Membrane and Desalination Technologies - TCE Moodle Website

630 P. Kajitvichyanukul et al.
4. DESIGN EXAMPLES
4.1. Design of RO in POU and POE Applications
The basic principles for system design of pressure-driven membrane filtration include flux,
recovery, and flow balance (66, 83).
Flux in filtration process is defined as the filtrate flow per unit of membrane filtration area,
which is the membrane filtration system throughput. Calculation of flux is shown in Eq. (3)as
shown below (66):
J ¼ Qp
; (3Þ
Am
where J = water flux (m 3 /m 2 h), Qp = filtrate flow produced by the membrane unit (m 3 /h), and
Am = wetted surface area of membrane a (m 2 ).
The recovery of a membrane unit is defined as the amount of feed flow that is converted to
filtrate flow, expressed as a decimal percent, as shown in Eq. (4) (66):
R ¼ Qp
(4Þ
Qf
Here, R = recovery of the membrane unit (decimal percent), Qp = filtrate flow produced by the
membrane unit (m 3 /h), and Qf = feed flow to the membrane unit (m 3 /h).
A general flow balance that can be applied to all membrane filtration systems is shown in
Eq. (5) (66):
Qf ¼ Qc þ Qp
(5Þ
Here, Qf = feed flow to the membrane unit (m 3 /h), Qc = concentrate flow from the
membrane unit (m 3 /h), and Qp = filtrate flow produced by the membrane unit (m 3 /h).
For the system operating without a concentration waste or “bleed” stream mode or crossflow
systems in which 100% of the concentrate is recirculated, the concentrate (i.e., bleed or
reject) flow, Qc, is zero. In determining of the filtrate flow, Qp, for the purpose of sizing a
membrane filtration system, the additional filtered water for both backwashing and chemical
cleaning should be included. Similarly, an estimate of the total required feed flow Qf to the
system should incorporate any raw water that may be used in these routine maintenance
processes.
In the RO process, the driving force for the transport of water is a pressure gradient across
the membrane. The resulting osmotic pressure gradient against the transport of water from the
feed to the filtrate side of the membrane has to be considered. Typically, the osmotic pressure
gradient is approximated from the concentration of TDS on the feed and filtrate sides of the
membrane. The corrected driving force across the semipermeable membrane in an RO
process is termed the net driving pressure (NDP) and can be calculated by Eq. (6). In this
equation, NDP is in a simplified form by combining the respective parameters associated with
two terms, the transmembrane differential pressure (DP) and the transmembrane osmotic
pressure differential (Dp).
NDP ¼ DP Dp (6Þ