# Municipal Sewage Sludge Disposal in the Republic of Poland
## 5 Methods of Disposal of Sewage Sludge
### Fertilizer Production  
Municipal sewage sludge may also be processed into fertilizers if it meets the requirements specified in the Regulation of the Minister of Agriculture and Rural Development on implementing specific provisions of the Act on Fertilizers and Fertilization [32]. According to the regulation, the permissible contents of pollutants in organic and organic-mineral fertilizers, organic and organic-mineral agents supporting plant cultivation, and post-fertimation products may not exceed 100 mg of chromium (Cr), 5 mg of cadmium (Cd), 60 mg of nickel (Ni), 140 mg of lead (Pb), and 2 mg of mercury (Hg) per kilogram of total solids of this fertilizer, which support plant cultivation of this post-fermentation product. In addition, these products may not contain live eggs of the intestinal parasites _Ascaris_ sp., _Trichuris_ sp., _Toxcara_ sp., and bacteria of the _Salmonella genus_. Fertilizers should also meet minimum quality requirements regarding organic matter content in total solids, i.e., at least 20%. The contents of individual components cannot be lower than 1% of total nitrogen (N), 0.5% of phosphorus in terms of phosphorus pentoxide (P\({}_{2}\)O\({}_{5}\)), or 1% of potassium in terms of potassium oxide (K\({}_{2}\)O) [32]. The Silesian Mining Institute developed the technological process of processing municipal sewage sludge into organic fertilizer [91; 92]. The fertilizer produced meets the procedure's requirements, enabling its introduction to sale in terms of parameters and form. This innovative approach to processing municipal sewage sludge is protected by a patent (PL 233754B1) [91]. Table 4 presents the basic parameters of the produced fertilizer. In addition, the produced fertilizer is characterized by the ability to retain water, a low content of carbonates and salts, and a low concentration of harmful substances. The technological process of producing fertilizer granules occurs in two stages. The first stage involves mixing and granulation, and the second requires drying and packaging. The technological chain of the first stage includes the following processes: dewatering in a centrifuge to a total solids content of approximately 22%; homogenization with calcium oxide, dolomite, gypsum flour, and cellulose fibers and redirection to the granulator. In the second stage, granulates with a dry matter content of approximately 40-45% are dried at an 80-110 \({}^{\circ}\)C temperature to a total solid content of approximately 75%. The dried granulate \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Unit** & **Value** \\ \hline Nitrogen & \% by weight TS & 1–1.3 \\ \hline Phosphorus & \% by weight TS & 1–1.92 \\ \hline Potassium & \% by weight TS & 0.2–0.5 \\ \hline Calcium & \% by weight TS & 15–19 \\ \hline Magnesium & \% by weight TS & 6–9 \\ \hline Organic substances & \% by weight TS & 35 \\ \hline pH & - & 6.0–9.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Parameters of organic fertilizer [91]. is then temporarily collected in a silo for cooling and directed to packaging in foil bags or large bags [91; 92]. Another proposal is to produce an organic-mineral fertilizer using sewage sludge from two sewage treatment plants in Zywiee and Kedzierzyn Kozle. In this case, research was conducted using dried and ground sewage sludge mixed with mineral fertilizers, namely, potassium chloride, diammonium phosphate, and poultry litter ash, to increase the potassium and phosphorus content. Mineral acids, namely, phosphoric acid (69-73% H\({}_{3}\)PO\({}_{4}\)) and nitric acid (55% HNO\({}_{3}\)), were used as binding agents necessary for mechanical granulation of the entire mixture [11; 93]. A different example of producing an organic-mineral fertilizer from sewage sludge is a method also patented by the Central Mining Institute under the patent number PL 234676B1 from 2020 [94]. In this case, stabilized municipal sewage sludge is mixed with an organic hydroabsorbing polymer in a ratio of 100 parts by weight of sludge to 1 part by weight of polymer.