Nanomembranes - Nanotechnologies for Water Environment Applications - page 383

assumed that the level of biodegradable organic matter (BOM) is the limiting nutrient
for bacterial growth (LeChevallier et al., 1991), although a small number of studies
suggest that phosphorus may be the limiting factor for some systems (Sathasivan et al.,
1997; Miettinen et al., 1999). Thus, BOM concentration, often measured as AOC and
BDOC, provides a good indicator for the bacterial regrowth potential of the treated
water. Furthermore, AOC has actually been correlated with bacterial counts in water
distribution systems (LeChevallier et al., 1987; Van der Kooij, 1992). Most commonly,
Van der Kooij (1992) suggested 10 g/L AOC (acetate equivalents) as the threshold
value for stability of non-chlorinated drinking water, while LeChevallier et al. (1996)
placed a limit of 100 g/L AOC on the potential regrowth of Escherichia coli in
drinking water. Similarly, Vital et al. (2007) reported 50-100 g/L AOC as a minimum
for Vibrio. cholerae regrowth.
Sibille et al. (1997) performed studies using groundwater which was treated via
ozonation followed by biological granular activated carbon filtration and NF in order to
investigate the effects of membrane treatment on biological regrowth potential using NF.
They concluded that NF produced a considerable gain in potable water quality by
decreasing the bacterial counts [from 820 to 340 colony forming units (CFU)/mL], DOC
(from 1.6 to 1.4 ppm) and BDOC (from 0.35 to 0.25 ppm). However, NF allows the
permeate containing a low concentration of biodegradable organic matters (BDOC 0.25
ppm) that might still have had a significant fraction of AOC in it. Unfortunately, AOC
was not detected. Noble et al. (1996) determined AOC removals by NF for treating raw
groundwater contained TOC at 11 ppm and AOC averaging 362 μg acetate-C/L. A
significant reduction in permeate TOC (0.63 ppm) was found, but no significant
difference between AOC values of the influent and permeate (AOC 334 μg acetate-C/L),
indicating that NF membrane did not produce biologically stable water. Escobar et al.
(2000) reported rejection > 90% for all the case at pH = 5.5 when RO membrane was
used to treat the artificial drinking water with AOC of ~256 μg acetate-C/L.
Two NF membranes (HL, NF270) and one high-flux RO (XLE) membrane have
been investigated by Meylan et al. (2007). The regrowth potential assessed by
measurement of the AOC showed, indeed, that AOC is poorly retained by NF. These
analytical findings suggest that microbiologically unstable water be produced by NF as
organic compounds of LMW are the preferred carbon source for microorganisms. The
high-flux RO membrane tested showed clearly a better retention of AOC and offered an
efficient barrier in terms of microbial stability by reducing the AOC concentration by
about a factor of 10 compared with the raw water (Figure 12.8).
Agbekodo et al. (1996) treated water from the River Oise at the Méry-sur-Oise
Plant (France) by NF using FilmTec NF70 (Dow Chemical, Germany) membranes. The
treatment process consisted of clarification and sand-filtration, followed by the addition
of sulfuric acid to drop the pH to 6.5, 10 and 5 m filters, and a three-stage NF process.