# Municipal Sewage Sludge Disposal in the Republic of Poland
## 1 Introduction  
In the case of municipal sewage treatment, biological methods are most often used, which always results in the production of waste sewage sludge. The amount of municipal sewage sludge produced depends on the reduced load of pollutants flowing into the treatment plant, including the content of suspensions and organic matter in the sewage. Depending on the sewage treatment technology adopted, 0.5 to 1.2 kg of total solids (TS) of excess sludge per 1 kg of removed biochemical oxygen demand (BOD\({}_{5}\)) is obtained during sludge production. Sludge resulting from the sedimentation of suspensions and sludge from the chemical precipitation of phosphorus should also be considered. In such a case, the amount of sludge may exceed 1.5 kg of TS of sludge per 1 kg of removed BOD\({}_{5}\)[1, 2]. With the increasing requirements for the quality of sewage discharged into the environment, the increased degree of biogenic compounds' removal, and the expansion of the sewage network, the amount of municipal sewage sludge produced has increased. This sludge must be subjected to processing and disposal processes. Sludge processing processes are currently known and commonly used in sewage treatment plants. The main purpose of sludge treatment is to reduce the volume of sludge and to stabilize and hygiene it. The management and disposal of municipal sewage sludge remain a problem. The final disposal method depends mainly on the quality and composition of the sludge [1, 2, 3, 4]. The quality of sewage sludge is determined by the type and the composition of wastewater flowing into the treatment plant, the concentration of pollutants in the raw sewage, the quantity and quality of industrial wastewater discharged into the sewer system, and the wastewater treatment technology and technological parameters, including the age of the sludge, the reactants used and the method of sludge treatment, especially the stabilization method [1; 2; 5; 6; 7]. The characteristics of municipal sewage sludge are shown in Table 1. The physicochemical parameters characterizing the resulting sludge include total solids (TS), volatile solids (VS), pH, fertilizing compounds (nitrogen, phosphorus, potassium), organic carbon, and toxic substances (heavy metals, polycyclic aromatic hydrocarbons, adsorbable organically bound halogens, polychlorinated biphenyl, furans, dioxins, pesticides, and others). A common feature of municipal sewage sludge is high hydration. Depending on the place of sludge generation in the technological system of sewage treatment, its hydration degree changes. Hydration can be over 99% for raw sludge and 96-97.5% for mixed sludge. On the one hand, fermented, dewatered sludge is characterized by hydration of about 80%. On the other hand, stabilized and dried municipal sewage sludge is characterized by an average hydration of 5% [1; 5; 6; 7; 8; 9; 10]. Preliminary sludge is characterized by the content of organic substances (VS) in the range of 60 to 80% of total solids. The content of organic substances in the excess sludge is at a similar level of 60-85% of total solids. On the other hand, after the drying process, the stabilized sludge has a content of organic substances of 50.6-71.8% of total solids [1; 8; 9; 10]. The maximum nitrogen content in sludge reaches 7%TS for excess sludge. In the case of stabilized sludge, the nitrogen content is about 2.5%TS. At the same time, phosphorus in stabilized sludge occurs at a level of 1.4-4.1%TS [1; 8; 9; 10]. Table 2 presents the indicative content of some heavy metals. The content of heavy metals in municipal sewage sludge may vary depending on the nature of the sewage treatment plant catchment area and the quantity and quality of industrial sewage flowing into the treatment plant [1; 8; 9; 10; 11; 12].