐ tation [83, 94, 95]. Magnesium chloride (MgCl2⋅6H2O) was considered a good source of Mg2+ for struvite formation and reduced the reaction time that was required to dissolve Mg2+ into solution for struvite formation [73]. Burns et al. [74] indicated that MgCl2 can force the precipitation of P from swine manure. Zeng and Li [83] also found that MgCl2 and MgSO4 provided the higher P removal efficiency, likely due to their highly water-soluble property. The advantage of using MgCl2 is its faster dissolution in water and hence a shorter reaction time. However, it decreases pH because it is slightly acidic. A pH of 8.5 or greater would be required for effective struvite precipitation [96]. Use of MgO or Mg(OH)2 can supply Mg2+ and raise solution pH, and they are normally less expensive than MgCl2 and MgSO4. Although MgO and Mg(OH)2 are both poorly soluble in water, MgO is still a good magnesium source for struvite precipitation and it was better than Mg(OH)2 for struvite precipitation from digested swine wastes after they incurred insolubility problems with Mg(OH)2 [76]. For struvite precipitation from digested cattle manure, MgO was slightly worse than MgCl2 and MgSO4 but much better than Mg(OH)2 and MgCO3. Due to its insolubility, MgCO3 was the least effective for struvite precipitation from manure among the salts used [83]. A fine particle size and vigorous agitation of the reaction solution were needed when using MgO or Mg(OH)2 for struvite precipitation.

### **3.4. Hydroxyapatite precipitation**

Except struvite, small amount of other phosphate precipitates are also formed, which include K-struvite (KMgPO4⋅6H2O), calcium phosphates, and magnesium phosphates. Zeng and Li [83] reported that less than 1.5% K-struvite was formed in precipitated solids although the original digested cattle manure effluent contains a high level of potassium. Burns et al. [74] detected brushite (CaPO3(OH)⋅2H2O) in precipitates from struvite precipitation of swine manure slurries due to the Ca2+ in the slurries. Magnesium phosphates possibly as MgHPO4⋅3H2O (newberyite), Mg3(PO4)2⋅8H2O (bobierrite), and Mg3(PO4)2⋅22H2O were formed [83].

As a matter of fact, phosphorus removal in high Ca2+ content wastewater also can be achieved by direct precipitation of calcium phosphate (hydroxyapatite, Ca5(PO4)3OH). For sludge side streams of wastewater treatment plant, phosphorus removal efficiencies by hydroxyapatite crystallization ranged from 75% to 85% with additional artificial crystal seed material, consisting of calcium silicate hydrate (tobermorite crystals, manufactured by mixing siliceous and calcareous raw materials, pelletizing, and autoclaving) [97]. However, few studies have been done to remove phosphorus via hydroxyapatite crystallization from manure slurries. When Harris and coworkers [84] recovered phosphorus from flushed dairy manure waste‐ water by precipitation, they found that phosphorus can be recovered from flushed dairy manure wastewater not only in the form of struvite, but also calcium phosphate. Presence or formation of carbonates inhibits hydroxyapatite formation and solution pH value, again, is a key factor influencing the precipitation process. Addition of MgSO4 can suppress carbonate precipitation and enhance Ca phosphate precipitation at elevated pH (>9) [84].

### **3.5. Commercialization and application of struvite precipitation**

digested dairy manure through precipitation of calcium carbonate at pH 9 to 10 while retaining

Technically, many magnesium sources that produce Mg2+ ions can be used for the struvite precipitation process. MgCl2, MgSO4, MgO, Mg(OH)2 and other low-cost sources of magnesi‐ um, such as bittern and seawater, were tried as magnesium amendment for struvite precipi‐ tation [83, 94, 95]. Magnesium chloride (MgCl2⋅6H2O) was considered a good source of Mg2+ for struvite formation and reduced the reaction time that was required to dissolve Mg2+ into solution for struvite formation [73]. Burns et al. [74] indicated that MgCl2 can force the precipitation of P from swine manure. Zeng and Li [83] also found that MgCl2 and MgSO4 provided the higher P removal efficiency, likely due to their highly water-soluble property. The advantage of using MgCl2 is its faster dissolution in water and hence a shorter reaction time. However, it decreases pH because it is slightly acidic. A pH of 8.5 or greater would be required for effective struvite precipitation [96]. Use of MgO or Mg(OH)2 can supply Mg2+ and raise solution pH, and they are normally less expensive than MgCl2 and MgSO4. Although MgO and Mg(OH)2 are both poorly soluble in water, MgO is still a good magnesium source for struvite precipitation and it was better than Mg(OH)2 for struvite precipitation from digested swine wastes after they incurred insolubility problems with Mg(OH)2 [76]. For struvite precipitation from digested cattle manure, MgO was slightly worse than MgCl2 and MgSO4 but much better than Mg(OH)2 and MgCO3. Due to its insolubility, MgCO3 was the least effective for struvite precipitation from manure among the salts used [83]. A fine particle size and vigorous agitation of the reaction solution were needed when using MgO or

Except struvite, small amount of other phosphate precipitates are also formed, which include K-struvite (KMgPO4⋅6H2O), calcium phosphates, and magnesium phosphates. Zeng and Li [83] reported that less than 1.5% K-struvite was formed in precipitated solids although the original digested cattle manure effluent contains a high level of potassium. Burns et al. [74] detected brushite (CaPO3(OH)⋅2H2O) in precipitates from struvite precipitation of swine manure slurries due to the Ca2+ in the slurries. Magnesium phosphates possibly as MgHPO4⋅3H2O (newberyite), Mg3(PO4)2⋅8H2O (bobierrite), and Mg3(PO4)2⋅22H2O were

As a matter of fact, phosphorus removal in high Ca2+ content wastewater also can be achieved by direct precipitation of calcium phosphate (hydroxyapatite, Ca5(PO4)3OH). For sludge side streams of wastewater treatment plant, phosphorus removal efficiencies by hydroxyapatite crystallization ranged from 75% to 85% with additional artificial crystal seed material, consisting of calcium silicate hydrate (tobermorite crystals, manufactured by mixing siliceous and calcareous raw materials, pelletizing, and autoclaving) [97]. However, few studies have been done to remove phosphorus via hydroxyapatite crystallization from manure slurries. When Harris and coworkers [84] recovered phosphorus from flushed dairy manure waste‐ water by precipitation, they found that phosphorus can be recovered from flushed dairy manure wastewater not only in the form of struvite, but also calcium phosphate. Presence or formation of carbonates inhibits hydroxyapatite formation and solution pH value, again, is a

magnesium and orthophosphate.

528 Biofuels - Status and Perspective

Mg(OH)2 for struvite precipitation.

**3.4. Hydroxyapatite precipitation**

formed [83].

Struvite precipitation is currently the most commercially adopted technology for phosphorus recovery from wastewater as fertilizer. Modification on waste streams' constituents or operating conditions, such as chemical dosing, temperature elevation, air stripping and reactor innovation, is made to improve struvite crystallization and precipitation, and therefore, to increase P removal and recovery efficiency. Based on internet search, we found that there are numeral commercially available processes which have been developed and marketed. These processes include AirPrexTM developed by Berlin Water (Wassmannsdorf, Germany), NuR‐ eSys® by Akwadok Company (Waregem, Belgium), Pearl® with WASSTRIP (patent pending) by Ostara Nutrient Recovery Technologies Inc. (Vancouver, Canada), Crystalactor® by Royal HaskoningDHV Company (Amersfoort, the Netherlands), PHOSNIX by Unitika Ltd (Osaka, Japan), and Quich WashTM by Renewable Nutrients LLC (Raleigh, NC). Struvite precipitation technology dominates the market compared to other types of technologies such as incineration, enzyme hydrolysis, critical point oxidation, and adsorption.

Phosphorus recovery from raw manure or anaerobically digested manure via struvite precipitation has been widely studied, and high phosphorus removal efficiencies were both obtained from the two kinds of manure. However, just few researches investigated the effect of anaerobic digestion on struvite precipitation. Moody et al. (2009) found that anaerobic digestion of swine slurry increased orthophosphate (PO4 3-) in solution by 25% (from 1.26 g/L to 1.59 g/L), but the amount of Mg2+ increased by 254% (from 88.3 mg/L to 313.3 mg/L), which indicated that anaerobic digestion have no significant effect on reactive phosphate releasing, but it significantly increased the amount of available Mg2+. Increasing the solution Mg2+ concentration means less amendment of magnesium salt and low cost. In addition, PO4 3 removal efficiency of struvite precipitation was increased by 36% with anaerobic digestion pretreatment.
