Civil Engineering Reference
In-Depth Information
TABLE 15-8. RO and NF Operating and Performance Criteria
Process
Operating Pressure
Product Water Flux
Recovery
Nanofiltration
80-150 psi
15-25 gfd
70-90%
Reverse osmosis
150-600 psi
3-20 gfd
30-85%
Note:
gfd
gallons / sq ft / day.
Osmonics
•
Dow
•
DuPont
•
These manufacturers can provide technical assistance on specific applications and
many have software available to design engineers. These computer models can be used
to help select membrane configurations and predict performance.
The operating pressures for RO and NF are significantly higher than for MF and
UF. However, recent advances in membrane technology have reduced operating pres-
sure requirements and several manufacturers have developed ultra-low-pressure mem-
branes (100-150 psi). In order to maintain the flux of product water during operation,
the operating pressure of a membrane system usually must be increased with time.
The accumulation of contaminants upstream of the membrane increases the osmotic
pressure with time. Also, solids accumulation on the membrane and membrane fouling,
compaction, and degradation can reduce productivity if the operating pressure is not
increased.
Product water flux varies from one membrane to another, primarily due to mem-
brane material and manufacture. For a given membrane, flux is dependent upon a
number of conditions, including feedwater quality and recovery, as well as operating
pressure. The temperature of the feedwater affects the flux of product water through
viscosity; a higher temperature feedwater results in a higher product water flux. The
concentration of contaminants in the feed stream also influences flux.
The recovery of product water through a RO or NF system is generally lower than
with MF or UF. High recoveries are more difficult to achieve with RO and NF mem-
branes because of the resulting high concentration of dissolved contaminants in the
concentrate.
The removal capabilities of RO and NF systems vary with several factors, including
the specific membrane used, contaminant of concern, operating conditions, and feed-
water quality. Generally, RO membranes are used if the removal of monovalent ions
is required. NF membranes are used for the rejection of divalent ions (e.g., softening)
and organics. Membrane manufacturers can give an estimate of rejection capabilities
of their membrane for some contaminants based on anticipated feedwater character-
istics and operating conditions.
Pretreatment Requirements
Most applications of membrane systems require pretreatment to ensure effective and
reliable treatment. Pretreatment may be used to aid in contaminant removal or mini-
mize plugging, scaling, and fouling. Pretreatment processes can be categorized as
follows:
