Application of Activated Carbon Materials in Tap Water Supply

Activated carbon is a type of carbon with a large specific surface area and a porous structure. It can be classified according to its raw materials into coal-based activated carbon, wood-based activated carbon, fruit shell activated carbon, and bone-based activated carbon; and according to its morphology into columnar activated carbon, crushed activated carbon, powdered activated carbon, and fibrous activated carbon. The main raw materials for activated carbon are carbon-rich organic materials such as coal, wood, and fruit shells, which are activated to form a complex porous structure with adsorption capacity. Pores with a radius greater than 20,000 nm are macropores, those between 150-20,000 nm are mesopores, and those less than 150 nm are micropores. The adsorption of activated carbon mainly occurs in these pores and on its surface. The numerous molecules on the pore walls of activated carbon generate strong attraction, drawing impurities from water and air into the pores.

The adsorption of activated carbon can be divided into physical adsorption and chemical adsorption. Physical adsorption mainly occurs in the abundant micropores of activated carbon, used to remove impurities from water and air. The molecular diameter of these impurities must be smaller than the pore size of the activated carbon. Different raw materials and processing techniques result in different microporous structures, specific surface areas, and pore sizes of activated carbon, making them suitable for different needs. Activated carbon not only contains carbon but also functional groups on its surface, which react chemically with the adsorbed substances, often occurring on the surface of the activated carbon. Impurities in the medium continuously enter the porous structure of the activated carbon through physical and chemical adsorption, causing adsorption saturation and a decrease in adsorption efficiency. Saturated activated carbon needs to be reactivated and regenerated to restore its adsorption capacity for reuse. The main indicators for evaluating the adsorption performance of activated carbon include methylene blue value, iodine value, and caramel adsorption value; the larger the adsorption capacity, the better the adsorption effect.

Activated carbon can be used in air purification and water supply and wastewater treatment to separate or collect impurities in air and water media. Granular and powdered activated carbon serve the same purpose and can both be used in water treatment. Granular activated carbon is less prone to loss and can be regenerated and reused, making it suitable for water treatment processes with less pollution and requiring continuous operation. Powdered activated carbon is not easily recovered and is generally for single use, used in intermittent water treatment processes with more severe pollution. Granular activated carbon for water treatment generally has well-developed micropores and mesopores and should meet three requirements: large adsorption capacity, fast adsorption rate, and good mechanical strength. In addition to the above characteristics, powdered activated carbon requires smaller particle size for better adsorption.

Waterworks under the Beijing Waterworks Group that use surface water as their source are equipped with 1.5-meter-deep granular activated carbon filters, designed for a filtration rate of 9.5 meters per hour. Activated carbon filters are a deep treatment process in water supply, effectively removing color, odors, and dissolved organic pollutants, thus improving water quality. Granular activated carbon requires periodic backwashing based on the raw water quality. Generally, water flushing is used, with backwashing every six days, achieving a backwash intensity of at least 30% of the filter bed's expansion rate. New activated carbon primarily uses physical and chemical adsorption; after a certain period, a biofilm forms on the carbon surface, leading to biodegradation. When the indicators for evaluating activated carbon adsorption fall below relevant standards, or when a pollutant penetrates the carbon bed, replacement or regeneration is necessary. New activated carbon requires soaking for 24-48 hours, followed by backwashing to remove residual tar and carbon powder. The number of backwash cycles and the turbidity of the effluent can be determined based on the intended use of the treated water. For water supply treatment, the turbidity of activated carbon backwash effluent can generally be controlled at 2-5 NTU.

Powdered activated carbon has excellent applications in treating sudden odors and industrial pollutants in water. From September to November 2005, due to high levels of odor-causing substances in the Miyun Reservoir, the Beijing Water Supply Group's Ninth Water Plant effectively removed the odor by adding powdered activated carbon to the water transmission pipeline. During the period of nitrobenzene and benzene pollution in the Songhua River, from November 26th to 30th, when the polluted water flowed through Harbin City, the Harbin Water Supply and Drainage Group, under the guidance of experts from the Ministry of Construction, used the technology of the Ninth Water Plant to promptly treat the nitrobenzene in the water, meeting water quality requirements.

When using powdered activated carbon, adsorption tests must be conducted based on the type and concentration of pollutants to be removed to determine the type of activated carbon and the required amount of powdered activated carbon. Before adding powdered activated carbon, it is important to prepare a carbon slurry and add it quantitatively and evenly to the water. The longer the contact time, the better the pollution removal effect. The following safety precautions should be taken when using powdered activated carbon: When the dust concentration reaches a certain level, it can easily explode upon contact with an open flame; therefore, smoking, sparks, and open flames are prohibited in the operating area. It should not be stored with oxidizers. Due to the small and light size of the powdered activated carbon particles, attention should be paid to dust pollution during use; operators must wear dust masks to avoid inhaling the dust.