Membrane fouling is one of the main obstacles for application of membrane distillation（MD)process.In this work

the most familiar organic solute

humic acid（HA)

and inorganic slightly soluble salt

calcium carbonate（CaCO3)

were selected as contaminants to study membrane fouling during MD process.The properties of feed solution during MD process

such as zeta potential and particle size distribution

were measured and used for illustrating the variation of feed solution properties.The variation of membrane surface morphology and contaminant chemical composition during MD process were characterized by Field emission scanning electron microscopy（FESEM)and X-ray photoelectron spectroscopy（XPS).Severe flux decline was observed for hybrid feed solution during DCMD

and could be roughly attributed into three reasons：（1)A considerable amount of HA aggregates aroused by concentration polarization in the feed decreased diffusion coefficient i.e.relative low mass transfer coefficient

of H2O molecules in boundary layer；(2)Fouling layer as filtration cake on membrane

consisting of deposits of HA aggregates and CaCO3

made mass transport resistance gradual increase; (3)As though all through DCMD manipulations any wetting was not detected（according to the permeate conductivity variations)

long-term DCMD operation may also induce pore wetting gradually especially in the pores located on membrane surface.At the time surface pores were filled with the feed

pores located inside membrane may still wonderfully dry

and this “semi-leakage” is of recoverability when process was shutdown.The effects of divalent metal counter ion Ca2+ of HA and its aggregation foulant on precipitation of CaCO3 onto membrane surface were studied.The results showed that Ca2+ could accelerate HA aggregation

HA aggregates with lower electronegativity adsorb preferentially onto hydrophobic poly(vinylidene fluoride)membrane surface and form organic contaminant layer

and CaCO3 could nucleate heterogeneously on the inner surface of the organic contaminant layer on hydrophobic membrane and crystal grows

and so modify crystal morphology.