Abstract: As an effective measure to improve water quality in water plants, powder activated carbon adsorption technology has flexible operation mode, low cost and obvious effect. By synthesizing the research results, the key problems that should be solved in the application of powder activated carbon adsorption technology in water plants were discussed.

Key words: powdered activated carbon; water quality; organic matter

1 Application Status

The use of powdered activated carbon in water treatment has a history of about 70 years. Since the United States first used powdered activated carbon to remove the odor caused by chlorophenols, activated carbon has become one of the effective methods for removing color, odor, taste and organic matter in water treatment. A large number of studies on the adsorption performance of powdered activated carbon abroad have shown that powdered activated carbon has a good adsorption effect on trichlorophenol, dichlorophenol, organic matter in pesticides, trihalomethanes and precursors, as well as disinfection by-products such as trichloroacetic acid, dichloroacetic acid and dihaloacetonitrile, etc. The removal effect on color, odor and taste has been widely recognized.

Powdered activated carbon is widely used in Europe, America, Japan and other countries. In the early 1980s, the United States used about 25,000 tons of powdered activated carbon in water treatment each year, and the amount has been increasing year by year. In China, the issue of deodorization and deodorization of polluted water sources began to be paid attention to in the late 1960s. Powdered activated carbon has been tested in Shanghai, Harbin, Hefei and Guangzhou. In recent years, China has gradually attached importance to the research and application of powdered activated carbon. Tongji University, Harbin Institute of Technology and other institutions have conducted in-depth research and achieved many practical results.

The main features of the application of powdered activated carbon are low equipment investment, low cost, fast adsorption speed, and strong adaptability to short-term and sudden water quality pollution. 2. Bottlenecks restricting the application of the technology According to our research, the application of powdered activated carbon adsorption technology in waterworks is a very promising technology. However, due to the failure to properly address the limitations of the technology in application, it is difficult to give full play to the advantages of powdered activated carbon technology, resulting in the technology application not achieving the actual effect. The application in waterworks must solve two types of problems: theoretical basis and application.

2.1 Problems that should be solved theoretically

(1) According to the water quality of raw water in the water plant, especially the distribution of molecular weight of organic matter, determine the type of carbon added with powdered activated carbon.

(2) According to the actual water quality of the water plant, determine a reasonable and economic dosage.

(3) According to the existing production process of the water plant, determine the appropriate and reasonable dosing point and dosing method to solve the contradiction between the adsorption competition between the powder activated carbon and the coagulant, and improve the use efficiency of the powder activated carbon.

As a general rule, under the same conditions, different powdered activated carbon species have a large difference in adsorption capacity of organic matter (removal rate difference of 16%). Similarly, according to the characteristics of the water production process of the water plant, the influence of different dosing points is also large, mainly due to the characteristics of raw water and the result of the competition between coagulation and adsorption, and the dosing amount should be determined according to the target expectation (factory water CODMn) and operating costs in engineering applications.

As an emergency measure, powdered activated carbon has been tried in some water plants, but the application effect of this technology is mixed. Our research shows that: according to the different actual situations of water plants, we must explore the appropriate ways to solve the above three problems; Especially for different processing processes, it is very important to choose a reasonable dosing point and dosing method. Therefore, enough attention must be paid to the application of this technology in order to economically and effectively play the role of powder activated carbon in removing pollution.

2.2 Problems to be solved in engineering application

(1) The pollution of dust flying in the application. In the application of water works, due to the high labor intensity of powdered activated carbon in many links such as loading and unloading, unpacking, preparation, and dozing process, it is easy to cause dust flying, resulting in poor working environment and strong resistance of operators, which has also become a key and substantive problem restricting the application of powdered activated carbon technology.

According to data reports, some waterworks use the negative pressure preparation dosing method to dosing powdered activated carbon. This method has basically solved the problem of dust pollution, but it is still difficult to avoid the problems of dust flying and labor intensity caused by powdered activated carbon (20 kg/ bag) in the process of handling and unpacking, especially for waterworks with a processing capacity of more than 100,000 m3/d. The amount of powdered activated carbon per hour is generally about 60 kg (calculated with the dosage of 15 mg/L).

(3) Precise preparation and quantitative dosing of powdered activated carbon in applications. In order to stabilize the effect of powder activated carbon adsorption and pollution removal, the accuracy of dosimetry should be ensured within a certain range, which is not only related to the treatment effect, but also closely related to the cost of water production. The entire powder activated carbon storage, preparation, dosing equipment or system built according to the appropriate parameters must be able to well prevent the instability factors caused by various links, such as the blockage problem in the transport dosing process, which will cause the flow instability, thus affecting the effect of pollution removal.

Automatic control of equipment or systems. In order to further reduce the operational strength of powder activated carbon dosing equipment, how to achieve automatic operation, match with the original automatic control system of the water plant and how to automatically track and adjust according to water quality changes to meet the purpose of stable effluent water quality is also a key factor restricting the application of this technology.

(4) Investment and cost control. The application of powdered activated carbon technology is the most critical issue of investment and cost control, in order to meet the new "drinking water hygiene code" (mainly CODMn < 3 mg/L, under special circumstances not more than 5 mg/L), most water departments are facing the problem of technological transformation. For most water departments, water pollution is generally intermittent or sudden, the conventional process in most of the time is able to meet the requirements of the new specifications, so powdered activated carbon technology is a very practical technology, its investment is relatively less, lower cost, flexible operation.

For example, a waterworks with a treatment capacity of 100,000 m3/d, the investment of equipment is about 1.2 million yuan, and the investment of 1 m3 water is about 12 yuan, which has great advantages compared with the investment of biological treatment methods (about 100 yuan for 1 m3 water) and ozone biological activated carbon process investment (about 250 yuan for 1 m3 water); At the same time, the increased treatment cost is about 0.02 yuan /m3(based on the average pollution period of the annual use of powdered activated carbon dotting equipment for 90 days, the average dotting amount is 15 mg/L). 

3 Conclusions

According to our long-term theoretical research and engineering practice, the addition of powdered activated carbon as an acute means of water quality improvement, as long as the correct solution to the technical use of carbon selection, adding point, adding method and other problems, can better improve the water quality of the water plant, especially the improvement of organic matter (CODMn), chroma and other water quality indicators; At the same time, the technology has been tested in engineering practice, and has solved many problems such as dust pollution in the process of use, accurate injection, reducing labor intensity and realizing automatic control, and the use of the technology has less investment, obvious effect and low operating cost.