Review on Pond Aquaculture Tail Water Treatment Technologies

Shunlong MENG Lihua REN Xi CHEN Liping QIU Chao SONG Limin FAN Dandan LI Jiazhang CHEN Pao XU

Abstract   Carrying out the treatment of aquaculture tail water and realizing the discharge and recycling of the aquaculture tail water are the inevitable trend in the development of the fishery industry at present and in the future. For this reason， this paper reviewed the characteristics of pond aquaculture tail water， the sources of pollutants in intensive aquaculture tail water， the important parameters in the water body that affect the growth of cultured organisms， and the water treatment methods in the process of aquaculture， and prospected from the technical level and industrial policy level， hoping to accumulate data for promoting the green development of aquaculture and cleaner production.

Key words   Pond aquaculture; Tail water treatment; Pollutant; Parameter

Aquaculture occupies an important position in China’s agricultural industrial structure and plays a huge role in accelerating the construction of the national economy  [1] . Pond aquaculture contributes 7/10 of China’s freshwater aquaculture production， in which fish production occupies the first position  [2] . At present， breeding methods have changed from the traditional extensive mode to the modern intensive and refined mode. Intensive aquaculture has two notable characteristics： one is the high breeding density， and the other is the difficulty in controlling the breeding environment， that is， through artificial control of environmental factors and management measures such as increasing the intensity of feed input， high-density biogroups can be raised in relatively small closed waters， so as to achieve a high-return breeding model in a short period of time.

However， since most intensive aquaculture ponds are semi-enclosed still water aquaculture， the lack of effective water purification facilities leads to the imbalance of the pond ecosystem structure， and a large amount of feed residues and fish excrement and other substances sink to the bottom of the ponds， further causing the accumulation of organic matter in the ponds. If the eutrophic water body is not discharged in time， it will lead to the continuous deterioration of the water body， which will affect the breeding environment at light level， and cause loss of appetite， morbidity， poisoning and even death of organisms at worst  [3-4] . At this stage， many farmers directly adopt the plan of changing the water， and discharging the untreated nutrient-rich water directly into the surrounding environment of the aquaculture will affect the quality of the surrounding waters， rivers and downstream water bodies， causing ecological and environmental problems.

With the further development of the intensive aquaculture technology， the problems of ecological environment pollution， water pollution and other ecological damage caused by aquatic product aquaculture are gradually heating up， and have attracted extensive attention from local environmental protection departments and scholars at home and abroad. How to repair the damaged environmental problems and reduce the water pollution and ecological pressure caused by aquaculture has become the top priority of current aquaculture  [3] . How to purify the water in the aquaculture process， improve the quality of the water environment for aquaculture， increase the yield and quality of aquaculture objects， and reduce the waste of water resources， especially freshwater resources， has become an important foothold for intensive pond aquaculture. The treatment of aquaculture wastewater and the in-situ restoration of aquaculture ponds have also entered a stage of vigorous development， providing theoretical support for solving pond water pollution in the future.

Characteristics of Tail Water from Pond  Culture

Tail water in the intensive farming process is mainly produced by precipitation， dissolution and certain chemical action of feed residues and excrement wastes in the water body. It has certain characteristics. First， it is rich in nutrients such as nitrogen and phosphorus， so it is not waste water in a pure sense， but rich water with high nutritional value that can be reused. Secondly， it also contains certain chemicals and solid waste， and if it is not treated for a long time， it may release toxic gases such as ammonia. In addition， it can pose a threat to human health through the food chain， and its processing requirements are higher. While satisfying the discharge of water purification systems， it is necessary to return it to the production process of aquaculture as much as  possible  to realize the multi-level utilization of water for aquaculture， so as to achieve the purpose of saving and protecting water resources and reducing production costs. Therefore， it puts forward requirements for the treatment of intensive aquaculture wastewater  [5] .

Sources of Pollutants in the Tail Water of Intensive Aquaculture

Traditional farming relies on natural bait and other resources in water bodies for farming. The advantages of this model are cost saving， low input， low output， and friendliness to the breeding environment and surrounding ecology. In highly intensive farming， in pursuit of higher yields， excessive feed is often used， and only a small portion of the feed is utilized. In the process of fish farming， only 20% to 30% of the nutrients in the feed fed into the aquaculture water are absorbed by the fish， and remaining components are dissolved in the aquaculture water or form tiny particles suspending on the surface of the aquaculture water  [6] . Finally， about 15%-65% of phosphorus and 20%-50% of nitrogen can be absorbed and utilized by aquaculture organisms， and the remaining nitrogen and phosphorus components become important contributors to eutrophic water bodies  [7] . In the process of shrimp farming， the nitrogen input by artificial bait accounts for 90%， but only 19% is absorbed， and the rest is dissolved in water， which aggravates the deterioration of the farming environment  [4] . In addition， in an intensive breeding environment， aquaculture organisms will increase their own feed intake and increase excrement， and the unused nitrogen and phosphorus components in the excreta are also quite high. The efficiency of feed utilization by fish is generally only 35% to 40%  [8] . The absorption rate of P in the feed by shrimp is only 7%， and the rest of the components will be discharged into the water body. During the whole process of shrimp farming， N and P discharged into the water account for 77%-94% of the total feed  [9] . In the high temperature season， excessive nitrogen and phosphorus and other nutrients are accumulated and prone to the outbreak of cyanobacteria， which will aggravate the deterioration of the water quality of ponds. In addition， antibiotics and other drugs used in aquaculture are very easy to remain in the water body， and they will not only cause the deterioration of the aquaculture water body， but also be easily absorbed by aquaculture organisms through the food chain， and finally threaten human health through market circulation  [10-11] .

Important Parameters Affecting the Growth of Cultured Organisms in the Water Body 

Dissolved oxygen

Dissolved oxygen is one of the most important factors affecting the growth and development of aquaculture animals and food organisms. No living things can survive without oxygen. Whether the dissolved oxygen in the water body is adequate or not is related to the growth process of the aquaculture objects. When the dissolved oxygen is lower than the lower limit of respiration of the farmed animals， it will lead to the occurrence of poor diet， floating heads and oxygen-driving slow swimming of the breeding objects， and the occurrence of death and floating in severe cases. When the dissolved oxygen is sufficient， it will promote the activities of the breeding objects. However， too high dissolved oxygen may lead to "gas disease" in farmed animals. According to the data， the concentration of dissolved oxygen in aquaculture water should be kept at 5 to 8 mg/L， which is more suitable for the growth and development of aquaculture organisms. When the dissolved oxygen is below 1 mg/L， most fish will show the symptom of head floating， and prolonged hypoxia will cause the fish to suffocate and die due to hypoxia  [12] .

pH

pH is an indicator of the acidity and alkalinity of water. Most fish are adapted to living in an alkaline water environment with a pH of 6.5-9.0. Too acidic or too alkaline aquaculture water will cause adverse reactions of cultured organisms， resulting in the stress phenomenon of the cultured objects. When the pH in the water body is too low， the osmotic effect of the aquaculture water body through their epidermal cells will greatly reduce the oxygen-carrying capacity of the blood. In this case， even if the dissolved oxygen in a water body is increased， it will cause the occurrence of hypoxia in the aquaculture objects. When the pH in the breeding environment is too high， the alkalinity of a water body will increase， which will affect various physiological phenomena of the breeding objects. In severe cases， it can cause damage to body tissues of the breeding objects and hinder their normal growth  activities.

Ammonia nitrogen

Ammonia nitrogen is one of the most concerned physical and chemical indicators in aquaculture. It has two states in water， one of which is the ionic state， existing in the form of NH +4-N， and the other is the non-ionic state， in the form of NH3·H2O and NH3. Harmful to aquaculture animals are mainly non-ionic NH3·H2O and NH3. In the intensive breeding mode， with the continuous increase of ammonia nitrogen concentration， when it exceeds a certain threshold， it will not only cause direct poisoning to the breeding objects， but also reduce their immunity， which will lead to the outbreak of various diseases. China’s fishery water quality standard stipulates that in the process of aquatic product aquaculture in China， the concentration of ammonia nitrogen in the water body should not be higher than 0.2 mg/L  [13] .

Nitrite

Nitrite is another important indicator that causes harm to aquaculture objects in the process of aquaculture. When dissolved oxygen is sufficient， ammonia nitrogen is directly converted into nitrate， but once dissolved oxygen is lacking or the concentration of ammonia nitrogen in water is too high， intermediate nitrite will be produced. The nitrite content in the water should be reduced as much as possible during the breeding process. Generally， when the nitrite content in the aquaculture environment is above  0.1  mg/L， the life of aquaculture organisms will be affected  [14] .

Phosphorus

Phosphorus is not only an essential mineral element for living things， but also an important component of bone development and fish scale growth. Therefore， a large amount of phosphorus needs to be supplemented in the process of aquaculture. However， the natural phosphorus element in the aquaculture environment is very small， so the supplementary phosphorus element for fish mainly comes from the feed. In order to meet the growth needs of farmed animals， artificial compound feed is generally added with a certain proportion of phosphorus. However， due to the difference in the effective utilization rate of feed by aquatic animals， a large amount of phosphorus will eventually be wasted in water bodies. Phosphorus is also an important nutrient element required by phytoplankton. In water bodies rich in phosphorus， algae and other plants will continue to grow wildly.

Organic matter

The organic matter in the aquaculture environment is an important factor that causes the changes in the physical and chemical indicators of a water body. It is mainly composed of residual aquaculture feed， the metabolites of plankton in water bodies， the excrement of aquaculture animals and the decomposition of corrupt corpses. When the organic matter is too high in aquaculture waters， on the one hand， it is easy to breed various pathogenic bacteria and cause diseases， and on the other hand， when the breeding environment is hypoxic， a large amount of organic matter will accelerate the consumption of dissolved oxygen and accelerate the deterioration of water quality  [15] .

Turbidity

Turbidity is one of the most intuitive indicators of water bodies. The degree of turbidity in the water environment， on the one hand， directly affects the depth of light entering the water body， and indirectly affects the changes in the number of cultured organisms and plankton in the water body， and on the other hand， suspended matter also directly affects the growth and development of cultured organisms and damages fish gills  [16-17] .

Sulfide

The concentration of hydrogen sulfide in aquaculture should not be too high， and hydrogen sulfide below 0.1 mg/L is safe for aquaculture objects. According to the data， the concentration is related to the pH value， and the toxicity increases as the pH decreases. Its mechanism of action is similar to that of nitrite， which causes tissue hypoxia by affecting the oxygen-carrying capacity of hemoglobin in cultured organisms  [16] .

Shunlong MENG  et al.  Review on Pond Aquaculture Tail Water Treatment  Technologies

Methods of Water Treatment in Aquaculture

The restoration of pond water quality is a high-tech field covering biology， microbiology， physics， aerodynamics， information and computer. With the development of science and technology， new technologies and processes will be applied to the improvement of pond water quality. Although the current aquaculture wastewater treatment methods emerge in an endless stream， according to the mechanism of remediation work， the treatment  technologies  are divided into four categories： physical， chemical， biological and comprehensive treatments  [17] . No matter what kind of water quality remediation treatment technology， it is to exchange the highest economic benefits with the smallest investment as much as possible.

Physical methods 

Changing the water

Changing the water is to discharge the nutrient-rich aquaculture wastewater from aquaculture ponds to the aquaculture environment and add clean and new water， which is the most effective measure to solve the water pollution in the aquaculture process. The aquaculture water in ponds should be replaced regularly every 10 to 15 d， and the amount of water to be replaced each time is about 1/3 to 2/3 of pond capacities  [18] . However， this method has disadvantages such as large waste of water resources and pollution of the surrounding water environment.

Adsorption and filtration

One of the important reasons for the deterioration of aquaculture water bodies is the decomposition of suspended organic matter composed of residual feed and aquaculture biological excrement in the water body. Adsorbing small particles in a water body on the surface or the gap between adsorbents through the adsorbents can reduce the concentration of these substances in the water body， so as to achieve the purpose of removing organic matter and avoiding water pollution.  Filtration is a processing technology that realizes solid and liquid separation under the action of external force. Common adsorption materials include activated carbon， medical stone，  etc. ， and commonly used filtration equipment include mechanical filter， sand filter，  etc.   [16] . Although this method has the advantages of simple operation and relatively low cost， the removal effect of nitrogen and phosphorus in water is not good  [27] .

Sediment dredging

The main component of the silt at the bottom of ponds is organic matter precipitated in the water bodies， which is stored as organic matter  [19] . If the dissolved oxygen in ponds decreases or the water quality deteriorates， it will directly affect the survival rate of plankton and cultured organisms in the water bodies， which is a potential threat to the cultured objects. Sludge removal is a technical measure for soil remediation， which restores the soil to its pre-farming state. It is of great value for restoring the vitality of aquaculture ponds， activating the aquaculture environment of water bodies， reducing potential hazards at the bottom of ponds， ensuring the stability of pond aquaculture ecology and increasing aquaculture production  [20] . However， pond dredging requires huge financial and material resources， and the operation is relatively difficult. It is generally used in pond cleaning， and is generally not adopted in the actual production process.

Magnetic separation

Electromagnetic separation method is a method for treating heavy metal pollutants in aquaculture water based on the electromagnetic principle. However， this method has too high cost and certain technical requirements， and the processing objects are more targeted， so it has not been widely used.

Aeration and blow-off method

Aeration and blow-off are to allow full contact between compressed air and aquaculture water， so as to increase dissolved oxygen in the aquaculture water and change the bottom materials of ponds. However， this method does not essentially discharge the organic matter in water bodies into the aquaculture waters， but only increases the dissolved oxygen in the water correspondingly. The decomposition of organic matter in water bodies is limited， so the treatment effect on the eutrophication of water bodies is general.

UV disinfection

Ultraviolet disinfection method is to use ultraviolet wavelength in the range of 200-400 nm to sterilize and disinfect aquaculture water. Although the ultraviolet treatment method has the advantages of high efficiency， it has requirements on the turbidity， flow rate， and depth of the water for aquaculture， and the treatment effect on closed recirculating aquaculture environments is not good  [16，20] .

Mechanical algae removal

The mechanical algae removal method is to use algae removal mechanical equipment to remove algae on the upper surface of a water body when cyanobacteria blooms break out in the high temperature season in summer. It is mostly used in large-scale aquaculture waters， and in the outbreak season of cyanobacteria. The single treatment object， large amount of engineering and high cost are the most intuitive shortcomings of this method  [21] .

Chemical methods 

Flocculation sedimentation

The flocculation sedimentation method is to sprinkle some chemical substances with flocculation into the water， so that the larger particles and some suspended solids in the water body are electrically neutralized or destabilized， forming a large polymer precipitation that is difficult to degrade， and achieving the effect of removing pollutants  [22-23] . The wastewater treated with the flocculants can be filtered and adsorbed again， and then returned to aquaculture ponds. However， in this method， the pollutants are still at the bottom of ponds， and follow-up work such as sediment removal is required， which increases the difficulty of the work.

Neutralization

Neutralization is a water treatment technology that adjusts the pH of aquaculture water. If the pH in a water body is too high， an appropriate amount of weak acid substances such as oxalic acid and acetic acid can be added to reduce the pH of the water. Otherwise， alkaline substances such as quicklime can be added.

Redox

Chlorine-containing disinfectants， ozone， hydrogen peroxide， potassium permanganate and other strong oxidizing substances can undergo redox reactions with nitrogen， phosphorus and some organic substances in aquaculture wastewater， so as to  achieve  the purpose of water purification. But generally， secondary treatment is needed. At present， the search for efficient and pollution-free chemical preparations has become one of the important research fields of chemical treatment of water pollution.

Complexing method

The complexing method mainly removes excess heavy metal ions in aquaculture waters. EDTA-Na2 is a chemical agent that can complex with heavy metals in water， especially in the process of seedling treatment， when the removal of heavy metal ions in water is necessary  [24] .

Biological methods

Biological treatment is the use of the growth characteristics of organisms， that is， through their own physiological processes such as digestion， absorption， transport and metabolism， to achieve the removal of excess organic matter in water. In the process of intensive farming， a large amount of residual feed and organic matter caused by inputting excessive feed is the most fundamental reason for the eutrophication of water bodies. Nitrogen， phosphorus and potassium are indispensable nutrients for aquatic plants and plankton to carry out life activities. Therefore， adding these organisms to aquaculture water can not only improve the utilization rate of feed， but also effectively eliminate the eutrophic state of water and save the cost of treatment in farming.  At present， the plant purification method  [25-28] ， the microbial purification method  [29-31]  and the animal purification method  [32]  used in the market have all  achieved  corresponding results， are the most popular tail water treatment methods in aquaculture at present.

Animal restoration technology

Animal restoration， that is， stocking an appropriate amount of aquatic animals different from the breeding objects， is adopted to achieve the improvement of pond culture water. The types of animals to be stocked should be based on the specific aquaculture water， such as adding filter-feeding fish such as silver carp and bighead carp or a certain density of cladocerans to reduce the number of algae in the water. The objects to be input should be controlled at a reasonable density. On the one hand， it is to prevent them from competing with the breeding objects for feed and affecting the normal feeding of the breeding objects. On the other hand， it will increase the load of ponds and reduce the effect of restoration  [33] . A reasonable amount of input can promote the restoration effect of ponds  [34] .

Phytoremediation technology

Phytoremediation is to use the growth characteristics of plants to absorb some organic substances such as nitrogen and phosphorus in water and sediments， so as to purify the tail water of aquaculture. In addition， the phytoremediation technology can also remediate heavy metal-contaminated waters， such as water spinach， which has a strong adsorption effect on heavy metals  [3] . Studies have shown that the removal rate of nitrogen and phosphorus by plant tissue is stronger than that of chemical oxygen demand  [35] . Different plants have different treatment efficiencies， and it is better to combine plants with different functions to treat aquaculture tail water  [36] .

Microbial remediation technology

Microbial remediation refers to artificial cultivation of microbes that have specific decomposition effects on specific pollutants by spreading in areas with serious pollutants. The use of the microbial enhancement technology can effectively remove the polluting components of black and odorous water， and achieve the effect of water purification  [37] . The application of the immobilized microorganism technology in the restoration of eutrophic water can effectively remove nitrogen and  phosphorus  pollutants in the water， so that the water can maintain certain biological activity  [38] .

Application of ecological floating island restoration technology

The ecological floating island restoration technology mainly relies on the huge plant roots scattered in water bodies to absorb suspended solids in water bodies， and meanwhile， microorganisms enriched on the surface of the root system form a biofilm that decomposes pollutants in water bodies， so as to transfer nutrients from aquaculture water to plants. The micro-nano aeration-ecological floating island combined technology can effectively increase the dissolved oxygen content of water bodies in a short period of time， and the degree of agitation to water bodies is small， which helps to improve the stability of the biological filler film， so the removal effect of nitrogen and phosphorus is significantly higher than that of traditional ecological floating islands  [39] . After adding water spinach ecological floating islands to turtle breeding ponds， the concentration of pollutants was significantly reduced， the aquaculture water environment was significantly improved， and the disease incidence was much lower than that of the control group  [40] . Reasonable plant-filler combined ecological restoration can significantly improve the removal effect of pollutants  [41] . The ecological restoration research of the hydrophilic river in Wuxi City showed that the purification effects of conventional physical and chemical indicators of water bodies were obvious compared with the control group， and most of the indicators differed by nearly 50% from the control group  [42] .

Comprehensive treatment method

This method mainly includes two modes： integrated farming using artificial wetland ecology and closed-cycle factory farming. Constructed wetland ecosystems are usually composed of surface-growing plants and plankton，  etc. ， and have certain purification  ability.  Constructed wetland systems have a relatively good treatment effect on aquaculture tail water， especially obvious removal effect of nitrogen and phosphorus， and the cost is low  [43] . The removal effects of constructed mangrove wetlands on nitrogen and phosphorus in aquaculture tail water showed that the average effective removal rates for NO -2-N， NH +4-N and NO -3-N were 61.23%， 49.01%， and 53.14%， respectively  [44] . The closed-cycle factory farming model is integrated into civil engineering， microbiology and other disciplines. Under the closed intensive factory breeding mode， the breeding mode of biological breeding， water purification and aquaculture water recycling can theoretically be realized. The closed circulating water industrial aquaculture systems have a certain removal effects on the nutrients in the aquaculture tail water. Studies have shown that the purification capacity of factory aquaculture systems gradually increases over time， and the data in this model tend to be stable， but the overall removal efficiency needs to be further improved  [45] .

Prospects

Under the guidance of the national strategy of ecological priority and green development， and under the specific guidance of the "Five Actions" for the promotion of green and healthy aquaculture technology for aquaculture， the regulation of aquaculture water quality and the treatment of aquaculture tail water will receive further attention. High-quality products and beautiful origins have become a broad consensus. The aquaculture industry must incorporate the concept of green development throughout the entire process of aquaculture production. It is necessary to speed up the transformation of methods， adjust the structure， actively promote green development and clean production， and promote the formation of green fishery production methods based on ecological environment friendliness and sustainable resource utilization. At the technical level， for concentrated contiguous aquaculture areas， we can implement ex-situ purification technologies such as artificial wetlands and ecological ditches， and build a circulating water aquaculture model. For sporadic aquaculture areas， in-situ purification technologies such as fish-vegetable symbiosis， microbial water transfer， and algae water transfer can be implemented to build an ecologically healthy farming model. At the policy level， government departments should provide policy support and industrial development guidance， and implement an ecologically healthy breeding system through policy guidance and financial support. Standard discharge and recycling of aquaculture tail water should be  achieved.

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 Editor： Yingzhi GUANG   Proofreader： Xinxiu ZHU