Chemical Engineering June 2016 - 45

Electropositive filters
The property of electronegativity
can be defined as a measure of the
tendency of an atom or a chemical
functional group to attract electrons
(electron density) toward itself. In
the context of a chemical bond, elements
that are highly electronegative
will attract electron density toward
their atomic nuclei, giving them a
slight negative charge, while leaving
less electronegative atoms, to which
they are bound, with a slightly positive
charge.
In the context of filtration, the overall
surface charge (the combination
of the electronegativity behavior of
larger groups of atoms) becomes important.
Small particles typically have
an overall negative charge. Electropositive
filters (EPFs) take advantage
of this fact by introducing an overall
positive charge to the filter media, in
the form of a surface coating (Figure
1). EPFs can be visualized as aggregated
or structured forms of flocculants.
Flocculants, including alumina,
are charged particles that will cause
colloidal particles to aggregate.
EPF advantages and limitations
The primary advantage of EPFs is
that they are effective at filtering colloidal
and nanometer-sized particles
at low head pressures. Those EPFs
that are fibrous-based depth filters
were initially designed for purifying
drinking water, where operating
pressures are less than 60 psi. Most
membrane systems require operating
pressures that are much higher
- in the range of hundreds of psi.
A second advantage is that EPFs
allow the elimination of water waste
because there is no concentrated
stream, as would be present in reverse
osmosis (Ro) filtration or as
would be the case of using crossflow
ultraporous membranes.
Third, EPFs achieve high filtration
efficiency. Their efficiencies are equivalent
to ultraporous membranes.
EPFs are so efficient because the
electropositive charge created by
their chemistry results in adhesive
forces on the surfaces of the many
pores that a particle must travel on
its tortuous path through the depth
media. Particles in aqueous media at
pHs between 4 and 10 are virtually
all electronegative. Certain aluminized
EPFs have such a high charge
that the combined electrostatic and
electrokinetic fields they create extend
up to 4 µm away from the surface
of the media inside the pore [4,
5], overlapping the pore network and
attracting the particle to the surface.
Fourth, EPFs have high dirt-holding
capacity. In order to work at high
efficiency, membranes, because
they are surface filters, must have
occluded particles swept away by
cross-flow (using water), or must
be washed out periodically. In contrast,
EPFs retain dirt in their depths
or amid sorbent grains of aluminized
diatomaceous earth (dE; to be discussed
more later) that are introduced
into the EPF media.
A final advantage of EPFs is that
they are capable of filtering certain
soluble contaminants directly or by
incorporating secondary sorbents.
Examples include removing polychlorinated
biphenyls (PCBs) by filtration
with EPFs. Although the mechanism
is not fully understood, it is believed
to be the result of solvated contaminant
molecules that are readily polarized,
exposing a negative face to the
electropositive field. Alternatively, a
sorbent particle, such as powdered
activated carbon (PAC), can be
added to adsorb Cl2. The advantage
is that the high surface area of the
PAC, as compared to granular carbon,
results in high kinetic adsorption
in thin layers.
operationally, EPFs can offer other
advantages, such as lowering capital
expenditure (capex) compared
to membrane systems when used
as a prefilter for Ro. In a University
of Wyoming (Laramie, Wy.; www.
uwyo.edu) study [6], substantial increases
in Ro membrane life were
demonstrated when an EPF was inserted
downstream of an ultraporous
membrane that had been previously
been inserted to protect the Ro.
Presumably, the EPF collected ultrafine
particles that passed through
the ultraporous filter. The study also
demonstrated improvements in Ro
filter lifetimes if the backwash fluid
was first filtered through an EPF.
The limitations of EPFs include that
they are not regenerable, because the
particles are retained and enmeshed
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