## 1 Introduction
It not only provides the growth medium for plants and microorganisms, but also directly removes pollutants through the effects of precipitation, filtration, and adsorption. The adsorption capacity of paddings to pollutants determines the overall treatment effect of constructed wetland [15]. The general selection of constructed wetland padding should meet the following conditions: (1) high mechanical strength; (2) large specific surface area, high porosity, and good adsorption capacity; (3) good permeability, not easy to plug; (4) stable chemical properties, no secondary pollution; (5) reasonable price. The most commonly used padding substrates are gravel, zeolite, soil, pebble, coal cinder, fly ash, and so on [16]. The ceramsite fired by sludge is porous, with large specific surface area and good adsorption ability, which just meets the requirements of constructed wetland padding. There have been pieces of research on the ceramics of constructed wetlands padding made by sludge [15; 16; 17; 18; 19]. The sludge from water works was used as raw material to fire the ceramsite. The surface porosity of the ceramsite was higher, and was more favorable to the growth of microorganisms. A constructed wetland system of \"sludge ceramsite + plant\" was constructed to treat domestic sewage. The removal effect of COD and TP was better than that of constructed wetland with coal cinder, shale, zeolite, and steel slag as paddings [15]. [PERSON] et al. [16] made biochar with sludge as raw material to adsorb heavy metals in the bottom sludge. While supplemented by the plant to establish the constructed wetland, which was used to treat the water of Xiangjiang River. The biochar could be reused, and the heavy metals in the effluent reached the standard. It has the advantages of low cost, relatively simple access to raw materials, and high treatment efficiency of heavy metal pollutants. [PERSON] et al. [17] used aluminum sludge as adsorbent to adsorb Cr(VI) in water through static experiments. Functional groups such as OH\({}^{-}\), SO\({}_{4}^{-2}\), and Cl\({}^{-}\) on the surface of aluminum sludge can undergo ligand exchange with Cr(VI) to achieve the purpose of Cr(VI) removal. The sludge from drinking water plant was used as raw material to make ceramsite, and _Oenanthe javanica_ was used to construct constructed wetland to treat rural domestic sewage. Removal efficiencies of 76.7% TP, 56.4% NH\({}_{4}\)-N, and 60.8% TN were obtained [19]. But mostly, the ceramics was fired by the aluminum sludge as raw material, because compared with residual activated sludge of sewage treatment plant, the amount of inorganic matter and heavy metals in aluminum sludge is relatively lower. As an adsorption material, heavy metals dissolution of the ceramics is easy to be controlled in the standard range. And the sludge of sewage treatment plant is enrichment of inorganic elements such as nitrogen, phosphorus, and potassium; Cd, Cu, Pb, and other heavy metals; organic pollutants such as pesticides, antibiotics, and pathogenic microorganisms; and other pollutants. The concentration of the pollutants causes great limitations to sludge disposal. The change of conditions can easily lead to the dissolution of heavy metals [5]. If necessary, a stabilizer should be added for stabilization [11]. In this study, the residual activated sludge from Chaozhou No. 1 Sewage Treatment Plant was used as raw material to fire sludge-based biochar ceramics under different sintering systems, and no other auxiliary material was added. Its physical properties were characterized, and the dual activity of ceramics and biochar was explored to adsorb Cr(VI) from waste water. The feasibility of the ceramics used as padding substrate in constructed wetland was discussed.