Seawater pretreatment by gravity-driven membrane (GDM)
fi
ltration at 40 mbar has been investigated.
In this system, a bene
fi
cial bio
fi
lm develops on the membrane that helps to stabilize
fl
ux. The effects of
membrane type, pre
fi
ltration and system con
fi
guration on stable
fl
ux, bio
fi
lm layer properties and dis-
solved carbon removal were studied. The results show that the use of
fl
at sheet PVDF membranes with
pore sizes of 0.22 and 0.45
m
m in GDM
fi
ltration achieved higher stabilized permeate
fl
uxes (7.3
e
8.4 L/
m
2
h) than that of
fl
at sheet PES 100 kD membranes and hollow
fi
bre PVDF 0.1
m
m membranes. Pore
constriction and cake
fi
ltration were identi
fi
ed as major membrane fouling mechanisms, but their
relative contributions varied with
fi
ltration time for the various membranes. Compared to raw seawater,
pre
fi
ltering of seawater with meshes at sizes of 10, 100 and 1000
m
m decreased the permeate
fl
ux, which
was attributed to removal of bene
fi
cial eukaryotic populations. Optical coherence tomography (OCT)
showed that the porosity of the biofouling layer was more signi
fi
cantly related with permeate
fl
ux
development rather than its thickness and roughness. To increase the contact time between the bio
fi
lm
and the dissolved organics, a hybrid bio
fi
lm-submerged GDM reactor was evaluated, which displayed
signi
fi
cantly higher permeate
fl
uxes than the submerged GDM reactor. Although integrating the bio
fi
lm
reactor with the membrane system displayed better permeate quality than the GDM
fi
ltration cells, it
could not effectively reduce dissolved organic substances in the seawater. This may be attributed to the
decomposition/degradation of solid organic substances in the feed and carbon
fi
xation by the bio
fi
lm.
Further studies of the dynamic carbon balance are required.