In many locations, fresh water resources are insufficient for local needs, and alternative sources with lesser water quality are being considered as drinking water supplies. In particular, the United States has many inland regions with untapped brackish water (500–10,000 mg/L total dissolved solids (TDS)) resources. Interest has increased for using these brackish water resources to produce drinking water. The two primary process choices for water desalination are thermal distillation or membrane filtration. Reverse osmosis (RO) desalination has become the primary choice for brackish water desalination due to lower energy requirements and a smaller footprint. However, from brackish water, the product recovery (volume of product water per volume of feed water) range is only 75 – 90%; i.e., at least 10% of the feed water becomes the RO waste stream, or concentrate. The costs and technical feasibility of concentrate disposal are the key limitations to widespread application of inland RO. This research focuses on the development of a novel process to reduce the volume of brackish water RO concentrate.
Several critical differences between brackish and sea waters are the relative metal ion concentrations and the typical RO recovery ranges. While a typical brackish water (1,500 mg/L TDS) has a chloride ion concentration that is 30 times smaller than sea water and a sodium ion concentration that is 90 times smaller, the calcium concentration is only 2 – 5 times smaller and the carbonate concentration is equal to or larger than that of sea water. In addition, brackish water RO recovery is higher than sea water RO recovery (40-60%). These factors cause the recovery in brackish water RO systems to be limited by salt precipitation.
Chemicals called antiscalants are often used to complex with problematic salts (CaCO3, CaSO4, BaSO4, SrSO4, silica), delaying precipitation. However, salt concentration increases with recovery, and antiscalants work successfully within a limited concentration range. Therefore, eventually precipitation control is overcome. To increase system recovery and decrease the concentrate volume, a new approach is required.
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 paper 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 (I) antiscalant deactivation, (II) precipitation, and (III) 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.
Specifically, this study focused on changes to the precipitate particles and the extent of precipitation caused by prior antiscalant oxidation. Experiments were performed on a series of increasingly complex waters. First a simplified concentrate (~ 8,000 mg/L TDS), containing only sodium chloride (NaCl), sodium bicarbonate (NaHCO3), and calcium chloride (CaCl2*2H2O) was used. Subsequently, magnesium and sulfate were added, and finally, real water samples were tested. Four different antiscalants, including several phosphonates and one acrylic polymer blend were used during experiments.
Several oxidation parameters, such as ozonation time and antiscalant concentration, were varied. Ozonation times of 1, 10, and 30 minutes were tested. For the first antiscalant tested (amino tri(methylene phosphonic acid), or AMPA) and the simplified concentrate, results show increased calcium precipitation for all ozonation times and all antiscalant concentrations (4 – 85 mg/L). The simplified concentrate with 85 mg/L AMPA, treated with only 1 minute of ozonation and subsequent precipitation at pH 10.5 for 1 hour, had a final dissolved calcium concentration of 5.2 mg/L Ca2+. In comparison, the same precipitated solution with no antiscalant resulted in a final dissolved calcium concentration of 3.7 mg/L, while the same precipitated solution with 85 mg/L AMPA and no prior ozonation resulted in a final dissolved calcium concentration of 77 mg/L.
A laser granulometer Mastersizer S (Malvern Instruments) and a laser particle counter (Met One) were used to evaluate the effect of the ozonation step on precipitate particle size and number. Previous results have shown that certain antiscalants, such as AMPA (40 - 85 mg/L), can change the particle size range and the modality of the size distribution for calcium carbonate (CaCO3) precipitation. A solution of precipitated simplified concentrate with 40 – 85 mg/L AMPA is bimodal and has particle size ranges of 0.2 – 2 microns and 2 - 50 microns, whereas the same solution without antiscalant has a single particle size range between 10 and 100 microns. Results from combined ozonation-precipitation experiments show that ozonation causes the particle size distribution to shift towards that of a solution containing no antiscalant. Light microscope photos (25x) are consistent with the granulometer results and show particles that resemble a precipitated solution with no antiscalant present.
The fouling potential of precipitated solutions was evaluated through dead end filtration experiments (MWCO = 0.1 micrometers, Millipore nitrocellulose membranes). For antiscalant AMPA (85 mg/L), ozonation times of 1 and 10 minutes increase the permeate flux. An ozonation time of 30 minutes causes a higher initial permeate flux but the flux decline is greater overall and results in a lower final permeate flux. These differences in flux decline indicate different membrane fouling mechanisms. Moreover, this ozonation duration is not in agreement with an industrial development.
Future work includes manipulation of flux data to determine the type of membrane fouling occurring and continuing experiments with more complex waters.
[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.