Analysis of phosphorus recovery based on vivianite formation for practical applications

Análise da recuperação de fósforo baseada na formação de vivianita para aplicações práticas

ABSTRACT

Phosphorus (P) is considered a non-renewable resource. Owing to the increasing consumption of phosphorus in daily life, the “P crisis” is imminent. To address this crisis, it is urgent to find new phosphorus resources. This paper summarizes the research progress of P recovery based on vivianite formation from waste. Specifically, the advantages and disadvantages of optimizing the Fe source and pH in waste-activated sludge (WAS) and the co-fermentation of WAS and food waste are analyzed. Thereafter, the advantages and disadvantages of increasing the Fe dosage in full-scale wastewater treatment plants is discussed and an optimization scheme is proposed on this basis. By analyzing the advantages and disadvantages of comparative experimental results, two recovery methods are proposed to recover a large amount of P (≥ 83% total P) as high-purity vivianite (≥ 93%).

Keywords:
vivianite; engineering application; P recovery

RESUMO

O fósforo (P) é considerado um recurso não renovável. Dado seu consumo crescente na vida cotidiana, a “crise do P” é iminente. Para enfrentá-la, é urgente que novos recursos de P sejam encontrados. Este artigo resume o progresso das pesquisas sobre a recuperação de P baseada na formação de vivianita a partir de resíduos. Especificamente, são analisadas as vantagens e desvantagens de otimizar a fonte de Fe e o pH no lodo ativado residual (WAS) e a cofermentação de WAS e resíduos alimentares. Feito isso, são discutidas as vantagens e desvantagens de aumentar a dosagem de Fe em estações de tratamento de efluentes de grande escala, e com base nisso é proposto um sistema de otimização. Com a análise das vantagens e desvantagens dos resultados experimentais comparativos, dois métodos são propostos para recuperar grande quantidade de P ((≥ 83% P total) como vivianita de alta pureza (≥ 93%).

Palavras-chave:
vivianita; aplicação de engenharia; recuperação de P

INTRODUCTION

Status of phosphorus recovery

Phosphorus (P) is widely found in animals and plants, and it is also one of the most abundant elements in the human body. It is an important component of human bones and teeth as well as an important component of soft tissue, especially for the formation of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) (CAO et al., 2019). Phosphorus is ubiquitous, and agricultural production requires it. Phosphorus mainly exists in the earth’s crust in the form of phosphate rock, a non-metallic mineral resource that is difficult to regenerate. However, the distribution of phosphorus is very uneven, and the reserves are very limited. Approximately 20 million tons of phosphate materials is mined every year, and the price of phosphate materials continues to rise (CORDELL et al., 2011). It is worth noting that the Ministry of Electricity and Water Resources has studied the reserves and consumption of phosphorus (MEW et al., 2016), and they predicted that global phosphate reserves will be exhausted in the next 50–100 years, which is the reason for the surge in market quotations (CORDELL; DRANGERT; WHITE, 2009). The economic reserves of phosphorus only represent 1/3 of the basic reserves. Demand for phosphorus has increased with population growth and improvement in human living standards. According to statistical analysis, China’s consumption of phosphorus materials is increasing daily, and on the basis of this trend, China’s phosphate reserves will be exhausted 20 years later, followed by the “P crisis” (XIAODI et al., 2018).

It is precisely because phosphorus plays an indispensable role in nature and people’s lives that the “P crisis” is so urgent. Although we have developed several mature recovery methods for the high-efficiency recovery of phosphorus, these methods also have their own problems. The struvite recovery method has been widely accepted as effective for phosphorus recovery, whereby P and ammonium ions are removed simultaneously and fixed into struvite (MgNH₄PO₄•6H₂O) crystals, struvite being a commonly used slow-release fertilizer (KIM et al., 2018). However, struvite crystals can only be formed under strictly controlled conditions (i.e., high phosphorus concentration, appropriate Mg/NH₃/H₃PO₄ molar ratio, and appropriate pH (HUG; UDERT, 2013). Generally, the concentration of Mg is much lower than that of P in wastewater, and thus it is always necessary to add an additional dose of Mg (LIU et al., 2018), which reduces the economic feasibility of the technology. Moreover, the pH needs to be controlled in the range 8–9.5 for the formation of struvite, and the efficiency of the struvite recovery method for phosphorus recovery is low (10–50%). Vivianite has gradually attracted attention in recent years because of its high phosphorus recovery rate, high economic value, and broad utilization prospect. With increasing research, the characteristics of vivianite have gradually been determined. Although research on vivianite has been continuously reported in academic circles, further studies are required to elucidate the formation mechanism of vivianite and to explore crystal recovery.

The effect of temperature on the formation mechanism of vivianite has been investigated, and temperature was found to affect both the morphology of vivianite crystals (MADSEN; HANSEN, 2014) and the reduction effect of Fe(III) (CHENG et al., 2017). In addition, Zhang et al. (2020) and Yu et al. (2021) explained the influence of extracellular polymeric substance (EPS) on the formation mechanism of vivianite and the addition of graphite on the promotion of crystal size. To explore the feasibility of vivianite in practical applications, Wilfert et al. (2018) and Prot et al. (2020) demonstrated the importance of adding an Fe source, and Azam and Finneran (2014) confirmed through the addition of an Fe source that vivianite can be precipitated from nutrient-rich wastewater before it leaves the on-site wastewater treatment facility. Moreover, to improve the phosphorus recovery rate and the purity of vivianite, Cao et al. (2019) first separated the supernatant and then recrystallized the product. Wu et al. (2020) further optimized the recovery experiment and co-fermented food waste (FW) and waste-activated sludge (WAS) to obtain high-purity (93.90%) vivianite with a high total phosphorus (TP) recovery rate (83.09%).

Despite the slow research progress on the recovery methods of vivianite crystals, Prot et al. (2020) developed a wet separation technology based on paramagnetism, specifically Jones magnetic separators, to extract magnetic components from sludge flows on a laboratory scale. The separated vivianite mixture had a purity of up to 62% and an effective TP content of 80%. Jiazhou (2020) enriched vivianite by nitrogen gas flotation and designed a hydrocyclone to separate vivianite, with low separation effect of the sludge. This may be explained by the small particle size of the vivianite produced by digestion and the interference from other P components, such as aluminum phosphorus.

Although research on vivianite has made great progress, the application of vivianite in engineering is still sparse. Because engineering applications are dependent on the combination of various reaction conditions, the experiments of Zhang et al. (2020) are not suitable for large-scale engineering applications, and thus can only be used as a reference for optimizing the recovery processes. At present, methods based on relatively mature technologies which thus can achieve large-scale applications include the recrystallization method of Cao et al. (2019), the co-fermentation experiment of Wu et al. (2020), and the demonstration of Wilfert et al. (2018) and Prot et al. (2020) using external iron sources in wastewater treatment plants (WWTPs) for the formation of vivianite. The following sections compare the advantages and disadvantages of these methods, including proposals for optimal countermeasures. The purpose of this study was to propose optimized processes for the formation of vivianite and improved universal methods for engineering applications. Therefore, this article comprises a summary of the basic characteristics and formation processes of vivianite, an analysis and comparison of the advantages and disadvantages of the above three mainstream engineering application technologies, and proposed optimization schemes, specifically two methods, for the recovery of phosphorus as vivianite.

Characteristics of vivianite

Vivianite (Fe₃(PO₄)₂•8H₂O) was first discovered by British mineralogist John Henry Vivian (1785–1855) (ROTHE; KLEEBERG; HUPFER, 2016), so it was named vivianite. This mineral consists of single (FeO₂(H₂O)₄) and double (Fe₂O₆(H₂O)₄) octahedral groups, which are connected to each other by PO₄ groups and H₂O–H₂O hydroxyl bonds (ROUZIES; MILLET, 1993). The Fe²⁺ ion occupies two different positions in the structure. At site A, Fe²⁺ is surrounded by two oxygen atoms and four water molecules, forming an octahedral group. At site B, Fe²⁺ is surrounded by four oxygen atoms and two water molecules, forming an octahedral group. Oxygen is part of the P group (PO₄³⁻), forming a tetrahedron (WU et al., 2019). In addition, vivianite is paramagnetic, although it undergoes a magnetic transition at a Neel temperature of 12 K (MEIJER; VAN DEN HANDEL; FRIKKEE, 1967; FREDERICHS et al., 2003).

Vivianite is the end member of the iron-rich vanadium-titanium group M₃ (XO₄) ₂•8 H₂O, where M is divalent Mg, Mn, Fe, Co, Ni, Cu, and Zn and X is phosphorus or arsenic (FLEISCHER; MANDARINO JOSEPH, 1991); therefore, it can combine with other metal ions to form other substances. For example, baricite, the Mg analogue of vivianite, was discovered in an Fe-rich cold seep sediment off western Taiwan (HSU; JIANG; WANG, 2014). Vivianite exists in many natural environments, including river sediments (up to 20% by mass) (HEARN; PARKHURST; CALLENDER, 1983; WOODRUFF et al., 1999), canals (DODD et al., 2000), lakes (NRIAGU et al., 1972; NEMBRINI et al., 1983), and river delta mud (KONHAUSER, 1998), as well as wastewater sludge (FROSSARD; BAUER; LOTHE, 1997).

Vivianite is colorless and transparent in its original state and gradually turns green when exposed to air because of the gradual oxidation of Fe²⁺ in its crystals (ROTHE et al., 2014). Depending on the degree of oxidation of Fe²⁺, it occurs in a variety of colors, including light green, green blue, light blue, dark blue, and even black (HEARN; PARKHURST; CALLENDER, 1983; AMMARI; HATTAR, 2011;EGGER et al., 2015).

In nature, Fe originates from the dissolution of iron minerals as well as from domestic sewage, industrial wastewater, aquaculture, agriculture, animals, plants, and microorganisms. In the built environment, phosphorus is found mainly in wastewater from human excrement, household waste, P-containing detergents, and industrial pollution (SMIL, 2000; LOTZE et al., 2006; WITHERS; JARVIE, 2008). Phosphorus in water usually manifests as organic P, inorganic P, and poly P (MCMAHON et al., 2002).

Various investigations have found that vivianite mainly exists in a reducing environment. Under anaerobic conditions, Fe³⁺ reduces to Fe²⁺ by reducing bacteria, such as dissimilatory metal-reducing bacteria (DMRB), and then Fe²⁺ combines with PO₄³⁻ produced by anaerobic microorganisms. Under certain conditions, vivianite is precipitated when the target K_SP is reached (NRIAGU et al., 1972; WILFERT et al., 2018).

In summary, vivianite is formed in the bottom sediments of natural water bodies via two processes: the conversion of organic P in organic matter to inorganic phosphate (PO₄³⁻) and the reduction of iron (Fe³⁺to Fe²⁺). Vivianite is precipitated in the form of crystals, as shown in Figure 1 and Equations 1 to 4

(1) $$Porg\to P{O}_4^{3-}$$

(2) $$Fe{(OH)}_3+3H^{+}+e^{-}\to 3H_2\text{O}+F{e}^{2+}$$

(3) $$3F{e}^{2+}+2P{O}_4^{3-}+8H_{2}O\to F{e}_3{\left(P{O}_4\right)}_2\cdot 8H_{2}O$$

(4) $$3F{e}^{2+}+2HP{O}_4^{2-}+8H_{2}O\to F{e}_3{\left(P{O}_4\right)}_2\cdot 8H_{2}O+2H^{+}$$

Figure 1
Simplified vivianite formation process.

RESEARCH PROGRESS ON PHOSPHORUS RECOVERY FROM VIVIANITE

Advantages of Fe source and pH optimization and co-fermentation

Recrystallization

Vivianite is mainly formed in a reducing environment. Cao et al. (2019) and Wu et al. (2020) crystallized vivianite in controlled environments. In both experiments, the pH was controlled and an external Fe source was used to release Fe²⁺ and PO₄³⁻ into the anaerobic fermentation solution, and the supernatant was recrystallized after separation. The advantage of this method is that the content of other ions in the solution is relatively low, and because PO₄³⁻ and Fe²⁺ are the main ionic forms, the mechanism of recrystallization is not affected by various other factors. This process is easier to control than inducing the in situ formation of azurite in the sludge. Moreover, we do not yet fully understand the various properties of the sludge, and thus there are many uncertainties in the in situ formation of azurite in the sludge. The growth of microorganisms in the sludge, pH, and various complex ions, such as Mg, Al, Zn, affect the formation of vivianite. It has been reported that a magnesium analogue called maficite is found in the sediments of an iron-rich cold spring in western Taiwan (HSU; JIANG; WANG, 2014). In addition, co-existing S²⁻ competes with PO₄³⁻ for Fe²⁺ to produce FeS preferentially (GÄCHTER; MÜLLER, 2003; O’CONNELL et al., 2015). The experiment of Cao et al. (2019) avoided this point.

Fe source

In WWTPs, high phosphorus and low iron concentrations are common, and thus the addition of iron sources is very important for the effective formation of vivianite. This is very important in the experiments of Cao et al. (2019) and Wu et al. (2020). Generally, at low pH, most of the total water-soluble Fe in all samples is Fe²⁺ (> 95%), which facilitates the subsequent recovery of phosphorus as vivianite. In the experiment of Cao et al. (2019), the FeCl₃ dosed batch has a higher Fe²⁺ concentration than the ZVI (zero-valent iron)-dosed batch, and the time to reach the maximum Fe²⁺ concentration in the solution is the shortest. FeCl₃ in water is mainly hydrolyzed to form hydrous iron oxide (HFO) precipitates (CHEN et al., 2019). Under anaerobic reduction conditions, Fe³⁺ is reduced by DMRB to Fe²⁺(VUILLEMIN et al., 2013), and Fe²⁺ mainly exists in the form of Fe(OH)₂ precipitates, which are easier to dissolve under acidic conditions (ZOU et al., 2017) and are converted into the ionic state and released into the solution. In addition, because FeCl₃ is acidic when hydrolyzed in water, as shown in Equation 5, it also accelerates the dissolution of precipitates, as shown in Equation 6. Although in the experiment of Cao et al. (2019) ZVI is also selected as an additional Fe source, FeCl₃ batches obviously have a higher Fe²⁺ release than ZVI batches under the same conditions, for example, under the same pH3 condition. The reason for this result is the dissolution of ZVI in an acidic environment (LIU; WANG, 2019). However, in a weakly acidic environment, the dissolution rate of ZVI is relatively low, resulting in a relatively low Fe²⁺ concentration in the aqueous solution, as shown in Equation 7. Therefore, FeCl₃ is a better choice than zero-valent Fe.

(5) $$F{e}^{3+}+3H_{2}O\to Fe{(OH)}_3+3H^{+}$$

(6) $$Fe{(OH)}_{3(S)}+H^{+}+e^{-}\to Fe{(OH)}_{2(S)}+H_{2}O$$

(7) $$F{e}^0+2H^{+}\to F{e}^{2+}+H_2$$

Both experiments use FeCl₃ as the external iron source, which fully explains the effectiveness of FeCl₃, although there is no more explanation and exploration on the effect of adding other iron sources. The additional iron source has a profound impact on the benefit of phosphorus recovery, and the price is different. According to mainland China’s March quotation, FeCl₃ is $0.26163/kg, while FeSO₄ is $0.055328/kg. Therefore, FeCl₃ is relatively more expensive. If FeSO₄ can achieve similar results, it is more economical to choose FeSO₄ as the external iron source.

pH

The pH of an environment is known to affect the ion concentration. The experimental results of Cao et al. (2019) and Wu et al. (2020) both show a significant correlation with the pH, which directly affects the efficient recovery of P. Both have designed related pH control experiments, as shown in Tables 1 and 2. The pH experiment of Cao et al. (2019) shows that, with decreasing pH, the concentrations of Fe²⁺ and PO₄³⁻ in the solution increase because the solubility of iron and phosphorus compounds increases as the pH decreases.

Table 1
Experimental batches with pH ratio of Wu et al. (2020).

Reactors	pH uncontrolled	pH 3.0	pH 4.0	pH 5.0
WAS	R0-none	R0-pH 3	R0-pH 4	R0-pH 5
WAS + 10%FW	R1-none	R1-pH 3	R1-pH 4	R1-pH 5
WAS + 20%FW	R2-none	R2-pH 3	R2-pH 4	R2-pH 5
WAS + 30%FW	R3-none	R3-pH 3	R3-pH 4	R3-pH 5

• WAS: waste-activated sludge; FW: food waste.
et al.

Table 2
Experimental batches with pH ratio of Cao et al. (2019).

Fe source	pH 3.0	pH 5.0	pH 10.0	pH 12.0	NC
ND	ND-pH 3	ND-pH 3	ND-pH 3	ND-pH 3	ND-pH 3
ZVI	ZVI-pH 3	ZVI-pH 3	ZVI-pH 3	ZVI-pH 3	ZVI-pH 3
FeCl3	FeCl3-pH 3	FeCl3-pH 3	FeCl3-pH 3	FeCl3-pH 3	FeCl3-pH 3

et al.

Similar to the results of Cao et al. (2019), Fe²⁺ and PO₄³⁻ are released into the supernatant more efficiently under lower pH conditions in the controlled pH experiments of Wu et al. (2020). Moreover, as the concentration of added FW increases, the release rates of Fe²⁺ and PO₄³⁻ also increase to different degrees. Under controlled pH conditions, the Fe²⁺ and PO₄³⁻ release rates of the R3-pH3 batch and the R3-pH4 batch are almost the same, which are the highest among all batches. However, compared with the experimental batch that controls the pH, no additional reagents are added, and only FW co-fermentation is used to freely control the pH of the reactor to produce better results. The concentrations of Fe²⁺ and PO₄³⁻ in the R3-none batch are the highest among all the experimental batches. The experimental data also show that, in the R0-none batch, the concentrations of Fe²⁺ and PO₄³⁻ are the lowest among all batches, and as the fermentation progresses the concentrations of Fe²⁺ and PO₄³⁻ are the lowest among all batches and the concentrations of Fe²⁺ and PO₄³⁻ in the supernatant are negligible. This result also shows that joint fermentation using FW is more important than the control of pH. There is no need to deliberately control the pH, i.e., there is no need to add additional reagents, and there is no secondary pollution, thus reducing cost, manpower, and material resources. Compared with the experiment of Cao et al. (2019), this is also the greatest advantage of the experiment of Wu et al. (2020).

Furthermore, vivianite is mainly formed in the anaerobic fermentation process, and through the action of DMRB, Fe³⁺ is reduced to Fe²⁺ to form vivianite. Cao et al. (2019) effectively reduced the pH of the solution by adding ferric chloride, creating favorable conditions for the survival of reducing microorganisms. However, instead of an external iron source, Wu et al. (2020) used volatile fatty acids (VFAs) produced by co-fermentation to reduce the pH of the anaerobic fermentation environment, thus creating conditions conducive to the survival of reducing microorganisms. Experiments have demonstrated that, in WAS, the sludge with added FeCl₃ and controlled pH3 (CAO et al., 2019) has a relatively high abundance of Clostridiaceae (40.25%), which belongs to the order Clostridium (42.33%), and the Clostridium family is mainly composed of typical iron-reducing bacteria, namely Clostridium beckinii and metalloalkaliophilus (WEBER; ACHENBACH; COATES, 2006). These functional bacteria can use organic substrates as carbon sources and electron donors to reduce Fe³⁺. Therefore, under acidic conditions, abundant Clostridia in the FeCl₃-pH3 reactor can greatly accelerate Fe³⁺ reduction, resulting in a high concentration of Fe²⁺. Although Wu et al. (2020) did not discuss the dominant species of bacteria, the VFAs produced by co-fermentation also create an acidic environment to promote the growth of dominant bacteria.

Compared with the experiment of Cao et al. (2019), another advantage the experiment of Wu et al. (2020) is that co-fermentation also produces a large amount of VFAs. These are important intermediate products in the anaerobic digestion process. Methanogens (such as Methanomonsharveyi) (ROTARU et al., 2014) mainly use VFAs to form methane. Methane is an energy source with a high calorific value.

From the analysis of the above two experiments, the advantage of the co-fermentation experiment of Wu et al. (2020) is more obvious. For the time being, regardless of the phosphorus recovery rate and the purity of the crystallized vivianite, the experiment of Wu et al. (2020) simultaneously solves the problems associated with FW and WAS, i.e., there is no need for additional acidifiers and no occurrence of secondary pollution. Moreover, the intermediate product VFAs can be further used to produce methane, recycled as an energy source, which fits well with the idea of resource recycling.

Disadvantages of Fe source and pH optimization and co-fermentation

Microbial discomfort in engineering applications

In contrast to the experiment of Wu et al. (2020), that of Cao et al. (2019) seems to have a technological mismatch for enhanced biological phosphorus removal (EBPR) in large-scale WWTPs. These WWTPs generally use anaerobic digestion. Most of this process uses phosphorus-accumulating organisms (PAOs) to accumulate and release phosphorus. Neutral or slightly alkaline pH conditions are suitable for this kind of bacteria, which is why most WWTPs remain neutral. Overly acidic conditions dissolve PAOs. In the experiment of Cao et al. (2019), the release of PO₄³⁻ and Fe²⁺ is the highest in the FeCl₃-pH3 fermentation reactor. However, under such pH conditions, PAOs lose their activity, and thus there is no way to release phosphorus and subsequently absorb phosphorus. In addition, the dominant bacteria in the experiment is the Clostridium family, which is composed of typical iron-reducing bacteria. Under acidic conditions, it has a great positive effect on accelerating Fe³⁺ reduction. Although acidic conditions are conducive to the reduction of Fe³⁺ and the release of PO₄³⁻, because PAOs are moderately alkaline bacteria, there is always an opposing relationship between the two bacterial species in practice. Moreover, to control the pH, it is necessary to add reagents, which may cause secondary pollution, which should be avoided.

Incompleteness of experimental results

To better carry out reasonable resource allocation in actual engineering applications, and to recycle useful materials as much as possible, unnecessary resource consumption and waste should be reduced. Therefore, it is necessary to clarify the optimal ratio of the experiment to make recommendations for subsequent product processing and recovery.

As shown in Figure 2, analysis of the experimental data of Wu et al. (2020) shows that the addition of FW has a positive correlation with the degree of influence of Fe²⁺ and PO₄³⁻. The greater the addition of FW, the higher the concentrations of Fe²⁺ and PO₄³⁻. In addition, for the R3-none batch, the dissolved concentrations of Fe²⁺ and PO₄³⁻ also show a rising trend, which means that, for the result of co-fermentation, WAS + 30% FW may not be the most suitable fermentation ratio. Under this condition, the release of Fe²⁺ and PO₄³⁻ is already very high (accounting for 82.88% of total Fe²⁺ and 88.49% of TP). Therefore, a higher FW ratio may achieve better ion concentration release. Unfortunately, the authors did not provide further experimental data. If a higher FW ratio can achieve higher ion release results, then all results will change, and the purity of vivianite will become higher.

Figure 2
Fe and P release for different waste-activated sludge (WAS) + food waste (FW) batches.

Similarly, by analyzing the experimental results of Wu et al. (2020), as shown in Figure 3, the amount of VFAs produced also shows a rising trend regardless of whether the pH is controlled. Therefore, although the amount of VFAs reaches the maximum value in the reaction system under the conditions of R3-none and R3-pH5, whether this is the best ratio remains to be confirmed. Studies have shown that high concentrations of VFAs reduce the pH of the reaction system, and because low pH conditions affect the activity of methane bacteria, the production of methane is inhibited. This has an adverse effect on the final recovered products of the co-fermentation system.

Figure 3
Amount of volatile fatty acids generated under different waste-activated sludge (WAS) + food waste (FW) ratios.

Wu et al. (2020) did not analyze and discuss the product, and it was impossible to determine the method they used to treat the product. The method used to treat the final product is an important indicator of cost and is inevitably considered to optimize the engineering plan. Moreover, small differences in the experiment produce huge economic differences in large-scale engineering applications, and thus a complete analysis of the experiment is necessary.

Discussion of optimization for engineering utilization

Improvement of engineering operation procedures

In both the experiments of Cao et al. (2019) and Wu et al. (2020), enrichment of Fe²⁺ and PO₄³⁻ into the supernatant is followed by recrystallization, which significantly improves the purity of vivianite. Removal of biological phosphorus traditionally passes through the anaerobic stage of release and then the aerobic stage of phosphorus accumulation to finally obtain phosphorus-rich surplus sludge. There are two ways to use the Cao et al. (2019) experiment in engineering. The first is to re-collect the surplus sludge rich in phosphorus and then perform the Cao et al. (2019) experiment to finally obtain vivianite. However, this method is too cumbersome to operate. The second method is to change the process flow of the treatment plant. The final result of the traditional process is the removal of P, whereas the final result of the Cao et al. (2019) experiment is to obtain a high concentration of PO₄³⁻, so that the phosphorus can be absorbed through the aerobic stage while removing BOD and then released through the anaerobic stage. Finally, Fe is added to the phosphorus-rich wastewater and the Fe/P molar ratio is controlled for recrystallization. The advantage of this is that the sludge produced contains a small amount of phosphorus. In addition, because this method recovers nearly 82.60% of TP, the phosphorus content in the water after recrystallization is very low, which does not affect the secondary use of water. However, the operability of this improved method needs to be verified by experiments.

Control of the Fe/P molar ratio

In the Wu et al. (2020) co-fermentation experiment, the initial Fe/P molar ratio needs to be kept at 1.5. This is the theoretical ratio of vivianite. For all large-scale treatment plants, it is unrealistic to ensure precise control of this ratio, and thus excessive FeCl₃ and other flocculants need to be added. However, this increases the cost, and if this addition is excessive the effluent can easily contain a large amount of Fe. In fact, it is unnecessary to deliberately adjust the Fe/P molar ratio before co-fermentation. After co-fermentation, the supernatant is extracted, an appropriate external iron source is selected as the adding reagent, the pH is controlled, and then precipitation occurs. In this way, excessive addition of Fe can be avoided as far as possible, FeSO₄ can be chosen as the additional reagent to adjust the pH to 7, and then precipitation is performed. Therefore, it is possible to avoid waste of resources and a large amount of enrichment caused by excessive addition of Fe. In general, the two experiments have high reference value. The high purity of vivianite and high efficiency of phosphorus recovery have triggered further studies on vivianite. Solving the contradiction between using bacteria in WWTPs and ensuring a high PO₄³⁻ release rate is also worthy of further discussion.

Comprehensively increasing the amount of Fe in wastewater treatment plants

Advantages of increasing the amount of Fe

Wilfert et al. (2018) analyzed vivianite in the digested sludge of several comprehensive municipal WWTPs in Europe. They verified that these different WWTPs all use Fe to remove phosphate and that phosphorus can be recovered as vivianite using different Fe/P ratios. The data obtained by Wilfert et al. (2018) using X-ray diffraction, conventional scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX), environmental SEM-EDX, and Mossbauer spectroscopy indicate that, in digested sludge with the highest iron content, 70–90% of all P are combined in the form of vivianite. Wilfert et al. (2018) hypothesized that the iron content affects the recovery of phosphate, and, to verify this hypothesis, controlled increase of iron dosage was realized to determine the effect of iron dosage on the production of vivianite. Prot et al. (2020) doubled the iron consumption of the Nieuwveer WWTP, and the results confirmed their hypothesis that the iron content in the sludge is directly related to the content of vivianite.

The advantage of the Prot et al. (2020) experiment is that, by increasing the amount of iron in a comprehensive manner, the proportion of phosphorus as vivianite in the digested sludge can be increased from 20 to 50%. Notably, in this experiment, the concentration of H₂S in the biogas is reduced from 26 to 8 ppm mainly because, in the solution, S²⁻ preferentially combines with Fe²⁺ to form FeS precipitates, which is important in reducing H₂S emission. The added iron does not have any adverse effects on the removal of nitrogen, the production of biogas, the removal of chemical oxygen demand, and the dewaterability of sludge, as well as cost.

Advantages of FeSO₄

FeSO₄ is added in the experiment of Prot et al. (2020), which is different from the experiments of Cao et al. (2019) and Wu et al. (2020). Fe²⁺ can be directly complexed with PO₄³⁻ to form vivianite, without the need for complex biological and chemical reactions to reduce Fe³⁺ to Fe²⁺. Prot et al. (2020) found that external Fe added in the experiment is combined during the formation of vivianite. This is of great significance for studying the formation of vivianite.

As mentioned above, if FeCl₃ is replaced by FeSO₄ in the experiments of Cao et al. (2019) and Wu et al. (2020), then there is no need to consider the Fe/P molar ratio in advance. Therefore, Fe can be added after the supernatant is separated, and there is no need to reduce Fe³⁺. Because external iron is all used to form vivianite, only the pH needs to be controlled for recrystallization. In this way, the amount of added Fe can be effectively controlled, and resource waste can be avoided. Moreover, vivianite can be used to more thoroughly recover phosphorus in the supernatant. Because FeSO₄ is cheaper than FeCl₃, using FeSO₄ as the iron source reduces the production cost.

Disadvantage analysis

The shortcoming of the experiments is the purity of vivianite. Although Prot et al. (2020) did not specifically highlight it, we can speculate this from the experimental results. There are many metal ions in sewage that have adverse effects on vivianite formation and thus the purity and recovery of vivianite. The Mossbauer spectroscopy results show that different types of vivianite are formed in the Nieuwveer WWTP. The oxidation of vivianite and the replacement of Fe by non-magnetic cations (e.g., Mg²⁺ and Ca²⁺) change the structure of vivianite, thereby affecting the magnetic properties, recovery, and purity of vivianite. Wilfert et al. (2018) determined that the structure of this impure crystal is similar to that of vivianite. Moreover, by adding more Fe to the Nieuwveer WWTP than before, the proportion of P in the digested sludge can be increased from 20 to 50% for vivianite formation. Nevertheless, such results are still far from the experimental results of Cao et al. (2019) and Wu et al. (2020).

The experiment of Prot et al. (2020) was carried out in a WWTP using the AB method, while the experiment of Cao et al. (2019) was performed in a treatment plant using the A₂O method. The biggest difference between the two methods is the way in which phosphorus is removed. The main phosphorus removal stage of the AB method is in stage A, which is mainly based on biological flocculation and adsorption, and the amount of sludge produced is relatively large (approximately 80%). In the Prot et al. (2020) experiment, the Fe source is added in stage A, and the produced vivianite is deposited in the sludge. The purity of vivianite formed as a biological complex in stage A is not high.

The A₂O method requires anaerobic fermentation. This process is the opposite of the AB method. In the AB method, PO₄³⁻ is removed through adsorption, flocculation, and sedimentation in the sludge, whereas in the A₂O method, phosphorus is released in the anaerobic digestion stage. This difference affects the subsequent recovery method and the purity of vivianite. In particular, the Prot et al. (2020) experiment is not suitable for separating the supernatant and then precipitating vivianite, which is also the most fundamental reason affecting the purity of vivianite. The complexity of various components in the sludge hinders the purity of vivianite crystals.

Although the efficiency of phosphorus recovery is much lower with the Prot et al. (2020) experiment than with the Cao et al. (2019) and Wu et al. (2020) experiments, it is still higher than with the struvite recovery method (30–40%) commonly used internationally. In addition, the authors pointed out that stimulating the formation of vivianite combined with magnetic recovery from sludge is expected to recover 50–60% of phosphorus in the effluent of the Nieuwveer sewage treatment plant and 70–80% of blue iron ore in digested sludge.

The most important point is that Prot et al. (2020) carried out operational experiments in large-scale sewage treatment for the first time, and the effect is remarkable, which is a very suitable method for all sewage treatment plants with anaerobic digestion capacity.

PRACTICAL APPLICATIONS OF PHOSPHORUS RECOVERY FROM VIVIANITE

Although phosphorus recovered as vivianite has been studied for a long time, there is still a lack of systematic recommendations on how to proceed. In general, based on the above experimental analysis, there are two mainstream methods that can be adopted:

In situ vivianite formation

The first method entails the induction of in situ vivianite formation (in the sludge) by supplying an external source of iron for subsequent phosphorus removal operations. At present, FeCl₃ and FeSO₄ are the most widely used iron sources, and choosing different iron sources inevitably leads to different results. FeCl₃ reduces the pH of the solution and increases the concentration of released PO₄³⁻, although it affects the activity of alkaline bacteria (P-accumulating bacteria). Because Fe reduction requires the participation of metal-reducing bacteria, choosing the right bacteria is essential for effective reduction. The final product is sludge flocs rich in vivianite crystals. The sludge after the reaction can control the quality of the effluent and reduce the generation of H₂S gas. The main application is the Prot et al. (2020) experiment in sewage treatment plants using the AB method. Through the addition of an external Fe source, PO₄³⁻ in the sludge is co-precipitated to form vivianite, which greatly reduces the P content in the sewage. However, owing to the various physical and chemical changes and uncertain physical and chemical components in the sludge, precipitation cannot be controlled. Therefore, the purity of vivianite produced by this method is low, and Prot et al. (2020) did not provide corresponding data. In addition, vivianite is difficult to separate from sludge. In this regard, to obtain high-purity vivianite and highly efficient phosphorus recovery, we can only further optimize the method of vivianite recycling and the supply of additional iron sources.

Recrystallization

The second method involves the induction of large-scale phosphorus release in the sludge, followed by the extraction of the supernatant to synthesize and recrystallize vivianite. The main applications are discussed before. By controlling the pH, phosphorus can be efficiently released into the supernatant. An external iron source is then used for complexation, and the pH is adjusted to recrystallize vivianite. The advantage of this method is that the purity of the formed vivianite is relatively high. For example, the experimental results of Cao et al. (2019) show that approximately 82.60% of TP is recovered as vivianite (93.67%). This result is currently the highest purity reported for vivianite formation. This is mainly because the process of first releasing and then recrystallizing no longer uses digested sludge as the substrate, which eliminates the need for complex biochemical reactions in the sludge and avoids other physical and chemical effects to form vivianite, and thus the purity of vivianite is high. However, the disadvantage of this method is that it is not easy to induce the release of phosphorus in wastewater treatment plants using the A₂O method. As already mentioned, the Cao et al. (2019) method requires low pH conditions to ensure efficient phosphorus release, and these conditions are not possible in practice. Inevitably, the pH affects the P-accumulating bacteria in the anaerobic fermentation stage of the treatment plant and thus phosphorus release. From this point of view, phosphorus release cannot reach the theoretical maximum release. A feasible solution, mentioned before, is to enrich PO₄³⁻ first, and, after separating the supernatant, to adjust the appropriate Fe/P molar ratio to 1.5 and the pH to 7. FeSO₄ is then added for recrystallization. This can avoid inappropriate cost increases from quantitative uncertainties in practical applications.

CONCLUSION

From an analysis of existing research on vivianite, phosphorus recovery from vivianite has broader application prospects than previous methods in terms of economic value, operation method, and phosphorus recovery rate. The premise of the two methods described can be used as the focus of research on phosphorus recovery. For different systems, different vivianite formation methods and recycling methods are used. For example, external iron sources can be used to induce the in situ synthesis of vivianite in septic tank systems and wastewater treatment plants using the AB method. When a concentrated or pure vivianite stream is obtained, vivianite can easily be dissolved under alkaline conditions using KOH. Under these conditions, phosphate and potassium dissolve while Fe and heavy metals precipitate. For WWTPs that adopt the A₂O method and are equipped with anaerobic reactors, supernatant separation and then recrystallization can be used to obtain high-purity vivianite crystals. However, both these methods currently have operational problems that require further exploration. Optimizing the phosphorus recovery method based on vivianite formation and striving to achieve the best combination of environmental and economic benefits will be the focus of our future research.

• Funding: the initiative “Introducing talents to start scientific research projects” (NO:H19004).
• Reg. ABES: 20210164

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Publication Dates

• Publication in this collection
  28 Oct 2022
• Date of issue
  Sep-Oct 2022

History

• Received
  21 June 2021
• Accepted
  05 Aug 2022