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Overview: Water is a fundamental condition for human survival and the most crucial factor influencing and restricting social development. On the one hand, most parts of the world suffer from a severe shortage of water resources ； On the other hand, due to the rapid population growth and continuous industrial development, humans are producing large amounts of pollutants, which contaminate the already scarce water resources. Water scarcity and water pollution are two major challenges faced by society and humanity today. Since the last century, as the relationship between humans and nature has become increasingly tense, people have gradually realized that wastewater treatment and reuse are the most effective ways to address water scarcity and water pollution. As is well known, current wastewater treatment plants first use various physical, chemical, or biological methods to remove pollutants from the water, thereby turning the treated water into \"clean water\" that can be discharged back into water bodies ； Next, the separated “solid” that has concentrated the “essence” of pollution – sludge – is treated further. Obviously, unless the issue of sludge treatment is resolved thoroughly, the pollutants in the sludge will return to nature through various channels; in such a case, no matter how good the water treatment results may be, they are nothing more than a futile attempt to deceive oneself. Thoroughly removing pollutants from sludge is a key step in addressing water pollution problems. 1 Tasks and Methods for Sludge Treatment 1.1 The Behavior of Sludge in Practice Clean water becomes wastewater after being used, as it contains organic substances, heavy metals, viruses, bacteria, etc., which can cause harm to humans and the natural environment. In a sense, the purification of wastewater involves partially separating and transferring these pollutants into sludge. Compared to wastewater, sludge contains similar pollutants but at much higher concentrations; in practice this is manifested as: (1) strong odors: numerous perishable organic substances emit various foul smells during the treatment, transportation, and storage of sludge, thereby affecting the environment ； (2) High risk: Sludge contains a large amount of viruses, bacteria, protozoa, as well as high concentrations of heavy metals such as zinc, copper, chromium, lead, and cadmium, along with toxic organic compounds; these pose a potential threat to water resources and human health ； 1.2 Key tasks in sludge treatment Sludge treatment involves carrying out thorough decontamination of sludge in order to completely eliminate its environmental pollution and hazards to humans. The main tasks include the following: (1) Reducing the volume of sludge: ① Using biological or chemical methods in water treatment processes to directly reduce the generation of sludge, thereby avoiding and minimizing its production ； ② Increasing the solid content of sludge in the sludge treatment system ； (2) Stable sludge properties: Removal of organic matter in the sludge that is prone to decay and deterioration ； (3) Harmlessness of sludge: Removal of viruses, bacteria, protozoa, heavy metals, and other substances in the sludge that are harmful to humans or the natural environment ； (4) Resource utilization of sludge: ① Utilize the abundant N, P, K, etc. in sludge to produce organic fertilizers, improve soil conditions, and promote crop growth ； ② The heat stored in the large amount of organic matter in the sludge is utilized for combustion to recover thermal energy. At present, to achieve the ultimate harmless treatment of sludge, the complete process is as shown in the figure below: As can be seen from the figure, sludge treatment comprises four main stages: sludge concentration, mechanical dewatering, drying or incineration, and the final disposal of the waste. The main characteristics of the first three stages are as follows: 1.3 Disposal of the final waste from sludge treatment and related requirements (1) Agricultural use: ① Removal of pathogens and heavy metals to prevent adverse effects on human health and the land due to long-term use ； ② It features a short application period and a long storage period; the fertilizer should be easy to transport, store, and handle, in order to improve sanitary conditions ； ③ Establish comprehensive application standards to reduce adverse environmental impacts and eliminate users’ distrust ； (2) Landfilling: ① As small a volume as possible ； ② Prevent secondary pollution ； (3) Building materials: The materials have stable properties and are non-toxic ； (4) Dumping into the sea: Its use has been prohibited. 1.4 The necessity of advanced sludge treatment: The selection of sludge treatment processes is closely related to factors such as the properties of the sludge, the final disposal method for the sludge, local regulatory requirements, economic considerations, the scale of treatment, available treatment technologies, and site conditions. As can be seen from the above, the current methods of concentration and mechanical dewatering cannot reduce pollution nor meet hygiene requirements; therefore, they are insufficient for proper sludge disposal. Advanced treatment methods such as sludge drying or incineration are an inevitable choice for the development of sludge management. (1) Requirements for pollutant removal: Concentration and dehydration cannot remove pollutants; drying or incineration are effective ways to completely eliminate the pollution load ； (2) Requirements of local regulations: In some parts of Europe, **the disposal of sludge that has not undergone thorough treatment is prohibited ； (3) Requirements for final sludge disposal: For agricultural reuse of sludge, landfilling, and use as building materials, it is necessary that the sludge be non-toxic and meet health standards ； (4) Economic considerations: The volume of sludge after advanced treatment is only half to one-fifth of that of dewatered sludge, which reduces transportation and storage costs as well as landfilling expenses (in Europe, landfilling costs are based on volume and are very high, at 200–300 euros per cubic meter). (5) Assurance for technological advancement: The mature development of sludge drying and incineration technologies has reduced investment and operating costs, while improving treatment efficiency. 2 Sludge drying processes: Drying involves using a heat source to heat the dehydrated sludge, thereby removing the capillary water present in it and increasing the solid content of the sludge to 70%–90%. Currently, Anditec has developed and utilized various drying processes, such as the HELIANTS® process which uses solar energy for heating, the MINERALIS® process that employs high-temperature oxidation of organic substances, as well as various other forms of heat-based drying processes. 2.1 HELIANTS® solar drying process: The dehydrated sludge is placed in a greenhouse, where solar energy is used to evaporate the water contained in it; this results in sludge that is 60–80% dry. During operation, a mixing wheel can be used to turn the sludge over so that it lies flat on the floor, or forced ventilation can be enhanced to improve the evaporation efficiency. This process has a simple design and low investment and operating costs, but it requires a large amount of space. It is suitable for situations where the amount of sludge generated is low, the sludge is to be used in agricultural applications, and long-term storage is necessary. 2.2 MINERALIS® High-Temperature Wet Oxidation Process: Sludge and pure oxygen are simultaneously fed into a reaction vessel at a temperature of 2900°C and a pressure of 80–100 bar. Under these high-temperature and high-pressure conditions, pure oxygen can oxidize the vast majority of organic substances into CO2 and water without the need for a catalyst. The remaining sludge can achieve a dryness level of 50–60% or higher through mechanical dewatering. The wet oxidation process is suitable for scenarios where sludge is not used in agricultural purposes; the dewatered sludge cake can be landfilled or used as construction material. 2.3 NARATHERM® Impeller Heating and Drying Process The heating and drying method is the one that is more widely used in drying processes, and all forms of heating and drying require a heat source. Structurally, in addition to the dryer, it should also include a sludge feeder, auxiliary heat sources, an air supply system, a heat recovery device, dust control systems, ash discharge systems, and control systems. The currently used heat drying systems include various types such as drum-type, disk-type, propeller-type, film-type, and tubular-type drying systems. The selection of a drying system is closely related to the requirements of the final product, the disposal method, the type of sludge, etc., as shown in the figure below. 2.3.1 Process flow The NARATHERM® process is a paddle-type heating and drying process jointly developed by Delimann and GOUDA companies. It is the most widely used heating process in Europe at present, and its process flow is summarized as follows: 2.3.2 The NARATHERM® process mainly includes the following steps: (1) Feeding – A screw conveyor is used to transport sludge with a moisture content of 15–35% to the inlet of the dryer. Depending on the properties of the sludge, it may be decided whether to pre-mix the screened fine dried sludge with the sludge that is to be dried. (2) Heat source for generating NARATHERM® – Hot steam or oil heated by a boiler can be used as the heat source for heat exchange. The temperature of general hot fluids is 180–200°C. Hot fluid is introduced separately into the hollow dryer shell and all the heating metal surfaces that come into contact with the sludge for heat exchange, such as the rotating shaft and impellers. (3) Sludge drying: The heated metal surface comes into uniform contact with the sludge, heating it and evaporating the water contained within it. Along the axial length of the dryer, water vapor evaporation goes through two stages: a constant evaporation efficiency phase and a low evaporation rate phase ； Sludge drying goes through four stages: plasticization, deplasticization, lump formation, and granulation ； The temperature of the sludge shows a trend of rapid increase, stabilization, and then another rapid increase; ultimately, the temperature of the sludge at the outlet is around 100–110°C (see figure below). The drying time is 3 to 7 hours, with the solid content of the sludge at the outlet reaching over 90%. (4) Cooling and screening: There are two methods of cooling, namely direct cooling and indirect cooling. Used alone or in combination, these two methods reduce the temperature of the dry sludge from 110°C to around 40°C. After cooling, it is fed into a fine screen for screening, or it is shaped using a shaping machine to achieve the desired form for the dry sludge. (5) Condensation recovery: The hot steam inside the dryer is sent to a condenser for condensation. Condensate solvent recovery ； The non-condensable gases are sent to the boiler to be reheated as a heat source ； The condensate water is returned to the wastewater treatment plant. 2.3.3 Advantages of the process: The NARATHERM® process utilizes a cleverly designed paddle dryer that enables thorough mixing of the sludge, keeping it in constant contact with clean surfaces for heat exchange. This solves the problem of low heat transfer efficiency associated with conventional indirect heat transfer processes, while also reducing the amount of steam required compared to direct heating. The combination of the entire process system confers significant advantages: (1) High adaptability: suitable for various types and properties of dewatered sludge ； (2) Low consumption: ① Low steam usage, with partial recovery of heat energy to reduce thermal consumption ； ② The powder is eliminated, odor production is reduced, and end treatment becomes easier ； (3) High-quality dried sludge: ① Extremely low organic content, with stable sludge properties ； ② A long enough heating time to eliminate viruses, bacteria, etc., meeting hygiene standards ； ③ Of agricultural utilization value ； ④ High solid content (>90%), enabling granulation for easy storage, transportation, and flexible disposal ； (4) Safe operation: It requires less steam, operates at a lower temperature, and there is no direct contact, resulting in high safety levels. (5) Easy to operate: The system is fully automated; it can monitor parameters such as O2, CO, and inert gases in real time, and it features automatic safety protection. The superior performance of the NARATHERM process is primarily attributed to its unique paddle dryer (as shown in the figure below): the dryer consists of a heatable hollow tank, two or four pairs of hollow drive shafts mounted together, numerous wedge-shaped mixing paddles attached to these shafts, and scrapers at the tips of the paddles. In operation, the sludge enters at one end of the dryer and slowly flows to the other end due to gravity. The heat transfer fluid enters from the same side into the hollow grooves, shaft, and impellers to heat all metals that may come into contact with the sludge. At the same time, the two paired shafts rotate in opposite directions; the tooth shapes of the paddles are matched, and the shaft with adjustable speed drives the stirrer to slowly and evenly turn over and mix the sludge. This design offers the following advantages: (1) Large contact area: A higher area/volume ratio increases the contact area ； (2) High heat transfer efficiency: ① The fluidized state of the material ensures 100% contact for heat exchange with the heating surface ； ② The self-cleaning paddle shape keeps the heating contact surface clean at all times, improving heat transfer efficiency ； (3) Quality of highly dried sludge: ① Smoothly shaped impellers and a mixing mechanism that does not generate axial thrust prevent damage to fragile sludge ； ② The push-flow passing mechanism and uniform, continuous heating on both sides of the blades enable all dried sludge to have similar properties. The NARATHERM® heating and drying process boasts advantages such as relatively low investment and operating costs, strong adaptability, high sludge dryness, stable performance, as well as safety and hygiene, which have enabled it to capture a large market share in Europe and North America and accumulated extensive experience in operation and management. There are records of successful operation in many water treatment plants such as those in Saint Brieuc, Lavelanet, Barrow, Metz, and others. 2.4 IC850® Sludge Incineration Process Sludge incineration is the most thorough sludge treatment method; it allows 15% to 35% of dewatered sludge to be fed into an incinerator, where the organic matter is directly burned into CO2, NO, and inorganic substances. The incineration of final waste results in inorganic ash with a solid content of over 99%. Similar to sludge drying, the incineration of sludge also involves two stages: sludge drying and sludge incineration. The difference is that sludge incineration can be sustained using the thermal value of the sludge itself, which allows for significant savings in fuel. Sludge incineration includes two methods: incinerating sludge separately and incinerating it mixed with municipal waste. In fact, burning it alone is rarely used due to high costs. The IC850® developed by Anteo Delimann is currently the most widely used mixed combustion process. It feeds dewatered sludge with a moisture content of 15% to 35% into an incinerator with a minimum temperature of 850°C, in a mixing ratio of 1:10 to 1:5 with municipal waste, thereby rapidly burning the organic matter into inorganic ash. IC850® offers advantages such as a wide range of applicable scenarios, low operating costs, no clogging of the sludge nozzles, flexible system adjustment, simple operation, and automated system operation; it is suitable for non-agricultural uses of sludge as well as for the centralized treatment of sludge and waste. Currently, this process is in successful operation at treatment plants including AMSTERDAM, CENON, DINAN, SARCELLES, and others, with an annual processing capacity of over 20,000 tons of dry sludge. 3 Conclusion Over the past half century, with the continuous and rapid growth of population and industry, the amount of sludge generated has also increased significantly. According to statistics, each person in Europe generates 15–25 kg of dry sludge per year. In the European Union, the production of dry sludge was 6 million tons in 1992; this figure rose to 12 million tons by 2002. By 2007, the amount of sludge generated each year continued to increase at a rate of over 5%. The increase in sludge production, the increasing complexity of its contaminant components, and the increasingly strict regulations imposed by various countries on the final disposal of sludge mean that conventional sludge treatment methods are no longer sufficient. Meanwhile, drying and incineration processes represented by NARATHERM® and IC850®, among others, are continuing to develop and mature, gradually demonstrating strong technical and economic advantages; in the near future, they will be the inevitable choice for sludge treatment.