Applications of Chlorine Dioxide Generators in Drinking Water and Other Food Industry Water Treatment

Chlorine Dioxide as an Alternative to Chlorine Disinfection

For the past 50 years, chlorine has been widely used as a disinfectant in drinking water and food industry water treatment, and it remains one of the most common disinfectants. However, 20 years of research have shown that treating water containing organic matter, especially humic acid, with chlorine can generate trihalomethanes (trihalomethanes, chlorinated hydrocarbons, etc.), which are destructive to human tissues and carcinogenic. In 1985, the Federal Government of Germany set the limit for trihalomethanes and chlorinated hydrocarbons at 25 ug/L, and in 1991, this limit was further lowered to 10 ug/L. The European Union explicitly stipulates that the organic matter content should be 1 ug/L. Specific regulations exist not only for drinking water treatment but also for wastewater treatment. In 1989, a specific limit was set for the AOX value in wastewater, and in 1994, this value was lowered to 1 mg/L. If this limit is exceeded, additional equipment must be added to the wastewater treatment process to reduce the value to meet the standard. For these reasons, people invested heavily in researching ways to improve disinfection properties, and attention focused on chlorine dioxide. In the drinking water and food industries, chlorine dioxide replaced chlorine, not only for the reasons mentioned above, but also for its unique advantages.

The Generation of Chlorophenols

The limit concentration of phenols in surface water is 1 ppm, but when disinfected with chlorine, the concentration reaches 1 ppb (1000 times lower), at which point chlorophenols become noticeable. Chlorophenols emit a smell similar to medicine and iodoform. However, chlorine dioxide disinfection does not produce chlorophenols. For this reason, the Niagara Falls Waterworks in the United States first successfully applied chlorine dioxide generators as early as 1944, and by 1958, 150 waterworks were using chlorine dioxide generators. The earliest recorded use in Germany was in 1959. The amount of chlorine used is affected by NH3 and NH4+. During chlorination, due to the presence of ammonia, ammonium compounds, and organic nitrogen in the water, depending on the pH value, they react with chlorine to produce ammonium chloride, ammonium dichloride, and nitrogen trichloride, respectively. Although ammonium chloride also has a disinfecting effect, its efficacy is 10-15 times lower than that of hypochlorous acid. Furthermore, it is difficult to control the residual chlorine content to below the specified concentration. If the concentration of combined chlorine is too high, trihalomethanes will be produced.

The disinfection effect of chlorine depends on the pH value, while the disinfection effect of chlorine dioxide is almost unaffected by pH.

Let's first focus on an equilibrium: Cl2 - HOCl - OCl-. During Cl2 disinfection, Cl2 exists in three forms in aqueous solution: Cl2, HOCl, and OCl-. (See Figure 1). As can be seen from the figure, HOCl exists over a wide pH range, and its disinfection effect is equivalent to that of Cl2, while OCl- is 10-100 times less effective than chlorine. At pH 8, only 18% of chlorine gas is present, with the remainder replaced by inefficient OCl-. At pH 9, only 3.3% of Cl2 is present. This principle can explain the inefficiency of chlorine disinfection at higher pH values.

On the other hand, chlorine dioxide exhibits high bactericidal activity, which increases with increasing pH. As pH increases, the redox potential is directly related to the kill rate. When using chlorine, its potential drops rapidly, while chlorine dioxide maintains a constant, high redox potential.

Design, Function, and Safety of Chlorine Dioxide Generators

Principle of Chlorine Dioxide Generation

Chlorine dioxide is an unstable gas and cannot be concentrated or stored. Therefore, it must be prepared on-site and directly supplied to the water to be treated. One of the reaction equations is as follows: 5NaClO2 + 4HCl ------ 4ClO2 + 5NaCl + 2H2O

Design and Working Principle of the Chlorine Dioxide Generator

In the Prominent Bello Zon system, chlorine dioxide is produced by the reaction of 9% dilute hydrochloric acid and 7.5% diluted sodium chlorite solution, or by producing a 2% (20g/L) chlorine dioxide aqueous solution from 30% hydrochloric acid and 24.5% sodium chlorite.

Internal Control Operation:

The system uses a Prominent electronically controlled, electromagnetically driven diaphragm precision metering pump that has undergone over 600,000 tests. In free-running mode, the pump's metering can be adjusted to achieve the required output by regulating both the pump's stroke length and stroke frequency. This allows for the precise injection of two chemical solutions used in the reaction into the reactor in the required amounts. In the reactor, the two chemicals react to produce a chlorine dioxide solution of a specific concentration. Due to pump pressure, the produced chlorine dioxide solution is further mixed with water from the bypass in a mixer, and the diluted chlorine dioxide solution to a safe concentration is then injected into the water flow to be treated.

External Control Operation:

The operation of the chlorine dioxide generator is controlled by external signals such as contact water meters, frequency converters, or analog signals. The metering pump can accept external control pulses or analog signals, and the stroke frequency is proportionally adjusted according to the magnitude of the external control signal. This control ensures that the two chemical solutions used in the reaction are precisely injected into the reactor in the required amounts, allowing for the chemical reaction and thus regulating the chlorine dioxide dosage. The generated chlorine dioxide solution then enters the mixer, where it is further mixed with water from the bypass before being injected into the water flow to be treated. Furthermore, the integration of Prominent's BelloZon chlorine dioxide system with Prominent's measurement and control technology fully realizes a closed-loop control system of measurement-control-metering, thereby controlling the quantitative addition of chlorine dioxide aqueous solution. The Prominent DULCOTEST chlorine dioxide sensor has a measurement range of 0-20 mg/L. The output signal of the Prominent DULCOMETER measurement and control instrument is directly used to control the pump's stroke frequency, thereby determining the amount of chlorine dioxide generated.

Safety

The Prominent BelloZon system is an absolutely safe and reliable closed-loop system. The chemical reaction process takes place entirely in a fully enclosed reactor, and the dosing process is completed entirely in closed pipelines, ensuring that there is no potential for leakage. Simultaneously, the system is a highly automated control system that monitors the water flow rate in the pipelines. To prevent accidents, an exhaust device installed within the equipment periodically ventilates and exhausts the system, allowing any condensed residual liquid in the tank to be safely discharged outside the system via a bypass ventilation duct. A dosage monitor installed in the pressure circuit accurately detects faults, shuts down the entire equipment, and automatically issues an alarm signal.

The metering pump's suction assembly has a level switch. When the liquid level drops to 30cm, the chlorine dioxide generator alarms, prompting workers to add reagent. When the level continues to drop to 0cm, the system automatically shuts down. For safety reasons, the chlorine dioxide generator must be inspected by a designated person before starting and resuming use.

Applications of Chlorine Dioxide in Drinking Water and Food Industry Water Treatment

We have previously discussed the series of byproducts generated during chlorine disinfection, such as trihalomethanes, chlorinated hydrocarbons, and chlorophenols. Therefore, we prefer to use chlorine dioxide generators. Even among chlorine dioxide generators, different reactants result in different types. Sodium chlorite reacts with hydrochloric acid to produce chlorine dioxide. Because no byproducts are produced in this process, generators using this method are considered pure chlorine dioxide generators. Chlorine dioxide without other byproducts is the disinfectant required for drinking water, beer, and other industrial water treatment.

The dosage concentration of chlorine dioxide is generally 0.2-0.4 ppm. The limit for chlorine dioxide in drinking water is 0.4 ppm, and the concentration of chlorine dioxide in treated water should not be less than 0.2 ppm.

To achieve a good disinfection effect, effective mixing and sufficient reaction time are essential. Generally, chlorine dioxide needs to be kept in contact with the water to be treated or the process medium for 15 minutes to allow for a thorough reaction.