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Membrane modules

FIGURE 8.10 Principle of operation of a reverse osmosis (RO) system.

Lime pressure. In larger plants, it is economically viable to recover the rejected brine energy with a suitable brine turbine. Such systems are called energy recovery reverse osmosis (ER-RO) systems.

Solar energy can be used with RO systems as a prime mover source driving the pumps (Luft, 1982) or with the direct production of electricity through the use of photovoltaic panels (Grutcher, 1983). Wind energy can also be used as a prime mover source. Because the unit cost of the electricity produced from photovoltaic cells is high, photovoltaic-powered RO plants are equipped with energy-recovery turbines. The output of RO systems is about 500-1500 L/d/m2 of membrane, depending on the amount of salts in the raw water and the condition of the membrane. The membranes are, in effect, very fine filters and very sensitive to both biological and non-biological fouling. To avoid fouling, careful pre-treatment of the feed is necessary before it is allowed to come in contact with the membrane surface.

One method used recently for the pre-treatment of seawater before directed to RO modules is nano-filtration (NF). NF was developed primarily as a membrane softening process, which offers an alternative to chemical softening. The main objectives of NF pre-treatment are (Adams et al., 2003):

1. Minimize particulate and microbial fouling of the RO membranes by removal of turbidity and bacteria.

2. Prevent scaling by removal of the hardness ions.

3. Lower the operating pressure of the RO process by reducing the feed-water total dissolved solids (TDS) concentration.

Tabor (1990) analyzed a system using an RO desalination unit driven by PV panels or from a solar thermal plant. He concluded that, due to the high cost of the solar equipment, the cost of freshwater is about the same as with an RO system operated from the main power supply.

Cerci (2002) performed an exergy analysis of a 7250 m3/d reverse osmosis desalination plant in California. The analysis of the system was conducted using actual plant operation data. The RO plant is described in detail, and the exergies across the major components of the plant are calculated and illustrated using exergy flow diagrams in an attempt to assess the exergy destruction distribution. He found that the primary locations of exergy destruction were the membrane modules in which the saline water is separated into the brine and the permeate and the throttling valves where the pressure of liquid is reduced, pressure drops through various process components, and the mixing chamber where the permeate and blend are mixed. The largest exergy destruction occurred in the membrane modules, and this amounted to 74.1% of the total exergy input. The smallest exergy destruction occurred in the mixing chamber. The mixing accounted for 0.67% of the total exergy input and presents a relatively small fraction. The second-law efficiency of the plant was calculated to be 4.3%, which seems to be low. He shows that the second-law efficiency can be increased to 4.9% by introducing a pressure exchanger with two throttling valves on the brine stream; this saved 19.8 kW of electricity by reducing the pumping power of the incoming saline water.

8.4.5 Electrodialysis (ED)

The electrodialysis system, shown schematically in Figure 8.11, works by reducing salinity by transferring ions from the feedwater compartment, through membranes, under the influence of an electrical potential difference. The process utilizes a DC electric field to remove salt ions in the brackish water. Saline feedwater contains dissolved salts separated into positively charged sodium and negatively charged chlorine ions. These ions move toward an oppositively charged electrode immersed in the solution, i.e., positive ions (cations) go to the negative electrode (cathode) and negative ions (anions) to the positive electrode (anode). If special membranes, alternatively cation permeable and anion permeable, separate the electrodes, the center gap between these membranes is depleted of salts (Shaffer and Mintz, 1980). In an actual process, a large number of alternating cation and anion membranes are stacked together, separated by plastic flow spacers that allow the passage of water. The streams of alternating flow spacers are a sequence of diluted and concentrated water, which flow parallel to each other. To prevent scaling, inverters are used that reverse the polarity of the electric field every about 20 min.

Because the energy requirements of the system are proportional to the water's salinity, ED is more feasible when the salinity of the feedwater is no more than about 6000 ppm of dissolved solids. Similarly, due to the low conductivity, which increases the energy requirements of very pure water, the process is not suitable for water of less than about 400 ppm of dissolved solids.

Because the process operates with DC power, solar energy can be used with electrodialysis by directly producing the voltage difference required with photovoltaic panels.

+ ve (anode)

(cathode)

O*

- ve ions

+ ve ions

(anions)

(cations)

\

Freshwater

\

Anion permeable Cation permeable membrane membrane

Anion permeable Cation permeable membrane membrane

FIGURE 8.11 Principle of operation of electrodialysis (ED).

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