Brine reject

Concentrate recycle RO plants allow some of the brine reject water to recycle back through the plant, which improves the permeate recovery rate. (The reduced flow of brine reject water does of course have a proportionally higher TDS level.) Various types of high pressure, corrosion-resistant pumps are used, including multistage, centrifugal and plunger pumps, each with their own benefits and area of application. 

Energy recovery turbine systems are available for very high pressure RO (seawater) plants, whereby brine reject water under high pressure can be used to spin an energy recovery turbine, reducing the RW hp requirements (and hence electricity costs) by 30 to 32%. 

Acid addition is commonly used to convert bicarbonates into the more soluble sulfate salts to reduce the alkalinity of the RO RW, which in turn modifies the brine reject water LSI. Sometimes it is required to maintain the pH level within membrane limits. Additionally, it may be used in conjunction with a reduced dosage of antiscalent chemical to reduce the overall chemical treatment costs. 

It is necessary to obtain one or more computer projection printouts from RO plant manufacturers to ensure correct RO design with the best balance of variables (e.g., feed flow rate, brine reject water flow rate, recovery rates, flux rates, and maximum applied pressures). If, for example, flux rates on first-stage membranes are higher than the manufacturer s recommendations, membrane fouling is likely to occur. 

MOLECULAR SIMULATIONS OF WATER FJIEEZING BRINE REJECTION AND HOMOGENEOUS NUCLEATION... 

Water is probably the most important and the most intensely studied substance on Earth. It is the solvent of life and it is also of vital importance in many aspects of our existence, ranging from cloud microphysics to its key role as a solvent in many chemical reactions. The familiar process of water freezing is encountered in many natural and technologically relevant processes. In this contribution, we discuss the applicability of the methods of computational chemistry for the theoretical study of two important phenomena. Namely, we apply the molecular dynamics (MD) simulations to the study of brine rejection from freezing salt solutions and the study of homogeneous nucleation of supercooled water. 

Brine rejection is thus an extremely interesting and also important process with wide natural, atmospheric, and technological consequences (with desalination as a prominent example). It has, therefore, been subject to experimental as well as theoretical research. 

These simulations can be easily extended to systems containing solutes, namely simple salts. However, most of the reported simulation studies concerned only the behavior of the solute at the stable ice/water interface.Only recently results of successful simulations of the brine rejection process involving a moving ice/solution boundary have been reported. 

We applied the MD technique to the simulation of brine rejection from freezing salt solutions employing the rigid 3-site simple point charge (SPC/E) water model to obtain the microscopic picture of this very important natural process. Further, we used a recently... 

Figure 1 Snapshots from the MD simulation of the brine rejection with 4 NaCl pairs. Na and CT are given as light and dark spheres, respectively. Snapshots correspond to A) 200 ns, B) 400 ns, C) 600 ns, D) 815 ns simulation time.

Representative snapshots from brine rejection simulations are given on Figure 1. It can be clearly seen that both sodium and chloride are rejected by the freezing front into the remaining liquid solution (save a single chloride which got trapped in the ice lattice). At the end of the simulation, a thin layer of unfrozen concentrated brine solution with glassy character is formed. There is a slow tendency to incorporate more of the remaining water molecules into the ice lattice however this process exceeds the time scale of present simulations. 

We investigated freezing of water and salt solutions by means of molecular dynamics simulations. We first established a robust simulation protocol for water freezing and than applied this approach to the study of the brine rejection process. Brine rejection was observed for a series of systems with varying salt concentration. We showed the anti-freeze... 

BRO = Brine/reject RO unit P1 = High-pressure pump... 

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