Lime sludge is a byproduct of drinking water production from the water softeningprocess. As the traditional land-based disposal of lime sludge can hardly meet the need of
society in a sustainable manner, alternative treatment methods are about to be developed
in order to appropriately solve the issue. Fortunately in recent decades, the emergence
and development of membrane technologies has provided people with advanced ways to
handle environment problems. Advances in the application of ion exchange membrane
processes for material separation exhibit promise for providing new approaches for
improving the sustainability of industrial operations.
The objectives of this study include the investigation of the fundamental
mechanism of Donnan dialysis with a cation exchange membrane and sustainable
treatment of lime sludge by this method. The research was designed to evaluate the
change in different ion concentrations on both sides of the Donnan cell as a function of
time under a variety of experimental conditions. After understanding mechanisms of the
system, Donnan dialysis was applied for the treatment of real lime sludge. Ideally, after
treatment, the product could be reused as material for the water softening process and
thus forms a sustainable recycling system.
Initial experiments tested the process using synthetic samples and various types of
acids. Based on the results, CaSO4 and HCl were chosen for further investigation. The
experiments presented a high recovery of calcium cations and most of the hydrogen ions
from the acid were exchanged to the other side for recovery. The results also verified
Donnan equilibrium theory for this cation exchange system and provided evidence for the
possible extension of the theory to more heterogeneous solids.
According to the fundamental experiments with synthetic samples, the process
was applied for the treatment of real lime sludge. During the kinetic experiment, an
interesting phenomenon was found that the magnesium cations on the feed side
exchanged the calcium cations back as the hydrogen ions from the acid depleted. In order
to improve the quality of the sweep side solution by increasing calcium and decreasing
magnesium, the cation exchange membrane selectivity for calcium and magnesium was
evaluated. Based on the test of membrane selectivity as interpreted by fundamental ion
exchange principles with respect to ion diffusion and binding strength, a batch sequence
process was tested to evaluate the exchange of Ca2+ for Mg2+. The results of the batch
sequence experiment revealed that the depletion of hydrogen ion on sweep and the
comparatively high concentration of magnesium on the feed side promote the exchange
of Ca2+ for Mg2+.
Although at present it is difficult to achieve complete recycling due to the product
of the process under these conditions, an improved system was proposed to achieve the
incorporation of this technique into the water softening process to realize the sustainable
usage of most materials.