# Municipal Sewage Sludge Disposal in the Republic of Poland
## 5 Methods of Disposal of Sewage Sludge
### Phosphorus Recover  
One of the trends that fits into the assumptions of the National Waste Management Plan is the recovery of valuable chemical elements from municipal sewage sludge. Different technologies are known for recovering phosphorus from municipal sewage sludge, both after the thickening and dewatering process, as well as from ashes after the incineration process. There are several examples of phosphorus recovery from municipal sewage sludge. One of them is the AQUA RECI technology, which consists of the oxidation of municipal sewage sludge in the presence of pure oxygen to gaseous products. Phosphorus is precipitated in the form of calcium hydrogen phosphate. The efficiency of this technology is estimated at 70% phosphorus recovery [61; 62; 63; 64]. Another technology called PHOXNAN is carried out in two stages. In the first stage, at a pressure of 12-28 bar and a temperature of 160-220 \({}^{\circ}\)C, the pH value of the municipal sewage sludge decreases to less than 1.5 when sulfuric acid is added. A significant degree of removal of organic compounds, polycyclic aromatic hydrocarbons (PAHs), pharmaceuticals, and organic micropollutants occurs. The second stage of PHOXNAN technology involves the following membrane processes: ultrafiltration (separation of liquid from solids) and nanofiltration. Struvite is precipitated from permeate rich in phosphates, whereas heavy metals are precipitated from the concentrate obtained after nanofiltration. The potential for phosphorus recovery is estimated at 50% [61; 63; 64; 65]. The KREPRO process was developed by the Kemira Company, Helsinki, Finland, (Kemwater REcycling PROcess). After the pH value decreases to 1-3 (using sulfuric acid), municipal sewage sludge is thickened to 5-7% TS and heated in a reactor to 140 \({}^{\circ}\)C for approximately 30-40 min. During this process, more than 40% of the organic matter is decomposed, and most of the inorganic compounds dissolve. The remaining part of the suspension was separated via centrifugation. The supernatant from centrifugation is directed to a reactor, where orthophosphates precipitate [63; 64; 66]. Another technology for phosphorus recovery from municipal sewage sludge is the KEMI-COND process. This is a technology based on KREPRO technology. The decomposition of organic compounds occurs at pH values ranging from 3 to 4 and a temperature of 20 \({}^{\circ}\)C in the presence of hydrogen peroxide [63; 64]. In turn, Seaborne (Gifhorner) technology is used in the Gifhorn sewage treatment plant. It consists of the following stages: acid leaching, extraction, removal of heavy metals, and two-phase recovery of nutrients, together with struvite precipitation. The digested sewage sludge is mixed with ash generated during the thermal treatment of sewage sludge and subjected to sulfuric acid action to decrease the pH value to 4.5. Biologically bound phosphorus is subsequently released from the solid phase of the sludge. Heavy metals precipitate in the reactor as metal sulfides because of the addition of sodium sulfide. This process takes place at an increased pH (up to 5.6). Struvite precipitation is possible at a pH of 9.0, and sodium and magnesium hydroxides are added for this purpose. The efficiency of this technology is estimated at 40% phosphorus recovery from municipal sewage sludge. The final product contains 28% P\({}_{2}\)O\({}_{5}\), significant amounts of hydroxyapatite, and no heavy metals. Gifhorn technology is expensive because of difficulties with pH adjustment [63; 64; 66]. Another method used is the recovery of phosphorus from the ashes after incineration the municipal sewage sludge. This kind of method can be used in the case of sewage treatment plants via the mono-incineration process. It is applied in municipal sewage treatment plants where sewage sludge is incinerated in a fluidized bed, in plate furnaces, in furnaces with a mechanical grate, or in rotary kilns. Ashes resulting from the incineration of municipal sewage sludge contain approximately 20% P\({}_{2}\)O\({}_{5}\).