Membrane and Desalination Technologies - TCE Moodle Website

302 J. Paul Chen et al.
Fig. 7.13. Cutaway view of hollow-fibre membrane module.
pipe is used to uniformly distribute the feed solution throughout the module. This is to avoid
the problem of “channelling” in “outside-in” model, which means that the feed has a tendency
to flow along a fixed path, thus reducing the effective membrane surface area. In gas
separation, as shown in Fig. 7.15b, the “outside-in” model is used to avoid high-pressure
losses inside the fibre and to attain a high membrane area (13).
6.3. Design Considerations
6.3.1. System Design
The design of the membrane operation system can be very different for different applications
and module configurations (13). A number of modules are connected together to form a
stage. It is desirable to optimize the whole separation system in order to achieve low capital
and operational costs and good quantity and quality of treated water. Normally, MF and UF
do not require pre-treatment units; however, the feed must be pretreated before entering NF or
RO unit.
The simplest design is a dead-end operation, as shown in Fig. 7.16a. As the feed is forced
through the membrane, the concentration of rejected components in the feed increases and
accumulates at the membrane interface; hence, the permeate quality decreases with time.
Therefore, for industrial applications, a cross-flow operation, as shown in Fig. 7.16b, is
preferred for its lower fouling tendency comparing with the dead-end mode.