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Dewatering of sludge by freezing
2010 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

The aim and scope of the project is to study a freezing and thawing apparatus developed by FriGeo AB, company located in Kiruna, in order to enhance the deliquorability of workshop sludges collected from the airport of Arlanda, Stockholm, and other minor sources. Two sludges with different initial characteristics have been analysed. The first sludge (S1) had the following composition: 60 wt% water content, 28 wt% organic matter (oil plus organic substances), and 12 wt% inorganic material (principally fine- grained material). The second sludge (S2) underwent to different pre- treatments to remove part of its water and oil content: 35 wt% water content, 11 wt% organic matter and 54 wt% inorganic material (principally coarse-grained material). Despite the different inorganic composition, the fine-grained material of both the sludges had a similar grain size distribution. The principal internal characteristics and external conditions that influence the dewatering and consolidation of sludges and soils when frozen and thawed have been researched in literature: sludges consolidation is principally enhanced by slow freezing rate (obtained with indirect freezing method at low temperature): while the ice front is moving, the sludge particles are compacted and the oil droplets coalesce, resulting in higher deliquorability while thawing. This effect becomes negligible with high concentration of dissolved substances, salts and impurities. The compaction of soils depends principally on their grain size distribution: three freezing/thawing cycles allow the fine-grained material to reach its maximum compaction: impurities, fast freezing and fast thawing rate contrast this effect. The most suitable parameters among freezing temperature and number of freezing cycles have been chosen varying their values, with the aim to obtain the best deliquorability degree. Soil tubes containing 300 ml of sludge have been placed into a laboratory freezer: 1, 3 or 5 freezing cycles at 5, 10, 20 and 28°C have been performed: the freezing time and the thawing time have been set at 24 hours. Every “24 hours” frozen sample have been let thawed over a 1 mm sieve: after 24 hours, moisture and organic content of the retained material are analysed to evaluate the differences in deliquorability achieved with different freezing parameters. The variation in freezing temperature doesn’t let to a significant change in dewaterability. The first sludge treated with one freezing cycle loses, in average, 60% of its initial water after 24 hours of leaching: 65 and 68% if treated with 3 and 5 freezing cycles, respectively. The second sludge treated with one freezing cycle loses, in average, 33% of its initial water after 24 hours of leaching: 35.4% if treated with 3 freezing cycles. Very little oil flowed away from every sludge sample: indeed the organic content resulted unaltered. The final product is a solid soil like material. The main conclusion from the project is that workshop sludge acts similarly to a soil: the increase of freezing cycles alters the fine-grained material skeleton, resulting in higher compaction and dewaterability, but the differences between one or three freezing cycles is negligible: furthermore, the use of an elevated number of freezing cycles is not viable. The most efficient parameters for the apparatus are one freezing cycle and –10°C. Future developments are needed, especially a cost analysis and a comparison between the results obtained with the freezing apparatus and the results obtained with traditional mechanical and chemical sludge dewatering systems.

Place, publisher, year, edition, pages
Keyword [en]
Life Earth Science, Freezing, sludge, dewatering, freezing cycles, temperature, oil sludges
Keyword [sv]
Bio- och geovetenskaper
URN: urn:nbn:se:ltu:diva-48949ISRN: LTU-EX--10/024--SELocal ID: 65e7c35e-eacb-4fcf-a3d7-b98a1592148eOAI: diva2:1022294
Subject / course
Student thesis, at least 30 credits
Educational program
Civil Engineering, master's level
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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