Permeate recovery

Let us first evaluate the composition and flowrate of the permeate using solute rejection and permeate recovery ... 

NOTE Generally speaking, the higher the TDS of the RW source to the RO, the higher the applied pressure required to produce a constant permeate water TDS. Also, for practical reasons, the rate of permeate recovery tends to decrease with increase in source water TDS. 

Tap water. As city water, mains supply, and the like, tap water is typically under 1,000 to 2,000 ppm TDS. It requires an RO plant operating with an applied pressure of 150 to 300 psig and permeate recovery rates of 80% down to perhaps 50% of the RW supplied... 

Brackish water. Usually associated with salty water, brackish water TDS levels range from 2,000 to 20,000 ppm or more. Most industrial sources of RW supply may be well water, surface waters, or the like, but do not specifically have to contain high levels of sodium chloride. The RO applied pressure required is from 250 to 600 psig, and the permeate recovery rates are typically 60% down to perhaps 40%. There is a tremendous variety in so-called brackish water sources, and correct membrane selection and other design criteria are critical to manufacturing an efficient RO plant. 

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. 

Rejection of sodium sulphate was exceptional at the lower temperatures but permeate recovery was low. The testing at 81°C fulfilled Kvaerner s need to reduce overall skid costs and provide good rejection with an acceptable recovery rate. The use of high temperature membranes has provided the means to optimise the number of membrane elements, reduce the equipment costs and ensure that the SRS skid is an economically attractive package. 

With respect to RO process, MD does not suffer osmotic-pressure limitation and can be therefore employed when high permeate recovery factors or retentate concentrations are required. 

However, technological advances related to the development of new membranes operations and innovative strategies of process design, have partially overcome this limitation. Membrane distillation, for example, is not subject to osmotic-pressure limitation and can be therefore employed in integrated systems when high permeate recovery factors or retentate concentrations are requested. 

Permeate recovery. To achieve high conversions, it is often desirable to maintain a very low permeate partial pressure which leads to an increase in the permeation rate. Vacuum or a sweep gas is usually employed to attain a low permeate pressure. Vacuum adds some energy cost while the use of a sweep gas may require further downstream processing for the recovery of the permeate (if it contains the desired species) or the separation of the permeate from the sweep gas. The use of a condensable gas or vapor as the sweep gas will facilitate the recovery of the permeate. For example, steam can be condensed at a relatively lower temperature and easily separated from many other gases. However, the issue of hydrothermal stability of the membrane, discussed in Chapter 9, can be critical. Air, on the other hand, is a convenient sweep or carrier gas to use because... 

Various performance indicators, such as VRF, percent solute rejection (SR), percent permeate recovery (R) are computed as follows ... 

The objectives of the experiments carried out in this pilot plant were to select a membrane and to simulate industrial operation, varying the percentage of permeate recovery. 

According to the foreseen design for the industrial plant of reverse osmosis, this would be formed by three blocks of membranes, each block containing two filtering stages the hrst working at a pressure of 15 bar and the second at about 23 bar. Each stage comprises several modules, each of which is formed by six membranes in series. The first unit shows a permeate recovery of 50%, which continues to increase until a value of 80% in the sixth unit. 

Second, using the selected membrane, a wide sweep of the permeate recovery percentage was made, to simulate the different units of the industrial modules. 

In view of the results, it can be concluded that conductivity increases with permeate recovery, that is, the higher the ratio between the permeate and concentrate flows, the lowest the quahty of the permeate stream. As a consequence of this, there was a concentration of salts in the concentrate stream, because a higher flow of permeate than of concentrate was being separated from the feed. 

Figure 15. Permeate flow rate per unit membrane area (gallons/day/ft ) and NaCl rejection of brackish water membranes offered by GE (<>), FilmTec/Dow (x), Koch (o), Toray (a), Trisep (+), andNitto Denko/Hydranautics (Is). All values taken from the manufacturers web sites. Test conditions for all membranes were 2000 ppm NaCl feed concentration, 225 psi feed pressure, 77 F feed temperature, and 15% permeate recovery. Feed pH for test conditions varied slightly in the tests from 6.5-8. Note that a 1500 ppm NaCl feed was used to obtain the Nitto Denko/Hydranautics values normalization to 2000 ppm NaCl reduces permeability by S%.

In order to avoid excessive concentration polarisation at the membrane surface, permeate recovery per membrane element should not exceed 18%. In the case of brackish water RO systems, the average recovery per 100 cm (40-in.) long membrane element is usually about 9%. The overall recovery for a staged system with pressure vessels containing six elements is usually as follows [46] ... 

Feed pressure Feed water temperature 6.0 bar 25.0°C (77°F) Permeate recovery ratio 75.0%... 

---