The pulp and paper mill requires a lot of water during the production. Stora Enso Skoghall requires about 33m3 of water for 2204 lb produced carbon or paper. Before the wastewater returns to the recipient, the water must be purified from organic material. Organic material is naturally found in wood, and if a high level of organic material is added to the recipient it will lead to deficiency of oxygen in lakes. In order to avoid deficiencies of oxygen in lakes, the wastewater must go through a biological treatment, Stora Ensos biological treatment is consisting a long-term aerated active sludge, LAS, which is similar to a large pond. During a period, Stora Enso Skoghall constructed a pre-treatment step fort their LAS which is a Moving Bed Biofilm Reactor, MBBR. Stora Enso has been struggling to aerate the wastewater in the LAS, which has affected the production of the industry badly. The purpose of building the MBBR is to facilitate the LAS, in order to deliver full production. The biological treatment requires a lot of energy to operate the aeration system, about 50 percent of the total energy demand for a treatment plant goes to the biological treatment. The purpose of this study is to examine the impact on the LAS if a MBBR-process is connected as a pre-treatment step. The aim of this study is to examine how the new biological treatment effects the energy demand considering aeration, compared to before the MBBR was connected. Wastewater samples were collected at Stora Enso Skoghall and taken to a laboratory at Karlstads University for analysis and tests. The wastewater samples have been studied regarding its content of TOC and how it affects the possibility of aerating the water. The a- and b-values, and as well the theoretical oxygen demand, were experimentally determined in cylinder aerators that contains 30 liters. The energy use for aeration of the MBBR and the LAS has been estimated. In order to compare if the new biological treatment effects the ability of purification in the existing LAS and if the energy demand changes, wastewater samples were taken before and after the MBBR was connected. The result of this study shows that the MBBR has a positive impact on the existing LAS at Stora Enso Skoghall. There is a higher reduction of COD and the oxygen uptake capacity is better in the LAS, compared to before the MBBR was connected as a pre-treatment step. The total energy demand has increased with 58 percent, compared to the results before the MBBR were connected. The wastewater from the CTMP production contains high levels of TOC. It contains high levels of surfactants and high ionic strength, which usually worsen the ability of aeration. The flow into the MBBR contains mainly wastewater from the CTMP and the aeration tests showed that only 1-6 percent of the oxygen dissolved in it. As a result, the estimated energy demand remains very high. In conclusion, with the new biological treatment system the industry can now deliver full production of pulp and cardboard, while the LAS gets enough oxygen to effectively purify the wastewater. The MBBR relives the LAS through reduce the right amount of COD so the LAS can aerate the wastewater more easily. Moreover, the total energy demand of the biological treatment has increased when it comes to aerate the wastewater due to the fact that the production of pulp and cardboard has increased with 46 percent.
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:kau-78789 |
| Date | January 2020 |
| Creators | Klarström, Diana |
| Publisher | Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013) |
| Source Sets | DiVA Archive at Upsalla University |
| Language | Swedish |
| Detected Language | English |
| Type | Student thesis, info:eu-repo/semantics/bachelorThesis, text |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
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