What are the treatment technologies for water eutrophication?

The production and use of large amounts of nitrogen- and phosphorus-containing fertilizers, industrial wastewater generated by food processing and livestock product processing, as well as large volumes of urban domestic wastewater – particularly wastewater from phosphorus-containing detergents – are discharged without treatment. This results in seawater and lake water becoming enriched with plant nutrients such as nitrogen and phosphorus, a condition known as eutrophication of water bodies. In eutrophic water bodies, with an ample supply of nutrients, phytoplankton such as algae reproduce rapidly as soon as suitable conditions arise. At this point, the consumption capacity of organisms such as fish cannot keep up with the growth rate of algae; as a result, more and more algae accumulate in the water. These algae concentrate near the surface of the water, and the oxygen produced through photosynthesis dissolves in the upper layers of seawater, turning those layers into saturated solutions. This prevents oxygen from the atmosphere from dissolving into the deeper layers of seawater. At the same time, the large amount of dead seaweed consumes the dissolved oxygen in the water as it decomposes, causing the level of dissolved oxygen to drop sharply (even to zero), which leads to the mass suffocation and death of fish and other aquatic animals. Some algae even release toxic substances that cause fish to die from poisoning. Furthermore, as dead algae decompose, gases such as CH4 and H2S are released, causing the seawater to become foul-smelling. In the ocean, if this happens, a red tide occurs ； If it occurs in freshwater, it is also called a \"water bloom\". The main causes of eutrophication include air pollution, urban sewage discharge, excessive use of fertilizers in agricultural fields, severe damage to the ecological environment, and aquaculture. A study on the mechanisms for controlling eutrophication in the Taihu Lake basin states that the transport fluxes of non-point source pollutants at the water-soil interface help to further understand the mechanisms behind algal blooms, and it proposes the use of artificial wetlands and hydraulic engineering to regulate and improve the water environment. In the research, a new approach to environmental management that combines watershed control with regional control was proposed. With regional management as its goal, this approach aims to achieve overall management of the entire watershed. The Hefei Institute of Physical Science, Chinese Academy of Sciences, utilized biosynthesized surfactants and chitinase, taking advantage of the exchange properties and high adsorption capacity of natural nanomaterial rod crystals to \"capture\" the nutrient elements in fertilizers. Chitin and other materials were then used to \"trap\" these rod crystals, thereby effectively fixing the fertilizer nutrients and reducing their loss

Abstract: Eutrophication of water bodies, which leads to excessive algae growth, is a global problem. Many lakes and reservoirs in China are severely polluted, and scientists both at home and abroad have conducted extensive research on this issue. The article provides a comprehensive overview of existing techniques for controlling eutrophic water bodies, with a focus on the new ultrasonic algae removal and killing technology. Keywords: eutrophication, detergents, ultrasound. Water resources are essential for human survival. With population growth and rapid socioeconomic development, the demand for water is increasing sharply, while water pollution is also becoming more severe. Since the 1980s, due to rapid economic development and relatively slow progress in environmental protection, many lakes and reservoirs in our country have become eutrophic, or even severely eutrophic, including Dianchi Lake, Taihu Lake, West Lake, East Lake, South Lake, Xuanwu Lake, the Bohai Bay, Laizhou Bay, the Jiulong River, and the Huangpu River. A survey of 18 major lakes in China in 2000 showed that 14 of them were already in a state of eutrophication. 2 Hazards of water eutrophication: The occurrence of algal blooms makes the water taste foul and unpleasant, reduces its transparency, and increases its turbidity. The water surface is covered with algae, preventing sunlight from reaching the water, which severely hinders photosynthesis in the deeper layers of water and reduces dissolved oxygen levels. The dead algae continue to sink to the bottom, accelerating the consumption of oxygen there and causing the water below the surface to become anaerobic, which leads to the death of aerobic organisms. In addition to emitting odors, damaging the landscape, and disrupting aquatic ecosystems, some algae can also secrete algotoxins that cause poisoning in animals such as birds, cows, and sheep; they may also have mutagenic effects and pose a significant potential risk to humans. Eutrophication has a significant impact on aquatic ecosystems and people’s lives, and the situation is particularly severe for cities that rely on eutrophicated water bodies as their source of drinking water. Algae in water will **increase the concentrations of chemical oxygen demand (COD), biological oxygen demand (BOD), suspended solids (SS), etc., thereby increasing the burden on water treatment.** Algae can clog the filter media during filtration, and produce toxic by-products such as trihalomethanes (THMs) during chlorination disinfection. Algal metabolites such as sugar acids react with coagulants during the coagulation process, reducing the efficiency of treatment and increasing the amount of coagulant required; moreover, the complexes formed can cause corrosion in the pipeline network. Algal toxins cannot be removed by conventional methods. Therefore, using eutrophic water bodies as drinking water sources can severely affect the operation of water treatment plants, cause corrosion of pipelines, and degrade the quality of the treated water. 3 Treatment Processes 3.1 Control of Nutrients 3.1.1 Control of Industrial and Agricultural Wastewater Improving fertilization methods to reduce the levels of nitrogen and phosphorus in agricultural wastewater, as well as strengthening soil and water protection, is a consensus worldwide and represents the best approach for protecting the environment and preventing eutrophication of water bodies. China has also made continuous efforts in this regard. However, for various reasons, the results have been poor, and soil erosion is becoming increasingly severe in some areas. Considerable progress has been made in the treatment of industrial wastewater in recent years, enabling effective control of water eutrophication. 3.1.2 Phosphorus ban in detergents: 25% of the phosphorus in domestic wastewater comes from phosphorus-containing detergents. Many places have policies prohibiting or restricting the use of such detergents, and similar measures have been adopted in Shenzhen in China, as well as in the basins of Taihu Lake and Dianchi Lake. However, monitoring of Lake Biwa in Japan before and after the phosphorus ban shows that since phosphates in detergents account for a low proportion of the total phosphorus pollution in the water, this policy does not significantly change the phosphorus content in the water. At the same time, zeolites, which are alternatives to phosphates in detergents, significantly increase the volume of sludge from wastewater treatment plants, posing difficulties in sludge disposal. Therefore, there is considerable debate regarding the environmental effects of banning phosphorus in detergents. 3.1.3 Nitrogen and phosphorus removal in municipal wastewater treatment – Nitrogen and phosphorus removal in municipal wastewater treatment is also known as tertiary treatment, and it is widely used in developed countries such as those in Europe and America. There are two types of tertiary treatment: chemical and biological methods. The chemical method involves using coagulants to precipitate soluble phosphorus, which is then treated through nitrification and denitrification processes ； Biological methods utilize microorganisms to remove nitrogen and phosphorus; common processes include AO, AAO (A2O), OAO (AO2), etc. To promote phosphorus removal, some processes also involve the addition of volatile organic acids or sugars. Tertiary treatment is primarily aimed at removing nitrogen; its effect on phosphorus removal is not significant, and some of these processes can cause secondary pollution. Furthermore, the tertiary treatment process is complex and costly; the volume of municipal wastewater treated centrally in China is still very low, making it difficult to implement tertiary treatment on a large scale in addition to conventional treatment. Therefore, the control of nitrogen and phosphorus in domestic wastewater remains difficult to implement in most areas of China. With the progress of urbanization and the improvement of residents’ living standards, nitrogen and phosphorus levels in domestic wastewater will continue to rise. 3.1.4 Pollution diversion: Measures such as diverting pollutants, partially removing water from the polluted water body, and introducing clean water to dilute the pollution can alleviate the pressure on the polluted water body to some extent, but they require massive engineering efforts; moreover, transferring the pollution to the diverted areas may create new polluted zones. The experiences with Xuanwu Lake and West Lake show that sewage separation and flushing with water fail to achieve the desired results, as algae growth resumes to its normal level after being temporarily suppressed for 3 months. Techniques for controlling eutrophic water bodies Source: Free Paper Network www.shu1000.com 3.1.5 Sediment excavation Nutrient-rich sediment can release nitrogen and phosphorus under certain conditions, becoming an endogenous source of pollution in water bodies; therefore, sediment excavation was once considered an important measure for addressing eutrophic water bodies. However, the sediment excavation is a massive undertaking, and the excavated sediment is difficult to further process; economically speaking, this is likely the most expensive measure. Due to the complex processes of nitrogen and phosphorus uptake and release in sediment, there is currently no clear understanding, and sediment excavation often fails to achieve the desired results. It even exacerbates algal blooms for a period of time by disrupting the environment of organisms and aquatic plants on the water bottom, exposing the deep sediment and causing the nitrogen and phosphorus contained in it to dissolve into the water. The experiences with Xuanwu Lake and West Lake have shown that this method has many drawbacks, and it must be considered carefully. 3.1.6 Coagulation phosphorus removal: Coagulants are added to precipitate soluble phosphorus, preventing it from being utilized by algae. This method is widely used in the United States and Australia, with iron and aluminum salts being the commonly used coagulants. This method is effective, especially in deeper lakes, where phosphate complexes can settle to the thermocline at the bottom of the lake and not return to the surface. However, under conditions of hypoxia or a reduced redox potential, these complexes become unstable and release soluble phosphorus. Furthermore, when coagulants are used in large water bodies, a large amount of them is required, and they may react adversely with other substances present in the water, thus posing certain potential risks. 3.2 Algae suppression and killing 3.2.1 Deep aeration: To address the anaerobic conditions that occur below the surface water due to excessive algae growth, which leads to the death of other organisms, people attempt to increase the dissolved oxygen level in the water through mechanical stirring or aeration. However, the main source of oxygen in water bodies is the photosynthesis of aquatic plants; the surface of eutrophic water bodies is not deficient in oxygen. The water beneath the surface lacks oxygen because it is covered by algae and thus unable to receive sunlight. Mechanical stirring or aeration cannot change this fundamental cause, and their effects are minimal. 3.2.2 Chemical algaecide treatment: Commonly used algaecides include copper sulfate, chlorine, and chlorine dioxide; in addition, ozone and potassium permanganate have also been studied as algaecides. These oxidants can quickly kill algae, and further oxidize the metabolic substances and toxic harmful substances released as a result of damaged algae cells, achieving significant effects. However, these agents are expensive, and their impact on aquatic organisms as well as their reactions with soluble ions in river water have not been ruled out, which may lead to secondary pollution. 3.2.3 Biological control: Utilize the predation or competitive effects of aquatic organisms on algae by introducing these inhibitory organisms, followed by regular harvesting. This method requires less investment and helps to establish a proper aquatic ecological cycle; therefore, extensive research has been conducted both domestically and internationally since the 1970s. Based on an analysis of fish populations, appropriate fish species can be selected according to actual conditions to filter feed on algae and algae-eating microorganisms, including common species in China such as silver carp, bighead carp, and grass carp. Available economic aquatic plants include water hyacinth, lotus seed grass, arrowhead, water bamboo, duckweed, and water caltrop. However, while reducing algae, these organisms also excrete a considerable amount of nutrients, which means that a large proportion of these nutrients enter the mineralization cycle without being truly removed. Aquatic ecosystems are highly complex; intense human interference can lead to system instability and make it difficult to maintain control. Introduced species that are not part of the local natural population may pose long-term risks. Therefore, when using biological control, the adverse ecological consequences must be carefully considered. 3.2.4 Mechanical harvesting: Algae are harvested using boats when algal blooms occur; the collected algae can be processed into fish feed and is used in places such as Shanghai. This method is easy to control and yields significant short-term results, but removing the algae after they have multiplied in large numbers requires a tremendous amount of effort, resulting in less effective outcomes. 3.2.5 Ultrasonic alga removal: Japan began researching ultrasonic technologies for suppressing and killing algae in the 1990s, and large-scale tests are currently being conducted at Lake Chiba. Units such as Tsinghua University in our country have also conducted some research. Preliminary results show that 5 minutes of ultrasonic treatment at an appropriate frequency and intensity can significantly suppress algae growth (by 50%). Its significant advantages such as high efficiency, speed, simplicity, and no secondary pollution make ultrasonic algae suppression and killing highly attractive. Ultrasonic waves refer to sound waves with frequencies above 16 kHz; they are elastic mechanical waves that travel through material media. In water, they can create a range of extreme conditions, such as particle accelerations several tens of thousands of times greater than the acceleration due to gravity, high temperatures and pressures resulting from the collapse of cavitation bubbles (4000 K, 500 atmospheres), intense discharges, as well as strong shock waves and jets. The secondary waves, radiation pressure, acoustic trapping, free radicals, oxidants, etc., derived from this may also significantly alter the properties of the medium. The possible mechanisms by which ultrasound inhibits and kills algae include: destroying cell walls, destroying gas vesicles, and destroying active enzymes. High-intensity ultrasound can destroy biological cell walls, causing the contents of the cells to leak out, a principle that has been applied in industry. A special feature of algal cells is a gas vacuole that accounts for 50% of the cell’s volume; this gas vacuole controls the upward and downward movement of the algal cell. Shock waves, jets, radiation pressure, etc., induced by ultrasound may destroy air bubbles. At appropriate frequencies, the air bubbles can even turn into cavitation bubbles and burst. At the same time, the high temperature and pressure generated by cavitation, along with the large number of free radicals, can destroy the active enzymes and substances within algae cells, thereby affecting the cellular physiological and biochemical activities. Furthermore, the chemical effects induced by ultrasound can also break down algal cell secretions and metabolites such as algotoxins. The effect of ultrasound is influenced by various factors, among which frequency and intensity are the most important; optimizing the process conditions is the focus of subsequent research