# Municipal Sewage Sludge Disposal in the Republic of Poland
## 5 Methods of Disposal of Sewage Sludge
### Biochar/Waste Adsorbent Generation/Preparation  
Currently, great emphasis is placed on technological solutions aimed at activities related to the so-called circular economy. These activities should consider the minimization of waste generated, as well as the reduction in the amount of raw materials used. Concerning the management of sewage sludge, research has been carried out for some time on the transformation of this waste into biochar. These activities can contribute to a reduction in the amount of raw materials used for the production of classic commercial adsorbents. In addition, the transformation of sewage sludge (which is waste in the meaning of the law) into porous materials is part of the recovery of raw materials and the production of new products through waste processing. It can be said that waste is given \"new life\" in this way. The transformation of sewage sludge, as well as excess activated sludge (which has not been subjected to stabilization processes) into biochar and waste adsorbents, has not yet been introduced within the framework of technologies used on an industrial scale. They are still in the phase of action; however, they are an exciting alternative for the management and disposal of these types of waste. Biochar is a porous material produced by partial oxidation (mainly by pyrolysis at temperatures above 700 \({}^{\circ}\)C in the absence of or with limited access to oxygen) of charcoal and organic carbon-rich material [72]. Therefore, various types of biomass are used to produce biochar, such as products from the industrial sector, agricultural and forestry biomaterials, crops, leaves, wood and other waste from human activities, including sewage sludge [72; 73]. A biochar derived from sewage sludge or activated sludge, due to its porous structure with a highly developed surface area, can be a type of adsorbent which is also called sludge-based adsorbent or waste adsorbent. Various procedures are used to transform sludge into porous materials. However, in general, they most often include common steps [74]:  
* Pre-dewatering;
* Drying the material;
* Carried out the carbonization process, usually in an inert gas atmosphere;
* Chemical or physical activation process;
* Washing the material after the activation processes;
* Final drying. The pre-dewatering sludge aims to reduce the amount of water as much as possible to carry out the next stage of sludge transformation into adsorbents more efficiently, faster, and at the lowest possible operating costs. The drying process includes both the stage of removing water from sludge and often consists of a step of appropriately crushing the dried material to the required grain size (usually in a powdery form). Properly effective drying of sludge before the next step of the preparation of sludge-based activated carbons is crucial to avoid significant energy losses. On the other hand, crushing the material increases the speed and efficiency of the material transformation. Dried and crushed municipal sewage sludge is then subjected to thermal processing, i.e., carbonization. Carbonization is carried out at temperatures ranging from 400 \({}^{\circ}\)C to 1200 \({}^{\circ}\)C. It aims to transform materials rich in organic matter into carbon-rich biochar. This process is usually carried out in oxygen-free conditions, where a neutral gas, e.g., nitrogen or argon, is most often supplied in the flow. These activities aim to avoid the occurrence of classic material incineration. The authors [PERSON] and [PERSON] [75] in their studies produced a porous material from excess sludge subjected to dehydration, drying and grinding for the adsorption of two synthetic dyes, Acid Red 18 and Acid Green 16. In addition, [PERSON] et al. [76] also produced an adsorbent from excess sludge by preliminary dehydration, drying, fragmentation, and activation using [PERSON]'s reagent. Biochar can also be produced in the process of hydrothermal carbonization.