Observed rejection

Solute separation is measured in terms of observed rejection, R, defined as... 

In a pressure-driven membrane process the molecules are generally rejected by the membrane and therefore their concentrations in the permeate are lower than those in the feed solution. However, an accumulation of excess particles can occur at the membrane surface with the creation of a boundary layer. This phenomenon, called concentration polarization, causes a different membrane performance. In particular, with low molecular weight solutes the observed rejection will be lower than the real retention or, sometimes, it could be negative. 

The observed rejection [Eq. (8a)j is clearly the important one for a practical separation operations, but it includes the confounding effects of concentration polarization. Since C k < Cj, the observed rejection is less than the intrinsic rejection and can be determined by estimating the solute wall concentration with Eq. (7). 

This novel integer approach makes it possible to determine today an NF most efficient membrane for the elimination of excess F in a Senegalese water sample taken from the endemic region of Fatick. This membrane denoted NF90 works very well due to its diffu-sional behaviour for fluoride rejection, its high hydraulic permeability and sufficient observed rejection for F in comparison to RO. 

According to the film theory, the relation between the observed rejection rate and the true rejection rate may be expressed as... 

To determine the relative degree of purification in a given UF process or to estimate the period of UF processing required to achieve a certain degree of separation or purification, the UF process must be mathematically modeled (29). The observed rejection coefficient at any point in the UF process is defined as... 

Membrane technology is used to remove oil particles from industrial wastewater. The initial feed tank volnme is 8640 m /d and after treatment the retained volume is required to be 50% of the initial volume entering to the basin. If the observe rejected coefficient is... 

The real rejection can be calculated from the observed rejection using Eqs. (12.1) to (12.3). 

Ultrafiltration membranes have pore diameters in the range of 1-100 nm and separate primarily by size exclusion. In this process, a liquid containing dissolved or suspended matter is driven through the membrane by an applied pressure difference across the membrane, and any species larger than the pore size of the membrane is rejected (prevented from passing through the membrane). The observed rejection R of a species is given by... 

The boundary-layer theory used here to correct observed rejection coefficients is an improvement over thin-film theory, but it ag ears limited to filtrate velocities, J, below about 0.5 X 10 cm/sec for highly rejected solutes. Xn exact theory for incomplete rejection by hollow fibers is needed to define the validity of Equation 23, over the range of conditions of the experiments. 

FIGURE 22.6 Effect of operating pressure AP on permeate flux and observed rejection for anserine-carnosine R/,r, creatinine Rc and sodium ion R, . with NFT-50 membrane. 

FIGURE 22.7 Effect of flow rate on permeate flux 7, and observed rejection for anserine-carnosine creatinine and sodium ion iJfja " ith NFT-50 membrane. 

The performance equation of a membrane can be derived by introducing intrinsic rejection (R = 1 — c lcf) and observed rejection (Robs = 1 — 0 /02) with c5 being the bulk concentration at the permeate side ... 

One may argue that the ion rejection is mainly caused by surface charges of the separation layer, because the concentration of excess charges in the membrane might be too low [16] to account for the observed rejection. In that case one should expect that the Pr values depend on... 

Consider ultrafiltration based separation of a protein in the configuration of Figures 6.3.26(a) and (b). Suppose now another membrane, identical to the first membrane, is located below the first membrane at some distance from it. Permeate generated from the first membrane having a protein concentration of Cip becomes the feed to the second UF membrane. The protein concentration in the permeate from the second membrane is Cjp. The protein concentration in the feed to the first membrane is Assume a pseudosteady state the macrosolute observed rejection value Ri for the first membrane may be assumed to be valid for the second membrane as well, in relation to its feed and permeate. 

Verify the accuracy of the graduation marks on the trap by adding 0.05-mL increments of distilled water, at 20 C, from a 5-mL microburet or a precision micro-pipet readable to the nearest 0.01 mL. If there is a deviation of more than 0.050 mL between the water added and water observed, reject the trap or recalibrate. 

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