Sewage sludge is known to be a poorly dewaterable material. In order to improve the dewatering performance, new techniques have been explored in recent years. A frequently studied technique is electrodewatering, in which the pressure-driven dewatering operation is assisted by an electric field, thus yielding faster dewatering kinetics and improved final cake dry matter contents by means of electroosmotic water transport. As special setup has been build to measure this proces in the NMR. This setup is given in figure 1.

Fig 1: The setup for measuring the sludge moisture during electro dewatering

Using this setup the dewatering was measured for various electric currents. These currents were applied after 45min (2700 s) of pressure dewatering, and the applied pressure was maintained until the end of the dewatering run. The result are given in figure 2.

Fig. 2 : Spatio-temporal sludge solidosity profiles during (electro)dewatering, as determined by the NMR , for 4
electric currents (0, 15, 30 and 45 mA). The currents were applied after 2700 s of pressure dewatering, as indicated by the thick
black lines in the center parts of the subplots. The colorbar represents the solidosity color code used in all subfigures.

In the mere pressure dewatering run (Fig. 2a), the expression slows down quickly, resulting in a profile with moderate solidosity at the filter medium and lower but rather uniform solidosity above the filter medium. In the cases where an electric current is applied from 45min onwards (Figs.2b–d,), it is seen that near the end of the pressure dewatering stage the profiles seem to slow down significantly, because of the limitations of the pressure dewatering operation. Once the electric current is applied, at 2700 s, the profiles show a quick decrease again in cake thickness and an overall increase in solidosity. The speed of the decrease in cake thickness and increase in solidosity is clearly linked to the applied current as can be seen from a comparison of the profile evolution for 15, 30 and 45mA.

H. Saveyn, D.Curvers, L. Pel, P. De Bondt and P. Van der Meeren, In situ determination of solidosity profiles during activated sludge electrodewatering, Water Research 40,  2135-2142 (2006).

E.J. La Heij, K. Kopinga, L. Pel and P.J.A.M Kerkhof, Determination of porosity profiles during filtration and expression of sewage sludge by use of NMR imaging, AIChE Journal. 42, 953-959 (1996).