6.1. External and internal fouling

The classification of the fouling phenomenon depends on the location of the accumulated rejected salts; it can be viewed as [40, 41]:


EF involves accumulation of rejected salts on the surface of the membranes by three distinct paths:


Typically, the three mechanisms can occur in any combination at any given time. However, external membrane fouling of ROMs is most frequently caused by biofouling.

IF is a regular loss of membrane productivity due to changes in its chemical structure either by physical compaction or by chemical degradation. Physical deterioration of the membrane may result from long-term application of feed stream at pressures higher than that designed for the ROMs; they are designed to handle 83 bars for sea water reverse osmosis membranes and/or by their continued setup at source water temperatures above 45C, the limit of safe membrane operation. Chemical deterioration results from continuous exposure to strong oxidants, e.g., chlorines, bromines, ozones, permanganates, peroxides chemicals, and very strong acids, typically pH < 3 and alkali at pH > 12.

leads to critical negative significances on the ROM function. They include increased osmotic pressure, increased salt extract, creation of hydraulic opposition of water stream, and Induc-

Concentration polarization cannot be evaded; it can only be reduced before taking any corrective measures; concentration polarization should be quantified. This quantification occurs in three separate consecutive paths. They can be emphasized as balancing the chemical and mass balance equations across the boundary layer, balancing the transport equations over the ROM and determination of solute transport equations within the pores of the ROM. System performance can be predicted by simultaneous solution of all these three equations. Based on the type of concentration polarization, there are two classes of models: an osmotic pressure-

In this situation, solute particles form a viscous boundary layer concluded on the surfaces of ROMs [45–47]. Solute concentration increases from the bulk to membrane surface concentration across the mass transfer barrier layer. In this case, the width of the mass transfer boundary layer is constant. At any cross section of the boundary layer for the concentration gradient, dc

/s.

dy <sup>¼</sup> <sup>0</sup> (2)

.s; c and cp are the bulk and permeate concentrations

Recent Drifts in pH-Sensitive Reverse Osmosis http://dx.doi.org/10.5772/intechopen.75897 13

vw c � vw cp <sup>þ</sup> <sup>D</sup> dc

/m<sup>2</sup>

dy,

tion of scale and fouling on the ROM.

controlled model and a gel layer-controlling model.

Figure 6. Boundary layers in a membrane-feed spacer. RO, reverse osmosis.

6.3. Osmotic pressure controlled model [OPCM]

at the steady state, the solute mass steadiness leads to

; and D is the solute diffusivity in m<sup>2</sup>

where vw is the permeate flux in m3

in kg/m<sup>3</sup>

The difference between EF and IF is somewhat clear; EF could be completely reversed by chemical cleaning, while IF causes permanent damage of the micropores, resulting in an irreversible changes.
