Chlorine Dioxide (chemical formula ClO2, molecular weight 68) is known to be a unique and effective oxidizer and biocide. When Chlorine Dioxide is dissolved in water, unlike chlorine or bromine, it does not dissociate into ions. Instead, it remains as a nonionic substance. This property is responsible for many of its practical uses as an oxidizer and biocide, such as biofilm removal.
In addition, the unique mechanism by which it destroys microorganisms allows it to produce desirable results at over five times lower concentrations than chlorine. For example, the US EPA equates 4 mg/l of chlorine with 0.8 mg/l of Chlorine Dioxide. However, in practice, dosage concentrations of 0.2–0.4 mg/l are effectively applied in disinfection applications, such as in biofilm destruction and Legionella control.
As a dissolved gas, if the solution is not kept under pressure, or if there is space above the solution, the Chlorine Dioxide gas will come out of solution. If the solution is in an enclosed space, such as a tank or chemical tote, the gas will accumulate in the space above the solution. At a concentration as low as 3,000 mg/l, this accumulation of gas is right on the edge of the “Safe Zone” at 77°F, as shown by the Fig. 1 chart. If the temperature of solution rises above this threshold, the potential for explosion increases significantly. However, at 700 mg/l, the temperature of solution must be much greater than 86°F to venture into an area of caution.
- Effective over pH range 2-10
- Does not produce THM’s
- Reacts with odor-causing compounds like sulfides
- Breaks down phenols
- Precipitates iron and manganese
- Is not affected by organic loading
- Penetrates and destroys biofilms
Chlorine Dioxide works so well primarily because it is a non-ionic dissolved gas in water that selectively reacts with certain other molecules. As such, it is able to migrate through biomasses and cell walls without being consumed by most organic material. It reacts through oxidation with sulfur compounds, amines, and cytoplasm. These properties translate into a situation where only a small amount of Chlorine Dioxide is required to effectively destroy microorganisms - even in the presence of organic materials.
These properties are especially valuable where biofilm is concerned. As a dissolved gas in solution, Chlorine Dioxide will migrate via molecular diffusion through the biofilm and inactivate the polysaccharide-producing microbes, thus killing the colony. Chlorine and bromine have no ability to do this, so they merely bounce off of the surface of the biofilm as shown in Fig. 2. Evidence of Chlorine Dioxide’s penetration can be seen when it is first applied. A visible sloughing off of the biofilm will seemingly increase the presence of organic and microbiological material in the water where none may have been visible before. Once the biofilm is removed from surfaces, the water becomes clear again. Biofilms on heat transfer surfaces will reduce efficiency of transfer, which translates to increased energy consumption. Biofilms are also responsible for harboring pathogenic organisms such as Legionella within water distribution systems, reverse osmosis membrane systems, and cooling towers.
The simple answer is that it is hard to make. Everything comes with a price, and the price of using the ultimate biocide is that you have to make it where you use it or suffer the consequences of trying to move it. Chlorine Dioxide has been made by mixing concentrated chemicals and by using electricity. Both come with hazards, as described in the Fig. 3 chart.
Once it is made, however, storage of Chlorine Dioxide is a real issue. UV light, heat, and time all degrade it, and the dissolved gas does not want to stay in solution. In addition, these problems are more severe in concentrated solutions than they are in dilute solutions. Although it is possible to store Chlorine Dioxide safely at normal temperature for short periods of time, the storage solutions require additional engineering and cost.
These reasons are why the ClO2IX systems have been engineered to dispense an inherently safe 700 mg/l solution on demand without the need for storage.