# Regeneration of Activated Carbons Spent by Waste Water Treatment Using KOH Chemical Activation
## 1 Introduction  
Activated carbons (ACs) have been widely used for the water treatment process, which includes the removal of odors, heavy metals, and organic compounds. In particular, drinking water treatment plants (DWTP) and waste water treatment plants (WWTP) require various types of ACs in large quantities [1; 2; 3]. Granular activated carbon (GAC) is commonly used in the filtration process of DWTP [2; 4]. This process can effectively remove organic matter, taste and odor-causing compounds. Powdered activated carbon (PAC) is usually used in final stage of WWTP. The treated water should meet the stringent effluent standard before discharging the water to the river. The PAC filtration process efficiently removes pollutants, so should be required in the WWTP [1; 2]. Water treatment plants require a large amount of fresh ACs, but this means that large amounts of spent ACs could be generated after losing the adsorption capacity. The spent ACs are classified into hazardous solid wastes, and are thus prohibited to dispose directly into landfill sites [5; 6]. The regeneration of ACs is more economically and environmentally effective than replacing fresh ACs [7; 8]. Various regeneration methods have been investigated, including wet oxidation, classical solvent regeneration, and thermal regeneration [9]. Thermal regeneration in particular has been widely used due to its simplicity, low cost, and versatility [7; 10; 11]. During thermal regeneration, spent ACs are exposed to high temperatures (\(\sim\)850 \({}^{\circ}\)C) in the inert gas, and followed by oxidizing conditions [12]. When thermally heated, any volatile compounds adsorbed in the pores are eliminated, and less-volatile compounds are decomposed [3; 5]. Thus, the specificsurface area and pores of spent ACs are efficiently regenerated [13]. The surface area of spent ACs using heat treatments was observed to increase compared with their initial surface area of untreated spent ACs [3; 14]. This is because metals accumulating in pores and surface during the water purification process could react as catalysts at a high temperature. The accumulation of metals catalyzes the reaction between surface carbon and oxidizing agents, leading to a pore development and increased surface area. Thermal regeneration of spent ACs usually results in a lower carbon yield and ash generation [15]. This in turn leads to a negative effect of AC regeneration. The oxidative reaction at high temperatures results in the pyrolysis of the surface carbons, which are then gasified and evaporate, leading to the low carbon yield [3; 15]. Ashes are generated during the thermal treatment which usually block the pores and covers the carbon surface [16]. While the carbon atoms on the exposed surface can be used the active site for adsorption, the ashes covering the surface will reduce the active sites, as well as the adsorption capacity [17]. In this study, a chemical activation method was used to overcome the limitations of thermal regeneration. The spent ACs from WWTP were regenerated by KOH chemical activation, and compared to regenerated ACs using the thermal method. Increased surface area of spent ACs using thermal regeneration was highly dependent on the adsorbed materials on the carbon surface and reaction conditions. However, during chemical activation, the chemical agent reacted with carbons on the surface and pores, and was not significantly affected by the adsorption materials. Similarly, previous research has investigated the chemical regeneration of spent ACs using the reaction between chemical and adsorbed materials [18], but the chemical activation carried out in this study used the reaction between chemical agent and surface carbons at the high temperature. In our knowledge, the regeneration of spent ACs using KOH chemical activation was not published previously. The KOH activation of activated carbon was common and widely investigated, but the regeneration of ACs using KOH activation is a new concept.