Eight methods of reducing ammonia nitrogen in Aquaculture

According to the China Fisheries Channel, there is a high concentration of ammonia nitrogen in many industrial waste water. If the water rich in ammonia nitrogen is discharged into other water bodies, especially slow-moving lakes and bays, it is easy to cause a large number of algae and other microorganisms in the water to multiply and form eutrophication pollution. In serious cases, it will reduce the dissolved oxygen in the water and kill a large number of fish.

Nitrogen in water mainly exists in the form of ammonia nitrogen, nitrate nitrogen, nitrite nitrogen and organic nitrogen.

Under aerobic condition, ammonia nitrogen can be oxidized to nitrite nitrogen and nitrate nitrogen by nitrifying bacteria.

Nitrogen exists in many forms in wastewater, such as molecular nitrogen, organic nitrogen, ammonia nitrogen, nitrate nitrogen, nitrite nitrogen, thiocyanate and cyanide, and ammonia nitrogen is one of the most important forms.

The concentration of ammonia nitrogen in water is not fixed, but can be transformed into each other among various forms of nitrogen. Nitrogen is a necessary major element for algae. It is also a common nutrient element that limits primary productivity in aquaculture water, which has a great impact on production.

In the culture water of artificial pond, nitrogen exists in the form of molecular nitrogen, inorganic nitrogen and organic matter (such as urea, amino acid, protein) and so on.

Under the influence of biological, abiotic and man-made factors, they are constantly transformed, migrated and dynamically cycled in the water. Among them, ammonia nitrogen in water has the greatest impact on production, and it is an essential nutrient for algae, and almost all algae can be used directly, quickly and preferentially.

On the one hand, the existence of ammonia nitrogen inhibits the utilization of nitrite nitrogen and urea by algae, and the dissolved oxygen in water is consumed during the conversion of ammonia nitrogen into nitrate. in particular, molecular ammonia is highly toxic to fish and other aquatic animals, even if the concentration is very low, it will inhibit growth, damage Gill tissue, aggravate fish disease, and adversely affect aquaculture.

The main source of ammonia nitrogen in pond water is the decomposition of nitrogenous organic compounds in pond water and sediment and the metabolism of aquatic organisms, which is the main way to increase ammonia content in pond water.

Especially in the ponds with high input and high output, large amounts of artificial bait and fertilization increase the amount of nitrogenous organic waste in the ponds.

The density of stocking is high, the biological metabolism is exuberant, and the amount of ammonia excreted by waste is increased. The increasing rate of ammonia greatly exceeds the limit of phytoplankton utilization, so that ammonia accumulates in water.

Ammonia nitrogen exists in two forms of ammonia and ammonium in water, and it is constantly transformed into each other according to the following formula to achieve dynamic equilibrium under complex water environment conditions.

When the pH value is less than 7, almost all the ammonia in the water exists in the form of ammonium, and when the pH is more than 11:00, it almost exists in the form of ammonia, and the proportion of ammonia increases with the increase of temperature. In other words, under alkaline conditions, the higher the water temperature, the greater the proportion of ammonia molecules, the stronger the toxicity.

Some studies have shown that the maximum ammonia concentration that fish can tolerate for a long time is 0.025 mg ammonia / liter.

① fish metabolism in the form of ammonia through the gills into the water, water organic matter, including fish feces, residual bait and other decomposition to produce ammonia.

③ is also the largest source of ammonia nitrogen in fish culture, which is caused by the pollution of surface water in areas where fish culture is used.

Ammonia nitrogen is a common substance in water, and high ammonia nitrogen can cause poisoning and death of fish. Excessive ammonia nitrogen can cause damage to the gills of fish, affecting the respiration of the gills of fish, resulting in "anoxia".

At present, there is no statistics on the safe concentration of molecular ammonia to fish, but it is generally based on 0.05-0.1 mg / L molecular ammonia as the allowable limit.

The concentration of molecular ammonia in the pond is related to the temperature, pH value and total ammonia concentration of the pond. when the concentration of ammonia is constant, the higher the pH value, the greater the toxicity.

According to the above general change law of ammonia in water, we can formulate specific ammonia reduction measures to reduce the impact of molecular ammonia on aquaculture production.

There are many methods of ammonia nitrogen removal, which can be divided into physical and chemical method, biological denitrification method and so on in industry and tap water process.

Physical and chemical methods include break point chlorination, air stripping, chemical precipitation, liquid membrane, ammonia removal by electrodialysis, catalytic wet oxidation, soil irrigation and ammonia removal by circulating cooling water system.

Biological denitrification can remove many kinds of nitrogen compounds, and the removal rate of total nitrogen can reach 70%-95%. There are traditional nitrification and denitrification, short-cut nitrification and denitrification, simultaneous nitrification and denitrification and anaerobic ammoxidation. Sometimes it is necessary to adopt the joint treatment of a variety of technologies in order to learn from each other to achieve better processing results.

Planting floating plants such as water lilies or water hyacinth in a corner of the pond can effectively reduce ammonia in the water.

It is proved by experiments that when the transplanted hyacinth covers 10% of the water surface, the total ammonia in the water can be reduced from 8 mg / L to 3 mg / L after five days, and the ammonia reduction effect is obvious.

Controlling the number of zooplankton can reduce the source of ammonia in water. According to some data, the metabolic waste ammonia excreted by crustaceans is 1 mg / g every day, and the flea can discharge about 5.11 grams of ammonia per kilogram every day.

Therefore, proper stocking of fish that feed on zooplankton or timely killing of a certain number of water fleas can reduce the source of ammonia nitrogen in water.

Increasing the amount of dissolved oxygen in water can promote the nitrification of ammonia and transform ammonia into nitrate nitrogen and nitrite nitrogen.

The results show that when the dissolved oxygen is less than 5mg / L, the nitrification rate of nitrification formed by nitrifying bacteria and nitrobacteria increases with the increase of dissolved oxygen, and the nitrification is the most suitable for pH=8.4, and the nitrification is accelerated with the increase of temperature in the range of 5: 30 ℃.

The results show that nitrate nitrogen is the main available nitrogen when dissolved oxygen is high, and ammonia nitrogen is dominant under anoxic condition. Therefore, improving the status of dissolved oxygen in water can reduce the content of ammonia and the harm of ammonia to some extent.

The principle of its action is that the algae and nitrifying bacteria attached to the biological turntable or drum are used to absorb and transform ammonia in the water, and the efficiency of ammonia removal can reach more than 80%.

Zeolite powder, a porous aluminosilicate, has high ability of ion exchange and absorption of toxic metabolites to reduce the content of ammonia in water.

When the ammonia reduction by phytoplankton assimilation or other ammonia reduction measures can not be implemented, zeolite powder can be applied in the pond, the dosage is generally 25~50ppm (25-50 grams per cubic meter of water), and the good effect of reducing ammonia by 90% can be achieved. And zeolite does not absorb nitrate and nitrite and does not affect other chemical indexes of water quality.

In addition, adding 5% zeolite powder to aquatic animal feed can also reduce the content of ammonia in water.

Photosynthetic bacteria are used to improve water quality. Many studies have shown that the application of photosynthetic bacteria in aquaculture water can significantly reduce the content of organic matter in sediment and water quality. As a result, the release of ammonia from mineral decomposition products is reduced. from this point of view, the application of photosynthetic bacteria also plays an auxiliary role in ammonia reduction.

According to foreign reports, formaldehyde can react with ammonia in water to form hexamethylenetetramine and formamide. The two products of ammonia and formaldehyde are quite stable, and the effect of ammonia removal is obvious in 7 days in the laboratory.

However, formaldehyde can kill phytoplankton in the pond and cause a decrease in dissolved oxygen. On the second day after treatment, the ammonia nitrogen in the water immediately decreased by 40%, and the ammonia nitrogen content changed greatly in the next few days. Ten days later, the concentration of ammonia nitrogen in the pond treated with formaldehyde was 0.3 mg / L while that in the control pond was 2.1 mg / L.

The treatment of diseases in fish ponds with 20-25 mg / L formaldehyde in culture ponds is successful.

At present, countries in Southeast Asia have long used formaldehyde to remove ammonia nitrogen from shrimp ponds. There are few reports on this aspect in our country, and the experiment should be passed before application.

In short, there is a lack of quantitative prevention and control measures to reduce ammonia in water. In addition, it is reported that the use of water purification tank and the scientific use of biochemical methods such as activated sludge and anaerobic digestion to treat aquaculture water is a promising method for water-saving and fish culture.