Summary of activated carbon for advanced treatment of tap water:
The application of activated carbon to the deep treatment of tap water began in the 1920s abroad, and China's Shenyang Water Company tried to use activated carbon filter in the 1970s. Since the 1980s, Beijing Water Company began to conduct research on the removal of odor with activated carbon, and Beijing Water Company Tiancun Shanshui Plant and the ninth water plant successfully used activated carbon filter, effectively reducing the content of odor and organic matter in water.
Beijing Water Group Co., Ltd. has been using activated carbon to treat tap water for nearly 20 years. Many other large and medium-sized cities across the country have conducted various studies and discussions to varying degrees, but have not yet truly entered the practical stage. On September 1, 2001, the Ministry of Health of the People's Republic of China promulgated and implemented the new "Hygienic Standards for Drinking Water Quality", replacing the old standards that had been in use since 1985. The new standards set higher requirements for the quality of tap water, which has brought new pressure and impetus to the water industry across the country. In recent years, Beijing has been expanding the scale of water plant renovations and increasing the use of activated carbon. The Zhoujiadu Water Plant in Shanghai has implemented the biological activated carbon and ozone water treatment process. Large cities such as Tianjin, Gansu, Shenyang, Fushun, Jinan, Guangzhou, Chongqing, Kunming, and Shenzhen have been gradually testing the use of activated carbon for advanced tap water treatment or source water purification. Water companies in Hangzhou, Jiaxing, Tongxiang, Pinghu, and other places in Zhejiang Province, considering the severe pollution of water sources by the local dyeing and leather industries, have conducted extensive experimental and demonstration work and are either building or have already built ozone biological activated carbon water treatment processes. Driven by large cities, the use of activated carbon for advanced tap water treatment processes is being widely promoted in cities across the country, which is bound to significantly increase the use of activated carbon in the water treatment field.
In 2003, when visiting the water production process of the Ninth Water Plant in Beijing, the following information about the activated carbon filter was learned: The treatment capacity is 1.5 million cubic meters per day, and the water source is from Miyun Reservoir. The raw water goes through initial filtration, coagulation sedimentation, sand filtration, and coal filtration before entering the activated carbon filter. There are a total of 72 activated carbon filters in this process, each with a water surface area of approximately 95 square meters, a single filter bed depth of 1.5 meters, and a total activated carbon usage of 5,200 tons. (Note: A rough calculation can be made as follows: 95m² × 1.5m × 0.5t/m³ × 72 = 5,130t. Assuming the bulk density of activated carbon is 0.5t/m³.) In the mid-1990s, the calculation of the additional production operation cost for adding the activated carbon treatment process was that the average cost increase per ton of water leaving the plant was 0.02 yuan.
The filtration rate of the activated carbon filter is 8 m/h, with a contact time of 13.5 minutes. It is backwashed with sand-filtered water. As the original backwash water tower is used for backwashing, the backwash intensity is controlled by adjusting the valve opening. According to the actual situation, the backwash cycle is temporarily set at 48 hours. The backwash intensity for broken carbon is 10.5 L/m².s, with a backwash time of 6 minutes and an expansion rate of 17.8%. The backwash intensity for columnar carbon is 16.0 L/m².s, with a backwash time of 6 minutes and an expansion rate of 17.6%.
The pretreatment tank was put into operation on April 19, 2003, and started aeration on April 21. It began full-load operation on June 18, with a maximum inflow of 4,600 m³/h. One week later, the ammonia nitrogen removal rate reached over 70%. After full-load operation, the average ammonia nitrogen removal rate was 91%, the turbidity removal rate was 27%, the CODMn removal rate was 10%, the iron removal rate was 22%, and the manganese removal rate was 56%. After the pretreatment operation, the effect of conventional treatment was improved. Under the same amount of alum addition, the turbidity removal rate reached 99.9%, the turbidity at the outlet of the sedimentation tank decreased from 3 to 4 NTU to less than 2 NTU, and the turbidity of the water after sand filtration was less than 0.5 NTU. The average CODMn removal rate increased from 30% to 44%, and the average CODMn of the water after sand filtration was less than 3 mg/L. The manganese removal rate increased from 20% to 75%.
The deep treatment ozone-activated carbon system was put into operation on May 1st. Ozone was added starting from May 17th, and full-load operation began on June 18th. One week after operation, the removal rate of residual ammonia nitrogen reached about 70%, and the biofilm was basically formed. The removal rate of CODMn was over 70% within one month and gradually decreased to stabilize at around 40% after one month. This indicates that in the initial stage, the adsorption effect of activated carbon was the main factor, while in the later stage, the biological effect played a dominant role. Judging from the treatment effects of the two types of carbon, crushed carbon was slightly better than columnar carbon, and the difference was basically negligible.
The power cost of pretreatment is composed of two parts. One part is the power cost of increasing the water head of the intake pump house by 1.5m because the pretreatment process is increased before the conventional treatment process. The other part is the power cost of blast aeration (the gas-water ratio is calculated according to 1.5:1), and the total unit cost is about 0.044 yuan/ton.
The advanced treatment includes: electricity charge for raising water intake in ozone contact pool and electricity charge for ozone generation (0.09 yuan/ton), liquid oxygen cost (including gas station rental fee, 0.049 yuan/ton), activated carbon (calculated according to service life of 1.5 years, unit cost 0.046 yuan/ton), the total unit cost is about 0.185 yuan/ton.
The total operating cost is 0.229 yuan/ton, plus depreciation 0.076 yuan/ton, and the water production cost of pre-treatment and advanced treatment is 0.305 yuan/ton.
● Quartz sand mat for activated carbon in BAC filter
Some water companies in the south add quartz sand mat under the activated carbon layer, which can reduce the turbidity of the effluent and have a certain interception effect on the bacteria in the water. Of course, the backwashing cycle should be controlled.
● Several BAC filters are also provided for reference.
Niu Kazan
Dai Xiuze
July 24, 2006
