Environmental Engineering Reference
In-Depth Information
10 and above, and therefore requires acidiication or carbonation. All this adds to the
cost and complexity of the method, which results in dificulties in ield operation. The
acid requirement for neutralization could be as high as 300 mg/L expressed in terms of
CaCO
3
per liter.
17.4 Membrane-Based Defluoridation Processes
Membrane-based processes have grown manifold during the last two decades and are
inding important applications in water treatment as well as desalination. A variety of
polymeric and ceramic membranes have been developed and commercially produced in
recent years, leading to more interest in membrane-based water puriication technologies.
The speciic applications of membrane luoride removal include reverse osmosis (RO),
nanoiltration (NF), electrodialysis, and Donnan dialysis. These are briely discussed in
the following sections.
17.4.1 Reverse Osmosis
Reverse osmosis is a technique in which a solvent is forced through a semipermeable mem-
brane by applying pressure greater than the osmotic pressure of the solution. Membrane
iltration processes usually remove the target particulate constituents on the basis of size
exclusion; however, RO does not use the same principle. The RO process rejects the select
constituents due to electrostatic repulsion at the membrane interface, chemical solubil-
ity, and diffusivity as well as straining of solutes (Boysen, 2008). The US Environmental
Protection Agency lists RO as a “best available technology” for luoride removal (Angers,
2001). Limitations of RO include a higher energy requirement associated with operation of
high-pressure pumps, complexity of the process, water quality issues due to removal of all
dissolved constituents in an uncontrolled separation, waste management challenge, and
the loss of water rejection as well as that associated with the disposal of the high solute
concentrate. However, these processes are increasingly improved and inding use for the
inal polishing of the treated efluent (Tetra Chemicals Europe, 2010) as well as for water
treatment.
17.4.2 Nanofiltration
Nanoiltration is a low-pressure (compared with RO) membrane separation process that
removes constituents on the basis of size exclusion and ion rejection principles. NF mem-
branes have slightly larger pores than those used for RO processes. This results in a
low-pressure requirement, making this a relatively low-energy process. Removal of sol-
utes is much less complex, and outputs are faster. The selectivity of NF relative to RO is
yet another distinct advantage, and much experimental and theoretical research work
is being focused to obtain a better idea of the mechanism of solute retention to facili-
tate production and selection of targeted membranes and optimization of conditions
for their most eficient use. Retention of solutes is attributed mainly to steric and charge
effects. Fluoride is a small ion; however, it is more strongly hydrated than other monova-
lent anions owing to its high charge density. The resulting steric effect leads to luoride
being more strongly retained on NF membranes than competing monovalent anions like
