Page 211 - Membranes for Industrial Wastewater Recovery and Re-Use

180 Membranes for Industrial Wastewater Recovery and Re-use

Figure 4.4 Array design dialogue box (Winflows, Osmonics)

incompatibilities in the design), and these can be attended to directly. For
example, if the solubility limit of calcium carbonate has been exceeded, acid may
be added as pretreatment. If the membrane module is hydraulically overloaded
more membranes can be added. If the design selected is entirely inappropriate,
calculation of the parameters (which relies on convergence) may not be possible
and the software will then produce a warning and halt.
An optimised RO system design is generally considered to be one for which the
lowest specific energy demand is incurred and/or the lowest membrane area is
demanded at the highest recovery, although these parameters obviously imply a
trade-off between operating and capital costs. Although there is no one method
of performing the iterative procedure to obtain the optimised system, a few
general points can be made:

(i) A high CP level, always greatest at the module exit, should be avoided a
warning is given when the p-coefficient value in the last membrane of a pressure
vessel exceeds the pre-set maximum value. CP can be reduced by increasing the
cross-flow velocity and/or reducing the flux. A number of options exist in this
regard

a reduction of overall recovery
a addition of a stage
a increase of feed flow