# Municipal Sewage Sludge Disposal in the Republic of Poland
## 5 Methods of Disposal of Sewage Sludge
### Phosphorus Recover
The use of ash for fertilizer purposes is possible after the contents of heavy metals, mainly Zn, Pb, and Cu, are reduced [64; 67]. One of the technologies for recovering phosphorus from municipalsewage sludge is BIOCON. The BIOCON process consists of recovering phosphorus compounds from ash resulting from sewage sludge incineration at a temperature of 850 \({}^{\circ}\)C. Ash, as in the case of sewage sludge, is mixed with sulfuric acid to obtain a pH value of approximately 1. In this process, municipal sewage sludge is combusted in a grate furnace for approximately 2 s at a temperature of 850 \({}^{\circ}\)C. After the sludge is combusted, the ash is treated with sulfuric acid to a pH of approximately 1. The solution then flows through a series of ion exchangers. The first is a cation exchanger separating iron ions; the next is an anion exchanger separating potassium ions; after regeneration with sulfuric acid, potassium hydrogen sulfate is created. The third anion exchanger separates phosphorus ions, creating a stream of phosphoric acid. Heavy metal chlorides are removed in the cation exchanger in the last stage. This technology has been tested on a pilot scale in the Bronderslev treatment plant in Denmark. Due to the high ion concentration in the exchangers and the necessity of frequent regeneration, BIOCON technology is economically unprofitable [63; 64]. Another example is an ASH DEC process. This process is carried out in two stages. The first stage involves the mono-incineration of sludge. As a result, incineration residues, which are ash with high phosphorus contents containing heavy metal compounds, are obtained. In the second stage, the sewage sludge ash is mixed with solid chlorine donors (MgCl\({}_{2}\) and CaCl\({}_{2}\)) and exposed to a temperature of 1000 \({}^{\circ}\)C for 20-30 min. At this temperature, heavy metals, usually mercury, cadmium, lead, copper, and zinc, react with salts, become gaseous, and evaporate. The amount of chlorine donors added depends on the concentration of heavy metals in the ash and the target removal rates required by national fertilizer regulations in European countries. In addition to removing heavy metals, the process produces phosphorus mineral compounds, which improve the bioavailability of phosphorus. These compounds include calcium phosphate chloroapatite (Ca\({}_{5}\)(PO\({}_{4}\))\({}_{3}\)Cl\({}_{1-x}\)(OH)\({}_{x}\)), magnesium phosphate farringtonite (Mg\({}_{3}\)(PO\({}_{4}\))\({}_{2}\)) and calcium magnesium phosphate stanfieldite (Ca\({}_{4}\)Mg\({}_{5}\)(PO\({}_{4}\))\({}_{6}\)). The ash is then mixed with other nutrients (NH\({}_{4}\)NO\({}_{3}\), K\({}_{2}\)SO\({}_{4}\), KCl), which is pelletized via special mixers. This way, the ASH DEC NPK fertilizer is produced and sold under the brand name \"PhosKraft\". Research institutes in Germany, Switzerland, the Netherlands, and Austria have tested and confirmed the quality of the product in numerous pot and field tests. As a result, the Austrian and German governments approved \"PhosKraft\" fertilizers for use on pastures and arable land [68]. Another example of phosphorus recovery from sludge is a solution used in Germany called RecoPhos. In this case, the ash after incineration sewage sludge, which is relatively rich in phosphorus content reaching up to 11%, is transformed into fertilizer. However, phosphorus in the ash after burning sewage sludge usually occurs in difficult forms for plants to access. Therefore, in this solution, it was decided to transform phosphorus into calcium and magnesium dihydrogen phosphate, the main nutrients of the fertilizer called RecoPhos P 38. The processing process consists of preliminary mixing of the ash after burning sewage sludge with phosphoric acid. The reaction of transforming phosphorus forms into the compounds, as mentioned earlier, takes place; then, the whole is subjected to the drying and granulation process [69].