pressure of 900 kPa. Further increasing the concentration
factor to FC=5 decreases the limiting flux to a very low
value of 14 l m-2 h-1. Clearly, at this high concentration factor,
the reduction of the trans-membrane pressure cannot be
explained only by the increase of the effluent osmotic pressure.
Based on the approximate values of the osmotic pressure
of the effluent at FC=1 and FC=2, the osmotic pressure
at FC=5 should be about 250 kPa, which is relatively low
compared with the trans-membrane pressure of 800 kPa.
Therefore, the permeate flux is also controlled by a fouling
or gel layer of concentrated organic matter that is deposited
on the membrane surface. To reduce this additional mass
transfer resistance it would be necessary to increase the
mass transfer coefficient in the nanofiltration membrane
module. This is not possible with the spiral wound module
used, because it was operated already at the maximum possible
recirculation flow-rate. Therefore, to improve the permeate
flux and increase further the concentration factor of
the effluent by nanofiltration it would be necessary to use
tubular modules that enable the operation at higher recirculation
velocities.
The gel or fouling layer was easily removed after cleaning
the membrane during 10 min. with a cleaning solution
of Ultrasil 11, and this shows that the membrane initial permeation
fluxes can be effectively recovered through cleaning.
The rejection coefficients of the TOC and conductivity
are displayed in Fig. 2 versus the concentration factor. As
shown in this figure, there is no significant change in the
conductivity and TOC rejection coefficients with CF during
NF of cork processing wastewaters. The TOC rejection
coefficient presents an average value of 95% and the conductivity
rejection coefficient value is around 60%. The
permeation of cork processing wastewaters by membranes,
namely ultrafiltration [16-18], has shown that there is a formation
of a dynamic layer over the membrane surface that
acts as a new surface which is tighter than the original. The
effect of this phenomenon is more pronounced when fouling
is more severe and expected results like, for example,
the decrease of salt rejection with the increase of its concentration,
are not observed.
These results show that nanofiltration has the capacity
of concentrating the organic matter of the effluent with
simultaneous partial demineralization of the final concentrate.
The NF permeate has a far lower pollutant charge than
the original effluent, and its further treatment or disposal
should not pose severe problems, as almost all the compounds
that are difficult to treat, namely the tannins, were
retained and concentrated by the NF step in the concentrate
stream.
As shown in Table 1, the TOC content of the concentrated
effluent is almost threefold of the initial TOC value.
pressure of 900 kPa. Further increasing the concentrationfactor to FC=5 decreases the limiting flux to a very lowvalue of 14 l m-2 h-1. Clearly, at this high concentration factor,the reduction of the trans-membrane pressure cannot beexplained only by the increase of the effluent osmotic pressure.Based on the approximate values of the osmotic pressureof the effluent at FC=1 and FC=2, the osmotic pressureat FC=5 should be about 250 kPa, which is relatively lowcompared with the trans-membrane pressure of 800 kPa.Therefore, the permeate flux is also controlled by a foulingor gel layer of concentrated organic matter that is depositedon the membrane surface. To reduce this additional masstransfer resistance it would be necessary to increase themass transfer coefficient in the nanofiltration membranemodule. This is not possible with the spiral wound moduleused, because it was operated already at the maximum possiblerecirculation flow-rate. Therefore, to improve the permeateflux and increase further the concentration factor ofthe effluent by nanofiltration it would be necessary to usetubular modules that enable the operation at higher recirculationvelocities.The gel or fouling layer was easily removed after cleaningthe membrane during 10 min. with a cleaning solutionof Ultrasil 11, and this shows that the membrane initial permeationfluxes can be effectively recovered through cleaning.The rejection coefficients of the TOC and conductivityare displayed in Fig. 2 versus the concentration factor. Asshown in this figure, there is no significant change in theconductivity and TOC rejection coefficients with CF duringNF of cork processing wastewaters. The TOC rejectioncoefficient presents an average value of 95% and the conductivityrejection coefficient value is around 60%. Thepermeation of cork processing wastewaters by membranes,namely ultrafiltration [16-18], has shown that there is a formationof a dynamic layer over the membrane surface thatacts as a new surface which is tighter than the original. Theeffect of this phenomenon is more pronounced when foulingis more severe and expected results like, for example,the decrease of salt rejection with the increase of its concentration,are not observed.These results show that nanofiltration has the capacityof concentrating the organic matter of the effluent withsimultaneous partial demineralization of the final concentrate.The NF permeate has a far lower pollutant charge thanthe original effluent, and its further treatment or disposalshould not pose severe problems, as almost all the compoundsthat are difficult to treat, namely the tannins, wereretained and concentrated by the NF step in the concentratestream.As shown in Table 1, the TOC content of the concentratedeffluent is almost threefold of the initial TOC value.
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