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The principle by which chlorine dioxide (ClO2) can selectively eliminate harmful bacteria while protecting beneficial bacteria (active microorganisms) in wastewater treatment processes, particularly the activated sludge process (aerobic microbial treatment), is based on highly sophisticated microbiological and chemical mechanisms.

The core principle regarding the 'balance between the protection of beneficial bacteria and the elimination of harmful bacteria' is as follows:

1. Selective Reaction of Oxidation Potential

- The most important characteristic of chlorine dioxide is that while its oxidizing power is strong, its oxidation potential (V) is low.

- Ozone (O3) and chlorine (Cl2) have very high oxidation potentials (V) and destroy both organic and inorganic matter indiscriminately. This leads to the elimination of even the beneficial bacteria within the sludge.

- Principle: The oxidation potential of chlorine dioxide (ClO2) is approximately 0.95V, which is lower than that of chlorine (1.36–1.47V) or ozone (2.07V). Thanks to this low potential, ClO2 does not react with all organic matter but selectively reacts only with specific bonds (phenols, amines, sulfides, harmful substances, etc.) that have high electron density. In particular, it acts strongly on the cell membrane proteins and genetic material of harmful bacteria.

2. Differences in Biofilm Permeability and Sludge Structure

- In the activated sludge process, beneficial bacteria generally form large microbial clumps called flocs.

- Beneficial Bacteria (Inside the Floc): Aerobic beneficial bacteria inhabit the inside of the flocs, which are surrounded by a protective layer called Extracellular Polymerase (EPS). Since ClO2 is used at low concentrations, it cannot immediately destroy this entire EPS layer. Therefore, the ecosystem of beneficial bacteria located in the center of the aggregate is not directly affected.

- Harmful Bacteria (None): On the other hand, filamentous fungi or pathogenic harmful bacteria that reduce treatment efficiency protrude outside the aggregate or float as single cells. ClO2 preferentially attacks the cell membranes of exposed microorganisms, causing them to die.

3. Selective Disruption of Cell Membrane Transport Systems

- ClO2 directly penetrates the cell walls of microorganisms and oxidizes amino acids (cysteine, tyrosine, tryptophan, etc.).

- Metabolic Inhibition: In the case of harmful bacteria, ClO2 destroys transport proteins that regulate cell membrane permeability, thereby halting protein synthesis.

- Energy Depletion: In particular, it immediately stops the process of intracellular ATP (energy source) production, leading to the death of microorganisms. However, beneficial bacteria in activated sludge maintain viability even in environments exposed to low concentrations of chlorine dioxide (ClO2) through high metabolic activity and protective mechanisms.

4. pH Independence and Suppression of Harmful Byproducts

- Conventional chlorine-based disinfectants exhibit rapid fluctuations in bactericidal efficacy depending on pH changes and leave residues that are highly toxic to beneficial bacteria.

- Stable Ecosystem: Chlorine dioxide maintains a consistent bactericidal effect within a pH range of 2 to 10. Even if the pH of the sewage treatment tank fluctuates, it can gently regulate the density of harmful bacteria without causing a sudden shock to the microbial ecosystem. - Non-toxic residue: Since it does not produce carcinogens such as chloroform or trihalomethanes (THMs), the risk of secondary contamination resulting in mass microbial death due to toxic byproducts is significantly low.

* Summary: The reason liquid chlorine dioxide preserves beneficial bacteria during wastewater treatment is due to "selective reactivity resulting from a low oxidation potential" and the "physical protective structure of microbial aggregates."

-When administered at an appropriate concentration, filamentous fungi or pathogens exposed to the outside of the aggregates are oxidized and killed, whereas core beneficial bacteria protected by the EPS layer survive, allowing for the continuous maintenance of biological treatment efficiency.

- Due to these characteristics, a high-efficiency quantitative method for precisely injecting a high concentration of 6,000 ppm liquid chlorine dioxide is essential for wastewater treatment facilities.

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