Dewatering of sludge by freezing

Abstract: 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.