Permeate total dissolved solids

Applications RO is primarily used for water purification seawater desalination (35,000 to 50,000 mg/L salt, 5.6 to 10.5 MPa operation), brackish water treatment (5000 to 10,000 mg/L, 1.4 to 4.2 MPa operation), and low-pressure RO (LPRO) (500 mg/L, 0.3 to 1.4 MPa operation). A list of U.S. plants can be found at www2.hawaii.edu, and a 26 Ggal/yr desalination plant is under construction in Ashkelon, Israel. Purified water product is recovered as permeate while the concentrated retentate is discarded as waste. Drinking water specifications of total dissolved solids (TDS) < 500 mg/L are published by the U.S. EPA and of < 1500 mg/L by the WHO [Williams et ak, chap. 24 in Membrane Handbook, Ho and Sirkar (eds.). Van Nostrand, New York, 1992]. Application of RO to drinking water is summarized in Eisenberg and Middlebrooks (Reverse Osmosis Treatment of Drinking Water, Butterworth, Boston, 1986). 

A "Permasep" permeator reverse osmosis (RO) installation, capable of producing 1500 cubic meters of product water per day, was successfully operated for six months with RO feed water drawn from cUi estuary cuid having 1200 to 1400 mg/H total dissolved solids (TDS). 

Higher recovery results in the need to dispose of less reject water. However, higher recovery also results in lower-purity permeate. Consider the example shown in Figure 3.1. At the influent end of the membrane, the influent concentration is 100 ppm, while the recovery is 0%, and the membrane passes 2% total dissolved solids (TDS) (see Chapter 3.3). The permeate right at this spot would be about 2 ppm. As the influent water passes across more and more membrane area, more water is recovered. At 50% recovery, the concentration factor is 2, so the influent water now has a concentration of about 200 ppm. The permeate water at this point would now have a concentration of 4 ppm. At 75% recovery, the concentration factor is 4, so the influent water now has a concentration of about 400 ppm. The permeate water at this point would have a concentration of 8 ppm. Hence, higher recovery results in lower product purity. 

Champion International s pulp mill in Arapoti (Brazil) tested a ZeeWeed MBR followed by reverse osmosis polishing to recycle thermomechnical pulp mill wastewater within the mill (Table 35.1). Over 4 months of tests showed that the system decreased the COD from 7800 to <20 mg/L. In addition, the total dissolved solids (TDS) content was reduced from 2000 to <300 mg/L in the RO permeate [65]. 

Total dissolved solids (TDS) in mg/1 or ppm is a measure of the total weight of impurities found in a water supply. The measure permits a quick, rough estimate of RO/NF permeate quality and feed pressure requirements. 

Figure 5.3 shows how concentration changes through an RO system (assuming 50% recovery per stage, as in Figure 5.2). In this example, assume a feed concentration of 100 ppm total dissolved solids (TDS) and a membrane rejection of 98% TDS (2% salt passage). The permeate concentration out of the first stage would be 2% of 100 ppm or 2 ppm. To calculate the reject concentration, use the concentration factor. Recall from Table 3.1 concentration factor as a function of recovery. At 50% recovery after the first stage, the concentration factor is two. Thus, the reject concentration...

The needle valve in the reject line is throttled to obtain a suitable pressure in the range of 7 to 25 bar. The initial permeate was recycled until a steady permeate flux was obtained. The reject was partially circulated back to the feed tank through the heat exchanger. The flow rates of the two streams such as permeate and reject streams were measured at regular time intervals using the rotameters. The conductivity, total dissolved solids (TDS) and flux values of feed and permeate were recorded as a function of time until the desired water recovery was achieved. 

Interestingly, product permeation rate varies considerably depending on the solute in the feed solution. A more than twofold change was observed from MgCb to K2SO4. They have also attempted the purification of wastewater from the Alberta heavy oil fields containing 5 000 to 15 000 ppm of total dissolved solids. 

RO is used to treat 1.31 m3/s of seawater at 293 K containing 3.5 wt% dissolved solids to produce 0.44 m3/s of potable water with 500 ppm of dissolved solids. The feed-side pressure is 138 bar, while the permeate pressure is 3.4 bar. A single stage of spiral-wound membrane is used that approximates crossflow. If the total membrane area is 0.10 km2, estimate the permeance for water and the salt rejection. Assume that the densities of the seawater and of the brine are approximately equal to the density of pure water. 

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