Efficiency and environmental effects of peat dewatering by mechanical pressing

Abstract

Changes in the chemical and physical structures of three types of Finnish peat (C, B and S) were studied during mechanical pressing. The effects of these changes on the efficiency of pressing and on the emissions into the pressing water were also investigated. Pressing was done at pH 3.2 by using Fennopol A3006 (a weakly anionic polyacrylamide-type chemical), a combination previously found inexpensive and effective. The dry solids load was 2.4 kg m −2. Clear differences were found in the water-binding capacities of the peats. The dry solids contents (wt%) of the peats after pressing for 500 s at 2.0 MPa by a computer-controlled piston press with two-way water outlet were: C, 37.0, B 33.5, S 27.5. A high content of humic substances and < 10 μm particles increased the water binding capacity of the peats. A large amount of Fennopol was bound to the peat during flocculation, making its recovery difficult. 20–40% of Ca and Mg were released from the peat owing to the low pH, and > 80% of Fe and Al remained in the peat. Most of the S of the H 2SO 4 used for pH lowering was found in the pressing water. Neutralization of the pressing water by Ca(OH) 2 to pH 7 costs ~US$8 per 1000 m 3. The chemical costs of pressing are ~US$5 per tonne of dried peat.