3. The biocide

The biocide is the water treatment agent which is used in the circulating cooling water system to kill the microorganisms (the bacteria and the algae), in order to prevent the metal equipment cooling water system from the corrosion and the accident, and the failure of the equipment operation which is caused by the blooms of the microorganisms. According to the killing mechanism, the biocide is divided into the oxidizing biocide and the non oxidizing biocide.

3.1 The oxidizing biocides

The chlorine is a kind of strong oxidizing biocide; it has the strong bactericidal power, the low price and the simple usage, so it is one of the most widely used biocides nowadays, but it is not suitable for the treatment of the alkaline water. In addition, it may be react with the organic matter in the water and generate the carcinogen Trihalomethanes which limits its application. So, the bromine, the ozone and the chlorine dioxide have been paid much attention in practical applications.

The bromine kind of biocides mainly include the sodium bromide, the DBDMH bromide, the active bromide and the brominating acrylic amide etc. the brominating acrylic amide is a kind of oxidizing biocide which is developed in recent years, including the 1, 2-dibromo, -3-propionamide which is a very effective broad-spectrum biocide. Along with the increase of the PH value and the temperature of the cooling water, its half-life period rapidly becomes short; thereby the pollution to the environment is very low.

The ozone has very excellent bactericidal activity and very strong slime stripping effect, meanwhile, it has the corrosion and scale inhibition effect, using the ozone in the treatment of the circulating cooling water, the concentration treatment can reach 30 ~ 50 times. However, due to its high cost, at present it has not been widely adopted.

The chlorine dioxide has the strong attraction ability and the ability to penetrate through the cell wall, it has the very good killing effect to the main harmful strain in the cooling water such as the heterotrophic bacteria, the iron bacteria and the sulfate reducing bacteria etc. Its characteristics are the less dosage, the high efficiency, the fast speed and the long duration of the drug action. For example, the effect of 2 mg/L the chlorine dioxide in 30 seconds can kill mearly 100% of the microbes. In the water at the PH value 8.6 and with the viable bacteria amount up to 71 x 10⁴ / ml, the effect of 0.5 mg/L the chlorine dioxide in 12 hours can remain the sterilization rate of the heterotrophic bacteria at over 99%. In addition, it can not be affected by the PH value, and does not react with the ammonia, the amines and the phenol in the water; it can not only kill the microorganisms, but also can decompose the residual cell structure, therefore, it has the effect to kill the spores and the virus. It is suitable for the treatment of the alkaline water, without any threat to the environment. In our country, its instability limited its popularization and its application in the past years. In recent years, some manufacturers have been in the mass production of the stable chlorine dioxide, a company even have launched the chemical method chlorine dioxide generator, it has the unique design with the simple operation, it is safe and reliable. There are many advantages to use the chlorine dioxide as the bactericide instead of the chlorine in the industrial circulating cooling water, especially for the cooling water system in the ammonia synthesis plant, the chemical plant and the oil refinery plant, because in such system the content of the organic matter and the ammonia is higher, the demand for the chlorine is also higher, the PH value is of alkaline, the using of the chlorine dioxide instead of the chlorine can be more beneficial in economy and in the environment.

3.2 The non oxidizing biocide

There is a broad variety of the non oxidizing biocides, the chlorophenols was used in the earlier application, but because it is of high toxicity and can easily pollute the water, it is gradually abandoned now. The organic amines are also used very rarely.

The dithiocyano-methane is a kind of organic sulfide biocide which was used earlier. It is very effective for the inhibition of the algae, the fungi and the bacteria, especially the fulfate reducing bacteria. But it is not suitable to be used in the alkaline cooling water system.