One of the harmful factors to the aquaculture process is an alum. Because it is a factor, affecting the productivity and quality of aquatic products. Moreover, most farmers still do not know a method to handle it in ponds properly. It leads to many consequences in the farming season. Therefore, it is necessary to identify the cause clearly to take appropriate measures.

1. Causes of alum in ponds

There are many causes of alum contamination in ponds. However, the main reason is that the soil in aquaculture contains large amounts of sulfate. Therefore, the sulfate will be reduced to release sulfur and the form of hydrogen sulfide (H2S) in anaerobic conditions and strong microorganisms. This gas will combine with iron (II) present in the ceiling of the pond bottom and form FeS2 (alum-pyrite).

In addition, prolonged rain is also the cause of alum contamination of ponds by washing away alum on the pond banks.

2. Signs of alum contaminated ponds

Farmers can identify alum in shrimp ponds through the following phenomena:

  • The gills and body of the shrimp turn yellow, the shell is harder than usual, the shrimp stop eating after the rain.
  • Pond water turns paler, clearer, and has a golden yellow floating on the water surface, the pH decrease.
  • Alum soils are often dark gray, due to high FeS2 content. This is also an important feature to identify alum contaminated soil. For shrimp ponds built on such lands, it will be difficult to handle alum.

3. Effects of alum water on shrimp pond water

a) Difficult to make watercolor

The important factor is watercolor in ponds. Because it shows the indicator of environmental factors, is at an appropriate or alarming level for the health. However, when contaminated with alum, it is difficult to cause coloration by Fe2+, Al 3+, which are difficult to dissolve impurities, limiting the growth of algae. As a result, the watercolor is not satisfactory, causing much impact on the crop.

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b) Incomplete molting of shrimp

To change weight and size, shrimp will molt. However, the characteristic of alum in the pond is low pH (<7.0), which will cause many obstacles in the molting process in shrimp: incomplete molting, sticking to the shell, reducing the survival rate of shrimp.

c) Reduced farming productivity

The cause comes from the acidic environment in the pond water, which limits the exchange of enzymes in the body, slowing the shrimp’s growth. Besides, alum water also reduces the binding capacity between oxygen and Hb compounds in the blood. As a result, the respiration process in shrimp increases, causing energy loss. Thereby, seriously affecting the productivity of farming.

d) Shrimp with soft shell

During the molting stage, shrimp need a large amount of calcium for shell formation. However, in alum-contaminated ponds, the Ca and Mg concentrations are very low, causing the shrimp to become soft. If too much lime is added to the pond, it will create gypsum which is not beneficial for the pond

4. How to handle alum in ponds

Although, alum water in ponds is very difficult to treat. However, if the correct technique is applied, it will solve the above situation and bring high efficiency to the aquaculture process.

a) Treat by fertilizing with phosphorus

Applying phosphorus to the bottom of the pond will help reduce Fe and release Phosphorus in the bottom of the pond effectively, making it easier to color the water. However, this also leads to toxic algae growth in the pond, so people need to treat the algae after treating the pond with alum.

b) Treatment by lime application

The purpose is to raise the pH of the pond bottom, remove alum and create a buffer system in the pond, lime is spread evenly to the bottom and the pond bank. Besides, providing calcium and magnesium content helps increase the alkalinity and hardness of the water. However, when applying limestone or hot lime, it should be done in the cool of the afternoon. Then supply water and ponds the next day. Do not spread lime, and dry the pond for too long.

c) Microbiological treatment

This is the solution that many people believe in today. Because probiotics in aquaculture bring high efficiency and cost savings. Besides, natural microbial components are capable of decomposing alum. However, you should consult with experts for support in choosing the right quality products and treatment dosage.