Analysis of the Status and Improvement of Microalgal Phosphorus Removal from Municipal Wastewater

Abstract

Phosphorus, as one of the main pollutants in municipal sewage, has received increasing attention recently. Phosphorus recovery also increases the sustainable development of municipal wastewater. Since algae have the ability to effectively redirect nutrients, including phosphorus, from municipal sewage to algae biomass, municipal sewage treatments involving microalgae have piqued the interest of many researchers. The phosphorus removal depends on the potential of the microalgae to absorb, preserve, or degrade phosphorus in municipal wastewater. It is, therefore, of great interest to study the mechanisms underlying the absorption, storage, and degradation of phosphorus by microalgae to ensure the viability of this phosphorus removal process in wastewater. The objectives of this review were to summarize phosphorus metabolism in microalgae, examine key external and internal factors impacting phosphorous removal by microalgae from wastewater, and examine the status of phosphorous-metabolism-related research to improve our understanding of microalgae-based municipal wastewater treatments. In addition, the methods of recovery of microalgae after phosphorous removal were summarized to ensure the sustainability of municipal wastewater treatment. Finally, a potential approach using nanomaterials was proposed to enhance the overall phosphorous removal performance in municipal wastewater through the addition of nanoparticles such as magnesium and iron.

1. Introduction

Cities are generally densely populated and have great demands for water. For some cities, the efficient treatment of urban sewage to achieve water recycling has become a strategic solution to water shortages [1]. In China, the amount of wastewater treated has increased by more than 50% in the last decade, and the increase in the volume of wastewater has significantly endangered human and environmental welfare [2]. The fecal sludge collected by the sewage systems in some industrial cities is untreated, causing significant damage to water supplies across such cities [3]. While urban sewage treatment in developed European countries strongly reduces pollution in wastewater, the amount of sludge generated is still growing year on year. Finding methods to efficiently treat sludge is an urgent challenge [4]. In comparative studies and simulation forecasts regarding the statuses of cities in Nanjing (domestic), South Asia, and Southeast Asia, the results indicated that by 2030 the water quality in samples sites in Manila and Jakarta will have deteriorated further [5].

Phosphorus is an essential basic element in organisms that is also commonly present in water bodies and primarily occurs in a dissolved form and in association with other particles via chemical adsorption; however, regardless of whether it originates from the natural world or via human activities, phosphorus significantly impacts the environment [6]. Excessive phosphorus in a water body, for example, accelerates the growth of algae and other microorganisms, subsequently creating nutrient imbalance in the water body and accelerating eutrophication [6]. Some phosphorous-containing compounds in water can be converted into minerals and may block water supply lines and sewage treatment facilities [7,8]. In municipal drainage systems, phosphorus originates primarily from domestic sewage, with 85% of the total phosphorus entering the wastewater from domestic sewage containing human excrement and detergents [6]. Phosphorus-containing sewage compounds in urban life exist mainly in the form of orthophosphoric acid, tripolyphosphate, and pyrophosphate [9,10]. Modern technologies used for the elimination of phosphorus are primarily categorized into biological, chemical, and physical technologies [11]. The removal and recovery of phosphorus from municipal sewage not only provides conditions for the utilization of phosphorus resources, but also reduces the eutrophication and increases the sustainability of municipal sewage; however, many conventional phosphorous reduction processes used for urban sewage entail high running and maintenance costs and are not sustainable, and may even cause other pollution to the water body [12,13].

Phosphorus also has numerous constructive uses. For example, certain phosphates can be applied to water supply pipes, where they combine with heavy metals in rainwater, lowering the concentration of heavy metals in the water while still acting as a corrosion inhibitor [14,15]. To avoid direct interactions between the metal and the food in metal food containers, phosphides are often added to form an inert coating [16]. Furthermore, household detergents, toothpaste, and shampoos are often also incorporated phosphorous compounds to improve their washing efficiency, with sodium tripolyphosphate being the most popular phosphorous-containing compound in detergents [17]. The overall amount of phosphorus in the world is small and nearly 40 million tons of phosphorus is absorbed worldwide per year, making the recycling of phosphorus critically significant [18].

CiteSpace is a tool used for the analysis of scientific literature. It can help an author to explore research hotspots and research frontiers in a certain research field and to predict future development trends. The Web of Science database was used to perform a comprehensive study of the literature, using the search terms “microalgae” and “phosphorous removal”. In total, 678 manuscripts published in the past five years were retrieved and a visual study of the keywords in the manuscripts was carried out using CiteSpace (version 5.7.R2), with the results shown in Figure 1. Based on the findings of the search, CiteSpace was used again to analyze clusters, with the results shown in Figure 2. Figure 1 and Figure 2 demonstrate that the study hotspots regarding the reduction of phosphorus by microalgae focus primarily on the treatment of municipal sewage, recovery, and resource utilization. The results of this analysis provided theoretical significance for this review of relevant research frontiers regarding the treatment of phosphorus in urban sewage and the prediction of future development trends for microalgal phosphorus removal technology.

This review summarizes phosphorus elimination from municipal wastewater by microalgae and analyzes the factors influencing this phosphorous removal and the associated methods. The aim of this review is to further understand the process of phosphorus removal by microalgae in municipal wastewater, so as to optimize the process and sustainability. In this paper, the problems related to microalgae dephosphorization are introduced for researchers who are interested in municipal wastewater dephosphorization technology.

2. Various Systems and Strategies for the Reduction of Phosphorus in Municipal Wastewater

Phosphorus removal from municipal sewage is mainly categorized into three categories of processes: physical, chemical, and biological. The respective technological methods and respective advantages and drawbacks of each process are shown in

Table 1. Different physical, biological, and chemical phosphorus removal technologies used in wastewater treatment processes [20,37,38,39].

Methods	Technologies	Advantages	Disadvantages
physical methods	physical absorption	widely used for phosphorus removal	not yet perfect for phosphorus adsorption
	sand filtration	removes all P compounds	only for the primary stage
	the membrane purification	simple and efficient	high operation and maintenance costs
	ion exchange	can treat hazardous waste and higher concentrations of phosphorus	lack of selectivity for specific ions and complex process
chemical methods	by precipitation of metal salts and lime	high phosphorus removal efficiency and economical	may cause secondary contamination
	crystal	reusable, little environmental harm	need to add chemicals and low stability
	Coagulation and flocculation	can be used for reaction by adding metal ions such as polymers or aluminum	need high charge for salt ions
biological methods	artificial aeration	mainly used for dephosphorization of lakes	no significant effect in shallow lakes
	enhanced biological phosphorus removal	no chemicals need to be added	low stability and biological population competition
	photosynthetic microorganisms immobilized on cellulose, ceramic, or gel carriers	can effectively immobilize and remove more than one type of microorganism or contaminant	not easily removed for most phototrophs
	phosphoric acid binds proteins	can work in low phosphorus environments	the use of this protein is limited

. Physical phosphorus removal technologies can eliminate all forms of particulate phosphorus compounds [19]. Membrane technology not only extracts phosphorus from complete suspended solids but also removes dissolved phosphorus. Membrane bioreactors (MBRs) and reverse osmosis (RO) devices with good phosphorous removal capacity have been widely used in full-scale sewage treatment plants [20]. The fundamental theory behind the elimination of chemical phosphorus is to crystallize or condense phosphorous compounds by adding chemical agents or by modifying certain reaction conditions. For example, in the coagulation and flocculation process, phosphorous-containing materials are flocculated by the addition of polymers or metal ions. This approach is effective for removing larger molecules, and its quality is determined by the charge of the salt ions [21]. Since biological phosphorus removal is used to treat municipal sewage, anaerobic or anoxic treatment is usually required first, followed by aerobic treatment and other procedures to remove phosphorus from activated sludge in municipal sewage. Microorganisms have been shown in studies to have the largest reduction effect on total phosphorus under anaerobic conditions, with removal rates exceeding 80–90% [21]. Compared to the large operational and repair costs of conventional physical and chemical phosphorous removal systems and the complexities of certain biological treatment processes, the use of microalgae has proved to be a cost-effective and long-term alternative for biological phosphorus removal, which is now commonly utilized [22]. The following is an introduction to the microalgae dephosphorization technology mechanism.

2.1. Microalgae Culture Methods

There are two types of microalgae culture structures: open and closed systems. The term “open systems” refers to growing systems of outdoor waters, such as lakes and reservoirs. In order to provide adequate light for the microalgae, the system’s water depth is usually no greater than 0.5 m [23]. While the open system layout is simple and easy to manage, nutrients can become diluted by pollution when exposed to the open environment over a long period [24]. Closed devices are segregated from the external environment, thereby shielding the system from the harmful effects of the external environment. Photobioreactors (PBRs) are widely used as closed structures for microalgae cultivation. PBRs are usually classified into stubs and flat plates, and are generally made from glass or plastic, air, carbon dioxide (CO₂), and other gases may be fed into the PBRs [23,25]. On the basis of being highly controlled, the closed method can be used to evaluate the characteristics of microalgae and the effects on the purification of wastewater under different sets of conditions. It also provides a culture system for improving the conditions for the absorption of phosphorus by microalgae in urban wastewater.

Microalgae culture modes can be separated into continuous and semi-continuous batch modes [24]. A lot of the management costs are avoided in closed batches because the culture material does not always need to be replaced. The growth of the microalgae, however, will be inhibited if the nutrients in the batch system are depleted or if certain factors occur, such as cell self-shading, pH variations, and contamination, inhibiting the growth. In addition, the device must ensure a successful exchange of gas [23]. Compared to the batch model, the semi-continuous model can achieve higher biomass despite the need for periodic substitution of culture material and the continuous removal of wastewater [26].

2.2. Phosphorus Uptake and the Metabolism Mechanism of Microalgae

The absorption and metabolism of phosphorus by microalgae are often distinct for different types of phosphorus or under different environmental conditions. Microalgae can induce phosphatase to absorb external organophosphorus and synthesize high-affinity inorganic phosphorus transporters to assist inorganic phosphorus absorption [27]. The absorption of inorganic phosphorus also relies on the inorganic phosphorus charge and the pH of the microalgae cell membrane [28,29,30]. In general, the lower the molecular charge, the higher the bioavailability of inorganic phosphorus for the microalgae [28]. Most microalgae assimilate inorganic salts such as HPO⁻, HPO²⁻, and PO₄³⁻ [31].

Polyphosphates include acid-soluble and acid-insoluble polyphosphates. Although certain microalgae do not use polyphosphates as their primary supply of phosphorus [32], in the absence of phosphorus, microalgae can assimilate and metabolize polyphosphate [31]. In addition, under the condition of excess inorganic phosphorus, microalgae can take up excess phosphorus and deposit it in the form of insoluble polyphosphate acid, where it can be used for cell metabolism when inorganic phosphorus is lacking [33]. Excessive phosphorus and high light intensity in municipal wastewater tend to facilitate the removal of phosphorus by microalgae [31,34]; however, some studies have shown that excessive phosphorus can impede the growth of some microalgae due to excessive accumulation of polyphosphate in the cells [35].

Photosynthesis is the basis of the metabolism of microalgae. Over the entire photosynthesis process, phosphorus is required in the reaction that produces the energy substance ATP. The equation for this reaction is as follows:

2 H₂O + 2 NADP⁺ + 3 ADP + 3 P_i + 8–10 Photons (light) → 2 NADPH + 2 H⁺ + 3 ATP + O₂

To summarize, the electrons in the water are transferred to NADP⁺ after the absorption of light energy by the microalgae. H⁺ formed by water allows ADP and inorganic phosphorus to form ATP on the thylakoid membrane in the cell [31]. Phosphorus is also important for the synthesis of DNA, RNA, and cell membranes [36].

3. Factors Impacting the Elimination of Phosphorus from Municipal Wastewater by Microalgae

There are several internal and external factors influencing the removal of phosphorus by microalgae, such as the temperature, strength and period of illumination, and pH [31,40]. These common factors have been extensively studied and summarized, although a review regarding their impacts on the treatment of urban sewage by microalgae is lacking. In this chapter, the impacts of microalgae on the removal of phosphorus in municipal wastewater will be examined in terms of the hydraulic retention time, ratio of nitrogen to phosphorus, carbon dioxide concentration, species of microalgae, and different types of wastewater.

3.1. Hydraulic Retention Time

The hydraulic retention time (HRT) for microalgae in bioreactors influences their growth and phosphorous removal performance. An appropriate HRT not only improves the efficiency of the microalgae wastewater treatment but also reduces the operating and maintenance costs of the system. In the batch regime pilot scale photobioreactor system, activated microalgae sludge is used for the treatment of municipal sewage. Findings have shown that when the hydraulic retention time is 2–6 days, the phosphorous removal efficiency of the microalgae is improved by around 30% to 90% [41]; however, owing to the deterioration of the switch virtual interface (SVI) and other factors in the later stage, the phosphorous removal performance of the microalgae does not improve with the increase in hydraulic retention time [41]. Similarly, the optimum hydraulic retention time in the high-level algae pond also occurs on the sixth day when the bacteria and algae system is used to treat sewage [42].

The short HRT means that ammonia nitrogen ions in steam pools used for wastewater treatment cannot be fully nitrified [43,44,45]. By separating HRT and sludge retention times (SRT), a next-generation anaerobic–aerobic algal bioreactor was developed to solve the problem of inadequate HRT [44,45,46]. Toledo-Cervantes et al. [45] used a new form of photobioreactor involving hypoxia–aerobic algae to investigate the removal of phosphorus in water under varying hydraulic retention periods. Their findings revealed that when the HRT of the bioreactor decreased from 4 to 2 days, the removal rate of P-PO₄³⁻ decreased from about 22% to approximately 11%. A schematic diagram of the photobioreactor involving anoxic–aerobic algae–bacteria is shown in Figure 3. Similarly, when a mixed-culture microalgae membrane bioreactor was used to treat secondary wastewater from municipal wastewater, the optimal retention time was decreased to 1 day [39]. In general, the optimum hydraulic retention time for the removal of phosphorus by microalgae is approximately 6 days. By enhancing the treatment technology, such as the selection of a suitable bioreactor, the hydraulic retention time can be reduced, along with the running and repair costs of the treatment.

3.2. The Ratio of Nitrogen to Phosphorus

Nitrogen and phosphorus are important urban wastewater elimination indices, as well as important microalgae nutrient sources. An appropriate N/P ratio provides a good growth environment for microalgae and increases the phosphorous removal performance of the microalgae. The optimal N/P ratio range for microalgae development in freshwater ranges is 6.8 to 10 [47]. In the photobioreactor method, when microalgae are used to treat urban wastewater, the optimum N/P ratio range for the elimination of total phosphorus is 5–30 [48].

Molazadeh et al. [49] performed post-screening, biological treatment, and disinfection treatment of wastewater drained from sedimentation tanks of urban wastewater treatment plants. They controlled the N/P ratio and CO₂ concentration through the injection of potassium dihydrogen phosphate and used a high CO₂ concentration. The analyses showed that Chlorella vulgaris demonstrated strong potential to remove phosphorus under all concentrations of CO₂ and N/P, exhibiting a removal range of 70.0–96.0%. Under conditions with 16% CO₂ and a ratio of 10:1 N:P, algae biomass was the highest, with an increase in lipid productivity, which makes a powerful contribution to the eventual recovery of microalgae for biofuel [49]. To summarize, the reduction of total phosphorus from urban wastewater by microalgae is not only related to the required N/P ratio but is also directly proportional to the biomass of the microalgae [48]; however, as microalgae process urban waste, the feedbacks between the N/P ratio, the concentration of CO₂ in the water body, and the regulation of the optimum concentration remain unclear [49].

3.3. Carbon Dioxide Concentration

Carbon is the most fundamental component of living things. When microalgae are used to treat urban waste, the source of carbon comes not only from sewage but also from CO₂ in the air. Shanshan Ma [50] added 10% CO₂ mixed gas to unsterilized sewage to support Tetradesmus obliquus PF3 for the treatment of sewage and nutrient recovery. Compared to the addition of air, sewage with 10% CO₂ added shows greater TP (99 ± 0%) removal performance under unsterilized conditions. This is due to the increased supply of carbon and the high concentration of CO₂ changing the pH to an optimal growth range (6.8–7.8) [50]. Chaudhary et al. [51] used Chlorella ATCC13482 for the treatment of urban wastewater in bubble column photobioreactors at a volume of 7 liters. The findings revealed that the rate of microalgae orthophosphate elimination with 5% CO₂ air was as high as 92.8%.

Increasing CO₂ concentrations not only enhances phosphorous reduction by the microalgae but also increases the biomass of the microalgae. The higher the biomass of the microalgae, the higher the phosphorous removal performance [48]. Studies have shown that in the case of Nannochloropsis sp., where the concentration of CO₂ was 15%, the biomass of the microalgae and intracellular lipids was dramatically increased [52]. In general, high CO₂ concentrations not only boost the phosphorous removal performance of microalgae but also improve the lipid content of microalgae cells and increase the recovery value of the microalgae.

3.4. Species of Microalgae

The optimal growth conditions for each microalgae species are different, such that suitable algae species are chosen for different initial concentrations of urban waste in order to achieve the maximum benefit of phosphorous elimination; however, in experimental or practical applications, a single type of algae is rarely used to treat municipal wastewater.

By using mixed microalgae for the treatment of municipal wastewater, dominant algae species can be chosen on the basis of the sewage characteristics. For example, Toledo-Cervantes et al. [45] increased the rate of phosphate elimination from a water body from about 10% to around 50% by reducing the C/N ratio from 9 to 7. At the same time, Chlorella vulgaris, the dominant species of algae, was eventually replaced by Phormidium sp. [45]. This type of research approach can reliably and efficiently find appropriate microalgae for certain sewage treatment plants by screening the dominant algae species and adding mixed microalgae for realistic conditions.

More studies have shown that relative to single algae species in wastewater treatment, there is a cooperative or competitive partnership between mixed algae species, resulting in biodiversity and making the treatment system more stable and efficient. Paches et al. [53] performed batch and mixed cultures for four types of microalgae using anaerobic membrane bioreactors. Their findings showed that the mixed microalgae culture could increase the rate of phosphorous removal and the productivity of water by letting the species compete with each other [53]. Devi et al. [54] also revealed through their research that mixed microalgae showed a high degree of phosphorous elimination in wastewater and concluded that using mixed cultures was one of the better methods to handle municipal wastewater and other low-toxicity wastewater.

3.5. Different Municipal Wastewater Treatment Technologies

In a city sewage treatment facility, the municipal wastewater can be separated into three levels based on the extent of treatment, ranging from primary wastewater treatment, where the wastewater has not yet been deeply treated, up to tertiary wastewater treatment, in which the wastewater is at the final cleaning process. Although primary wastewater exhibits several negative factors, such as high optical density (OD) and bacterial contamination, the concentration of nutrient species in primary wastewater is much higher than in other wastewater treatment levels, making it more favorable for microalgae development [50,55]. Secondary wastewater is partially treated and most of the nitrogen at this stage is available as nitrate due to nitrification. This is a negative factor for microalgae, as microalgae preferentially absorb nitrogen in the form of ammonia [50]. Bellucci et al. [56] used microalgae to treat secondary wastewater and evaluated the combined function of microalgae as a disinfectant and nutrient remover. Their findings revealed that the microalgae contributed to an E. coli count equal to that of standard ultraviolet therapy in the batch disinfection test, and that the count was smaller than that of light experiments without microalgae. The E. coli population decreased by an order of magnitude in subsequent continuous studies. The rate of elimination of total phosphorus in the secondary wastewater was 100%. In addition to the level of municipal wastewater treatment, the forms of municipal wastewater can also be categorized according to the special new wastewater created by the treatment process. Various processes and techniques are used to treat different forms of wastewater with microalgae, as shown in

Table 2. Phosphorus removal effects of microalgae on municipal wastewater samples treated using different technologies.

Sewage Source	Microalgal Species	Initial Conditions	Experimental Conditions	Results	Cites	Notes
General municipal sewage	Mutant Chlorella	After 121 °C autoclave treatment CODCr: 190–230 mg/L TP: 4.5–5.6 mg/L TN: 40–60 mg/L NH3-N: 20–35 mg/L pH: 6.6–7.6	a symbiotic system of PAOs and bacillariophyta	absorb 3.05 mg/L phosphorus, keep TP below 0.46 mg/L	[69]
Synthetic domestic wastewater	Chlorella vulgaris and Phormidium sp.	COD: 632 ± 45 mg/L TOC: 196 ± 9 mg/L IC: 195 ± 12 mg/L TN: 43 ± 3 mg/L N-NH4+: 24 ± 3 mg/L P-PO43−: 13.1 ± 0.8 mg/L	Anoxic–aerobic algal–bacterial photobioreactor structure	the maximum removal rate of P-PO43− was 47 ± 5%	[45]	low C/N ratio, Chlorella is the main algae, otherwise Phormidium SP will be dominant
	Chlorella vulgaris	COD: 300 mg/L TN: 30 mg/L TP: 10 mg/L	the new MAIFAS SBR	more than 51% phosphorus was removed without mechanical aeration	[70]
Aerobic wastewater	Mixed microalgae collected in lakes	pH: 7.7 ± 0.2 TN: 99.5 mg/L TP: 5.5 mg/L COD: 475 mg/L TOC: 245.6 mg/L pH: 7.2	Photoperiod:12 h/d, immobilized microalgae, operated at 5 different HRTS for 2–10 days	the removal rate of phosphorus was 93%	[71]	collected in an aeration tank of a distributed domestic sewage treatment plant based on ASP
Unsterilized sewage	Tetradesmus obliquus	N-NH4+: 28 mg/L	the mixed gas containing 10% CO2 was added to the unsterilized sewage	The removal rate of TP was 99.0%	[50]
Anaerobic digester	Chlorella sp. and Scenedesmus sp.	COD: 12600 ± 300 mg/L TKN: 1692 ± 256 mg/L NH3-N: 900 ± 62 mg/L NO3-N: 0.13 ± 0.02 mg/L TP: 105 ± 7.5 mg/L PO4-P: 64 ± 6 mg/L TSS: 15880 ± 932 mg/L pH: 9.00–9.15	in an adaptive room with continuous illumination: 150 mol photon M−2 S−1, 25 ± 2 °C. cultured at a dilution ratio of 2%, 5%, 7% and 10%	reaction rate coefficient: 0.21 mg PO4-Pmg−1 CHl a day−1, saturation constant:2.94 mg L−1, yield coefficient: 5.03 mg CHL A mg−1 PO4-P	[60]
	Chlorella	activated sludge: COD: 500 mg/L; NH4+1-N: 40 mg/L; NO3–-N: 2 mg/L; PO43−-P: 8 mg/L	treated in a membrane photobioreactor (MPBR) in a continuous mode	the removal rate of orthophosphate exceeded 99%	[72]	schematic diagram is shown in Figure 4
		anaerobic digester: COD: 5–10 g/L; NH4+1-N: 0.7–1.2 g/L; NO3–-N: 90–300 mg/L; PO43−-P: 60–190 mg/L
Ozonation sludge wastewater	Scendesmus sp. is the dominant species	MLSS: 1500 mg/L algae: sludge = 1:3 (w/w).	run for 10 days, under 2500 lx on the inner wall of the reactor, photoperiod: 12 h/d (from 5:00–17:00), magnetic stirring rod (80 RPM)	the removal rate of TP was 53.9 ± 1.4%, higher than microalgae alone	[64]	sludge is obtained from secondary sedimentation tanks
Secondary wastewater from sewage treatment plants	Natural algal bloom (Chlorella mainly)	TP: 0.43 mg/L TN: 7 mg/L Mg: 0.45 mM Ca: 1.12 mM	continuous bubbling tower photobioreactor (BCPBR), flocculation–precipitation method	the removal rate of total dissolved phosphorus was greater than 99% under continuous operation	[73]
Secondary wastewater from sewage treatment plants	Chlorella	COD: 111 mg/L pH: 7.9 ± 0.9 NH3-N: 22 ± 2.6 mg/L NO3-N: 0.30 ± 0.42 mg/L PO4−3-P: 3.2 ± 1.3 mg/L Turbidity: 184 ± 23 FAU E. coli: 4.7 × 106 ± 3 × 106 CFU 100 m/L	laboratory-scale photobioreactor, 10% of the effluent mixed with secondary effluent from a large municipal wastewater treatment plant, tertiary disinfection by ultraviolet treatment	The removal rate of TP was 100%	[56]
Sewage discharged from sedimentation tanks of municipal wastewater treatment plants	Common Chlorella	NH4+1-N: 64.84 mg/L NO3−1-N: 4.21 mg/L PO4−3-P: 3.78 mg/L COD: 82.00 mg O2/L pH: 8.52 Alkalinity: 91.80 mg CaCO3/L	Through the different concentrations of CO2 and different N/P ratios	Absorbance of 95.00% phosphorus for the medium supplemented under 16% CO2 and N:P ratio of 10	[49]	the wastewater was screened, biotreated, and disinfected.
Synthetic wastewater from municipal wastewater and laterite nickel mine	Chlorella	The two types of sewage were mixed in different proportions	temperature: 25 °C, light intensity: 4000 lux, Photoperiod:14 h/d, sterilized before experiment, added after sampling high-pressure deionized water of the same volume 6 times	The removal rate of TP was 39.3%	[74]
Primary sedimentation tank wastewater	Chlorella	NH4+1-N: 25 ± 1.24 mg/L TKN: 42.0.47 mg/L NO3-N: 2.5 ± 0.39 mg/L sCOD: 156 ± 2.6 mg O2/L pH: 6.7 ± 0.05 DO: 3.5 ± 0.08 mg/L sBOD: 65 ± 3.4 mg/L TOC: 45.3 ± 1.12 mg/L TIC: 1.24 ± 0.07 mg/L TN: 46 ± 1.25 mg/L	carried out in a 7 L bubbling photobioreactor, temperature: 25 ± 2 °C, Photoperiod:14 h/d, pumped into the air with different concentrations of CO2	the removal rate of orthophosphate was 92.8% under 5% CO2 (v/v) for 7 days	[51]
Settlement of sewage	Mixed algae	/	Wastewater Treatment and Resource Recovery (STaRR) system	the phosphorus recovery content was 71.6%	[75]

.

Anaerobic digestion is a mechanism that converts polluted waste to energy materials; however, this method volatilizes harmful gasses such as high-viscosity, high-moderation, and highly volatile fatty acids. In addition, these reactive compounds are also poisonous to plants and microalgae. Products from untreated anaerobic digests cannot, therefore, be released immediately into nature [57]. The immediate discharge of untreated anaerobic digestive fluid into bodies of water can cause eutrophication [57]. The integrated technology of using microalgae to treat digested products is a technology that can offer economic gains, while also being environmentally sustainable [58]. In regards to the uptake of nitrogen and phosphate from wastewater, algae have demonstrated higher removal efficiency than other microorganisms [59]. Ermis et al. [60] used an experimental batch sequencing device to investigate the use of mixed microalgae in the treatment of anaerobic liquor digestion. The digestive juice was diluted to 2%, 5%, 7%, and 10%, so that the original concentrations of ammonia nitrogen and phosphate in the digestive juice were regulated at 18.6–87.1 mg L⁻¹ and 1.85–6.88 mg L⁻¹, respectively. It was found that the absorption of nitrogen by mixed microalgae was 10 times greater than that of phosphorus. Based on a biokinetic coefficient of the phosphorus measurements, the reaction rate coefficient was 0.21 mg PO₄-P mg⁻¹ chl a day⁻¹ and the saturation constant was 2.94 mg L⁻¹, with a yield coefficient of 5.03 mg chl a mg⁻¹ PO₄-P.

The main goal of treating eutrophic water bodies is to remove organic and inorganic compounds from the wastewater. Nitrogen and phosphorus, however, are not readily eliminated [61]. The utilization of photosynthetic–autotrophic digestion by microalgae means that CO₂ or inorganic carbon in the water or air can be used as a carbon source and source of energy. Autotrophic microalgae release extracellular organic matter (EOM) that converts inorganic carbon to organic carbon, increasing the concentration of organic carbon in the water [62]. While microalgae do not specifically remove organic matter from eutrophic water sources, they can be mixed with bacteria and other heterotrophic microorganisms to treat bodies of water with high amounts of organic matter; this specific topic will be detailed in the next chapter.

Sludge ozone technology can not only degrade several refractory organic compounds so that the production of the sludge can be decreased to 50–100%, but no harmful by-products are generated during the application of this technology [63,64]; however, owing to the high concentrations of nitrogen, ammonia, COD, and heavy metals in the excess ozone sludge, an additional burden is placed on the sewage treatment plant, which decreases the effectiveness of the sewage treatment [64,65,66,67,68]. Lei et al. verified the possibility of growing algae in sludge-concentrated wastewater ozone. While generating biomass, the microalgae can also extract nutrients from the water [64]. Their findings revealed that the bacteria–algae system had greater elimination effects in terms of total phosphorus removal than the pure microalgae system, with the systems showing 93% and 53.9% elimination effectiveness, respectively.

4. Research Status Analysis of Phosphorus Removal from Municipal Wastewater by Microalgae

4.1. Symbiotic Systems of Bacteria and Algae

The treatment of wastewater by pure microalgae is usually limited to laboratory conditions, while sterile water is difficult to find in sewage treatment plants. Microalgae often work with endophytic bacteria to purify the wastewater [76]. There are high concentrations of activated sludge in some urban sewage treatment plants. Urban sewage provides a culture substrate for microalgae, which can reduce the high costs associated with microalgal artificial culture medium. Cultures of microalgae can also be mixed with heterotrophic microorganisms in activated sludge to meet the sustainability requirements for urban sewage purification [77]. Bacteria and algae can form a good symbiotic relationship [78], enhancing the effects of the microalgae in the purification of urban sewage. For the treatment of municipal wastewater and industrial wastewater, there is a trend of combining selected algal and bacterial species [79,80]. For example, a culture made up of C. vulgaris and P. putida can remove organic matter and other nutrients and shows good performance in synthetic municipal wastewater [81,82]. Lananan et al. [83] co-cultured Chlorella and effective microorganisms (EM-1), and their findings revealed that this mix could extract 99.15% of the total phosphorus from domestic sewage. Qing et al. [69] screened Klebsiella from activated sludge and treated municipal wastewater with C. pyrenoidosis. Their findings revealed that the phosphorous microbe not only boosted the phosphorous absorption performance of the microalgae (up to 3.05 mg/L), helping to regulate the total phosphorous concentration in the water to 0.46 mg/L, but also increased the lipid yield and the average productivity of the microalgae (90.1% and 13.6%, respectively).

4.2. Adding Metal Compounds

Magnesium ions (Mg²⁺) are some of the most essential components for microalgae photosynthesis. P in wastewater can be removed by trimagnesium diphosphate (Mg₃(PO₄)₂) and MgNH₄PO₄ precipitation with other ions such as NH₄⁺-N and Mg²⁺. The assimilation of PO₄³⁻-P could be hindered to some degree under Mg²⁺ deficiency [54,84,85,86,87]. Studies have shown that the development of C. vulgaris is inhibited in media without Mg²⁺, whereas microalgae grown in media with Mg²⁺ are four times more productive than the blank group [88,89]. The concentration of Mg²⁺ has a significant influence on the metabolism of microalgae in urban wastewater treatment [59]. Nickel laterite ore wastewater (NLOWW) provided by the hydrometallurgical recovery of the nickel contains high concentrations of Mg²⁺ in the range of 20–40 g L⁻¹ [74]. Conventional NLOWW treatment for recovery of Mg²⁺ consists of a series of integrated chemical–physical processes requiring investment in equipment and chemicals that are energy intensive and produce solid waste requiring further treatment [90]. Chen et al. [74] mixed urban and lateral nickel ore wastewater to cultivate C. sorokiniana. Their findings showed that the growth of microalgae cells in a culture without nickel laterite ore wastewater was slower and had a low biomass yield, whereas the microalgae biomass production rate in mixed wastewater containing nickel laterite ore increased by 1.89 times, the photosynthetic activity (Fv/Fm value) increased by 3.77 times, and the phosphorus removal rate increased by 39.3%; however, for 100% nickel laterite ore wastewater, excess Mg²⁺ can contain high amounts of reactive oxygen species, which inhibit the growth of microalgae.

As an essential micronutrient for the growth of algae, iron ions also play an important role in the physiological synthesis and enzymatic reactions of algae. Iron can coordinate active oxygen in algal cells and take part in electron transport, enzyme reactions, photosynthesis and respiration, and the synthesis of proteins and nucleic acids, and can promote the metabolism and absorption of nutrients [91,92]. As mentioned earlier, phosphorus is an important nutrient for synthesizing cell proteins and nucleic acids. Qiu et al. [93] compared the effects of various forms of iron on the growth of Anabaena flos-aquae, and the results revealed that ferric ammonium citrate, EDTA-Fe, iron ions, and ferric oxalate are the forms of iron that can stimulate the development of microalgae. When the iron concentration was regulated in the range of 0.1 mg L⁻¹ to 0.8 mg L⁻¹, the impact of the iron type on microalgae growth was still greater than that of the iron concentration.

4.3. Biofilm Technology

In the 21st century, several wastewater treatment plants in the United States found that combining mobile bed bioreactor and fixed-membrane-activated sludge technologies not only enhanced the wastewater nitrification technology but also reduced the footprint of the facilities [70,94]. The integrated fixed-membrane-activated sludge process is an innovative biological wastewater treatment process that incorporates biofilm carriers into conventional activated sludge to eliminate nitrifiers, resulting in an improved retention time for the heterotrophic bacteria [70]. The nitrifying bacteria can be applied to the biofilm without being affected by the washing of the nitrifying agent, while the biological nitrogen can be eliminated by the nitrifying reaction [70]. Compared to the moving bed bioreactors (MBBR) system, the Integrated Fixed-Film Activated Sludge (IFAS) system can decouple the SRT of nitrifiers and polyphosphate bioaccumulators (PAO) by maximizing the elimination performance of biological nitrogen and phosphorus [70].

Jared Church et al. [70] incorporated microalgae into an optimized fixed-membrane-activated sludge configuration for photooxygenation and examined the symbiotic reactions of microalgae and bacteria to suspended matter and IFAS biofilms. In sequential batch mode, the microalgae were combined with the IFAS method to remove 51% of the phosphorus without mechanical aeration. This study also showed that the addition of microalgae to the IFAS system modified the metabolic function of multiple bacterial populations. This study was not only desirable for the reduction of phosphorus in water sources, but it also offers new research ideas for the improvement of various water bodies, the use of microalgae–IFAS technologies to modify the behavior of bacterial species, and the evolution of water quality.

Abeysiriwardana-Arachchige et al. [75] suggested a research approach for the treatment and recovery of wastewater based on algae (STaRR). Their findings revealed that the STARR device had a recovery output of 71.6% of nutrient phosphorus and that the removal of phosphorus per unit of energy consumption was calculated to be 0.1 g/kJ. This indicates that the STaRR system could be a green alternative for water treatment and nutrient recovery.

Anaerobic membrane bioreactor technology has the benefits of absorbing less energy and producing less sludge relative to more conventional aerobic systems, while still producing biomethane.; however, inorganic contaminants such as nitrogen and phosphorus cannot be removed from anaerobic reactors. Other nitrogen-containing compounds convert ammonia, which increases the concentration of ammonia in the water, one of the major microalgae nutrients [53,95,96]. Microalgae, thus, play an important role in the production of this system. Microalgae demonstrate good growth conditions in 5–30% nitrified digestion solutions combined with municipal wastewater. For example, the addition of a 10% nitrification solution for digestion in a two-stage bacterial-microalgal phase can eliminate 77% of the phosphate. Under the same conditions, the continuous use of a microalgae-based photobioreactor (MPBR) membrane will extract more than 99% of the phosphate [72].

4.4. Recovery Technology

From an economic and sustainable development point of view, the recovery of microalgae is a significant link in the treatment of wastewater by microalgae [97,98]. Currently, more mainstream approaches for the recovery of microalgae include centrifugation, filtration, sedimentation by gravity, and flocculation [98,99]. In addition, flocculant recovery technology and immobilized recovery technology are used.

4.4.1. Flocculant Recovery Technology

Microalgae can also be flocculated and retrieved by inorganic coagulants, such as aluminum sulfate or ferric chloride, polymeric flocculants, mixtures of the two components, or by using automated chemical flocculation methods such as pH modification [100,101,102]. Mennaa et al. [73] researched the continuous activity of the BCPBR (bubble column photobioreactor) using a flocculation–precipitation system to examine the effects of phosphorous removal and recovery of natural microalgae plants in urban wastewater. Their findings showed that continuous-mode experiments extract up to 99% of the total dissolved phosphorus without controlling the volume of CO₂ or regulating pH. PAC, Fe₂(SO₄)₃, and Al₂(SO₄)₃ were found to have positive effects on the recovery of microalgae; however, the addition of a flocculant not only increases the cost, but also may cause other pollution to the water body, reducing the sustainability of urban sewage.

4.4.2. Immobilized Recovery Technology

Compared with flocculant recovery technology, immobilized recovery technology can avoid secondary water pollution and increase the possibility of the sustainable development of urban sewage. The elimination of sewage from suspended algae systems can result in a low concentration of algae in the reactors, resulting in a reduction in the treatment rate [33,103,104]. The use of alginate beads to immobilize microalgae cells helps to retain a high concentration of microalgae in the reactor, which can easily remove nutrients from the water body, while the hydraulic retention period is less than 12 h [80,105]. Immobilized microalgae beads can settle rapidly to promote screening and regeneration, and these beads can be used directly as fertilizer or biomethane production after digestion [106,107,108]. In addition, beads produced by immobilized microalgae can also shield the culture from harmful contaminants in wastewater [109]. Kube [110] demonstrated that different concentrations of nitrogen and phosphorus can influence the absorption of phosphorus by Chlorella and that the immobilization of microalgae does not hinder the rate or ratio of nitrogen and phosphorous absorption. In addition, cell immobilization and co-cultivation of Bacillus vulgaris and P. brasiliensis can boost the removal rates of ammonia and phosphorus [111].

Katam et al. [71] used mixed microalgae to investigate the removal efficiency of carbon and nutrients in the treatment of real wastewater in an activated sludge reactor and set up two separate treatment systems for the simultaneous treatment of domestic wastewater. Their findings revealed that the total phosphorus removal performance of the immobilized microalgae system was as high as 93%, which was higher than the suspended activated sludge system. In addition, the microalgae developed higher lipid and carbon contents than the suspended activated sludge solution in the immobilized microalgae system.

4.5. Other Improved Technologies

Photobioreactors used to grow microalgae also have a major effect on the treatment of urban wastewater by microalgae. For example, relative to other reactors, BPBR has the advantages of high heat and mass transfer speeds, compact construction, and low operational and maintenance costs. Since the reactor has a higher surface-to-volume ratio, good mixability, lower shear stress, high scalability potential, simple sterilization, low emissions, and decreased photoinhibition, it can better monitor the growth parameters (such as temperature) of photooxidation [112].

The microalgal elimination of phosphorus can also be improved via genome building. Guerra-Renteria et al. [113] developed a genome-scale biochemical reaction network for the co-cultivation of Chlorella spp. and Pseudomonas aeruginosa bacteria using a metabolic pathway analysis (MPA). This analysis considers the metabolic ability of co-cultivation and determines the best conditions for the removal of nutrients. The theoretical phosphorous removal yield under photoheterotrophic conditions was determined as follows: 0.042 mmol of PO₄^3– per g DW of C. vulgaris, 19.43 mmol of phosphorus (Pi) per g DW of C. vulgaris, and 4.90 mmol of phosphorus (Pi) per g DW of P. aeruginosa. These theoretical yields are important because they can help in the design of biological systems and in the understanding of the theoretical requirements of oxygen and carbon dioxide in order to achieve maximum nutrient absorption. In this system, other by-products containing nitrogen or phosphorus may not even be formed, and all nutrient absorption is directed toward the growth of microalgae and bacteria [113].

5. Conclusions and Perspectives

While microalgae-based technologies provide a sustainable alternative for the removal of phosphorus from urban wastewater, the substitution of conventional water treatment technologies remains a major challenge. In this paper, we present the microalgae culture methods and the microalgae dephosphorization process. In addition, considerations influencing the elimination of microalgae phosphorus in urban water include traditional factors, but also the species characteristics of the microalgae and urban sewage types. This study, however, cannot completely summarize all of the factors influencing the dephosphorization of microalgae. It is necessary to further study the mechanisms and factors impacting microalgae dephosphorization from a microscopic perspective.

The study of phosphorous removal by microalgae showed that microalgae were often combined with other municipal wastewater treatment systems. Symbiotic relationships between bacteria and microalgae are common in municipal wastewater treatment plants. Biofilm has commonly been used in the treatment of sewage in the 21st century, and its combination with microalgae has encouraged the elimination of phosphorus from wastewater. While the microalgae biomass may be improved and phosphorous removal efficiency may be increased by increasing the concentrations of magnesium and iron ions in water, the mechanism for the removal of phosphorus by the inclusion of certain metals is less studied. In addition, certain metal nanomaterials have good adsorption and other characteristics, although the study of metal nanomaterials on microalgae is still lacking. This study also shows that immobilized microalgae technology can not only solve the problems of microalgae recycling and urban sewage sustainability, but can also improve the efficiency of phosphorus removal. This immobilization technology also offers a research concept to solve secondary contamination caused by the addition of metal ions to support the growth of microalgae.