What indicators should we look at when buying activated carbon?

Author: Hongjie Water

The structure of an activated carbon filter is basically the same as that of a multi medium filter, except that it is internally equipped with activated carbon with strong adsorption function, which is used to remove organic matter that has not been removed after filtration, adsorb residual chlorine in water, and make the residual chlorine in water ≤ 0.1mg/L, SDI ≤ 4. Chlorine is a strong oxidant and has a destructive effect on various membranes, especially the reverse osmosis membrane, which is very sensitive to residual chlorine. Therefore, for pure water production lines using urban tap water as the water source, the main function of activated carbon is to adsorb residual chlorine in the water and protect the RO membrane.

In the process of producing pure water, the adsorption effect of activated carbon is manifested in four aspects: reducing the oxidation requirements of water; Avoiding the entry of organic matter into contaminated ion exchange membranes and reverse osmosis membranes; Remove residual chlorine from water and protect the reverse osmosis membrane; Improve the overall quality of water by removing trihalides (THM) and other impurities.

The specific surface area of good activated carbon is generally greater than 1000m2/g, and the total volume of fine pores can reach 0.6-1.8mL/g. The adsorption capacity of activated carbon is mainly physical adsorption, without polarity, and is generally reversible. Experiments have found that it can also undergo some selective adsorption, namely chemical adsorption. When the production temperature is between 300-500 ℃, acidic oxides dominate, and when these acidic oxides ionize in water, activated carbon carries a negative charge; When the production temperature is between 800-900 ℃, activated carbon has amphoteric properties. From the measurement of potential, it can be concluded that activated carbon generally carries negative charges and exhibits weak acidity in solution. Under acidic conditions with lower pH values, it exhibits good adsorption performance; Under alkaline conditions with high pH values, the adsorption is poor.

What indicators should we look at when buying activated carbon?

Pouring high-quality activated carbon into water will produce a large number of small bubbles, and the stronger the phenomenon, the more bubbles, and the longer the duration, the better the effect. When the bubbles are almost released, it can be seen that the water of inferior activated carbon has turned black, while the water of high-quality activated carbon is still relatively clear and transparent. The cost of real activated carbon is very high. Activated carbon is obtained by activating carbonized materials at a high temperature of around 1000 degrees Celsius, which is a key step in the production of activated carbon.

The selective adsorption of activated carbon can not only adsorb electrolyte ions, but also perform ion exchange adsorption. Activated carbon adsorption can also reduce the oxygen consumption (COD) of potassium permanganate from 15mg/L (O2) to 2-7mg/L (O2). Due to the adsorption effect, the concentration of adsorbed substances on the surface increases, which can also play a catalytic role. Although activated carbon can adsorb a considerable amount of inorganic and organic substances, it does not have the ability to adsorb all organic substances, such as alcohols, high molecular weight organic matter, or colloidal organic matter, which are difficult for activated carbon to adsorb.

When using activated carbon in the water treatment system, pre-treatment is first required. Before entering the column, granular activated carbon should be soaked in clean water and washed to remove dirt. After column installation, 2% HCl and 4% NaOH solution should be alternately dynamically treated 1-3 times, with a flow rate of 18-21m/h, and the dosage should be about 3 times the volume of activated carbon. After each treatment, it should be washed until neutral. Secondly, the applicable inflow conditions require that the suspended solids content in the inflow be less than 3-5 mg/L. Once again, the control of the adsorption endpoint is to control the residual chlorine content in the effluent to be less than 0.1mg/L based on the nature of the removed material. If the leakage of organic matter is used as the control point, the ratio of oxygen consumption to total anion content in the effluent should be controlled to be less than 0.004 or COD<2mg/L.

There are many forms of activated carbon adsorbers, which can be divided into fixed bed, expanded bed, and moving bed according to the carbon bed form. Currently, fixed bed adsorption devices are widely used, usually pressure type. The structure of the pressure type adsorber is similar to that of the pressure filter. It can be made into a simple adsorber or combined with quartz sand to form an adsorption filter. Both can remove organics and filter suspended solids. The bottom is filled with 0.2-0.3m thick support layer and quartz sand filter material layer, and the upper part of quartz sand is generally filled with 1.0-1.5m thick activated carbon as an adsorption filter, with a filtering speed of 6-10m/h generally; When only used as an adsorber, there is no quartz sand filter material layer on the support layer. The activated carbon layer is 2-3m high, with a filtering speed of 3-10m/h and a backwashing strength of 4-12L/(S · m2).