Phosphorus Recovery through Waste Transformation: Implication for an Alternative Fertilizer

Abstract

Presently, the recovery and reuse of phosphorus are still far from being a mainstream practice. Yet, the techniques already accepted and applied differ by the origin of the used matter (wastewater, sludge, ash) and are mainly focused on the process of precipitation. One of these techniques is struvite (magnesium ammonium phosphate; MAP; MgNH4PO4·6H2O) precipitation, which can be implemented in wastewater treatment plants that use enhanced biological or semi-biological/chemical phosphorus (P) removal. Struvite/MAP is formed by a basic precipitation reaction in different stages of the wastewater treatment process, where magnesium (Mg2+), ammonium (NH4+), and orthophosphate (PO4−3). This chapter aims to discuss: (i) the progress in extraction of P from sewage sludge and animal manure; (ii) the methods to create optimal conditions for struvite precipitation in such media; (iii) the avenues for overcoming the problems associated with choosing the right Mg source, pH adjustment and the non-acceptable level of organic matter in the initial suspension; and (iv) the implication of struvite as an alternative fertilizer for the global agriculture sector.

1. Introduction

Phosphorus (P) is an essential nutrient for all forms of life. Specifically, P is an important nutrient element in agriculture and a major limiting factor for plant growth and the entire food production chain. Currently, both P and nitrogen (N) are the basic components of mineral fertilizers viewed as an irreplaceable part of modern agriculture. P is a resource obtained mainly from phosphate rocks located in a few regions of the world [1]. More than 87% of all mined and processed phosphate rock is turned into fertilizers while few phosphate rock is used for additives in livestock feed and food.

There are evidence that P reserves used for mineral phosphate fertilizers as a primary source of P input to agricultural lands are steadily decreasing with time, with the expectation to be depleted in a few centuries or less [2, 3, 4]. More striking is the view of the United States Geological Survey according to which the phosphate deposits will last about 50 years at the current rate of extraction [5]. Such a perspective can be reasonably explained by the continuously growing population and rising global demand for food [6]. Global production of phosphate rock coupled increased world populace is shown in Figure 1.

Figure 1.

Global production of phosphate rock (blue) coupled with world population (red) in time [6].

The expected P rocks deficiency is supported by the current monetary evaluation of the global phosphate rock market. It shows an annual growth rate higher than 5% in recent years [7] reaching $25.49 billion in 2023.

Recognizing that phosphate rock is a limited resource for mineral fertilizers production, the necessity of identifying P-rich waste streams and finding technical ways for P extraction into valuable products is of extreme importance. Part of the increased pressure on P resources could be alleviated by recycling P contained in various agricultural, industrial, and urban wastes [8, 9]. Excess P during fertilization and the non-proper treatment of wastes, ground an imbalance of the biosphere following severe negative environmental effects. While previous efforts have been directed toward removing P from the wastewater discharged into surface water, current efforts are directed to recover P as useful species [10]. The potential of the waste P is high as the global mass balance shows that up to 20% of the world’s P-production (about 3Mt P/year) is currently lost [11]. On the other hand, resource recovery from waste streams is increasingly seen as one option to improve the economics of wastewater treatment. Domestic sewage, industrial wastewater, and manure are no longer considered waste but just a resource of different origins [12]. The green line in Figure 2 (below) outlines the recovery route in addressing the P-resource recycling in the framework of the circular economy concept.

Figure 2.

Phosphorus production, consumption, and recycling pathways [13]. The color code indicates the impact in terms of environmental health and sustainability: Blue – Neutral (currently applied practices); red – Negative practices to be avoided or minimized; green – Processes with positive impact.

2. Phosphorus recovery as struvite

The shift into circular use of P passes through the implementation of multidimensional innovations focused on the creation of P recovery/recycling technologies [14]. The main well-recognized “waste” streams containing nutrients and causing negative environmental effects include animal manure, urban wastewater and sewage sludge, and food processing wastewater [15]. In the following sections, this chapter discusses two specific potential sources of P, namely the sewage and manure waste streams.

Domestic wastewater and sludge could be regarded as an important secondary source of P. The sludge water from municipal wastewater treatment plants (WWTP) is a fluid containing a reasonable level of P (120–160 mg/l PO₄³⁻), which allows the extraction of this nutrient with good economic effect, especially taking into account the global volumes of this waste stream. According to many authors, its potential counts for 15–20% of the global phosphorus demand [8, 16, 17]. For this reason, phosphate recovery has high priority in sewage treatment. European Union estimations show that the sludge amount produced annually is over 7 Mt. dry solid (DS), while globally, 1.3 Mt. P/year is treated in WWTPs worldwide [8, 17]. Animal manure production is another significant waste stream containing P. The amount of P in manure compared to municipal wastewater depends strongly on the manure source and type. Common o-phosphate concentrations of manure liquid fraction varied in a wide range of 50–1200 mg/l. However, the high suspended and dissolved organic matter should be taken into consideration in choosing the P recovery option. The available data show that in evaluating the by-products and waste sources of P in Mt. per year (Table 1) P in animal manure is almost one order of magnitude higher in sewage sludge (20–30 Mt. vs. 3–5 Mt., respectively) [19].

Molar ratio Mg^2+: PO4^3−	1:1	2:1	3:1	1:1	2:1	3:1	1:1	2:1	3:1
pH	8	8	8	9	9	9	10	10	10
P Removal efficiency, %	45.6	44.7	44.9	76.1	76.3	78.4	79.9	89.9	89.2

Table 1.

Series of experiments using supernatants produced after centrifugation of sludge from WWTP-Burgas [18].

Interestingly, several technologies for P recovery from wastewater and sludge are operating at either full or demonstration scale. However, P-recovery technologies are focused mainly on the aqueous phase of sludge (the so-called sludge water). In the solid phase, the technologies are directed either to recover P from the dewatered sewage sludge or from mono-incinerated sewage sludge ashes [20]. The accepted means differ in respect of technology choice, costs, efficiency, and product purity.

Chemical precipitation of P into salts of low solubility is a common method for removing dissolved phosphorus from wastewater: in the form of magnesium ammonium phosphate (MAP) hexahydrate (MgNH₄PO₄·6H₂O), also called struvite, or calcium phosphates [21]. Struvite has the advantage of being a slow-dissolving salt, while calcium phosphates are characterized with extremely low water solubility. P-recovery is usually based on the chemical precipitation of struvite from concentrated wastewater or the liquid fraction remaining after anaerobic digestion of sludge in WWTP. The chemical precipitation of struvite removes up to 98% of the soluble phosphates and 20–30% of the soluble ammonia contained in the targeted liquor [22]. Notably, the main challenge in relation to struvite precipitation is the P-recovery from wastewater characterized by phosphorus concentration of less than 50 mg/L and suspended solids concentration (TSS) above 1000 mg/L [23].

The struvite formation process is a function of pH and the molar ratio between magnesium, ammonium, and phosphate ions. The precipitation occurs in alkaline conditions and optimal crystal formation is observed at pH above 9 and equimolarity of the constituent ions [24]. However, several studies show that struvite can be synthesized in a wide range of pH values. Even experiments applying a pH slightly above 8 show positive results, but the formation and precipitation rates are lower compared to the optimal pH range. This is confirmed by studies with real dewatered sludge liquor (DSL) taken from the MWWTP of Burgas, Bulgaria (initial PO₄³⁻concentration of 86.7 mg/l) [18]. In the same study, MgCl₂ was used as a precipitation agent while different pH and molar ratio Mg:PO₄ were applied (Table 1). Obviously, both pH and mole ratio Mg:PO₄ are crucial factors affecting precipitation efficiency. However, the highest effect is observed at pH near and above 9.0. At extremely high pH values (above 11), the struvite yield decreased due to the formation of Mg(OH)₂ and transformation of ammonia ions into free NH₃ (reducing the general availability of Mg²⁺ and NH₄⁺ in the medium). Experiments carried out by Saidou et al. [25] showed that at initial solution pH of 10, another phosphate mineral, namely Mg₃(PO₄)₂.2H₂O starts precipitating. Such formation results in other dominant species formation. In fact, varying the pH levels results in different species of phosphate ions. The precipitation of struvite requires equal molar ratios of its components: magnesium, ammonia, and phosphate ions. Different molar ratios have been shown to influence several characteristics of the struvite. It could be argued that as more Mg is available, more crystal units are generated, hence larger crystals could be formed. As reported by Merino-Jimenez et al. [26], several studies have applied the ratios of phosphate and magnesium ions around 1:1.2 for optimum struvite yield. The latter result confirms the effects shown in Table 1.

As it was shown above, one of the main factors for struvite formation is the pH. In the targeted P-containing waste streams, approximately 90% of P is trapped in sewage sludge following primary and secondary sedimentation [27]. The pH of these fluids (usually centrate) is in the range of 7.3 and 7.5; far from the optimal pH values providing technologically acceptable conditions for MAP crystals formation. In this case, as a general option struvite crystallization is achieved through alkalization by reagents such as NaOH.

Besides the direct reagent alkalization there, pH can be elevated by carbon dioxide (CO₂) stripping through aeration of the reagent mixture. Figure 3 shows the principal scheme of the striping system [28]. The mechanism of this treatment is based on the manipulation of the dissolved carbon dioxide mass balance in the liquid. The aeration of the solution leads to the decomposition of the carbonic acid (H₂CO₃) to H₂O and CO₂ which results in pH elevation. The described mechanism seems to be appropriate in the case of sludge water treatment as it contains high amounts of organic matter which potentially enriches the liquid with CO₂ due to the microbial activity occurring in the suspension. It was found that the rate of pH elevation depends strongly on the spangling area of the air distribution system while the air flow rate does not influence considerably the dissolved oxygen level which governs the CO₂ stripping process [28]. The theoretically calculated values of the volumetric mass transfer coefficient have been compared with those obtained experimentally. Based on the data obtained, relationships of pH/k_La (mass transfer coefficient) were developed. These correlations serve as a tool for the prediction of pH during the struvite precipitation process. The pH dependence of air rate is given in Figure 4.

Figure 3.

Air stripping unit design [28].

Figure 4.

Elevation in pH by air stripping through applying different volumetric air rates [28].

The results showed evidently that within the volumetric air rates applied pH elevation from 7.5 to 8.4 was achieved by CO₂ stripping in less than 25 minutes. Such a retention time in the reactor is technologically applicable. Actually, a pH value around 8.5 is high enough for effective struvite precipitation even in several studies the optimal pH for the process is shown to be in the range of 9 to 9.5. The targeted pH 9 could be also achieved in an acceptable retention time of less than 1 hour. On the other side, the slow pH change during the aeration could be considered an advantage of the CO₂ stripping process because it restricts the rapid increase of solution saturation. At such conditions, the struvite crystallization process predominates, and in addition, Ronteltap et al. [29] reported that the crystal sizes at pH values of 7–11, and found that the largest crystals occurred at pH 8.

The concentration of magnesium is the other limiting factor for struvite crystal formation, the choice of Mg source is important as it forms nearly 75% of the struvite production costs [21]. Several commercial magnesium salts such as MgSO₄, MgCl₂, MgO, or Mg(OH)₂ have been used to precipitate struvite from different liquid wastes [30]. Cheaper options based on seawater as a source of Mg ions are thoroughly studied as well demonstrating a comparatively high phosphorus recovery rate (80–90%) and precipitation of several different products with the domination of struvite (but also containing magnesium calcite and calcite due to the complex ion composition of the seawater) [31].

Similar results were obtained in the research group of the authors, where seawater brine was used to precipitate P from WWTP sludge centrate [32]. Also, the seawater brine used was found to contain significant amounts of calcium and potassium (Table 2). However, the magnesium concentration is higher (59.5 g/l) than the calcium concentration (3.5 g/dm3) which resulted in effective struvite precipitation with minimal calcium phosphate co-production during the process.

Elements	Na^+	Mg^2+	SO4 ^2−; (as S)	K^+	Ca^2+	Cl^−
Concentration ins seawater brine [g/L]	11.34	59.5	19.17	16.03	3.5	161.97

Table 2.

The concentration of main elements present in the seawater brine [32].

The MAP crystals obtained by sweater brine used during this study were compared with struvite produced via the application of a conventional magnesium source (MgCl₂.6H₂O). Starting from a model solution of (NH₄)₂HPO₄ the precipitation was carried out at a mole ratio Mg:P = 2:1 and pH of 9.5, within 15 minutes of continuous slow mixing (50 rpm). The crystals obtained following a period of 30 minutes for struvite agglomeration, were studied under a scanning electron microscope (SEM) (Figures 5 and 6).

Figure 5.

Struvite crystals precipitated by seawater brine.

Figure 6.

Struvite crystals precipitated by MgCl₂.6H₂O.

The SEM images showed that in both cases the crystals obtained are of typical struvite morphology. Brine-induced crystals have an average size (length) of 280–300 μm, while the crystals precipitated by MgCl₂ are of size close to 200 μm. The difference can be explained by the calcites produced during the precipitation of seawater brine.

3. Influence of organic matter content

The particulate and colloidal organic matter is known to slow the kinetics of struvite formation. Even a low concentration of organic matter has an adverse, inhibiting effect on crystal growth [31, 33, 34, 35]. To this end, some promising results for the separation of the nutrients ions from the organic matter dissolved and suspended in the wastewater streams were already obtained in our recent studies [36]. Membrane separation was used as a process to improve wastewater characteristics for further struvite crystallization. The most commonly used membrane processes for organic matter separation are Microfiltration (MF) and Ultrafiltration (UF). It was found that if dead-end MF membranes are used a fast membrane permeability decrease and sharp loss of phosphates is observed (case study for filtration of municipal WWTP sludge dewatering liquid and swine). Positive results were obtained by the application of ceramic ultrafiltration membranes (Figure 7). The UF unit has been operated in a cross-flow mode at a pressure of up to 6 bars and a recirculating flow rate of 20%. The UF module was loaded with sludge water with the following characteristics: COD – 78.1 gO₂/l and TSS - 2.6 g/L; phosphorus and ammonia levels of 250 mg/L and 2.4 g/l, respectively. Two types of ceramic tube membranes, namely with pore sizes of 50 nm and 200 nm, were used [32].

Figure 7.

Membrane filtration unit [32].

The volumetric rate of the permeate stream did not change during all filtration runs. In the case of usage of a 200 nm UF module, the permeate parameters were: phosphates −195 mg/l, COD - 642 mgO2/L, TSS - 0.15 g/L, and ammonium ions - 224 mg/l. The organic matter and TSS removal rates observed were over 99% while the phosphate reduction was as low as 22% (Table 3). In the case of the 50 nm UF module, the removal efficiencies for COD and TSS were again over 99%, however, the phosphates in the filtration product were much lower (only 48% of the initial total amount). In summary, the results obtained show that the 200 nm UF module is more appropriate as it keeps a higher level of phosphorus in permeate, i.e. higher recovery and struvite yield is expected.

UF membrane (pore size, nm)	Initial COD in the sludge, mgO2/l	Residual COD in permeate, mgO2/l	COD removal level, %	Concentration of PO4 in permeate, mg/l
50	78,100	414	99.47	118
200	78,100	642	99.18	195

Table 3.

Results obtained through the application of the nano-filtration process.

4. Case studies of struvite production

The most common methods for P-removal from domestic wastewater treatment are Chemical Phosphorus Removal (CPR) and enhanced biological P removal (EBPR) [37]. Currently, the biological process for phosphate removal is more attractive as the rejected sludge liquor after digestion units have a higher phosphorus content with higher phosphorus recovery. In addition, the CPR technique has the disadvantage of an increased sludge volume (by 26% compared to EBPR) with reduced dissolved P content. However, in several cases, the less complicated approach, the CPR, is still the preferred approach, mainly due to its high efficiency in achieving the requirements for residual P in the WWTP outlet streams. Contrary, even taking into account its higher sustainability, the EBPR can rarely provide P removal efficiency higher than 60%. It is well recognized that the combination of EBPR and anaerobic sludge digestion offers a better opportunity for P extraction because, during the anaerobic stage, P is released to a high extent and makes the following struvite precipitation more efficient. The eventual struvite precipitation after the combination of chemical and biological phosphorus removal is shown herein by two case studies aiming at the WWTP of Burgas City in Bulgaria (applying both chemical and biological P removal) and the municipal WWTP in the town of Pomorie. Bulgaria which applies solely biological process for P removal. Both plants are serving settlements located on the South-East Coast of the Black Sea. The process used for both is a conventional activated sludge system with denitrification/nitrification zones (Figure 8). The conditions in this system configuration allow partial bio-dephosphatation. However, in the case of Burgas WWTP, FeCl₃ is added before the activated sludge basin (doses ranging from 1.7 to 2.4 mgFe³⁺/mgP) for the chemical precipitation of phosphates. This method has one additional drawback i.e. locking the phosphates into the insoluble form of FePO₄, which needs to be treated for re-mobilization of the phosphate ions.

Figure 8.

Principle scheme of the WWTPs under study.

The data shown in Table 4 reveals that a considerable part of phosphates remain in the returned streams and are available to be recovered.

WWTP; Name	Inlet P; [kg P/d]	Outlet P; [kg P/d]	P directed to dewatering system [kg P/d]	Solid waste [kg P/d]	Returned P load; [kg P/d]
Burgas	103	23	120	80	40
Pomorie	29	15	26	14	12

Table 4.

Phosphorus balance.

Based on the average phosphate and ammonia levels in the centrate, the amount of struvite produced in the corresponding plants has been calculated (Table 5). The struvite production estimation is based on experimental results obtained for optimal Mg/P molar ratios and pH 9 when seawater brine is used as a source of magnesium ions (according to the characteristics mentioned above in the text).

WWTP	Phosphate ions, mg/l	Ammonium ions, mg/l	Expected struvite production from*, kg per day; sewage waste
Pomorie	138 ÷ 250	250 ÷ 600	104
Burgas	86 ÷ 131	250 ÷ 500	368

Table 5.

Phosphate and ammonia levels in the centrate.

*

If the Average dewatering liquor P concentration for WWTR Pomorie and Burgas are 200 mg/l and 100 mg/l respectively.

5. Struvite as fertilizer

One of the main concerns of applying alternative fertilizer obtained by P-recovery from waste streams is related to the potential presence of pathogens, heavy metals, and also low levels of bio-utilization [38]. However, these drawbacks are case-specific and could be solved on a technological level. Usually, the heavy metal content of struvite is below the detection limit and it is suggested that is mainly linked to total organic matter in the samples and not to the struvite crystals themselves [39]. It is already known that MAP is suitable for feeding decorative plants like grass, tree seedlings, decorative plants, vegetables, flowers, and grass gardens [40]. The main question for the agriculture industry remains the efficiency of alternative fertilizers, such as struvite, when they are used to replace (or complement) commercially available products.

The chemical nature of struvite defines it as a combined fertilizer source of nitrogen, phosphorus, and magnesium which is an important microelement for the photosynthetic systems of plants [41]. Due to the limited water solubility (K_sp value of 7.59 × 10–14), it is also a natural slow-releasing fertilizer that could provide a steady supply of plant nutrients over an extended period of time. The nutrient release could be significantly influenced and regulated by the soil microbiology and physical characteristics matching the plant utilization rate [42]. Contrarily, conventional mineral fertilizers are readily soluble and could be washed out by rainfalls and irrigation increasing the risk of surface and groundwater pollution and eutrophication [43].

Recently, the author’s research group tested struvite fertilization toward maize growth which was followed for 6 months of vegetation in a test field [44]. The quantity and quality of the crops harvested were compared with control samples obtained by cultivation with conventional fertilizers. The evaluation was performed based on the yield and nutritional characteristics of the corn. According to the results obtained, the struvite is a very efficient complement to the nitrogen fertilizers (such as ammonium nitrate and carbamide) as an alternative to the normal superphosphate and triple superphosphate in the role of P source. Considering overall P availability, different struvite samples show patterns with a continuous soil release of phosphorus without peak concentrations followed by fast decline which is typical for super phosphates or rock phosphate for instance [45] (Tables 6 and 7).

Fertilizer	Yield, kg/ha	M1000, g
Control sample	51.43	238.92
Ammonium nitrate	55.87	246.40
Carbamide + Ammonium nitrate	56.64	249.20
Struvite	54.60	245.27
Struvite + Ammonium nitrate	55.11	246.36

Table 6.

Productivity of maize applying different fertilizers (M₁₀₀₀ – The absolute weight of 1000 grains).

Fertilizer	Wet, %	Proteins, %	Fats, %	Starch, %
Control sample	14.1	8.8	3.4	58.9
Ammonium nitrate	14.2	9.5	3.3	59.0
Carbamide + Ammonium nitrate	14.1	9.5	3.2	58.5
Struvite	14.0	8.8	3.2	58.2
Struvite + ammonium nitrate	14.1	9.7	3.2	57.5

Table 7.

Qualitative characteristics of maize while using different fertilization options.

Acknowledgments

The authors wish to heartily acknowledge the support under the BG05M2OP001-1.002-0019 Clean technologies for sustainable environment – waters, waste, energy for the circular economy“(Clean&Circle) Project financed by the Operational program “Science and Education for Smart Growth” 2014-2020, co-financed by the European Union through the European structural and investment funds.