The Reasons For Choosing Sodium Hypochlorite Generators For Disinfection Technology In Rural Water

As of 2024, China has made significant progress in addressing the issue of drinking water safety for rural residents and teachers and students in rural schools. According to the information released by the Ministry of Water Resources, by 2023, the national rural tap water penetration rate will reach 90%, which means that the goal of achieving a national rural tap water penetration rate of 88% has been achieved two years ahead of schedule. In addition, in 2023, 23000 rural water supply projects will be started nationwide, improving the water supply guarantee level for 110 million rural population. The proportion of rural large-scale water supply projects covering the rural population has reached 60%. Minister of Water Resources Li Guoying stated at the 2024 National Water Resources Work Conference that by 2024, the rural tap water penetration rate in China will increase to 92%, and the proportion of rural population covered by large-scale water supply projects will reach 63%.

In terms of ensuring the drinking water safety of rural residents, the Ministry of Water Resources has comprehensively solved the drinking water safety problem of 17.1 million impoverished people and properly solved the problem of 10.95 million rural residents drinking high fluoride water and brackish water. These measures and achievements reflect China's emphasis on rural water supply safety and rural revitalization, as well as the efforts made to improve the level of rural water supply security.

The Necessity of Disinfection in Rural Water Supply Projects

According to a World Health Organization survey, 80% of human diseases and 50% of child mortality rates are related to drinking water quality. In its "Drinking Water Quality Guidelines," it is clearly stated that microbial issues still top the list of safety issues related to drinking water, both in developing and developed countries. In rural areas of our country, due to the impact of water body pollution and the lack of source water protection, microbial pollution is the main type of drinking water source pollution and the primary reason for the low water quality compliance rate in rural water supply projects. Water bodies contaminated with pathogenic microorganisms contain a large number of pathogens and viruses, which can cause diseases such as typhoid, dysentery, cholera, polio, hepatitis A, and diarrhea, and can even lead to death in severe cases.

The correct choice and application of disinfection technology can ensure the microbial safety of drinking water. The "Hygienic Standard for Drinking Water" (GB5749), "Outdoor Water Supply Design Code" (GB50013), and "Village Water Supply Engineering Design Code" (SL 687) all make mandatory provisions for the disinfection of drinking water.

At present, the water sources of rural water supply projects are small and scattered, the protection of water source areas is not strong, there are many sources of water source pollution, and the existing water treatment methods are not enough to ensure the microbial safety of drinking water for users at the end of the pipeline. Through disinfection, on the one hand, it can effectively kill the microorganisms in the water source, ensuring the microbial safety of the water leaving the plant; on the other hand, it can retain some residual disinfectant, which plays a role in defending against microbial pollution in the water distribution pipelines and containers after the water leaves the plant.

Main Disinfection Technologies and Equipment Suitable for Rural Water Supply

1.Chlorine and Chlorine Compound Disinfection and Its Application Forms

Chlorine and chlorine compound disinfection includes sodium hypochlorite, liquid chlorine, bleaching powder, bleaching powder essence, and calcium hypochlorite, etc. After they dissolve into the water body, the effective disinfection component is hypochlorous acid, so the disinfection mechanism is also the same. This disinfection technology is suitable for source water with a pH value not higher than 8.0, good water quality, and no pollution. Among them, sodium hypochlorite and liquid chlorine are used the most. The application form of sodium hypochlorite disinfection is: commercial sodium hypochlorite solution is used in conjunction with its dosing device, or sodium hypochlorite solution is prepared on-site by electrolyzing brackish water with a sodium hypochlorite generator. The application form of liquid chlorine is: liquid chlorine steel bottles are used in conjunction with chlorinators, which include positive pressure chlorinators, rotor chlorinators, vacuum chlorinators, and other forms. Bleaching powder, bleaching powder essence, and calcium hypochlorite are solid preparations in different forms, and their application form is: manual or automatic drug dissolution devices are used to dissolve the solid preparations, which are then added to the water body through dosing devices.

2.Chlorine Dioxide Disinfection and Its Application Forms

Chlorine dioxide disinfection relies on molecular chlorine dioxide. Chlorine dioxide disinfection can be considered for rural small and medium-sized water supply projects in the following situations: ① Surface water sources contaminated by organic matter; ② Water sources with color, odor, and taste caused by algae and fungi; ③ Source water with high pH value and ammonia nitrogen content; ④ Groundwater sources with high iron and manganese content.

Chlorine dioxide is generally prepared on-site, and according to the different generation technologies, it includes chemical chlorine dioxide generation devices and electrolytic chlorine dioxide generation devices. The latter has high electricity consumption and salt consumption, low chlorine dioxide yield and purity, difficult maintenance and management, and is not recommended for use in rural water supply projects. Chemical chlorine dioxide generation devices can be divided into composite chlorine dioxide generators and high-purity chlorine dioxide generators according to the different final products. The former's product contains both chlorine dioxide and chlorine, while the latter's product contains only one disinfectant, chlorine dioxide. The composite type generator generally uses the sodium chlorate + hydrochloric acid method, while the high-purity type generator is divided into binary high-purity chlorine dioxide generators and ternary high-purity chlorine dioxide generators. The former has two preparation processes: hydrochloric acid + sodium chlorite method, chlorine + sodium chlorite method, the former has more application examples in domestic water supply projects, and the latter is rarely used in China because it still needs to use liquid chlorine in the raw materials. The preparation process of the binary high-purity chlorine dioxide generator includes sodium chlorite + sodium hypochlorite + hydrochloric acid method, sodium chlorate + sulfuric acid + hydrogen peroxide method, sodium chlorate + sulfuric acid + sucrose method, and sodium chlorate + sulfuric acid + urea method, and other different reaction principle generation methods. The ternary high-purity chlorine dioxide generator is rarely used in rural water supply projects due to its complex raw materials.

3.Ultraviolet Disinfection and Its Application Forms

Ultraviolet disinfection uses ultraviolet light in the 200~280 nm (especially 253.7 nm) band to kill bacteria. Ultraviolet disinfection is only suitable for rural small-scale centralized or decentralized water supply projects without clear water pools and short pipelines. When groundwater is used as the water source, water quality indicators such as turbidity, color, iron, manganese, and hardness should be better; when surface water is used as the water source, particulate matter and colloids in the water need to be removed first. The most widely used ultraviolet disinfection equipment in rural small-scale centralized or decentralized water supply is the flow-through pressure-type ultraviolet sterilization equipment. In addition, there are also intermittent ultraviolet disinfection equipment applications in single-family projects.

4.Ozone Disinfection and Its Application Forms

Ozone disinfection mainly relies on [O] and ·OH generated during the decomposition process of ozone entering the water body. Like ultraviolet disinfection, ozone disinfection is only suitable for rural small-scale centralized or decentralized water supply projects without clear water pools and short pipelines. In addition, it should be noted that when the bromide content in the water is above 20 μg/L, there may be a risk of exceeding the standard for the by-product bromate, and it is recommended to conduct an ozone dosing test in advance. Ozone is generally prepared on-site by ozone generators, which mainly include corona discharge method ozone generators that generate ozone by corona discharge with air or pure oxygen as raw materials, and electrolytic pure water method ozone generators that generate ozone by electrolysis with pure water as raw material.