Page 212 - Membranes for Industrial Wastewater Recovery and Re-Use

System design aids 18 1

Chemical
r G Nme
,
i r Sulfutic Acid
' 1 r Hydrochloric Acid
f Sdim carbate
r Lme
r Sodm Hydroxide

Figure 4.5 Pretreatment dialogue box (ROPRO, Koch-Fluid Systems)

0 reduction of the number of pressure vessels per bank whilst increasing the
number of membranes per pressure vessel
0 recycling of part of the concentrate
0 decreasing the element size.

(ii) Permeate quality changes with recovery. In principle, solute diffusion
increases with higher recoveries but permeate flux also increases. These
synergistic phenomena mean that permeate quality can increase until some
specific recovery value, beyond which it may decrease again. It is rarely the case
that the highest permeate water quality, as may be demanded by, for example,
microprocessor applications, coincides with the highest recovery.
(iii) The specific energy demand increases with increasing recovery.
(iv) Calcium carbonate scaling can be very substantially reduced through
pretreatment by acid dosing or hardness removal, allowing much greater
recoveries and thus demanding smaller membrane areas. Hardness reduction can
be achieved by classical chemical softening, by ion exchange or lime dosing, or by
blending the feedwater with some of the permeate product.
(v) Scaling by non-hydrolysable scalants (Table 2.14) can be suppressed through
the use of appropriate scale inhibitors. Dedicated software from anti-scalant
chemical suppliers is available (Fig. 4.7) and can be used in parallel with the RO
software to calculate scaling propensity of the chemically treated water. Scaling
from these contaminants is otherwise reduced only by their removal or by
reducing CP.