# Municipal Sewage Sludge Disposal in the Republic of Poland
## 5 Methods of Disposal of Sewage Sludge
### Thermal Treatment  
Thermal conversion methods are used to process municipal sewage sludge. These methods include incineration and co-incineration processes and so-called alternative methods. Alternative methods include gasification, pyrolysis, and wet oxidation. The possibility of disposal of municipal sewage sludge in incineration processes depends on the content of mineral substances, the content of volatile substances, hydration, ash, and its composition. However, if the dried sewage sludge is high in calories, it can be used as fuel in various processes [4; 54; 55]. These parameters also affect the selection of the thermal process method that allows for maintaining minimum emissions and avoiding operational hazards [56]. Owing to the high content of nitrogen and sulfur, when considering the possibility of incineration and co-incineration of sewage sludge, the possibility of probable emissions of sulfur and nitrogen oxides, as well as heavy metals, dioxins, and furans, should be analyzed. Notably, the thermal processing process is not neutral to the environment, causing the emission of these compounds [56; 57]. In Poland, municipal sewage sludge is disposed of by incineration in large sewage treatment plants, as shown in Table 3.  
An example of municipal sewage sludge co-incineration in rotary kilns is the Rudniki and Chelm cement plants. In both cement plants, the sludge combines with coal dust, and the addition of sewage sludge does not exceed the emission standards. The amount of municipal sewage sludge co-incineration in both cement plants in 2011 was 6251 thousand Mg of total solids in the Chelm cement plant and 1406 thousand Mg of total solids in the Rudniki cement plant [56; 57]. Technical and economic analysis should precede the selection of thermal conversion technology. First, it should be assessed whether it is possible to thermally process the sludge without additional energy, the necessity of using it, and the degree of drying of the sludge before incineration. Incineration significantly reduces the volume of stored sewage sludge. This is important in densely populated countries such as Japan. Large amounts of sludge are produced there, and it cannot be used for agricultural purposes. The amount of sludge incinated is 55% [4; 58]. The incineration process must be preceded by drying the sewage sludge to 18-35% TS (usually about 25%). The waste products of all incineration processes are ash, which must be recycled or used in another method [4; 42; 59]. In the case of the low calorific value of the municipal sewage sludge, a possible solution is the process of co-incineration with coal, fuel oil, or natural gas. An alternative is also to transfer the sewage sludge to a combined heat and power plant, which can be used as an admixture (approximately 10%) [4; 59]. One of the methods of thermal conversion of municipal sewage sludge is pyrolysis. This process involves burning sewage sludge in conditions of limited access or lack of air. Materials processed this way can be further used to produce absorbents [4; 60]. Another product of this process is pyrolytic liquids, so-called bio-oil or pyrolytic oil, which can be used as fuel [4; 58]. In Czechia, municipal sewage sludge is co-incinated and used to produce electricity, heat, or cement. One of the Bohuslavice-Trutnov sewage treatment plants uses the pyrolysis process. However, there is no municipal sewage sludge incinerator [43]. In Japan, the situation is quite the opposite, where almost two-thirds of the municipal sewage sludge produced is incinated. Most of the ash generated after incineration is used in the construction industry (47%) or stored in landfills (36%)--building artificial islands at sea (28%) or directed to landfills (8.4%) [43].