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.