Aerobic Treatment

Aerobic Treatment

Cultivation and Wastewater Treatment of Aerobic Bacteria

Aerobic bacteria are organisms that require oxygen for their survival and growth. By utilizing oxygen and breaking down organic matter, they obtain a significant amount of energy. Examples of aerobic bacteria include Escherichia coli, Mycobacterium tuberculosis, and Nocardia. In contrast, bacteria that do not use oxygen are called anaerobic bacteria. Aerobic respiration in aerobic bacteria yields more energy compared to anaerobic respiration.
One application of aerobic bacteria is in wastewater treatment. In the activated sludge process, aerobic bacteria decompose organic substances in water, purifying the water. Supplying oxygen (air) into the water as bubbles enhances the activity of aerobic bacteria, promoting the decomposition of organic matter.
In the cultivation of aerobic bacteria, it is necessary to create the appropriate conditions (temperature and pH of the culture medium, aeration, and agitation of the culture medium) depending on the type of bacteria being cultured. As the bacteria multiply, the viscosity of the culture medium increases. Insufficient agitation leading to poor mixing of the liquid reduces oxygen concentration significantly, resulting in a decrease in the growth rate of bacteria. To maintain a high level of oxygen concentration in the liquid, aeration and agitation of the liquid become crucial.

Biological methods combining nitrification and denitrification are commonly used as nitrogen removal techniques in wastewater treatment. To efficiently carry out biological nitrogen removal, effective oxidation and digestion of organic matter are essential. Bacteria involved in the nitrification process (nitrifying bacteria) are chemolithoautotrophs and generally have much lower growth rates compared to heterotrophic BOD-oxidizing bacteria. Therefore, to enhance efficient nitrification, methods such as extending the sludge retention time, reducing the ratio of organic matter to nitrogen in wastewater, or promoting nitrification after reducing organic matter concentration are employed.

Biofilm methods offer advantages such as the ability to set longer sludge retention times and easier operation compared to suspended growth methods. Consequently, new biofilm methods like immersed biofilm reactors are being researched and developed. However, in traditional biofilm methods, competition for oxygen within the microbial film between BOD-oxidizing bacteria and nitrifying bacteria necessitates multi-staging of the apparatus. This is done to create conditions with low organic matter concentration and high oxygen concentration to facilitate nitrification.

In contrast, if it becomes possible to form a microbial film on an oxygen-rich membrane, the supply directions of organic matter and oxygen become opposite to each other. This allows for the creation of conditions where the microbial film surface (liquid side) has high organic matter concentration and low oxygen concentration, while the interior of the microbial film (enrichment membrane side) has low organic matter concentration and high oxygen concentration. In other words, it becomes feasible to efficiently oxidize and digest BOD without the need for multi-staging the apparatus.

By using an oxygen-enriched microorganism attachment carrier made of woven polyester fiber around a silicone hollow fiber membrane, the direction of oxygen and organic matter supply to the biological membrane is reversed, enabling simultaneous removal of organic matter and nitrogen. The attached figure shows the appearance and dimensions of the oxygen-enriched microorganism attachment carrier used. The carrier is made of silicone hollow fiber (inner diameter: 0.25 mm, outer diameter: 0.41 mm) coated with polyester fine-wired fiber around the fiber layer, with a diameter of approximately φ1.72 mm. (Figure 1) This fiber was used to fabricate a collective tube bundle type module (Figure 2) and a plain weave type module (Figure 3).

Figure 1
Figure 2
Figure 3