Eight practical methods for reducing ammonia nitrogen in aquaculture

According to a report by the China Aquatic Products Channel, high concentrations of ammonia nitrogen already exist in many industrial wastewaters. If water bodies rich in ammonia nitrogen are discharged into other water bodies, especially slow-flowing lakes and bays, they can easily cause algae and other microorganisms in the water to multiply in large numbers, forming eutrophication pollution. In severe cases, this reduces dissolved oxygen in the water and causes mass fish deaths.

Nitrogen in water mainly exists in several forms: ammonia nitrogen, nitrate nitrogen, nitrite nitrogen and organic nitrogen.

Under aerobic conditions, ammonia nitrogen can be oxidized by nitrifying bacteria into nitrite nitrogen and nitrate nitrogen.

In wastewater, nitrogen exists in many forms, including molecular nitrogen, organic nitrogen, ammoniacal nitrogen, nitrate nitrogen, nitrite nitrogen, as well as thiocyanates and cyanides, and ammonia nitrogen is one of the most important forms.

The concentration of ammonia nitrogen in water is not fixed either; it can transform among the various forms of nitrogen. Nitrogen is an essential macroelement for algae. It is also a relatively common nutrient element limiting primary productivity in aquaculture water bodies, and it has a great impact on production.

In the aquaculture water of artificial ponds, nitrogen exists in forms such as molecular nitrogen, inorganic nitrogen and organic matter (such as urea, amino acids and proteins).

Under the influence of biological, abiotic and anthropogenic factors, they continuously transform and migrate in the water body and are constantly engaged in a dynamic cycle. Among them, ammoniacal nitrogen in water has the greatest impact on production; all are essential nutrients for algae, and almost all algae can directly, rapidly and preferentially utilize them.

Its unfavorable aspect is that the presence of ammoniacal nitrogen inhibits the utilization of nitrite nitrogen and urea by algae; moreover, the process of converting ammoniacal nitrogen into nitrate also consumes dissolved oxygen in the water. In particular, molecular ammonia is highly toxic to fish and other aquatic animals. Even at very low concentrations, it inhibits growth, damages gill tissue, aggravates fish diseases, and adversely affects aquaculture production.

The main sources of ammonia nitrogen in pond water are the decomposition of nitrogen-containing organic matter in the pond water and bottom sediment and the metabolic activities of aquatic organisms. This is the main pathway by which ammonia content in water increases.

Especially in high-input, high-output ponds, artificial heavy feeding and fertilization increase the amount of nitrogen-containing organic waste in the pond.

The stocking density is high, biological metabolism is vigorous, and the amount of ammonia excreted as waste increases. The rate of ammonia increase greatly exceeds the utilization limit of phytoplankton, causing ammonia to accumulate in the water.

Ammoniacal nitrogen exists in water in two forms, ammonia and ammonium, and under complex water environment conditions it continuously interconverts according to the following formula to reach a dynamic equilibrium.

When the pH value is less than 7, ammonia in water exists almost entirely in the form of ammonium; when the pH is greater than 11, it exists almost entirely in the form of ammonia, and as temperature rises the proportion of ammonia increases. That is to say, under alkaline conditions, the higher the water temperature, the greater the proportion of ammonia molecules and the stronger the toxicity.

Some studies have shown that the maximum ammonia concentration that fish can tolerate over the long term is 0.025 mg ammonia/L.

① Fish metabolism excretes ammonia into the water through the gills, and the decomposition of organic matter in the water, including fish feces and residual feed, produces ammonia.

③ The source of ammonia nitrogen that currently causes the greatest losses in fish farming is also the excessively high exogenous ammonia nitrogen caused by surface water pollution in areas where surface water is used for fish farming.

Ammonia nitrogen is a substance often present in water, and excessively high ammonia nitrogen can cause fish poisoning and death. Excessively high ammonia nitrogen can damage the gills of fish and affect the respiratory function of the gills, thereby producing "hypoxia."

At present, the safe concentration of molecular ammonia for fish has not yet been statistically determined, but generally 0.05-0.1 mg/L of molecular ammonia is taken as the permissible limit.

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

Based on the above general patterns of ammonia changes in water, we can formulate specific ammonia-reduction measures in a targeted manner, striving to reduce the impact of molecular ammonia on aquaculture production.

There are many methods for removing ammonia nitrogen. In industry and in tap water processes, they are divided into physicochemical methods and biological denitrification methods.

Physicochemical methods include breakpoint chlorination, air stripping, chemical precipitation, liquid membrane method, electrodialysis for ammonia nitrogen removal, catalytic wet oxidation, soil irrigation, and ammonia removal in circulating cooling water systems.

Biological denitrification can remove a variety of nitrogen-containing compounds, with a total nitrogen removal rate of up to 70%-95%. It mainly includes traditional nitrification-denitrification, shortcut nitrification-denitrification, simultaneous nitrification-denitrification, and anaerobic ammonia oxidation. Sometimes combined treatment using multiple technologies is needed to make up for each other's shortcomings and achieve a better treatment effect.

Planting floating aquatic plants such as water hyacinth or water lettuce in an enclosed area at one corner of the pond can effectively reduce ammonia in the water.

Tests have confirmed that when transplanted water hyacinth covers 10% of the water surface, the total ammonia in the water can drop from 8 mg/L to 3 mg/L after five days, showing an obvious ammonia-reduction effect.

Controlling the number of zooplankton can reduce the source of ammonia in the water. According to available data, crustaceans excrete 1 mg of ammonia per gram per day as metabolic waste; Daphnia pulex can excrete about 5.11 grams of ammonia per kilogram per day.

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

Increasing the dissolved oxygen content in the water can promote the nitrification of ammonia, converting ammonia into nitrate nitrogen and nitrite nitrogen.

Studies have shown that in nitrification formed by nitrifying bacteria and nitrosifying bacteria, when dissolved oxygen is less than 5-6 mg/L, the nitrification rate accelerates as dissolved oxygen increases; the optimum pH for nitrification is 8.4; within the temperature range of 5-30°C, nitrification accelerates as temperature rises.

Measurement results show that when dissolved oxygen is plentiful, available nitrogen is mainly nitrate nitrogen, while under oxygen-deficient conditions it is mainly ammoniacal nitrogen. Therefore, improving the dissolved oxygen status of the water body can, to a certain extent, reduce ammonia content and the harm caused by ammonia.

Its principle of action is to use the algae and nitrifying bacteria attached to biological rotating disks or rotating drums to absorb and transform ammonia in the water, and the ammonia removal efficiency can reach more than 80%.

Zeolite powder, a porous aluminosilicate with high ion exchange capacity and the ability to absorb toxic metabolites, is used to reduce the ammonia content in water.

When ammonia reduction through phytoplankton assimilation in the pond or other ammonia-reduction measures cannot be implemented, zeolite powder can be applied to the pond, generally at 25-50 ppm (25-50 grams per 1 cubic meter of water), which can achieve a good effect of reducing ammonia by 90-97%. Moreover, zeolite does not absorb nitrate and nitrite and does not affect other chemical indicators of water quality.

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

Using photosynthetic bacteria to improve water quality: many studies have shown that applying photosynthetic bacteria to aquaculture water can significantly reduce the organic matter content of the substrate and water quality. This reduces the release of ammonia as a mineral decomposition product. From this perspective, applying photosynthetic bacteria also has a certain auxiliary effect on ammonia reduction.

According to foreign reports, formaldehyde can react with ammonia in water to form hexamethylenetetramine and formamide. These two products of the reaction between ammonia and formaldehyde are quite stable, and the ammonia removal effect was obvious during 7 days of laboratory treatment.

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

Using 20-25 mg/L formaldehyde in aquaculture ponds has been successful in treating fish pond diseases.

At present, countries in Southeast Asia have long used formaldehyde to remove ammonia nitrogen in shrimp ponds. There are relatively few research reports on this in China, and trials should be conducted before application.

In short, there is still a lack of quantitative prevention and control measures for reducing ammonia in water. In addition, according to reports, using water purification ponds and scientifically applying biochemical methods such as the activated sludge process and anaerobic digestion to recycle aquaculture water is a very promising water-saving fish-farming method for purification.