(5ai) A Novel Three-Stage Treatment for Brackish Water Reverse Osmosis Concentrate: Treatment and Analysis of a Real Brackish Water

Reverse osmosis (RO) membrane
desalination has been increasingly used to produce drinking water from salt
water. 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. Antiscalants are typically
synthetic organic phosphonates, acrylic polymers, or polymer blends and are
used, along with pH adjustment, to prevent precipitation. However, as recovery
is increased, antiscalant control is overcome and precipitation occurs. An
alternate approach is thus required for further recovery augmentation.

Previous research using
precipitation and separation to treat concentrate has shown that significant
increases in total system recovery are possible [1]. However, the presence and influence of antiscalants and natural organic matter (NOM) during RO concentrate treatment have not been investigated.

This study presents the
development of a novel three-stage process to treat the concentrate from a
brackish water RO system. The process achieves problematic salt removal through
three treatment steps: antiscalant deactivation, precipitation, and
solid/liquid separation. Antiscalant deactivation is performed using ozone (O₃)
and hydrogen peroxide (H₂O₂). pH elevation is used to
precipitate salts, and solid/liquid separation is achieved through
sedimentation and filtration. While technologies for solid/liquid separation
are well-established, the combination of antiscalant oxidation and
precipitation represents a new system; research on antiscalant oxidation has
been limited [2], and the effect of ozonation on precipitation has not been investigated.

Previous results with synthetic
concentrates have shown that ozonation prior to precipitation increases calcium
precipitation.  For a simplified concentrate containing only sodium chloride
(NaCl), sodium bicarbonate (NaHCO₃), and calcium chloride (CaCl₂*2H₂O),
ozonation times as small as one minute increased calcium precipitation from 94%
to 99.6%, for precipitation performed at pH 10.5 for 1 hour.  In comparison,
precipitation of the same solution without antiscalant results in 99.7% calcium
precipitation.  For a more complex water containing magnesium, sulfate, and
other metals, 10 minutes of ozonation increased calcium precipitation from 81%
to 87% after 30 minutes precipitation at pH 10.5.  These results show the
three-stage process allows increased calcium removal and thus a larger possible
recovery for the overall process.

After tests and process
optimization with synthetic RO concentrates, experiments were performed on a
real water sample.  The purpose of testing a real water sample was to evaluate
the effect of NOM on the concentrate treatment process.  The water sample
contained similar concentrations of calcium, magnesium, and carbonate, with
larger concentrations of sodium, chloride, and sulfate.  A laboratory-scale RO
pilot was used to make concentrate; a Koch ultra-low pressure spiral wound RO
membrane module was used during experiments (operating pressure = 12 bar).  Several
antiscalants were tested, including phosphonate and acrylic polymer blend
products.  Measures for dissolved metal ions, total carbonate, conductivity,
total organic carbon and COD were taken.  In addition, changes in the
precipitate were evaluated; particle size and particle
number distributions were obtained, using a laser granulometer
Mastersizer S (Malvern Instruments) and a laser particle counter (Met One). 

[1] Rahardianto, A.; Gao, J.; Gabelich, C.J.; Williams, M.D.; Cohen, Y., High recovery
membrane desalting of low-salinity brackish water: Integration of accelerated
precipitation softening with membrane RO. Journal of Membrane Science 2007,
289, 123-137.

[2] Yang, Q.; Ma, Z.;
Hasson, D.; Semiat, R., Destruction of Anti-Scalants in RO Concentrates by
Electrochemical Oxidation. Journal of Chemical Industry and Engineering
(China) 2004, 55(2), 339-340.