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Plan for the protection of landscape water bodies in residential communities: Based on data analysis, as society develops and people’s material and spiritual standards of living improve, there is an increasing demand not only for intelligent hardware facilities in residential areas, but also for attractive external landscapes, green spaces, and a natural environment. When planning residential communities, it is necessary to reserve certain areas for green spaces and water bodies in order to maintain a serene and natural environment there. Maintaining the cleanliness and freedom from pollution of the water bodies in residential areas or rivers is one of the key factors in improving the living conditions of residents and attracting buyers. However, since these water bodies are enclosed, they generally have characteristics such as small size, susceptibility to pollution, limited capacity to handle pollutants, and low self-purification ability. If proper management is not carried out for these water bodies, and if emissions of pollutants from surrounding areas are not strictly controlled, they can easily become receptacles for domestic wastewater, rainwater, and household waste from the residents, leading to varying degrees of water pollution. In severe cases, this can result in eutrophication of the water, with an excessive growth of algae, causing the water to turn dark and smelly, thereby severely affecting the surrounding natural environment and the quality of the residents’ living conditions. Therefore, the discharge of pollutants into the water bodies and rivers within residential areas must be strictly controlled, and appropriate protective measures must be taken to maintain a pleasant natural environment and a good living condition in those areas. Otherwise, if the water bodies and rivers in residential areas become polluted to varying degrees, it will not only fail to improve the natural environment of the area but may also have adverse effects on the surrounding environment, even affecting the normal lives of residents living nearby. Therefore, how to ensure the cleanliness and pollution-free status of enclosed water bodies in residential areas has become a topic of concern. Control measures for pollution of landscape water bodies 1. Main sources of pollutants For landscape water areas in residential communities, the main sources of pollutants are domestic wastewater generated by residents’ daily activities, wastewater from car washing in community parking lots, rainwater, household waste, construction waste and its leachate, floating debris, and construction dust. In particular, domestic wastewater contains large amounts of organic pollutants as well as plant nutrients such as nitrogen and phosphorus. Once these plant nutrients enter natural water bodies, they degrade the water quality, accelerate the process of eutrophication, and affect the utilization of those water bodies. 2. Pollution source control measures To maintain the cleanliness of enclosed, slowly flowing water bodies (rivers, lakes) and ensure that they meet the specified water quality standards for use in landscape and recreational purposes, it is necessary to strictly control those pollution sources that may release the aforementioned pollutants into such water bodies. The main recommended measures are as follows: (1) Domestic wastewater from residential areas must be discharged into the urban sewer system and treated at urban wastewater treatment plants; it cannot be directly released into the enclosed landscape water bodies within those areas ； (2) When the community’s sewer system cannot be connected to the existing municipal sewer system, a community wastewater treatment plant must be established to treat all wastewater within the community. The water purified by this plant can be reused for flushing toilets, watering green spaces, cleaning roads, washing cars, etc., or it can be used, after further treatment, as water to replenish closed landscape water bodies ； (3) Surface runoff water contains a high amount of organic matter and inorganic dust; in particular, the pollutant concentration in surface runoff water is higher during the first 10 minutes of rainfall. It should be discharged into urban stormwater systems and not directly into enclosed water bodies, as doing so can cause siltation or varying degrees of pollution of the water bodies. If conditions permit, it is possible to consider using rainwater that has been collected and left to settle as supplementary water, which can then be discharged into that enclosed body of water ； (4) A dedicated person must be assigned to remove floating objects from the water surface promptly; organic matter such as weeds and leaves, if not removed in time, remaining on the water surface not only affects the natural oxygenation process of the water but also, after decaying and accumulating on the lake bottom, causes the water quality to become foul-smelling ； (5) It is strictly prohibited to pile up domestic or construction waste in the areas surrounding the lake, so as to prevent waste debris from being blown onto the lake’s surface by the wind or waste leachate from flowing directly into the lake, thereby causing varying degrees of pollution to the water quality of the lake ； (6) The lake slopes should be lined with rubble or precast concrete blocks to prevent the soil on the slopes from being washed away by water waves, which could affect the sensory parameters of the water quality. 3. Measures for pollution control Methods for treating polluted landscape water bodies include: (1) Physical methods Physical methods for purifying landscape water bodies include mechanical filtration, dredging of bottom sediment, light regulation, water level control, high-voltage discharge, ultrasound, etc. These methods are effective, but they are not easy to implement on a wide scale. Removing lake bottom sediments on a regular basis and suppressing the release of nitrogen and phosphorus from the sediment are effective ways to control internal loading. Regular water replenishment is one of the most basic methods for maintaining the water quality of landscape waters. Its main mechanism is dilution, which is a physical purification process. Dilution does not change the properties of pollutants, but it creates conditions for further purification processes; for example, it reduces the concentration of harmful substances, allowing other purification processes in the water body, especially biological purification, to resume normally. The method of regularly replenishing water is an effective approach for landscape water bodies with smaller surface areas. Even if full water replacement is considered, it will not result in excessive waste of water sources; it is economically viable, easy to manage, and can achieve the desired results. However, for larger landscape water bodies, the only option is to replenish water regularly; replacing the water all at once would result in significant waste of water resources, which is not economically viable. Therefore, regular water replenishment can help reduce the increase in salinity in water bodies caused by evaporation and seepage, as well as dilute the concentrations of harmful pollutants in them. It can only serve to delay the deterioration of water quality and the occurrence of eutrophication, but it cannot fundamentally solve the problem of gradual degradation of water quality. The amount of water to be added should be determined based on local conditions. In southern regions, where rainfall is abundant, only a small amount of water needs to be added during certain periods; artificial supplementation is not required in most months. In the northern regions, where the annual evaporation rate is much higher than the annual rainfall, supplementation is necessary to maintain the desired water level in the bodies of water. In areas with high groundwater levels, groundwater replenishment is also an important source; however, if the groundwater is brackish, an appropriate amount of fresh water needs to be added to dilute the accumulation of salts and maintain the relative stability of the ecosystem in the water body. (2) Chemical methods For slow-flowing water bodies such as lakes and rivers, where eutrophication has occurred due to the excessive discharge of plant nutrients like nitrogen and phosphorus, resulting in bad water quality, chemical agents can be added directly to the water to kill algae. After natural sedimentation, the sediment layer can be removed, thereby preventing eutrophication of the water body. Common agents used for killing algae include copper sulfate and bleaching powder. When adding the chemical to reservoirs or lakes, it can be placed in a cloth bag, tied to the stern of a boat, submerged in the water, and then the boat can be moved through the water along a specific route. The dosage depends on the type and quantity of algae, as well as other relevant conditions. Generally speaking, copper sulfate is more effective and has a longer duration of action; adding 0.3–0.5 mg per liter of water can kill most odor-producing algae within a few days, but it often fails to eliminate the foul-smelling substances released by the dead algae. Chlorine bleach or chlorine can remove these odor-causing substances released, but a larger amount is required, such as 0.5–1 mg. It should be noted that too much chlorine should not be added, as this will instead increase the odor of the water. The correct dosage of the agent can be determined through experimentation. Furthermore, since copper sulfate is toxic to fish as well, its lethal dose varies depending on the species of fish, ranging from approximately 0.15 to 2.0 mg/l. This figure is within the range of doses required to eliminate algae; therefore, caution should be exercised when using algicides to kill algae in ponds used for both landscaping and fish farming, to avoid the death of fish in the water. (3) Aquatic plant system Aquatic plant technology is guided by ecological principles, applying the structure and functions of ecosystems to water quality purification. It makes full use of natural purification processes as well as the synergistic interactions among various aquatic organisms within the aquatic plant system to improve water quality. By leveraging the competitive relationships between organisms, water quality can be adjusted, and resources can be effectively recovered and utilized, thereby achieving benefits such as improved water quality, resource utilization, and enhanced landscape aesthetics. Since water pollutants include not only some easily decomposable organic compounds but also plant nutrients such as nitrogen and phosphorus, planting aquatic vascular plants in the lake and carrying out regular cleaning can improve the water body’s ability to remove organic pollutants as well as inorganic nutrients like nitrogen and phosphorus. Additionally, the aquatic plants can be utilized for various purposes after being harvested, thereby generating certain economic benefits. Commonly used planting varieties with good effects include: water hyacinth, lotus, reed, cattail, etc. It is estimated that the yield of water hyacinth is around 5–10 kilograms per square meter of water surface. Assuming that water hyacinth contains 1.2% nitrogen and 0.79% phosphorus, the nitrogen removal capacity and phosphorus removal capacity per square meter of water hyacinth are respectively 0.06–0.12 Kg/year and 0.04–0.08 Kg/year. However, it is necessary to control the planting density of aquatic plants to prevent excessive growth, which could have the opposite effect. (4) Biological barriers and biological floating islands A biological barrier is a facility that provides attachment and growth conditions for microorganisms, protozoa, small zooplankton, and other organisms involved in pollutant purification. It involves installing rope-like biological contact materials on fixed supports, allowing a large number of organisms involved in pollutant purification to grow there. Due to their attached growth pattern, they are not easily consumed by large aquatic animals and fish, which results in a geometric increase in the number of organisms per unit volume of water, **enhancing the lake’s purification capacity. Ecological floating islands represent a new ecological technology used for water purification in enclosed water bodies. Plants can be grown on their upper surface, which helps to absorb and degrade pollutants in the water; at the same time, it also enhances the aesthetic appearance of the lake surface, creating a three-dimensional landscape effect. Plants that can be grown include: canna, water spinach, wheat, etc. (5) Aquatic animals Fish are at the top of the aquatic food chain; they rely on algae as food for plankton, which in turn serves as prey for fish, thus forming an ecosystem consisting of bacteria—algae—plankton—fish. Fish are generally kept in water bodies with aquatic plants, or released directly into the water without any bait. Since the water quality standards for scenic waters are stricter than those for fishery waters, they can fully meet the needs of fish survival. The species suitable for stocking in landscape water bodies should mainly include silver carp and white lotus carp, supplemented by bighead carp, grass carp, common carp, tilapia, etc. They can feed on algae, controlling their excessive growth and playing a significant role in preventing eutrophication of water bodies. According to experiments, even in fish farming with wastewater, the algae count in the water can be reduced to around 1,000 cells per ml. Therefore, stocking an appropriate number of fish in landscape water bodies is a good approach, as it not only helps to purify the water quality but also enhances the fishing functionality of those water bodies. The BOD5 load for fish ponds can be set within the range of 20–35 kg/ha.d. (6) Aeration for oxygenation Aeration is primarily used to supply oxygen to the water, ensuring an adequate supply of oxygen for the metabolic activities of aquatic organisms and for microorganisms to oxidize and decompose organic matter; it also helps to stir the water, thereby promoting water circulation. If large amounts of plant nutrients such as nitrogen and phosphorus enter slow-flowing bodies of water like lakes and ponds, it will boost the activity of various aquatic organisms, primarily algae, stimulating their excessive growth. The overgrowth of algae leads to a sharp drop in dissolved oxygen in the water, resulting in a severe oxygen deficiency in the water body for a certain period of time, which causes large numbers of fish to die. Therefore, using aeration to oxygenate enclosed water bodies can, to a certain extent, prevent fish deaths caused by excessive algae growth, and plays a role in maintaining the ecological balance of the water body. Aeration can only delay the occurrence of eutrophication in water bodies, but it cannot resolve it fundamentally. The main methods of aeration currently in use are natural fall water aeration and mechanical aeration. Natural fall water aeration has a low oxygenation efficiency, but it requires no energy and is simple to maintain; however, it generally struggles to meet the high oxygenation requirements ； Mechanical aeration offers high oxygenation efficiency and flexible options, and is widely used for oxygenating lakes or ponds. To ensure adequate oxygen supply for fish, the dissolved oxygen level in the water should generally be above 3 mg/l (which corresponds to the standard value for Class B water quality). Typically, one aeration unit is required for every 10–15 mu of water surface. (7) Micro-pollution biological treatment This treatment method is one of the most commonly used, efficient, and advanced treatment techniques both domestically and internationally at present. Biological treatment of slightly polluted water generally employs the biological contact oxidation method. The mechanism of this method involves allowing bacteria and fungi, as well as microorganisms such as protozoa and metazoans, to attach to fillers or certain carriers where they can grow and multiply, forming a biofilm. When the wastewater comes into contact with this biofilm, the organic pollutants and plant nutrients like nitrogen and phosphorus present in the wastewater are utilized as nutrients by the microorganisms in the biofilm, thereby purifying the slightly polluted water; at the same time, the microorganisms themselves also reproduce. This treatment method can effectively remove organic pollutants from wastewater, reduce the total amount of pollutants, and achieve complete purification of the water body. It is widely used in the treatment of wastewater and slightly polluted water, and constitutes an effective treatment approach. According to domestic and international reports, biological contact oxidation can achieve removal rates of 20–30% for COD and 80–90% for NH3-N in slightly polluted water bodies. The experimental results from our institute show that removal rates of 20% and 30% can also be achieved for these two parameters, and such removal rates will have a significant impact on maintaining the water quality of lakes. (8) Addition of PSB This method is currently widely used in countries such as Japan, South Korea, and Australia. It also belongs to the category of biological treatment methods; in this approach, photosynthetic bacteria are added to water on a regular basis. Photosynthetic bacteria are microorganisms that grow in aquatic systems. The cells of these bacteria contain 60% (by mass) protein, and when the amount of protein is equivalent to that found in yeast protein or fish meal protein, they also contain abundant vitamins and folic acid. Since photosynthetic bacteria can use light energy and oxygen to convert inorganic and organic carbon sources as well as other nutrients in slightly polluted water or wastewater into bacterial biomass, they can thus help purify the water quality. Adding PSB is a novel treatment method featuring a simple process, no need to construct separate treatment facilities, and low initial investment. However, the cost of adding bacterial strains is high, which in turn increases the treatment costs. At the same time, since photosynthetic bacteria are photoautotrophic organisms and do not contain nitration or denitrification strains, they have a high removal efficiency for organic pollutants in slightly polluted water or wastewater. However, for plant nutrients such as nitrogen and phosphorus, they can only remove them at a ratio of COD:N:P=100:5:1, resulting in a relatively low removal efficiency. The fundamental cause of eutrophication in slow-flowing water bodies such as lakes and ponds is the excessive influx of plant nutrients like nitrogen and phosphorus. Since photosynthetic bacteria do not possess the ability to remove nitrogen and phosphorus, using these bacteria as a treatment method for slightly polluted water cannot fundamentally resolve the issue of eutrophication in such water bodies. The methods for the protection of landscape water bodies and pollution control mentioned above were derived from our institute’s practical experience in the conservation projects of the lakes in Meijiang Community and Meijiangnan Community in Tianjin, as well as from engineering examples both domestically and internationally; they hold certain guiding significance for the protection and management of landscape water bodies.