Natural Sludge Dewatering. I: Combination of Freezing, Thawing, and Drying as Dewatering Methods

Abstract

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.