[Smart Farmers] New technology helps microorganisms settle in water

China is the world's largest aquaculture country and has made important contributions to world food security and nutrition supply. my country's traditional marine aquaculture is mainly based on nearshore aquaculture. With the improvement of people's living standards, my country's demand for aquatic products continues to rise, and the aquaculture industry has also shifted from low-yield nearshore stocking to high-density, intensive recirculating aquaculture. In high-density, intensive recirculating aquaculture systems, residual bait and feces are easily converted into soluble inorganic pollutants such as ammonia nitrogen and nitrite nitrogen, which exist in the aquaculture water body and will have a serious negative impact on the growth and reproduction of aquaculture products. Therefore, it is crucial for recirculating aquaculture systems to use efficient wastewater treatment technology to remove excessive nitrogen-containing inorganic pollutants in the water.

At present, the processes used in aquaculture wastewater treatment can be mainly divided into three types: physical method, chemical method and biological method. The physical method is suitable for treating particulate pollutants in water, but has poor removal effect on soluble pollutants in water; the chemical method can remove soluble pollutants in water, but requires the real-time addition of chemical reagents, which not only increases the operating cost, but also introduces foreign substances.

Therefore, biological methods using microbial degradation are widely used in aquaculture wastewater treatment due to their good removal effect and low operating costs. In the biological wastewater treatment process, the main body of pollutant removal is microorganisms, so the number, activity and population type of microorganisms have a direct impact on the removal effect of the process.

The traditional method is to increase the number of functional microorganisms in the system by adding free bacteria to the wastewater treatment system to achieve the purpose of enhancing the wastewater treatment effect. However, the microorganisms inoculated in this way have poor resistance to unfavorable environments and are easily lost with the water body, often failing to achieve the ideal treatment effect. Therefore, the microbial immobilization technology that uses immobilized materials to provide attachment carriers for functional microorganisms to maintain a high population density and biological activity of microorganisms has received more and more attention and has been widely used in biological wastewater treatment processes (Shi Guanghui et al., 2015).

Microbial encapsulation and immobilization is a process that uses high-molecular gel polymer materials with high porosity to adsorb and retain microorganisms in pores for fixation. The encapsulation beads have the characteristics of high mechanical strength, strong resistance to impact loads, and microorganisms are not easy to fall off, which has unique advantages in the field of water treatment. As a carrier for microorganisms to attach and grow, the physical and chemical properties of the selected materials will directly affect the mass transfer performance and microbial activity of the system. The carrier material used for immobilization needs to have sufficient pores to ensure the inflow of substrates and the outflow of products after microbial removal. It is also necessary to limit the loss of fixed microorganisms and have little effect on the activity of microorganisms. In addition, it is also necessary to consider the difficulty of immobilization operation, carrier stability, reusability and economic cost. Therefore, finding a suitable and effective carrier is one of the key factors affecting the removal performance of the system.

Currently, carriers can be divided into two categories: inorganic carriers and organic carriers.

Inorganic carriers mainly include ceramics, zeolites, quartz sand, sand, activated carbon, foamed metal, inorganic glass, etc. Inorganic carriers have high mechanical strength, good mass transfer performance, stable chemical properties, and relatively cheap materials, but their cell binding force is poor, which can easily cause the shedding and loss of microorganisms.

Organic carriers can be divided into natural organic carriers including agar, alginate, chitosan, rice husk and other carriers, and synthetic organic carriers including high molecular materials such as polyethylene (PE), polyvinyl chloride (PVC), polyvinyl alcohol (PVA), polyethylene glycol (PEG) and other polymer materials (Zhu Gangli, 2010). Natural organic carriers have good mass transfer performance, simple immobilization operation, and are non-toxic to organisms, but they have low mechanical strength and poor stability, which can easily cause the loss of microorganisms. Synthetic organic carriers have high mechanical strength, good stability, and resistance to biodegradation, but poor mass transfer performance, a more complicated immobilization process, and some carriers have a small amount of toxicity.

It is difficult to meet multiple needs by using only a single carrier. In recent years, experts and scholars have modified and combined the carriers, and improved the cross-linking methods, etc., all of which have achieved relatively ideal results. However, the production environment is diverse. In actual application, the carrier type or combination should be changed according to the needs to ensure the best removal performance of the system.

Microorganisms are the main body of biological pollutant removal. Therefore, the type and characteristics of microorganisms are also one of the key factors affecting the removal performance of the system. Microorganisms used in aquaculture wastewater treatment should be safe, non-toxic and harmless, and rapidly degrade pollutants. Microorganisms used in marine aquaculture also need to be salt-tolerant. Currently, the commonly used microorganisms are photosynthetic bacteria that use light energy and nitrogen cycle bacteria that use chemical energy.

Photosynthetic bacteria are a type of prokaryotic microorganism that uses light energy and inorganic carbon sources such as carbon dioxide to synthesize organic matter to maintain its own growth and development in an anaerobic environment. Photosynthetic bacteria have strong environmental tolerance and can degrade inorganic nitrogen, sulfur and other pollutants in water. They can also be cultivated as feed additives for farmed fish and have received much attention and research in the aquaculture industry.

Nitrogen cycle bacteria are a type of prokaryotic microorganisms that use chemical energy and have the ability to remove nitrogen-containing inorganic substances such as ammonia nitrogen, nitrate nitrogen, and nitrite nitrogen. They mainly include anaerobic ammonia oxidizing bacteria, nitrifying bacteria, and denitrifying bacteria. Anaerobic ammonia oxidizing bacteria do not require a carbon source or aeration, have good removal effects, and low operating costs, but they grow slowly, the system startup time is long, and the growth conditions are harsh. Nitrifying bacteria can remove ammonia nitrogen and nitrite nitrogen from water, which is of great significance to aquaculture. Denitrifying bacteria can remove nitrate nitrogen from water, and the final product is nitrogen gas. It can reduce TN in water and reduce subsequent treatment procedures. Traditional denitrification theory believes that denitrification can only be carried out in an anaerobic environment. With the discovery of aerobic denitrifying bacteria, it is proved that the denitrification process can also occur in an aerobic environment. On this basis, nitrifying bacteria and denitrifying bacteria can be inoculated in the same reactor, which not only reduces the floor space, but also reduces the operating cost. Therefore, it is worth further studying to prepare a composite bacterial agent by fixing nitrifying bacteria and denitrifying bacteria and applying it to improve the removal efficiency of aquaculture wastewater (Zhang, 2022).

The encapsulation and immobilization technology can effectively increase the abundance of microorganisms, enhance the activity of microorganisms, improve the effluent quality, and is non-toxic and harmless to aquaculture organisms. It has a huge application space in the treatment of aquaculture wastewater. However, due to the complexity of the aquaculture system, the diversity of microbial communities, the wide range of pollutants in the water, and the variable operating environment of the system, the encapsulation and immobilization technology is mostly used to simulate aquaculture wastewater as a research object and has not been widely used in the actual environment. Therefore, further research can be carried out in the screening of efficient composite strains, the development of high-performance and low-cost carriers, and the optimization of fixation methods, so as to realize the widespread application of encapsulation and immobilization technology in aquaculture wastewater treatment.

References:

[1] Ma Hongjing, 2022. Research on enhanced treatment of marine aquaculture wastewater by encapsulated and immobilized aerobic denitrifying bacteria[D]. Dalian Ocean University.

[2] Shi Guanghui, Liu Qingsong, Zhang Xufeng, et al., 2015. Research progress of encapsulated immobilized microorganism technology in aquaculture water treatment[J]. Water Treatment Technology.

[3] Zhu Gangli, 2010. Study on the embedding and immobilization characteristics of anaerobic ammonium oxidation mixed culture[D]. South China University of Technology.

[4] Shuai Zhang, Amjad Ali, Junfeng Su, et al., 2022. Performance and enhancement mechanism of redox mediator for nitrate removal in immobilized bioreactor with preponderant microbes[J]. Water Research, 209, 117899.

Author: Liu Peiwu, Su Xin (graduate students at Dalian Ocean University Aquaculture and Equipment Engineering Research Center)

Scientific review: Liu Ying (doctoral supervisor at the School of Biosystems Engineering and Food Science, Zhejiang University), Han Rui (associate professor and graduate supervisor at the Aquaculture and Equipment Engineering Research Center, Dalian Ocean University)