|
Application of Powder Activated Carbon in Drinking Water Treatment in ShanghaiTime£º2023-11-03 The application of activated carbon in water treatment has a long history. Since the first use of powdered activated carbon to remove the odor generated by chlorophenols at the New Milford Water Plant in the United States in 1929, powdered activated carbon has been used in water treatment for more than 80 years. Research has found that its treatment effect on color, odor, and taste in water is very significant. The powder activated carbon adsorption treatment technology has become an effective method for removing color, smell, taste, and organic matter in water treatment. PAC has a well-developed microporous structure, large specific surface area, and excellent adsorption performance, which can effectively remove odors, chromaticity, chlorinated organic compounds, pesticides, natural organic compounds, and artificially synthesized organic compounds. PAC is a hydrophobic adsorbent prepared by high-temperature carbonization and activation of substances containing carbon as the main raw material. During the activation process of the raw material, the removal of carbon containing organic matter causes the formation of pores between the basic lattice, forming many fine pores of various shapes and sizes. The total area of the pore walls is the specific surface area. Due to its high specific surface area, activated carbon has strong adsorption capacity. However, activated carbon with the same specific surface area may not have the same adsorption capacity, which is caused by different pore structures and distributions. The pore structure of PAC varies with the raw materials, activation methods, and activation conditions. Generally, its pores can be divided into three types: 1) small pores (micropores) with a radius of less than 2 nm, and their surface area accounts for over 95% of the specific surface area, which has the greatest impact on the adsorption amount and exhibits strong adsorption effect; 2) Mesopores (transition pores), with a radius of 2-50 nm and a surface area accounting for less than 5% of the specific surface area, not only provide diffusion channels for adsorbates and affect diffusion speed, but also facilitate the adsorption of macromolecular substances. They can be used for adding catalysts and deodorizing chemicals, and can have different functions depending on the type of chemicals added; 3) Macropores, with a radius greater than 50 nm and a surface area of only 0.5-2m2/g, accounting for less than 1% of the specific surface area, mainly provide diffusion channels for adsorbates. Macropores mainly serve as channels for solutes to reach the interior of activated carbon, and have little effect on liquid phase physical adsorption. However, as catalyst carriers, the role of macropores is very significant. The mesopores act as both adsorption and channels, so the diffusion rate of the adsorbate is also influenced by the cross pores; Micropores account for the main part of the specific surface area of activated carbon and are the main point of action for activated carbon to adsorb micro pollutants. The adsorption of PAC is not only influenced by its pore structure, but also by factors such as adsorbate characteristics, natural organic matter, and residual chlorine concentration. PAC is not suitable for adsorbing high solubility organic compounds due to its strong affinity with polar solvents, while PAC is a hydrophobic adsorbent, making it difficult to adsorb such organic pollutants. The size of adsorbate molecules has a significant impact on the adsorption capacity of PAC. When the size of adsorbate molecules approaches the pore size of PAC, the adsorption capacity of PAC is strongest. Research has shown that PAC can effectively adsorb odorants. The adsorption capacity of PAC for five odorants is in the order of trichloroacetic acid (TCA)>2-methoxy-3-isobutylpyrazine (IBMP)>2-methoxy-3-isopropylpyrazine (IPMP)>soil odor (Geosmin)>2-methylisobutyl alcohol (2-MIB), and the adsorption capacity of 2-MIB is much lower than that of Geosmin. This is because Geosmin has low solubility and its structure is easily adsorbed by the narrow pores of PAC. The influencing factors of PAC adsorption capacity are shown in Table 1. Table 1 Factors affecting the adsorption capacity of PAC
The standard specifications for PAC in drinking water treatment at home and abroad include the American Water Association Standard (AWWA B600-10), the British Standard (BS EN 12903:2009), the Japanese Sewer Association Standard (JWWA K113:2005), the domestic activated carbon standard for wooden water purification (GB/T 13803.2-1999), and the coal based activated carbon standard for drinking water purification plants (CJ/T 345-2010). The requirements for adsorption capacity of each PAC standard are shown in Table 2. Table 2 Specification for PAC adsorption capacity
There are two ways to add powdered activated carbon, namely dry dosing and wet dosing. The dry dosing system uses equipment such as a dry powder dosing machine to directly add powdered activated carbon into the treated water body through a water jet. The main units generally include five parts: storage room, feeding unit, storage bin, metering and dosing equipment, and automatic control system. The wet dosing system first modulates PAC into 5-10% carbon slurry, and then adds it to the water through a metering pump. The main equipment units generally include six parts: storage room, feeding unit, storage bin, carbon slurry mixing equipment, carbon slurry dosing equipment, and automatic control system. The dry dosing system is relatively simple and covers a small area, but dry PAC has a flammable and explosive risk, and the equipment is prone to malfunctions. Professional maintenance personnel are needed to cooperate. The wet dosing system has precise measurement and uniform mixing, but requires a dedicated carbon slurry tank, which occupies a large space area and the equipment is also complex. When adding PAC, the method of adding needs to be selected based on site conditions and dosage. Figures 1 to 4 show different dosing systems.
When the dosage of PAC is small, its adsorption capacity can be fully utilized, but when the concentration of organic matter is high, it is difficult for the effluent to meet the standard; Excessive dosage results in low effluent concentration of the target substance and water quality standards, but PAC is not fully utilized, resulting in high water production costs. Due to the different pore size distribution, surface functional group distribution, ash composition and content of PAC, the adsorption characteristics are different, and the carbon species suitable for each water source quality are different. Therefore, it is necessary to conduct coagulation and stirring tests to determine the appropriate dosage and carbon species. Add 1 L of raw water with odor pollution to each cup and bottle, turn on the stirring device to 60-80 rpm, and add 0 mL, 0.5 mL, 1 mL, and 2 mL of PAC to each cup and bottle. The PAC concentrations are 0 mg/L, 5 mg/L, 10 mg/L, and 20 mg/L, respectively. Stir at 60-80 rpm for 5 minutes. After completion, add 0.175 mL of coagulant solution to each bottle at the same time. The coagulant concentration in each bottle is 20 mg/L, reduce the speed to 30 rpm, stir for 20 minutes, then stop stirring, precipitate for 1 hour, take 600 mL of the supernatant and filter it. Analyze the MIB/Geosmin odor concentration using SPME-GC/MS. The relationship between PAC dosage and odor removal rate is shown in Figure 5, with PAC dosage as the x-axis and removal rate as the y-axis. Taking the removal rate of 90% as an example (with an initial concentration of 100 ng/L and an odor threshold of 10 ng/L), the optimal dosage of PAC required is 19 mg/L. The above experiments were conducted with four different types of PACs (PAC-A, PAC-B, PAC-C, PAC-D), and the removal rate PAC dose map was established as shown in Figure 6.
Taking the removal rate of 90% as an example, the optimal doses for PAC-A, PAC-B, and PAC-C are 14 mg/L, 15.5 mg/L, and 19 mg/L. Based on the minimum required dose, the efficiency factors for PAC are calculated as follows: PAC-B=14/14=1.00, PAC-C=15.5/14=1.11, PAC-A=19/14=1.36, and dosing cost=unit cost ¡Á The efficiency factor, calculated as the cost of adding each type of PAC, is shown in Table 3, so the optimal PAC is PAC-B. Table 3 Cost of each PAC addition
The optimal location for adding powdered activated carbon is: (1) the raw water intake; (2) Rapid mixing tank; (3) Activated carbon slurry contactor (specially designed for powder activated carbon). The advantages and disadvantages of different dosing positions of powdered activated carbon are summarized in Table 4. Table 4 Advantages and Disadvantages of Adding PAC at Different Feeding Positions
¢Ù Wet powder activated carbon can cause powder dispersion during bag opening operations, so it is necessary to operate the dust collection device and wear dust protection equipment for operation. ¢Ú During the operation of adding powdered activated carbon, attention should be paid to the protective measures of dust collection devices and environmental hygiene management. If necessary, various dust hazard prevention measures in the "Dust Hazard Prevention Standard" should be referred to for handling. ¢Û After or during the injection of powdered activated carbon, there may be residual liquid in the dissolution tank. The mixer should be started at an appropriate time to prevent the deposition of powdered activated carbon and its solidification. ¢Ü When using dry dosing, attention should be paid to fire and dust prevention, and the adverse effects on power equipment should be evaluated. When injecting dry powder activated carbon into the liquid, it should be injected below the liquid level, and attention should be paid to whether there is any floating of powder activated carbon in the dissolution tank. It is necessary to confirm the appropriate stirring state to allow the activated carbon to flow in. ¢Ý Powdered activated carbon is usually removed by coagulation sedimentation and filtration units. However, due to the relatively fine powder, when adding at high concentrations, if the dosage of coagulant is insufficient, the sedimentation time is too short, or the continuous high filtration rate passes through the fast filter for a long time, the fine activated carbon will penetrate the filter bed, causing the filtered water to contain activated carbon. ¢Þ When powdered activated carbon and chlorine disinfection are added simultaneously, usually every 1 mg/L of powdered activated carbon will consume 0.2-0.3 mg/L. Therefore, to avoid affecting the adsorption effect of activated carbon and reducing the chlorine consumption of activated carbon, it is necessary to consider adjusting the pre chlorination point to an appropriate position. ¢ß Activated carbon containing water powder has corrosive properties, so important pipeline equipment should be made of 316 stainless steel. After the addition of powdered activated carbon is stopped (during the shutdown period), the feeding pipes, piping, and other facilities should be cleaned with clean water. At an appropriate time, maintenance operations such as general inspection, repair, and improvement of facilities and equipment should be carried out for the next use. ¢à When adding powdered activated carbon on a high platform, safety protective equipment and other facilities should be prepared to prevent falling. Operators adding powdered activated carbon should wear protective equipment such as dust jackets and masks to prevent dust hazards. ¢á The workplace where powdered activated carbon is added should comply with the relevant provisions of the Labor Safety Law. When hiring workers, general physical examinations should be carried out, while on-the-job workers should undergo regular special health examinations and implement health management. The supervisor responsible for adding powdered activated carbon should receive safety and health education and training from the dust operation supervisor. The Shanghai water supply system has installed PAC dosing devices at both the water intake head and the head of the water plant, playing an important role in emergency response to odor and other sudden water quality pollution problems caused by seasonal algae, greatly improving the emergency guarantee capacity of Shanghai's water supply quality. Both the Songpu Bridge water intake pump station and the Qingcaosha Reservoir No. 5 pumping station in the upstream water source of the Huangpu River have been constructed into large-scale powder activated carbon dosing systems; The head of the water plant is equipped with powder activated carbon dosing points, mainly to deal with poor raw water quality during on-site water intake. By adding powder activated carbon, the chromaticity and odor in the water can be effectively removed, and the level of water supply safety assurance can be improved. Various water plants such as Yangshupu Water Plant, Changqiao Water Plant, Jinshan No.1 Water Plant, Linjiang Water Plant, and Fengxian Water Plant have installed powder activated carbon dosing devices. The feeding equipment is shown in Figures 7 to 12, respectively.
Figure 7 Schematic diagram of PAC dosing at the Songpu Bridge water intake pump station
Figure 8 Mixing Tank, Dosing Pump, and Carbon Liquid Dosing Pipe of Songpu Bridge Water Intake Pump Station
The eutrophication of water bodies directly leads to the prominent problem of seasonal algae pollution in raw drinking water, among which the odor of water quality caused by algae pollution is the main problem faced by water supply enterprises. Powder activated carbon is one of the effective means to deal with odor removal. The setting of powder activated carbon dosing points in Shanghai is scientific and reasonable. The raw water intake head is equipped with a powder activated carbon dosing point, which can effectively prolong the adsorption time of powder activated carbon and greatly improve the treatment efficiency. The head of the water plant is equipped with powder activated carbon dosing points, mainly to deal with the poor quality of raw water during on-site water intake. By adding powder activated carbon, the chromaticity and odor in the water can be effectively removed, and the level of water supply safety assurance can be improved. Although powdered activated carbon has been used for many years in China, there is still no unified technical specification and standard from an industry perspective. This article suggests that relevant regulatory departments or industry leaders in Shanghai should study and develop technical guidelines for powdered activated carbon for water purification. A standardized and scientific technical standard should be established comprehensively from the selection of powdered activated carbon, setting of dosing points, dosage testing, dosing methods, operation management, transportation and storage, safety emergency response, etc., to provide support for Shanghai's drinking water treatment technology. |
