| Abstract [eng] |
Environmental degradation and climate change threaten not only Europe, but also the whole world. The Green Deal has set ambitious targets for the European continent to become a climate neutral continent by 2050 and to decouple economic growth from resource use. The operations of paper mills generate a large amount of sewage sludge, which has a high potential to be used as a secondary raw material, thus contributing to economic growth by decoupling the use of new resources. The Master's thesis analyses the process of wastewater sludge generation in a papermaking company and analyses the possible uses of the sludge, as well as reviews the scientific literature and the legal framework. The paper manufacturing company selected for the final project is AB Grigeo. To assess the company's problems and their causes, an analysis was carried out, the material and energy flows of the process were determined and a balance sheet was prepared. It was identified that the company's activities generate over 17 000 t/y of dewatered sewage sludge, and that this amount is expected to increase and may reach 22 000 t/y in the next few years. A laboratory analysis of the dewatered sludge has also been carried out to determine the main parameters. The following were identified: the dewatered sludge is characterised by high carbon and low nitrogen content, as is typical of paper mill sludge, low concentrations of heavy metals and a relatively high content of calcium carbonate. The dewatered sludge is currently being used to produce technical compost, which is then used for the restoration of damaged areas. It has been found that the treatment of the sludge by Grigeo AB, although called composting, is not composting due to the inadequate maintenance of the technological parameters. Compost (soil conditioner) is not produced in this way. Following the analysis and identification of the problems, two alternatives for the use of the sludge were proposed: 1) Production of a biofuel derivative - mixing the drained sludge with wood biofuel used in the plant (sludge fraction up to ≤ 15 % by weight). 2) Production of ceramic bricks using the dewatered sludge as a secondary raw material (sludge fraction up to 10 % by weight). The first option would use the waste generated in one process of the company to create a new value-added product within the company. The production of a biofuel derivative (ratio: 15 % dewatered sludge and 85 % wood biofuel) would use up to 16.300 t/y of the dewatered sludge produced, increase the environmental performance to 0.363 t/t of the company's production, reduce the company's wood biofuel consumption to 7.400 t/y, and increase the environmental performance to 0.165 t/t PP. This would save the company around EUR 0.8 million/y. It would also increase the environmental efficiency of direct and indirect GHG emissions to 0.107 t CO2e/t. In the second proposed alternative, the dewatered sludge would be transferred to a ceramic brick manufacturing company and a new product would be produced. The sludge would be used as a secondary raw material in the production of bricks (ratio: 10 % dewatered sludge and 90 % clay). The production of ceramic bricks could use up to 13.500 t/y of dewatered sludge and save the company up to EUR 294.900 per year. However, the life cycle analysis shows that the environmental benefits of sludge recovery depend strongly on the distance between the plants. In this case, where the brick manufacturing plant is 287 km away from the sludge production site, in almost all categories assessed, the brick with sludge has a higher environmental impact than the brick without sludge and the transport of sludge generates the highest environmental impact in the system assessed. The analysis of the two alternatives has led to recommendations for the alternative use of the dewatered sludge for the paper mill. |
