Aquaculture wastewater treatment

Vietnam is one of the leading seafood exporters in the world, the current seafood industry accounts for 4% of GDP, 8% of exports and 9% of the labor force. However, the Seafood Processing industry is also one of the industries that causes serious pollution to the environment.

Let’s learn about the seafood processing process?

Depending on the type of seafood as well as processing requirements, seafood processing processes may be different. However, below is a common process commonly encountered in seafood processing:

Aquaculture processing process diagram

Throughthe processing process we can see that the source of wastewater during the production process mainly arises during the defrosting, cleaning and preliminary processing stages.

What are the characteristics and pollutants of aquatic wastewater?

Waste source: Wastewater arises from two sources: wastewater during the production process and during the daily activities of workers. The main source of pollution is from wastewater during the production process. production.

Aquaculture wastewater contains mostly organic substances originating from fish and seafood with main ingredients being proteins, carbohydrates, fats, etc. These substances when decomposed creates an unpleasant stench and reduces the oxygen concentration in the water. In addition, aquatic wastewater contains high levels of nitrogen and phosphorus. If not treated, it will cause eutrophication.

Below is the pollutant parameter table of aquatic wastewater:

Parameter table of pollution indicators compiled by hasy environment

Choosing aquatic wastewater treatment technology

The characteristic of aquatic wastewater is that it contains high levels of organic matter, total nitrogen and phosphorus. Therefore, the most suitable technology for treating aquatic wastewater today is microbiological technology.

To invest in an aquaculture wastewater treatment system with not too high initial investment costs, effective treatment and low operating costs, the following technology is used. The choice is the optimal technology.

Technological line diagram of aquatic wastewater treatment system

Aquaculture wastewater treatment technology line

Trash screen

Wastewater from the factory is sent directly to the treatment system, first passing through the trash screen.

The trash screen is responsible for retaining large-sized trash and waste such as fish scraps, fish bones, plastic bags, etc.

Periodically, the trash in the trash screen is collected and processed to avoid clogging and damaging the equipment.

Grease separator

Aquaculture wastewater contains a lot of animal fat and oil.

It is necessary to use a grease removal tank. Avoid clogging pipes and ensure equipment and downstream processing works.

Floation tank

After passing through the oil and grease separation tank, wastewater flows to the flotation tank. The flotation tank is installed with an aeration system in the form of small bubbles.

When wastewater enters the tank, air bubbles will penetrate the wastewater, bringing suspended substances that are difficult to settle to the top of the water and are collected and flowed into the sludge tank. The wastewater is then transferred to the equalization tank.

Air conditioning tank

Contains wastewater before entering the biological treatment stage. This is a place that helps stabilize the concentration and flow of waste streams. Here, an aeration system is arranged to create disturbance to avoid sedimentation and anaerobic decomposition in the tank, and a submersible pump is arranged. Submersible pumps pump wastewater from the equalization tank into the biological treatment system.

Anaerobic biological tank

The wastewater after being stabilized in the regulation tank flows into the anaerobic tank. Here, under the action of anaerobic microorganisms, organic substances are decomposed into nutrients for the growth and development of microbial cells. After this process, the organic matter content will decrease significantly.

Anoxic and aerobic tanks

Aquaculture wastewater has very high nitrogen and phosphorus content. It is necessary to use an anoxic combined aerobic tank to treat these substances.

At the anoxic tank, the wastewater first passes through containing nitrogen in the form of NH₄⁺. Going through the anoxic tank, no reactions will occur, flowing into the aerobic biological tank.

In the aerobic biological tank, under the influence of aerobic microorganisms in an environment always supplied with oxygen from an air blower, the microorganisms oxidize NH₄ ⁺ becomes NO₂^– and finally becomes NO₃^–. Wastewater containing NO₃^– is recirculated back to the anoxic tank, under the action of anoxic microorganisms that reduce NO₃^– becomes N₂ and flies up.

At the aerobic tank, the oxidation of other organic substances is carried out simultaneously under the influence of aerobic microorganisms.

Biological settling tank

The mixture of sludge and wastewater after aerobic treatment flows into the biological sedimentation tank. The sludge will settle to the bottom of the settling tank due to the effect of gravity. One part is circulated by a submersible pump to the aerobic tank. The remainder is discharged into the sludge tank. The remaining clear water will be collected and led to the disinfection tank.

Sludge tank

Sludge after flotation and biological sedimentation is collected and led to the sludge tank. Sludge will be periodically collected and treated to avoid anaerobic decomposition of excess sludge.

Disinfection tank

After treatment, wastewater will be directed to an ozone disinfection tank. Ozone is injected here to disinfect the fish from bacteria. The wastewater after disinfection is led outside.

Advantages and disadvantages of wastewater treatment systems

• Advantages:

– Treatment efficiency is quite high for the main criteria of aquatic wastewater, effectively removing suspended solids (SS) and high organic loads (BOD5, COD) in seafood processing wastewater.

– The biological treatment facility includes two aerobic activated sludge tanks (two units). Thus, if one of the two tanks has a microbiological problem, the system can still continue to operate with the remaining unit while fixing the problem.

– Moderate investment costs for construction and equipment installation are 4,500,000 VND/m3 of wastewater.

• Disadvantages:

– High operating costs compared to other wastewater treatment systems that are equivalent in terms of post-treated wastewater quality requirements and use more dynamic equipment.