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A Swedish full-scale pilot plant employing all-year-round natural sewage sludge dewatering is presented in this paper. The treatment includes three different types of outdoor ditches: a freezing ditch for the winter months, a drying ditch for the early summer months, and a combined drying-freezing-thawing-drying (DFTD) ditch for sludge produced in late summer and early autumn. The test period included two consecutive winters. Complete freezing of the sludge was achieved in the first winter in contrast to the second when incomplete freezing of the sludge occurred due to an unusually warm winter. The dry matter content for the freezing ditch was, at the harvest in August, 30-70% of the first test year. The second test year yielded a sludge with a dry matter content of 20-40% in the freezing ditch. The final dry matter result for the DFTD ditch was 20-40%. The summers included were similar to the extent that both late summers were unusually warm, helping to produce sludge of high final dry matter content. The first summer, being somewhat warmer and with a lower sludge loading, yielded a sludge of 60-90% dry matter in the drying ditch. The second summer, when the sludge load was approximately double the preceding year, resulted in a sludge of 20-60% dry matter.
The treatment could reasonably be only applied on sludge generated during the cold season. However, the land space used for freeze-thaw treatment in the winter could be potentially used as a sludge drying bed during the summer ( Martel, 1993;Hellström and Kvarnström, 1997). The purpose of this study was to investigate various combinations of dewatering and anaerobic digestion of mill biosludge in combination with freeze-thaw treatment to harness the benefits of this method to its fullest.
According to Martel and Diener (1991), odor can be kept at an acceptable minimum, if the meltwater is drained efficiently by means of draining pipes. Leaching of potential harmful substances into the ground can be prevented by applying lining underneath a gravel/sand bed ( Hellström and Kvarnström, 1997), or an impermeable layer of clay ( Penman and Van Es, 1973). Freeze-thaw dewatering of sludge may be more cost effective than mechanical dewatering.
It is worthwhile mentioning that freeze-thaw treatment can lead to a notably higher solid content than mechanical dewatering. Previous pilot-scale and field-scale tests indicate that a combination of freezing-thawing and drying beds can provide sludge with a solid content of 20-70%, mainly depending on the type of sludge, and the duration of freezing, thawing, and drying ( Reed et al., 1986;Martel and Diener, 1991;Hellström and Kvarnström, 1997).