A Review of the Efficiency of Phosphorus Removal and Recovery from Wastewater by Physicochemical and Biological Processes: Challenges and Opportunities

Abstract

Phosphorus (P) discharge from anthropogenic sources, notably sewage effluent and agricultural runoff, significantly contributes to eutrophication in aquatic environments. Stringent regulations have heightened the need for effective P removal technologies in wastewater treatment processes. This paper provides a comprehensive review of current P removal methods, focusing on both biological and chemical approaches. Biological treatments discussed include enhanced biological P removal in activated sludge systems, biological trickling filters, biofilm reactors, and constructed wetlands. The efficiency of microbial absorption and novel biotechnological integrations, such as the use of microalgae and fungi, are also examined. Chemical treatments reviewed encompass the application of metal salts, advanced oxidation processes such as chlorination, ozonation, and the Fenton reaction, as well as emerging techniques including the Electro-Fenton process and photocatalysis. Analytical methods for P, including spectrophotometric techniques and fractionation analyses, are evaluated to understand the dynamics of P in wastewater. This review critically assesses the strengths and limitations of each method, aiming to identify the most effective and sustainable solutions for P management in wastewater treatment. The integration of innovative strategies and advanced technologies is emphasized as crucial for optimizing P removal and ensuring compliance with environmental regulations.

1. Introduction

The discharge of phosphorus (P) as a result of human activities encourages eutrophication in aquatic environments. According to Boer et al. [1], the primary sources of P entering rivers in the UK are sewage effluent and agricultural run-off, with sewage discharges accounting for up to 70% of the total. Due to this fact, P discharge rules have been tightened, and the water sector is under more pressure to minimize P loads entering rivers, especially in environmentally vulnerable areas [2]. As a result, wastewater treatment processes (WWTPs) focused on P-removal have become more prevalent (Figure 1).

Chemical treatments, including the use of metal salts [4] and advanced oxidation processes (AOPs) such as chlorination [5], ozonation [6], and the Fenton reaction [7], provide effective means for P removal. However, these methods often require additional stages to manage byproducts and maintain water quality [8]. Emerging techniques such as membrane bioreactors [9] and integrated biological-chemical processes enhance the effectiveness of P removal while addressing environmental and operational challenges [10].

Biological P removal is implemented by introducing anaerobic or anoxic zones ahead of aerobic stages in the activated sludge system. In these zones, bacteria such as Acinetobacter uptake volatile fatty acids (VFAs) and release P into the solution in the absence of oxygen and nitrates. In the subsequent aerobic stage, these bacteria luxuriously uptake P, achieving removal rates of 80–90%. Recent advancements in biological P removal include the development of enhanced biological P removal (EBPR) techniques, such as biological trickling filters and biofilm reactors, which utilize support particles and biofilm surfaces for pollutant absorption [11]. Additionally, biological nitrification and denitrification processes, such as the Sorption-Denitrification-P Removal (SDPR) approach, optimize biofilm functionality to achieve extensive denitrification and increased biological P removal [12]. The use of Biologically Activated Carbon (BAC) merges activated carbon technology with biodegradation to purify water, providing efficient P removal [13]. Moreover, microalgae and fungi-based treatments offer promising avenues for industrial-scale applications, enhancing nutrient removal efficiency [14]. Constructed wetlands have also proven effective in P removal, employing mechanisms such as sorption, precipitation, plant uptake, and soil accretion [15]. The substrate used in these systems plays a critical role, with materials such as laterite and natural pyrite achieving high P removal efficiency. Integration of innovative strategies, such as intermittent aeration and construction waste substrates, further enhances the performance of constructed wetlands in P removal [16].

This paper aims to review the current technologies for P removal from wastewater, focusing on both biological and chemical methods. These technologies are critical for maintaining water quality and protecting aquatic ecosystems from the adverse effects of P pollution. By examining the strengths and limitations of each method, this study seeks to identify the most effective and sustainable solutions for P management in wastewater treatment processes.

2. Different Methods of P Detection and Removal

2.1. Analytical Techniques for P Detection

The ability to distinguish between the types of P present in a water sample is very important for understanding and predicting the fate of P in fresh waters and associated sediments. Several scientific protocols have already indicated that all water samples should be filtered through a 0.45 or 0.2 µm cellulose acetate membrane immediately after sampling. This procedure eliminates the solids but fails to represent the difference between the dissolved and particulate P components. Moreover, the freshwater P fractions associated with particles may be examined across a wide range of particle sizes as well [17]. As a result, P fractions might be divided into four categories: dissolved (0.001 µm), colloidal (0.001–1 µm), supra-colloidal (1–100 µm), and settleable (>100 µm). For instance, Jarvie et al. [18] observed that 87% of the organic P present was in a colloidal state, which is defined as >1 kDa and 0.45 µm, in a river in the UK that had been impacted by industrial discharges. Overall, both samples (the unfiltered and the dissolved subsample) can be subjected to P-fraction analysis. The difference between the concentrations of the complete and dissolved samples or the examination of the suspended material on the filter, as detailed below, can be used to determine P associated with particulates or suspended materials. It may be important to distinguish between organic and inorganic forms of P in addition to dissolved and suspended or particulate forms. In this section, a review is given of the techniques used to draw such differences. In Standard Methods for the Examination of Water and Wastewater [19], these analyses are covered in further depth. According to Worsfold et al. [20], the majority of P measurement techniques rely on the spectrophotometric detection of a colored phosphomolybdate complex. For instance, the colorimetric malachite green approach assays P by creating a complex of the aromatic amine malachite green with P and molybdate [21]. The most commonly used colorimetric technique for calculating P is the molybdate-blue approach [22]. The latter technique involves reducing molybdenum within the complex using diverse reductants, such as ascorbic acid, facilitated by antimony as a catalyst, resulting in the formation of a blue P-molybdate complex [23]. Using a spectrophotometer, the absorbance is determined at 880 nm. Although this method shows insensitivity to temperature variation during color development, its implementations can be difficult and time-consuming [20]. Additionally, because certain reagents are not stable at ambient temperature for longer than 24 h, they may lead to a considerable amount of chemical waste [24].

2.2. Removal of Different P Forms

Effective P management requires not only the identification and quantification of different P forms in wastewater but also the application of removal techniques tailored to each specific form. The following subsections provide a categorized discussion of the most effective removal methods for orthophosphate, polyphosphate, and organic P.

2.2.1. Orthophosphate Removal

Orthophosphate is the most reactive and readily available form of P in wastewater, commonly originating from human and animal waste, agricultural runoff, and household detergents. Its presence in wastewater can significantly contribute to eutrophication in receiving water bodies, leading to harmful algal blooms when concentrations exceed 0.1–0.5 mg L⁻¹. Given its environmental impact, the removal of orthophosphate is a critical objective in wastewater treatment processes. Orthophosphate is the most straightforward form of P to remove due to its high reactivity. Chemical precipitation is the primary method for its removal, involving the addition of metal salts such as aluminum or iron to form insoluble phosphate compounds that can be separated from the water. Biological methods, such as those used in EBPR processes, can also directly uptake orthophosphate, making it available for removal during the wastewater treatment process [8].

2.2.2. Polyphosphate Removal

Polyphosphates are more complex, being stored in the cells of microorganisms, particularly P-accumulating organisms (PAOs). The most effective removal strategy involves EBPR, where PAOs release polyphosphates under anaerobic conditions and then uptake them in aerobic conditions, leading to significant P reduction in the treated effluent. Chemical treatments can also assist in breaking down polyphosphates into orthophosphates, which are then more easily removed through precipitation [7,25].

2.2.3. Organic P Removal

Organic P is typically part of larger organic molecules, making it more challenging to remove. AOPs such as ozonation and the Fenton reaction are effective in breaking down these complex molecules, freeing P for removal. Biological processes, such as those utilized in constructed wetlands, also play a role in the degradation of organic P, relying on microbial activity to break down the organic compounds and facilitate P removal [26,27].

3. Removal Efficiency of P by WWTPs

When evaluating various new technologies for delivering P removal in small-scale operations, multiple crucial aspects must be considered. To fulfill the current discharge limits of 1–2 mg L⁻¹ P, removing up to 90% of the incoming P load is imperative. Cornel and Schaum [8] reported that biological processes and solids settling may remove between 10% and 30% of the P in conventional WWTPs. Therefore, assuming a daily contribution of 2 mg of P per person, an additional 50% removal of P is needed [28]. However, the majority of innovative P-removal techniques fail short in meeting these stringent demands. A small-scale system is also probably going to be situated far away from any significant civil infrastructure. Therefore, any technology used in rural and distant areas must only require little maintenance to maintain operating performance. This can only become a reality if the system is easy to build and operate and stabilizes quickly. A possible option on these grounds is the use of adsorptive filter media in the context of a created wetland or as a standalone system. These possibilities could be constrained, though, by high capital costs, space limitations, and long-term viability. Additionally, it has not been demonstrated that P-removal employing active media is reliable for long periods in large-scale operations. Repeated wetland restoration might not be practical due to spatial limitations in smaller locations. As a result, approaches using adsorptive media for P removal may be preferable in certain situations. Thorough investigation is required to ascertain the long-term efficacy of such systems for small-scale treatment applications and the performance of absorptive medium under fast changes in flow and nutrient loadings to make such systems an alternative. Sludge handling causes another critical consideration for small-scale systems. Therefore, to cut down on storage and transportation expenses, technologies with reduced sludge generation are needed. While increasing the sludge production in larger treatment plants aids energy generation through anaerobic digestion, it might not be cost-effective to transport small amounts of sludge to central collection points. In a similar vein, further research is required to address sludge-related difficulties, including modeling various scenarios to ascertain whether increasing or decreasing sludge output is desirable. The development of systems that take advantage of microbial absorption may be acceptable if sludge generation is needed. Literature demonstrates that even in small-scale treatment applications, a strong and stable microbial population is necessary for successful biological P elimination. According to Belivermiş et al. [29], microbial biofilm systems are also probably more robust than floating cultures.

4. P Retention Process

P control has long been achieved via physicochemical methods of P elimination. Such procedures have several limitations but are often trustworthy and efficient. These methods sometimes need additional processing stages due to the production of certain byproducts during treatment. These byproducts can impact effluent pH, requiring chemical adjustments before discharge [8]. The most common chemical P removal method is introducing metal salts into pretreated influencers, conventional activated sludge (CAS) reactors, or secondary clarification effluents [4]. Improvements to P removal in filter systems with active media have received attention recently [30]. Reactive media filters, as opposed to conventional filtration methods, rely on the P-sorption capabilities of certain materials to remove P from wastewater in a targeted manner rather than just attaching biomass using filter media [31]. The materials used to create adsorbent media can be either natural (such as apatite, bauxite, or limestone), industrial waste (such as fly ash, ochre, or steel slag), or man-made (such as filtrated). There are several commercially available products, but Polonite stands out as one of the most extensively researched options (

Table 1. Examples of cutting-edge P-removal technologies and their respective performance at various scales.

Technology	Explanation	Rates of TP Deletion	Developmental Stage	References
Algal systems
Algal biofilm reactors	Fixed-growth algal bioreactors	41–97%	Bench-scale	[32,33]
		Not reported	Full-scale	[34]
Algae immobilized	Algal species are into sheets or beads and immobilized	62–90%	Bench-scale	[35,36]
Suspended growth photo-bioreactors	Suspended growth algal bioreactors	61%	Pilot-scale	[37]
Membrane photo biofilm reactors	Membrane bioreactors with algae seed; operating promote phototrophic growth	66–97%	Bench-scale	[38,39]
Osmotic MPBR	Photo-bioreactor with osmotic membrane	90–100%	Bench-scale	[40,41]
EBPR systems
MBR-UCT	Integrating a membrane bioreactor into an EBPR with continuous flow	Up to 88%	Up to 88%	[42,43]
Sequencing batch moving bed membrane bioreactor	Moving-bed Carriers Sequencing batch reactor with integrated membrane	84%	Bench-scale	[44]
MB-SBBR	Batch biofilm reactor with moving bed sequencing	97%	Bench-scale	[45]
SBBR	Sequencing batch reactor with fixed biofilm	90%	Bench-scale	[46]
		70–90%	Pilot-scale	[47]
Granular sludge	SBR advanced activated sludge process	87%	Full-scale	[48]
MABR-SBR hybrid	Membrane-aerated biofilm reactor operating in combination with a sequencing batch reactor	90%	Bench-scale	[49]
AnoxAn	Vertical flow anaerobic-anoxic reactor	89%	Bench-scale	[50]
Physico-chemical systems
Active filter media	Materials that remove P by precipitation or absorption, whether naturally occurring or man-made.	95% (PO4)	Bench-scale	[51,52]
		77–91%	Full-scale (other)	[53]
The exchange of ions		80–90%	Bench-scale	[54,55]

).

4.1. Removal of P by Biological Treatment Technologies

In recent years, there has been extensive research on biological P removal, particularly EBPR utilizing activated sludge systems. Chemical treatment is thought to be unnecessary, and this method is ecologically beneficial [56]. However, EBPR is also seen as being somewhat unreliable due to variable performance and a significant reliance on competent operators, which makes process management challenging [57]. This might make it less appropriate for use in decentralized treatment facilities when scaled down [58], but given its expanding variety of uses and ongoing improvements in efficiency and dependability, it merits consideration for small-scale deployment. Additionally, the main advantage of EBPR systems is in situations where space is at a premium and multifunctional solutions are preferred.

The molecular processes of luxury P-uptake, reliant on PAOs for EBPR, are becoming more understood. Modifications in operating circumstances, particularly related to metabolism prerequisites such as carbon, glycogen, and electron acceptor needs, aim to encourage the growth and proliferation of PAOs, even though this is not yet completely understood. Smaller treatment facilities are characterized by lower flow rates and varying organic and nutrient loads. Here is a succinct explanation of what is now known about EBPR biochemistry and microbiology, which must be taken into account when this therapeutic method is scaled down.

EBPR relies on the unique metabolism of PAOs, which are capable of luxury P uptake. During the anaerobic phase, PAOs absorb VFAs from wastewater and store them as polyhydroxyalkanoates (PHAs), utilizing energy from the hydrolysis of intracellular polyphosphate to orthophosphate. This process results in the release of P into wastewater [59]. In the subsequent aerobic phase, PAOs use the stored PHAs as an energy source to take up P from the wastewater and store it again as intracellular polyphosphate. This cyclical process, facilitated by alternating anaerobic and aerobic conditions, is essential for the efficient removal of P.

To maximize the effectiveness of EBPR, specific operational conditions need to be optimized. Anaerobic conditions are crucial for the uptake of VFAs by PAOs and the release of P [60]. Maintaining sufficient anaerobic retention time and ensuring low nitrate levels are essential, as nitrates can inhibit P release by PAOs. Adequate oxygen levels in the aerobic phase are needed to support the oxidation of PHAs and P uptake, with a dissolved oxygen concentration of around 2 mg L⁻¹ typically recommended. Additionally, a high ratio of VFAs to P (typically above 10:1) is essential to favor PAO activity over glycogen-accumulating organisms (GAOs), which do not contribute to P removal [61].

Other important factors include pH and temperature, where optimal conditions for PAOs include a pH range of 7 to 8 and temperatures between 20 °C and 30 °C, which support enzymatic activities critical for PAOs metabolism. Hydraulic Retention Time and Solids Retention Time (SRT) are also critical, with an SRT of 10-20 days recommended to maintain a stable PAO population and balance the growth rates of PAOs and other microbial populations. Strategies such as optimizing anaerobic conditions, managing carbon to P ratios, and adjusting SRT can help suppress the activity of GAOs, which compete with PAOs but do not aid in P removal [62].

4.1.1. Biological Trickling Filter and Biofilm Reactor

Biological trickling filters constitute a filtration methodology employing a layer of support particles through which wastewater flows. This system utilizes either stationary or rotating distributors to evenly disperse water across the support layer, mitigating the formation of channels and dry pockets. Such a design ensures comprehensive utilization of biofilm surfaces for pollutant absorption. The support structure serves as a medium for biofilm growth, which is pivotal in capturing pollutants from wastewater and facilitating the removal of organic substances and nutrients as the water traverses through the biofilms. Subsequently, the lower part of the collected water is directed for further treatment processes (Figure 2).

Biofiltration, aimed at purifying industrial waste gases, involves the use of microorganisms to convert pollutants into less harmful compounds. These compounds include carbon dioxide, water, and minerals. This transformation occurs as waste gases percolate through filters or granular materials layered with immobilized microorganisms. P elimination is achieved via sorption or direct precipitation, a process contingent on the filter medium’s mineral composition [11]. Initial research focused on the use of local sands and gravels for P removal through sorptive mediums [64], while recent advancements have expanded to include a variety of organic or synthetic materials, enhancing the feasibility of this technology on a smaller scale [65]. The development and implementation of biofilm systems, especially those tailored to algal treatment, underscore the potential for process optimization to achieve superior P removal rates with minimal energy and chemical input. Despite existing knowledge gaps, commercially viable algal biofilm technologies, such as Clearas Advanced Biological Nutrient Recovery, have emerged, demonstrating the efficacy of these systems in urban wastewater nutrient removal.

Recent studies have shed light on the capabilities of moving bed biofilm reactors for achieving simultaneous partial nitrification and denitrification alongside biological P removal under specific conditions, reporting removal efficiencies between 83–86% for P-PO₄³⁻ when utilizing acetate and ethanol as feed [66]. Similarly, sequencing batch membrane-aerated biofilm reactors have shown promise in enhancing biological nitrogen (N) and P removal, maintaining total P (TP) removal efficiencies above 85% over an operational period of 112 days through the integration of intermittent aeration and biofilm shedding [49]. Sequencing batch biofilm reactors (SBBR), governed by intelligent control systems, have demonstrated comparable efficiencies, highlighting the potential for energy conservation without compromising P removal efficiency [67]. Furthermore, the application of sequencing batch moving bed biofilm reactors for P removal in municipal wastewater has proven effective, with removal efficiencies reaching up to 81% under moderate temperatures and with the addition of VFAs [68].

Innovative approaches for P recovery from sewage treatment, such as the use of PAO-enriched biofilm reactors, have been proposed, showing the ability to recover phosphate in concentrated solutions without generating excess sludge. The reactor achieved phosphate concentrations of >100 mg P L⁻¹ [69]. Moreover, laboratory-scale fixed bed biofilm reactors employing sequencing batch reactor (SBR) strategies have successfully demonstrated the process’s efficacy in biological phosphate removal [70].

4.1.2. Biological Nitrification and Denitrification

Nitrite plays a crucial role in both nitrification and denitrification, serving as a substrate that is directly reduced to nitric oxide (NO) and subsequently to N gas, thereby bypassing the conversion of nitrite to nitrate. The anammox process, performed by autotrophic anaerobic bacteria, is recognized for its efficiency and energy savings over traditional N removal methods. It uses nitrite as an electron acceptor and ammonium as an electron donor in a unique form of autotrophic denitrification [71,72].

While nitrification and denitrification are typically conducted in separate stages in both heterotrophic and autotrophic systems, these processes can occur concurrently within a single biofilm layer. This co-occurrence is often enhanced by intermittent aeration, aimed at facilitating either partial nitrification or the anammox process within the biofilm. According to Ruiz et al. [73] and Metcalf et al. [71], organic matter is the reducing agent, here indicated as C₁₀H₁₉O₃N, which leads to the overall generalized equation.

The SDPR approach is a biofilm-based method aimed at extensive denitrification and increased biological P removal. It maintains anaerobic and anoxic conditions throughout the denitrification phase to optimize biofilm functionality, functioning such as a modified SBR. This process involves a preliminary sedimentation phase followed by denitrification [12]. Denitrifying organisms develop through a sequence of anaerobic/anoxic conditions, with the biomass alternating between exposure to primarily treated and nitrified wastewater. During the sorption phase, biodegradable substrates are absorbed and stored by PAOs, promoting P removal.

Unlike traditional methods, this process utilizes nitrates instead of oxygen for respiration during the denitrification phase, merging aerobic and anoxic stages into a single, anoxic phase. This integration allows for the efficient removal of P from wastewater, involving the stored degradable substances from the anaerobic phase in both denitrification and P removal activities [71].

Li et al. [74] examined a SBR operating at low temperatures, achieving notable success in simultaneous nitrification, denitrification, and P removal (SNDPR), with removal efficiencies of 89.6% for total N (TN) and 97.5% for TP. This study underscored the importance of denitrifying PAOs, particularly PAOII, which uses nitrite for P uptake, enhancing the SNDPR process’s efficiency at low temperatures. Further research by Du et al. [75] explored deep-level nutrient removal in a single-sludge SBR, coupling denitrifying P removal (DPR) with simultaneous partial nitrification-endogenous denitrification (SPNED), showing significant pollutant reductions and identifying key microbial contributors to the process. This process achieved significant reductions in PO₄³⁻-P and NO₃⁻-N anoxically, with TN and PO₄³⁻-P removal efficiency of 90.8% and 97.5%, respectively. The microbial analysis revealed the dominance of Dechloromonas in the DPR process and the enrichment of Nitrosomonas and Candidatus Competibacter in the SPNED process.

Additionally, research on a lab-scale SBR highlighted the feasibility of integrating simultaneous nitrification and denitrification (SND) through the nitrite pathway with EBPR, achieving comprehensive nutrient removal with minimal chemical oxygen demand (COD) requirements. This system operated in an alternating anaerobic-aerobic mode, effectively reducing P levels to below 0.5 mg L⁻¹ by the cycle’s end [76]. Zeng et al. [77] also investigated the impact of nitrite accumulation on P removal in an A₂O process, noting that a pre-anoxic zone significantly improved TN and TP removal efficiencies. The results indicated that the configuration of a pre-anoxic zone significantly increased TN and P removal efficiencies to 75% and 98%, respectively. However, increased nitrite levels inhibited P uptake, a challenge overcome by adding extra carbon sources to boost denitrification and P release, thus enhancing overall removal efficiency.

4.1.3. Biologically Activated Carbon

The BAC process merges the principles of activated carbon technology with biodegradation to purify water. Leveraging the large surface area and intricate pore structure of activated carbon, this technology excels at adsorbing dissolved organics and oxygen from raw water. Activated carbon serves as a scaffold, supporting the growth and reproduction of microorganisms under optimal conditions of temperature and nutrition. These microorganisms form a biofilm on the activated carbon, creating a dual-functioning system capable of both adsorption and biodegradation [13].

This technology relies on the interaction between dissolved oxygen (DO), pollutants, microorganisms, and activated carbon particles within a water solution [63]. Figure 3 describes a model illustrating how these four components interact, highlighting that adsorption by activated carbon is a key initial step influenced by the nature of both the carbon and the pollutants. Furthermore, activated carbon absorbs DO, which is then utilized by the adherent microorganisms to break down pollutants. This synergy allows for the efficient removal of contaminants from raw water through a combination of physical and biological processes [78].

Research by Tong and Chen [80] has investigated optimal carbon sources to enhance the EBPR process. Studies have found that short-chain fatty acids (SCFAs), derived from the alkaline fermentation of waste activated sludge, are effective carbon sources for EBPR, achieving up to 98% P removal efficiency in SBRs. This performance significantly surpasses that obtained using acetic acid, where only 71% efficiency was noted. The superior results with SCFAs are linked to their reduced usage for glycogen synthesis and higher efficiency in utilizing polyhydroxyalkanoates (PHAs) for the uptake of soluble ortho-P (SOP).

Furthermore, Granular Activated Carbon (GAC) has been used in advanced wastewater treatment as a filtration medium. When used alongside coagulation, GAC can remove organic micropollutants and P, reducing TP concentrations to as low as 0.1 mg L⁻¹. This combined approach presents a space- and energy-saving alternative to traditional wastewater treatment methods [81].

Innovations in EBPR have led to processes operating at shorter Sludge Retention Times (SRTs), enhancing energy efficiency. A novel high-rate Bio-P removal process, operating with an SRT of less than four days, has achieved over 90% phosphate removal efficiency. The Comamonadaceae family has been identified as a critical group of polyphosphate-accumulating organisms under these conditions [82]. Additionally, using waste activated sludge as the sole carbon source for EBPR has been explored, demonstrating not only effective P removal, dominated by Tetrasphaera, but also a reduction in sludge volume by approximately 44.14% [83].

4.1.4. Microalgae/Fungi-Based Treatment

Microalgae cultivation and application in industrial-scale operations, including wastewater treatment, have garnered significant attention. Microalgae offer the dual benefits of generating valuable biomass while removing excess nutrients from wastewater, contributing to the mitigation of eutrophication in natural water bodies [14]. Studies, such as those by Tang et al. [84], have shown that incorporating microalgae into wastewater treatment can enhance N and P removal efficiencies significantly, by 39 to 66% and 32 to 89%, respectively.

Despite these advantages, the cost of biomass harvesting, which may constitute up to 30% of total production expenses due to the small size (5–30 μm) and colloidal stability of microalgae cells, presents a substantial challenge [85]. An innovative solution involves bio-flocculation with filamentous white-rot fungi such as Irpex lacteus, Trametes versicolor, Pleurotus ostreatus, and soil fungi such as Trichoderma reesei, which have demonstrated potential for degrading pharmaceuticals in wastewater and exhibit high enzymatic activity. This approach suggests a promising avenue for harvesting microalgae from wastewater and producing lignocellulose-degrading enzymes for bioethanol production [86,87,88].

Microalgal biofilms, specifically, have shown remarkable P removal capabilities, achieving target effluent P concentrations of 0.15 mg L⁻¹, thus proving effective in nutrient management [89]. Co-immobilization techniques pairing microalgae with microalgae growth-promoting bacteria have further enhanced nutrient removal from municipal wastewater. For instance, co-immobilizing Chlorella species with Azospirillum brasilense significantly improved P removal rates (up to 36% P removal within 6 days) compared to microalgae alone [90]. Additionally, microalgal biofilms in novel photobioreactors have achieved up to 97% (during 24 h) total P removal under continuous artificial illumination [32].

Explorations into microalgal-bacterial granular sludge processes have also been promising, with one study showing that about 86% of influent P could be removed within 6 h, including a 2-h dark phase and a 4-h light phase. Efficient P removal is facilitated by specific microalgal genera such as Pantanalinema, which accumulate poly-P within their cells [91]. Furthermore, research has identified certain microalgae strains, such as Botryococcus braunii, that were able to remove 79.63% of N and P from treated domestic wastewater while accumulating significant amounts of lipids [92]. The impact of light wavelengths on microalgae growth and nutrient removal has also been investigated, revealing that specific light mixing ratios can significantly enhance both biomass production rates and P removal efficiency (45% and 90%, respectively) [93]. A six-species screen identified two dual-purpose microalgae candidates, Monoraphidium minutum and Tetraselmis suecica, which showed high rates of P removal and growth, making them suitable for nutrient removal and biomass production schemes [94].

In the context of EBPR in Danish wastewater treatment plants, studies have highlighted the prevalence and stability of two PAO genera, Accumulibacter and Tetrasphaera, underscoring their critical role in the EBPR process [95]. The co-immobilization of Chlorella vulgaris with Azospirillum brasilense in alginate beads presents a novel method for wastewater treatment, showing increased efficiency in removing soluble P ions [96]. Lastly, lab-scale studies on continuous microalgal cultures grown on sterile-filtered wastewater have elucidated that P removal mechanisms include both assimilation by algal biomass and biologically mediated chemical precipitation, facilitated by photosynthesizing algae [97].

4.1.5. Activated Sludge Process

The Activated Sludge Process (ASP) stands as a cornerstone in biological wastewater treatment, widely recognized for its adaptability to implementing EBPR to combat eutrophication in aquatic ecosystems. This method hinges on the intricate dynamics between microbial communities and operational conditions. Despite its efficacy, the process is sometimes challenged by fluctuations in performance.

Recent literature has extensively documented the biological extraction of phosphates within ASP [98]. Research indicates that biological absorption is the principal method of P removal in activated sludge, complemented by calcium phosphate precipitation, which accounts for a minor portion (15–27%) of total P elimination [99]. The EBPR process, particularly within SBRs that facilitate aerobic granule formation, showcases remarkable P removal efficiencies, occasionally surpassing 95% [100]. Studies by Benammar et al. [101] have highlighted the role of specific bacterial strains, such as Pseudomonas aeruginosa and Acinetobacter junii, in boosting phosphate uptake, with certain cultures achieving up to 83.36% P removal.

In summary, the ASP is a robust solution for biological P elimination, often achieving removal efficiencies above 95%. The effectiveness of ASP is influenced by various factors, including bacterial populations, organic matter availability, and operational parameters. Through advanced monitoring and control, along with the potential for polyphosphate recovery and recycling, ASP presents a sustainable approach to wastewater management [102,103].

4.1.6. Biosorption

It is reported that using biosorption techniques, cost-effective, efficient, and effective biomaterials were created for wastewater treatment [104]. Biosorption has earned high hopes in academia, research, and industry because of its intriguing characteristics. It was thought that harmful contaminants might be selectively removed from aqueous solutions to desired low levels by employing this innovative technique, which uses biomass as a sorbent. The biosorption idea has gained the utmost relevance in several industries since biomass exhibits a wide range of desirable qualities. Over the past several decades, significant progress has been made in our understanding of the intricate biosorption mechanism, techniques for quantifying it (equilibrium and kinetics), and our ability to identify the variables that affect the efficiency and speed of the process. Additionally, the pilot and industrial-scale implementation of this technology was examined. The removal or binding of desirable chemicals from an aqueous solution by biological material is known as biosorption. Such compounds come in both liquid and solid forms and can be either organic or inorganic [105]. In the literature, the sorptive characteristics of a variety of natural biomasses are frequently investigated for wastewater treatment, particularly when the pollutant concentration is less than 100 mg L⁻¹ and alternative treatment options are inefficient or prohibitively expensive [106]. A process known as “biosorption” involves the quick and irreversible attachment of ions from aqueous solutions to functional groups found on the surface of biomass. According to Davis et al. [107], this mechanism is not dependent on cellular metabolism. The literature describes biosorption as an effective and discerning mechanism. The pH range for biosorption is 3 to 9, and the temperature range is 4 to 90 °C. As long as the biosorbent particle size is between 1 and 2 mm, both the adsorption and desorption equilibrium states may be reached fairly quickly. The functional costs are reasonable since this method does not need a huge capital investment. Additionally, the biological elements are frequently cheap and may be found in industrial waste or in agriculture [108].

Research has explored various biosorbents and biological processes for P removal from wastewater. For example, Genz et al. [109] investigated the use of granulated ferric hydroxide (GFH) and activated aluminum oxide in removing P from membrane bioreactor (MBR) effluents. GFH showed superior phosphate adsorption capacity and affinity at low P concentrations, achieving effluent criteria of 50 µg P L⁻¹, with potential for adsorbent regeneration using sodium hydroxide.

Magnesium oxide nanoparticle-containing biochar composites (MgO-biochar) have been synthesized, displaying significant P adsorption capabilities attributed to their external surface area, with a maximum capacity of 60.95 mg P g⁻¹ [110]. Zirconium-loaded okara (ZLO) has also proven effective for P capture from aqueous solutions, with a maximum adsorption capacity of approximately 44.13 mg PO₄ g⁻¹, demonstrating rapid, feasible, spontaneous, and endothermic removal processes [111].

Furthermore, dolomite-modified biochar derived from urban dewatered sewage sludge has shown a high phosphate removal efficiency of 96.8%, with the adsorption process being endothermic and well-described by the Langmuir model, indicating electrostatic attraction and the formation of new surface complexes as mechanisms for phosphate removal [112].

4.1.7. Membrane Bioreactor

MBRs represent a sophisticated approach to wastewater treatment, merging biological degradation processes with membrane filtration to treat wastewater efficiently. In MBR systems, biomass degradation occurs within a bioreactor tank, followed by the separation of treated wastewater from microorganisms through a membrane bioreactor. This integration has led to the production of high-quality effluent, positioning MBR as a leading technology in wastewater treatment over the past two decades. The MBR market has witnessed significant growth, with predictions indicating a compound annual growth rate of 22.4% due to global population increase and the consequent demand for advanced water treatment solutions [113,114].

MBR technology efficiently removes both organic and inorganic pollutants, making it suitable for municipal and industrial applications. However, achieving EBPR in MBRs poses challenges, particularly under conditions of low wastewater concentrations and extended treatment times. MBRs’ unique features, such as membrane-based solid-liquid separation and high mixed liquor volatile suspended solids (MLVSS) concentrations, significantly influence sludge properties and system operation, differing from CAS processes [115].

P removal in MBR systems can be optimized in environments conducive to PAOs. These systems, capable of retaining PAOs within the bioreactor due to their size and compatibility with microfiltration membranes, can lead to more effective P removal [9]. Despite the challenges, several innovative approaches have been developed to enhance nutrient removal efficiency in MBR systems:

Sequencing Batch Moving Bed Membrane Bioreactor: This approach has demonstrated significant improvements in removing carbon, N, and P, with an average TP removal efficiency of 84.1% under specific operational conditions [44].

Hybrid Microfiltration-Forward Osmosis Membrane Bioreactor: Utilizing seawater brine as a draw solution, achieve direct P recovery from municipal wastewater. This process demonstrated a high rejection rate (97.9%) of phosphate P and offered an overall P recovery efficiency. More than 90% P recovery was achieved at pH 9.0, resulting in an overall P recovery of 71.7% over 98 days [116].

Algae-Based MBRs: These systems leverage algae-induced phosphate precipitation for chemical P removal, producing P-rich algal biomass. Algae-facilitated chemical P removal has also been explored in high-density Chlorella emersonii cultivation within an A-MBR. The A-MBR system was capable of removing 66 ± 9% of the total P from the water, with the algal biomass containing significant amounts of extracellular and intracellular P, suggesting that algae-induced phosphate precipitation is key to P removal [38].

Anoxic/Oxic-Membrane Bioreactor: Capable of simultaneous nitrification and denitrification alongside P removal, this system highlights the role of PAOs in achieving high P removal efficiencies (with average removal efficiencies of 94.6% for COD and 90.0% for PO₄³⁻P). The presence of PAOs in the reactor contributed to the high P removal efficiency [117].

Membrane bioreactors offer a promising solution for efficient P removal from wastewater, supported by various biological processes and innovative configurations. By controlling key environmental conditions and integrating advanced technologies, MBRs not only ensure high-quality effluent but also open avenues for resource recovery, making them a viable option for modern wastewater treatment strategies.

4.1.8. Constructed Wetland

Constructed wetland systems originally went into operation in the late 1960s, and ever since then they have proliferated all over the world. The treatment of wastewater now uses a variety of constructed wetlands. Although vertical flow systems that are intermittently supplied have quickly developed and expanded due to the rising need for ammonia removal, subsurface flow systems are often constructed with horizontal flow. As a result, the bed might be more oxygenated, which increases nitrification. Although treating many different types of industrial and agricultural wastewater, stormwater runoff, and landfill leachate has lately become more widespread, treating municipal and household wastewater is still the main application of most man-made wetlands across the world. Constructed wetlands have been extensively recognized across the world and have developed into an effective option for wastewater treatment, despite the skepticism of many civil engineers and water authorities. It was not surprising that the first full-scale constructed wetlands were built outside of Germany because Seidel’s concept to apply macrophytes to sewage treatment was challenging for sewage engineers who had eradicated any visible plants on a treatment site for more than 50 years [15].

P removal in CWs is achieved through mechanisms such as sorption, precipitation, plant uptake, and soil accretion, with efficiencies ranging between 40 and 60% [118]. The substrate used plays a vital role in the removal process. For instance, laterite, a material rich in iron and aluminum, has shown potential in achieving up to 99% P removal in both laboratory tests and pilot-scale wetlands. Similarly, natural pyrite used as a substrate has enhanced long-term P and TN removal without hindering plant growth or other removal processes, achieving average removals of 87.7% for TP and 69.4% for TN [119,120].

Innovative strategies have been developed to optimize P removal in CWs. A three-stage pilot-scale system employing Myriophyllum aquaticum demonstrated significant TP removal efficiencies (89.8%) from swine wastewater, highlighting the role of harvested biomass in the process [121]. Additionally, the integration of intermittent aeration and construction waste substrates into vertical flow CWs has improved P removal. Fly-ash bricks, in particular, enhanced TP removal efficiency, and aeration promoted plant growth, increasing P uptake [16]. The potential of Brachiaria-based CWs for the removal of P and N was investigated, showing that the removal efficiency of Brachiaria increased with influent phosphate concentration, with average removal of total phosphate ranging from 55.2% to 85.6% across different seasons [122].

The selection of vegetation in CWs also influences P removal efficiency. Studies have identified Typha latifolia as an exceptionally effective species for PO₄-P removal. Furthermore, a novel simultaneous N and P removal (SNPR) process has been developed, combining nitrification, endogenous denitrification, and denitrifying P removal, achieving high levels of N (83.73%) and P removal efficiency (87.84%) [123,124].

The dynamics of P removal in a constructed treatment wetland treating agricultural irrigation return flows were evaluated by Betul et al. [125], with concentration removal efficiency averaging 21% for TP in the sedimentation basin and 37–43% in the wetlands. The mechanisms of P removal in a wetland of Hovi, Finland, constructed on former arable land were clarified by Liikanen et al. [126], with the retention of P from runoff water being 68% for total P load. The subsoil of the former arable land, rich in reactive oxides, played a crucial role in P sorption.

Constructed wetlands represent a versatile and sustainable approach to wastewater treatment, offering not only high P removal efficiencies but also the potential for integration into various environmental and operational settings. Their continued development and implementation highlight the importance of CWs in achieving global water treatment objectives.

4.2. Chemical Treatment Technologies

Since water is so crucial to our everyday lives, there is a constant need to conserve and improve its quality. Unfortunately, it faces contamination from various sources, both local and diffuse. Industrial, domestic, and agricultural activity, coupled with environmental and global changes, are the major contributors to water pollution. Consequently, numerous regions across the world have polluted surface and groundwater, unfit for human consumption. The major goal of wastewater treatment is to eliminate the solid waste present in wastewater. Its purpose is to effectively eliminate things such as bits of clothing, paper, wood, cork, hair, and fiber, as well as kitchen waste and fecal solids. Typically, screening comes first in the process of treating wastewater. For this reason, screens of different sizes are employed, and the size of the screen is chosen based on the requirements or the size of the particles contained in the wastewater. For screening, water is passed through a material with tiny holes as part of the filtering process. Typically, a setup featuring pores between 0.1 and 1.5 mm is employed. It is used to remove germs, grease, oils, and other suspended particles. Various filters, including membranes and cartridges, are utilized in this process. Filtration may be used to remove particles smaller than 100 mg L⁻¹ and oil smaller than 25 mg L⁻¹, which can have their concentration decreased by up to 99%. Water treatment involves the filtering process. Adsorption, ion exchange, or membrane separation techniques utilize filtered water. Moreover, filtration devices play a crucial role in producing potable water. The cost of filtration per million liters of treated water spans between 25 and 450 US dollars [127].

4.2.1. Chlorination

The use of chlorine for disinfection has a long history. When added to water, it dismutases to hypochlorous acid and the chloride ion. To create hypochlorite, hypochlorous acid can be further ionized. In the context of disinfection, hypochlorous acid is typically considered the primary disinfectant. According to Fuzawa et al. [128], chlorine inactivates viruses by degrading the exposed nucleic acids and harming the viral capsids. According to [129], certain viral capsid locations are extremely reactive, explained by chlorine’s variable reactivity to amino acids [130]. Additionally, the viral genome has a sensitive region to chlorination. The 1-671 nt sequence in the 50 non-coding region of HAV is especially susceptible to chlorine [5]. A stem-loop structure found in this sequence region is linked to viral propagation. Because chlorine is less reactive than ozone, their methods of inactivating bacteria differ. Because the diffusion of chlorine into cells is less hampered by contact with cell walls, chlorine reacts more with intracellular components, and the cells exhibit higher structural integrity and less plasma leakage following chlorination [131]. Additionally, the viral genome has a sensitive region to chlorination. The 1-671 nt sequence in the 50-non-coding region of HAV is especially susceptible to chlorine [5]. A stem-loop structure found in this sequence region is linked to viral propagation. Because chlorine is less reactive than ozone, their methods of inactivating bacteria differ. Because the diffusion of chlorine into cells is less hampered by contact with cell walls, chlorine reacts more with intracellular components, and the cells exhibit higher structural integrity and less plasma leakage following chlorination [131]. An increase in turbidity had no effect on or only marginally decreased the coxsackievirus B5’s susceptibility to chlorine disinfection in the 0.2–5 NTU range. The CT value necessary for 4-log inactivation was, however, at least two times higher at high turbidity (20 NTU) than it was at low turbidity (5 NTU). Turbidity’s effect on composition is also a factor. When humic acid was used to enhance turbidity, chlorine’s ability to disinfect the water drastically declined once the turbidity level reached 1 NTU. However, when chalk was introduced in its place, even at 5 NTU, the turbidity had no impact on the disinfection effectiveness [132].

Li et al. [133] have demonstrated that a two-step addition of chlorine surpasses the efficacy of a single-step approach, achieving up to 1.02-log improvements in disinfection across various chlorine doses and contact times. They advocate for the fine-tuning of time intervals and dosage ratios as a strategy to bolster the effectiveness of chlorination in wastewater treatment.

Furthermore, research has illuminated the multifaceted advantages of chlorination beyond mere disinfection, particularly in the context of sewage sludge-derived biochar production. Xia et al. [134] noted remarkable enhancements in P solubility in neutral ammonium citrate following chlorination during sludge pyrolysis at 700 °C. The most pronounced solubility increase was observed with MgCl₂, leading to the formation of magnesium phosphate (Mg₃(PO₄)₂), which raised the P solubility from 40.08% in the sludge biochar to 72.07%, 74.00%, 74.05, and 76.57% in the biochar produced after adding PVC, NaCl, CaCl₂, and MgCl₂.

Huang et al. [135] explored the combined technology of using bittern as a magnesium source in struvite precipitation, paired with the internal recycling of the chlorination product of recovered struvite, for the recovery of total orthophosphate (PT) and removal of total ammonia-N (TAN) from swine wastewater. Their findings indicate that PT recovery efficiency and struvite purity are primarily influenced by wastewater pH and the Mg:PT molar ratio, with co-precipitations at higher pH levels (>9) reducing struvite purity. However, the feasible decomposition of recovered struvite by sodium hypochlorite (NaClO) and the subsequent recycling of the chlorination decomposition product significantly reduced the TAN concentration in swine wastewater over seven cycles, showcasing a 37% reduction in treatment costs compared to using pure chemicals for struvite precipitation.

Zhang et al. [136] reported a noticeable decline in P removal efficiency in WWTPs as chlorine disinfection levels increased. Specifically, a reduction in P removal efficiency by 44.1% was observed at an active chlorine concentration of 5.0 mg g⁻¹ SS and a significant decrease in P release and uptake activities by 49.6% and 100%, respectively, at 10.0 mg g⁻¹ SS. Moreover, P removal activities completely vanished at an active chlorine concentration of 25.0 mg g⁻¹ SS. These observations highlight the critical sensitivity of P removal processes to chlorine concentration, presenting potential challenges in maintaining effective P removal in WWTPs amidst heightened chlorine disinfection practices.

These findings highlight the sensitivity of P removal processes to chlorine concentration, suggesting potential challenges in maintaining effective P removal in WWTPs with increased chlorine disinfection.

4.2.2. Ozonation

Ozone interaction with water initiates a swift oxidative reaction, yielding ions and free radicals such as HO, HO₂, O, and O₂. Nonetheless, the ozone molecule itself is the principal agent in the disinfection process. This rapid reaction leads to a significant decrease in residual ozone concentration, halving within the initial 30 s and accomplishing at least a 1-log inactivation of microorganisms [137]. Ozone decomposition accelerates at elevated pH levels, generating more radicals but diminishing disinfection efficiency. Although both ozone molecules and hydroxyl radicals (OH^•) are capable of inactivating microorganisms, direct interaction with ozone has been identified as more effective [138]. Staehelin and Hoigne [139] noted that the ozone half-life in water significantly decreases with increasing pH, being tenfold shorter at a pH of 10 than at a pH of 9, and a hundredfold shorter at a pH of 8.

Ozonation’s application in advanced sewage treatment promotes the solubilization of P and organic matter. A substantial fraction of the solubilized P consists of acid-hydrolyzable P (AHP), crucial for PAOs. This process has demonstrated a 30% solubilization degree with an ozone dosage of 30 mgO₃/gSS, highlighting its potential for sludge reduction and P recovery [140]. Xu et al. [6] reported that ozonation of 2-Phosphonobutane-1,2,4-tricarboxylic acid led to the conversion of P into organic and inorganic forms, with subsequent coagulation treatments effectively removing total and inorganic P.

Additionally, combining ozonation with microwave treatment effectively released P from waste activated sludge, with ozonation alone boosting reactive P content by 89.5% and a COD release of 19.4%. An acidic pretreatment followed by ozonation and microwave treatment yielded the most significant nutrient release [141].

The use of the magnesium ammonium phosphate method for P removal from ozonated sewage sludge indicated increases in TP concentrations in the aqueous phase and orthophosphate in the ozonized sludge supernatant correlating to the ozone dose. Optimized removal conditions achieved P removal efficiencies of 43.1% for total P and 52.2% for orthophosphate [142].

Importantly, Nagare et al. [25] found that ozonation of sludge within a wastewater treatment framework reduced excess sludge volume without compromising P removal efficiency. This study quantified the solubilization degree per ozone consumption for general sludge in a range from 2.4 to 5.8 g SS O₃⁻¹ and for organic matter (COD) from 4.1 to 7.7 gCOD g⁻¹O₃⁻¹, showcasing the nuanced efficiency of ozonation in sludge treatment and P removal processes. This highlights the importance of integrating ozonation with other treatment processes, such as biological nutrients removal or chemical precipitation, to optimize wastewater treatment strategies.

In conclusion, ozonation presents a vital strategy for P removal in wastewater treatment, with its effectiveness and optimal implementation contingent upon the specific conditions and objectives of the treatment process.

4.2.3. Fenton Process

The Fenton process, a method for breaking down organic materials, is explained through two primary mechanisms. The first, known as the Haber-Weiss mechanism, suggests that the Fenton reaction generates OH^•, which are pivotal in decomposing organic substances [143]. The second mechanism, proposed by Bray and Gorin, indicates the creation of potent oxidizing iron compounds (FeO²⁺ and FeO³⁺) rather than OH as the primary agents of the Fenton reaction [144]. However, contemporary advancements in spectroscopy and chemical probes have led to the consensus that the initiation of Fenton oxidation primarily involves the generation of OH (Figure 4).

Fe²⁺ + H₂O₂ + H⁺→Fe³⁺ + H₂O + OH

According to Lucas et al. [7], the OH^• may be produced by the interaction of aqueous ferrous ions with hydrogen peroxide (H₂O₂) and can eliminate harmful and recalcitrant organic contaminants from wastewater. Typically, the pH of the solution used in the Fenton process is 3. The pH of the solution affects the oxidation activity of OH.

The Fenton process is recognized as an advanced oxidation technique, where OH^• is produced through the interaction between aqueous ferrous ions (Fe²⁺) and hydrogen peroxide (H₂O₂), efficiently eliminating persistent organic pollutants from wastewater. The optimal pH for the Fenton reaction is around 3, as the oxidation activity of OH^• is significantly influenced by the solution’s pH. Lower pH levels enhance the oxidation potential of OH^•, while higher pH leads to the precipitation of ferric hydroxide and iron oxyhydroxides, reducing the Fenton reagent’s activity due to the absence of active Fe²⁺. Moreover, at elevated pH levels, H₂O₂ tends to decompose autonomously [145,146,147].

In terms of P removal from wastewater, the efficiency of the Fenton process varies. One study indicated that using the Fenton reagent to degrade polyphosphonates achieved less than 20% transformation to orthophosphate (O-PO₄³⁻) in a pure water matrix. Conversely, the Photo-Fenton method yielded better outcomes, significantly increasing the formation rate of phosphonates with fewer phosphonate groups at pH 3.5. Despite the Fenton reagent’s incomplete transformation of organically bound P to O-PO₄³⁻ in heavily polluted wastewater, almost total removal of total P was achieved through a sequence of reaction steps, including sludge separation and neutralization [26].

Furthermore, the integration of ultrasound with Fenton oxidation as a pre-treatment has shown to significantly enhance the concentrations of soluble TP and PO₄³⁻ in sludge supernatant, outperforming the individual effects of ultrasound or Fenton oxidation alone [148]. Additionally, in treating triethyl phosphate wastewater, the Fenton process effectively reduced organic P concentrations from 58 to 5 mg/L under optimal conditions, adhering to a first-order kinetic model [149]. A combined approach of coagulation and Photo-Fenton processes has also proven effective in treating wastewater from an alkaline cleaning solution, meeting sewage discharge regulations, including a total P limit of 10 mg L⁻¹ [150].

In conclusion, the Fenton process, along with its variations such as the Photo-Fenton method, presents a valuable option for P removal in wastewater treatment. Its effectiveness is enhanced when integrated with other treatment technologies, demonstrating the Fenton process’s versatility and potential in achieving regulatory compliance for wastewater discharge.

4.2.4. Photolysis

Light-induced chemical reactions of pesticides significantly impact residues, effectiveness, toxicity, and the environment on the atmospheric surface of the waterbodies or other objects (such as plants and soil). According to Zuo et al. [151], photocatalytic degradation is a cost-effective and highly efficient degrading technique with excellent potential. Only pesticides that absorb light energy above 285 nm may be destroyed by natural sunlight since photodegradation requires the absorption of light energy. Therefore, high-intensity light is typically used for photocatalytic degradation investigations. Radiation from light was absorbed by pesticides, which can result in the production of hydroxyl, superoxide, and ozone radicals and degradation products. Isomerization, substitution, or oxidation are all possible photocatalytic degradation events. According to Ahmed et al. [152] and Devipriya and Yesodharan [153], the physical characteristics of pesticides, ambient variables, reactants, and other factors influence the light-induced chemical reaction. Photocatalysts are often needed for photocatalytic degradation processes. According to Ahmed et al. [152], the ideal substance for photocatalysts should have strong photoactivity, photocorrosion resistance, chemical inertia, a low cost, and low environmental toxicity in the near ultraviolet and visible spectrums. The two major catalysts used in photocatalytic reaction tests were titanium dioxide and zinc oxide [154,155]. However, recent studies have begun to use semiconductors as catalysts [156].

Recent advancements include Huang et al.’s [157] development of a novel photocatalytic approach for removing organic P contaminants from wastewater using a UCN/CdS NP photocatalyst activated under light exposure. This method breaks down organic P, such as glyphosate, by cleaving carbon-P (C-P) bonds through the generation of reactive OH^•, leading to the transformation of organic P into simpler molecules and the production of carbon monoxide (CO) gas in a process termed “carbon upcycling”.

Zarei et al. [158] demonstrated an 84% degradation efficiency of p-nitrophenol under optimal photolysis conditions within 60 min, achieving up to 89% mineralization efficiency. Similarly, Sun et al. [159] reported a significant reduction in total P using a proprietary process involving Fe(III) displacement, UV irradiation, and co-precipitation, transforming NTMP to phosphate.

These studies underscore that photolysis, particularly when integrated with other treatment methods or optimized conditions, offers an effective strategy for P removal. The approach not only addresses the issue of P in wastewater but also contributes to the sustainable management and valorization of organic pollutants, presenting a promising solution for environmental protection and resource recovery.

4.2.5. Electro-Fenton Processes

The most well-known electro-Fenton processing method is electrochemical advanced oxidation processes (EAOPs). The EAOPs method is the most recent of the advanced oxidation processes (AOPs) that have seen significant development during the past ten years. According to Oturan and Oturan [160], the EAOPs process is a novel technology that provides clean, efficient, and affordable processing for removing contaminants from water. In general, there are two alternative configurations for the electro-Fenton (EF) process: the first type uses Fenton reagent that has been introduced externally, and the second configuration uses H₂O₂ that has been introduced externally while Fe²⁺ is produced by the process of anode reduction [161]. The level of environmental contamination is influenced by the scale of the industrial parameters. Numerous studies have been conducted using a variety of techniques, including coagulation, adsorption, oxidation, biological processing, and electrochemistry, to minimize and even completely remove certain undesirable substances from water. Anodic oxidation by electro-regeneration of H₂O₂, electro-Fenton, photo-Fenton, and sun photo-Electro-Fenton are only a few of the several types of electro-AOP procedures. According to Moreira et al. [162], approaches may be employed alone or in conjunction with other processes, including biological ones, coagulation, electrocoagulation, and membrane filters. The Fenton reagent-based EAOPs are among the different EAOP kinds that are now receiving a lot of attention since they are used to getting rid of persistent organic pollutants. According to Ganiyu et al. [163], two processing modalities are extremely well known: electro-Fenton and photoelectron-Fenton.

The Electro-Fenton process is acclaimed for its ability to remove various pollutants, including P compounds and organic contaminants, from water. This process electro-catalytically generates OH^•, which are highly reactive and lead to the oxidation of pollutants to carbon dioxide and inorganic ions, signifying their complete elimination from the water [164]. Notably, the electro-Fenton process has achieved a 98% efficiency in removing P from filtered activated sludge effluent, demanding an energy input of 7.69 kWh per equivalent of removed P and 0.45 kWh m⁻³ [10]. This high level of efficiency, compared to traditional P removal methods, suggests the electro-Fenton process is both economical and effective. Additionally, the optimization of the electro-Fenton process for phenol removal resulted in over 98.5% elimination within just 1.5 h using a boron-doped diamond anode, with minimal energy consumption of 0.08 kWh per gram of Total Organic Carbon (g-TOC)⁻¹ [165], showcasing the process’s potential for the efficient and energy-efficient removal of micropollutants.

Further applications have demonstrated the electro-Fenton process’s versatility. For instance, the treatment of gas field wastewater mother liquor achieved a COD removal efficiency of 71.9% at an initial pH of 3 after 3 h of treatment [166]. Similarly, the removal of Rhodamine B (RhB) dye reached a maximum of 97.7%, with a TOC removal of 35.1% under optimized conditions [167]. The operational parameters, notably the initial pH and the concentration of iron, are crucial in influencing the current efficiency of the electro-Fenton process. Optimal conditions have been identified to significantly enhance the quantitative oxidation of P compounds, with an ideal iron concentration range being 50–150 mg L⁻¹ [168].

In conclusion, the electro-Fenton process demonstrates exceptional potential for the efficient removal of phosphorous and organic contaminants from water. Its effectiveness is highly dependent on the optimization of operational conditions, such as pH, current density, and iron dosage, underscoring the electro-Fenton process as a versatile and powerful approach in water treatment strategies.

4.2.6. Photo-Fenton Process

The Fenton reaction, which uses iron as a catalyst, has drawn a lot of interest recently in treating water. Fenton’s reaction has several advantages over other processes: (i) it is simpler to operate, can be carried out at room temperature, and does not require illumination; (ii) the reagents are affordable and readily available; they are also simpler to store and handle; (iii) H₂O₂ is environmentally friendly because it slowly breaks down into oxygen and water; and (iv) there are no mass transfer restrictions that are typically present in heterogeneous processes. Fe is the most often utilized transition metal for applications related to Fenton’s reaction because it is abundant, nontoxic, and easy to remove from water [169]. Additionally, compared to certain other AOPs, the creation of hazardous by-products linked to Fenton’s reaction is substantially reduced. Due to the potential advantages of using Fenton’s reaction as a remediation procedure for the treatment of a wide variety of water contaminants, extensive research and field investigations have been conducted. Many organic compounds can be detoxified and degraded using Fenton (H₂O₂/Fe₂) and photo-Fenton (UV/H₂O₂/Fe₂) AOP techniques, which are efficient and cost-effective [170]. This method has shown notable success in removing color from textile wastewater, achieving up to 96% color removal within 30 min, although challenges such as the photoreduction of ferric to ferrous ions, causing color resurgence, have been observed [171]. Studies have also reported high removal efficiencies for organic pollutants in industrial wastewater, further highlighting the process’s versatility [172].

Optimizing parameters such as pH, concentrations of H₂O₂ and iron salts, contaminant levels, and reaction time is crucial for maximizing the Photo-Fenton process’s effectiveness. Generally, acidic conditions (pH ~3) with tailored dosages of H₂O₂ and iron salts yield optimal results, depending on the wastewater’s specific characteristics [172,173].

In the context of P removal, the Photo-Fenton method has demonstrated significant potential. For instance, it achieved formation rates of orthophosphate (O-PO₄³⁻) up to 80% for phosphonates with fewer phosphonate groups in a pure water matrix, although challenges remain in transforming organically bound P to O-PO₄³⁻ in highly polluted wastewater. Despite this, nearly complete removal of total P has been reported under certain conditions [26]. Additionally, a combined Fe(III)/UV/co-precipitation process was shown to effectively reduce TP levels from 1.81 mg L⁻¹ to 0.17 mg L⁻¹, underscoring the process’s utility in P removal [159].

4.2.7. Photocatalysis

For the light-degradation of hazardous chemicals, photocatalysis is a very sophisticated oxidation process. The filtration of water is another use for it [174]. Heterogeneous catalysis and homogeneous catalysis are the two divisions of photocatalysis. Diverse families of hazardous compounds have been effectively degraded using heterogeneous catalysis. In comparison to more conventional wastewater treatment methods, including chemical oxidation [175], activated carbon adsorption [176], and biological treatment [177], photodegradation provides several benefits. The phase transfer of pollutants using the activated carbon adsorption technique results in another pollution without any breakdown. pollution problem. The chemical oxidation approach is suitable for the removal of contaminants at high concentrations but cannot completely remove all organic compounds. According to Robert and Weber [178], biological treatments are exceedingly laborious, produce a lot of sludge, and need careful monitoring of pH and temperature [178]. Accordingly, photocatalytic techniques provide benefits for the removal of contaminants from industrial waste water, even at low concentrations [57]. Additionally, photooxidation uses extremely active and affordable catalysts that may be utilized in properly designed reactor systems to completely oxidize organic pollutants in just a few hours, even at ppb levels, without the creation of secondary hazardous compounds [179]. Due to its high oxidation efficiency, non-toxicity, high photostability, chemical inertness, and environmentally favorable attributes, titanium dioxide is a commonly used photocatalyst [180].

The removal of contaminants, including P compounds from wastewater, via photocatalysis, primarily involves the absorption of light by a photocatalyst, leading to the generation of electron-hole pairs. These pairs then drive redox reactions that degrade pollutants present in the water. Titanium dioxide is a particularly favored photocatalyst for this purpose due to its high effectiveness and stability. Upon exposure to UV light, Titanium dioxide produces photo-generated holes (h⁺) and electrons (e⁻), capable of breaking down both organic and inorganic substances in wastewater [181,182].

The efficiency of this photocatalytic process depends on several variables, such as the choice of photocatalyst, the use of doping agents, the wastewater’s pH, temperature, and the presence of competing substances. For instance, In₂O₃ porous nanoplates have achieved 100% decomposition of per- and polyfluoroalkyl substances (PFAS) under UV light, demonstrating a rate constant (k_t) of 0.158 min⁻¹ and a half-time (τ_1/2) of 4.4 min [181]. Moreover, the photocatalytic degradation of phosphamidon, an organophosphorus insecticide, using TiO₂ under sunlight highlights the potential of utilizing solar energy for wastewater decontamination [182].

Enhancing photocatalysts through modifications such as N doping or supporting them on luminescent microparticles has proven to increase the photocatalytic reaction rates. This enhancement is particularly beneficial for removing organic dyes from wastewater [183]. Furthermore, innovative materials such as red P/hollow hydroxyapatite microspheres have shown remarkable photocatalytic performance and stability, capable of degrading certain antibiotics completely in minutes under full-spectrum light [184].

4.2.8. Solar Photocatalysis

Solar photocatalytic systems utilize high-energy, short-wavelength photons to facilitate photochemical reactions, in contrast to solar-thermal processes, which gather substantial amounts of photons at various wavelengths to attain a certain temperature range. However, the equipment required for solar photochemical applications shares many characteristics with those of thermal applications, such as solar thermal collector designs, parabolic trough collectors (PTC), and non-concentrating collectors, which have been used in both photochemical reactors. At this point, the two designs emerge because in photochemical processes, the fluid must be directly exposed to sun radiation, necessitating the need for an absorb that is photon-transparent. Additionally, temperature plays a less significant role, eliminating the need for insulation. Due to two factors, the first photoreactors for solar photocatalytic applications developed towards the end of the 1980s were based on PTCs. Firstly, this sort of collector has historically been preferred for solar thermal applications, making the technology very advanced and versatile. Second, PTCs were thought to be the most suitable focusing system for this kind of application out of all the ones that were readily accessible. Several years ago, Goswami provided a thorough study that included all the specifics of these early breakthroughs [185]. Later, at the start of the 1990s, some research teams attempted to replace PTCs with non-concentrating solar collectors because PTCs are unsuitable for photocatalytic applications for a number of reasons [186], including water heating, excessive radiation flux, inefficient use of the majority of photons [187], and high cost. Non-concentrating solar collectors are stationary devices without any sort of sunlight-tracking technology. They are typically static flat-plate devices angled towards the equator at a specific inclination based on the site’s latitude. Their simplicity and cheaper manufacturing costs are the key advantages. Additionally, the construction of non-concentrating collector support structures is less complex, more affordable, and requires a smaller installation space because there is less shade than with concentrating systems. Since there are no moving parts or tracking systems, non-concentrating static solar collectors are more affordable than PTCs. Given their fixed orientation to the incoming radiation, their beam-sunlight collection may be less energy-efficient. However, they may use diffuse radiation and are far more suited to small-scale operations. Numerous non-concentrating solar reactors for solar photocatalytic processes have been created and tested all over the world as a result of the significant work put into compact non-concentrating collector designs. Only a few studies concerning non-concentrating collectors have been published since our group examined the majority of this work [188].

Solar photocatalysis, particularly when employing titanium dioxide and oxidants under solar irradiation, presents a powerful method for reducing organic content in wastewater, which may encompass P-containing compounds. Kositzi et al. [189] highlight the significant potential of this approach. A noteworthy advancement in this field was demonstrated by a study that employed a single-compartment photoelectrocatalytic (PEC) cell system for the recovery of P from hypophosphite-laden wastewater. This innovative system made use of a TiO₂/Ni-Sb-SnO₂ bifunctional photoanode paired with an activated carbon fiber cathode, alongside the dosing of Fe²⁺ ions. Remarkably, this setup enabled the complete oxidation and recovery of hypophosphite within just 30 min at a voltage of 3.0 V. The observed pseudo-first-order rate constants for hypophosphite oxidation significantly surpassed those in systems relying solely on electrocatalytic or photocatalytic processes [190].

5. Comparative Analysis and Integration of P Removal Processes

5.1. Comparative Effectiveness of P Removal Methods

P removal from wastewater is a critical process in mitigating eutrophication in aquatic environments. Various methods have been developed, broadly categorized into biological, chemical, and hybrid approaches. Each method has its strengths and limitations, which must be understood to optimize wastewater treatment processes. This section provides a comparative analysis of these methods, focusing on their effectiveness, operational challenges, environmental impact, and potential for integration and innovation.

Biological P removal, particularly EBPR, is widely used in municipal wastewater treatment plants. EBPR relies on PAOs that uptake and store P under alternating anaerobic and aerobic conditions. This method is highly effective, achieving removal efficiencies of up to 90% under optimal conditions [82]. Biological trickling filters and biofilm reactors further enhance P removal by providing robust platforms for microbial growth [191]. Constructed wetlands, which utilize natural processes such as plant uptake and microbial activity, are effective in decentralized or rural applications, although their performance can be influenced by environmental factors such as climate and seasonal variations.

Chemical P removal methods, such as the use of metal salts (e.g., aluminum and iron) for precipitation, are effective in rapidly reducing P concentrations. These methods are particularly suitable for industrial wastewater treatment where high P loads are common. AOPs, including ozonation and the Fenton reaction, while primarily aimed at degrading organic pollutants, can incidentally contribute to P precipitation or release. However, these processes are not dedicated P removal methods and are best integrated into broader treatment schemes where P removal is a secondary benefit [192].

Hybrid approaches that combine biological and chemical methods offer a synergistic solution to P removal. For example, coupling EBPR with chemical precipitation can address the limitations of each method, such as the variability in biological process efficiency and the potential chemical byproducts from precipitation [193]. These integrated systems can achieve higher removal efficiencies, particularly in settings where P concentrations fluctuate or where stringent discharge limits are required.

5.2. Maintenance and Complexity

Biological systems such as EBPR and constructed wetlands generally require more complex operational management, including careful control of redox conditions and maintenance of microbial populations [194]. In contrast, chemical methods are relatively straightforward to operate but can generate secondary waste products that require additional treatment. Hybrid systems, while potentially more efficient, may increase operational complexity by requiring the integration and coordination of multiple processes.

The environmental impact of P removal methods varies significantly. Biological methods are generally more sustainable, relying on natural processes with minimal chemical inputs. However, their effectiveness can be compromised by environmental factors and operational variability. Chemical methods, while effective, often involve the use of reagents that can contribute to secondary pollution if not managed properly [195]. Hybrid approaches aim to balance these factors by optimizing the strengths of each method, reducing the overall environmental footprint of the treatment process.

Therefore, the cost of P removal is a critical consideration, particularly in large-scale or decentralized applications. Biological methods such as EBPR can be cost-effective over the long term, especially when integrated with renewable energy sources or resource recovery processes. However, they may require significant upfront investment in infrastructure and ongoing operational costs for monitoring and maintenance. Chemical methods, while potentially less expensive to implement initially, can incur higher costs due to the need for chemical reagents and the management of byproducts. Hybrid systems may offer a balanced cost structure, leveraging the efficiency of biological processes with the reliability of chemical methods.

P recovery is increasingly recognized as a critical component of sustainable wastewater treatment. Different forms of P, including orthophosphate, polyphosphate, and organic P, can be recovered through various methods, depending on the wastewater characteristics and the treatment process. Recovery efficiency is a key consideration, with advanced technologies focusing on maximizing the yield of usable P while minimizing energy consumption and costs. Economic analysis of P recovery methods suggests that while upfront costs may be high, the long-term benefits, including reduced environmental impact and potential revenue from recovered P, can make these processes economically viable.

Overall, the comparative analysis of P removal methods highlights the importance of selecting the appropriate technology based on specific operational conditions and treatment goals. Biological methods offer sustainability and long-term efficiency, while chemical methods provide rapid and reliable removal. Hybrid approaches, integrating the strengths of both, represent a promising direction for future development. Future research should focus on optimizing these integrated systems, exploring new materials and technologies for P recovery, and developing cost-effective solutions that meet increasingly stringent environmental regulations.

6. P Recovery from WWTP Streams

P removal in WWTPs primarily aims to reduce the P concentration in influent wastewater through its incorporation into biomass or precipitates during the mainstream treatment process. Consequently, the removed P typically accumulates in the sludge stream. Recovery of P is usually carried out from the liquid or solid side streams of the WWTP, such as digester supernatant or sludge dewatering liquor. The efficiency of P recovery is often enhanced when EBPR or chemical P removal methods are implemented in the mainstream treatment line. Additionally, incorporating a hydrolysis or anaerobic digestion step within the sludge processing line can further promote P recovery by facilitating the mineralization of organic P and the biological release of P from PAOs [196].

The recovery of P from wastewater is not only a crucial step in minimizing environmental impact but also presents significant opportunities for the reuse of P as a valuable resource. Numerous patented systems for P recovery have been developed, with some reaching a Technology Readiness Level (TRL) of 9, indicating successful full-scale implementation. These recovery processes vary depending on the source matrix of P, and the following subsections review technologies tested with real municipal WWTP streams, focusing on recovery from liquid streams such as digester supernatant and sludge dewatering liquor [197].

6.1. Systems for P Recovery from Liquid Streams

P recovery from liquid streams in municipal WWTPs typically involves processing the liquid fraction of anaerobically digested or hydrolyzed sewage sludge, which is obtained post-dewatering (e.g., centrate or filtrate). The application of sludge disintegration technologies, such as thermal hydrolysis, can enhance P solubilization, thereby improving recovery rates in the liquid fraction [198].

6.1.1. Crystallization Technologies

Crystallization is a widely applied method for P recovery from liquid streams, often conducted in fluidized bed reactors. Technologies such as the Crystalactor^® and Ostara Pearl^® systems facilitate the crystallization of P in forms such as struvite (NH₄MgPO₄·6H₂O) or calcium phosphate, which can then be harvested as pellets. These crystallization processes are effective in recovering P from digester supernatant, with recovery rates typically ranging from 10% to 40% of the influent P depending on the system used. The choice of crystallization reagent, such as lime or magnesium hydroxide, and the design of the reactor (e.g., fluidized bed, fixed bed) play crucial roles in the efficiency of P recovery [199,200,201].

6.1.2. Adsorption

Adsorption is another viable method for P recovery, involving the binding of phosphate ions to sorbent materials through Van der Waals and electrostatic forces. This process is influenced by factors such as pH, contact time, and the affinity of the sorbent surface. Various industrial by-products, such as fly ash, red mud, and slags, have been explored as cost-effective sorbents for phosphate removal. Additionally, ion-exchange resins and activated carbon are commonly used in adsorption processes due to their high surface area and regeneration potential. Recent advances in biochar production and modification have also shown promise for P recovery, particularly when applied in biofiltration systems [202,203,204].

6.1.3. Electrochemical Precipitation

Electrochemical precipitation is a newer approach to P recovery that leverages the local pH increase at the cathode during water electrolysis to precipitate calcium phosphate, typically as hydroxyapatite. This method does not require additional chemicals and simplifies the recovery process by allowing the direct harvesting of precipitates from the cathode. However, challenges remain in controlling the coprecipitation of by-products and optimizing P recovery efficiency. Despite these challenges, electrochemical precipitation represents a promising, cost-effective solution for P recovery from municipal wastewater [205].

6.2. Systems for P Recovery from Raw and Digested Sewage Sludge

P recovery from sewage sludge offers a higher yield than from influent wastewater, with up to 95% of the P removed in WWTPs ending up in the sludge. This makes sewage sludge an attractive resource for P recovery and reuse, particularly for land fertilization. However, concerns about pollutants, such as heavy metals and pharmaceuticals, necessitate stringent regulations and alternative recovery strategies.

6.2.1. Struvite Crystallization (MagPrex^® System)

The MagPrex^® system, formerly known as AirPrex^®, developed by Berliner Wasserbetriebe, treats digestate in an air-lift reactor where air is recycled to strip CO₂ and raise the pH. Magnesium chloride is then added to facilitate struvite formation, with crystals settling to the reactor bottom. Full-scale plants using MagPrex^® report P recovery rates exceeding 80%, though only about 40–50% of the struvite produced is recovered in the reactor [206].

6.2.2. Wet-Chemical Extraction

The Gifhorn and Stuttgart processes exemplify wet-chemical extraction methods for P recovery from sewage sludge. Both involve an initial acidification step to dissolve P, followed by metal separation and P precipitation. The choice of chemicals and pH conditions significantly impacts recovery efficiency, with the Stuttgart process achieving P recovery rates exceeding 65% in the form of low-metal-content struvite [207].

6.2.3. Aqua Reci^® and LOPROX Systems

The Aqua Reci^® process employs supercritical water oxidation (SCWO) before acid or alkaline leaching to extract P from sludge. SCWO mineralizes organic matter and oxidizes toxic metals and P, facilitating P recovery as sodium phosphate or other forms. Despite its benefits, the Aqua Reci^® process has significant energy demands, limiting its economic viability [208].

The LOPROX system uses low-pressure wet oxidation followed by ultrafiltration and nanofiltration to recover P. The recovery rate depends on the ion rejection properties of the nanofiltration membrane, with higher P recovery achieved through optimized membrane washing [209].

6.2.4. MEPHREC^® Process

The MEPHREC^® metallurgic process offers the highest P recovery rates among sludge-based technologies, recovering up to 75% of the P in the WWTP influent as a P-rich slag. The process involves melting the sludge at high temperatures, resulting in the formation of calcium silicon phosphate, although metal behavior during the process can impact depollution efficiency [210].

6.2.5. Vivianite Recovery

Vivianite recovery from sewage sludge is a promising area of research, particularly in WWTPs using iron-based chemical P precipitation. Magnetic separation has concentrated vivianite by a factor of 2–3 in lab-scale experiments, though further research is needed to optimize separation efficiency and product quality. Vivianite could be used as a slow-release fertilizer or a platform for P extraction while reducing sludge disposal volumes [211].

6.3. Systems for P Recovery from Sludge Ash

P recovery from sludge ash is a vital approach in wastewater treatment due to the significant P content that remains in the ash after incineration. Key methods for P recovery from sludge ash include wet-chemical extraction, wet-chemical leaching, and thermo-chemical treatments. These methods can achieve P recovery rates between 70% and 90% of the P present in the WWTP influent [212].

6.3.1. Wet-Chemical Leaching and Extraction

Wet-chemical processes, such as those used in the RecoPhos^® and Fertilizer Industry methods, involve dissolving P from sludge ash using acidic solutions. These methods can recover up to 85% of the P, resulting in a mineral fertilizer that meets agricultural standards such as those outlined in the German Fertilizer Ordinance. However, because sludge ash often contains heavy metals such as iron, zinc, and copper, additional purification steps are necessary to ensure the recovered P is safe for use in agriculture [213].

The PASCH and LEACHPHOS^® processes are examples of wet-chemical leaching techniques that focus on re-solubilizing P from incinerated sludge ash. The PASCH process, in particular, incorporates a liquid-liquid extraction step to remove heavy metals, enhancing the purity of the recovered P. However, both PASCH and LEACHPHOS^® processes have only been tested at the laboratory and pilot scales [197].

6.3.2. Thermo-Chemical Treatments

Thermo-chemical treatments offer an alternative method for recovering P from sludge ash. The AshDec^® system, which has been tested at the pilot scale, involves treating sludge ash in a rotary kiln at temperatures between 900 °C and 950 °C. In this process, the ash is mixed with potassium and/or sodium compounds and a reducing agent, such as dry sewage sludge. This reaction replaces calcium ions in phosphates with potassium or sodium, resulting in the production of soluble P that can be used in PK fertilizers. Heavy metals evaporate during the process and are subsequently removed from the gas stream using electrostatic precipitators [214].

Another approach, the Thermphos^® process, operates at higher temperatures (1500 °C to 1600 °C) to recover P as elemental white P (P₄). Although this method is highly energy-intensive, it produces a pure (99.99%) P product that can be used in the manufacture of various P-based chemicals. However, high iron content in the ashes can reduce the efficiency of P recovery by forming iron phosphides instead of elemental P. Additionally, the process generates dust rich in heavy metals, which requires periodic removal to prevent contamination [215].

6.4. Economic Analysis of P Recovery Processes in WWTPs

The economic viability of P recovery technologies in WWTPs is crucial for their practical adoption. Costs for recovering P from post-digestion supernatants typically range from 8 to 10 euros per kilogram. The NuReSys^® technology stands out as the most cost-effective, with expenses as low as 3 euros per kilogram, thanks to its use of continuous stirred-tank reactors (CSTRs), which are more economical than traditional fluidized-bed systems [201,216].

Fixed-bed technologies such as P-RoC^® are less attractive due to higher costs (around 6 euros per kilogram) and the production of calcium phosphate, which is less bioavailable to plants compared to struvite, especially in neutral and alkaline soils [199]. However, alternative calcium phosphates, such as hydroxyapatite, are being explored as potential substitutes for phosphate rock [217,218]. Implementing these technologies can also reduce maintenance costs by preventing issues such as struvite buildup in WWTP infrastructure.

The use of chemicals in recovery processes can significantly increase costs. For instance, the REM-NUT^® technology, which recovers P directly from WWTP effluent, is among the most expensive, with costs reaching up to 28 euros per kilogram, primarily due to the need for resins and reagents [197]. Wet-oxidation processes, such as Aqua Reci^®, also have high operational costs, ranging from 23 to 27 euros per kilogram, due to their energy-intensive nature [197,219].

Thermal treatment technologies, while costly, offer benefits such as heat recovery and sludge conversion into inert waste, which can lower overall treatment expenses. However, these technologies are still primarily at the pilot scale, making their full economic impact uncertain [197]. Potential revenues from P-rich products, reduced sludge disposal, and improved digestate dewaterability can help offset these costs, but challenges remain, such as the low market value of recovered P and concerns about pathogen contamination in products such as struvite [220].

7. Concluding Remarks and Perspective

The comprehensive review of P removal technologies underscores the complexity and variability of effective P removal across wastewater treatment plants. This variability is influenced by a range of factors, including the specific technologies employed, operational conditions, and the inherent characteristics of the wastewater being treated (Figure 5).

7.1. Effectiveness of Biological and Chemical Methods

EBPR systems, utilizing activated sludge processes, remain highly effective for municipal wastewater treatment. These systems leverage the metabolic activities of PAOs within specific anaerobic and aerobic zones to achieve high P removal efficiencies. Key to their success is the careful management of redox potential shifts, which facilitate the release and subsequent uptake of P by PAOs. Advanced biofilm reactors and biological trickling filters further enhance EBPR effectiveness by providing robust platforms for microbial activity. Constructed wetlands offer an environmentally sustainable option, particularly in decentralized and rural applications. These systems utilize natural processes such as plant uptake, microbial activity, and soil accretion to remove P. The choice of substrates, such as laterite and natural pyrite, plays a crucial role in optimizing P removal efficiency. However, the performance of constructed wetlands can be influenced by seasonal variations and local environmental conditions.

Chemical treatments, including the use of metal salts (e.g., aluminum and iron), and AOPs such as chlorination, ozonation, and the Fenton reaction, provide reliable options for achieving rapid and substantial P removal. These methods are particularly suitable for industrial applications where high P loads and stringent discharge requirements are prevalent. Ozonation and the Fenton process, in particular, are effective in breaking down complex organic P compounds, facilitating their removal. However, these methods often require careful management of byproducts and additional treatment stages to maintain water quality.

7.2. Operational Considerations and Redox Potential

A consistent finding across various treatment plants is the critical role of anaerobic zones in facilitating P release from sludge, which is essential for subsequent removal stages. In plug-flow plants, high oxygen demand in the initial sections of the aeration basin can create localized anaerobic conditions, promoting P release. Managing these redox conditions is crucial, particularly in systems where nitrification occurs, as nitrates can disrupt the anaerobic environment necessary for effective P removal.

To address these challenges, the integration of denitrification processes and the strategic return of nitrate-free sludge to the aeration basin inlet are essential. This approach ensures the maintenance of favorable redox conditions for P removal.

7.3. Challenges, Opportunities, and Future Directions

Despite significant advancements in P removal technologies, several challenges persist. The scalability of laboratory and pilot-scale processes continues to be a major barrier to widespread adoption. Furthermore, the high operational costs associated with advanced chemical treatments, as well as the complexities involved in managing byproducts, pose economic challenges, particularly for smaller WWTPs.

However, opportunities abound in the development of hybrid systems that integrate biological and chemical methods, aiming to maximize P removal efficiency while minimizing both costs and environmental impacts. An example of such an integrated approach is the “Phoredox” process, which optimizes conditions for P release and uptake by combining anaerobic basins with re-aeration zones. Exploring similar synergistic combinations of technologies could yield more robust and efficient P removal solutions.

Simultaneous removal and recovery of P represent the new frontier in modern WWTPs. While struvite remains the most commonly recovered product, its high cost relative to conventional fertilizers poses a significant challenge. Incentives from national governments to promote the use of struvite and sludge-derived fertilizers could greatly encourage this practice. Additionally, technologies for P recovery from sludge liquors are currently the readiest for full-scale implementation, offering high technical maturity and lower complexity and costs compared to methods that recover P from sludge and sludge ash.

The application of P recovery methods in soilless cultivation systems, particularly in greenhouse agriculture, presents another promising direction for future research. Greenhouse wastewater (GW), generated during soilless cultivation of plants, is rich in nutrients, including P. The recovery of P from GW can help mitigate the environmental impact of nutrient discharge while providing a valuable resource for agriculture. Methods such as crystallization, sorption, and chemical precipitation can be employed to recover P from GW, but further research is needed to optimize these processes for this specific type of wastewater. Given the high calcium and magnesium content in GW, chemical precipitation could be particularly effective, though the process parameters need to be carefully studied to ensure efficiency and practicality [221,222,223].

The volume of GW generated in greenhouse cultivation, such as in the tomato industry in Poland, underscores the potential scale of P recovery. For instance, the P recovered from GW in Poland could reach up to 30.2 MgP per day, demonstrating a substantial opportunity to recycle this critical resource. Moreover, utilizing pre-treated GW in a recirculation or cascade system could reduce production costs and diversify P sources in agriculture, moving closer to a circular economy model [224,225].

Future research should focus on optimizing the operational parameters of both biological and chemical P removal methods, enhancing the efficiency of existing technologies, and developing more cost-effective and sustainable treatments. There is also a critical need to improve the long-term stability of innovative systems, such as aerobic granular sludge and carrier-based biofilm systems, which show great potential but require further optimization to ensure consistent performance. Additionally, integrating microalgae with bacterial systems could further improve the efficiency and sustainability of P removal processes, although full-scale studies are needed to confirm the long-term stability of these consortia and their responsiveness to varying influent conditions.

P recovery technologies offer substantial potential for sustainable resource management in WWTPs and agricultural systems. While direct recovery from sludge presents challenges due to lower product quality and high costs, thermochemical recovery from sludge ash emerges as a promising alternative with high recovery efficiencies and the potential for cost savings through heat recovery and reduced disposal costs. As the transition to P recovery accelerates, it will be essential to carefully consider environmental impacts, economic feasibility, and regulatory frameworks to ensure the successful implementation of these technologies at scale.