Natural organics removal using membranes

Membrane processes are increasingly used in water treatment. Experiments were performed using
stirred cell equipment, polymeric membranes and synthetic surface water containing natural organics,
inorganic colloids and their aggregates, and cations.
All processes could remove a significant amount of natural organics. Pretreatment with ferric chloride
was required to achieve significant organic removal with MF and high MWCO UF.
Additionally, fouling mechanisms for the three processes were investigated. Crucial parameters were
aggregate characteristics (fractal structure, stability, organic-colloid interactions), solubility of organics
and calcium, and hydrodynamics.
In MF, fouling by pore plugging was most severe. Variations in solution chemistry changed the
aggregation state of the colloids and/or natural organic matter and dramatically affected rejection and
fouling behaviour.
UF membrane fouling was mainly influenced by pore adsorption and could improve natural organics
rejection significantly. Coagulant addition shifted fouling mechanism from pore adsorption to cake
formation. Aggregate structure was most significant for flux decline.
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In NF, rejection of natural organics involved both size and charge exclusion. Fouling was caused by
precipitation of a calcium-organic complex and could be avoided by pretreatment with metal salt
coagulants.
Thorough chemical characterisation of the organics used demonstrated that only size and aromaticity
can be related to fouling.
The study is concluded with a process comparison based on a water quality parameter and a cost
comparison. Treatment cost of microfiltration with chemical pretreatment was similar to that of
nanofiltration at a comparable natural organics rejection.