A membrane system should be designed such that each element of the system
operates within a recommended range of operating conditions to minimize the
fouling rate and to exclude mechanical damage. These element operating conditions
are limited by the maximum recovery, the maximum permeate flow rate, the
minimum concentrate flow rate and the maximum feed flow rate per element: the
higher the fouling tendency of the feed water, the more conservative the limits of
these parameters. The proposed limits are recommended guidelines based on
many years of experience with FILMTEC membranes.
The average flux of the entire system, i.e. the system permeate flow rate related
to the total active membrane area of the system, is a characteristic number of a
design. The system flux is a useful number to quickly estimate the required number
of elements for a new project. Systems operating on high quality feed waters are
typically designed at high flux values whereas systems operating on poor quality
feed waters are designed at low flux values. However, even within the same feed
water category, systems are designed with higher or lower flux values, depending
on the focus being either on minimizing the capital expenses or minimizing the longterm operational expenses. The ranges of flux values given in the tables below are
typical numbers for the majority of systems, but they are not meant to be limits.
A continuous RO/NF process designed according to the system design guidelines
and with a well-designed and operated pretreatment system will show stable
performance with no more than about four cleanings per year in standard applications.
Exceeding the recommended limits may result in more frequent cleanings, reduced
capacity, increased feed pressure and reduced membrane life. A moderate violation
of the limits for a short time may be acceptable as long as the physical limits—the
maximum pressure drop and the maximum feed pressure—are not exceeded. On
the other hand, a conservative approach is to anticipate a higher fouling tendency
and to design the system according to the stricter limits in order to enjoy a troublefree system operation and an increased membrane life.
Membrane System
Design Guidelines
D