Disinfection by Chlorine Dioxide
A common and effective approach to lowering DBP's is to optimise the physical removal of organic precursors before adding any disinfectant. This approach can also minimize the potential for episodic taste-and-odour events. It appears to be more effective in establishing a true disinfectant residual for C x T credits in the case of CIO2. This initial removal of most particulate-phase constituents before primary disinfection results in more cost-effective application of CIO2, but it does not override CIO2's use as a preoxidant in the treatment train.
During the past decade, improved and sophisticated analytical chemistry methods have led to a better understanding of the control of DBP's that accompany CIO2. Treatment of a variety of raw waters with CIO2 produces many oxidation by-products common to all major disinfectants, but CIO2's uses can result in far fewer chlorinated by-products (TOX), including haloacetic acids (HAA) and THM's, than if CI2 is used as the only disinfectant.
In the presence of humic or fulvic acids, potential precursors of THM formation, there is very little TOX and no THM formed in a direct manner when CIO2 is used instead of chlorine. Under some conditions, however, substitution reactions and incomplete phenol ring cleavage can occur when CIO2 is used. Even trace quantities of chlorine contaminating CIO2 solutions can influence the degree of chlorinated compounds formed, and such contamination must be carefully assessed.
CHEMICAL OXIDATION BY ClO2
Chlorine Dioxide is the active ingredient of many Scotmas products. ClO2 possesses a chemical reactivity that differs markedly from other oxidants (Such as chlorine). Commercial applications have shown that chlorine dioxide can effectively oxidise many compounds considered to be waste and water pollutants. The table below lists a number of pollutants found in various industries and demonstrates the wide range of possible applications for the product. Chlorine dioxide has been shown to be an effective treatment for the following pollutants.
ALDEHYDES
Aldehydes are produced by a number of common industrial processes. Their treatment is a common problem, especially so in the photographic industry. In general, ClO2 can oxidise an aldehyde to its corresponding carboxylic acid. Formaldehyde is a major component in the formulations used in photo processing. Chlorine Dioxide oxidises formaldehyde to formic acid and finally to carbon dioxide. Para formaldehyde can be depolymerised and eliminated completely by oxidation with chlorine dioxide.
AMINES AND MERCAPTANS
The major sources of odorous substances such as mercaptans and substituted amines include the chemical and petroleum industries, cooking and sanitary processes, animal feedlots and rendering plants.
Between pH 5 & 9, 4.5 parts by weight of chlorine dioxide instantaneously oxidises 1 part by weight of mercaptan (expressed as sulphur) to the respective sulphonic acid or sulphonate compound, thus destroying the mercaptan odour. Similarly, chlorine dioxide reacts with organic sulphides and disulphides destroying the original odour.
Secondary and tertiary amines are also present in many waste water's, causing their own unique odour problems. The oxidation of amines with chlorine dioxide depends on the pH of the reaction mixture and the degree of substitution of the amine.
Between pH 5 and 9, an average of 10 parts by weight of chlorine dioxide oxidises 1 part by weight of a secondary aliphatic amine (expressed as nitrogen) removing all traces of amine odour. The higher the pH of the reaction mixture (chlorine dioxide and tertiary and/or secondary aliphatic amines) the more rapidly oxidation proceeds.
THM PRECURSORS
The key to understanding why chlorine dioxide is so effective can be found in the differences in the reactions of chlorine dioxide and chlorine with Tri-halomethane (THM) precursors such as humic and fulvic acids.
Chlorine reacts with THM precursors by oxidation and electrophilic substitution to yield both volatile and non-volatile chlorinated organic substances (THMs).
Chlorine dioxide, however reacts with THM precursors primarily by oxidation to make them non-reactive or unavailable for THM production. This means that pre-treatment with chlorine dioxide has an inhibiting effect on THM formation when chlorine is subsequently used.
PESTICIDES
Some pesticides can be oxidised to less toxic materials by chlorine dioxide. Specifically, Methylchlor (DMDT) and Adrian react with ClO2. With parathion, the reaction is slow near to pH 7; however, when pH is above 8, less biodegradable herbicides such as paraquat and diquat are eliminated within a few minutes.
ALGAE/SLIME
Chlorine dioxide has been sown to be effective in controlling algae growth. In one study, chlorine dioxide was found to be more effective than copper sulphate, at comparable treatment costs. Chlorine dioxide is believed to attach the pyrolle ring of the chlorophyll. This cleaves the ring and leaves the chlorophyll inactive. Since algae cannot function without chlorophyll metabolism, they are destroyed. The reaction of chlorine dioxide with algae and their essential oils forms tasteless, odourless substances.
Algae control is carried out by adding chlorine dioxide to the reservoir at night (To prevent photolytic decomposition of ClO2 ) The algae killing action is fast enough to be effective before the sun rises. A dosage of 1 mg/litre has been reported to control algae populations
SULPHIDES
Many industrial processes produce sulphide-containing gases and waste products. These are generated, for example, during petroleum refining, coal coking, black liquor evaporation in kraft pulping, viscose rayon manufacture and natural gas purification. These gases and wastes are frequently scrubbed with alkaline solutions and require treatment before discharge.
Between pH 5 and 9, an average of 5.2 parts by weight of chlorine dioxide instantaneously oxidises 1 part by weight of hydrogen sulphide (expressed as sulphide ion) to the sulphate ion.
NITROGEN COMPOUNDS
Nitrogen oxides are dangerous and corrosive. Nitrous Oxide (NO) and nitrogen dioxide (NO2) are industrial effluents which result from fuel combustion, nitric acid manufacture and use, and from metal finishing operations which use nitrates, nitrites or nitric acid. Other sources include chemical processes in which nitrogen compounds are used as reagents.
Chlorine dioxide has been used to scrub these contaminants. Nitric oxide contained in gas discharges from coke kilns may be eliminated by oxidation by chlorine dioxide. This process is particularly convenient for continuous operation.
CYANIDES
Cyanide compounds originate from processes such as metal plating, steel case hardening, pickle liquor neutralising, gold and silver ore refining and blast furnace stack gas scrubbing. Chlorine dioxide oxidises simple cyanide to cyanate (a less toxic substance) and/or carbon dioxide and nitrogen. The end products depend on reaction conditions.
In neutral and alkaline solutions below pH 10, an average of 2.5 parts by weight of chlorine dioxide oxidises 1 part by weight of cyanide ion to cyanate. Above pH 10, an average of 5.5 parts by weight of chlorine dioxide oxidises 1 part by weight of cyanide ion to carbon dioxide and nitrogen. Chlorine dioxide does not react with cyanate ion, nor has it been observed to form cyanogen chloride during the oxidation of cyanide.
Chlorine dioxide also oxidises thiocyanate to sulphate and cyanate. In neutral solutions, an average of 3.5 parts by weight of chlorine dioxide oxidises 1 part by weight of thiocyanate ion.