Chlorine Dioxide FAQs

DPBs is short for disinfection by-products, which are a class of substances formed when naturally occurring organic compounds in water react with disinfection additives such as chlorine, hypochlorite, or bromine. DPBs include substances that are either known or suspected carcinogens. Two DPBs, THM (trihalomethane) and HAA (Haloacetic Acid), have recently been strictly regulated under the USEPA's Stage 1 of the Disinfection/Disinfection By-Product Rule (D/DPBR) in effect since November of 1998. Under this rule, public water supplies in the United States must employ treatment strategies to keep the two families of DPBs, THM (trihalomethane) and HAA (Haloacetic Acid) below specific maximum allowable limits. Stage 1 guidelines specify a maximum allowable concentration for HAA of 60 ppb; the maximum allowable concentration for TTHM (Total THM) is 80 ppb. Since replacing chlorine or bromine with ClO2 can prevent the formation of DPBs, chlorine dioxide can play a vital role in meeting Stage 1 D/DPBR guidelines. As stage two guidelines take effect in the near future, compliance will be even more difficult. Stage 2 guidelines specify a maximum allowable concentration for HAA of 30 ppb, and a maximum allowable concentration for TTHM of 40 ppb.

DPBs is short for disinfection by-products, which are a class of substances formed when naturally occurring organic compounds in water react with disinfection additives such as chlorine, hypochlorite, or bromine. DPBs include substances that are either known or suspected carcinogens. Two DPBs, THM (trihalomethane) and HAA (Haloacetic Acid), have recently been strictly regulated under the USEPA's Stage 1 of the Disinfection/Disinfection By-Product Rule (D/DPBR) in effect since November of 1998. Under this rule, public water supplies in the United States must employ treatment strategies to keep the two families of DPBs, THM (trihalomethane) and HAA (Haloacetic Acid) below specific maximum allowable limits. Stage 1 guidelines specify a maximum allowable concentration for HAA of 60 ppb; the maximum allowable concentration for TTHM (Total THM) is 80 ppb. Since replacing chlorine or bromine with ClO2 can prevent the formation of DPBs, chlorine dioxide can play a vital role in meeting Stage 1 D/DPBR guidelines. As stage two guidelines take effect in the near future, compliance will be even more difficult. Stage 2 guidelines specify a maximum allowable concentration for HAA of 30 ppb, and a maximum allowable concentration for TTHM of 40 ppb.

Disinfection is currently defined by the USEPA to mean a 99.9% reduction in Giardia lambia (3 log reduction), no lactose fermenting coliforms, less than 10 cfu/ml (colony forming units per ml) of non-lactose fermenting coliform bacteria, and a 99.99% reduction in enteric virus concentrations (4 log reduction).

ClO2 has many unique and beneficial characteristics. Among them are the following: ClO2 is a true gas in solution, and is therefore 100% available and does not lose any biocidal efficacy over the broad 3–10 pH range. Several papers have documented that weaker oxidizers such as ClO2 are much better than strong oxidizers at biofilm penetration and destruction. For this reason, ClO2 can remove and control biofilm better than any strong oxidizer including chlorine, ozone, and bromine. ClO2 has a higher oxidizing capacity and lower oxidation potential than other oxidizers. A unique feature of ClO2 is its ability to accept 5 electrons in an oxidation/reduction reaction versus 2 electrons for ozone, chlorine and bromine. At the same time it has a relatively low oxidation potential (ease of reactivity). This makes ClO2 available to function as a biocide, instead of getting "used up" before it gets to the target microorganisms. ClO2 penetrates the cell wall of the microorganism and disrupts metabolic functions of the cell. This process is more efficient than other oxidizers that "burn" whatever they come in contact with. This also allows for the use of lower effective biocide concentrations. ClO2 is a selective oxidizer that only reacts with certain organic compounds, and unlike chlorine, bromine, and ozone, chlorine dioxide does not attack most water treatment compounds. Bacterial recovery (the re-growth of the bacteria after sanitation) is slower with ClO2 than any other oxidizer.

Biofilm is similar to a spider web in its design and impact. When certain microbes land on hard surfaces they attach themselves by producing polysaccharides (the web). This material is sticky and very difficult to remove. Channels are formed in this film, through which water may flow. The sticky web catches nutrients, suspended solids, and other microbes that pass by, providing food and a quick growth mechanism for the entrained cells.

Once a biofilm is established it is very difficult to remove. Often a system must be taken off-line for manual cleaning. Problems associated with biofilms include: Formation of a habitat for pathogenic organisms Increased corrosion rates Fouling of heat exchangers by biofilms reduces heat transfer rates and results in higher operating costs, lowers productivity, and increased maintenance costs.

ClO2, like ozone, is a dissolved gas that penetrates the biofilm by molecular diffusion. Unlike ozone, ClO2 reacts slowly, allowing time for it to travel to the base of the film where it attacks the microorganisms and destroys the biofilm at its point of attachment. Strong oxidizers react mostly at the surface of the biofilm to form an oxidized layer, like charring on wood. This “burning” prevents further penetration and protects the underlying layer of biofilm from the biocidal agent in the bulk water.

ClO2 has been shown to be more effective than copper sulfate in controlling algae. ClO2 attacks the pyrrole rings of the chlorophyll. This attack cleaves the ring, which inactivates the chlorophyll. In comparison, chlorine and bromine have a limited impact on algae.

No. One of the greatest concerns in cooling towers, drinking water disinfection, wastewater disinfection, and many other industrial applications, is biofilm formation. Biofilms retards heat transfer, creates an environment capable of proliferating pathogenic bacteria, supports the growth of corrosive anaerobic bacteria, and is the main cause of scale, fouling, and underdeposit corrosion in water handling systems. No biocide can control biofilms better than chlorine dioxide.

ClO2 has no loss of efficacy over the wide pH range of 3 to 10! Chlorine and bromine maintain efficacy over a very narrow pH range of 1 to 2 pH units. The impact this has on biocidal efficacy of systems operating in the alkaline pH range, especially above pH 8.5, is tremendous. At pH 8.5, chlorine dioxide is at least five times more effective than chlorine.

ORP (Oxidation-Reduction Potential) is a measure of the strength of an oxidizer with regard to an oxidizable material. ORP can indicate the relative corrosivity of a material. Oxidation Capacity is the measure of how many electrons can be transferred in an oxidation/reduction reaction. The table below shows the uniqueness of ClO2. Chlorine dioxide is a 160% weaker oxidant (less corrosive) than chlorine, but has a 250% greater oxidation capacity than ozone.

Yes. ClO2 is reduced all the way to chloride while oxidizing iron. It is 250% more efficient than chlorine, and the reaction is almost instantaneous. At high a pH, ClO2 reacts with dissolved iron to form ferric hydroxide. When ClO2 is used as a pre-oxidant in drinking water and wastewater applications, this ferric hydroxide can aid coagulation in liquid/solids separation processes. ClO2 even oxidizes organically-bound iron. This has been documented in systems which had iron- metabolizing bacteria not effected by chlorine.

ClO2 readily oxidizes manganese. As with soluble iron, neutral to alkaline conditions are desirable for precipitation. Unlike chlorine and other halogen type oxidizers, the reaction time for chlorine dioxide to precipitate manganese is short, less than five minutes. Chlorine will take up to 24 hours to react, and the reaction will still not go to completion.

Yes. Chlorine dioxide is a widely used and a very effective bleaching agent. Because of its reaction process, it will remove color faster and more completely than chlorine.

Yes. Chlorine dioxide controls odor primarily through the following mechanisms: ClO2 oxidizes sulfides and other odor causing reduced species ClO2 controls microorganisms that produce hydrogen sulfide and volatile fatty acids (VFAs) ClO2 almost instantaneously oxidizes mercaptans at pH's greater than 9, and quickly destroys tertiary amines at pH's greater than 7.

No. ClO2 is a selective oxidizing agent that has little or no effect on most organic water treatment chemicals including amines, azoles, and most phosphonates. It will have no effect on any inorganic additives such as molybdenum. It is also unreactive with many other contaminants commonly found in industrial water handling systems, including ammonia, acids, alkanes, alkynes, alchohols, aldehydes, aliphatic amines, carbohydrates, ethers, fats, glycol, ketones, methanol, carbohydrates, polysaccharides, saccharides, unsaturated fatty acids (i.e., malaeic and fumaric acids), and unsubstituted aromatics. When used as a microbiocide, chlorine dioxide is left virtually unaffected by the classes of substances listed above, and will therefore be available to control the true target - microorganisms.

ClO2 reacts with amino acids (sulfur containing amino acids), proteins, cyanide, hydrogen sulfide, formaldehyde, simple sugars, and phenolic compounds.

Some pesticides can be oxidized to less toxic materials by ClO2, such as Methylchlor and Aldrin. At a pH greater than 8, less biodegradable herbicides such as Paraquat and Diquat are destroyed within a few minutes by ClO2.

Some of the more common substances chlorine dioxide can be used to control are sulfides, mercaptans, nitrogen oxides, cyanides, phenols, and aldehydes. When chlorine reacts with phenols, it forms chlorophenols, which have a worse taste and odor than phenols. When chlorine dioxide is used to destroy phenols, no chlorophenols are produced.