Comparison of the Advantages and Disadvantages of Algae Removal Technology and Its Development Status

Abstract

In recent years, the frequent outbreaks of cyanobacterial blooms have caused severe water pollution in many rivers and lakes at home and abroad, endangering drinking water safety and human health. How to remove cyanobacteria from water bodies safely, quickly, and economically has attracted the attention of many scientists. Currently, the typical treatment methods for algae in algae-bearing water bodies are physical, biological, and chemical methods. The physical method of algae removal is for both the symptoms and the root cause, but the workload is extensive, with high input costs, and should not be used on a large scale. The biological method is low-cost, but the removal efficiency is slow and unsuitable for the treatment of sudden water bloom. The chemical method can kill algae quickly, but it is easy to cause secondary pollution. These methods are relatively independent of each other, so the choice of a practical combination of technologies is essential for algal bloom removal and eutrophication management. This paper reviews the current application status and advantages and disadvantages of algae removal technologies at home and abroad; classifies them from physical, chemical, biological, and combined methods; and provides an outlook on the future development direction of algae removal technologies.

1. Introduction

As more and more wastewater is discharged into water bodies around the world, nitrogen (N) and phosphorus (P) levels in water bodies increase dramatically, exceeding the self-purification level of the environment itself, resulting in algal microbial blooms; lower DO; higher turbidity and pH; the disappearance of aquatic plants, especially submerged plants; and mass mortality of fish, ultimately leading to the eutrophication of freshwater resources and the collapse of aquatic ecosystems [1,2]. According to the total phosphorus (TN), total nitrogen (TP), and chlorophyll a (Chl-a) concentrations and transparency, the degree of eutrophication of water bodies can be classified as, poor, light, medium, medium-rich, heavy, and exceptionally eutrophic [3]; see

Table 1. Water body eutrophication degree division [3].

Nutritional Grading	Total Nitrogen (mg·L−1)	Total Phosphorus (mg·L−1)	Chlorophyll (mg·m3)	Transparent (cm)	Nutritional Status Evaluation
Poor nutrition	0.6	0.03	1.6	70	Excellent
Light enrichment	1.7	0.06	10	40	Good
Medium nutrition	2.2	0.09	26	30	Light pollution
Medium-rich nutrition	3.2	0.12	64	30	Moderate pollution
Heavy eutrophication	4.2	0.18	160	30	Severe pollution
Exceptional eutrophication	6.2	0.28	160	25	Extreme pollution

. Studies have shown that total nitrogen and total phosphorus are vital factors in inducing cyanobacterial water outbreaks [4]. When TN and TP exceed 0.5 mg·L⁻¹ and 0.02 mg·L⁻¹, respectively, they can promote water bloom formation.

With the increasing eutrophication of lakes, especially the increase in phosphorus concentration, cyanobacteria significantly dominate the phytoplankton community succession. Under the environment of global warming and the yearly increase in CO₂ and other greenhouse gas emissions, the scale and frequency of cyanobacterial blooms are expanding, which has attracted extensive attention and research from scholars at home and abroad. The outbreak of cyanobacterial blooms has caused many severe threats to human life and health, production, and living [5]. Visually, there is a reduction in water clarity and a deepening of color and turbidity [6], and olfactively, the water body emits a strong and irritating odor. In addition, the proliferation of cyanobacteria has led to the continuous deterioration of the water quality environment and eventually causes the collapse of freshwater ecosystems [7]. In waters with cyanobacterial bloom outbreaks, the dissolved oxygen (DO) levels in the water column are extremely low and the concentrations of cyanobacterial toxins (MCs) extremely high, and the biodiversity is severely damaged.

The current methods of removing cyanobacteria are mainly divided into chemical, physical, and biological methods, but they are relatively independent of each other [8,9]. In the face of large-scale cyanobacterial blooms, a single technology can provide a certain degree of removal, but there are some obvious drawbacks, and the effectiveness of different methods varies widely. Therefore, how to choose an effective algae removal technology to achieve the highest algae removal rate has become a key and difficult problem to be solved. In this paper, the advantages, disadvantages, and current status of various algae removal techniques are reviewed from physical, chemical, biological, and combined methods to provide relevant references for their subsequent research on algae removal techniques.

2. Status of Algae Removal Technology

The current basic measures to prevent and control cyanobacterial blooms are shown in Figure 1, and the specific steps are as follows: source control and pollution interception to reduce the input of exogenous pollutants from textile, paper, and chemical industries; timely dredging, using excavators and other equipment to remove silt from the water bottom and reduce endogenous pollutants such as carbon, nitrogen, and phosphorus; then, using chemical methods to substantially remove nitrogen, phosphorus salts, and algae density, and planting aquatic plants such as calamus, water lettuce, and Myriophyllum aquaticum to give full play to the restoration effect of biological methods, thus effectively avoiding the occurrence of cyanobacterial water blooms. In addition, the research of combined algae removal technology and advanced oxidation technology is still at an early stage, and its applicability in cyanobacterial water bloom treatment has to be evaluated gradually.

2.1. Physical Algae Removal Technology

The most commonly used techniques for algae removal by physical methods include mechanical methods, shading technology methods, air flotation, clay flocculation, ultrasonic methods, filtration, ultraviolet irradiation, and adsorption. The principle of the mechanical method is to use a power device to gather and salvage the algae in the lake, to quickly obtain a large amount of algal biomass and rapidly reduce the concentration of algae in the water body, which is one of the commonly used treatment means, but due to the high water content of the salvaged algae, it increases the difficulty of subsequent treatment and utilization, consumes a lot of manpower and material resources, and is difficult to implement on a large scale [11]. The shading technology method is mainly carried out by laying shading panels or shading cloth above the water surface to prevent the photosynthesis of algae to achieve the purpose of controlling algal blooms, but the technology is only applicable to the watershed area of small water bodies, since the watershed area of larger water bodies will cost a lot of human, material, and financial resources, so it has certain limitations.

Air flotation is a method of solid–liquid separation by using algal flocs attached to tiny bubbles to float upward, which has a good removal effect on the algal solution with low concentration and low turbidity [12,13]. However, because the algae cells are negatively charged and have the same charge as the bubbles, it is not conducive for the algae cells to attach to the bubbles to produce inhibition, and the actual algae removal rate is difficult to reach 90%. Yap et al. [14] used a novel posiDAF dissolved air flotation process to modify the bubbles by the cationic polymer polyDMAEMA to make the bubbles positively charged and to remove algal cells up to 95% efficiently. As a natural and nontoxic substance, algae can be removed by flocculation and precipitation of clay, but this technique may cause water blooms to erupt again. Modified clay flocculation and sedimentation are ecologically fast and safe for algae removal, but there are disadvantages such as large clay dosage, high cost, and an unstable effect when using this technology alone. Ahmad et al. [15] explored the optimal conditions for the removal of algal cells by chitosan and found that the removal rate of algal cells reached 90% after 20 min of sedimentation at a chitosan concentration of 10 mg·L⁻¹, 20 min of stirring time, and 150 r·min⁻¹ of stirring speed.

The ultrasonic method is a physical algae removal method developed in recent years, generally referring to the frequency of 20,000 Hz or more elastic mechanical waves, mainly using mechanical forces and cavitation effects generated by shock waves, high temperature, and pressure, jets, etc., to destroy the algal cell photosynthetic system and biological activity [16,17]. The ultrasonic method can further decompose algae cell secretions and algal toxins as well as other metabolic products; with good algae removal effects, simple installation and maintenance, fast removal speed, green environmental protection, and other advantages, it has a broad market prospect, but when ultrasonic algae control is used, the greater the ultrasonic intensity, the higher the energy consumption, the worse the economy, and the aquatic organisms are easily affected. Rajasekhar et al. [18] confirmed that the results showed that a 20 kHz ultrasound could kill algal cells rapidly and prolonged treatment can lead to exposure of microcystin, but ultrasound can remove microcystin simultaneously and selectively remove algae. Song et al. [19,20] demonstrated that microcystin can be rapidly degraded when the ultrasonic frequency is 640 kHz and that the microcystin (MC-LR) degradation pathway is the oxidation of the benzene ring by hydroxyl radicals and the diene bond on Adda peptide residues. In addition to this, the Mdha alanine peptide bond was also broken, probably by pyrolysis triggered by high temperatures generated at the cavitation site. Therefore, the main mechanism of ultrasonic degradation is hydroxyl radicals, and hydrolysis/cleavage processes are also present.

Filtration methods generally include sand filtration and membrane filtration, which can remove algal cells intact without breaking the cells [21]. Sand filtration is usually used after coagulation and sedimentation or air flotation processes and is suitable for raw water with low concentrations of suspended matter and algae cells; raw water with high algae cell concentrations may clog or infiltrate the filter bed. The membrane filtration process is based on the selective permeation membrane as the separation medium, with the pressure difference or concentration difference driving the raw material side components selectively through the membrane, thus achieving the purpose of separation or purification. Among them, pressure-driven membrane filtration processes mainly include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). Among them, nanofiltration and reverse osmosis are high-pressure membrane filtration processes, which can effectively retain not only microorganisms and organic matter in water, but also ions and water-soluble salts in water to a large extent. This makes the water lack some beneficial trace elements and hardness and alkalinity, thus making it unsuitable for long-term drinking [22,23]. In addition, the high water purification efficiency of nanofiltration and reverse osmosis comes at the cost of correspondingly high operational energy consumption [24] and is not suitable for widespread application in water supply treatment. Microfiltration and ultrafiltration are both low-pressure membrane filtration processes with marketable material prices and operating costs, and have broad application prospects. However, microfiltration cannot completely retain pathogenic bacteria and viruses. Ultrafiltration technology, which is in the middle of microfiltration, nanofiltration, and reverse osmosis, makes good use of their respective advantages, and its membrane pore size is moderate, between 0.001 µm–0.02 µm, and its molecular weight is usually in the range of 1000–300,000 µm [25], so it can play a good role in the removal of some large molecules. MF and UF are effective in removing algal cells and microcystin [26]. Campinas et al. [27] achieved 93–98% removal of MCs by powdered activated carbon adsorption/ultrafiltration (PAC/UF) for the removal of microcystin.

Ultraviolet (UV) irradiation can cause significant damage to the genes, cell membrane integrity, and photosynthetic capacity of algal cells, as well as inhibit the production and release of toxins and increase the sedimentation capacity of algal cells [28,29]. Alam et al. [30] irradiated Microcystis aeruginosa with UV light at a wavelength of 254 nm and found that algal cell growth was significantly inhibited for 7 d when the UV intensity was 37 mJ·cm⁻². When the irradiation intensity increased to 75 mJ·cm⁻², all algal cells died. Further probing revealed that UV significantly disrupted the photosynthetic system of the algal cells, causing severe damage to DNA strands and impeding protein transcription and synthesis.

The adsorption method of algae removal is usually adsorption by activated carbon; powder activated carbon (PAC), granular activated carbon (GAC), and activated carbon fiber (ACF) are commonly used [31,32]. Activated carbon has a huge specific surface area and complex pore structure, which can easily adsorb organic substances with relative molecular masses between 500~3000 µm. Microcystins have a molecular weight close to 1000 µm and are easily adsorbed. Activated carbon is usually used as a coagulant in algae removal. Powdered activated carbon is added to strengthen the effect of coagulation and algae removal, while adsorbing microalgae, which is suitable for emergency treatment of high-algae water. Jiang et al. [33] developed an “activated carbon-membrane bioreactor” (PAC-MBR) to treat mildly eutrophic water bodies. After 4 months of continuous operation, it was found that algae, humic acid substances, and ammonia nitrogen (NH₄⁺-N) were effectively removed from the water.

The mechanism of action, advantages, and disadvantages of physical algae removal techniques are shown in

Table 2. The mechanism of physical algae removal technology and its advantages and disadvantages.

Method Categories	Mechanism of Action	Method Advantages	Method Deficiency
Mechanical method	Collection of water by algae removal equipment; algae accumulation in the domain	Emergency management of harmful algal bloom outbreaks	Large manpower consumption, suitable for small areas of water
Shading technology method	Laying shade cloth or shade board to stop algae photosynthesis	Low impact on the ecosystem	Suitable for water bodies with small watershed areas
Air flotation method	The combination of tiny bubbles and algal flocs makes the flocs follow the bubbles to float	High separation efficiency	Large amount of flocculant needs to be added
Clay flocculation method	Reduction in algal blooms on the surface of water bodies mainly by flocculation and sedimentation	Little pollution to the environment; easy to operate	It is not possible to prevent and control the reoccurrence of water bloom outbreak and algal bloom, generally as an emergency treatment measures
Ultrasonic method	Reciprocal movement to damage and rupture algal cells through physical action	Fast algae removal without additional chemicals, safe and harmless; simple operation technology and control method	Easy to lead to the release of intracellular organic matter; high cost and limited use
Filtration method	Algae removal using micromesh filtration	Complete removal of algal cells without cell rupture.	The cost is high, the technology is more complex, and there are secondary pollution problems
UV illumination method	UV light irradiation induces algae; degeneration of cytogenetic material and alteration of algal activity	No need to add other drugs; no secondary contamination	The device is complex and difficult to apply in practice
Adsorption method	Utilizing the huge surface area and complex pore structure of activated carbon to absorb microcystin molecules	Easy to operate	Higher dosage, difficult recovery, and higher cost

.

2.2. Biological Algae Removal Technology

The biological method uses the food chain relationship between organisms in the natural ecosystem and the principle of mutualism to inhibit the growth of algae and change the structure of biological communities, thus achieving the purpose of controlling and killing algae and restoring the health of water bodies [34]. The biological method has the advantages of simple operation, obvious effect, and little impact on the environment, and it plays an important role in the material cycle of the ecosystem [35,36]. At present, it mainly includes microbial action, aquatic plant action, and aquatic animal action, and the advantages and disadvantages of biological algae removal technology are shown in

Table 3. The principle of action and advantages and disadvantages of biological algae removal technology.

Method Categories	Mechanism of Action	Method Advantages	Method Deficiency
Microbial action	Inactivation of algae by microorganisms such as specific pathogenic bacteria and viruses	Specific strains of algae removal with high specificity	Not universal, need to strictly control the living environment of microorganisms
Aquatic plant role	Using plants to rob algae of the nutrients they need to grow	No chemical pollution problems, friendly to the environment	Unstable effect, need other engineering measures to assist
Aquatic animal role	Rebuilding biomes and controlling algae through biological competition	Easy to operate, environmentally friendly, and economically beneficial	High environmental impact and risk of damaging the ecosystem

[37,38].

2.2.1. Microbial Action

Microorganisms are the most abundant and widely distributed decomposers in the ecosystem. They play an important role in the degradation of organic pollutants and the recycling of materials. Mechanisms of microbial algae control can be divided into three categories:

• Microorganisms act directly on cyanobacteria, attacking cyanobacterial cells or secreting extracellular substances to destroy cyanobacterial cells, and some microorganisms can also inhibit photosynthesis in cyanobacterial cells, ultimately affecting cyanobacterial biomass [39];
• The use of microorganisms to absorb and degrade excess nutrients in the water column to achieve purification of the water column and thus reduce the biomass of cyanobacterial blooms;
• Reduction of algal cell biomass in the water column by bioflocculation techniques [40].

Cyanophage

Cyanophage is a special planktonic virus that uses cyanobacteria as hosts and is parasitic in cyanobacterial cells. It can cause specific lysis of cyanobacterial cells, which are widely distributed in the natural aquatic environment. Cyanophage is an important component of planktonic viruses in the water column and plays an important role in controlling cyanobacterial population structure and reducing the frequency of cyanobacterial blooms. Up to now, more than 20 species of cyanophages have been identified at home and abroad, and their hosts are spread over Anabaena, Microcystis aeruginosa, and Nostoc, etc. Cyanophages have been reported many times in ecology and algae removal studies as biological factors for the rapid lysis of algal cells [41].

Algae Removal by Algicidal Bacteria and Algicidal Viruses

Algicidal bacteria refer to the bacteria that inhibit the growth of algae or kill algae by direct or indirect means, thereby dissolving algae cells [42,43]. They are divided into directly and indirectly algicidal bacteria: directly algicidal means directly attacking the host and entering the algae cells; indirectly algicidal is when bacteria compete with algae for limited nutrients or lyse algae by secreting extracellular substances. Algicidal bacteria play an important role in the biodegradation of algae and their toxic by-products. Myxobacteria is the first reported algicidal bacteria. The mechanism of killing algae is speculated that when bacteria come into contact with algae cells, bacteria may secrete some enzymes that can dissolve cellulose, and achieve the purpose of algicide by digesting the cell wall of the host [44].

Microbial Flocculants for Algae Removal

Microbial flocculants are polymeric organic substances with a flocculation effect produced by microorganisms, and there are three main types: flocculants using microbial cells, flocculants using microbial cell wall extracts, and flocculants using microbial cell metabolites. The use of microorganisms themselves or their peptides, esters, glycoproteins, mucopolysaccharides, cellulose, and nucleic acids as flocculants can produce flocculation on most microorganisms, including algae, without secondary pollution to the environment.

The microbiological method is an ecofriendly method with low cost and no secondary pollution, mainly using bacteria, viruses, and flocculants to treat cyanobacterial blooms.

2.2.2. Aquatic Plants’ Role

Aquatic plants compete with cyanobacterial cells for light, nitrogen, phosphorus, and other nutrients, releasing allelochemicals that inhibit the metabolism and reproduction of cyanobacterial cells [45,46,47]. The mechanism of chemosensory algae inhibition by aquatic plants is as follows:

• It destroys the ultrastructure of algal cell membranes, chloroplasts, mitochondria, etc., and hinders the normal operation of various biochemical reactions in algal cells.
• It disrupts the light-trapping system of cyanobacterial cells, reduces the rate of oxygen release during photosynthesis, and affects electron transfer and adenosine triphosphate (ATP) synthesis during photosynthesis.
• It inhibits the metabolic activity of mitochondria in algal cells, interferes with the electron transfer process of mitochondria, and blocks the oxidative phosphorylation process of mitochondria.
• It disrupts the antioxidant system of algal cells and affects the expression of enzymes such as oxide dismutase (SOD), peroxidase (POD), and catalase (CAT).

Most of the aquatic plants, especially water-holding plants such as Eichhornia crassipes, Potamogeton maackianus, bitter weed, water lettuce, Myriophyllum aquaticum, etc., have continuously released polyphenols, polyacetylenes, oxygenated fatty acids, sulfur compounds, etc. into the water column during the process of growth and development. Sulfur compounds and other allelochemicals inhibit the metabolism and reproduction of algal cells. Hong et al. [48] found that gramine could disrupt the basic physiological functions of algal cells. β bad carotene and chlorophyll are important non-enzymatic antioxidants and photosynthetic pigments, respectively, and gramine significantly inhibited the synthesis of β carotene and Chlorophyll-a Chl-a).

It has the advantage that there is no chemical pollution problem and it is friendly to the environment. However, the effect is unstable and needs to be assisted by other engineering measures.

2.2.3. Aquatic Animals’ Role

Aquatic fauna action mainly refers to the methods of controlling algal populations by using the predation of aquatic animals [49], as well as controlling algae by rebuilding biological communities and using biological competition. Most algae are a food source for aquatic animals. The main aquatic animals that can feed on algae are protozoa, postlarvae, filter-feeding shellfish, and fish, etc. Among them, large filter-feeding fish such as tilapia, silver carp, and bighead carp can swallow algae in large quantities and can effectively control algal outbreaks. According to the uptake mechanism of the ecosystem food chain, there are two categories of classical biomanipulation techniques and nonclassical biomanipulation techniques.

Fish feeding behavior has a greater impact on prey zooplankton and phytoplankton at lower trophic levels in the food chain, and the degree of impact varies depending on fish habits. By releasing carnivorous fish, eliminating zooplankton-feeding fish, and rebuilding fish communities, the classical biomanipulation technique gives full play to the predatory role of zooplankton on cyanobacteria [50], creating a safe and stable environment for the growth and reproduction of algae-eating zooplankton.

Predation relationships are the most critical factor in determining the density and structure of planktonic communities and in causing planktonic community succession. Nonclassical biomanipulation technology uses the characteristics that the feeding rate of large zooplankton preying on cyanobacteria is much higher than that of small plankton, and they prey on cyanobacteria cells in large quantities through filter-feeding fish. To effectively reduce cyanobacterial density and regulate aquatic community balance, large pelagic fish such as silver carp, bighead carp, and carp have a significant advantage in the biological control of cyanobacterial blooms. However, the nonclassical biological manipulation method has some shortcomings [51], such as the introduction of bighead and silver carp in the United States, which are not the main target of fishermen due to the low market price of bighead and silver carp, leading to the proliferation of bighead and silver carp and their inclusion in the list of harmful wildlife.

The method is easy to operate, without any toxic substances, is friendly to the environment, economically efficient, and does not cause secondary pollution.

2.3. Chemical Algae Removal Technology

At present, chemical algae removal technology is effective in controlling algal blooms and is the most widely used and mature technology. Intensive coagulation and precipitation, as well as preoxidation, are the two most common methods of chemical algae removal technology, and advanced oxidation is still in the laboratory research stage [52,53,54]. The advantages and disadvantages of enhanced coagulation and precipitation, preoxidation, and advanced oxidation methods are compared in

Table 4. Advantages and disadvantages of chemical algae removal technology.

Method Categories	Mechanism of Action	Method Advantages	Method Deficiency
Intensive coagulation and sedimentation	Add coagulant or use new modified coagulant	Mature technology, easy to use	Possible increase in metal ion residues
Preoxidation method	Inactivate algae cells by adding oxidant	Reduces disinfection by-products and odors	High cost and production requirements
Advanced oxidation method	Using strong oxidizing hydroxyl radicals to carry out oxidation reaction with algae cells and eventually kill them	Efficient and pollution-free	Complex treatment process and generally high cost

.

Chemical algae removal technology has a certain mineralization effect on algae, with outstanding algae removal effects and short action time, but it also causes the generation of disinfection by-products and easily causes secondary pollution (as shown in

Table 5. The removal efficiencies of different reagents for algal bloom control.

Species	Reagent	Optimum Addition Rate	Removal Rate	Shortcomings	Reference
Microcystis aeruginosa	AlCl3	15 mg/L	99%	Residual metal ions are toxic	[55]
Microcystis aeruginosa	PACl	4 mg/L	90%	Increases the lysis of algae cells	[56]
Microcystis aeruginosa	FeCl3	100 mg/L	89%	Increases water chroma	[57]
Dunaliella salina	Al2(SO4)3	150 mg/L	95.2%	High costs	[58]
Dunaliella salina	FeCl3	150 mg/L	99.2%	Residual metal ions are toxic	[58]
Chlorella minutissima	FeCl3	750 mg/L	65.0%	Increases water chroma	[59]
Chlorella minutissima	Al2(SO4)3	750 mg/L	95.0%	High costs	[59]

). Therefore, when dealing with algae, careful consideration should be given, and the dosage should be strictly controlled.

2.3.1. Intensive Coagulation and Sedimentation

Conventional coagulants such as ferric sulfate, ferric trichloride, and aluminum sulfate are not effective at treating high algae water. To improve the efficiency of coagulation and algae removal, coagulation can be strengthened from the perspective of coagulants. Coagulant enhancement methods include increasing the coagulant dosage, adding coagulant aid, and developing new coagulants, all of which can strengthen the algae removal effect.

• Increasing the coagulant dosage, that is, increasing the concentration of counter ions in the water, destabilizes the algal cells and sinks them from the water under the adsorption effect of flocs. However, this method not only leads to an increase in the cost of water treatment, but may also endanger human health by increasing the concentration of coagulant residual ions, such as the health risks of Alzheimer’s disease caused by overdose [60,61].
• Adding coagulant develops new polymer coagulants (such as the addition of sodium alginate, activated silica gel) [62], and the use of the new organic coagulant polydiallyldimethylammonium chloride [63], the new inorganic coagulant polysilicon aluminum sulfate, etc., can improve the algae removal effect.
• Using modified coagulants, such as modified clay, compensates for the shortcomings of traditional coagulants with low algae removal efficiency and poor settling performance [64]. Anderson [65] has suggested that clay is the most promising material for algae flocculation, and many studies on the modification of clay by different methods have been reported since then, further confirming the excellent performance of modified clay in algae removal [66,67,68,69].

Intensive coagulation and sedimentation generally ensure the integrity of algal cells and is effective in removing intracellular microcystin, but drains the sludge in a timely manner to ensure that no algal toxins are released, but does not remove extracellular algal toxins.

2.3.2. Preoxidation Method

The preoxidation method refers to the pretreatment of high algal water using oxidants such as chlorine, chlorine dioxide, ozone, potassium permanganate (KMnO₄), and hydrogen peroxide (H₂O₂), which kill algal cells by destroying their structure, as detailed in Table 6.

Chlorine is the most widely used preoxidant in water treatment, and in practice, it is generally added to the water in the form of sodium hypochlorite or chlorine gas [70]. In the conventional pH range of water treatment, chlorine mainly exists in the water body in the form of hypochlorous acid with high oxidation capacity, which oxidizes pollutants through oxidation, addition, and electrophilic substitution reactions. However, the existing prechlorination-enhanced coagulation and algae removal mechanisms all achieve enhanced coagulation and algae removal by destroying, killing, or even disintegrating algal cells. This mechanism of prechlorination-enhanced algae removal methods can cause intracellular organic matter (IOM) release, which in turn leads to increased concentration of disinfection by-products (DBPs) and other drinking water safety issues, endangering human health.

Chlorine dioxide is a strong oxidizing agent that is readily soluble in water and can rapidly achieve the inactivation of cyanobacterial cells. Chlorophyll in algal cells has a pyrrole ring structure, similar to the benzene ring, so chlorine dioxide has the property of being close to the benzene ring [71], which can rapidly cause oxidative degradation of chlorophyll and destroy the photosynthetic system of algal cells and terminate cyanobacterial metabolism. Chlorine dioxide oxidizes algal cells very fast and has a significant algacidal effect [72], and can effectively control DBPs and odor, but its cost and production requirements are high, and harmful substances such as chlorite and chlorate are produced during use. Jin et al. [73] found that chlorine dioxide was effective in killing Microcystis aeruginosa within 10 min, but produced more disinfection by-products such as trihalomethanes (THM) and haloacetic acid (HAA), chlorate, and chlorite.

Ozone is a strong oxidizer and decomposes much faster in water than in the air. In acidic environments, ozone is predominantly in the molecular form, while in neutral and alkaline environments, ozone is highly susceptible to decomposition into superoxide: anionic radicals (O²⁻) and OH. Therefore, ozone can rapidly destroy the cell membrane of microorganisms, has a very high oxidative lethal effect on cyanobacteria, and can effectively remove algal toxins and odor while removing algae, but will produce organic by-products such as bromate, and ozone is not easy to store and it is more expensive to generate equipment; equipment investment and operating costs are very high [74]. Almomani et al. [75] demonstrated that ozone is effective in removing microcystin, but ozone disinfection also produces by-products. Rodríguez et al. [76] demonstrated that ozone was more effective in degrading MC-LR algal toxins and other algal toxins than chlorine, chlorine dioxide, and permanganate.

Potassium permanganate (KMnO₄) is a purple-black crystal at room temperature, easily soluble in water, and manganese is presented in various valence states in water, which is capable of redox reactions with organic substances [77]. KMnO₄ itself has a super-high oxidizing property, which can quickly achieve the inactivation of algae cells, while its reduction product, MnO₂, also has the effect of adsorption and flocculation on the suspended algae cells in water. KMnO₄ can play the roles of oxidation, disinfection, and microflocculation at the same time, which has significant economic practicality and easy production and use compared with ozone [78], but its aqueous solution is purple, and improper use can cause the problem of watercolor and manganese ions to exceed the standard. Fan et al. [79] found that when the algal cell concentration was 0.7 × 10⁶ cells·mL⁻¹ and the KMnO₄ dose was less than 2 mg·L⁻¹, the algal cell removal rate was extremely low; when the KMnO₄ dose was 5 and 10 mg·L⁻¹, respectively, the algal cell removal rates were 26% and 100% after 6 h, respectively. However, overdosing on KMnO₄ will result in higher manganese content in the effluent, a significant decrease in transparency, higher chromaticity and turbidity, and the production of environmentally toxic disinfection by-products such as trihalomethanes (THMs) and haloacetic acids (HAAs) [80].

Hydrogen peroxide (H₂O₂) is a green oxidizer that is widely used for its high oxidation and reduction products of O₂ and H₂O [81]. Oxidative stress by H₂O₂ leads to damage of algal cells, which is manifested by reduced photosynthetic rate, gene base deletion, and eventually algal cell inactivation and death, but the effect of algae removal by H₂O₂ application alone is not satisfactory [82]. Ding et al. [83] found that when H₂O₂ was administered at 4 μg·mL⁻¹ and the cell density of Microcystis aeruginosa was 10⁶ cells·L⁻¹, the algal cell removal rate was 50% after 6 h. The content of formic acid, L-gamma glutamate-L-leucine, putrescine, and docosa amide in algal cells increased, indicating that H₂O₂ could effectively inhibit the metabolism of algal cells.

Table 6. Advantages and disadvantages of the preoxidation method.

Preoxidation Method	Advantages	Disadvantages
Chlorine oxidizer	Has a high oxidation capacity	Susceptible to chlorine disinfection by-product generation [84]
Chlorine dioxide oxidizer	High efficiency of algae cell oxidation; effective control of disinfection by-products and odor	High cost and production requirements; also produces harmful substances such as chlorite and chlorate
Ozone oxidizer	High oxidative lethal effect on cyanobacteria; effective removal of algal toxins and odor while removing algae	Organic by-product bromate will be produced, and ozone is not easy to store and the generation device is more expensive; the equipment investment and operating costs are very high
Potassium permanganate oxidizer	It can play the roles of oxidation, disinfection, and microflocculation at the same time, and quickly achieves the inactivation of algae cells.	The aqueous solution is purple in color, and improper use will cause the problem of excessive color and manganese ions in the water body
Hydrogen peroxide oxidizer	Green oxidizer with high oxidizing power	H2O2 alone is not ideal for algae removal

Table 6. Advantages and disadvantages of the preoxidation method.

Preoxidation Method	Advantages	Disadvantages
Chlorine oxidizer	Has a high oxidation capacity	Susceptible to chlorine disinfection by-product generation [84]
Chlorine dioxide oxidizer	High efficiency of algae cell oxidation; effective control of disinfection by-products and odor	High cost and production requirements; also produces harmful substances such as chlorite and chlorate
Ozone oxidizer	High oxidative lethal effect on cyanobacteria; effective removal of algal toxins and odor while removing algae	Organic by-product bromate will be produced, and ozone is not easy to store and the generation device is more expensive; the equipment investment and operating costs are very high
Potassium permanganate oxidizer	It can play the roles of oxidation, disinfection, and microflocculation at the same time, and quickly achieves the inactivation of algae cells.	The aqueous solution is purple in color, and improper use will cause the problem of excessive color and manganese ions in the water body
Hydrogen peroxide oxidizer	Green oxidizer with high oxidizing power	H2O2 alone is not ideal for algae removal

2.3.3. Advanced Oxidation Method

The advanced oxidation method of algae removal refers to a process of algae removal by reacting to produce hydroxyl radicals with strong oxidizing properties and oxidizing with algae to oxidize organic matter, including algal cells and extracellular materials, to low or nontoxic small molecules [85]. According to the different ways of free radical generation, advanced oxidation can be divided into electrochemical algae removal technology, electro-Fenton technology, photochemical oxidation, etc. Although advanced oxidation is efficient and nonpolluting, the low concentration of hydroxyl radicals generated by the reaction and the long reaction time are the key problems encountered so far.

Electrochemical Algae Removal Technology

In the second half of the last century, the United States and the former Soviet Union conducted intensive and extensive research on electrochemical technology and accumulated a large amount of relevant knowledge. However, electrochemical water treatment technology has not been widely used due to high investment in infrastructure and high energy consumption of electrical energy. At present, due to the improvement of drinking water quality standards and increasingly serious water environment problems, as well as the rapid development of the electric power industry and new energy bringing down the cost of electricity, electrochemical technology has once again attracted the attention of researchers internationally. Currently, electrochemical technology is mainly used to treat waste leachate, organic pollutants, printing and dyeing wastewater, disinfection, sterilization, etc. [86,87]. The basic principles of electrochemical technology for treating algae-containing water include electrochemical oxidation technology, electroflocculation technology, electric flotation technology, and the synergistic effect of electric fields. Compared with conventional algae removal process, electrochemical algae removal has the significant advantages of no secondary pollution and good removal effect.

During electrochemical oxidation, organics can be oxidized directly by electron transfer on the anode surface and indirectly by weakly physically adsorbed ∙OH on the anode surface or by reagents in the native solution, such as active chlorine species, O₃, persulfates, and oxidants such as H₂O₂ (Equations (1)~(4)) [88,89]. The basic principle is shown in Figure 2.

$${\mathrm{H}}_2\mathrm{O}\to \mathrm{OH}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$

(1)

$$\mathrm{OH}\to \mathrm{O}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$

(2)

$$3{\mathrm{H}}_2\mathrm{O}\to {\mathrm{O}}_3+6{\mathrm{H}}^{+}+6{\mathrm{e}}^{-}$$

(3)

$${\mathrm{O}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(4)

Electrochemical oxidation technology has unique advantages over traditional algae removal methods.

• Additional chemicals are generally not required, which reduces the cost of storage and transportation of drugs and other costs, as well as the risk of secondary contamination after the reaction [91].
• The strong oxidizing substances produced by the electrochemical process not only inactivate algae cells in the water, but also oxidize and mineralize most of the organic matter and repair the water quality condition of polluted water bodies.
• The generation equipment required for electrochemical technology is small in size, and the equipment is highly automated and easy to operate.

The working principle of electroflocculation makes the anode the metal electrode, and the metal anode generates electrons and forms a large number of dissolved metal ions, which further act as a flocculant to combine with organic matter to achieve flocculation and removal of organic matter [92]. In the electroflocculation process, gas is generated by electrolysis of the water when energized, which results in air flotation for algae removal. In addition, the electroflocculation process is often accompanied by electro-oxidation and electroreduction processes [93]. In general, the electrically driven anodic flocculation process is the main mode of pollutant removal by electroflocculation, as shown in Figure 3.

The main advantages of electroflocculation technology are that flocculants can be generated in situ without introducing other substances, low production of electroflocculated sludge, and simple and easy-to-operate electroflocculation equipment, while obvious disadvantages are mainly electrode passivation and power consumption, which limit its use.

Electrical flotation refers to a process in which algal flocs attach to bubbles generated by electrolysis, and then the algae are removed by solid–liquid separation. The bubbles produced by electrical flotation are characterized by a narrow distribution range, small diameter, and large specific surface area, and have good adhesion properties to suspended particles in water, thus obtaining a very high separation efficiency.

Electro-Fenton Technology

Electro-Fenton technology is an electrochemical advanced oxidation technology based on the Fenton reaction [95]. The strong degradation ability of most organic compounds, including toxic and nonbiodegradable compounds, has received a lot of attention. Electricity is a clean energy source, so the electro-Fenton process does not produce secondary pollution [96]. The basic principle of the reaction is shown in Figure 4. The electro-Fenton technique is the electrolysis of water on the anode in the presence of electricity to produce oxygen; see Equation (5). H₂O₂ is generated by a two-electron oxygen reduction process of O₂ or air at the cathode; see Equation (6). By the external addition of Fe²⁺, the Fenton reaction produces active oxides in situ in solution, and Fe²⁺ is oxidized to Fe³⁺ at the end of the reaction, with the chemical equation shown in Equation (7). During the experiment, Fe³⁺ may still be converted to Fe²⁺ again at the cathode by the Ferric iron reduction reaction (FRR) process to reach the Fenton reagent cycle; see Equation (8). In addition, Fe²⁺ can be regenerated by the reaction of Fe³⁺ with H₂O₂, as shown in Equation (9).

Anode:

$$2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{O}}_2+4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}$$

(5)

Cathode:

$${\mathrm{O}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(6)

Fenton reaction:

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{Fe}}^{2+}\to \mathrm{OH}+{\mathrm{Fe}}^{3+}+{\mathrm{OH}}^{-}$$

(7)

Conversion of Fe³⁺ to Fe²⁺:

$${\mathrm{Fe}}^{3+}+{\mathrm{e}}^{-}\to {\mathrm{Fe}}^{2+}$$

(8)

$${\mathrm{Fe}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Fe}}^{2+}+{\mathrm{H}}^{+}+\mathrm{OOH}$$

(9)

The electro-Fenton method has its unique advantages:

• The in situ generation of H₂O₂ avoids the risks associated with storage, transport, and handling [98];
• The Fe²⁺/Fe³⁺ cycle reaction on the cathode can effectively reduce the production of iron sludge [99];
• In addition to the strong oxidation by reactive radicals generated by the Fenton reaction, it is accompanied by the synergistic effect of anodic oxidation, electro-adsorption, and electrical flotation processes [100].
• Due to the existence of heterogeneous Fenton reactions, the application range of pH can be widened to some extent [101].

At present, the application of electro-Fenton technology to remove algae is still in the initial stage of exploration. Zhang et al. [102] studied the electro-Fenton-like system constructed by iron (Fe) anode and activated carbon fiber (ACF) cathode to remove algae cells and algae metabolic pollutants from seawater. Under the optimum conditions, the inactivation rate of algae can reach 98%. The coupling of indirect oxidation of OH, direct oxidation of free chlorine, and coagulation and adsorption of iron and ferrous hydroxy complexes during the electro-Fenton-like process was achieved under neutral pH conditions, which promoted the deactivation of algae and the removal of metabolic contaminants. Long et al. [100] found that the removal of Microcystis aeruginosa was significantly enhanced by the addition of Fe²⁺ to the electrochemical system of boron-doped diamond (BDD) anode and carbon felt (CF) cathode at neutral pH, and revealed that the removal mechanism was a nonhomogeneous electro-Fenton effect. An et al. [103] pioneered a switchable-electrode three-electrode (anode: Fe, Ti/IrO₂, cathode: carbon black-graphite) electroflocculation-electro-Fenton system for the removal of Microcystis aeruginosa and Microcystis aeruginosa toxins by switching electrodes to achieve free alternation between electroflocculation and electro-Fenton methods.

The above-mentioned studies on the use of electro-Fenton technology for algae-containing water treatment have shown that this technology can effectively remove algal cells.

• Selection of electrode materials: Electrode materials with good catalytic and stability properties are usually expensive. Although the soluble sacrificial anode greatly enhances the electroflocculation effect, it also causes the generation of anode sludge, and the frequent replacement of electrodes increases the operating cost.
• The implementation of electro-Fenton technology also faces two severe tests, which are limited by the H₂O₂ electrocatalytic performance of the cathode material on the one hand and the more stringent pH conditions on the other.

Photochemical Oxidation

Photochemical oxidation refers to the oxidation–reduction reaction under the action of a photocatalyst, which is excited by light to produce oxidizing radicals (O₂⁻ and OH) to decompose the hard-to-degrade organic pollutants into small-molecule substances. At present, there are two main directions for the design of photocatalysts: the first is to dope precious metals (such as Pt, Pd, Ag, Au) or nonmetals (such as N, F, Br) in semiconductor materials for modification; the second is to combine carbon materials (such as graphene, carbon nanotubes, etc.) to inhibit charge recombination and accelerate electron transfer.

Hao et al. [104] prepared an efficient photocatalyst (TiO₂/WO₃/GO) using tungsten trioxide doping and graphene oxide hybridization, which exhibited good light absorption and inhibition of charge recombination in the visible region and showed good photocatalytic performance (93.2% removal rate) for the degradation of bisphenol A (BPA) under visible light and sunlight irradiation. Photochemical oxidation has great potential for algae removal, but the improvement of the catalyst utilization rate and its practical application still need more in-depth exploration and research.

2.4. Comparison and Analysis of Algal Bloom Control Technologies Based on Algal Density

Currently, many researchers are choosing strategies to control algal blooms based on the advantages and disadvantages of different methods. Some studies have reported that the inhibitory effect of hydrogen peroxide on algal cells depends largely on the initial algal density, and the inhibition of P. aeruginosa growth using hydrogen peroxide should be at the beginning of the algal bloom, and the inhibition rate of hydrogen peroxide decreases with the increase in algal density [105]. Different concentrations of copper-based algacides have been reported to have different inhibitory effects on Pseudomonas aeruginosa [106]. The density of algae in the water column varies with time at different stages, so the density of algae in the water column at that time should be considered when selecting the control method.

Table 7. The control performance of different methods on algal blooms with the different algal densities.

Density (cells/mL)	Species	Method	Efficiency (%)	Time	References
1.0 × 104	Alexandrium minutum	Bacillus subtilis	80.0	48 h	[107]
1.0 × 105	Microcystis aeruginosa	Air floatation	93.0	8 min	[108]
1.0 × 106	Microcystis aeruginosa	K2FeO4	70.3	10 min	[80]
1.0 × 106	Microcystis aeruginosa	UV	100.0	3 d	[109]
1.8 × 106	Microcystis aeruginosa	Electrochemical	93.0	180 min	[110]
2.0 × 106	Microcystis aeruginosa	Oxidation-coagulation	95.0	25 min	[111]
4.8 × 106	Microcystis aeruginosa	Ag/AgCl	93.1	6 h	[112]
1.1 × 107	Microcystis aeruginosa	Flocculation	99.0	3 min	[113]
3.0 × 107	Microcystis aeruginosa	CuSO4	80.0	4 d	[114]
1.4 × 108	Microcystis aeruginosa	Cu2O/SiO2	100.0	5 d	[115]

summarizes the control effects of different algal species at different initial densities and different treatment times. The density of algal blooms to be prevented and controlled usually ranges from 1.0 × 10⁴ cells/mL to 1.4 × 10⁸ cells/mL. When the algal density is less than 1 × 10⁷ cells/mL, inactivation of algal cells by physical, chemical, and biological methods is an effective way to control algal blooms. Although these methods can inhibit the growth of algal cells for a short period of time, most of them do not exceed 95% inhibition efficiency. The results show that when the algal density is below 1.0 × 10⁷ cells/mL, controlling the development of algal blooms is the key, i.e., how to precisely act on algal cells to deactivate them and slow down their growth. In general, the deactivation efficiency of algal cells decreased with increasing algal density. When the density of algae is higher than 1.0 × 10⁷ cells/mL, it takes a relatively long time to achieve a good inhibition effect. Chemical agents can slow down the growth of algae for a short period of time, but they can cause secondary pollution to the aquatic environment.

2.5. Application of Algae Removal Technology in Engineering

The pond is located in Zhejiang Province, with a total area of about 2 to 3 acres, and the water body is a flowing water body. The pond is quadrangular, with the north corner of the pond as the inlet and the northeast corner as the outlet. The inlet end and the outlet end are each connected to the farmland [116]. Figure 5 shows the condition of the pond before treatment; the pond is blue and has a lot of suspended algae, and the cyanobacteria outbreak is serious.

Ltd. independently developed a high-efficiency microbial compound bacterial agent [116] and algacide combination program for comprehensive treatment. It is monitored daily for water quality. The dosage is 0.2% to 0.5% of the volume of water in the pond. After the algacide was added, the area covered by cyanobacteria on the surface of the pond water body was reduced by about 30% the next day. On the third day, there were no more patches of aggregated cyanobacteria, but there were still fine cyanobacterial particles visible to the naked eye. On the fourth day, there were no cyanobacterial particles visible to the naked eye, indicating that the cyanobacteria were gradually declining. The pond water remained in good condition on the fifth day. The pond was tracked and retested, and it was found that there was no recurrence of cyanobacteria, and the water quality indicators were still maintained above the surface water environmental standard III. As shown in Figure 6, the cyanobacterial bloom in the pond was significantly eliminated, and all the water body indicators of the pond were significantly improved.

Although the chemical algacide is fast, low cost, and easy to operate, most of them are used for emergency treatment of cyanobacterial blooms and cannot solve the eutrophication problem at the root. In order to prevent future outbreaks of cyanobacterial blooms in the lake, it is necessary to strengthen the prevention and control of water pollution in the lake.

3. Combined Algae Removal Technology

3.1. Photocatalytic/Ozone Combined Oxidation

The principle of photocatalytic/ozone complex oxidation is mainly based on the free radical chain reaction, which is divided into photocatalytic and ozone-catalytic oxidation, as well as combined photocatalytic/ozone oxidation [117].

In the photocatalytic process, the catalyst forms electron (e⁻) and hole pairs (h⁺) under UV/visible light irradiation. h⁺ oxidizes OH⁻ or H₂O in water to produce strong oxidizing hydroxyl radicals (OH), while e⁻ can also react with O₂ to produce superoxide radicals (O²⁻), and O²⁻ under acidic conditions H₂O₂ can be further generated under acidic conditions, and H₂O₂ can generate OH under UV irradiation, and OH reacts with organic pollutants in water in a chain reaction (see reaction Equations (10)–(14)) and finally degrades them [118].

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{h}}^{+}\to \mathrm{OH}+{\mathrm{H}}^{+}$$

(10)

$${\mathrm{OH}}^{-}+{\mathrm{h}}^{+}\to \mathrm{OH}$$

(11)

$${\mathrm{O}}_2+{\mathrm{e}}^{-}\to {\mathrm{O}}_2^{-}$$

(12)

$${\mathrm{O}}_2^{-}+{\mathrm{e}}^{-}+2{\mathrm{H}}^{+}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(13)

$${\mathrm{H}}_2{\mathrm{O}}_2+\mathrm{h}\mathsf{\upsilon }\to 2\mathrm{OH}$$

(14)

The main principles of O₃-catalyzed reactions are divided into O₃ direct reactions and indirect reactions. The direct reaction uses the O₃ oxidizing property to bring the ozone into direct contact with organic pollutants to oxidize and degrade them. Although ozone has a high oxidation potential of 2.07 eV, it is selective in degrading organic pollutants, while the reaction rate remains slow. The indirect reaction uses O₃ to react with the catalyst to decompose OH or O²⁻, especially OH has strong oxidation properties (2.8 eV) and low selectivity, which can degrade or even mineralize most organic pollutants. Therefore, the indirect reaction efficiency of O₃ is much greater. O₃ direct oxidation and indirect oxidation exist simultaneously, and there are many ineffective decomposition reactions of O₃, and its overall reaction effect is not satisfactory.

The combined photocatalytic/ozone oxidation technology highlights the synergistic advantages of the coupling [119]. First, the photocatalyst produces (e⁻)-hole pairs (h⁺), as shown in reaction Equations (10)–(13), to produce OH. Moreover, catalytic O₃ can provide more OH, as in the reaction Equations (15)–(17). It can be seen that one molecule of O₃ can produce raw 2OH under the conditions of obtaining one photon or catalysis, which is very efficient. Second, the O₃ molecule also reacts with e⁻ to form O₃⁻; see reaction Equation (18). The photogenerated electron e⁻ reaction can reduce the complexation rate of e⁻ with h⁺, which is conducive to the efficient reaction of h⁺ and the generation of more OH. Furthermore, O₂ can react with e⁻, as in the reaction Equations (12)–(14). O₃⁻ in an acidic solution will also react to form OH, as shown in the reaction Equations (19)–(20). Of course, other reactions occur in addition to the above, such as in reaction Equations (21)–(24).

$${\mathrm{O}}_3+\mathrm{h}\mathsf{\upsilon }\to {\mathrm{O}}_2+\mathrm{O}$$

(15)

$${\mathrm{O}}_3+\mathrm{Catalyst}\to {\mathrm{O}}_2+\mathrm{O}$$

(16)

$$\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{OH}$$

(17)

$${\mathrm{O}}_3+{\mathrm{e}}^{-}\to {\mathrm{O}}_3^{-}$$

(18)

$${\mathrm{O}}_3^{-}+{\mathrm{H}}^{+}\to {\mathrm{HO}}_3$$

(19)

$${\mathrm{HO}}_3\to {\mathrm{O}}_2+\mathrm{OH}$$

(20)

$${\mathrm{O}}_3+{\mathrm{H}}^{+}\to {\mathrm{HO}}^{+}+{\mathrm{O}}_2$$

(21)

$${\mathrm{O}}_3+{\mathrm{OH}}^{-}\to {\mathrm{HO}}_2^{-}+{\mathrm{O}}_2$$

(22)

$$2{\mathrm{O}}_3+{\mathrm{OH}}_2^{-}\to {\mathrm{O}}_3^{-}+2{\mathrm{HO}}_2$$

(23)

$${\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{O}}_2+{\mathrm{H}}_2{\mathrm{O}}_2$$

(24)

From the above, it can be seen that photocatalytic/ozone composite oxidation technology can produce more OH, and its overall reaction effect is higher than that of photocatalytic and O₃ catalytic oxidation technology alone, which is a very promising water treatment method. However, the photoexcitation of the photocatalyst to produce e⁻ and h⁺ in the reaction and the performance of the ozone catalyst directly determine the chain initiation rate of the free radical chain reaction. Therefore, the development of catalysts that can make full use of sunlight, have high O₃ catalytic performance, and are stable and easy to recover is one of the bases for the practical diffusion of this technology.

An et al. [120] constructed a synergistic photocatalytic–catalytic ozone oxidation system using Ag₃PO₄ as a photocatalyst and passing ozone, which substantially improved the degradation efficiency of phenol. A 30 mg/L solution of phenol was completely degraded after 6 min of cotreatment, with a 3-fold reduction in time compared to the single oxidation technique for the same degradation efficiency. Kang et al. [121] investigated an effective method to improve photocatalytic ozone oxidation mass transfer performance using spiral photocatalytic modules (HPM) in an annular UltraViolet C radiation (UVC) reactor. The total organic carbon (TOC) removal rate was 91.5% using the photocatalytic ozone oxidation process with HPM at a hydraulic retention time of 19 min and a feed water phenol concentration of 33 mg/L (TOC of 26 mg/L). After biological treatment of steel rolling wastewater, the effluent chemical oxygen demand (COD) was reduced to 45.8 mg/L at a hydraulic retention time of 57 min and an initial COD of 124 mg/L, which met the discharge standard for pollutants from urban wastewater treatment plants in China. Sanchez et al. [122] found that the application of ozone oxidation technology to the pretreatment stage of wastewater, coupled with photocatalytic oxidation technology, can effectively improve the pollutant degradation efficiency, and the combination of the two processes will significantly improve the TOC removal rate.

3.2. UV/Hydrogen Peroxide Synergistic Oxidation

Ultraviolet/hydrogen peroxide (UV/H₂O₂) technology is one of the photochemical oxidation methods. Hydrogen peroxide itself, as a strong oxidant, can effectively remove most organic pollutants in water, but using only H₂O₂ as an oxidant, the oxidation rate is low and cannot effectively decompose pollutants, while using the UV/H₂O₂ process can generate hydroxyl radicals (OH) with extremely strong oxidation, thus effectively decomposing some organic matter that cannot be decomposed by using H₂O₂ alone, and the H₂O₂ hydrolysis products are water and oxygen, and no new pollutants will be generated. The whole process is shown in Figure 7. It is generally accepted that the UV/H₂O₂ process is achieved in three main ways.

• Direct oxidation of organic matter using the strongly oxidizing properties of H₂O₂;
• Photodegradation of organic matter by direct excitation of molecular bond dissociation through effective photons by UV light;
• H₂O₂ generates hydroxyl radicals (OH) under UV irradiation, and OH reacts with organic matter in a redox reaction, thus removing pollutants.

The main role is played by the third article above, and the following reactions occur in water [123].

$${\mathrm{H}}_2{\mathrm{O}}_2\stackrel{\mathrm{UV}}{\to }2\mathrm{OH}$$

(25)

$${\mathrm{H}}_2{\mathrm{O}}_2\leftrightarrow {\mathrm{HO}}_2^{-}+{\mathrm{H}}^{+}$$

(26)

$$\mathrm{OH}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{HO}}_2$$

(27)

$${\mathrm{HO}}_2\leftrightarrow {\mathrm{O}}_2^{-}+{\mathrm{H}}^{+}$$

(28)

$$2\mathrm{OH}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(29)

$$2{\mathrm{HO}}_2\to {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2$$

(30)

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{HO}}_2+{\mathrm{O}}_2^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2+{\mathrm{OH}}^{-}$$

(31)

The advantages of UV/H₂O₂ co-oxidation are low cost, simple operation of H₂O₂ dosing and UV irradiation, and no secondary pollution to the environment. Therefore, the combined application of the above technologies for the treatment of cyanobacteria in water bodies is much less likely to cause harm to nontarget organisms.

Hai et al. [124] studied the effect of UV/H₂O₂ process on the removal of 2,4-dichlorophenoxyacetic acid (2,4-D) from water. The study showed that the UV radiation intensity was increased from 183.6 μW·cm⁻² to 1048.7 μW·cm⁻², and the 2,4-D concentrations were 78.3 μg·L⁻¹ and 7.6 μg·L⁻¹ after 60 min of reaction, respectively. The improvement of the target pollutant removal rate is very obvious. Xu et al. [125] conducted an experiment on the degradation of diethyl phthalate (DEP) by UV/H₂O₂ process and found that the DEP removal rate was 51.6% when the UV radiation intensity was 21.2 W/cm², and increased to 98.6% when the UV radiation intensity was 133.9 W/cm² within 40 min reaction time.

Figure 7. Principle of UV/H₂O₂ combination technology [126].

Figure 7. Principle of UV/H₂O₂ combination technology [126].

3.3. Hydrodynamic Cavitation/Hydrogen Peroxide Synergistic Oxidation

Hydrodynamic cavitation is a hydrodynamic phenomenon. The extreme conditions such as local high temperature and pressure, luminescence, electrical discharge, strong shock waves, and high-speed jets provided by the movement and collapse of the vacuoles in the liquid generated during hydrodynamic cavitation can strengthen the physical and chemical processes that can cause the water molecules to break chemical bonds and generate free radicals within the cavitation bubbles.

In research, simple hydrodynamic cavitation can usually be achieved in this way: when the liquid flows through a contraction device, the kinetic energy (velocity) of the liquid increases; by the contraction device of the flow blocking effect, the pressure of the liquid decreases; when the pressure drops below the saturation vapor pressure of the solution, the fluid vaporizes and a large number of cavities are formed; with the change in pressure around the liquid, the cavities grow further into a mature cavitation bubble. When the pressure reaches a critical value, the cavitation bubble instantly breaks and collapses, releasing a large amount of energy and thus generating cavitation. Physical effects such as strong shock waves, high-speed jets (400 km/h), and cavitation luminescence can be formed at the moment of collapse of the vacuole. In addition, under these extreme conditions, high temperatures, high pressures, and chemical effects with strong oxidizing OH can be generated. These physical and chemical effects are collectively known as the cavitation effect.

Under cavitation, H₂O₂ decomposes to OH. Therefore, the additional OH can accelerate the degradation rate of pollutants. The use of H₂O₂ alone may result in a poorer ability to dissociate H₂O₂ into OH, but under the cavitation effect, the rate of OH production is enhanced due to the extreme conditions created by temperature, pressure, and energy dissipation, and can be properly dispersed in the liquid to improve the overall oxidation rate. Pang et al. [127] found that the dissociation energies of the O-H bond in H₂O and the O-O bond in H₂O₂ is 418 KJ/mol and 213 KJ/mol, respectively, and thus H₂O₂ readily decomposes to OH under cavitation conditions, thereby increasing the concentration of OH.

Many authors have studied the effect of different concentrations of H₂O₂ to achieve maximum degradation of pollutants by hydrodynamic cavitation. Saharan et al. [128] investigated the effect of H₂O₂ addition on the hydrodynamic cavitation degradation of reactive red 120 dye (RR120). The degree of degradation increased with the concentration of H₂O₂, and complete decolorization and a 60% reduction in total organic carbon (TOC) were obtained at an optimal ratio of 1:60 (RR120:H₂O₂). Due to the scavenging effect of excess H₂O₂, the degradation rate and reduction of TOC were not further increased at higher H₂O₂ concentrations. Thus, this combined effect significantly improves the degradation efficiency of the process. Gore et al. [129] also observed that at an optimal H₂O₂ loading of 1:30 molar ratio of reactive orange four dye to H₂O₂, reactive orange 4 was almost completely decolorized, while at higher concentrations, the degradation rate did not increase further due to the scavenging effect of H₂O₂ itself. It was concluded that this combined method is 2.5 times more efficient than the hydrodynamic cavitation method alone. However, studies on the effect of hydrodynamic cavitation/hydrogen peroxide synergistic oxidation technology in algae suppression and the changes of intracellular toxins and environmental toxins in the water column during algae suppression have not been reported.

Overall, the combination of hydrodynamic cavitation with H₂O₂ demonstrated the presence of foreign OH and the ability of these radicals to be completely dispersed in the hydrodynamic cavitation, thus increasing the rate of oxidation reactions between the radicals and the pollutant molecules. To take full advantage of these oxidants in combination with organic pollutants, their concentrations need to be optimized, as excess organic pollutant molecules reduce their degradation efficiency and may affect their economic viability.

4. Conclusions and Perspectives

Frequent outbreaks of algae have brought great trouble to people’s productive lives, and research on algae removal at home and abroad mainly focuses on single physical, chemical, or biological methods; although all these methods can play a certain effect in algae removal, there are some limitations, and different methods of algae removal vary greatly.

In weighing the social, economic, and environmental benefits and other comprehensive factors, deep research should be carried out on the integration of multiple methods. In this field, photocatalytic/ozone combined oxidation, UV/H₂O₂ synergistic oxidation, and hydrodynamic cavitation/hydrogen peroxide synergistic oxidation have promising applications. With the advantages of a high mineralization rate and rapid reaction, photocatalytic/ozone combined oxidation has good development prospects in algae removal, but its industrial application has two major problems: the development of high-efficiency and low-consumption reactors and high-efficiency catalysts. Compared with reactor development, high-efficiency catalyst development can systematically optimize the technology in terms of reaction mechanism and improve pollutant degradation and mineralization efficiency, which is an important way to a breakthrough in this technology. The main direction of catalyst development and research is to reduce the photoexcitation energy level and increase the photogenerated carrier mobility and synergistic effect. UV/H₂O₂ has the advantages of strong oxidation capacity, high treatment efficiency, no selectivity, no secondary pollution, etc. It shows great potential for development, but there are also problems such as low H₂O₂ utilization and H₂O₂ residue, which can be improved by adding catalysts, and finding economical and efficient catalysts to improve H₂O₂ utilization is the focus of the current UV/H₂O₂ process research. Hydrodynamic cavitation technology is a new technology to treat microorganisms (algae) in water, with the advantages of no secondary pollution, high treatment efficiency, simple equipment, suitability for industrial applications, etc., and the combination with conventional treatment technology can achieve better treatment results, with broad application prospects. The future development direction aims to overcome the shortage of microbial (algae) treatment technology and explore new treatment technology and combination processes that are efficient, economical, and clean. At present, research on cavitation technology focuses on the cavitation mechanism, exploring the factors affecting the cavitation effect and parameter optimization. As cavitation theory and technology continue to mature, cavitation technology and its combination will play an important role in the field of microbial (algae) and other treatments.

In summary, the author believes that some efficient, low-cost, environmentally friendly new algae removal methods and their combination processes are the future development direction: a variety of methods used together to achieve the ideal algae removal effect and minimize the harm of cyanobacteria water bloom. However, combined technologies such as photocatalytic/ozone combined oxidation, UV/H₂O₂ synergistic oxidation, and hydrodynamic cavitation/hydrogen peroxide synergistic oxidation are not very mature for the treatment of eutrophic water bodies. Most of the current research stays in the laboratory, and further evaluation is needed to guide the implementation of engineering for water purification. The biological method has the advantages of simple operation, low impact on the environment, etc. A combined technology and biological method to remove algae will be the future development trend. Applying these methods in the field requires designing strategies based on local watersheds, infrastructure, resources, and other factors. The treatment of algal blooms and microcystins in surface waters is a global and local challenge, and there are still many technical and economic issues that require in-depth research.