Nanomembranes - Nanotechnologies for Water Environment Applications - page 402

more than 80% were achieved with a total hardness rejection of 86.5%. The success of
this pilot-scale testing led to a field-scale demonstration at the existing Umm Luji
SWRO plant, Saudi Arabia. In this field-scale project, one of the conventional SWRO
process was converted to an NF-SWRO unit, while the other conventional SWRO unit
was retained in operation; the performances of the two different operating modes were
compared while operating concurrently. The pretreatment prior to the NF-SWRO and
the conventional SWRO mode are the same, consisting of gravity filters and pressure
filters. This earlier pretreatment was able to produce feed quality with SDI of less than
3.5 to both NF-SWRO and the conventional SWRO units. The removal of hardness and
TDS achieved by the NF membrane at pilot-scale and commercial-scale are summarized
in Table 12.16. The rejection of divalent ions such as Mg 2+ , Ca 2+ and CO 3 2- were higher
in the commercial scale compared to the pilot scale NF membrane. Thus, this gave rise
to a higher hardness removal at commercial scale as reported by Hassan et al. (2002).
Table 12.16 NF membrane rejection rates for some parameters.
Rejections (%)

Parameter

Pilot-Scale 1 Field-Scale 2

TDS 37.3 38

Hardness 86.5 97

Monovalent Ion

Cl - 26.7 24

HCO 3 - 63.3 56

Divalent Ion

Mg 2+ 87.7 98

Ca 2+

80.7 92

SO 4 2- 93.3 99.9

1 Hassan et al. (1997). 2 Hassan et al. (2002).

The NF-SWRO mode (56%) was able to double the recovery ratio of the
conventional SWRO unit (28%) at the commercial-scale (Hassan et al., 2002). This NF-
SWRO recovery was even higher than the 84% increase in the recovery tested at the
pilot scale (Hassan et al., 1997). This observation shows a promising potential on better
performance if the NF-SWRO were to be applied at full scale for similar seawater
quality as feed. The pilot- and the commercial-scale NF-SWRO plants demonstrated that
the NF pretreatment of seawater feed to the RO plants was able to: (i) reduce the
potential of biofouling and scaling due to the high removal of microorganisms and scale
forming hardness ions without the addition of antiscalant; and (ii) lower the required
pressure to operate the SWRO unit due to the ability of the NF to reduce the TDS in the
seawater by ~37%. These factors, in turn, contributed to a lower operating cost and