Chlorine dioxide systems

Lactide/glycolide polymers have been investigated for delivery of agents in applications outside the pharmaceutical field. For example, the microbiocidal properties of chlorine dioxide disinfectants have been improved by formulating a long-acting chlorine dioxide system based on lactide/glycolide copolymers. Blends of microspheres based on 50 50 and 87 13 copolymers were developed to afford the release of chlorine dioxide over several months (114). 

The ACTIV-OX system has been developed to meet the needs for a safe and controllable chlorine dioxide system for application in small water using systems. The system instantaneously delivers over 90% of the available chlorine dioxide at a pH of 4 compared to other systems which require lower pH and or longer reaction times (Fig 1 and 2). 

The ACTIV-OX chlorine dioxide system evaluated in this trial overcomes many of the problems associated with chlorine dioxide for the small water user. A chlorine dioxide precursor solution and a dilute acid solution are mixed in a 1 1 ratio immediately prior to injection into the water to be treated. The dose rate of chlorine dioxide is controlled by water meter signal to two proportioning pumps. The mixing of the two chemicals immediately produces a chlorine dioxide solution which is diluted to the required strength by injection into the water to be treated (Fig 3). 

The assessment of the performance of the new chlorine dioxide system was carried out over a continuous period of 39 weeks. 

The newly developed ACTIV-OX chlorine dioxide system has effectively delivered continuous low levels of chlorine dioxide which were effective in controlling Legionella in a hot and cold water system without the need for prior disinfection. 

The chlorine/chlorine dioxide system was modelled with real water data. A maximum in chlorite formation was found (Schmidt et al. 1999). 

The Baxter Water Treatment Plant, Philadelphia, Pennsylvania, is a 12.35-m /s (282-MGD) conventional water treatment plant built in 1960. The plant supphes drinking water from the Delaware River to a population of over 800,000. Chemicals used in treatment include chlorine, ferric chloride or ferrous sulfate, hme, fluoride, and ammonia. Powdered activated carbon is used on demand for control of taste and odor, and chloride dioxide is used for control of THMs, tastes, and odors. The chlorine dioxide system was left over from the previous water treatment plant on that site. In the 1950s, it was used to oxidize phenolic compounds found in the watershed, which have since been eliminated. 

Batch chlorine dioxide systems use ideal conditions to produce a high-purity solution that is stored in a tank and fed into the water by a chemical metering pump. The concentrated chlorine dioxide solution is rather stable, and storage of a few days is usually acceptable. 

Chlorine Dioxide Systems Using Uquid Generator Chemicals... 

The common oxidants are ozone, hydrogen peroxide, H2O, catalyzed usually with ferrous iron, Fe , and ia some cases chlorine dioxide and uv light. Advanced oxidation systems iaclude H2O2 + uv ozone + uv and H2O2, ozone, and uv. Depending on the appHcation, the oxidation can be complete to end products as in a contaminated groundwater or partial to degradable intermediate products as in a process wastewater. 

Chlorine dioxide decomposition occurrences in the generator system ate less explosive when operating under reduced pressure with water vapor... 

The threshold limit value—time integrated average, TLV—TWA, of chlorine dioxide is 0.1 ppm, and the threshold limit value—short-term exposure limit, STEL, is 0.3 ppm or 0.9 mg /m of air concentration (87,88). Chlorine dioxide is a severe respiratory and eye irritant. Symptoms of exposure by inhalation include eye and throat irritation, headache, nausea, nasal discharge, coughing, wheezing, bronchitis, and delayed onset of pulmonary edema. Delayed deaths occurred in animals after exposure to 150—200 ppm for less than one hour. Rats repeatedly exposed to 10 ppm died after 10 to 13 days of exposure. Exposure of a worker to 19 ppm for an unspecified time was fatal. The ingested systemic effects of low concentration chlorine dioxide solutions are similar to that of chlorite. 

Acid—Sodium Chlorite System. The addition of a strong inorganic acid into an aqueous sodium chlorite solution produces chlorous acid, which rapidly disproportionates into chlorine dioxide. One proposed set of reactions using hydrochloric acid is (110) ... 

Hypochlorous Acid—Sodium Chlorite System. In this method, chlorine gas is educted into water forming a hypochlorous acid solution which then reacts with aqueous sodium chlorite to produce chlorine dioxide (114—116). Hypochlorous acid, formed from the disproportionation of chlorine gas in water ... 

Demonstrated chlorine dioxide yields from chlorite are 95% or higher in properly operated systems. Excess hypochlorous acid is commonly used to achieve a high conversion. 

Chlorine Gas—Sodium Chlorite System. In this method, chlorine gas reacts direcdy with a concentrated sodium chlorite solution under a vacuum and chlorine dioxide gas is removed from the reaction chamber using a water-based eductor (117). The reaction has a 100% theoretical molar conversion of chlorite ... 

The pH of the chlorine dioxide reaction mixture must be maintained in the 2.8—3.2 pH range, otherwise decreased conversion yields of chlorite to chlorine dioxide are obtained with by-product formation of chlorate. Generator efficiencies of 93% and higher have been demonstrated. A disadvantage of this system is the limited storage life of the sodium hypochlorite oxidant solution. 

Sodium chlorite has also been used for treatment and removal of toxic and odorous gases such as hydrogen sulfide and mercaptans. Chlorine dioxide from chlorite is also useful for microbial and slime control in paper mills and alkaline paper machine systems (164,165). The use of sodium chlorite in textile bleaching and stripping is well known. Cotton is not degraded by sodium chlorite because the oxidation reactions are specific for the hemiceUulose and lignin components of the fibers. 

Oil Field and Petroleum Processing. Sodium chlorite is finding increasing use as the choice precursor for generating chlorine dioxide for biocidal control in the production of cmde oil (see Petroleum). The use of water in the oil field pumping and processing systems presents significant... 

B. Jackson, Sodium Chlorate System for Direct Feed to Chlorine Dioxide Generators and Chlorine Femoral Reyck Process, Huron Technical Corp. commercial brochure, Eeb. 13, 1979. 

At present, chlorine dioxide is primarily used as a bleaching chemical in the pulp and paper industry. It is also used in large amounts by the textile industry, as well as for the aching of flour, fats, oils, and waxes. In treating drinking water, chlorine dioxide is used in this country for taste and odor control, decolorization, disinfection, provision of residual disinfectant in water distribution systems, and oxidation of iron, manganese, and organics. The principal use of chlorine dioxide in the United States is for the removal of taste and odor caused by phenolic compounds in raw water supplies. 

Figure 4. Components of a gaseous sodium chlorite-chlorine dioxide generation system.

A major disadvantage of this system is the limitation of the single-pass gas-chlorination phase. Unless increased pressure is used, this equipment is unable to achieve higher concentrations of chlorine as an aid to a more complete and controllable reaction with the chlorite ion. The French have developed a variation of this process using a multiple-pass enrichment loop on the chlorinator to achieve a much higher concentration of chlorine and thereby quickly attain the optimum pH for maximum conversion to chlorine dioxide. By using a multiple-pass recirculation system, the chlorine solution concentrates to a level of 5-6 g/1. At this concentration, the pH of the solution reduces to 3.0 and thereby provides the low pH level necessary for efficient chlorine dioxide production. A single pass results in a chlorine concentration in water of about 1 g/1, which produces a pH of 4 to 5. If sodium chlorite solution is added at this pH, only about 60 percent yield of chlorine dioxide is achieved. The remainder is unreacted chlorine (in solution) and... 

Excess sulfur dioxide feed to a chlorine dioxide reactor, leading to excessive exothermic reaction, combined witli failure of the cooling system... 

Hydrochloric acid in combination with chlorine dioxide can be used as a treatment fluid in water-injection wells that get impaired by the deposition of solid residues [332,333]. The treatment seems to be more effective than the conventional acidizing system when the plugging material contains iron sulfide and bacterial agents because of the strongly oxidative power of chlorine dioxide. Mixtures of chlorine dioxide, lactic acid, and other organic acids [1172,1173] also have been described. 

A. Cavallaro, E. Curd, G. Galliano, M. Vicente, D. Crosta, and H. Leanza. Design of an acid stimulation system with chlorine dioxide for the treatment of water-injection wells. 7th SPE Latin Amer Caribbean Petrol Eng Conf (Buenos Aires, Argentina, 3/25-3/28), 2001. 

The use of chlorine dioxide in water systems results in its reduction to chlorite and chloride. In the UK the Drinking Water Inspectorate (DWI) restricts the use of chlorine dioxide in potable water supplies to a maximum of 0.5ppm total oxidants expressed as chlorine dioxide. This ensures that chlorite (and any chlorate) concentrations do not reach levels of potential harm to humans. 

Solutions of stabilised chlorine dioxide have been claimed to provide a simpler system for delivering chlorine dioxide. However the effectiveness of such solutions depends on the amount of chlorine dioxide released, which depends on the strength of acid used and length of time for reaction(8). 

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