## 1 Introduction  
Sludge is the solid waste generated in the process of sewage treatment. With the rapid development of the sewage treatment industry, the amount of sludge produced in the process is also increasing, and the problem of sludge treatment and disposal is becoming increasingly prominent. Sludge contains a large number of organic matter, insoluble inorganic substances, and bacterial micelles formed by various microorganisms, etc. Untreated sludge entering the environment will cause great harm to the environment [1]. There are many types of sludge, which can be roughly divided into three types according to their sources: water supply sludge, domestic sewage sludge, and industrial wastewater sludge [2]. Different sludges have different compositions and different disposal methods. At present, commonly used sludge disposal methods mainly include incineration, landfill, composting, ocean discharge, and so on [3]. However, these methods have certain limitations, and the exploration of new methods for sludge disposal and new ways for comprehensive utilization of resources is of great significance in solving the problem of sludge disposal [4]. Considering that it contains a large number of viruses and bacteria, high temperature is a better treatment method, which can not only kill the viruses and bacteria in the sludge, but also achieve the effect of volume reduction [1]. At present, the common high-temperature treatment methods of sludge include incineration [5], high-temperature hydrolysis [6], preparation of ceramics and activated carbon [1, 2, 7]. Due to the low calorific value and high moisture content of sludge, the traditional incineration needs to be mixed with household waste and coal or other high calorific value materials, and the heat utilization rate is low, and at the same time, a large amount of fly ash, HCl gas, and dioxins are generated [8]. Ceramsite as a light aggregate possesses the porous, light weight, anti-seismic, heat preservation, sound insulation, and other advantages, and is widely used in construction, environmental protection, chemical industry, horticulture, and other industries. The sludge is rich in inorganic components such as SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and Fe\({}_{2}\)O\({}_{3}\), which can be used as filler or adsorbent to realize the resource utilization of sludge [9; 10]. The use of sludge to prepare ceramics can not only reduce a large amount of sludge but also produce lightweight material as water treatment filler or building supplies, which is the current research hotspot of sludge resource utilization [2]. There are more and more patents for making ceramics from sludge [2; 4; 6; 11], but most of them are mixed with kaolin, fly ash, cement, gypsum, or other materials to increase the firing strength [2; 7]. Sludge is rich in carbonaceous organic matter, which is the raw material required for the manufacture of activated carbon. The preparation of activated carbon with sludge as raw material can save coal and wood and other precious resources, and solve the environmental problem of sludge disposal, curb waste with waste, turn waste into resource, and realize the resource utilization of sludge [12]. Wetland has a strong purification function, known as the lung of the earth. Constructed wetland is an environmental ecosystem constructed by an artificial simulation method. It uses the organic combination of packing matrix permeability, aquatic or marsh plants, and the microbial matrix attached on the padding to reach the effect of sewage purification, which is like a compound ecological system with physical (precipitation, filtration, adsorption), chemical (ion exchange), and the biological (plants extract nutrients, microbial metabolism) properties [10; 13]. Constructed wetlands are widely used because of their advantages such as good pollutant purification efficiency, low cost, and easy maintenance [14]. Wetland padding is the crucial part of constructed wetland system.