Activated Carbon in Water Treatment Plants
Effects of using activated carbon in water treatment
Activated carbon adsorption is one of the most mature conventional water treatment technology. It was first used in the sugar industry then widely used to remove organic and certain inorganic substances from wastewater. Many water plants are equipped with activated carbon filters. Activated carbon has a developed microporous structure and a huge specific surface area which provide a wide range of removal effects on a variety of pollutants organic pollutants, substances that cause color and odor in water, such as geosmin, 2-methylisoborneol (MIB). It also has good adsorption ability for aromatic compounds, pesticides, heavy metal ions like mercury, hexavalent chromium, cadmium, lead. Several studies have shown that the mutagenic activity of water tested positive turns negative after the adsorption by activated carbon.
As a kind of non-polar adsorbent with good adsorption capacity for non-polar and weakly polar organic substances in water, regarding physical adsorption, activated carbon has low selectivity, can effectively adsorb in a multilayer structure and has relatively easy desorption, which is conducive to the regeneration of activated carbon after saturation. During the preparation of activated carbon at high temperature, a variety of functional groups are formed on the surface of the carbon, and these functional groups have a chemical adsorption effect on ions in water, so activated carbon can remove heavy metal ions. Its mechanism of action is through complexation and chelation, and it is with high selectivity, monolayer adsorption, and relatively difficult to desorb.
Selection of activated carbon for the filter of water treatment plant
Granular activated carbon (GAC), extruded activated carbon (EAC) or powdered activated carbon (PAC), which one is more suitable for water treatment plants?
Usually, the activated carbon filter in the initial operation of water treatment mainly relies on the micro-pores of the activated carbon itself. After the activated carbon has been in operation for a period of time, a biofilm is formed on the surface of the activated carbon and in the external pores, and the adsorption and degradation of microorganisms gradually take the leading role. At the same time, a large number of micro-pores inside the activated carbon can also adsorb organic matter, thus forming a biological activated carbon with both micro-pore adsorption and biodegradation. After the activated carbon treatment, COD, ammonia nitrogen, etc. can be significantly decreased. However, the water purification effect of different types of activated carbon is not the same.
According to the appearance, there are granular activated carbon (GAC), extruded activated carbon (EAC), and powdered activated carbon (PAC).
Granular activated carbon (GAC) versus extruded activated carbon (EAC)
Granular activated carbon and extruded activated carbon are both used in water plants, each has its own advantages. Granular activated carbon has an irregular shape and rough surface, and its specific surface area is much larger than that of extruded activated carbon, so microorganisms are easily adsorbed on its surface, and the removal effect is better, especially for organic matter, odor indicators. The choice of activated carbon is also related to the flow direction of the filter. In the upward flow filter, if extruded activated carbon is used, it is easy to retain turbidity and increase the head loss, so it is recommended to use granular activated carbon in the upward flow filter.
Granular activated carbon (GAC) versus powdered activated carbon (PAC)
Granular activated carbon is more used for advanced treatment of water, and its treatment method is generally carbon filter bed. Powdered activated carbon is more used for pretreatment of water, for example, it is added to the water during coagulation, and the organic matter in the water is adsorbed and removed from the water together with alum flocculation when sedimentation. Compared with powdered activated carbon, granular activated carbon filtration provides stable treatment effects and good water quality, and after a period of adsorption, the activated carbon can be regenerated and reused after saturated, so the material cost is lower. The disadvantage of granular activated carbon is that a filter or filter tank is required, and the investment in equipment is higher than that of powdered activated carbon.
For long-term adsorption in water treatment, generally granular carbon filtration is used, whereas powdered carbon is mainly used in seasonally feeding.
The process of adding activated carbon in the filter
The process is generally divided into the following steps.
(1) Before the activated carbon is put into the filter, strict disinfection measures should be taken. All impurities in the filter should be clean out with factory water. Chlorine water with a concentration of 15mg/L should be injected into the filter, and discharged after soaking for 24 hours.
(2) After putting in activated carbon, Immediately add a certain amount of post-filtration water without residual chlorine and soak it for more than 24 hours to wet the activated carbon. In some water plants, activated carbon media is usually mixed with water (about 15% of water is added to the activated carbon) and pumped into the filter. This method is convenient for putting in and taking out activated carbon, and the activated carbon is already wet in the process of putting in. However, this method requires additional auxiliary equipment and pipes, and the cost of the filter is increased accordingly, and it also has a high requirement for the hardness of carbon granules.
(3) During the soaking process, all the fine particles and the carbon not soaked through will slowly float to the water surface. After several backwashes, the fine particles in the carbon layer and the carbon that has not been soaked through are removed along with the air in the pores of the carbon particles, so that the adsorption capacity of the activated carbon can be given full play. The activated carbon produced is generally alkaline, and its PH value reaches neutral after flushing, which does not cause the pH value of the effluent to exceed the standard during operation.
(4) During backwashing, the flushing intensity should be gradually increased until the expansion rate of the carbon bed is about 30%, and it should be kept stable for 10-15 min. During this process, the carbon bed is stratified, i.e. the particles of large and small are redistributed, and the fine particles flow up with the flushing water.