(624f) The Effect of the Presence of Antiscalants on Calcium Carbonate Precipitation: Evaluation of Calcium Precipitated, Particle Characteristics, and Fouling Potential

Reverse
osmosis (RO) membrane desalination has been increasingly used to produce
drinking water from salt water.  Most of the United States has relied on fresh
water resources to produce drinking water, but even water-rich regions, such as
states in the northeast, are experiencing a strain on traditional resources. 
Furthermore, many regions lack sufficient fresh water resources and are turning
to alternate resources, such as seawater or brackish water, to sustain water
needs.  In particular, a growing number of inland communities have both
insufficient fresh water and unused brackish water (500 ? 10,000 mg/L total
dissolved solids) resources. A key financial and technical limitation to inland
RO desalination is disposal of the waste stream (concentrate); typically, 10 ?
25% of the influent volume becomes the RO concentrate stream. This waste volume
is large when compared to the waste volume produced by traditional fresh water
treatment (less than 1%).  To improve the feasibility of RO desalination, RO
system recovery (volume of product water per volume of feed water) must be increased
to decrease the concentrate volume.

Brackish
water RO treatment recovery is limited by sparingly soluble salt (CaCO₃, CaSO₄,
BaSO₄, SrSO₄, silica) precipitation. Specifically,
calcium carbonate (CaCO₃) is known to be a key, omnipresent
precipitate. Salt precipitation can be limited or prevented through a
combination of chemical addition, pH control, and decreased recovery. 
Chemicals called antiscalants are dosed prior to membrane treatment and delay
precipitation through association with and modification of the salt crystals in
solution.  Antiscalants are typically synthetic organic phosphonates, acrylic
polymers, or polymer blends.  Precipitation prevention through antiscalants can
be achieved within a limited range of specific salt concentrations.  As
recovery is increased, antiscalant control is overcome and precipitation
occurs.  An alternate approach is thus required for further recovery
augmentation.  

This
research focuses on the development of a novel process to treat brackish water
RO concentrate and increase overall recovery.  This research focuses on a novel
process and includes three stages: antiscalant degradation, salt precipitation,
and solid/liquid separation.

This study
focused on the second stage, salt precipitation, using model synthetic
concentrates and several antiscalant types to evaluate the precipitation of
calcium carbonate.  The model concentrate is based on data from a brackish
water source in Arizona, assuming an 80% recovery and 100% ion retention.  First,
the precipitation of calcium carbonate as a function of pH was determined for
solutions with a phosphonate antiscalant and with no antiscalant.  For the
model concentrate used (Saturation Index, SI, for CaCO₃ = 2.3 at pH
8), samples without antiscalant resulted in 87% calcium precipitation, while,
as expected, samples with antiscalant showed 0% calcium precipitation.  This
result corresponds to the recommended SI limit for successful precipitation
prevention by an antiscalant (2.5 for CaCO₃) [1].  As pH increased, samples with antiscalant showed increased calcium precipitation (92 ? 93% at pH 10.5).  However, the same sample without antiscalant always resulted in a higher calcium precipitation (99.7% at pH 10.5).  Subsequent experiments with varied water composition showed a decrease in calcium precipitation with the addition of magnesium and sulfate.

The effect
of antiscalants on CaCO₃ precipitation was also evaluated through particle size and particle number distributions, using a laser granulometer Mastersizer S (Malvern Instruments) and a
laser particle counter (Met One).  Several antiscalants, including organic
phosphonates and one acrylic polymer blend, were tested.  Most samples showed
particle size distributions similar to samples without antiscalant. However, two
antiscalants (amino tri(methylene phosphonic acid), or AMPA) and
diethylenetriamine penta(methylene phosphonic acid), or DTPMP), showed markedly
different results at the highest antiscalant concentration tested (85 mg/L and
100 mg/L, respectively, i.e. 17 mg/L and 20 mg/L in the hypothetical RO
influent). Particle size measures taken directly after precipitation in the
presence of 85 mg/L AMPA and 100 mg/L DTPMP showed a bimodal particle size
distribution. Both peaks represented smaller sizes than peaks obtained for
samples without antiscalant. Particle size measures taken 3 days after
precipitation showed a significant increase in particle size, and results also
varied as a function of pH. Photos taken with a light microscope (10x and 25x)
confirm the results obtained from the laser granulometer.
Particle counter results showed the majority of particles are within a particle
size range of 2 ? 50 micrometers; the number of larger particles (50 ? 300
micrometers) increased with time for samples with 85 mg/L AMPA.

Light
microscope photos also showed differences in particle shape and size with
different antiscalants, as well as for solutions with and without antiscalant
present.  These photos, along with flux data manipulation, were used to provide
explanations for the type of membrane fouling during dead end filtration.

Dead end
filtration (MWCO = 0.1 micrometers, Millipore nitrocellulose
membranes) was performed to compare the fouling potential of the
precipitated solutions as a function of water composition, antiscalant type,
and antiscalant concentration.  A simplified concentrate, containing sodium chloride
(NaCl), sodium bicarbonate (NaHCO₃), and calcium chloride (CaCl₂*2H₂O)
was first tested.  The water composition was then changed by separately adding
magnesium chloride (MgCl₂*6H₂O) and sodium sulfate (Na₂SO₄). 
Finally, the full water data set was tested.  Results showed differences
between antiscalants and water compositions.  AMPA and DTPMP showed flux
decreases of approximately 30% over a period of 7 minutes for the simplified
concentrate, while the same two antiscalants showed only a 19% flux decrease
when magnesium was added to the simplified concentrate.  For the four antiscalants
tested, there was no consistent relationship between antiscalant concentration
and membrane flux.  For all antiscalants tested, while a greater antiscalant
concentration resulted in less calcium precipitated, a greater antiscalant
concentration did not result in lower permeate flux decline. Subsequent tests
on solutions containing only antiscalant and deionized water indicated membrane
fouling occurred through antiscalant adsorption onto the membrane surface.

Future
experiments will focus on tests with real water samples to evaluate the effect
of natural organic matter.

[1] Chemical Pretreatment for
RO and NF. Hydranautics, Technical Application Bulletin No. 111, Rev. B,
2003.