NAMS 2002 Workshop - ICOM 2008

days of filtration from 120 to 7-10 L/hm 2 and remained stable for the studied
period of 76 days independently on fluctuations of feed water quality. Upon
variations of the pressure, the flux varied temporarily, but afterwards stabilized
again to a value around 7-10 L/hm 2 . This implies that the permeability decreases
with increasing pressures. Sodium azide (1.5%) was added to the feed in order
to suppress biological activity without changing the NOM composition. The
results show that the initial flux decline was similar to the first experiment, but no
flux stabilization was observed and the flux continuously declined. From these
results it can be concluded that biological activity has an important influence on
the flux stabilization. Therefore, microbiological parameters were studied more in
detail. The bioactivity in the feed and on the membrane was measured using the
ATP method. The results show that the bioactivity on the membrane increases
within the first 2 days and then stabilizes. The ATP material balance shows that
during time the proportion of active cells on the membrane decreases, indicating
cell die-off in the depth of the fouling layer. In the presence of sodium azide,
biological activity (ATP) on the membrane was much lower. In the absence of
sodium azide, the concentration of assimilable organic carbon (AOC) was
determined in the feed water and it was observed that AOC correlates with the
activity on the membrane (ATP). Considering that during time increasing
amounts of suspended material from the feed water are deposited on the
membrane, the stabilizing flux implies that the specific resistance of the fouling
layer (m/m 2 ) decreases. Based on the results, we postulate that this is based on
biologically induced cell die-off, resulting in cavity formation and increased
porosity.
More detailed results which support these mechanisms will be presented.
In contrast to conventional membrane plants, neither the membrane surface nor
the capacity is a limiting factor for application of POU systems. While the
required productivity of POU system is approx. 20-50 L/day, and assuming the
flux of 7-10 L/hm 2 the required membrane surface to provide the required
capacity is 0.12-0.21 m 2 . Assuming a membrane price of 40 US$/m 2 , this
corresponds to US$ 4.8-8.4 of membrane costs per POU system. Thus, in case
the membrane life time is several years, the membrane costs are not prohibitive
for application in developing and transition countries. Moreover, the system can
be operated without pumps under hydrostatic pressure of 40 mbar or less,
without regular backflushing, any addition of chemicals or multi-stage
pretreatment.